Crystal Structure and Photoluminescence Properties of an

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

Crystal Structure and Photoluminescence Properties of an Incommensurate Phase in EuO- and P2O5‑Doped Ca2SiO4 Yuya Hiramatsu,† Yuichi Michiue,‡ Shiro Funahashi,‡ Naoto Hirosaki,‡ Hiroki Banno,† Daisuke Urushihara,† Toru Asaka,† and Koichiro Fukuda*,† †

Department of Life Science and Applied Chemistry, Nagoya Institute of Technology, Nagoya 466-8555, Japan National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

‡

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S Supporting Information *

ABSTRACT: We have for the first time clarified the incommensurately modulated crystal structure as well as the photoluminescence properties of Eu2+-activated Ca2SiO4 solid solution, the chemical formula of which is (Ca1.88Eu2+0.01□0.11)(Si0.78P0.22)O4, where □ denotes vacancies in Ca sites with the replacement of Si4+ by P5+. The emission spectrum upon the 335 nm excitation showed a relatively broad band centered at ca. 490 nm and a full width at half-maximum of ca. 80 nm. The crystal structure was made up of the four types of β-Ca2SiO4-like layers with one type of interlayer. The incommensurate modulation with superspace group Pnma(0 β 0)00s was induced by the long-range stacking order of these layers. The modulation wavevector was 0.27404(2) × b*, with the basic unit-cell dimensions being a = 0.68355(2) nm, b = 0.54227(2) nm, and c = 0.93840(3) nm (Z = 4). The basic structure contained two nonequivalent Ca sites. One site was fully occupied by Ca2+ and free from Eu2+ in the overall incommensurate structure. The occupational modulation at the other site was so significant that the sum of site occupation factors for Ca2+ and Eu2+ as low as 0.5 was seen at the interlayer. This site was too large for accommodation of Ca2+ but was suitable for Eu2+. Thus, the Eu2+ ions would exclusively concentrate at the relevant site, which would cause the emission peak of the incommensurate phase to be shifted to the shorter wavelength ranges as compared with those of the other commensurate phases such as β and α′L.

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INTRODUCTION The Ca2SiO4 polymorphs established so far are γ (orthorhombic), α′L (orthorhombic), α′H (orthorhombic), and α (trigonal) with increasing temperature at ordinary pressure, with the β-phase (monoclinic, space group P121/n1) being stable under high pressures.1 In the binary system Ca2SiO4− Ca3(PO4)2, the end member Ca2SiO4 incorporates certain amounts of P2O5 component to stabilize the high-temperature modifications as well as the β-phase at ambient temperature and pressure.2 The constituent phases of (Ca2−x/2□x/2)(Si1−xPx)O4 (□ denotes a vacancy in Ca site), when quenched from the stable temperature region of the α-phase, have changed depending on the x-value (≤0.3) from β (x = 0.03− 0.08), β and α′L (x = 0.1), and α (x = 0.275−0.3). With x = 0.125−0.25, the quenched crystals have demonstrated an incommensurate (IC) structure. Its modulation wavevector N−1 × b* has been along the b-axis of the orthorhombic basic © XXXX American Chemical Society

structure, the unit-cell dimensions of which is equivalent to those of the α′H-phase. The noninteger N is defined by L/l, where L represents the distance between the main reflections, and l is the distance between the main reflections and the nearest satellites.3 The crystal grains consisting of the IC-phase necessarily showed the 3-fold cyclic twinning around the triad axis of the former α-phase. In the binary system Ca2SiO4− Ba2SiO4, the α′H-phase has been successfully stabilized at ambient temperature for (Ba0.24Ca0.76)2SiO4.4 Its disordered crystal structure (space group Pnma) has been suggested to be made up of the two mirror-related structural configurations with P21/n11 symmetry, the atom arrangements of which were very similar to that of the β-phase. The doped Ca2SiO4 crystals have been reported as promising host materials for a lightReceived: February 12, 2019

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DOI: 10.1021/acs.inorgchem.9b00408 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry emitting diode (LED) phosphors.5−9 When the γ-, α′L-, α′H-, α-, and β-phases were activated with Eu2+ ion, the emission spectra, induced by the 5d → 4f transition of Eu2+, have been dependent on the phase compositions.9 However, the photoluminescence (PL) properties of the IC-phase have, to the authors’ best knowledge, never been reported so far. Aperiodic structure is generally described as a threedimensional section of a periodic structure in higher-dimensional space, which is usually the (3 + 1) dimensional superspace.10,11 An atom in the superspace is expressed by the string extending along the fourth direction. Thus, we have to determine the shape of strings (i.e., modulation functions) in the superspace to correctly understand the 3-dimensional details of the incommensurately modulated structures. Saalfeld and Klaska have prepared the cyclically twinned crystal grains of IC-(Ca1.855□0.145)(Si0.706P0.294)O4 with N = 3.75.12 They have, assuming N = 4, derived the commensurate 3dimensional crystal structure based on the space group Pnm21 and unit-cell dimensions a = 0.940 nm, b = 2.171 (≈ 0.543 × 4) nm, and c = 0.683 nm (Z = 16). However, their commensurate approach would be insufficient for the detailed structural discussion especially on the coordination environments of Ca/Eu sites as well as the tilting of [MO4] tetrahedra (M = Si or P). In the present study, we have for the first time reported the PL properties of the Eu2+-activated IC-phase of doped Ca2SiO4. We have also determined the incommensurately modulated crystal structure using a (3 + 1)-dimensional description based on the superspace formalism.

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Table 1. Crystallographic Data and Conditions for Data Collection and Refinement for (Ca1.88Eu2+0.01□0.11)(Si0.78P0.22)O4a formula weight Mr crystal system superspace group a (nm) b (nm) c (nm) modulation wavevector V (nm3) Z Dx (Mg m−3) μ(Mo Kα) (mm−1) radiation scan mode θmax. ranges of h, k, l, m reflections measured independent reflections observed reflections (Io > 2σ (Io)) Rint refinement on Robs(F), wRobs(F2) Rall(F), wRall(F2) number of parameters Sobs, Sall weight factor Δρmin (1000 nm3), Δρmax (1000 nm3)

EXPERIMENTAL SECTION

Material and Sample Selection. The reagents used were CaCO3 (99.5%, Kishida Chemical Co. Ltd., Osaka, Japan), Eu2O3 (99.99%, Kishida Chemical Co. Ltd., Osaka, Japan), SiO2 (99%, Kishida Chemical Co. Ltd., Osaka, Japan), and CaHPO4·2H2O (98%, Kishida Chemical Co. Ltd., Osaka, Japan). They were weighed in molar rations of 1.69:0.005:0.8:0.2 (CaCO3/Eu2O3/SiO2/CaHPO4·2H2O), corresponding to the cation stoichiometry of 1.89:0.01:0.80:0.20 (Ca/Eu/Si/P). In anticipation of the formation of various phosphor particles with different chemical compositions and PL properties,13 we roughly mixed and ground the weighed reagents using an agate mortar and pestle, and the mixture put into a Pt crucible. The whole assembly was placed in a box furnace, heated at 1773 K (stable temperature region of the α phase) for 100 h, followed by quenching in air. The sample was subsequently annealed in a tube furnace at 1473 K for 3 h under the reductive atmosphere of 90% N2 and 10% H2 for the reduction of Eu3+ to Eu2+. The slightly sintered polycrystalline material as obtained was composed of the microcrystalline phosphor particles with various emission colors, when excited at 365 nm ultraviolet light. We picked up a green-light-emitting crystal grain of approximately 68 μm × 69 μm × 10 μm in size (see Figure S1) under an optical microscope (BX51M, Olympus), and mounted it with epoxy resin on the end of a soda glass capillary that was attached to the goniometer head. The selected crystal grain was, as discussed below, found to be composed exclusively of the IC-phase with N ≈ 3.649. Characterization. The elemental composition of the crystal grain was determined using a scanning electron microscope (SU1510, Hitachi) operated at 15 kV and equipped with an energy-dispersive Xray analyzer (EDX; XFlash SSD, Bruker). We collected the diffraction intensities on a single-crystal X-ray diffractometer (Smart Apex II Ultra, Bruker AXS) using Mo Kα radiation (50 kV × 50 mA). The conditions and parameters for data collection and refinements are listed in Table 1. The reliability indices for main and satellite reflections are given in Table S1. An initial structure was drawn by the charge-flipping method using SUPER-

169.6 orthorhombic Pnma(0 β 0)00s 0.68355(2) 0.54227(2) 0.93840(3) 0.27404(2) × b* 0.34784(2) 4 3.238 3.497 Mo Kα (0.071073 nm) ω 34.66° −10 ≤ h ≤ 10 −9 ≤ k ≤ 8 −12 ≤ l ≤ 14 −3 ≤ m ≤ 3 24688 5007 3311 0.0536 F2 0.0710, 0.1345 0.1172, 0.1410 204 2.74, 2.32 1/[σ (Io)2 + 0.0004Io2] −3.75, 3.25

a

Reliability factors for main and satellite reflections are given in Table S1. FLIP.14 A list of structural parameters (see Table S2), plots of modulation functions for displacements (see Figure S2), and electron densities along the fourth direction of each ion (Figure S3) are given in the Supporting Information. We used the programs JANA200615 for structural refinements and VESTA16 for crystal-structure drawing. The monochromatized 150 W Xe light was introduced onto the crystal-grain surface through an optical bundle fiber, and the emission and excitation spectra were obtained using a multichannel photo detector (MCPD-7700, Otsuka Electronics) coupled with the microscope. The excitation spectrum was restricted within the wavelength range of 335−460 nm by the absorption of lens glass and the sensitivity of the detector.

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RESULTS AND DISCUSSION Chemical Formula. The molar ratios of Ca/(Ca + Eu) and Si/(Si + P) determined by EDX were, respectively, 0.993(4) and 0.780(3), corresponding to the atom ratios of 1.88:0.01:0.78:0.22 (Ca/Eu/Si/P). Thus, we determined the chemical formula of the selected crystal grain to be (Ca1.88Eu2+0.01□0.11)(Si0.78P0.22)O4 on the basis of 4 oxygen atoms, where □ denotes vacancies in Ca sites with the replacement of Si4+ by P5+. Determination and Description of Crystal Structure. The precession images (Figure 1), which were constructed from the single-crystal XRD data, showed that the crystal grain was composed of the orthorhombic domains in three orientations, each of which was related by 120° rotation around the common a-axis.3,4 This cyclic twinning, which is B

DOI: 10.1021/acs.inorgchem.9b00408 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

peculiar to the Ca2SiO4 solid solution crystals quenched from the stable temperature region of the α-phase, was probably formed by the phase transition from trigonal α to orthorhombic IC on quenching. Each twin variant showed the first-, second-, and third-order satellite reflections along the b*-axis of the orthorhombic subcell (a ≈ 0.684 nm, b ≈ 0.542 nm, and c ≈ 0.938 nm). The distance from a main diffraction spot to the first-order satellite was approximately (1/3.65)|b*|. Since the atom arrangements was incommensurately modulated, we refined the crystal structure using a (3 + 1)dimensional description based on the superspace formalism.10,11 The systematic reflection conditions are k + l = 2n for 0 k l m and h + m = 2n for h k 0 m (n: integer). Thus, the possible superspace groups are Pnma(0 β 0)00s and Pn21a(0 β 0)00s, with the latter being one of the subgroups of the former. The final structural model was satisfactorily obtained for the superspace group Pnma(0 β 0)00s. The projection of the partial structure along the a-axis is drawn in Figure 2a. Each cation at the Si/P site is coordinated by 4 O ions to form an isolated [MO4] tetrahedron. One of the most characteristic features of the structure is the tilting of [MO4] tetrahedra. For convenience of discussion, we define a plane P that is parallel to the a-axis and contains two O2 ions for each [MO4] tetrahedron (Figure 2b). Tilting of a [MO4] tetrahedron is simply evaluated by the angle (= Θ) between the plane P and the ab plane (see Figure S4). When viewed along [1̅00], the tetrahedra of clockwise tilting (5° ≤ Θ < 15.2°) are condensed at the specific regions in Figure 2a,

Figure 1. Precession image showing the overlapping of three reciprocal nets 0kl. The crystal grain is composed of the three orientational domains I, II, and III, each of which is related by 120° rotation around the common a-axis.

Figure 2. (a) Projection of a partial structure along the a-axis. (b) Clockwise and counterclockwise tilting of the [MO4] tetrahedra. Plane P is parallel to the a-axis and contains two O2 ions. (c) Structure of β-Ca2SiO4 bearing a resemblance to the S layer in the partial structure. (d) Ca1− O1 distances extended by the tilting of [MO4] tetrahedra. C

DOI: 10.1021/acs.inorgchem.9b00408 Inorg. Chem. XXXX, XXX, XXX−XXX

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same in both IC- and α′H-phases, although the long-range order of modulation is absent in the latter. As stated previously, it is also noted that the structure of IC-(Ca1.88Eu2+0.01□0.11)(Si0.78P0.22)O4 is closely related to that of β-Ca2SiO4 in that the simple repetition of S (or T) layer results in the construction of comparable monoclinic structure with the angle β = 94.59°.17 Occupational Modulation at Ca1 Site. The basic structure contains two nonequivalent Ca sites, Ca1 and Ca2. The Ca2 site was fully occupied by Ca ion, hence the bond valence sum (BVS)18 at this site was close to 2 in all range of t (= x4 − q·r) (see Figure S6). This implies that the Ca2 site is free from Eu2+ ion. With the Ca1 site in the average structure, we assumed the site occupation factors (sof’s) to be 0.88 for Ca2+, 0.01 for Eu2+, and 0.11 for vacancy. The occupational modulation at Ca1 site is so significant that the sum of sof’s for Ca2+ and Eu2+ (site occupancy) goes down as low as 0.5 near t = 0.7 (Figure 3). (Strictly, the minimum is at t (= 0.75 − β ×

forming the layers denoted by S. As the structure is incommensurately modulated, all of the layers categorized as S resemble but slightly differ from each other. There is another type of layer, T, containing tetrahedra of counterclockwise tilting (−15.2° < Θ ≤ −5°). The atom arrangements in the S and T layers, which are similar to those of β-Ca2SiO4,17 are approximately related to each other by a mirror plane normal to the b-axis of Pnma(0 β 0)00s. Both layers have the thickness corresponding to one and a half the d100-value of β-Ca2SiO4 (Figure 2c). In a few cases, there are S′ and T′ layers with the thickness that are almost equal to the d100-value. The incommensurately modulated structure can be regarded as being composed of the four types of β-Ca2SiO4-like layers (S, S′, T, and T′) and one type of interlayer denoted by U as shown in Figure 2(a). The interlayer U, which is characterized by the hardly or slightly tilted [MO4] tetrahedra with |Θ| < 5°, seems to play a role of buffer layer, although the direct connection of S and T layers without the interlayer is also possible and actually seen in a few parts of the structure. It is noted that the modulation with wavelength λ (= 1/|q|) ≈ 3.649 × b is underlying as schematically illustrated in the upper part of Figure 2a. Thus, the modulated structure of the present IC-(Ca1.88Eu2+0.01□0.11)(Si0.78P0.22)O4 is described by an alternate sequence of the S (or S′) and T (or T′) layers separated mostly by the interlayer U. This feature seems common among related structures with different modulation wavevectors including those of commensurate phases. It is consequently assumed that a structure of a commensurate phase is simply described, in connection with its modulation wavevector (q = β·b*) of β = m/n, as a supercell of n × b containing m sets of clockwise and counterclockwise tilting layers mostly accompanying an interlayer of untilted tetrahedra. In a phase of β = 3/11, for example, three sets of clockwise and counterclockwise tilting layers in an alternate sequence are contained in the supercell of 11 × b (see Figure S5). With β = 1/4, the crystal structure corresponds to that having the supercell of 4 × b.12 It is noted that the space group of commensurate phases changes depending on the β value, odd or even for numerator and denominator, and the 3d section as summarized in Table 2. As modulated structures are

Figure 3. Occupational modulation at the Ca1 site.

y0) = 0.6815, where β (= 0.2740) is the b* component of the modulation wavevector and y0 (= 0.25) is the fractional coordinate y for Ca1 in the basic structure.) In the threedimensional space, the nearly half occupied Ca1 sites are mostly found in the U layers (Figure 2a). As a common feature among these Ca1 sites, the deformation as illustrated in Figure 2d is seen in coordination environments. An O1 ion in one of the neighboring [MO4] tetrahedra moves away from the Ca1 site according to the tilting of the tetrahedron. The similar deformation occurs at the opposite side of the interlayer. As two Ca1−O1 bonds are significantly elongated by clockwise and counterclockwise tilting of tetrahedra at both sides (Figure 2d), this Ca1 site seems to become too large to stably accommodate the Ca ion. Consequently, these Ca1 sites are partly vacant, resulting in the site occupancy far less than 1. Variations of Ca1−O1 distances affect the BVS at the Ca1 site, as the BVS is calculated by interatomic distances. The BVS of the Ca1 (Figure 4) is as low as 1.2 around t = 0.7, and roughly close to 2 in other regions. The contribution of the bond valence (BV) for the Ca1−O1 bonds was also calculated and plotted in Figure 4. It is clear that the decrease of the BVS at the Ca2 site around t = 0.7 is due to the decrease of BV for Ca1−O1 bonds. Comparing plots in Figures 3 and 4, the modulation of the BVS is synchronized with the modulation of the site occupancy. The Ca1 site in Figure 2d corresponds to the situation of t ≈ 0.7 in these plots; the site occupancy about 0.5 and BVS as low as 1.2. All of these results are consistent with each other. That is, Ca1−O1 distances in the interlayer U are so extended as to give considerably smaller BVS than 2. The site occupancy is far less than 1 in these Ca1 sites, because the coordination environment of the sites is rather unsuitable

Table 2. Space Groups Derived from the Superspace Group Pnma(0 β 0)00sa m/n (= β) odd/odd odd/even even/odd

t0 = 0 (modulo 1/2n) t0 = 1/4n (modulo 1/2n) P121/m1 P1121/n P21/n11

P212121 P21mn Pnm21

t0 = general P1211 P11n Pn11

a

3d section is given by t = t0.

ordered structures, the connecting sequence of the β-Ca2SiO4like layers and interlayer in the IC-phase is unambiguously defined by the modulation function for each atom. However, the random connection of S and T layers, together with interlayer U, would cause an orthorhombic disordered structure, since the layer T is approximately the transformation of the layer S by a mirror plane normal to the b-axis and eventually compensates the deviation of the α angle from 90°. Thus, the atoms in the resulting crystal structure would be described with large atomic displacement parameters or statistically distributed at split positions as reported for α′H(Ba0.24Ca0.76)2SiO4.4 It seems that local structures including the coordination environment of Ca2+/Eu2+ ions are basically the D

DOI: 10.1021/acs.inorgchem.9b00408 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Bond valence sum of the Ca1 site as depicted by the blue solid line. Contribution of the bond valence for Ca1−O1 bonds is shown by the red solid line.

Figure 5. Emission and excitation spectra of (Ca1.88Eu2+0.01□0.11)(Si0.78P0.22)O4 single particle.

for a Ca ion. The site occupancy is close to 1 at Ca1 sites in the middle of S and T layers, indicating that the Ca ion vacancies are concentrated in the vicinity of the interlayers. Coordination Environments. The variation of M−O distances (= dM−O) are moderate: 0.154 < dM−O < 0.163 nm (see Figure S7). The BVS of the M site (see Figure S8) was calculated using a parameter set for the Si4+−O2− pair.18 The BVS ranging from 4.2 to 4.5 are larger than that expected for a pure Si4+site. This is because P5+ ions of an ionic radius 0.017 nm in the coordination number IV19 are partially substituted for Si4+ ions of an ionic radius 0.026 nm at M sites. The BVS calculated by a parameter set for P5+−O2− (Figure S8) is far smaller than 5 in all of t. As variations of BVSs are moderate, the occupational modulation of Si/P at the M site would be basically insignificant for the construction of the present modulated structure. A coordination environment of the Eu2+ ion is one of the most important points in the research of phosphors. However, the precise determination of structural parameters for the Eu2+ ion in the present crystal is practically impossible by the following two reasons. First, the estimated molar ratio Eu/(Ca +Eu) in a whole crystal is only 0.0053 (= 0.01/1.89). Second, a considerable amount of vacancies is assumed at the Ca/Eu sites. Thus, in the structure refinement, we located the Eu2+ ions at the same position to one of the Ca sites, Ca1, with the constant molar ratio Ca/Eu = 0.88/0.01 in all t. Details are given in Supporting Information (see the section “Refinement for occupation parameters of Ca and Eu ions”). Under these circumstances, we considered the coordination environment of Eu2+ ions on the basis of BVS at the Ca1 site calculated by a parameter set of Eu2+−O2− (Figure S9). The BVS is roughly close to 2 in a region around t = 0.7, which corresponds to sites in the interlayer U as mentioned above. These Ca1 sites are too large for accommodation of Ca2+ ions, but suitable for Eu2+ ions. In contrast, the BVS of t regions corresponding to sites in the middle of S and T layers is as high as 3, implying these Ca1 sites are too small for Eu2+ ions to stably occupy. Therefore, it is expected that Eu2+ ions concentrate at around the interlayers, which is supported by the results of PL spectra as mentioned in the following subsection. Single-Particle Photoluminescence Properties. The excitation spectrum of the present IC-(Ca1.88Eu2+0.01□0.11)(Si0.78P0.22)O4 grain displayed a band with a tail extending to 400 nm, which matches well with the emission of infrared LEDs (Figure 5). The emission spectrum upon the 335 nm excitation showed a relatively broad band centered at ca. 490 nm and a full width at half-maximum (fwhm) of ca. 80 nm.

This emission peak was almost the same in position and in fwhm as that of α′H-(Ca1.98□0.02)(Si0.88P0.12)O4:0.02Eu2+ when excited by 350 nm.9 Both crystal structures were made up of the β-Ca2SiO4-like layers of S and T, which would cause the closely related local environments of Eu2+ ions in the Ca/Eu sites of IC- and α′H-phases. As compared with these two phosphors, the emission peaks (λem) were, when excited at 350 nm, considerably red-shifted for the β (λem = 510 nm for Ca 1.98 SiO 4 :0.02Eu 2+ ) and α′ L (λ em = 539 nm for Ca1.48Sr0.5SiO4:0.02Eu2+) phases.9 Actually, the Eu2+ ions in the IC-phase would preferentially occupy the relatively large sites in the U interlayer. This would induce the smaller crystal field splitting and weaker nephelauxetic effect, and eventually cause the emission peak that was located at the shorter wavelength ranges than those of the β- and α′L-phases. Comparison of BVS between the IC phase and the α′L-phase supports the above speculation. As the α′L-phase has a superstructure of 3 × a, three Ca sites, Ca11, Ca12, and Ca13, correspond to the Ca1 site in the IC-phase. The BVSs at these sites were calculated using a parameter set of Eu2−O2− and structural parameters from the literature,20 and found to exceed 2, the formal charge of Eu2+; 2.79 at Ca11, 2.40 at Ca12, and 2.27 at Ca13. In contrast, the BVS calculated in a similar manner for the IC phase (Figure S9) is approximately equal to 2 in a region of t corresponding to sites in the interlayer U, suggesting that the relevant sites are more suitable for the Eu2+ ions.

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CONCLUSION We prepared the single-crystal grain of (Ca1.88Eu2+0.01□0.11)(Si0.78P0.22)O4 with IC structure. The X-ray diffraction data showed the satellite reflections along the b*-axis of the orthorhombic subcell (a ≈ 0.684 nm, b ≈ 0.542 nm, and c ≈ 0.938 nm). The modulated structure with wavelength of 3.649 × b was determined using a (3 + 1)-dimensional description based on the superspace group Pnma(0 β 0)00s. The crystal structure was found to be composed of the four types of βCa2SiO4-like layers (S, S′, T, and T′) and one type of interlayer (U). The former layers were classified into two by the tilt orientation of the constituent [MO4] tetrahedra (M = Si or P). The layers of S and S′ contained the [MO4] tetrahedra of clockwise tilting when viewed along the a-axis, and those of T and T′ included the tetrahedra tilted in a counterclockwise manner. The interlayer U, which was characterized by the hardly or slightly tilted [MO4] tetrahedra, would play a role of the buffer layer. The basic structure contained two nonequivalent Ca sites, Ca1 and Ca2. The Ca2 site was fully E

DOI: 10.1021/acs.inorgchem.9b00408 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry occupied by Ca2+ and free from Eu2+ in all range of t (= x4 − q· r). The occupational modulation at the Ca1 site was so significant that the sum of sof’s for Ca2+ and Eu2+ decreased to as low as 0.5 at t = 0.6815. Because the relevant site was too large for accommodation of Ca2+ but suitable for Eu2+, the latter ions would concentrate at this site located in the interlayer U. The emission spectrum upon the 335 nm excitation showed a relatively broad band centered at ca. 490 nm and a fwhm of ca. 80 nm. This emission peak of the ICphase was shifted to the shorter wavelength ranges as compared with those of the β- and α′L-phases.

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(6) Kim, J. S.; Park, Y. H.; Kim, S. M.; Choi, J. C.; Park, H. L. Temperature-dependent emission spectra of M2SiO4:Eu2+ (M = Ca, Sr, Ba) phosphors for green and geenish white LEDs. Solid State Commun. 2005, 133, 445−448. (7) Sato, Y.; Kato, H.; Kobayashi, M.; Masaki, T.; Yoon, D.-H.; Kakihana, M. Tailoring of deep-red luminescence in Ca2SiO4:Eu2+. Angew. Chem., Int. Ed. 2014, 53, 7756−7759. (8) Nakano, H.; Yokoyama, N.; Banno, H.; Fukuda, K. Enhancement of PL intensity and formation of core-shell structure in annealed Ca2‑x/2(Si1‑xPx)O4:Eu2+ phosphor. Mater. Res. Bull. 2016, 83, 502− 506. (9) Mao, Z.; Lu, Z.; Chen, J.; Fahlman, B. D.; Wang, D. Tunable luminescent Eu2+-doped dicalcium silicate polymorphs regulated by crystal engineering. J. Mater. Chem. C 2015, 3, 9454−9460. (10) Smaalen, S. V. Incommensurate Crystallography; Oxford University Press: New York, 2007. (11) Janssen, T.; Chapuis, G.; de Boisseu, M. Aperiodic Crystals; Oxford University Press: New York, 2007. (12) Saalfeld, H.; Klaska, K. H. The crystal structure of 6Ca2SiO4· 1Ca3(PO4)2. Z. Kristallogr. - Cryst. Mater. 1981, 155, 65−73. (13) Hirosaki, N.; Takeda, T.; Funahashi, S.; Xie, R. J. Discovery of new nitridosilicate phosphors for solid state lighting by the singleparticle-diagnosis approach. Chem. Mater. 2014, 26, 4280−4288. (14) Palatinus, L.; Chapuis, G. SUPERFLIP - a computer program for the solution of crystal structures by charge flipping in arbitrary dimensions. J. Appl. Crystallogr. 2007, 40, 786−790. (15) Petricek, V.; Dusek, M.; Palatinus, L. Crystallographic Computing System JANA2006: General features. Z. Kristallogr. Cryst. Mater. 2014, 229, 345−352. (16) Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272−1276. (17) Jost, K. H.; Ziemer, B.; Seydel, R. Redetermination of the structure of β-dicalcium silicate. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1977, 33, 1696−1700. (18) Brown, I. D. VALENCE: a program for calculating bond valence. J. Appl. Crystallogr. 1996, 29, 479−480. (19) Shannon, R. D.; Prewitt, C. T. Effective ionic radii in oxides and fluorides. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1969, 25, 925−946. (20) Itoh, H.; Nishi, F.; Kuribayashi, T.; Kudoh, Y. Orientational ordering of three SiO4 tetrahedra in α′L-Ca1.5Sr0.5SiO4 that satisfies bond-valence requirements and avoids O-O repulsion. J. Mineral. Petrol. Sci. 2009, 104, 234−240.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00408. Reliability indices for main and satellite reflections, and structural parameters of the basic structure, optical micrographs, modulation functions for the displacement of ions, electron densities along the fourth direction of ions at selected sections, tilting of a [MO4] tetrahedron evaluated by the angle Θ, structure for the commensurate phase of b = 3/11, interatomic distances, and bond valence sum (PDF) Accession Codes

CCDC 1895361 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Koichiro Fukuda: 0000-0001-8591-2786 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Professor H. Nakano, Toyohashi University of Technology, for valuable discussion on silicate phosphors. This research was supported by NIMS Joint Research Hub Program FY2018.

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REFERENCES

(1) Taylor, H. F. W. Cement Chemistry, 2nd ed.; Thomas Telford Services Ltd.: London, 1997. (2) Fukuda, K.; Maki, I.; Ito, S.; Miyake, T. Structural change in phosphorus-bearing dicalcium silicate. Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi 1997, 105, 117−121. (3) Fukuda, K.; Maki, I. Transitional phase of Ca2SiO4 solid solution with incommensurate superstructure. J. Am. Ceram. Soc. 1989, 72, 2204−2207. (4) Fukuda, K.; Hasegawa, H.; Iwata, T.; Hashimoto, S.; Inoue, K. Structural disorder and intracrystalline microtexture of α′H(Ba0.24Ca0.76)2SiO4. J. Am. Ceram. Soc. 2007, 90, 925−931. (5) Kim, J. S.; Jeon, P. E.; Choi, J. C.; Park, H. L. Emission color variation of M2SiO4:Eu2+ (M = Ba, Sr, Ca) phosphors for lightemitting diode. Solid State Commun. 2005, 133, 187−190. F

DOI: 10.1021/acs.inorgchem.9b00408 Inorg. Chem. XXXX, XXX, XXX−XXX