Mechanism of Crystal Structure Transformation and Abnormal

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Mechanism of Crystal Structure Transformation and Abnormal Reduction in Ca5−y(BO3)3−x(PO4)xF (CBPxF):yBi3+ Xue Li, Zhijun Wang,* Jinjin Liu, Xiangyu Meng, Keliang Qiu, Qi Bao, Yuebin Li, Zhipeng Wang, Zhiping Yang, and Panlai Li*

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College of Physics Science & Technology, Hebei Key Laboratory of Optic-Electronic Information and Materials, Hebei University, Baoding 071002, China ABSTRACT: Tricoordinated planar triangle (PO4)3− may be formed due to the structural differences between planar triangular (BO3)3− and tetrahedral (PO4)3− when (BO3)3− is gradually substituted by (PO4)3−. This transformation of structure may affect the luminescence properties of phosphor. Therefore, a series of Ca5−y(BO3)3−x(PO4)xF (CBPxF):yBi3+ (y = 0.05, 0.15; x = 0−3), Ca5−y(PO4)3−X(BO3)XF (CPBXF):yBi3+ (y = 0.05, 0.15; X = 0−1), Ca4.9(PO4)3F (CPF):0.1Eu3+, Ca4.95(PO4)3F (CPF):0.05Bi3+, and nCaF2/CaCl2 (n = 0−0.1) are synthesized to explore transformation of the crystal structure on luminescence properties. In CBPxF:0.15Bi3+ (x = 0−3), (PO4)3− is doped to substitute for (BO3)3−, the position of emission spectra remains unchanged and the emission intensity decreases rapidly with increasing x. The underlying main reason for that is formation of the triangular plane (PO4)3−, which has been verified by performing a series of verification experiments of CPBXF:yBi3+ (y = 0.5, 0.15; X = 0−1). In CPBXF:yBi3+ (y = 0.5, 0.15; X = 0−1), (BO3)3− is doped to substitute for (PO4)3−, P− O2 bond breaks and the coordination of (PO4)3− varies from four to three when 0.5 < X < 1; meanwhile, the crystal structure transforms from Ca5(PO4)3F (ICSD-9444) to Ca5(PO4)3F (ISCD-30261), which impedes abnormal reduction from Bi3+ to Bi2+. Furthermore, Bi3+ should non-luminance in the plane triangular (PO4)3−, but luminescence in (BO3)3−. Therefore, the emission intensity starts to increase and the emission position suddenly changes from 553 to 474 nm in CPBXF:yBi3+ (y = 0.05, 0.15; 0.5 < X < 1). From this, the crystal structures of CBPxF:yBi3+ (y = 0.05, 0.15; x = 0−3) has been inferred to transform from Ca5(BO3)3F (ISCD-65763) to Ca5(PO4)3F (ISCD-30261), and then to Ca5(PO4)3F (ISCD-9444) with x increasing. Emission position remains unchanged and the emission intensity decreases rapidly in CBPxF:yBi3+ (y = 0.05, 0.15; x = 0−3) do to formation of the triangular plane (PO4)3−. In addition, the rate of abnormal reduction from Bi3+ to Bi2+ can be improved by reducing the electronegativity of the environment around the activator or increasing the ionization energy of the activator, which has been confirmed by verification experiments of CPF:0.05Bi3+, nCaF2/CaCl2 (n = 0−0.1), and CPF:0.1Eu3+. tuned from 421 to 463 nm.4 It is found that all of the above phosphors are based on the isostructural phase to modify the luminescence properties. (Sr,Ba)3SiO5 and (Sr,Ba)3AlO4F are the isostructural phases in (1 − x)Sr3SiO5x(Sr,Ba)3AlO4F:Ce3+, α-Sr2SiO4 (JCPDS 38-271) and βSr2SiO4 (JCPDS 39-1256) in LuxSr1.97−xSiNxO4−x:Eu2+, and La5Si2BO13 and La5Si3O12N in La5(Si2+xB1−x)(O13−xNx):Ce3+.2−4 That is to say, there is a stringent specification for the construction of a host via an anionic polyhedral substitution modification approach. However, there are a number of exceptions: for instance, although both Ca5(PO4)3F and Ca5(VO4)3F are assigned to the hexagonal crystal system, they are located in different group spaces. The variations of the emission and excitation spectra of Ca5[(P,V)O4)]3F:Eu3+ can still be observed depending on the varied (PO4)3−/(VO4)3− substitution amounts.6 Similarly, in spite of

1. INTRODUCTION Recently, as a modification approach, anionic polyhedral substitution has been become a suitable strategy to develop new phosphor materials. The variable composition can provide rich crystal-field environments and a wide range of cationic substitution, which contributes to alteration of the crystal-field environment and coordination of the activator in the compound. Luminescence properties, such as the thermal stability and spectral position, can be effectively improved. Thus, anionic polyhedral substitution has become an effective way of designing a new host to modify the luminescence properties.1−9 For example, through chemical unit cosubstitution of (Al3+-F−) for (Si4+-O2−), the emission peak shifts from 520 to 490 nm with increasing (Al3+-F−) content in (1 − x)Sr3SiO5-x(Sr,Ba)3AlO4F:Ce3+.2 The emission peak of LuxSr1.97−xNxO4−x:0.03Eu2+ redshifts from 563 to 583 nm with increasing (Lu3+-N3−) content.3 The thermal stablility of La5(Si2+xB1−x)(O13−xNx):Ce3+ can be improved by (B3+-O2−) substitution for (Si3+-N3−), and the emission peak can be © XXXX American Chemical Society

Received: August 16, 2018

A

DOI: 10.1021/acs.inorgchem.8b02317 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

concentration quenching in Ca5(PO4)3F when the concentration of Bi3+ is 0.15; however, it is denied by a verification experiment on CBPxF:0.05Bi3+ [x = 0−3; Bi3+ luminescence suffers concentration quenching when Bi3+ is 0.05 in Ca5(PO4)3F]. Hypothesis 2 is that the structural difference between (BO3)3− and (PO4)3− results in no change of the emission position and a decrease of the emission intensity in CBPxF:0.15Bi3+ (x = 0−3), which is confirmed to be correct through the verification experiment on CPBXF:yBi3+(y = 0.05, 0.15; X = 0−1). (BO3)3− is doped to substitute for (PO4)3−, the P−O2 bond breaks gradually, and the coordination of (PO4)3− varies from four to three when 0.5 < X < 1; meanwhile, the crystal structure will transform from the α phase of Ca5(PO4)3F (ICSD-9444) to the β phase of Ca5(PO4)3F (ISCD-30261), which impedes abnormal reduction from Bi3+ to Bi2+; furthermore, Bi3+ should not luminesce in a triangular plane (PO4)3−, but (BO3)3− is responsible for Bi3+ luminescence. Therefore, it is seen that the emission intensity starts to increase and the emission peak suddenly changes from 553 to 474 nm in CPBXF:yBi3+ (y = 0.05, 0.15; 0.5 < X < 1) because of the formation of three coordinations of (PO4/BO3)3−. Also, it is found that the emission peak (462 nm) in CBPxF:yBi3+ (y = 0.05, 0.15; x = 0−3) remains unchanged and the emission intensity continuous decreases because of the formation of three coordinations of (PO4)3− with decreasing of (BO3)3−. On the basis of verification of the above two hypotheses and a series of analyses, it can be inferred that the crystal structure should transform from Ca5(BO3)3F (ISCD-65763) to Ca5(PO4)3F (ISCD-30261) and then to Ca5(PO4)3F (ISCD-9444), with x increasing in CBPxF:yBi3+(y = 0.05, 0.15; x = 0−3) because of formation of the triangular plane (PO4)3−, which results in no change of the emission peak and a decrease of the emission intensity. In addition, the rate of abnormal reduction from Bi3+ to Bi2+ can be improved by reducing the electronegativity of the environment around the activator or increasing the ionization energy of the activator, which has been confirmed by verification experiments of CPF:0.05Bi3+, nCaF2/CaCl2 (n = 0−0.1), and CPF:0.1Eu3+. This research has certain reference values to design the host via anionic group substitution.

the fact that the hosts of Sr5(PO4)3Cl and Sr5(BO3)Cl do not belong to the isostructural phase nor are they in the same crystal system and space group, the color-tunable emission of Sr5(PO4)3−x(BO3)xCl:Eu2+ can still be obtained via (BO3)3− substitution for (PO4).3−7 The above results reveal that the luminescence properties are closely related to the host characteristics, but it does not necessarily require that the host belong to the isostructure and be in the same crystal system. In recent years, borate and phosphate host materials have attracted great attention because of the facts that borate contains abundant cation sites and phosphate can provide abundant crystal-field environments. In order to explore the relationship between the crystal structure and luminescence properties, Ca5(BO3)3F and Ca5(PO4)3F are selected as hosts. Ca5(BO3)3F contains three cation sites, which can provide a wide range of possible cationic substitutions.10,11 Ca5(PO4)3F incorporates various foreign ions, which provide rich crystalfield environments. In addition, Ca5(PO4)3F has a characteristic crystal structure and contains tetrahedral anion groups PO4 surrounding cations, which will bring about abnormal reduction.12−14 Abnormal reduction has been reported in some materials, such as CaAl2O 4:Eu3+ , CsAlSi2O 6:Eu3+, and SrB4O7:Eu3+.15−20 They shared a common feature that contains tetrahedral anion groups, such as PO4, BO4, SiO4, and AlO4, by which the substituted cation is surrounded.15−20 Meanwhile, in this work, Bi3+ is selected as the activator 3+ because of the fact that the difference between Bi3+ (rBi = 1.03 2+

Å) and Ca2+ (rCa = 1.00 Å) is small in radius and will not cause significant changes in the crystal structure. Furthermore, Bi3+ is a variable-valence ion and satisfies a condition for abnormal reduction in Ca5(PO4)3F; namely, low-valence-ion Ca2+ is substituted by high-valence-ion Bi3+. Besides, Bi3+ is easily affected by the external environment, including the coordination environment and anionic polyhedron around Bi3+, because of the sensitivity of Bi3+ naked 6s electrons and 6p electrons to the crystal field particularly surrounding the Bi3+ ions. In Ca5(BO3)3F, the luminescence properties of Bi3+ with different coordination environments have been studied in detail in previous research. However, the effects on the luminescence properties of Bi3+ of changing the anionic polyhedron have not been studied so far. Therefore, a series of CBPxF:yBi3+ (y = 0.05, 0.15; x = 0−3), CPBXF:yBi3+ (y = 0.05, 0.15; X = 0−1), CPF:0.1Eu3+, CPF:0.05Bi3+, and nCaF2/CaCl2 (n = 0−0.1) were synthesized to investigate the relationship between the structural variation of the anionic polyhedron and luminescence properties. In contrast, the luminescence properties of phosphors, such as the thermal stability and spectral position, can be tuned by anion group substitution. The position of emission spectra remains unchanged, and the emission intensity decreases rapidly with increasing x in CBPxF:0.15Bi3+ (x = 0−3). The underlying reasons for that are discussed in detail. It is known that (BO3)3− is usually three coordination to form a triangular plane, and (PO4)3− is usually four coordination to form a tetrahedron. Thus, three coordination of (PO4)3− may be formed due to the structural difference between (BO3)3− and (PO4)3− when (BO3)3− is gradually substituted by (PO4)3−. This transformation of the structure may destroy the luminescence properities of phosphors. In order to find the real causes, two hypotheses were built, and a series of validating experiments were carried out. Hypothesis 1 is that Bi3+ luminescence has been

2. EXPERIMENTAL SECTION 2.1. Material and Synthesis. A series of CBPxF:yBi3+ (y = 0.05, 0.15; x = 0−3), CPBXF:yBi3+ (y = 0.05,0.15; X = 0−1), CPF:0.05Bi3+, nCaF2/CaCl2 (n = 0−0.1), and CPF:0.1Eu3+ were synthesized by a high-temperature solid-state reaction. Raw materials H3BO3 (analytical reagent, A.R.), NH4H2PO4 (A.R.), CaCO3 (A.R.), CaF2 (A.R.), CaCl2 (A.R.), Eu2O3 (99.99%), and Bi2O3(99.99%) were thoroughly mixed in a stoichiometric ratio, ground for more than 30 min in an agate mortar, put in an alumina crucible, and sintered at 1150 °C for 4 h. Finally, all of the obtained samples were cooled to room temperature in the furnace and ground into powder for further measurement. The chemical reactions can be expressed as follows:

(4.5 − y)CaCO3 + 0.5CaF2 + (3 − x)H3BO3 + x NH4H 2PO4 y + Bi 2O3 → Ca5 − yBi y(BO3)3 − x (PO4 )x F 2

(4.5 − y)CaCO3 + 0.5CaF2 + X H3BO3 + (3 − X )NH4H 2PO4 y + Bi 2O3 → Ca5 − yBi y(PO4 )3 − X (BO3)X F 2 B


Article

Inorganic Chemistry

Figure 1. (a) Crystal structure of Ca5(PO4)3F (ICSD-9444) and coordination environment of Ca1 and Ca2. (b) Crystal structure of Ca5(PO4)3F (ICSD-30261) and coordination environment of Ca1* and Ca2*. (c) Crystal structure of Ca5(BO3)3F (ICSD-65763) and coordination environment of Ca1**, Ca2**, and Ca3**.

Figure 2. XRD patterns and the standard of Ca5(BO3)3F (ICSD-65763) and Ca5(PO4)3F (ICSD-30261 and ICSD-9444): (a) CBPxF:0.15Bi3+; (b) CBPxF:0.05Bi3+. n 1+n )CaCO3 + CaF2 + 3NH4H 2PO4 + 0.025Bi 2O3 2 2 → Ca4.95Bi 0.05(PO4 )3 F1 + n

3. RESULT AND DISCUSSION 3.1. Phase Characterization and Crystal Analysis. Parts a−c of Figure 1 depict the crystal structures of Ca5(PO4)3F (ICSD-9444), Ca5(PO4)3F (ICSD-30261), and Ca5(BO3)3F (ISCD-65763), respectively. Ca5(PO4)3F (ICSD9444) and Ca5(PO4)3F (ICSD-30261) have the same XRD diffraction pattern; however, they possess different crystal structures, and both crystallize in a hexagonal structure with a P63/m space group. The reason is that one of the Ca−O bonds will break or connect when Ca5(PO4)3F is located in different synthetic environments or contains different concentrations of (PO4)3−. Thus, Ca5(PO4)3F (ICSD-9444) and Ca5(PO4)3F (ICSD-30261) should be defined as the α and β phases of Ca5(PO4)3F, respectively. There are two cationic sites in the α phase of Ca5(PO4)3F (ICSD-9444), which are referred to as Ca1 and Ca2 and locate in the 4f and 6h sites, respectively. Ca1 coordinates with six O atoms, which is surrounded by six (PO4)3− groups. P coordinates with four O atoms, forming a tetrahedron. Ca2 coordinates with five O atoms and one F atom. The β phase of Ca5(PO4)3F (ICSD30261) shows two cationic sites as well, named Ca1* and Ca2*, which locate in the 4f and 6h sites, respectively. Ca1* coordinates with six O atoms, which is also surrounded by six (PO4)3− groups; however, the coordination of (PO4)3− is three in which three O atoms connect with P and the other O atom is above P, forming a triangular plane. Ca2* coordinates with

(4.45 −

n 1 )CaCO3 + CaF2 + 3NH4H 2PO4 + 0.025Bi 2O3 2 2 n + CaCl 2 → Ca4.95Bi 0.05(PO4 )3 FCl n 2

(4.45 −

2.2. Material Characterization. The crystal structures were identified by X-ray diffraction (XRD; a D8-A25 Focus diffractometer at 40 kV and 40 mA). In order to determine the change in the crystal structure, Rietveld structure refinement was performed by General Structure Analysis System (GSAS). Meanwhile, high-resolution transmission electron microscopy (HRTEM) images were obtained using 200 kV (JEOL-2010UHR). Spectral properties were detected at room temperature on a Hitachi F-4600 fluorescence spectrophotometer. The decay curve was collected using 300 nm pulse laser radiation (nano-LED) as the excitation resource at room temperature. Chemical compositions were detected by a Nova Nano SEM 650 instrument and an attached electron-dispersive X-ray (EDX) spectrometer. The thermoluminescence spectra of the samples were measured using a FJ-427A1 TL dosimeter with a fixed heating rate of 1 °C s−1 within the range of 50−300 °C. X-ray photoelectron spectroscopy (XPS) was performed on a Quantera II X-ray photoelectron spectrometer with an excitation source of Al Kα. C


Article

Inorganic Chemistry

Figure 3. HRTEM images of CBPxF:0.15Bi3+, where (a) x = 0.7, (b) x = 1.2, (c) x = 1.5, (d) x = 1.7, (e) x = 2.5 and (f) x = 3. HRTEM images of CBPxF:0.05Bi3+, where (a′) x = 0.7, (b′) x = 1.2, (c′) x = 1.5, (d′) x = 1.7, (e′) x = 2.5, and (f′) x = 3. D


Article

Inorganic Chemistry five O atoms and one F atom. Compared with the α and β phases of Ca5(PO4)3F, Ca5(BO3)3F shows a different crystal system, space group, and crystal structure. Ca5(BO3)3F crystallizes in a monoclinic structure with a C1m1 space group, which has three cationic sites, referred to as Ca1**, Ca2**, and Ca3**, and locates in the 4b, 4b, and 2a sites, respectively. Ca1** coordinates with five O atoms and one F atom. Ca2** coordinates with six O atoms. Ca3** coordinates with four O atoms and two F atoms. Also, they are not surrounded by (BO3)3− groups; B coordinates with three O atoms, forming a triangular plane. XRD patterns of CBP x F:0.15Bi 3+ (x = 0−3) and CBPxF:0.05Bi3+ (x = 0−3) are shown in Figure 2. It can be shown that the diffraction peaks of two sets of samples all gradually vary from the standard card of Ca5(BO3)3F (ICSD65763) to those of Ca5(PO4)3F (ICSD-30261 and ICSD9444) with increasing (PO4)3− concentration. It is seen that the diffraction peaks of two sets of samples basically match well with the standard card of Ca5(BO3)3F (ICSD-65763) when the (PO4)3− content is less than 0.7, and then part of the diffraction peaks begin to appear at about 28° and 37° when the (PO4)3− content is between 0.7 and 2.5. The diffraction peaks of two sets of samples completely change from Ca5(BO3)3F (ICSD-65763) to Ca5(PO4)3F (ICSD-30261 and ICSD-9444), and there is no impure phase when the (PO4)3− content is more than 2.5. It is known that (PO4)3− coordinates with four, forming a tetrahedron, and (BO3)3− coordinates with three, forming a plane triangle generally; thus, (PO4)3− of the plane triangle may form when (BO3)3− is substituted by (PO4)3−. This infers that two sets of samples should be changed from Ca5(BO3)3F (ICSD-65763) to Ca5(PO4)3F (ICSD-30261) and then Ca5(PO4)3F (ICSD9444) when the (PO4)3− content is varied from 0 to 3. Because the raw material is NH4H2PO4 and (PO4)3− of a plane triangle will formation when a part of (BO3)3− is substituted by (PO4)3− gradually; thus, new diffraction peaks at about 28° and 37° should belong to (PO4)3− of the plane triangle. Generally, information about the internal structure of the crystal can be obtained by HRTEM;20−26 in order to further confirm the transformation from Ca5(BO3)3F (ICSD-65763) to Ca5(PO4)3F (ICSD-30261) and then to Ca5(PO4)3F (ICSD-9444), the microstructures of CBPxF:0.15Bi3+ (x = 0.7, 1.2, 1.5, 1.7, 2.5, and 3) and CBPxF:0.05Bi3+ (x = 0.7, 1.2, 1.5, 1.7, 2.5, and 3) are characterized by HRTEM, and the fast Fourier transformation (FFT) pattern is measured according to the HRTEM measurement, as shown in Figure 3. Figure 3a (CBP0.7F:0.05Bi3+) exhibits the morphology and lattice fringes with a d spacing of 7.1123, 8.0256, and 2.9766 Å based on the (110), (020), and (150) planes of Ca5(BO3)3F (7.1465, 8.0225, and 2.9783 Å; ICSD-65763), respectively. Figure 3a′ (CBP0.7F:0.15Bi3+) presents lattice fringes with observed interplanar distances d of 2.9329, 2.8300, and 3.1994 Å, which are keeping with the lattice distance on the (111), (240), and (021) planes of Ca5(BO3)3F (2.9349, 2.8296, and 3.1887 Å; ICSD-65763). Also, the corresponding FFT images are symmetrically arranged on different crystal faces. The results of the HRTEM and FFT images confirm that CBPxF:0.05Bi3+ (y = 0.5, 0.15; x = 0−0.7) is a single phase when the (PO4)3− content is less than 0.7. The crystal structure corresponds to Ca5(BO3)3F (ICSD-65763). Figure 3b clearly displays the resolved lattice fringes of CBP1.2F:0.05Bi3+, and the values of the associated interplanar spacing d are about 7.2202, 3.3558, and 4.5024 Å, which

correspond to the (110), (−111), and (130) crystal planes of Ca5(BO3)3F (ICSD-65763; d(110) = 7.1465 Å, d(−111) = 3.3573 Å, d(130) = 4.4441 Å, respectively). Figure 3b′ clearly shows well-resolved lattice fringes with the estimated interplanar spacings d of 4.4523, 7.9872, and 2.9295 Å, which are consistent with the (130), (020), and (111) planes of Ca5(BO3)3F (ICSD-65763; d(130) = 4.4441 Å, d(020) = 8.0255 Å, and d(111) = 2.9349 Å), respectively. Meanwhile, the FFT images are also symmetrically arranged on different crystal faces. Although a part of diffraction peaks which belong to (PO4)3− of plane triangle gradually get appear and a part of diffraction peaks which corresponds to Ca5(BO3)3F (ICSD65763) gradually get weaker, the results of HRTEM and FFT images indicate that CBPxF:0.05Bi3+ (y = 0.5, 0.15; x = 0.7− 1.2) is still single phase of Ca5(BO3)3F (ICSD-65763) when the concentration of (PO4)3− is 0.7−1.2. Figure 3c illustrates that CBP1.5F:0.05Bi3+ exhibits lattice fringes with d spacings of 3.4946, 2.8070, and 2.7797 Å, corresponding to the (201), (211), and (112) plane lattices of Ca5(PO4)3F (ICSD-30261) (d(201) = 3.4949 Å, d(211) = 2.8013 Å, and d(112) = 2.7728 Å). Figure 3c′ shows the resolved lattice fringes of CBP1.5F:0.15Bi3+, and the values of the associated interplanar spacing d are about 3.4464, 3.0846, and 5.2521 Å; compared with the theoretical values of Ca5(BO3)3F (ICSD-65763) and Ca5(PO4)3F (ICSD-30261 and ICSD-9444), the interplanar spacing d of CBP1.5F:0.15Bi3+ tends to match well with the theoretical values of Ca5(PO4)3F (ICSD-30261), corresponding to the (002), (210), and (101) crystal planes of Ca5(PO4)3F (ICSD-30261) (d(002) = 3.4400 Å, d(210) = 2.0670 Å, and d(101) = 5.2452 Å). The corresponding FFT images are arranged symmetrically on different crystal faces as well. The results of HRTEM and FFT reveal that CBPxF:yBi3+ (y = 0.5, 0.15; x = 1.2−1.5) is still single phase and the crystal structure has changed from Ca5(BO3)3F to Ca5(PO4)3F (ICSD-30261) when the concentration of (PO4)3− is 1.2− 1.5. The HRTEM image (Figure 3d) of CBP1.7F:0.05Bi3+ shows a d spacing of 2.6223 Å, which is consistent with that of the (202) crystal face of Ca5(PO4)3F (ICSD-30261; d(202) = 2.6239 Å). Figure 3d′ shows the typical HRTEM image of CBP1.7F:0.15Bi3+, which has a d spacing of 4.0559 Å, corresponding to that of the (200) layer of the Ca5(PO4)3F (ICSD-30261) structure (d(200) = 4.0573 Å). Furthermore, the corresponding FFT images are also arranged symmetrically. The results of the HRTEM and FFT images indicate that CBPxF:yBi3+ (y = 0.5, 0.15; x = 1.5−1.7) is still a single phase corresponding to Ca5(PO4)3F (ICSD-30261) when the (PO 4 ) 3− content is 1.5−1.7. The HRTEM image of CBP2.5F:0.05Bi3+ (Figure 3e) shows lattice fringes with observed interplanar distances (d) of 2.7100, 2.8149, and 4.0576 Å, which is well in line with the lattice distances of the (300), (211), and (200) planes of Ca5(PO4)3F (d(300) = 2.7049 Å, d(211) = 2.8013 Å, and d(200) = 4.0573 Å; ICSD-30261). The d spacings of CBP2.5F:0.15Bi3+ (Figure 3e′) are 3.8759, 3.0769, and 3.5026 Å, which are indexed as (111), (210), and (201) with the interplane distances of Ca5(PO4)3F (d(111) = 3.8724 Å, d(210) = 3.0670 Å, and d(201) = 3.4949 Å; ICSD-30261). The corresponding FFT images are also arranged symmetrically. The result confirms that CBPxF:yBi3+ (y = 0.5, 0.15; x = 1.7− 2.5) is still a single phase and the crystal structure belongs to Ca5(PO4)3F (ICSD-30261) when the (PO4)3− content is 1.7− 2.5. The HRTEM image (Figure 3f) of CBP3F:0.05Bi3+shows d spacings of 1.5313, 2.3413, and 1.7953 Å, corresponding to the the (240), (220), and (231) plane lattices of Ca5(PO4)3F E


Article

Inorganic Chemistry

Figure 4. (a) Emission spectra of CBPxF:0.15Bi3+ (λex = 320 nm). (b) Gaussian fitting of Ca4.95(PO4)3F:0.05Bi3+. Inset: Emission intensity of Ca5−z(PO4)3F:zBi3+. (c) Emission spectra of CBPxF:0.05Bi3+ (λex = 320 nm). (d) Emission spectra of CBPxF:0.05Bi3+ (λex = 300 nm).

corresponds to a 3P1 → 1S0 electronic transition. There is still no emission band of Ca5−z(PO4)3F:zBi3+ when (BO3)3− is gradually substituted by (PO4)3−, as shown in Figure 4c. Upon 300 nm excitation, which is the best excitation of Ca5−z(PO4)3F:zBi3+, the position of the emission peak is still consistent with that of CBPxF:0.05Bi3+ at 320 nm, as shown in Figure 4d, which suggests that concentration quenching of Bi3+ in Ca5−z(PO4)3F:zBi3+ is not the cause of the stationary emission position and reduced emission intensity. Thus, hypothesis 1 is confirmed by the results to be wrong. To further clarify the reason for the unchanging emission position and decreasing emission intensity in CBPxF:0.15Bi3+ (x = 0−3), the source of the emission band of Ca5−z(PO4)3F:zBi3+ (553 nm) is analyzed in detail. Figure 5a displays the emission spectra of Ca4.95(PO4)3F:0.05Bi3+ under different excitations of 250, 255, 260, 300, and 320 nm, and it can be found that there is a remarkable change in the emission spectra with changing excitation wavelength, which implys that Ca4.95(PO4)3F:0.05Bi3+ has a site-selection excitation.31 As is known from the above crystal structure analysis, there are two Ca2+ sites with different coordination environments in Ca5(PO4)3F (ICSD-30261 and ICSD-9444) and the Bi3+ ions are sensitive to the surrounding coordination environment. Thus, the broad emission band may originate from different emission centers. In order to determine the source of the emission peak, Gaussian fitting is performed. Taking into account the two kinds of Ca2+ ion sites in Ca5(PO4)3F (ICSD30261 and ICSD-9444), the emission peak of Ca4.95(PO4)3F:0.05Bi3+ could be fitted to two subemission peaks by Gaussian functions, whose wavelengths are 527 and 583 nm, respectively, as shown in Figure 4b. The excitation spectra for both emission peaks are shown in Figure 5b; clearly, there is an excitation peak at 300 nm for the emissions at 527 and 583 nm, while for the 538 nm emission, small excitation peaks can be observed at 428 and 478 nm. According to the report, this infers that the excitation spectra for 527 and 583 nm should correspond to the excitation spectra of Bi3+ and Bi2+, respectively.32−34 Therefore, Ca4.95(PO4)3F:0.05Bi3+ is further examined by XPS, so as to confirm the valence of Bi. Figure 5c depicts the Bi 4f peak in Ca4.95(PO4)3F:0.05Bi3+, and two broad bands correspending to Bi 4f7/2 and Bi 4f5/2 orbital splitting are observed in Ca4.95(PO4)3F:0.05Bi3+. The Bi 4f7/2 and Bi 4f5/2 XPS peaks are located at about 158.7 and 164.1 eV, respectively, with a peak separation of 5.31 eV, while the Bi

(ICSD-9444; d(240) = 1.5324 Å, d(220) = 2.3407 Å, and d(231) = 1.7957 Å). The HRTEM image (Figure 3f′) of CBP3F:0.05Bi3+ shows d spacings of 2.1381, 2.0585, and 1.7137 Å, corresponding to the (311), (113), and (141) plane lattices of Ca5(PO4)3F (ICSD-9444; d(311) = 2.1376 Å, d(113) = 2.0590 Å, and d(141) = 1.7136 Å). The corresponding FFT images are also arranged symmetrically. The result confirms that CBPxF:yBi3+ (y = 0.5, 0.15; x = 3) is a single phase and the crystal structure belongs to Ca5(PO4)3F (ICSD-9444). Thus, through the results of the HRTEM and FFT images, the conclusion can be drawn that the crystal structure should be gradually changed from Ca 5(BO3)3F (ICSD-65763) to Ca5(PO4)3F (ICSD-30261) and then to Ca5(PO4)3F (ICSD9444) in CBPxF:yBi3+ (y = 0.05, 0.15; x = 0−3) with increasing (PO4)3− concentration, which coincides with the changing trend of XRD. The two sets of samples are always single phase, with the (PO4)3− content varying from 0 to 3. 3.2. Luminescence Properties and Transformation of the Crystal Structures in CBPxF:yBi3+ (y = 0.05, 0.15; x = 0−3) and CPBXF:yBi3+ (y = 0.05, 0.15; X = 0−1). Figure 4a shows the emission spectra of CBPxF:0.15Bi3+ (x = 0−3). Upon 320 nm excitation, CBPxF:0.15Bi3+ (x = 0−3) displays a board emission band in the range of 350−750 nm, which originates from the 3P1 → 1S0 electronic transition of Bi3+ ions.27−29 When (BO3)3− is gradually substituted by (PO4)3−, the position of the emission peak is basically unchanged at 462 nm; however, the emission intensity suffers a monotonous decrease and eventually becomes zero. Upon 300 nm excitation, Ca4.85(PO4)3F:0.15Bi3+ shows a board emission band in the range of 350−850 nm and presents a yellow-green light; meanwhile, the emission center locates at 553 nm.13,30 The result indicates that the electronic transitions of Ca4.85(PO4)3F:0.15Bi3+ and CBPxF:0.15Bi3+ are different. However, there is no the emission band of Ca4.85(PO4)3F:0.15Bi3+(553 nm) when (BO3)3− is gradually substituted by (PO4)3−. The inset of Figure 4b shows that the emission intensity of Ca5−z(PO4)3F:zBi3+ suffers concentration quenching when the concentration of Bi3+ is 0.05. Hence, hypothesis 1 is proposed in which Bi3+ is concentrationquenched in Ca5(PO4)3F when the concentration of Bi3+ is 0.15. In order to verify the correctness of hypothesis 1, CBPxF:0.05Bi3+ (x = 0−3) is synthesized. Upon 320 nm excitation, the emission peak of CBPxF:0.05Bi3+ (x = 0−3) still belongs to the electronic transition of the Bi3+ ions, which F


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Inorganic Chemistry

in Figure 5d. The average lifetimes are calculated as 1.08 and 5.42 μs at the monitoring of 527 and 583 nm, respectively. Generally, the emission peak of Bi3+ can be seen in the ultraviolet, blue, and green areas in different hosts, and the average lifetime of Bi3+ is about a few microseconds.31,37−41 The fitted peak at 527 nm is not only located in the green area but the value of the average lifetime is consistent with the average lifetime of Bi3+. Thus, the fitted peak at 527 nm should be derived from the 3P1 → 1S0 electronic transition of Bi3+ ions. In addition, it is known that the emission peak of Bi2+ is generally located in the yellow and orange-red areas, and the average lifetime of Bi2+ is about tens of microseconds.32−34 The fitted peak at 583 nm is located in the yellow area, and the value of the average lifetime is inkeeping with that of Bi2+. The electron transition of Bi2+ is usually 2S1/2 → 2P1/2, 2P3/2(2) → 2 P1/2, and 2P3/2(1) → 2P1/2, but the excition to 2S1/2, 2P3/2(2), and 2P3/2(1) always results in the emission 2P3/2(1) → 2P1/2. Hence, the fitted peak at 583 nm may be derived from the 2 P3/2(1) → 2P1/2 electronic transition of Bi2+ ions.32−34 The above results further suggest that Bi3+ may be reduced to Bi2+ in air. Generally, abnormal reduction must satisfy the following three conditions. First, a trivalent cation is used to replace the divalent ion in the host. Second, the radius of the trivalent cation should be similar to that of the divalent ions in the host, and, moreover, the efficiency of abnormal reduction will get stronger when the difference between the trivalent cation and divalent ions is smaller in radius. Third, the host should contain tetrahedral anion groups, such as PO4, BO4, SiO4, and AlO4; meanwhile, the substituted divalent cation should be surrounded by tetrahedral anion groups.15−19,42,43 From the above analysis of the crystal structure, it is known that the crystal structure of Ca5(PO4)3F (ICSD-9444) accords with the basic conditions of abnormal reduction in air. Thus, abnormal reduction should appear in Ca5−z(PO4)3F:zBi3+. Figure 5e shows the abnormal reduction mechanism of Bi3+ to Bi2+. In order to maintain a charge balance, two Bi3+ ions need to replace three Ca2+ ions when Ca2+ is substituted by Bi3+. Therefore, a vacancy is formed at Ca2+, which is represented by VCa″. Because of the lack of Ca2+, the vacancy has two negative charges and becomes an electron donor. Meanwhile, two Bi3+ ions act as electron acceptors. Hence, two electrons shift from the vacancy to the position of Bi3+, resulting in Bi3+ being reduced to Bi2+. This process can be expressed by the following formulas:44,45

Figure 5. (a) Emission spectra of Ca4.95(PO4)3F:0.05Bi3+ under different excitation wavelengths. (b) Excitation spectra of 527and 583 nm, respectively. (c) XPS spectra of Ca4.95(PO4)3F:0.05Bi3+ (d). Average lifetimes monitored at 527 and 583 nm, respectively. (e) Abnormal reduction mechanism of Bi3+ to Bi2+ in Ca5−z(PO4)F:zBi3+.

peaks are located at 159.8 and 165.2 eV in Ca4.95(PO4)3F:0.05Bi3+, respectively. The shift nature of the peak should be acribed to the various types of Bi in Ca4.95(PO4)3F:0.05Bi3+. Thus, according to the bonding energy of the different valence-state Bi ion and the excitation spectra of two emission peaks, two broad bands can be divided into four peaks, which correspond to Bi2+ and Bi3+, respectively. The information on the four peaks is listed in Table 1 because the bonding energy of the low valence state of Table 1. XPS Information on Four Peaks in Ca4.95(PO4)3F:0.05Bi3+ peak

position (eV)

peak area

1 2 3 4

159.447 160.260 164.824 165.649

762.667 777.371 551.598 601.885

2Bi3 + → 2Bi•Ca + V″Ca

(1)

× V″Ca → VCa + 2e′

(2)

2e′ + 2Bi3 + → 2Bi2 +

(3)

The (PO4)3− groups around Bi2+ are similar to a protective cover, which can avoid Bi2+ being oxidized by the surrounding environment and ensure Bi2+ stablility at high temperature in air. Through the above analysis, it can be concluded that the fitted peak at 583 nm does derive from the 2P3/2(1) → 2P1/2 electronic transition of Bi2+ ions, which occupy the Ca1 site in Ca5(PO4)3F (ICSD-9444). However, the Ca2 site connects with a F atom and five O atoms, which is not surrounded by (PO4)3− groups. Thus, the formation of Bi2+ is easily reduced to Bi3+ in the Ca2 site. Moreover, F−, O2−, and Bi3+ can maintain a partial charge balance in the Ca2 site, which causes Bi3+ to be stable in the Ca2 site. It can be expressed by the following formula:

Bi ion was lower than that of the high valence state of Bi ion, and the higher binding energy peaks at 160.26 and 165.649 eV should be attributed to Bi3+ (peaks 2 and 4). The lowerbinding-energy peaks at 159.447 and 164.824 eV are assigned to Bi2+ (peaks 1 and 3). The result indicates that Bi2+ is generated in Ca4.95(PO4)3F:0.05Bi3+. The ratio of Bi2+ to Bi3+ can be obtained through the area ratio of Bi2+ and Bi3+, which is calculated by the formula Speak 1 + peak 3/Speak 2 + peak 4. The proportion of Bi2+ and Bi3+ is 95.28%.35,36 Moreover, the fluorescent decay curves of the two peaks are measured under 300 nm pulse laser radiation (nano-LED) excitation and given G


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Inorganic Chemistry Bi3 + ↔ F− + O2 −

site, which impedes part of Bi3+ from reducing to Bi2+ in the Ca1 site. When the concentration of CaF2 is higher, parts of F− transfer electrons to Bi3+, which results in Bi3+ reducing to Bi2+ in the Ca1 site. In order to further determine the change in the proportion of Bi2+ and Bi3+ , XPS was performed in CPF:0.05Bi3+,0.1CaF2, as shown in Figure 6f. Two broad bands also can be divided into four peaks, which correspond to Bi2+ and Bi3+, respectively. The XPS information on four peaks in CPF:0.05Bi3+,0.1CaF2 is listed in Table 3. The Bi peaks are located at 159.77 and 164.98 eV in CPF:0.05Bi3+,0.1CaF2, respectively. Compared with Ca4.95(PO4)3F:0.05Bi3+, the Bi peak slightly shifts to lower binding energy, and the positions of the four emission peaks also shift slightly to the lower energy side in CPF:0.05Bi3+,0.1CaF2. The reason may be ascribed to the increase of Bi2+ and the decrease of Bi3+. The ratio of Bi2+ to Bi3+ is also calculated by the formula Speak 1 + peak 3/ Speak 2 + peak 4, and the proportion of Bi2+ and Bi3+ is 96.45%, which increases slightly in comparison with that of Ca4.95(PO4)3F:0.05Bi3+.35,36 The conclusion can be drawn that abnormal reduction in air is indeed caused by charge compensation. The schematic diagram of abnormal reduction in CPF:0.05Bi3+,nCaF2 (n = 0−0.1) is shown in Figure 6g. In addition, the fitted peak at 527 nm is partly derived from the Ca1 sites because of the limited ability of electron transfer, which mainly derives from the Ca2 site, corresponding to the 3 P1 → 1S0 electronic transition of Bi3+ ions. Through analysis of the luminescence properties of CPF:0.05Bi3+,nCaF2 (n = 0−0.1), it can be inferred that abnormal reduction in air may also be associated with the electronegativity of the environment around the activator and the ionization energy of the activator. In order to verify the validity of this inference, a verified experiment on CPF:0.05Bi 3+ ,nCaCl 2 (n = 0−0.1) is performed. The concentration of CaCO3 is also decreased in order to maintain the balance of Ca2+ in the host. It is known that the electronegativity of Cl− is smaller than that of F−; thus, Cl− is more likely to lose electrons than F−. Compared with CPF:0.05Bi3+,nCaF2 (n = 0−0.1), the generation rate of Bi2+ should be higher in CPF:0.05Bi3+,nCaCl2 (n = 0−0.1). XRD Rietveld refinement of CPF:0.05Bi3+,0.1CaCl2 is performed by the GSAS program, with single crystallographic data of Ca4.95(PO4)3F (ISCD-9444) as an initial model to determine whether an excess of Cl− enters into the crystal interstitial.46,47 Table 4 shows the Rietiveld refinement results of CPF:0.05Bi 3+ ,0.1CaCl 2 and crystallographic data of Ca4.95(PO4)3F (ISCD-9444). The final reliability factors for the complete patterns are Rp = 13.27%, Rwp = 9.80%, and χ2 = 3.149. The result indicates that the results of refinement are reliable and CPF:0.05Bi3+,0.1CaCl2 is still a single phase without any impurity phase. It is seen that the cell parameters (a, b, and c) and cell volume V (Å3) all get larger, which suggests that an excess of CaCl2 may enter into the crystal interstitial of Ca5(PO4)3F (ICSD-9444). Because there is F− in the Ca2 sites, Cl− might substitute for F−, leading to a larger cell volume V (Å3). However, the bond length just increases by 0.002 Å in Ca2O5F and is basically unchanged via refinement, which suggests that F− is not substituted by Cl− in the Ca2 sites, as shown in Table 5. Therefore, Cl− should enter into the crystal interstitial. The emission peaks of CPF:0.05Bi3+,nCaCl2 are also fitted to two subemission peaks by Gaussian functions, whose wavelengths are 527 nm (peak 1) and 583 nm (peak 2), respectively, as shown in Figure 7a. Figure 7b displays the position of the emission peak, which remains unchanged at

(4)

Hence, the fitted peak at 527 nm should be derived from the P1 → 1S0 electronic transition of Bi3+ ions, which mainly occupy the Ca2 site in Ca5(PO4)3F (ICSD-9444). Therefore, it can be inferred that abnormal reduction in air is the structural self-reduction that is caused by charge compensation. To verify the correctness of this inference, an excess of CaF2 is added to the host so that it enters into the crystal interstitial of Ca4.95(PO4)3F:0.05Bi3+. The concentration of CaCO3 should be decreased in order to maintain the balance of Ca2+ in the host. In order to determine whether there is an excess of F− entering into the crystal interstitial, XRD Rietveld refinement of CPF:0.05Bi3+,0.1CaF2 is performed by the GSAS program, with the single crystallographic data of Ca5(PO4)3F (ICSD-9444) as the initial model.46,47 As shown in Table 2, the final reliability factors for the complete 3

Table 2. Rietveld Refinement Results of CPF:0.05Bi3+,0.1CaF2 and Crystallographic Data of Ca5(PO4)3F (ICSD-9444) Ca5(PO4)3F (ICSD-9444)

CPF:0.05Bi3+,0.1CaF2

Rp (%) Rwp (%) χ2 a (Å) b (Å) c (Å) cell volume V (Å3) VCa1O6 (Å3)

9.363 9.363 6.878 522.180 15.598

13.33 9.83 3.170 9.372 9.372 6.882 523.568 15.639

VCa2O5F (Å3)

14.954

14.994

formula

pattern are Rp = 13.33%, Rwp = 9.83%, and χ2 = 3.170. The result demonstrates that the result of refinement is reliable and CPF:0.05Bi3+,0.1CaF2 is still a single phase without any impurity phase. The refined lattice parameters (a, b, and c) and cell volumes [V (Å3), VCa1O6 (Å3), and VCa2O5 F (Å3)] all get larger when a excess of CaF2 is added, which means that an excess of CaF2 does enter into the crystal interstitial of Ca5(PO4)3F (ICSD-9444). Through the above analysis, the emission peaks of CPF:0.05Bi3+,nCaF2 are fitted to two subemission peaks by Gaussian functions, whose wavelengths are 527 nm (peak 1) and 583 nm (peak 2), respectively, as shown in Figure 6a. With an increase in the concentration of CaF2, the position of the emission peak remains unchanged at about 553 nm in CPF:0.05Bi3+,nCaF2, as shown in Figure 6b. However, it is seen that the emission intensity and quantum yield gradually increase in the inset of Figure 6b, which can be attributed to a change in the ratio of Bi2+ to Bi3+ due to the introduction of excessive F−. The positions of emission peaks 1 and 2 are basically unchanged and still located at about 527 and 583 nm, respectively, as exhibited in Figure 6c,d. From the results of Figure 5, it is known that peaks 1 and 2 correspond to Bi3+ and Bi2+, respectively. However, the emission intensity of Bi3+ slightly increases first and then decreases when the CaF2 content is more than 0.03. On the contrary, the emission intensity of Bi2+ slightly decreases and then keeps continuously increasing when the CaF2 content is over 0.03, as shown in Figure 6e. The reason is follows: it is well-known that F− has a greater electronegativity, which makes electron loss difficult. Thus, when the concentration of CaF2 is lower, F− primarily maintains a partial charge balance with O2− and Bi3+ in the Ca1 H


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Inorganic Chemistry

Figure 6. (a) Gaussian fitting of CPF:0.05Bi3+,nCaF2 (n = 0, 0.03, 0.05, and 0.1). (b) Position of the emission peak in CPF:0.05Bi3+,nCaF2 (n = 0, 0.03, 0.05, and 0.1). Inset: Emission intensity and quantum yield of CPF:0.05Bi3+,nCaF2 (n = 0, 0.03, 0.05, and 0.1). (c) Position of the emission peak 1 at different concentrations of CaF2. (d) Position of the emission peak 2 at different concentrations of CaF2. (e) Emission intensities of Bi3+ and Bi2+ at different concentrations of CaF2, respectively. (f) XPS spectra of CPF:0.05Bi3+,0.1CaF2. (g) Schematic diagram of abnormal reduction in CPF:0.05Bi3+,nCaF2 (n = 0−0.1).

Table 5. Bond Lengths of Ca2O5F in CPF:0.05Bi3+,0.lCaCl2

Table 3. XPS Information on Four Peaks in CPF:0.05Bi3+,0.1CaF2

bond length (Å) (Ca2/Bi2+-O2−)

peak

position (eV)

peak area

1 2 3 4

159.400 160.153 164.752 165.498

3495.732 3339.090 2569.990 2949.803

O3 O3 O2 O3 O3 F1

Table 4. Rietveld Refinement of CPF:0.05Bi3+,0.1CaCl2 and Crystallographic Data of Ca5(PO4)3F (ICSD-9444) Ca5(PO4)3F (ICSD-9444)

CPF: 0.05Bi3+,0.1CaCl2

Rp (%) Rwp (%) χ2 a (Å) b (Å) c (Å) cell volume V (Å3) VCa1O6 (Å3)

9.363 9.363 6.878 522.180 15.598

13.27 9.80 3.149 9.371 9.371 6.882 523.377 15.634

VCa2O5F (Å3)

14.954

14.988

Ca5(PO4)3F (ICSD-9444)

CPF:0.05Bi3+,0.1CaCl2

2.395 2.395 2.384 2.384 2.384 2.229

2.397 2.397 2.386 2.386 2.386 2.231

Figure 7b, the emission intensity and quantum yield gradually increase; moreover, the quantum yield of CPF:0.05 Bi3+,nCaCl2 is larger than that of CPF:0.05Bi3+,nCaF2 at the same doping concentration. The reason is that more Bi2+ is produced in CPF:0.05Bi3+,nCaCl2. The positions of emission peaks 1 and 2 are also basically unchanged and still located at about 527 and 583 nm, respectively, as exhibited in Figure 7c,d. However, as shown in Figure 7e, it is found that the emission intensity of Bi3+ gradually decreases and the emission intensity of Bi2+ gradually increases with increasing concentration of CaCl2. If an excess of CaCl2 is used as the solvent, the emission intensity of Bi3+ should also increase. This indicates that an excess of CaCl2 does not act as the solvent in CPF:0.05Bi3+,nCaCl2 (n = 0−0.1). Also, the intensity ratio of

about 553 nm in CPF:0.05 Bi3+,nCaCl2 with an increase in the concentration of CaCl2. However, as shown in the inset of I


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Inorganic Chemistry

Figure 7. (a) Gaussian fitting of CPF:0.05Bi3+,nCaCl2 (n = 0, 0.03, 0.05, and 0.1). (b) Position of the emission peak in CPF:0.05Bi3+,nCaCl2 (n = 0, 0.03, 0.05, and 0.1). Inset: Emission intensity and quantum yield of CPF:0.05Bi3+,nCaCl2 (n = 0, 0.03, 0.05, and 0.1). (c) Position of the emission peak 1 at different concentrations of CaCl2. (d) Position of the emission peak 2 at different concentrations of CaCl2. (e) Emission intensity of Bi3+ and Bi2+ at different concentrations of CaCl2, respectively. Inset: Intensity ratio of Bi3+ to Bi2+. (f) XPS of CPF:0.05Bi3+,0.1CaCl2. (g) Schematic diagram of the generation rate of Bi2+ with n increasing in CPF:nCaF2/CaCl2 (n = 0−0.1). (h) Schematic diagram of abnormal reduction in CPF:hEu3+ (h = 0−0.15).

Bi3+ to Bi2+ is smaller in CPF:0.05Bi3+,nCaCl2 (n = 0−0.1) when the same concentration of CaCl2 is added with CaF2, compared with CPF:0.05Bi3+,nCaF2 (n = 0−0.1), as shown in the inset of Figure 7e. The result demonstrates that more Bi3+ is reduced to Bi2+ in CPF:0.05Bi3+,nCaCl2 (n = 0−0.1) in comparison with CPF:0.05Bi3+,nCaF2 (n = 0−0.1). Thus, XPS is measured in CPF:0.05Bi3+,0.1CaCl2 to confirm that there is indeed more Bi2+ produced. Figure 7f displays the XPS spectra of CPF:0.05Bi3+,0.1CaCl2. From the above analysis, two broad bands are also divided into four peaks, which correspond to Bi2+ and Bi3+, respectively. The XPS information on four peaks in CPF:0.05Bi3+,0.1CaCl2 is listed in Table 6. The Bi peaks are located at 159.68 and 164.95 eV, and it is found that the Bi peaks and positions of the four emission peaks all slightly shift to lower binding energy in CPF:0.05Bi3+,0.1CaCl2, compared with CPF:0.05Bi3+,0.1CaF2. The reason should be attributed to the emergence of more Bi2+ in CPF:0.05Bi3+,0.1CaCl2. The

ratio of Bi2+ to Bi3+ is also calculated by the formula Speak 1 + peak 3/Speak 2 + peak 4, and the proportion of Bi2+ and Bi3+ is 99.03%, which increases slightly in comparison with that of CPF:0.05Bi3+,0.1CaF2.35,36 The result demonstrates that abnormal reduction is indeed associated with the electronegativity of the environment around the activator, and the rate of abnormal reduction in air can be improved by reducing the electronegativity of the environment around the activator; a schematic diagram of the generation rate of Bi2+ is shown in Figure 7g. It is known that the electronegativity indicates the ability of atoms to attract electrons. The more electronegative an atom is, the greater its ability to attract electrons. Therefore, the bigger the activator’s electronegativity and the larger the ionization energy, the more likely abnormal reduction in air will occur. A schematic diagram of abnormal reduction in Ca5−h(PO4)3F:hEu3+ (h = 0−0.15) is given in Figure 7h. It is seen that the emission peak is accordance with the characteristic emission of Eu3+, and there is no emission peak of Eu2+ in air. The radii of Bi3+ and Eu3+ are basically the same (rBi3+ = 1.02 Å; rEu3+ = 0.95 Å); moreover, Bi2+ and Eu2+ also have the similar ionic radii (rBi2+ = 1.14 Å; rEu2+ = 1.17 Å).32,48 However, the electronegativity of Bi3+ [χ(Bi3+) = 2.02] is larger than that of Eu3+ [χ(Eu3+) = 1.2]; therefore, the ionization energy of Bi3+ (703 kJ mol−1) is larger than that of Eu3+ (547.1 kJ mol−1), which makes it easier for Bi3+ to get an electron into Bi2+. Thus, abnormal reduction cannot be observed in Ca5−h(PO4)3F:hEu3+ (h = 0−0.15) in air. The conclusion

Table 6. XPS Information on Four Peaks in CPF:0.05Bi3+,0.1 CaCl2 peak 1 2 3 4

position (eV)

peak area

159.374 160.137 164.726 165.494

2398.635 2167.375 1613.255 1883.941 J


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Inorganic Chemistry

Figure 8. (a) Emission spectra of CPBXF:0.15Bi3+ (X = 0−1; λex = 300 nm). (b) Emission spectra of CPBXF:0.05Bi3+ (X = 0−1; λex = 300 nm). (b) Excitation spectra of CPBXF:0.15Bi3+ (X = 0−1; λem = 474 nm). (d) Excitation spectra of CPBxF:0.05Bi3+ (X = 0−1; λem = 474 nm).

Figure 9. (a) Rietveld refinement results of CPB0.3F:0.05Bi3+/0.15Bi3+. (b) EDX pattern detected in CPB0.3F:0.05Bi3+/0.15 Bi3+. (c) Process simulation diagram of CPBXF:0.05Bi3+/0.15Bi3+ (0 < X < 0.3). Inset: Emission intensity of CPBXF:0.05Bi3+/0.15Bi3+ (0 < X < 0.3). (d) Bond length of (PO4)3− in CPBXF:0.05Bi3+/0.15 Bi3+ (X = 0, 0.3).

can be drawn that abnormal reduction in air is related to the ionization energy of the activator ions as well. It is known that (BO3)3− is a flat triangle with three coordination, which is stable and hard to be destroyed. However, (PO4)3− is a tetrahedral structure with four coordination, which is highly different from the crystal structure of (BO3)3−. Therefore, in CBPxF:yBi3+ (y = 0.05, 0.15; x = 0−3), (PO4)3− may be formed with three coordination, which leads to production of the crystal structure Ca5(PO4)3F (ICSD-30261) with increasing x. Thus, the crystal structure should vary from Ca5(BO3)3F (ICSD-65763) to Ca5(PO4)3F (ICSD-30261) and then to Ca5(PO4)3F (ICSD9444) in CBPxF:yBi3+ (y = 0.05, 0.15; x = 0−3) when (BO3)3− is gradually substituted by (PO4)3−. Through the analysis of abnormal reduction in air above, one can find that Ca5−z(PO4)3F:zBi3+ can form abnormal reduction in air with the appropriate structure of Ca5(PO4)3F (ICSD-9444) and activator Bi3+. Thus, in CBPxF:yBi3+ (y = 0.05, 0.15; x = 0−3), abnormal reduction from Bi3+ to Bi2+ cannot form because of the production of Ca5(PO4)3F (ICSD-30261), which results in no changes of the emission position and a decrease of the emission intensity.49−53

Hypothesis 2 is that no changes of the emission position and a decrease of the emission intensity are caused by the structural difference between (BO3)3− and (PO4)3− in CBPxF:yBi3+ (y = 0.05, 0.15; x = 0−3). In order to verify the correctness of hypothesis 2, CPBXF:0.15Bi3+ (X = 0−1) and CPBXF:0.05Bi3+ (X = 0−1) were synthesized. As shown in Figure 8a,b, when the (BO3)3− contents are less than 0.3, the emission peak wavelength of CPBXF:yBi3+ (y = 0.05, 0.15; X < 0.3) is consistent with that of Ca5−z(PO4)3F:zBi3+ at 300 nm excitation; meanwhile, the emission intensity continues to decrease. When the (BO3)3− contents are between 0.3 and 0.5, the emission peak wavelength of CPBXF:yBi3+ (y = 0.05, 0.15; X = 0.3−0.5) is in accordance with that of Ca4.85(BO3)3F:0.15Bi3+, but the emission intensity slightly increases and remains basically unchanged. When the (BO3)3− contents are more than 0.5, it can be found that the position of the emission peak of CPBXF:0.05Bi3+ (y = 0.05, 0.15; X > 0.5) still matches well with that of Ca4.85(BO3)3F:0.15 Bi3+; furthermore, the emission intensity suddenly begins to increase. Parts c and d of Figure 8 shows the excitation spectra of CPBXF:0.15Bi3+ (X = 0−1) and CPBXF:0.05Bi3+ (X = 0−1) for 474 nm emission; the excitation spectra are keeping K


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Inorganic Chemistry

Figure 10. (a) XRD pattern of CPBXF:0.05Bi3+ (y = 0.05, 0.15; 0.3 < X < 0.5) and the standard of Ca5(PO4)3F (ISCD-9444 and ISCD-30261). (b) Rietveld refinement results of CPB0.5F:0.05Bi3+/0.15Bi3+. (c) Bond length of (PO4)3− in CPBXF:0.05Bi3+/0.15Bi3+ (X = 0.3, 0.5). (d) Process simulation diagram of CPBXF:0.05Bi3+/0.15Bi3+ (0.3 < X < 0.5).

with the theory that of Ca5−z(PO4)3F:zBi3+, when the (BO3)3− contents are less than 0.3, but in line with that of Ca4.85(BO3)3F:0.15Bi3+, when the (BO3)3− contents are over 0.3. The above results demonstrate that hypothesis 2 is correct and also further confirm that hypothesis 1 is wrong. In order to further determine the relationship between the luminescence properties and transformation of the crystal structure, variation of the bond length in (PO4)3− was analyzed in detail in CPBXF:yBi3+ (y = 0.05, 0.15; X = 0−1). From the above results of Figure 8, variation of the luminescence properties can be divided into three parts to analyze in CPBxF:yBi3+ (y = 0.05, 0.15; X = 0−1): CPBXF:yBi3+ (y = 0.05, 0.15; X = 0−0.3), CPBXF:yBi3+ (y = 0.05, 0.15; X = 0.3−0.5), and CPBXF:yBi3+ (y = 0.05, 0.15; X = 0.5−1). In the first part, CPBXF:yBi3+ (y = 0.05, 0.15; X = 0−0.3), the calculated and experimental results as well as their differences in the XRD refinement of CPB0.3F:0.05Bi3+ and CPB0.3F:0.15Bi3+ are shown in Figure 9a. The initial structure mode was established by the standard crystallographic data of Ca5(PO4)3F (ISCD9444). It is seen that the experimental patterns (black lines) matched well with the calculated patterns (red lines), which demonstrates that the results of the refinement are reliable and the samples are purity phases.46,47 The chemical compositions of CPB0.3F:0.05Bi3+ and CPB0.3F:0.15Bi3+ are characterized by EDX, as shown in Figure 9b. It can be found that CPB0.3F:0.05Bi3+/0.15Bi3+ contains Ca, P, F, O, and Bi elements; however, there is no B element, which indicates that when the concentration of (BO3)3− is less than 0.3, a few of (BO3)3− do not substitute for (PO4)3− but volatilize as a flux. The emission intensity should be strengthened when H 3 BO 3 acts as a flux; however, it gets weaker in CPBXF:0.05Bi3+/0.15 Bi3+ with increasing H3BO3 (X = 0− 0.3), as shown in the inset of Figure 9c. This may be caused by the lower symmetry of the PO4 crystal structure with decreasing content of (PO4)3−. Figure 9d shows the bond length of (PO4)3−; the bond lengths of P−O1 and P−O2 get shorter and that of P−O3 remains stable in CPB0.3F:0.05Bi3+/ 0.15Bi3+, respectively. Thus, the symmetry of the (PO4)3− crystal structure gets lower, which causes a slight distortion of the (PO4)3− crystal structure. With decreasing (PO4)3− concentration, the protective cover, formed by the (PO4)3− groups, is slightly damaged, which leads to a few of O2− entering into the protective cover and contacting with Bi2+.

Ultimately, a few of Bi2+ are oxidized to Bi3+; therefore, the emission intensities of CPBXF:0.05Bi3+/0.15Bi3+ (X = 0−0.3) get weaker because the concentration of Bi2+ decreases. The process simulation diagram of CPBXF:yBi3+ (y = 0.05, 0.15; 0 < X < 0.3) is shown in Figure 9c; (BO3)3− is not substituted for (PO4)3− and acts as a flux when the concentration of (BO3)3− is less than 0.3. There is no significant change in the crystal structure of (PO4)3−; therefore, the crystal structure of CPBXF:0.05Bi3+ (y = 0.05, 0.15; 0 < X < 0.3) should be Ca5(PO4)3F (ISCD-9444). In the second part, CPBXF:0.05Bi3+ (0.3 < X < 0.5), Figure 10a displays the XRD patterns of CPBXF:yBi3+ (y = 0.05, 0.15; 0.3 < X < 0.5). It can be seen that when the (BO3)3− content is less than 0.3, there is no impurity peak and the diffraction peaks well match with the standard of Ca5(PO4)3F (ISCD9444). When the (BO3)3− content is more than 0.3, an impurity peak gradually appears at 30.78°, which should belong to Ca5(BO3)3F (ISCD-65763). Meanwhile, a detailed Rietveld structure analysis of the selected CPB0.5F:0.05Bi3+/ 0.15Bi3+ was performed with the single crystallographic data of Ca5(PO4)3F (ISCD-9444) as the initial model, as shown in Figure 10b. The final reliability factors for the complete pattern are R p = 14.3%, R wp = 8.93%, and χ 2 = 4.634 in CPB0.5F:0.05Bi3+. The final reliability factors are Rp = 14.29%, Rwp = 9.03%, and χ2 = 5.834 in CPB0.5F:0.15Bi3+. This indicates that the results of refinement are reliable and the small impurity peak has not significant influence on the crystal structure.46,47 In addition, the bond length is obtained by the resulting refinement, as shown in Figure 10c. In CPB0.5F:0.05Bi3+, the bond length of P−O3 gets longer, from 1.535 to 1.554 Å; meanwhile, the bond length of P−O2 gets longer rapidly, which changes from 1.540 to 1.574 Å. On the contrary, the bond length of P−O1 gets shorter, which changes from 1.534 to 1.530 Å. The same results were obtained in CPB0.5F:0.15Bi3+. The bond lengths of P−O2 and P−O3 all get longer gradually, and that of P−O1 gets shorter. Because the bond lengths of P−O2 and P−O3 rapidly get longer and the bond length of P−O1 gets shorter, the symmetry of (PO4)3− suffers from severe damage, leading to extreme distortion of the (PO4)3− crystal structure. The protective cover, formed by (PO4)3−, is completely destroyed, which leads to Bi2+ being completely oxidized to Bi3+ in CPBXF:yBi3+ (y = 0.05, 0.15; 0.3 < X < 0.5).52−55 Therefore, as L


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Figure 11. (a) Rietveld refinement results and bond lengths in CPB0.6F:0.05Bi3+/0.15Bi3+ with the standard crystallographic data of Ca5(PO4)3F (ISCD-9444) as the initial model. (b) Rietveld refinement results and bond lengths in CPB0.6F:0.05Bi3+/CPB0.6F:0.15Bi3+ with the standard crystallographic data of Ca5(PO4)3F (ISCD-30261) as the initial model. (c) XRD patterns of CPBXF:0.05Bi3+/0.15Bi3+ (X = 0.6−1).

by the results of refinement as well; it was found that the bond length of P−O3 gets longer and that of P−O1 gets shorter, while P−O2 bond breaking appears in CPB0.6F:0.05Bi3+/ 0.15Bi3+, which indicates that the crystal structure of (PO4)3− has been destroyed and the crystal structure Ca5(PO4)3F (ISCD-9444) may become its isomer Ca5(PO4)3F (ISCD30261). The bond length of P−O2 may exceed the critical bond length, leading to bond breaking. It is known that a covalent bond should be formed between the two atoms when the electronegativity difference of the two atoms is less than 1.7. The difference between P [χ(P) = 2.19] and O [χ(O) = 3.44] is 1.25 in electronegativity. Thus, the covalent bond should be formed between the P and O atoms. For a covalent bond formed by different atoms, the length of the covalent bond should be less than or equal to the sum of the covalent radius of the two atoms.The covalent radii of P and O are 1.06 and 0.66 Å, respectively. Thus, the length of the covalent bond between P and O should be 1.72 Å. It can be inferred that the bond length should be more than 1.72 Å in CPBXF:yBi3+ (y = 0.05, 0.15; 0.5 < X < 1), which causes P−O2 to break. This change trend of the bond length may be caused by the structural difference between (BO3)3− and (PO4)3−. Generally, (BO3)3− is a flat triangle and coordinates with three O atoms, which is stable and hard to destroy. However, (PO4)3− is a tetrahedral structure and coordinates with four O atoms

shown above in Figure 8a,b, the position of the emission peak of CPBXF:yBi3+ (y = 0.05, 0.15; 0.3 < X < 0.5) is consistent with that of Ca4.85(BO3)3F:0.15Bi3+, and there is no characteristic emission peak of Bi2+; meanwhile, the emission intensity of Bi3+ gets slightly stronger because Bi2+ is completely oxidized to Bi3+ . The process simulation diagram of CPBXF:yBi3+ (y = 0.05, 015; 0.3 < X < 0.5) is shown in Figure 10d, and the protective cover, formed by (PO4)3−, is completely destroyed, with (BO3)3− substitution for (PO4)3−, which leads to Bi2+ being completely oxidized to Bi3+. Despite the fact that the crystal structure of (PO4)3− is seriously distorted, there is no break bond in (PO4)3−. Therefore, the crystal structure should be Ca5(PO4)3F (ISCD-9444) in CPBXF:yBi3+ (y = 0.05, 0.15; 0.3 < X < 0.5). In the third part, CPBXF:yBi3+ (y = 0.05, 0.15; 0.5 < X < 1), Figure 11a displays the results of Rietveld refinements of CPB0.6F:0.05Bi3+/0.15Bi3+ with the single crystallographic data of Ca5(PO4)3F (ISCD-9444) as the initial model. It is seen that the experimental patterns (black lines) coincide with the calculated patterns (red lines). The final reliability factors for the complete pattern are Rp = 14.04%, Rwp = 9.19%, and χ2 = 6.064 in CPB0.6F:0.05Bi3+ and Rp = 14.15%, Rwp = 9.74%, and χ2 = 5.115 in CPB0.6F:0.15Bi3+, which demonstrates that the refinement is reliable,46,47 and the corresponding XRD is shown in Figure 11c. The bond length of (PO4)3− is obtained M


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Inorganic Chemistry generally, which is much different from the crystal structure of (BO3)3−. Thus, with increasing (BO3)3− concentration, one of bonds will break in (PO4)3− because of the difference in crystal structure between (BO3)3− and (PO4)3−. Meanwhile, the coordination of (PO4)3− should be changed from four coordination to three coordination, and the crystal structure should be transformed from Ca5(PO4)3F (ISCD-9444) to Ca5(PO4)3F (ISCD-30261). To further verify the transformation of the crystal structure from Ca5(PO4)3F (ISCD9444) to Ca5(PO4)3F (ISCD-30261), Rietveld refinement of CPB0.6F:0.05Bi3+/0.15Bi3+ was performed by the GSAS program with the single crystallographic data of Ca5(PO4)3F (ISCD-30261) as the initial model. The results of refinement are shown in Figure 11b. The calculated results agree well with the experimental results in the XRD refinement of CPB0.6F:0.05Bi3+/0.15Bi3+, which indicates that the results of refinement are reliable.46,47 Also, the bond lengths of P−O1 and P−O3 are 1.288 and 1.595 Å in CPB0.6F:0.05Bi3+, respectively, which are similar to the results of refinement of CPB0.6F:0.05Bi3+ with the standard crystallographic data of Ca5(PO4)3F (ISCD-9444) as the initial model. Meanwhile, the bond lengths of P−O1and P−O3 are 1.275 and 1.581 Å in CPB0.6F:0.15Bi3+, respectively, which are also similar to the results of refinement of CPB0.6F:0.15Bi3+ with the standard crystallographic data of Ca5(PO4)3F (ISCD-9444) as the initial model. Thus, the conclusion can be drawn that the crystal structure does transform from Ca5(PO4)3F (ISCD-9444) to Ca5(PO4)3F (ISCD-30261) in CPBXF:yBi3+ (y = 0.05, 0.15; 0.5 < X < 1), Furthermore, it can inferred that Bi3+ should luminesce in three coordination of (BO3)3− and not luminesce in three coordination of (PO4)3−. Therefore, it is seen that the emission intensity starts to increase amd the position of the emission peak suddenly changes from 553 to 474 nm in CPBXF:yBi3+ (y = 0.05, 0.15; 0.5 < X < 1) because the four coordination of (PO4)3− changes to three coordination of (PO4/BO3)3−. Here, the effect of Bi3+ on the luminescence is the three coordination of (BO3)3−. On the contary, it is seen that the position of the emission peak (462 nm) in CBPxF:yBi3+ (y = 0.05, 0.15; x = 0−3) remains unchanged and the emission intensity continues to decrease because three coordination of (BO3)3− changes to three coordination of (PO4)3−. Furthermore, it can be inferred that Bi3+ should produce luminescence in three coordination of (BO3)3− and nonluminance in three coordination of (PO4)3−. Therefore, it is seen that the emission intensity starts to increase and the position of te emission peak suddenly changes from 553 to 474 nm in CPBXF:yBi3+ (y = 0.05, 0.15; 0.5 < X < 1) because four coordination of (PO4)3− changes to three coordination of (PO4/BO3)3−. Here, the effect of Bi3+ on the luminescence is three coordination of (BO3)3−. Also, it is seen that the position of the emission peak (462 nm) in CBPxF:yBi3+ (y = 0.05, 0.15; x = 0−3) remains unchanged and the emission intensity continues to decrease because three coordination of (BO3)3− changes to three coordination of (PO4)3− with decreasing (BO3)3−. Hypothesis 2 is confirmed to be correct. The decreasing emission intensity may also be caused by deterioration of the crystallinity or a large number of defects in CPBXF:yBi3+ (y = 0.05, 0.15; 0 < X < 0.5) due to structural differences, compared with CPBXF:yBi3+ (y = 0.05, 0.15; 0.5 < X < 1). The crystallinity refers to the percentage content of the crystalline phase in the sample, which can be roughly calculated by the following formula:

IC = IA /(IA + IB)

(5)

where IA is the intensity value of the crystalline phase and IB is the intensity value of the amorphous phase in the sample. Jade software can be used to fit the peaks, and the areas of the diffraction peaks of the crystalline and amorphous phases can be calculated after deduction of the background. This area is the value of the intensity. Thus, in order to exclude the influence of crystallinity, the crystallinity of CPBXF:yBi3+ (y = 0.05, 0.15; 0 < X < 1) is roughly calculated by Jade software. It is seen that the crystallinity of CPBXF:yBi3+ (y = 0.05, 0.15; 0 < X < 1) slightly decreases without significant changes from that in Table 7. However, the emission intensity gradually increases Table 7. Crystallinity of CPBXF:yBi3+ (y = 0.05, 0.15; X = 0, 0.3, 0.5, 0.6, 0.8, 1) CPBXF:0.15Bi3+

crystallinity (%)

CPBXF:0.05Bi3+

crystallinity (%)

0 0.3 0.5 0.6 0.8 1

88.05 87.75 87.64 86.80 86.52 85.57

0 0.3 0.5 0.6 0.8 1

93.70 91.88 91.75 90.58 90.17 89.86

in CPBXF:yBi3+ (y = 0.05, 0.15; 0.5 < X < 1). Furthermore, the thermoluminescence spectra of CPBXF:yBi3+ (y = 0.05, 0.15; 0 < X < 1) were measured, as shown in Figure 12a,b. It is found that there is no influence on the samples. Through the results of calculating the crystallinity and measuring the defect of the sample, it is further confirmed that the unchanging position of the emission spectra and the decrease of the emission intensity in CBPxF:0.15 Bi3+ (x = 0−3), indeed, are caused by the formation of plane triangle (PO4)33−, which will change the crystal structure from Ca 5 (BO 3 ) 3 F (ICSD-65763) to Ca5(PO4)3F (ISCD-30261) instead of Ca5(PO4)3F (ISCD9444).

4. CONCLUSIONS In order to explore the relationship between variation of the crystal structure and luminescence properties via anionic group substitution, a series of CBP x F:yBi 3+ , CPF:0.1Eu 3+ , CPF:0.05Bi3+,nCaF2/CaCl2, and CPBXF:yBi3+ are synthesized via a solid-state method. In CBPxF:0.15Bi3+ (x = 0−3), the emission peak remains unchanged and the emission intensity decreases with (PO4)3− substitution for (BO3)3−. The underlying reasons for this are discussed in detail and proven by the verification of two hypotheses. Hypothesis 1 proposes that the results are from concentration quenching of Bi3+ in Ca5(PO4)3F; however, it is denied by the verified experiment CBPxF:0.05Bi3+ (x = 0−3; Bi3+ luminescence has been concentration quenched when Bi3+ is 0.05 in Ca5(PO4)3F). Moreover, abnormal reduction occurs in Ca5−z(PO4)3F:zBi3+, and it is found that the rate of abnormal reduction can be improved by reducing the electronegativity of the environment around the activator or increasing the ionization energy of the activator through the experiment of CPF:0.05Bi3+,nCaF2/ CaCl2 (n = 0−0.1) and CPF:0.1Eu3+. Hypothesis 2 proposes that the results may be caused by the structural difference between (BO3)3− and (PO4)3−, which is confirmed to be correct by performing the verification experiment CPBXF:yBi3+ (y = 0.05, 0.15; X = 0−1) and analyzing the results of the luminescence properties, XRD, EDX, and Rietveld refinement. N


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Figure 12. Thermoluminescence spectra of (a) CPBXF:0.05 Bi3+ (0 < X < 1) and (b) CPBXF:0.15Bi3+ (0 < X < 1). Photoluminescence in the Ca2(Al1−xMgx)(Al1−xSi1+x)O7:Eu2+ Phosphor. J. Am. Chem. Soc. 2015, 137, 12494. (2) Yu, Z.; Xia, Z. G.; Chen, M. Y.; Xiang, Q. C.; Liu, Q. Insight into the preparation and luminescence properties of yellow-green-emitting [(Sr,Ba)3AlO4F-Sr3SiO5]:Ce3+ solid solution phosphors. J. Mater. Chem. C 2017, 5, 3176−3182. (3) Xia, Z. G.; Miao, S. H.; Molokeev, M. S.; Chen, M. Y.; Liu, Q. L. Structure and luminescence properties of Eu 2 + doped LuxSr2−xSiNxO4−x phosphors evolved from chemical unit cosubstitution. J. Mater. Chem. C 2016, 4, 1336−1344. (4) Xia, Z. G.; Molokeev, M. S.; Im, W. B.; Unithrattil, S.; Liu, Q. L. Crystal Structure and Photoluminescence Evolution of La5(Si2+xB1−x)(O13−xNx):Ce3+ Solid Solution Phosphors. J. Phys. Chem. C 2015, 119, 9488−9495. (5) Deng, T. R.; Xia, Z. G.; Ding, H. Effect of [PO4]3− /[VO4]3−, substitution on the structure and luminescence properties of Ca5[(P,V)O4)]3F:Eu3+, phosphors. Chem. Phys. Lett. 2015, 637, 67−70. (6) Barykina, Y. A.; Medvedeva, N. I.; Zubkov, V. G.; Kellerman, D. G. Luminescence of (VO4)3−, centers in LiMgPO4, and LiMgVO4: Effect of [PO4]3−/[VO4]3−, substitution on the structure and optical properties. J. Alloys Compd. 2017, 709, 1−7. (7) Dai, P. P.; Li, C.; Zhang, X. T.; Xu, J.; Chen, X.; Wang, X. L.; Jia, Y.; Wang, X.; Liu, Y. C. A single Eu2+-activated high-color-rendering oxychloride white-light phosphor for white-light-emitting diodes. Light: Sci. Appl. 2016, 5, e16024. (8) Guo, Q.; Liao, L.; Mei, L.; Liu, H. Structures and luminescent properties of single-phase La 5.9‑x Ba4+x (SiO4)6‑x (PO4)xF2: 0.1Ce3+ phosphors. J. Lumin. 2016, 172, 191−196. (9) Xia, Y.; Chen, J.; Liu, Y. G.; Molokeev, M. S.; Guan, M.; Huang, Z.; Fang. Crystal structure evolution and luminescence properties of color tunable solid solution phosphors Ca2+xLa8‑x(SiO4)6‑x(PO4)xO2: Eu2+. Dalton Trans.. 2016, 45, 1007−1015. (10) Yi, L. P.; Zhang, J. L.; Qiu, Z. X.; Zhou, W. L.; Yu, L. P.; Lian, S. X. Color-tunable emission in Ce3+, Tb3+ co-doped Ca5(BO3)3F phosphor. RSC Adv. 2015, 5, 67125−67133. (11) Xu, K.; Loiseau, P.; Aka, G.; Lejay, J. A New Promising Nonlinear Optical Crystal for Ultraviolet Light Generation: Ca5(BO3)3F. Cryst. Growth Des. 2009, 9, 2235−2239. (12) Qiao, X. B.; Seo, H. J. Luminescence and Structural Features for Eu3+ Ions in Ca5(PO4)3F Phosphor. Sci. Adv. Mater. 2017, 9, 575− 581. (13) Zhang, B. H.; Qiu, J. B.; Li, Q. Y.; Yu, X.; Xu, X. H.; Wu, Y. M. Improved photoluminescence behavior of Eu3+-activated Ca5(PO4)3F red nanophosphor by adding Li+, Au3+, and Bi3+ as co-dopants. Chin. Opt. Lett.. 2014, 12, 82−85. (14) Yu, M.; Zhang, W.; Qin, S.; Li, J.; Qiu, K. Synthesis and luminescence properties of single-component Ca5 (PO4)3 F:Dy3+, Eu3+ white-emitting phosphors. J. Am. Ceram. Soc. 2018, 101, 4582− 94590. (15) Peng, M. Y.; Pei, Z. W.; Hong, G. Y.; Su, Q. Study on the reduction of Eu3+→Eu2+, in Sr4Al14O25: Eu prepared in air atmosphere. Chem. Phys. Lett. 2003, 371, 1−6.

When the concentration of (BO3)3− is 0.5 < X < 1, the bond P−O2 appears to break in CPBXF:yBi3+ (y = 0.05,0.15; X = 0− 1) because of the structural difference between (BO3)3− and (PO4)3−, which results in the crystal structure transforming from Ca5(PO4)3F (ISCD-9444) to Ca5(PO4)3F (ISCD30261); thus, abnormal reduction from Bi3+ to Bi2+ cannot form. Furthermore, Bi3+ should not luminesce in three coordination of (PO4)3−. Thus, the emission intensity starts to increase and the position of the emission peak suddenly changes from 553 to 474 nm in CPBXF:yBi3+ (y = 0.05, 0.15; 0.5 < X < 1) because of the formation of three coordinated (PO4)3−, and the position of the emission peak (462 nm) in CBPxF:yBi3+ (y = 0.05, 0.15; x = 0−3) remains unchanged and the emission intensity continues to decrease because three coordination of (BO3)3− changes to three coordination of (PO4)3− with decreasing (BO3)3−. On the basis of verification of the above two hypotheses and a series of detailed analyses, the crystal structure of CBPxF:yBi3+ (y = 0.05, 0.15; x = 0−3) can be inferred to be gradually changed from Ca5(BO3)3F (ICSD-65763) to Ca5(PO4)3F (ISCD-30261) and then to Ca5(PO4)3F (ISCD-9444), which leads to the position of that emission peak remaining unchanged and the emission intensity decreasing. Furthermore, it is worth mentioning that the transforming process of the crystal structure is continuous and the crystal structure is arranged regularly and periodically throughout. The research work will provide important references for design of the host via anionic group substitution.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86 312 5977068. *E-mail: [email protected]. Tel.: +86 312 5977068. ORCID

Zhijun Wang: 0000-0002-3574-3985 Panlai Li: 0000-0003-0972-9343 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by the National Natural Science Foundation of China (Grant 51672066), the Funds for Distinguished Young Scientists of Hebei Province, China (Grant A2018201101), and the personnel training project of Hebei Province, China (Grant A2016002013).

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REFERENCES

(1) Xia, Z. G.; Ma, C. G.; Molokeev, M. S.; Liu, Q. L.; Rickert, K.; Poeppelmeier, K. R. Chemical Unit Cosubstitution and Tuning of O


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Inorganic Chemistry

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Inorganic Chemistry (53) Xia, Y. F.; Chen, J.; Liu, Y. G.; Molokeev, M. S.; Guan, M.; Huang, Z. H.; Fang, M. H. Crystal structure evolution and luminescence properties of color tunable solid solution phosphors Ca2+xLa8‑x (SiO4)6‑x (PO4)xO2:Eu2+. Dalton Trans.. 2016, 45, 1007− 1015. (54) Cozzan, C.; Laurita, G.; Gaultois, M. W.; Cohen, M.; Mikhailovsky, A. A.; Balasubramanian, M.; Seshadri, R. Understanding the links between composition, polyhedral distortion, and luminescence properties in green-emitting β-Si6−zAlzOzN8−z:Eu2+ phosphors. J. Mater. Chem. C 2017, 5, 10039−10046. (55) Miao, S. H.; Xia, Z. G.; Molokeev, M. S.; Chen, M. Y.; Zhang, J.; Liu, Q. L. Effect of Al/Si substitution on the structure and luminescence properties of CaSrSiO4:Ce3+phosphors: analysis based on the polyhedra distortion. J. Mater. Chem. C 2015, 3, 4616−4622.

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