High-Pressure Synthesis of CeOCl Crystals and Investigation of Their

Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

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High-Pressure Synthesis of CeOCl Crystals and Investigation of Their Photoluminescence and Compressibility Properties Leilei Zhang,† Ya Cheng,‡,§ Li Lei,*,† Xianlong Wang,*,‡,§ Qiwei Hu,† Qiming Wang,† Hiroaki Ohfuji,∥ Yohei Kojima,∥ Qiang Zhang,† Zhi Zeng,‡,§ Fang Peng,† Zili Kou,† Duanwei He,† and Tetsuo Irifune∥ †

Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, China § University of Science and Technology of China, Hefei, 230036, China ∥ Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan ‡

S Supporting Information *

ABSTRACT: The well-crystallized mixed anions compound cerium oxychloride (CeOCl) was successfully synthesized by a high-pressure solid-state metathesis reaction. The photoluminescence experiment shows that the CeOCl, with a band gap of ∼3.05 eV, has good violet-blue emission properties. And first-principles calculations of the band structures show that CeOCl is an indirect (direct) band gap semiconductor for the spin-up (spin-down) branch. This suggests that the CeOCl can be expected to be a semiconductor material. In addition, in situ high-pressure angle-dispersive X-ray diffraction experiment reveals that the bulk modulus of CeOCl is 52.8(8) GPa, which is close to our first-principles calculations, giving that B0 = 47.6(5) GPa.

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INTRODUCTION Rare-earth oxychlorides (REOCl) in general crystallize in a tetragonal PbFCl-type structure with space group P4/nmm.1−4 As shown in Figure 1, the structural feature of REOCl is a network of alternating layers of (REO)nn+ cations and (Cl)nn− anions along the c-axis. The (REO)nn+ unit is very rigid and stable, which is assumed to be the reason for the efficient luminescence.3,5 For example, photoluminescence (PL) properties of LnOCl:RE3+ (RE = Pr, Sm, Eu, and Er) have been studied in detail.6−9 To the best of our knowledge, however, there is no detailed research on the PL properties of CeOCl. Presently, many methods have been reported to prepare REOCl materials, such as solid-state reaction method for REOCl (RE = Ce, Ho, Yb, Tm, and Er),10,11 liquid-phase high temperature calcination method for LaOCl:Eu3+,12 hydrothermal-solvothermal method for REOCl (RE = La, Ce, Gd, and Dy),13 sol−gel method for LaOCl,14,15 etc. However, it is still a great challenging to make well-crystallized larger scale CeOCl materials. The high-pressure solid-state metathesis (HPSSM) reaction, reported in 2009 by Lei and He, could provide advantages over © XXXX American Chemical Society

conventional solid-state metathesis synthetic methods because of the high-pressure confinement environment and wellcrystallized stoichiometric production.16,17 Over the past 8 years, our laboratory has reported on HPSSM reactions between diverse metal oxides and BN that form well-crystalline Fe3N,18 Re3N,19 VN,20 CrN,21 W2N3,22 MoN2,23 and ternary Fe3−xCoxN and Fe3−xNixN.24 All in all, the HPSSM route was proved to be an effective route for accessing novel metal nitrides. Besides the study of catalytic properties,11 little is known about other properties for CeOCl due to the poor quality of the crystal. However, as is well-known, detailed studying on undoped CeOCl is necessary for our better understanding of doped CeOCl. Therefore, in the present work, we reported the synthesis of well-crystallized CeOCl employing a modified HPSSM reaction. The purified CeOCl powders were fully characterized by powder X-ray diffraction (PXRD), fieldReceived: December 8, 2017 Revised: January 31, 2018 Published: February 5, 2018 A

DOI: 10.1021/acs.cgd.7b01712 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Figure 1. (a) The local coordination environment of RE-ions. (b) Layered structure of the tetragonal forms of REOCl.

Figure 2. (a) PXRD pattern of CeOCl under laboratory conditions. Black line: observed curve; red short vertical lines: all possible Bragg positions. Inset in (a) is the crystal structure of CeOCl. (b) Optical microscope photograph of CeOCl crystals. (c) SEM image of a CeOCl crystal. (d) The SAED pattern of CeOCl crystal. CeOCl crystals. The experiment details have been described elsewhere.18,25 Characterization. The structural characterization and phase determination of the prepared samples were first performed by PXRD (Fangyuan DX-2500, China) with Cu Kα radiation (λKα1 = 1.54056 Å, λKα2 = 1.54440 Å) at 0.03°/s steps over the range of 2θ = 10−90°. FE-SEM (JSM-7000F, JEOL), operated at 15 kV, was used for secondary electron image (SEI) observations. The equipped EDX detector (SSD, Oxford Instruments, INCA E250) was used to perform elemental analysis. Before EDX measurement, osmium surface coating (5 nm thick) was carried out with an osmium coater (Neoc-STB, Meiwafosis Co., Ltd.). Pure CeO2 and NaCl were used as the EDX standards for the quantification of Ce, O, and Cl, respectively. The accuracy of the quantification was estimated to be no more than 0.5%.26−28 The selected area electron diffraction (SAED) was carried out to confirm the crystal structure. The PL spectra were performed at ambient temperature in a custom-built spectroscopy system29 with a 325 nm He-Cd laser line. The high-pressure in situ ADXRD experiment was performed at room temperature at the 4W2 beamline of the Beijing Synchrotron Radiation Facility (BSRF, China). A symmetric type diamond-anvil cell (DAC) with 300 μm culets was used as the pressure-generating assembly, and a methanol−ethanol mixture (4:1) was used as the pressure mediator. The as-prepared CeOCl powders were loaded in an ∼100 μm diameter hole which was drilled in a preindented to ∼50 μm thickness T301 inconel gasket. Pressure in the sample chamber was determined by the conventional ruby fluorescence method.30 The

emission scanning electronic microscopy (FE-SEM), energydispersive X-ray (EDX), and high-resolution transmission electron microscopy (HRTEM). Specifically, we did a detailed study on PL properties of CeOCl by a custom-built spectroscopy system with 325 nm excitation light, which was qualitatively confirmed by the first-principles calculations. Besides, the compressibility of the obtained CeOCl was investigated by in situ high-pressure angle-dispersive X-ray diffraction (ADXRD) experiment and the first-principles calculations in this work.

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EXPERIMENTAL AND CALCULATION SECTION

Sample Preparation and Purification. The well-crystallized CeOCl were synthesized by an HPSSM reaction16,17 between MgO (98.0%, Aladdin) and CeCl3 (99.5%, Alfa Aesar) powders at 5 GPa and 1873 K. The chemical reaction equation can be depicted as follows

CeCl3 + MgO = CeOCl + MgCl2

(1)

The high-pressure synthesis experiments were conducted on a large volume press (DS6 × 14MN, China). Before HPHT experiments, the reaction precursors were mixed in the optimized molar ratio (MgO:CeCl3 = 1:1) and then were placed in a cylindrical hBN crucible encircled by a graphite sleeve used to heat the samples in the high-pressure experiments. After the HPHT experiments, an ultrasonic washer was used to remove the remnants and to obtain the pure B



Article

incident X-ray was monochromatized to 0.6199 Å. The diffraction patterns were recorded by a Mar345 image plate detector and analyzed with the FIT2D program.31 Calculation. First-principles calculations were used to provide qualitative confirmation of experimental observations. First-principles calculations were carried out within the framework of the density functional theory (DFT)32,33 implemented in the Vienna Ab-initio Simulation Package (VASP) software34−36 with pseudo-potential method and plane-wave basis sets. The 4f15d16s2, 2s22p4, and 3s23p5 electrons were included in the valence space for Ce, O, and Cl atoms, respectively. An energy cutoff of 800 eV was used for plane-wave basisset expansion. k-point meshes of 8 × 8 × 5 were employed to sample the first Brillouin zone (BZ) and ensured that energies converged to within 0.03 meV/atom. All investigated structures are fully relaxed until the forces acting on atoms were less than 0.001 eV/Å, and the total stress tensor was reduced to the order of 0.01 GPa. In this work, all calculations were performed using generalized gradient approximation plus Hubbard U (GGA + U) correction for the Ce f states. A metallic state is observed when the applied U values are smaller than 4.5 eV. In the work, we use a U value of 4.8 eV, which can produce a comparable band gap of CeOCl with that of experimental observations.

Figure 3. PL spectra of CeOCl under excitation with 325 nm light at room temperature comparing with GaN and ZnO.

For further clarifying the electronic structure of CeOCl, we performed the first-principles simulations, and the calculated band structures are presented in Figure 4. We can find that CeOCl is an indirect (direct) band gap semiconductor for the spin-up (spin-down) branch with a band gap of 3.14 eV comparable to experimental observation, 3.05 eV. In the spinup branch shown in Figure 4a, an approximately flat band with ∼0.2 eV dispersion is observed just below the Fermi level. Depending on the partial density of states shown in Figure 4c, we can find that the approximately flat band is contributed mainly by the Ce f orbital, since the sharp peak close to the Fermi level originates from the Ce f orbital. Furthermore, our results show that the localization of the Ce f orbital in the valence band is not sensitive to the applied U values. In general terms, the electrons transition between the valence band and the conduction band in the direct band gap semiconductors is easier than that of the indirect band gap semiconductors. Moreover, in the case of an approximately flat band with small dispersion, the energy differences between the valence band maximum and other reciprocal points in the approximately flat band is small. Therefore, as shown in Figure 4b, the possibility of the electron transition between the G point of the valence band and the conduction band minimum is also large, and CeOCl can be expected to illustrate good PL properties even though it is an indirect band gap semiconductor. Compressibility Analysis. It is necessary to analyze the compressibility for our better understanding of the material. Figure 5a shows the representative in situ high-pressure ADXRD patterns of CeOCl under pressure up to 21.8 GPa. Taken as a whole, all the diffraction peaks shift to larger diffraction angles and become broader with increased pressure. Figure 5b shows the pressure dependent volume of the CeOCl. The appearance of a large error above 10 GPa resulted from the solidification of the pressure-transmitting media. The P−V data at 300 K were analyzed with a third-order Birch−Murnaghan equation of state (BM-EOS) described as follows40

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RESULTS AND DISCUSSION Phase Analysis. Figure 2a shows the PXRD pattern of the obtained CeOCl samples. All the diffraction peaks can be indexed by a tetragonal unit cell in space group P4/nmm. The size and surface morphology of CeOCl crystals were investigated by optical microscope (Figure 2b) and FE-SEM (Figure 2c). From the optical microscope photograph and the SEM morphology, we can see that the prominent features observed are the presence of well-defined grains. Homogeneity and composition of the products were determined by EDX measurements. The obtained typical Ce/ O/Cl atomic ratio is about 33.18/33.24/33.58 (Figure S1 and Table S1), which is approximately equivalent to the ideal stoichiometric ratio (1:1:1). That is to say, the EDX data reveal that the samples prepared in this work present a nearstoichiometric composition. The well-defined grains and nearstoichiometric composition can be attributed to the well highpressure confinement environment in the HPHT experiments. Diffraction points are clearly identified from the SAED pattern (see Figure 2d) taken of the products, and they correspond to the planes (11̅0), (110), and (200). The tetragonal symmetry and space group P4/nmm were used to assign the reciprocal lattice points. The results obtained in this work are a = b = 4.0711 Å, c = 6.8341 Å, and V = 113.2 Å3. The data are well consistent with the previous work (Table S2). Photoluminescence Properties. The luminescence properties of materials are determined by their electronic structure; therefore, we performed PL test on CeOCl. The PL properties of CeOCl were compared with semiconductor materials GaN and ZnO. Under the excitation of 325 nm laser, as shown in the inset in Figure 3, CeOCl samples will emit violet-blue light. The curve of CeOCl (black line) displays an intensity peak at about 3.05 eV (∼417 nm) which is lower than intrinsic GaN (about 3.47 eV) and ZnO (about 3.25 eV). However, the peak intensity of CeOCl is considerably higher than GaN and about the same of ZnO. Hence, CeOCl is an underlying semiconductor material. Here, the broadband excitation and emission in the UV−visible spectral region may result from the large absorption cross section due to optical transitions between the 4f 1 ground and the first excited 4f 0 5d 1 configurations are parity allowed.37−39

P(V ) = 1.5B0 [(V /V0)−7/3 − (V /V0)−5/3 ]· {1 + 0.75(B0 ′ − 4)[(V /V0)−2/3 − 1]}

(2)

where V0, B0, and B0′ are the zero pressure volume, the zero pressure bulk modulus, and the bulk modulus pressure derivative, respectively. There, considering the accuracy of the results, we only fitted the datum below 10 GPa to analyze the C



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Figure 4. Electronic structure of CeOCl at 0 GPa. Panels (a) and (b) are the band structures of spin-up and spin-down electrons, respectively. The projected density of state is plotted in (c).

Figure 5. (a) Selected high-pressure in situ ADXRD patterns of CeOCl (Miller indices refer to the tetragonal P4/nmm symmetry). “↓” means the decompression progress. (b) Pressure−volume curves of CeOCl. The solid lines are guides to the eye.

compressibility of CeOCl, as shown of the inset in Figure 5b. The result of least-squares fit of a BM-EOS of our experimental data gave B0 = 52.8(8) GPa at fixed B0′ = 4, and our firstprinciples calculations gave that B0 = 47.6(5) GPa (with B0′ = 4), which is close to 51(3) GPa for tetragonal PbFCl and is larger than 39.9(4) GPa for tetragonal PbFBr.41

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AUTHOR INFORMATION

Corresponding Authors

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*E-mail: [email protected] (L.L.). *E-mail: [email protected] (X.W.).

CONCLUSION In summary, the well-crystallized CeOCl product was synthesized through an HPSSM reaction under 5 GPa and 1873 K. The PL experiment showed that CeOCl, with ∼3.05 eV band gap, can be expected to be a semiconductor material, which was supported by our first-principles calculations. In addition, the bulk modulus of CeOCl, B0 = 52.8(8) GPa for experiment and B0 = 47.6(5) GPa for the first-principles calculations, was given for the first time in this work.

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and (200) (Table S2); the calculated CeOCl density of states by using a 6 eV U value (Figure S2) (PDF)

ORCID

Leilei Zhang: 0000-0002-5296-8430 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 11774247 and 11674329), the Science Foundation for Excellent Youth Scholars of Sichuan University (Grant No. 2015SCU04A04), the Science Challenge Project (No. TZ2016001), the Chinese Academy of Sciences (Grant No. 2017-BEPC-PT-000568), and the Joint Usage/ Research Center PRIUS (Ehime University, Japan).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b01712. Different EDX measurement points on the surface of a CeOCl crystal (Figure S1); elements content of CeOCl at different measurement points (Table S1); the lattice parameters (a, c, and V) and d values of the planes (110)

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REFERENCES

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High-Pressure Synthesis of CeOCl Crystals and Investigation of Their

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