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

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Synthesis, Crystal Structure, and Optical Properties of Layered Perovskite Scandium Oxychlorides: Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2 Yu Su,†,‡ Yoshihiro Tsujimoto,*,†,‡ Kotaro Fujii,§ Makoto Tatsuta,∥ Kengo Oka,⊥ Masatomo Yashima,§ Hiraku Ogino,*,# and Kazunari Yamaura†,‡ †

Research Center for Functional Materials, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Graduate School of Chemical Sciences and Engineering, Hokkaido University, North 10 West 8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan § Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-W4-17, Ookayama, Meguro-ku, Tokyo 152-8551, Japan ∥ Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ⊥ Faculty of Science and Engineering, Chuo University, 112-8551, 1-13-27, Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan # National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan S Supporting Information *

ABSTRACT: We report the successful synthesis of three new Ruddlesden− Popper-type scandium oxychloride perovskites, Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2, by conventional solid-state reaction. Small single crystals of Sr2ScO3Cl were obtained by a self-flux method, and the crystal structure was determined to belong to the tetragonal P4/nmm space group (a = 4.08066(14) Å, c = 14.1115(8) Å) by X-ray diffraction analysis. The scandium center forms a ScO5Cl octahedron with ordered apical oxygen and chlorine anions. The scandium cation, however, is shifted from the position of the octahedral center toward the apical oxygen anion, such that the coordination geometry of the Sc cation can be effectively viewed as an ScO5 pyramid. These structural features in the oxychloride are different from those of octahedral ScO5F coordinated with a partial O/F anion order at the apical sites in the oxyfluoride Sr2ScO3F. Rietveld refinements of the neutron powder diffraction data of Sr3Sc2O5Cl2 (I4/mmm: a = 4.107982(5) Å, c = 23.58454(7) Å) and Ba3Sc2O5Cl2 (I4/mmm: a = 4.206920(5) Å, c = 24.54386(6) Å) reveal the presence of pseudo ScO5 pyramids with the Cl anion being distant from the scandium cation, which is similar to the Sc-centered coordination geometry in Sr2ScO3Cl with the exception that the ScO5 pyramids form double layers by sharing the apical oxygen. Density functional calculations on Sr2ScO3Cl indicate the strong covalency of the Sc−O bonds but almost nonbonding interaction between Sc and Cl ions.

1. INTRODUCTION

Ruddlesden−Popper (RP)-type layered perovskite with the general chemical formula of An+1BnX3n+1 (A = electropositive cation, B = transition metal, X = anion, n = perovskite block number) constitutes an interesting platform for investigating the relationship between the anion order and the physical and chemical properties. The ideal RP-type structure without octahedral distortion possesses more than one different anion site; specifically, for n = 1 phase, the apical and equatorial anion sites are surrounded by AB5 and A2B4 polyhedra, respectively, in contrast to six equivalent anion-centered coordination with A4B2 in the ideal cubic Pm3̅m perovskite system ABX3. Therefore, these structural features of the RP phase favor

Recently, mixed anion compounds have been drawing attention from solid-state scientists because the incorporation of mixed anions with different ionic radii, electronegativities, oxidation states, and polarizabilities can lead to the emergence of novel properties that differ from those of the compounds with one type of anion.2−5 One of the most important aspects of studies on mixed anion compounds is to control and understand the arrangement of anions that dictate their electronic structures, for example, the ferroelectric characteristics of SrNbO2N with cis-configuration of O/N anions in the NbO4N2 octahedra3 and the two-dimensional quantum antiferromagnetism in Sr2CuO2Cl2 with trans-configuration of Cl ions in the CuO4Cl2 octahedra.6 © XXXX American Chemical Society

Received: March 5, 2018

A

DOI: 10.1021/acs.inorgchem.8b00573 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

2. EXPERIMENTAL SECTION

ordered arrangements of mixed anions, for example, selective occupation of equatorial sites in the oxide sublattices by hydride and nitride ions,7,8 whereas apical sites are occupied by halide and sulfide ions.1,4,9−11 Common across RP-type layered oxyhalides, halide ions occupy apical sites, which leads to rearrangement of the coordination geometry for the transition metal centers with octahedral symmetry. For example, Sr2BO2Cl2 (n = 1; B = Mn, Co, Ni, Cu, Zn) exclusively takes a BO4 square unit in the basal plane,6,12−14 while Sr2BO3X (n = 1, B = Mn, Fe, Co, Ni, X = F, Cl),15−19 Sr3B2O5Cl2 (n = 2, B = Fe, Co),16,20 Ba3B2O5Cl2 (n = 2, B = Gd−Lu),21 and Sr4B3O7.5Cl2 (n = 3, B = Mn, Co)15,16 have BO5 distorted square pyramids (see Figure 1). The high

Synthesis. Single crystals of Sr2ScO3Cl were grown by the alkalineearth-metal chloride flux method28−31 using SrCl2. A stoichiometric amount of SrCO3, Sc2O3, and SrCl2 was mixed thoroughly with an additional amount (50 mol %) of SrCl2 in an Ar-filled glovebox. The mixed powder (1 g) was loaded into a platinum crucible placed in a covered alumina crucible. Then, the crucible was heated to 1273 K at the rate of 200 K h−1 in air and held there for 1 day. Subsequently, the crucible was cooled to 973 K at the rate of 6.25 K h−1 and finally cooled down to room temperature naturally. The resultant products were washed with water to remove the remaining flux. Powdered samples of Sr2ScO3Cl (n = 1), Sr3Sc2O5Cl2 (n = 2), and Ba3Sc2O5Cl2 (n = 2) were prepared by conventional high-temperature solid-state reaction. Stoichiometric molar ratios of the starting materials, (Sr/Ba)O, Sc2O3, and (Sr/Ba)Cl2, were ground thoroughly using an agate mortar and pestle, pelletized, and placed in alumina crucibles in an Ar-filled glovebox. SrO was prepared by heating SrCO3 at 1573 K for 10 h under a flow of O2 gas. The pellets were heated at 1173 K for 1 day in air in an electrical furnace. Further heating was necessary for obtaining Sr3Sc2O5Cl2 and Ba3Sc2O5Cl2. The pellets were crushed into fine powders, reground, and pressed into pellets, and then heated at 1273 K for 2 days in air. We also attempted to synthesize Ba2ScO3Cl and (Sr/Ba)4Sc3O7.5Cl2 (n = 3) under similar reaction conditions; however, we always obtained the n = 2 phase for the former, and n < 3 phases for the latter, with unreacted starting materials. Sr2ScO3Cl powder is white, Sr3Sc2O5Cl2 is pale pink, and Ba3Sc2O5Cl2 is pale green. Powder X-ray and Neutron Diffraction. Synchrotron X-ray powder diffraction (SXRD) data on Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2 were collected at room temperature using a onedimensional X-ray detector installed on BL15XU, NIMS beamline at SPring-8 in Japan. The synchrotron radiation X-rays were monochromatized to the wavelength of 0.65298 Å. The samples were loaded in 0.1 mm diameter glass capillaries, and the diffraction data were recorded in 0.003° increments over the range 2 ≤ 2θ ≤ 50°. Time-offlight neutron powder diffraction (NPD) measurements were conducted at 300 K with iMATERIA32 installed at J-PARC in Japan. Each of the samples (1.2−1.9 g quantity) was loaded into a vanadium can with 5.8 mm inner diameter, and the diffraction data were collected with a backscattering detector bank. The NPD data were analyzed by Rietveld refinement using the program, Z-Code.33,34 Single-Crystal X-ray Diffraction. A single crystal of Sr2ScO3Cl with dimensions of 20 μm × 20 μm × 5 μm was picked up from the partially melted products and mounted on a MiTeGen microloop for the single-crystal X-ray diffraction measurements. All the data were collected at 299 K on a Rigaku XtaLAB PRO diffractometer equipped with a PILATUS 200 K hybrid pixel array detector and a microfocus Mo Kα radiation (MicroMax007HF, 50 kV, 24 mA). A total of 672 oscillation images, covering an entire sphere of 8.6 < 2θ < 72.6°, were collected using the ω-scan method. The crystal-to-detector distance was set at 35 mm. The data were processed using the CrysAlisPro program (Rigaku Oxford Diffraction, Version 38.46) and corrected for Lorentz-polarization and absorption effects. The structures were solved by the direct method and refined on F2 with full-matrix leastsquares techniques with SHELXL-201635 and WinGX package.36 Scanning Electron Microscopy and Energy Dispersive X-ray Analysis. Elemental analysis on the single crystals of Sr2ScO3Cl was carried out using a scanning electron microscope (SEM, HITACHITM3000) equipped with an energy dispersive X-ray (EDX) spectrometer (Oxford Instruments, Swift ED3000). The accelerating voltage was 15 keV. Density Functional Theory (DFT) Calculations. First-principles DFT calculations were performed for Sr2ScO3Cl using the Vienna Ab initio Simulation Package (VASP). The Perdew−Burke−Ernzerhof (PBE) generalized gradient approximation (GGA) was employed for the exchange and correlation functional. Projector augmented-wave (PAW) potentials were used for Sr, Sc, O, and Cl atoms. A plane wave basis set with a cutoff of 500 eV was used (k-point mesh 7 × 7 × 3). The self-consistent field tolerance was 1.0 × 10−7 eV/atom. The cell

Figure 1. Crystal structures of anion ordered oxyhalide compounds with the Ruddlesden−Popper-type structure. Selective occupation of apical sites by halide ions (X) can yield a square pyramidal coordination for B cations with the surrounding five O anions.

electronegativity of fluoride and the large ionic radius of chloride are important factors causing the reduction in the coordination number of the metal centers. However, there are some exceptions such as (Ba/Sr)2ScO3F22,23 and Sr2MnO3F24 in which an octahedral coordination center can be stabilized in n = 1 oxyfluoride phases with O/F anion disorder. In the case of the manganese oxyfluoride, the MnO5F octahedra are elongated along the c-axis because of the first-order Jahn− Teller effect, and it contributes to the formation of octahedral coordination. In contrast, although Sc3+ is not Jahn−Teller active, its larger ionic size than that of Mn3+ ion favors the octahedral coordination rather than a square pyramidal one. In fact, scandium-based compounds typically take more than fivefold coordination.25−27 In this paper, we report the successful synthesis of new Scbased layered oxychlorides, namely, Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2. Single crystals of Sr2ScO3Cl were obtained using SrCl2 as a flux. Single-crystal X-ray analysis and neutron powder diffraction studies demonstrate not only the O/Cl order at the apical sites but also the formation of strongly distorted ScO5Cl octahedra, which is different from the structural features observed for (Ba/Sr)2ScO3F. The firstprinciples calculations reveal covalent bonding between scandium and oxygen atoms but a more ionic bonding for the Sc−Cl bonds, and thus, the Sc-based coordination geometry is effectively a square pyramid with five oxygen anions. B


Article

Inorganic Chemistry

attributed to the large ionic radius of Ba2+ compared to that of Sr2+.37 The SXRD data of Sr2ScO3Cl could be assigned to a primitive tetragonal cell with the space group P4/nmm, unlike Sr2ScO3F with the I4/mmm tetragonal cell: some sets of reflection conditions were violated in comparison with I4/ mmm: 0kl, no condition; hhl, no condition, 0k0: k = 2n; 00l, no condition. These reflection conditions should be uniquely satisfied by the space group P4/nmm, which is typically observed in n = 1 layered oxychlorides with O/Cl-site order at the apical sites (for example, Sr2FeO3Cl17 and Sr2NiO3Cl18). The calculated lattice constants of Sr2ScO3Cl are a = 4.0804(2) Å and c = 14.0894(9) Å, which are larger than those of Sr2ScO3F because of the larger ionic radius of Cl− than that of F−.37 Single-Crystal Structure Analysis of Sr2ScO3Cl. Figure 3 shows a typical SEM image of a single crystal of Sr2ScO3Cl.

parameters and atomic positions were relaxed until the maximum force on each atom was less than 0.02 eV/Å. UV−vis−NIR Absorption Measurements. Diffuse-reflectance spectra were recorded in the region of 220−1600 nm at room temperature using a UV−vis−NIR spectrometer (Shimazu UV-2600). The reflectance spectrum of the BaSO4 powder was collected as the baseline.

3. RESULTS AND DISCUSSION Synthesis and Synchrotron X-ray Powder Diffraction. Figure 2 shows the SXRD patterns of Sr2ScO3Cl and

Figure 3. SEM image of a rectangular single crystal of Sr2ScO3Cl grown from SrCl2 flux.

EDX analysis reveals the average atomic ratio of Sr:Sc:Cl = 2.07:1:1.02, which is in good agreement with the nominal composition of Sr2ScO3Cl. The single-crystal X-ray diffraction patterns could be indexed to the primitive tetragonal cell with a = 4.08066(14) Å and c = 14.1115(8) Å, consistent with the cell parameters calculated with the SXRD data. Single-crystal structure analysis revealed that Sr2ScO3Cl crystallized in the P4/nmm Sr2FeO3Cl-type structure. No appreciable deviation from the full occupancy for all the atoms is detected. The final results of the structure refinements are presented in Tables 1 and 2, and the crystal structure is shown in Figure 4. Bond valence sum (BVS) calculation38 was carried out, obtaining BVS values of 1.47 for Sr1, 2.18 for Sr2, and 3.06 for Sc. Although the BVS values for Sr2 and Sc atoms are consistent with the chemical formula, the BVS value for Sr1 is much lower than that expected. This will be discussed in the following section. Oxygen and chlorine atoms are fully ordered at the apical sites in the stacking sequence of -(SrO)2-(ScO2)-(SrCl)2(ScO2)-. The scandium atom is surrounded by four equatorial oxygen atoms (Oeq) at 2.0652(5) Å, one apical oxygen atom (Oap) at 1.981(6) Å, and the other apical chlorine atom at 2.975(3) Å. The Sc−Oeq bond lengths are consistent with the sum of the ionic radii of 6-fold Sc3+ (rSc3+ = 0.745 Å) and O2− (rO2− = 1.4 Å);37 however, the Sc−Oap bond length is much shorter than that in the ionic model because of the shifting of the Sc atom toward the Oap atom from the O4 basal plane. As a result, the ScO4 basal plane is significantly corrugated with the O1−Sc−O1 bond angle of 162.20(17)°. The Sc−Cl bond length is much longer than the sum of the ionic radii of Sc3+

Figure 2. Synchrotron X-ray powder diffraction patterns of Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2 collected at room temperature. The wavelength is 0.65298 Å. Spikes marked by asterisks are artifacts from the X-ray detectors.

A3Sc2O5Cl2 (A = Sr, Ba). The SXRD peaks of Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2 could be readily assigned to a simple bodycentered tetragonal cell with the space group I4/mmm. The lattice constants calculated by the least-squares method are a = 4.1072(3) Å and c = 23.587(1) Å for Sr3Sc2O5Cl2 and a = 4.2037(2) Å and c = 24.578(1) Å for Ba3Sc2O5Cl2. The larger lattice constants for Ba3Sc2O5Cl2 than those for Sr3Sc2O5Cl2 are C


Article

Inorganic Chemistry Table 1. Results of Structure Refinement of Sr2ScO3Cl Using Single-Crystal XRD Data formula formula weight radiation T (K) crystal system space group a (Å) c (Å) V (Å3) Z Dcalc (g/cm3) F000 no. of measured reflns no. of unique reflns no. of observed reflns (F2 > 2σ(F2)) Rint (%) R[F2 > 2σ(F2)]/wR(F2) (%) GoF

Sr2ScO3Cl 303.6468 Mo Kα (λ = 0.71073 Å) 299(2) tetragonal P4/nmm (No. 129) 4.08066(14) 14.1115(8) 234.986(18) 2 4.292 276 1540 398 342 2.67 2.92/6.45 1.091

and Cl− (rCl− = 1.81 Å).37 The longer bond length between the Sc and Cl ions is correlated with the relatively strong Sc−Oap bond in addition to the charge difference between O2− and Cl−. As a result, the coordination geometry for the Sc ions can be effectively viewed as square pyramidal. The Sr2 atom is coordinated with four oxygen atoms at 2.5704 Å, four in-plane chlorine atoms at 3.0851 Å, and an apical chlorine atom at 3.179 Å. The deviation between these bond lengths and the values expected from the ionic model is only within ±5%, leading to the reasonable BVS value. In contrast, the BVS value for Sr1 suggests that the SrO9 polyhedron is under tension: eight of the Sr1−(Oeq/Oap) bonds lengths are much longer by 6−7%, whereas one Sr1−Oap bond length along the c-axis is shorter by 11% than the expected value (rSr2+ + rO2− = 2.71 Å).38 The elongation of the Sr1−Oap bonds in the plane is caused by the lattice mismatch between the (SrO)2 and (SrCl)2 rock-salt layers where Oap and Cl sites have the same x and y coordinates. In fact, the anisotropic atomic displacement parameters (U) of Sr1 and Oap atoms, particularly U11 and U22, are found to be much larger than those for Sr2 and Cl atoms. Neutron Powder Diffraction. Figure 5 shows the neutron powder diffraction patterns collected at 300 K from Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2. For Sr2ScO3Cl, the model determined by the single-crystal structure analysis was employed for the structure refinement. The atomic coordinates, isotropic (Uiso) and anisotropic atomic displacement parameters, and site occupancies of all the atoms were allowed to vary during the refinements. The fitting for Sr2ScO3Cl converged smoothly with reasonable reliability factors, and no site deficiencies were found for all the sites within the error margin.

Figure 4. Crystal structure of Sr2ScO3Cl. White, blue, red, and green spheres represent strontium, scandium, oxygen, and chlorine, respectively. Thermal ellipsoids are shown at the 99% level. Selected bond lengths and angles are given in angstroms and degrees, respectively.

The final obtained Rietveld refinements are shown in Figure 5. The local coordination environment around the Sc and Sr cations is presented in Figure 6. Table 3 lists the crystallographic parameters, which are consistent with the results obtained from the single-crystal analysis. The anisotropic displacement parameters of all the atoms are presented in Table S1 (Supporting Information). For A3Sc2O5Cl2 (A = Sr, Ba), a model based on the crystal structure of Sr3Fe2O5Cl2 with the Cl ions fully occupying the terminal apical sites was refined against the NPD data. Sr1/Ba1 and Sr2/Ba2 were placed at 4e and 2a sites, respectively, with Sc at 4e, Oeq at 8g, Oap at 2b, and Cl at 4e sites, respectively (Figure 6), in the I4/mmm space group, yielding the stacking sequence of -(AO)-(ScO2)-(ACl2)2-(ScO2)-. Refinements for both compounds readily converged with reasonable reliability factors, except for the rather high Uiso parameters for Oap and Sr1 atoms located on the same plane in Sr3Sc2O5Cl2. The refined anisotropic atomic displacement parameters reveal that the thermal ellipsoids are substantially extended in the ab plane for Oap and along the c-axis for Sr1. In contrast, the Ba counterpart exhibits reasonable values of the atomic displacement parameters for all the atoms. In general, a high Uiso value suggests site deficiencies or displacement to sites with a lower

Table 2. Structural Parameters for the Single Crystal of Sr2ScO3Cl at 299 K atom

site

x

y

z

Uiso/Å2

U11/Å2

U22/Å2

U33/Å2

Sr1 Sr2 Sc Oeq Oap Cl

2c 2c 2c 4f 2c 2c

1/4 1/4 3/4 3/4 3/4 3/4

1/4 1/4 3/4 1/4 3/4 3/4

0.09540(4) 0.34869(4) 0.21526(8) 0.2379(2) 0.0749(4) 0.42606(13)

0.0113(2) 0.00811(14) 0.0059(2) 0.0099(6) 0.0203(11) 0.0151(3)

0.0121(2) 0.0068(2) 0.0043(3) 0.0080(13) 0.024(2) 0.0149(5)

0.0121(2) 0.0068(2) 0.0043(3) 0.0039(12) 0.024(2) 0.0149(5)

0.0098(3) 0.0106(3) 0.0091(5) 0.017(2) 0.012(2) 0.0155(7)

D


Article

Inorganic Chemistry

Figure 5. Observed (crosses), calculated (upper solid line), and difference (bottom solid line) plots obtained from the Rietveld analysis of the powder neutron diffraction data collected at 300 K from Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2. Vertical lines represent expected Bragg peak positions.

overbonding of the barium atoms, respectively. The Ba2 atom is coordinated to one chloride ion at 3.3520(8) Å, four chloride ions at 3.1611(2) Å, and four oxide ions at 2.7020(4) Å, the bond lengths of which are 2.2% longer for the former, and 3.6% and 6.0% shorter for the latter two than those predicted from the ionic model. Because of the dominance of the short bond lengths, the local coordination environment leads to the compression of the barium atom. Despite the low BVS value for the Ba1 site, not only are the bond lengths of Ba1−Oap and Ba1−Oeq in the BaO12 polyhedron consistent with the sum of the ionic radii of Ba2+ and O2− within 1% but also the Uiso values for the Ba1 atom and the surrounding oxygen atoms are also reasonable. This situation is different from that observed for the Sr1 atom with the elongated Sr−O bonds and high Uiso values in Sr3Sc2O5Cl2. Thus, the extraordinarily low BVS value for the Ba1 atom could not be due to underbonding. The reason is not clear yet, but a similar relationship has been observed for the indium analogue, Ba3In2O5Cl2,39 for which the value of BVS for the Ba1 atom is as small as 1.36 despite the Ba−O bond lengths being consistent with the ionic model. These unusual BVS values for the Ba1 site appear to be common to related oxychloride phases containing Ba cations.

symmetry. However, the examination of possible site deficiencies and displacement to more general sites for the strontium and oxygen atoms did not improve the refinement. Thus, both atoms are placed on the original special positions. No anion deficiencies were detected within the error margin in Ba3Sc2O5Cl2. The refined crystallographic parameters of Sr3Sc2O5Cl2 and Ba3Sc2O5Cl2 are listed in Tables 4 and 5 and Tables S2 and S3. The results of the Rietveld refinements and local coordination environment around the Sr/Ba and Sc cations are presented in Figures 5 and 6. It is likely that the high Uiso values for Oap and Sr1 in Sr3Sc2O5Cl2 are due to the lattice mismatch between the SrO layer in the perovskite blocks and the (SrCl)2 rock-salt layers, as discussed during the structure analysis of Sr2ScO3Cl. The BVS value for Sr1 is 1.16, much lower than the expected oxidation number, in contrast to the consistent values of BVS for Sr2 (2.16) and Sc (3.09) atoms. This result suggests the underbonding of the Sr1 site. Four Sr1−Oap and eight Sr1− Oeq bond lengths are longer by 2.3% and 6.6% than the lengths expected from the ionic model. In contrast, the relationship between the Uiso and BVS values of barium sites for Ba3Sc2O5Cl2 is somewhat puzzling. The BVS values for Ba1 and Ba2 are 1.61 and 2.60, implying the under- and E


Article

Inorganic Chemistry

Figure 6. Schematic view of the crystal structures and local coordination environment around scandium and strontium/barium cations in Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2, determined by the Rietveld analysis of the neutron powder diffraction data. Thermal ellipsoids are shown at the 99% level. Selected bond lengths and angles are given in angstroms and degrees, respectively.

Table 3. Structural Parameters Obtained by the Rietveld Refinements Using the Neutron Powder Diffraction Data of Sr2ScO3Cl at 300 Ka

Table 5. Structural Parameters Obtained by the Rietveld Refinements Using the Neutron Powder Diffraction Data of Ba3Sc2O5Cl2 at 300 Ka

atom

site

x

y

z

Uiso/Å2

atom

site

x

y

z

Uiso/Å2


2c 2c 2c 4f 2c 2c

1/4 1/4 3/4 3/4 3/4 3/4

1/4 1/4 3/4 1/4 3/4 3/4

0.09552(3) 0.34875(4) 0.21502(2) 0.23781(3) 0.07472(5) 0.42529(3)

0.00928(11) 0.00613(11) 0.00727(8) 0.00760(7) 0.0164(12) 0.01115(1)

Ba1 Ba2 Sc Oeq Oap Cl

2a 4e 4e 2b 8g 4e

0 0 0 1/2 0 0

0 0 0 0 0 0

0 0.15993(3) 0.41941(1) 0.09083(2) 1/2 0.29650(2)

0.0064(2) 0.00453(13) 0.00757(8) 0.00687(7) 0.0090(2) 0.01210(10)

a

a

P4/nmm (No. 129). a = 4.081406(4) Å and c = 14.09368(3) Å. Rwp = 3.71%, Rp = 3.04%, RB = 2.70%, and S = 1.61.

I4/mmm (No. 139). a = 4.206920(5) Å and c = 24.54386(6) Å. Rwp = 4.85%, Rp = 4.07%, RB = 3.35%, and S = 1.67.

Table 4. Structural Parameters Obtained by the Rietveld Refinements Using the Neutron Powder Diffraction Data of Sr3Sc2O5Cl2 at 300 Ka atom

site

x

y

z

Uiso/Å2


2a 4e 4e 2b 8g 4e

0 0 0 1/2 0 0

0 0 0 0 0 0

0 0.15961(2) 0.41613(1) 0.09434(2) 1/2 0.29435(2)

0.0161(2) 0.00625(11) 0.00742(6) 0.01088(10) 0.0243(2) 0.01211(2)

and chloride ions is very weak. In addition, the bond lengths of Sr−Oap are contracted by 7.9% for Sr3Sc2O5Cl2 and 7.8% for Ba3Sc2O5Cl2. The bond length of Sc−Oeq is 2.06931(6) Å for Sr and 2.11841(5) Å for Ba, both of which are consistent with the sum of the ionic radii within 1%. A similar coordination environment is confirmed in related oxychloride phases, Sr3B2O5Cl2 (B = Mn, Fe, Co).15,20,11 Band Structure Calculations. As far as we know, there have been a few examples of square pyramidal coordination for scandium in layered oxypnictide and oxysulfide compounds.10,40−42 However, these electronic structures have not been studied well by theoretical calculations. To investigate the chemical bonding in Sr2ScO3Cl, the first-principles DFT calculations were performed for Sr2ScO3Cl. The computationally optimized crystal structure yielded the lattice constants of a = 4.1145 Å and c = 14.2614 Å, which are consistent with those determined by XRD analyses. Figure 7 shows the total and partial density of states (DOS) and the band dispersions of Sr2ScO3Cl. The results suggest a semiconducting state with an indirect band gap of 3.75 eV between the valence band maximum (VBM) at the M point and the conduction band minimum (CBM) at the Γ point. The VBM and CBM are mainly composed of the O 2p and Sc 3d orbitals, respectively. The O 2p band is highly dispersive and ranges from −3.5 to 0.0 eV, whereas the Cl 3p band is relatively narrow and is located

a

I4/mmm (No. 139). a = 4.107982(5) Å and c = 23.58454(7) Å. Rwp = 4.87%, Rp = 3.98%, RB = 2.43%, and S = 1.71.

As compared in Figure 6, the coordination geometry for the Sc cation in Sr3Sc2O5Cl2 and Ba3Sc2O5Cl2 is very similar to that in Sr2ScO3Cl, as determined by the single-crystal and powder diffraction studies. The preferential occupation of the anion sites by the chloride ions in the n = 2 phases results in ScO5 square pyramids with the corrugated ScO4 squares (∠Oap−Sc− Oeq ≈ 97°), but the apically connected double layer of cornersharing ScO5 pyramids is constructed. The bond lengths of Sc− Cl for the two n = 2 phases are significantly elongated by 12% for Sr and 18% for Ba in comparison with those of the ionic model, indicating that the interaction between the scandium F


Article

Inorganic Chemistry

3p band between −3.5 and −2 eV, which apparently contradicts the long Sc−Cl bond. As shown in Figure S1, the result of the crystal orbital overlap population (COOP) calculations revealed the nature of the Sc−O and Sc−Cl bonds: the Sc− O bonds with positive COOP values are bonding, but the Sc− Cl bond with negligibly small COOP values is nonbonding. Therefore, we can conclude that the Sc-centered coordination geometry is square pyramidal. Further, the calculated band structures and dispersions of A3Sc2O5Cl2 (A = Sr, Ba) are presented in Figure S2. These electronic structures are qualitatively similar to those of Sr2ScO3Cl, and the magnitude of the indirect band gaps is 3.85 eV for Sr and 3.57 eV for Ba. Optical Properties and Band Gap. Figure 8 shows the UV−vis−NIR absorption spectra of all the compounds, obtained by the conversion of the diffuse reflectance spectra by the Kubelka−Munk transformation. The absorption spectrum of the related scandium oxide LaSrScO4 with ScO6 octahedra,26 which was prepared according to the procedure in ref 23, is also shown for comparison. LaSrScO4 exhibits a sharp absorption above Eg = 5.26 eV corresponding to its band-gap energy, whereas all the oxychloride compounds studied here show complex absorption behaviors. The absorption curve of Sr2ScO3Cl has two sub-bands centered at 3.40 and 4.83 eV, followed by a steep increase to higher energy. For Sr3Sc2O5Cl2 and Ba3Sc2O5Cl2, no clear absorption edges are observed, although Ba3Sc2O5Cl2 exhibits a broad band at ∼4.0 eV and a steep increase in the absorption above 5.0 eV. It is likely that the sub-bands observed in the low energy regions for Sr2ScO3Cl and Ba3Sc2O5Cl2 do not involve transitions from the VBM to CVM but are related to impurity bands derived from surface defects or oxygen vacancies. It seems difficult to determine the band gaps explicitly by the absorption experiments. However, given that band-gap energies calculated by

Figure 7. (a) Total and (b) partial density of states, and (c) band dispersions of Sr2ScO3Cl.

between −4 and −2 eV. It is clear that the Sc 3d band ranging from −3.5 to −0.5 eV overlaps with the O 2p band, suggesting the presence of covalent interaction between Sc 3d and O 2p orbitals, as expected from the X-ray structure analysis. However, note that the Sc 3d band also overlaps with the Cl

Figure 8. UV−vis−NIR absorption spectra of (a) Sr2ScO3Cl, (b) Sr3Sc2O5Cl2, (c) Ba3Sc2O5Cl2, and (d) LaSrScO4. G


Article

Inorganic Chemistry Author Contributions

DFT calculations tend to be much smaller than the experimental values,10,14 it is natural to assume that the bandgap energies of the Sc-based oxychlorides are larger than the calculated values (∼4 eV). In this context, it is likely that the sharp increase in the absorption curves observed above 5 eV for Sr2ScO3Cl and Ba3Sc2O5Cl2 is associated with the transitions from the VBM to CVM. The energy of the band gap calculated by the extrapolation method (Figure 8) is 4.93 eV for Sr2ScO3Cl and 4.61 eV for Ba3Sc2O5Cl2, somewhat lower than that of LaSrScO4.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This study was supported in part by the Japan Society for the Promotion of Science (JSPS) through Grants-in-Aid for Scientific Research (15K14133, 17K14466, 17H05493, 17H05489, 16K05731, 16H04501, 16H06438, 16H06439, 16H06440, 26390045, 26800180) and JSPS Bilateral Open Partnership Joint Research Projects from MEXT.

4. CONCLUSION We successfully synthesized three new members of the Ruddlesden−Popper-type scandium oxychloride perovskites, Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2. A combination of structure analyses using neutron powder and/or X-ray singlecrystal diffraction and the DFT calculations revealed the full O/ Cl order at the apical sites and rare ScO5 square pyramidal coordination. The calculated band dispersions suggest indirect band-gap semiconducting characteristics common across Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2. As demonstrated in this study, the use of mixed anions is an effective approach to stabilizing unusual coordination around the metal center. Our compounds are expected to be good starting materials for investigating the electronic structures originating from ScO5 pyramids and their relevant physical or chemical properties. Recently, related Sc-based oxyfluorides, A2ScO3F (A = Sr, Ba), were reported to show a photocatalytic activity under UV light irradiation for A = Sr23 and high-efficient photoluminescence properties for Eu3+-doped Ba2ScO3F.43 We have been investigating the photoluminescence properties of our scandium oxychlorides, which will be reported elsewhere.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Y. Katsuya, Dr. M. Tanaka, and Prof. O. Sakata for their assistance in performing the SXRD experiments at SPring-8 (Proposal No. 2016B4504, 2017A4503). We also acknowledge Dr. Ishigaki for his assistance in the NPD experiments with iMATERIA with the approval of J-PARC (Proposal No. 2017A0023).

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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.8b00573. Anisotropic displacement parameters for Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2. Crystal orbital overlap population for Sr2ScO3Cl. Total and partial density of states, and band dispersion of Sr 3 Sc 2 O 5 Cl 2 and Ba3Sc2O5Cl2 (PDF) Accession Codes

CCDC 1827269 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 data_ [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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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.T.). *E-mail: [email protected] (H.O.). ORCID

Yoshihiro Tsujimoto: 0000-0003-2140-3362 Kotaro Fujii: 0000-0003-3309-9118 Masatomo Yashima: 0000-0001-5406-9183 Kazunari Yamaura: 0000-0003-0390-8244 H


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