Self-Luminescence of Perovskite-Like LaSrGaO4 via Intrinsic Defects

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

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Self-Luminescence of Perovskite-Like LaSrGaO4 via Intrinsic Defects and Anomalous Luminescence Analysis of LaSrGaO4:Mn2+ Zhenhua Xing, Panlai Li,* Danjie Dai, Xiaotong Li, Chunjiao Liu, Li Zhang, and Zhijun Wang* College of Physics Science and Technology, Hebei Key Lab of Optic-Electronic Information and Materials, Hebei University, Baoding, Hebei 071002, China

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S Supporting Information *

ABSTRACT: A series of deep red phosphors with perovskite-like oxide LaSrGaO4 as host are synthesized by a high temperature solid state method, and the luminescence properties and mechanisms have been investigated in detail. LaSrGaO4 presents self-luminescence at 722 nm, and it is proved that the self-luminescence comes from two kinds of electronic defects and three kinds of vacancy defects, which are anti-occupation defects LaSr•, strontium gap defects Sri••, the oxygen interstitial defects Oi″, substitution defects SrLa ′ , and strontium vacancy defects V″Sr. In addition, when Mn2+ ions are doped in LaSrGaO4, interestingly, the shape of the emission spectra of LaSrGaO4:Mn2+ is the same as that of LaSrGaO4, and the emission intensities are enhanced greatly. Luminescence of Mn2+ ions has been confirmed by doping Mg2+ into LaSrGaO4 and measuring the lifetimes of host LaSrGaO4, LaSrGaO4:Mg2+, and LaSrGaO4:Mn2+ for comparison. The mechanisms of host self-luminescence and Mn2+ luminescence are discussed by detecting the luminescence centers with the low temperature spectra, calculating the forbidden bandwidth with the diffuse reflectance spectra, and calculating the trap depths with the thermoluminescence spectra and further depicted by establishing the transition model. LaSrGaO4:Mn2+ can emit strong deep red light about 722 nm, so the phosphor will have a good application prospect in the field of plant lighting.

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INTRODUCTION As is well-known, the unique properties of perovskite and perovskite-like materials such as ferroelectricity, superconductivity, ferromagnetism, giant magnetoresistance oxygen ion conduction, and high dielectric1−8 constant have attracted more and more research interest. The perovskite composite is a large class of compounds having the same structure with the chemical formula XYO3 as the perovskite material CaTiO3.9−11 When the composition of the material deviates from the chemical formula of 1:1:3, defects occur in the perovskite crystal lattice. The X and Y positions can be occupied by different elements at the same time, and the perovskite structure could be changed by changing the elemental composition of the X and Y positions; besides, some new properties can also be produced. Such compounds derived from perovskite structures are referred to as perovskite-like compounds.12−16 Perovskite-like compounds generally include perovskite structural defect compounds, layered perovskite compounds, and hexagonal perovskite compounds.9 They have a common host material LaSrGaO4 (LSG), which belongs to the group of compounds of the general formula ABCO4, where A = Ca, Sr, or Ba, B = some other rare earth metal, and C = Al, Ga, or some transition metal. These compounds belong to the tetragonal K2NiF4 structure type, and the space group is I4/mmm, which can be regarded as a two-dimensional layer of angularly connected GaO6 octahedrons, and La3+ and Sr2+ cations are randomly distributed between them.17−20 It can be seen that LSG is a perovskite-like oxide with some properties of perovskite oxide, © XXXX American Chemical Society

such as the defects in the crystal lattice and its catalytic activity caused by lattice oxygen and surface adsorption oxygen. In addition, the host LSG has been studied for many years, so its physical and chemical properties and applications in various aspects have been well-known, such as lattice constant,17,19 Raman scattering,20 infrared vibration,21 polarized infrared reflectance spectra,18 anisotropy of the elastooptic properties,22 energy band structure,19 etc. Some researchers also doped various ions in the host, such as Cr3+, Eu3+, Mn4+, Nd3+, Ho3+, Tb3+, and the lattice positions and luminescent properties were reported.23−28 However, the systematic study of self-luminescence and structural defects of LSG has not been reported so far. As is known, most hosts with the self-luminescence usually emit blue light under 254 nm excitation because of the oxygen deficiencies.29−32 However, in this research, it is found that LSG has the self-luminescence phenomenon, which emits deep red light at 722 nm under the wide excitation range from 250 to 650 nm. Therefore, in order to study the LSG special selfluminescence phenomenon, the intrinsic defects inside the LSG are verified reasonably, and the photoelectrons transitional process about self-luminescence is clearly explained and described in detail. The Mn2+ ion, whose luminescence is greatly influenced by the crystal environment, usually emits orange-NIR light at the wide excitation band,33−36 so it is selected as the activator to obtain the Received: December 13, 2018

A

DOI: 10.1021/acs.inorgchem.8b03470 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

CN = 6 where CN means coordination number) are substituted by Mn2+ ions with larger size (r = 0.083 nm, CN = 6) and shrink when La3+ ions (r = 0.1216 nm, CN = 9) or Sr2+ ions (r = 0.136 nm, CN = 9) are substituted by Mn2+ ions with smaller size. Figure 3 shows the changes in the cell parameters (a, b, c), unit cell volume, and the volume of Ga3+ in LSG:xMn2+ with x increasing. From Figure 3 and Table S1, it can be obviously seen that the cell parameters (a, b, c) and unit cell volume gradually increase with increasing Mn2+ concentration, which indicates that Mn2+ is more likely to replace Ga3+ ions. Luminescence Properties of LaSrGaO4. Figure 4a shows the photoluminescence excitation (PLE) and photoluminescence emission (PL) spectra of LSG host sintered in air and measured at room temperature. It can be seen that the PLE spectrum consists of a broadband absorption ranging from 250 to 650 nm with two peaks at 354 and 527 nm. Under excitation at both absorption peak wavelengths, the host LSG exhibits a narrow emission band in the range from 675 to 800 nm with a single peak at 722 nm. It is worth mentioning that the PL spectrum is asymmetrical and has two slight packets on the left side, which may be the result from the different luminescence centers in the host. Therefore, the low temperature spectra of LSG are tested to determine luminescence centers in the host, as shown in Figure 4b. It can be seen that there are two emission bands peaking at 718 and 729 nm under different low temperatures, and as the temperature increases, an emission band appears at around 690 nm, which may be associated with the intrinsic defects of the host LSG.37 In order to learn where the three luminescence centers are from, the crystal structure of host LSG was analyzed clearly. The crystal structure of LSG is shown in Figure 5. It can be seen that LSG is a perovskite-like oxide with some properties of perovskite oxide, such as the defects in the crystal lattice, and its catalytic activity is caused by lattice oxygen and surface adsorption oxygen. Thus, LSG easily displays the self-luminescence phenomenon. Many books about defects have mentioned a kind of host La2CuO4 whose structure is similar to that of LaSrGaO4. For instance, in “Inorganic Materials Chemistry”38 referring to unequal substitution, La3+ ions are replaced by Sr2+ in the La2CuO4 host, 3Sr2+ + 2La3+ → 2SrLa ′ + Sri••, and it forms the La2−xSrxCuO4 structure; as a result, the hole defects are introduced to change Cu2+ to Cu3+ or increase the valence of Cu2+ ions by adding oxygen 1 interstitial ions, 2 O 2+CuCu X → Oi″ + 2CuCu•. Another example, the “Defects in Solids”,39 also refers to misplaced defects of the point defects, mentioning anti-occupation defects; that is, an atom is likely to appear in a position occupied by another different chemical atom in the compound under the normal conditions, such as 2La3+ + 3Sr2+ → 2LaSr• + VSr ″ in the La2CuO4 host. On the basis of the above, it is learned that: (1) The valence state of La3+ ions is different from that of Sr2+ ions, but La3+ and Sr2+ occupy the same lattice position in LSG, where they are randomly distributed. It is likely that the anti-occupation phenomenon between La3+ and Sr2+ occurs, resulting in the occurrence of occupied and anti-occupation defects in the crystal. (2) It is found that La2CuO4, LaSrCuO4, and LaSrGaO4 all belong to the tetragonal K2NiF4 structure, and their XRD diffraction patterns are similar, as shown in Figure S1. (3) Due to the Jahn−Teller effect, Cu2+ may occupy a distorted octahedral six-coordinate or a planar tetragonal coordination. Further, the possible intrinsic defects inside the crystal of the LSG host are as follows: (1) the oxygen gap defects (Oi″); (2) the substitution defects (SrLa ′ ) and strontium interstitial defects (Sri••) in pairs;

brighter deep red light. It is quite interesting that the shape of the LSG:Mn2+ emission spectra is the same as that of LSG. Thus, whether Mn2+ ions present luminescence is analyzed systematically, and the luminescence mechanisms of LSG and LSG:Mn2+ are reasonably explained by establishing a transition model.

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EXPERIMENTAL SECTION

Sample Preparation. A series of LaSrGaO4 (LSG) phosphors with doping Mn2+ or Mg2+ were synthesized by a high temperature solidstate method. Starting materials La2O3 (99.99%), SrCO3 (99.99%), Ga2O3 (99.99%), MnCO3 (A.R.), and MgO (A.R.) were weighed in stoichiometric proportion, mixed homogeneously and ground by an agate mortar for 20 min, and loaded into corundum crucible to sinter. All of the samples were heated at 1450 °C for 4 h. The obtained phosphors were naturally cooled to room temperature and then were again ground to a powder for measurement. Materials Characterization. The phase formation of samples was examined by X-ray powder diffraction (XRD) performed on a Bruker D8 X-ray diffractometer with Ni-filtered Cu Kα radiation (λ = 0.15405 nm), operating at 40 mA and 40 kV. Step length was 0.05°, and the diffraction range was 10−80°. Room-temperature luminescence spectra of samples were recorded with a Hitachi F-7000 fluorescence spectrophotometer using a 450 W Xe lamp as the excitation source, with a scanning wavelength from 200 to 900 nm, scanning at 1200 nm/min. Decay curves of samples were obtained using a Multifunction Steady-State/ Transient Fluorescence spectrophotometer. Diffuse reflection spectra of samples were surveyed on a Hitachi U4100 UV−vis-NIR spectrophotometer using BaSO4 as a reference, with scanning wavelength from 200 to 800 nm. Thermoluminescence spectra of samples were measured using a FJ-427A1 TL dosimeter with a fixed heating rate of 1 °C/s within the range of 30−300 °C.

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RESULTS AND DISCUSSION Phase Characterization. Figure 1 depicts the XRD patterns of LaSrGaO4(LSG):xMn2+ (x = 0, 0.0005, 0.001, 0.002, 0.003,

Figure 1. XRD patterns of LSG:xMn2+ (x = 0−0.01) and the standard card of LaSrGaO4 (ICSD#54130).

0.01) and the standard data for LaSrGaO4 (ICSD#54130). It can be seen that the XRD patterns of LSG:xMn2+ (x = 0, 0.0005, 0.001, 0.002, 0.003, 0.01) are indexed to the standard data for LaSrGaO4 (ICSD#54130), and no impurity phase was detected, indicating that doping Mn2+ ions does not cause significant changes to the crystal structure. In order to further prove that the samples are purity phase and investigate the Mn2+ position in LSG, the XRD Rietveld refinements of LSG:xMn2+ (x = 0, 0.001, 0.002, 0.003) are performed by the GSAS program with the single crystallographic data of LaSrGaO4 as the initial model. The results of refinement are presented in Figure 2. It is seen that the calculation is in good agreement with the experiment, which further proves that LSG:Mn2+ phosphors are single phase. It is well-known that the unit cell volume will expand when Ga3+ ions (r = 0.062 nm, B


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Figure 2. Rietveld refinement of LSG:xMn2+ with (a) x = 0, (b) x = 0.001, (c) x = 0.002, and (d) x = 0.003.

(3) the anti-occupation defects (LaSr•) and strontium vacancy ″ ) appearing in pairs. defects (VSr Many host materials emit light because of oxygen vacancy defects. When the raw materials are in a high temperature and high pressure environment during the sintering process, oxygen vacancies are easily generated, and defect levels are formed inside the crystals, which could capture and release electrons to emit light. As is known, the raw materials are in an oxygen-free or oxygen-little atmosphere when they are sintered in the reducing atmosphere, so the interstitial oxygen in the crystal of the prepared material is also reduced. Therefore, the host LSG was synthesized in the reducing atmosphere (10% H2 + 90% N2) and in air, respectively, for comparison. The prepared samples are all pure phase, whose XRD patterns are shown in Figure S2, and the PLE and PL spectra are shown in Figure 6a. It is found that the spectra of the host sintered in the reducing atmosphere cannot be observed. Therefore, it can be preliminarily determined that the cause of the LSG self-luminescence is not oxygen vacancy defects but may be oxygen interstitial defects. To further illustrate the existence of oxygen interstitial defects in the crystal, the host is tested by XPS, as shown in Figure 6b. According to some reports,40−42 the electron energy corresponding to the adsorbed oxygen (O2−, O22−, O−, OH−, CO32−) in the crystal structure is about 531.2 eV, the electron energy of the adsorbed water molecule (H2O) is about 533.0 eV, and the electron energy of the lattice oxygen is about 529.1 eV. It can be seen from the electron energy spectrum after peak separation that the electron energies of peak1, peak2, and peak3 are Eb = 531.1 eV, Eb = 532.9 eV, and Eb = 529.0 eV, respectively, which correspond to the electron energy of adsorbed oxygen, adsorbed water molecules, and lattice oxygen, respectively. The adsorbed oxygen means that gaseous oxygen is adsorbed on the surface of the material, interacts with surface ions to obtain electrons as

the temperature increases, and is converted from electrophilic O2− to nucleophilic O2−. Chemically adsorbed oxygen is commonly found in metal oxides in which adsorbed states can be molecular, atomic, and interstitial oxygen. From the XPS data, there is the adsorbed oxygen, meaning that oxygen interstitial ions indeed presented in the LSG. In order to clarify whether the substitution defects and antioccupation defects exist in the LSG host or not, two sets of experiments are designed by fine-tuning the La:Sr ratio. The XRD of samples of La:Sr > 1 and La:Sr < 1 shows that they are all pure phase in Figure S3. The PLE and PL spectra of the prepared samples are shown in Figure 7. It is noted that the PLE spectra and PL spectra intensities of La:Sr > 1 samples and La:Sr < 1 samples both increase. As is known about the intrinsic defects of LSG, as La3+ ions and Sr2+ ions occupy the same positions and randomly distribute in the crystal structure, the substitution defects Sr′La (3Sr2+ + 2La3+ → 2Sr′La + Sri••) are likely to occur; likewise, for the anti-occupation defects LaSr• (2La3+ + 3Sr2+ → ″ ), when La:Sr < 1, the Sr2+ ions easily occupy the 2LaSr• + VSr ′ and the position of La3+. Then, the substitution defects SrLa strontium gap ions Sri•• increase, so the spectra intensity is increased, as shown in Figure 7a. When La:Sr > 1, the La3+ ions are more likely to occupy the position of Sr2+, so the anti″ occupation defects LaSr• and the strontium vacancy defects VSr increase, which results in an increase of spectra intensity, as shown in Figure 7b. Thus, it can been seen that the host LSG has two kinds of electron defects and three kinds of hole defects: the two electron defects are LaSr• and Sri••, and the three hole ″ , SrLa ′ , and Oi″. defects are VSr Luminescence Properties of LaSrGaO4:Mn2+. The PLE and PL spectra of LSG:xMn2+ (x = 0, 0.0005, 0.001, 0.002, 0.003, 0.005) are measured and shown in Figure 8a,b. It can be seen that the peak position and shape of the excitation and C


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Figure 3. Changes in the cell parameters (a, b, c), unit cell volume, and the volume of Ga3+ in LSG:xMn2+ with x increasing.

Figure 4. (a) PLE and PL spectra of LSG measured at room temperature. (b) PL spectra of LSG at different temperatures.

emission spectra of LSG:Mn2+ are substantially the same as that of the host LSG. To compare LSG with LSG:Mn2+, the normalized PLE and PL spectra of LSG:xMn2+ (x = 0, 0.0005, 0.001, 0.002, 0.003, 0.005) are given in Figure 8c,d. It can be seen from Figure 8c that the first absorption peak (λex1) red shifts from 354 to 366 nm and broadens on the right side. When the concentration of Mn2+ ions increased, the intensity of the second absorption peak (λex2) at 527 nm increases faster than that of the first absorption peak at 366 nm. It can be seen from Figure 8d that the quenching concentration of the emission spectra is 0.002, and moreover, it is interesting that the shape of PL spectra of LSG:Mn2+ completely coincides with that of the host, which also bulges twice on the left side of the emission peak. The PL stability of LSG and LSG:0.002Mn2+ samples is also tested, as shown in Figure 9, showing a high photostability. Even

repeatedly irradiating the same phosphor in 1 h, the luminescence intensity of phosphor still remains unaltered, further indicating the high reproducibility upon excitation with xenon light. Since the peak pattern of the PL spectra between LSG and LSG:Mn2+ is exactly the same, but the intensity of LSG:Mn2+ is enhanced a lot compared to that of LSG, it is impossible to distinguish the luminescence center of LSG from that of Mn2+ ions. Thus, when the Mn2+ ions are doped into the host, do the Mn2+ ions really emit? There are two kinds of conjectures: (1) Mn2+ ions enter the crystal interior, which only affects the crystal structure of the host and improves the luminescence intensity of LSG; (2) Mn2+ ions have their own luminescence center, but the luminescence intensity of them is weak and covered by that of LSG. The following experiments were conducted to answer these questions. D


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are measured and shown in Figure 11. All the decay curves can be fitted successfully by the second-order exponential mode, and the average lifetimes are calculated and shown in Table 1. Figure 11a shows the fluorescent decay curve of the host LSG under 350 nm excitation. Figure 11b displays the fluorescent decay curves of LSG:yMg2+ also measured under 350 nm excitation. It is found that the lifetimes of LSG:Mg2+ are basically the same as that of the host LSG, which are both in nanosecond order and have no regular change with the changes of Mg2+ concentration. Figure 11c,d illustrates fluorescent decay curves of LSG:Mn2+ under 366 and 527 nm excitation, respectively. It is seen that the lifetimes of LSG:Mn2+ are both in millisecond and gradually shorten as the concentration of Mn2+ increases. Hence, it can be confirmed that the luminescence of Mn2+ does exist. In order to distinguish the luminescent centers of Mn2+ and the host, the low temperature spectra of LSG:0.002Mn2+ are measured and shown in Figure 12a. It is surprisingly found that LSG:0.002Mn2+ also has two emission bands peaking at 718 and 729 nm under lower temperatures and has an emission band around 690 nm as the temperature increased, which are very similar to these of the LSG host. For the sake of contrast, the low temperature spectra of LSG and LSG:0.002Mn2+ at 10 and 250 K are shown in Figure 12b,c. It can be clearly seen that the shape of the low temperature spectra of the LSG host and LSG:0.002Mn2+ is exactly the same. From the above results, it can be inferred that the luminescence of Mn2+ is much weaker than that of LSG. As a result, the PL spectra of Mn2+ are completely covered by that of LSG. Luminescence Mechanisms of LSG and LSG:Mn2+. In order to understand the luminescence mechanisms of LSG and LSG:Mn2+, it is necessary to understand its forbidden bandwidth. Figure 13a shows the diffuse reflectance spectra and the PLE spectra of LSG and LSG:0.002Mn2+. Additionally, the forbidden bandwidths are calculated using eq 143,44 and shown in Figure 13b,

Figure 5. Crystal structure of LaSrGaO4, La2CuO4, and LaSrCuO4.

In order to clarify whether there is the luminescence of Mn2+, a reverse verification method is adopted. Since Mg2+ is divalent and has a similar radius with Mn2+ and, moreover, it cannot emit light, here it is doped into the host LSG. It can be hypothesized that, if the luminescence phenomenon of the LSG:Mg2+ is the same as LSG:Mn2+, it can be proved that there are no luminescence of Mn2+ ions. Mn2+ ions just affect the internal crystal structure of LSG to enhance the self-luminescence intensity. If there is a difference in the PLE and PL spectra of LSG:Mg2+ and LSG:Mn2+, it can be testified that Mn2+ ions may give out light. Figure S4 depicts the XRD patterns of LSG:yMg2+ (y = 0, 0.0005, 0.001, 0.002, 0.003, 0.005); it can be found that the diffraction peaks of all samples are well matched with the standard cards of LaSrGaO4 (ICSD#54130), which means that all of the samples are in purity phase. Figure 10 shows PLE and PL spectra of LSG:Mg2+ with different Mg2+ concentrations, and the insets are the wavelength and intensity change diagram. Figure 10a illustrates that there are no obvious changes in the PLE spectra of LSG:Mg2+ compared with that of the host LSG. The PL spectra have no great improvement in intensity compared to that of LSG, as shown in Figure 10b. Obviously, there are differences in the excitation and emission spectra of LSG:Mg2+ and LSG:Mn2+, which suggests that Mn2+ ions have greater influence on the crystal lattice than Mg2+ ions. To further confirm the Mn2+ luminescence exists, the lifetimes of the host LSG, LSG:yMg2+ (y = 0.0005, 0.001, 0.002, 0.003, 0.005), and LSG:xMn2+ (x = 0.0005, 0.001, 0.002, 0.003, 0.005)

F (R ) =

(1 − R )2 K = 2R S

(1)

where F(R) is the Kubelka−Munk absorption coefficient and R is the measured reflectivity. The forbidden bandwidth of the host Eg is calculated to be 2.86 eV, which is consistent with the forbidden bandwidth of LaSrGaO4 in ref 19 (2.85−2.87 eV). The forbidden bandwidth of LSG:0.002Mn2+ E′g is calculated to be 2.56 eV, which is narrower than LSG. Figure 13c shows the thermoluminescence spectra (TL) of the LSG:xMn2+ (x = 0, 0.0005, 0.001, 0.002, 0.003, 0.005). It can be seen that there are

Figure 6. (a) PLE and PL spectra of LSG sintering in air and a reducing atmosphere. (b) XPS O1s electron energy spectrum of LSG. E


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Figure 7. (a) PLE and PL spectra of La:Sr < 1. (b) PLE and PL spectra La:Sr > 1.

Figure 8. (a) PLE spectra of LSG:xMn2+ (x = 0−0.005). (b) PL spectra of LSG:xMn2+ (x = 0−0.005). (c) Normalized PLE spectra of LSG:xMn2+ (x = 0−0.005). (d) Normalized PL spectra of LSG:xMn2+ (x = 0−0.005), and the inset is the relationship between emission intensity and Mn2+ concentration.

Figure 9. (a) PL stability of LSG during 6 on/off cycles over a period of 3600 s (λex = 354 nm, λem = 722 nm); (b) PL stability of luminescence of LSG:0.002Mn2+ during 6 on/off cycles over a period of 3600 s (λex = 366 nm, λem = 722 nm). F


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Inorganic Chemistry

Figure 10. (a) PLE spectra of LSG:yMg2+ (y = 0−0.005). (b) PL spectra of LSG:yMg2+ (y = 0−0.005).

Figure 11. (a) Fluorescent decay curve of single host LSG (λex = 350 nm). (b) Fluorescent decay curves of LSG:yMg2+ (y = 0.0005−0.005) (λex = 350 nm); the inset is the relationship of lifetime and Mg2+ concentration. (c) Fluorescent decay curves of LSG:xMn2+ (x = 0.0005−0.005) (λex= 366 nm); the inset is the relationship of lifetime and Mn2+ concentration. (d) Fluorescent decay curves of LSG:xMn2+ (x = 0.0005−0.005) (λex = 527 nm); the inset is the relationship of lifetime and Mn2+ concentration. δ

two kinds of electron trap defects and both of the intensities decrease with the increase of Mn2+, which indicates that the number of defects reduces with Mn2+ doping. Figure 13d shows the peak resolution of the TL spectra of LSG. The depths of the two electron trap defects are calculated using eq 245 E = [2.52 + 10.2 ×(μg − 0.42)]×

κBTm 2 − 2κBTm ω

δ

factor, μg = ω = δ + τ . Using eq 2, the depths of the two electron trap defects of the host LSG are calculated to be ETrap1 ≈ 0.491 eV and ETrap2 ≈ 0.778 eV. In order to better distinguish transition energy levels of host LSG and Mn2+ ions and to explain the photoelectron transition mechanisms clearly, the PLE and PL spectra of LSG:0.002Mn2+ material are subjected to peak separation, as shown in Figure 14. From Figure 4a, it can be seen that the PLE spectrum of host LSG consists of a broadband absorption ranging from 250 to 650 nm with two peaks at 354 and 527 nm, which means the host LSG has two different forms of upward transition. From Figure 8c, it can been seen that the first absorption peak (λex1)

(2) −23

where κB is the Boltzmann constant whose value is 1.38 × 10 , Tm is the temperature at the highest intensity, T1 and T2 are the two temperatures of the half intensity, τ = Tm − T1, δ = T2 − Tm, ω is the width of half intensity, ω = τ + δ, and μg is the symmetry G


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belongs to Mn2+ ions. The PL spectrum is also divided into three peaks, as shown in Figure 14b, and 730 nm (13686 cm−1), 718 nm (13921 cm−1), and 687 nm (14560 cm−1) are three different downward transition levels, which cause the asymmetry of the PL spectrum. The energies of the excitation peaks at 354 and 527 nm are calculated using eq 3: c E = hγ = h (3) λ

Table 1. Average Lifetimes of LSG, LSG:Mg2+, and LSG:Mn2+ sample LSG

concentration 0 0.0005 0.001 0.002 0.003 0.005

LSG:Mg2+

LSG:Mn2+

λex = 354 nm

λex = 366 nm

λex = 527 nm

λem = 722 nm

λem = 722 nm

λem = 722 nm

0.53757 ms 0.53394 ms 0.52887 ms 0.51557 ms 0.49932 ms

0.50047 ms 0.49256 ms 0.46837 ms 0.45937 ms 0.43677 ms

1.90031 ns 1.87990 ns 2.46621 ns 1.99427 ns 2.11115 ns 2.22990 ns

The results are Eex1 ≈ 3.503 eV and Eex2 ≈ 2.353 eV. It shows that the photoelectrons in the materials have two different forms of upward transition: one is that the photoelectrons coming from the valence band are directly excited to an energy level inside the conduction band because of Eex1 > Eg > E′g; the other is that the photoelectrons coming from the valence band are excited to the local energy level that the electron defects formed in the forbidden band because of Eex2 < Eg′ < Eg. Then, the excitation peak energy of Mn2+ (EMn‑ex) at 393 nm is calculated to be ∼3.155 eV. This indicates that the upper level of Mn2+ ions is located in the conduction band of LSG because of EMn−ex > Eg′, which leads to the red shift and the broadening of the first

red shifts from 354 to 366 nm and broadens on the right side, which means there is the upper level of Mn2+ ions on the right side to pull the first absorption peak from 354 to 366 nm and broaden the fwhm of the first absorption peak. Thus, the PLE spectrum is divided into three peaks, as shown in Figure 14a. The two peaks at 354 nm (28320 cm−1) and 527 nm (19008 cm−1) belong to host LSG, so the last peak at 393 nm (25433 cm−1)

Figure 12. (a) PL spectra of LSG:0.002Mn2+ at different temperatures. (b) PL spectra of LSG and LSG:0.002Mn2+ at 10 K. (c) PL spectra of LSG and LSG:0.002Mn2+ at 250 K.

Figure 13. (a) Diffuse reflectance spectra and PLE spectra of LSG and LSG:0.002Mn2+. (b) The forbidden band of LSG and LSG:0.002Mn2+. (c) TL spectra of LSG:xMn2+ (x = 0−0.005). (d) The peak resolution of TL spectra of LSG. H


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Figure 14. (a) Peak separation of the PLE spectrum of LSG:0.002Mn2+. (b) Peak separation of the PL spectrum of LSG:0.002Mn2+.

Figure 15. (a) Luminescence mechanism of LSG. (b) Luminescence mechanism of LSG:Mn2+.

conduction band are 0.491 and 0.778 eV; they can act as another upper level and the excited state for photoelectron transition; the three hole defects are V″Sr, Sr′La, and O″i , which can absorb the holes from the valence band to form the lower energy level. (4) The photoelectrons in the valence band absorb the external energy, can be excited to one upper energy level in the conduction band and another upper energy level formed by an electron defect, then nonradiatively relax to the excited state formed by another electron defect, and finally move down to recombine with the holes of the lower energy levels formed by three kinds of vacancy defects, releasing energy to emit light. Figure 15b shows the luminescence mechanism of LSG:Mn2+. It can be seen that the host LSG is excited by external energy and self-illumination, but when Mn2+ ions are doped in LSG, they emit light and have an influence on LSG, which can be explained

absorption peaks, as shown in Figure 8c. Further, it is indicated that Mn2+ ions may transfer energy to the host LSG through the conduction band. In addition, the transition energies of the three emission peaks are calculated, respectively: Eem1 ≈ 1.699 eV, Eem2 ≈ 1.727 eV, and Eem3 ≈ 1.804 eV, which corresponds to the three lower levels. The luminescence mechanisms of the host LSG and LSG:Mn2+ can be explained by the models in Figure 15.46,47 As shown in Figure 15a, the luminescence mechanism of the host LSG can be explained as follows: (1) The forbidden bandwidth of the host LSG is 2.86 eV. (2) There is an upper level in the conduction band of LSG for photoelectron transition. (3) The host LSG has two kinds of electron defects and three kinds of hole defects in the forbidden band: the two electron defects are LaSr• and Sri••, whose trap depths from the I


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Inorganic Chemistry as follows: (1) As photoelectrons of the Mn2+ ground state (6A1(6S)) absorb the external light source and are excited to the Mn2+ upper level (4A1(4G) + 4E(4G)), the 6A1(6S) → 4A1(4G) + 4 4 E( G) transition occurs. (2) Since the 4A1(4G) + 4E(4G) energy level of Mn2+ is inside the conduction band, the photoelectrons move down nonradiatively by two ways: ① the photoelectrons move down to the Mn2+ excited state (4T1(4G)), so the nonradiative transition of 4A1(4G) + 4E(4G) → 4T1(4G) can occur; ② the photoelectrons can move down two electron defect levels of the host LSG. (3) The photoelectrons of 4T1(4G) move down to recombine with the holes of 6A1(6S), releasing energy to emit light. What needs to be pointed out is, when Mn2+ ions enter the crystal structure to replace the Ga3+ ions, the substitution defects are produced: 3Mn2+ + 2Ga3+ → 2Mn′Ga + Mni•• and the generated Mni•• can fill V″Sr and inhibit Sri•• at the same time, as a result the number of defects in the LSG host is reduced, as shown in Figure 13c. However, because of the process ②, the number of photoelectrons on the two electron defect levels are increased, which causes the intensity of the second absorption peak at 527 nm to increase faster than that of the first absorption peak at 366 nm, as shown in Figure 8c. Consequently, due to the luminescence of Mn2+ ions and the great influence of Mn2+ on LSG, the luminescence intensity of deep red light at 722 nm is enhanced.

ORCID

Panlai Li: 0000-0003-0972-9343 Zhijun Wang: 0000-0002-3574-3985 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 (No. 51672066) and the Funds for Distinguished Young Scientists of Hebei Province, China (No. A2018201101).

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CONCLUSIONS A series of phosphors of LSG and LSG:Mn2+ were synthesized by a high-temperature solid-state method. The luminescence properties and mechanism of phosphors were investigated by the XRD, luminescence spectra, thermoluminescence, low temperature spectra, decay curves, diffuse reflection, and X-ray photoelectron spectroscopy. Two kinds of electron defects and three kinds of hole defects in the host LSG were proved, and the mechanism of self-luminescence was explained by establishing a transition model. For the phenomenon that the shape of the LSG:Mn2+ emission spectra were the same as that of LSG, it was proved that there is indeed the weak luminescence of Mn2+ ions, and the spectra of Mn2+ were covered by that of LSG. The luminescence mechanism of the material LSG:Mn2+ was also explained by establishing the transition model, including the Mn2+ luminescence mechanism and the influence of Mn2+ ions on the host LSG. LSG:Mn2+ phosphor can emit deep red light at 722 nm, and it may have potential application in the field of plant lighting.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03470. Refinement results of cell parameters (a, b, c), unit cell volume, and the volume of Ga3+ with increasing Mn2+ concentration; XRD patterns of La2CuO4, LaSrCuO4, and LaSrGaO4; XRD patterns of the host LSG synthesized in the reducing atmosphere and in air; XRD patterns of LSG with different La:Sr ratios; XRD patterns of LSG:yMg2+ (y = 0−0.005) (PDF)

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REFERENCES

(1) Hasni, W.; Boukortt, A.; Bekkouche, B.; Kacimi, S.; Djermouni, M.; Zaoui, A. B-cation effect on the electronic and magnetic properties of CeBO3(B= Ga, In) compounds from first principles study. Phys. B 2012, 407, 901−906. (2) He, T.; Huang, Q.; Ramirez, A. P.; Wang, Y.; Regan, K. A.; Rogado, N.; Zandbergen, H. W.; et al. Superconductivity in the nonoxide perovskite MgCNi3. Nature 2001, 411, 54. (3) Bousquet, E.; Dawber, M.; Stucki, N.; Lichtensteiger, C.; Hermet, P.; Gariglio, S.; Ghosez, P. Improper ferroelectricity in perovskite oxide artificial superlattices. Nature 2008, 452, 732. (4) Terki, R.; Feraoun, H.; Bertrand, G.; Aourag, H. Full potential calculation of structural, elastic and electronic properties of BaZrO3 and SrZrO3. Phys. Status Solidi B 2005, 242, 1054−1062. (5) Hashimoto, T.; Hirasawa, R.; Yoshida, T.; Yonemura, Y.; Tanaka, H.; Mizusaki, J.; Tagawa, H. Analysis of role of oxygen deficiency in crystal structure and conduction mechanism of BaBi0.25Pb0.75O3‑δ. J. Phys. Chem. Solids 1995, 56, 777−785. (6) Chen, Y. C.; Yashima, M.; Ohta, T.; Ohoyama, K.; Yamamoto, S. Crystal Structure, Oxygen Deficiency, and Oxygen Diffusion Path of Perovskite-Type Lanthanum Cobaltites La0.4 Ba 0.6 CoO 3‑δ and La0.6Sr0.4CoO3‑δ. J. Phys. Chem. C 2012, 116, 5246−5254. (7) Dang, Z. M.; Lin, Y. H.; Nan, C. W. Novel ferroelectric polymer composites with high dielectric constants. Adv. Mater. 2003, 15, 1625− 1629. (8) Subramanian, M. A.; Sleight, A. W. ACu3Ti4O12 and ACu3Ru4O12 perovskites: high dielectric constants and valence degeneracy. Solid State Sci. 2002, 4, 347−351. (9) Tobias, G.; Canadell, E. Nature of the bottom t2g-block bands of layered perovskites. Implications for the transport properties of phases where these bands are partially filled. J. Am. Chem. Soc. 2006, 128, 4318−4329. (10) Moreira, M. L.; Paris, E. C.; do Nascimento, G. S.; Longo, V. M.; Sambrano, J. R.; Mastelaro, V. R.; Longo, E.; et al. Structural and optical properties of CaTiO3 perovskite-based materials obtained by microwave-assisted hydrothermal synthesis: An experimental and theoretical insight. Acta Mater. 2009, 57, 5174−5185. (11) Wang, Y. X.; Zhong, W. L.; Wang, C. L.; Zhang, P. L. Electronic structure and properties in perovskite CaTiO3 from first principle. Ferroelectrics 2001, 259, 127−132. (12) Blasse, G. New compounds with perovskite-like structures. J. Inorg. Nucl. Chem. 1965, 27, 993−1003. (13) Righi, L.; Amboage, M.; Gutierrez, J.; Barandiaran, J. M.; Barquı ́n, L. F.; Fernández-Dı ́az, M. T. Magnetic and nuclear structure of the perovskite-like oxides (LaBi)0.7Ca0.3MnO3. Phys. B 2000, 276, 718−719. (14) Baran, E. J. Structural chemistry and physicochemical properties of perovskite-like materials. Catal. Today 1990, 8, 133−151. (15) McCormack, M.; Jin, S.; Tiefel, T. H.; Fleming, R. M.; Phillips, J. M.; Ramesh, R. Very large magnetoresistance in perovskite-like La-CaMn-O thin films. Appl. Phys. Lett. 1994, 64, 3045−3047. (16) Zhu, J.; Zhao, Z.; Xiao, D.; Li, J.; Yang, X.; Wu, Y. CO Oxidation, NO Decomposition, and NO + CO Reduction over Perovskite-like Oxides La2CuO4 and La2‑xSrxCuO4: An MS−TPD Study. Ind. Eng. Chem. Res. 2005, 44, 4227−4233.

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Inorganic Chemistry (17) Sangwal, K.; Surowska, B. Study of indentation size effect and microhardness of SrLaAlO4 and SrLaGaO4 single crystals. Mater. Res. Innovations 2003, 7, 91−104. (18) Kamba, S.; Buixaderas, E.; Pajaczkowska, A. Polarized infrared reflectivity spectra of SrLaAlO4 and SrLaGaO4 single crystals. Phys. Status Solidi A 1998, 168, 317−324. (19) Jezierski, A. Band structure of LaSrGaO4 and LaSrAlO4 compounds. Phys. Status Solidi B 1998, 207, 183−189. (20) Drozdowski, M.; Domagała, J.; Kozielski, M.; Szybowicz, M.; Pajaczkowska, A. Temperature study of lattice constants and Raman scattering of SrLaGaO4 single crystal. Solid State Commun. 1995, 96, 785−788. (21) Humlíček, J.; Henn, R.; Cardona, M. Infrared vibrations in LaSrGaO4 and LaSrAlO4. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 14554. (22) Drozdowski, M.; Kasprowicz, D.; Pajaczkowska, A. Characterization of SrLaAlO4 and SrLaGaO4 single crystals by Brillouin scattering method. Proc. SPIE 1998, 3724, 220−229. (23) Ryba-Romanowski, W.; Pisarski, W. A.; Dominiak-Dzik, G.; Paja̧czkowska, A.; et al. Investigation of the Cr3+ sites in SrLaGaO4 crystal. Chem. Phys. Lett. 1995, 242, 623−626. (24) Ryba-Romanowski, W.; Gołab, S.; Pisarski, W. A.; DominiakDzik, G.; Berkowski, M.; Pajaczkowska, A. Investigation of Eu3+ sites in SrLaGa3O7, SrLaGaO4 and SrLaAlO4 crystals. J. Phys. Chem. Solids 1997, 58, 639−645. (25) Jiang, C.; Zhang, X.; Wang, J.; Zhao, Q.; Wong, K. L.; Peng, M. Synthesis and photoluminescence properties of a novel red phosphor SrLaGaO4: Mn4+. J. Am. Ceram. Soc. 2019, 102, 1269−1276. (26) Liu, X. Q.; Chen, X. M. Microwave dielectric characteristics of SrLaGaO4 and SrNdGaO4 ceramics. J. Eur. Ceram. Soc. 2006, 26, 1969−1971. (27) Kaczkan, M.; Malinowski, M. Upconversion-induced ultraviolet emission in Ho3+ doped SrLaGa3O7 and SrLaGaO4 crystals. J. Alloys Compd. 2004, 380, 201−204. (28) Fu, X.; Zheng, S.; Shi, J.; Li, Y.; Zhang, H. Long persistent luminescence property of a novel green emitting SrLaGaO4: Tb3+ phosphor. J. Lumin. 2017, 184, 199−204. (29) Rotman, S. R.; Warde, C. Defect luminescence in cerium-doped yttrium aluminum garnet. J. Appl. Phys. 1985, 58, 522−525. (30) Luo, W.; Li, R.; Chen, X. Host-sensitized luminescence of Nd3+ and Sm3+ ions incorporated in anatase titania nanocrystals. J. Phys. Chem. C 2009, 113, 8772−8777. (31) Li, K.; Liu, X.; Zhang, Y.; Li, X.; Lian, H.; Lin, J. Host-sensitized luminescence properties in CaNb2O6:Ln3+ (Ln3+ = Eu3+/Tb3+/Dy3+/ Sm3+) phosphors with abundant colors. Inorg. Chem. 2015, 54, 323− 333. (32) Sun, Y.; Li, P.; Wang, Z.; Cheng, J.; Li, Z.; Wang, C.; Yang, Z.; et al. Tunable emission phosphor Ca0.75Sr0.2Mg1.05(Si2O6):Eu2+, Mn2+: luminescence and mechanism of host, energy transfer of Eu2+→Mn2+, Eu2+→host, and host→Mn2+. J. Phys. Chem. C 2016, 120, 20254− 20266. (33) Zhou, Q.; Dolgov, L.; Srivastava, A. M.; Zhou, L.; Wang, Z.; Shi, J.; Wu, M.; et al. Mn2+ and Mn4+ red phosphors: synthesis, luminescence and applications in WLEDs. A review. J. Mater. Chem. C 2018, 6, 2652−2671. (34) Zhang, J. C.; Zhao, L. Z.; Long, Y. Z.; Zhang, H. D.; Sun, B.; Han, W. P.; Wang, X.; et al. Color manipulation of intense multiluminescence from CaZnOS:Mn2+ by Mn2+ concentration effect. Chem. Mater. 2015, 27, 7481−7489. (35) Song, K.; Zhang, J.; Liu, Y.; Zhang, C.; Jiang, J.; Jiang, H.; Qin, H. B. Red-emitting phosphor Ba9Lu2Si6O24: Ce3+, Mn2+ with enhanced energy transfer via self-charge compensation. J. Phys. Chem. C 2015, 119, 24558−24563. (36) Ma, Y. Y.; Hu, J. Q.; Song, E. H.; Ye, S.; Zhang, Q. Y. Regulation of red to near-infrared emission in Mn2+ single doped magnesium zinc phosphate solid-solution phosphors by modification of the crystal field. J. Mater. Chem. C 2015, 3, 12443−12449.

(37) Zhydachevskii, Y.; Suchocki, A.; Pajączkowska, A.; Kłos, A.; Szysiak, A.; Reszka, A. Spectroscopic properties of Mn4+ ions in SrLaAlO4. Opt. Mater. 2013, 35, 1664−1668. (38) Weller, M. T. Inorganic Materials Chemistry; Oxford University Press: New York, 1994. (39) Tilley, R. J. D. Defects in Solids; John Wiley & Sons, 2008. (40) Liu, Y.; Dai, H.; Deng, J.; Zhang, L.; Zhao, Z.; Li, X.; Guo, G.; et al. Controlled generation of uniform spherical LaMnO3, LaCoO3, Mn2O3, and Co3O4 nanoparticles and their high catalytic performance for carbon monoxide and toluene oxidation. Inorg. Chem. 2013, 52, 8665−8676. (41) Pawlak, D. A.; Ito, M.; Oku, M.; Shimamura, K.; Fukuda, T. Interpretation of XPS O(1s) in mixed oxides proved on mixed perovskite crystals. J. Phys. Chem. B 2002, 106, 504−507. (42) López, G. P.; Castner, D. G.; Ratner, B. D. XPS O1s binding energies for polymers containing hydroxyl, ether, ketone and ester groups. Surf. Interface Anal. 1991, 17, 267−272. (43) Love, R. B.; Oglesby, S.; Gailey, I. The Relation between Dye Concentration and Reflectance−Amendments to the Kubelka−Munk Equation. J. Soc. Dyers Colour. 1965, 81, 609−614. (44) Tian, M.; Li, P.; Wang, Z.; Teng, X.; Li, Z.; Cheng, J.; Yang, Z.; et al. Synthesis, color-tunable emission, thermal stability, luminescence and energy transfer of Sm3+ and Eu3+ single-doped M3Tb(BO3)3 (M = Sr and Ba) phosphors. CrystEngComm 2016, 18, 6934−6947. (45) Kivits, P.; Hagebeuk, H. J. L. Evaluation of the model for thermally stimulated luminescence and conductivity; reliability of trap depth determinations. J. Lumin. 1977, 15, 1−27. (46) Xia, Z. G.; Meijerink, A. Ce3+ Doped garnet phosphors: Composition modification, luminescence properties and applications. Chem. Soc. Rev. 2017, 46, 275−299. (47) Jin, Y.; Hu, Y.; Chen, L.; et al. A Novel Orange Emitting Long Persistent Phosphor CdGeO3:Sm3+. Sci. Adv. Mater. 2017, 9, 386−391.

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Self-Luminescence of Perovskite-Like LaSrGaO4 via Intrinsic Defects

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