Synthesis, Crystal Structure, and Physical Properties of Two Polymorphs of CsGaSe2, and High-Temperature X‐ray Diffraction Study of the Phase Transition Kinetics

The light gray selenogallate CsGaSe2-mC64 was obtained by reaction of stoichiometric amounts of CsN3, GaSe, and Se at elevated temperatures. Its crystal structure was determined by single-crystal X-ray diffraction. The compound crystallizes in the monoclinic space group C2/c (No. 15) with a = 11.043(2) Å, b = 11.015(4) Å, c = 16.810(2) Å, β = 99.49(1) °, V = 2016.7(8) Å, and Z = 16 (powder data, ambient temperature). Its crystal structure features anionic layers ∞ 2 [Ga4Se8 4−] consisting of corner-sharing Ga4Se10 supertetrahedra. The compound undergoes a first-order phase transition at temperatures of 610 ± 10 °C. The hightemperature phase CsGaSe2-mC16 also crystallizes in the monoclinic space group C2/c (No. 15) with a = 7.651(3) Å, b = 12.552(4) Å, c = 6.170(3) Å, β = 113.62(4)°, V = 542.9(5) Å, and Z = 4 (powder data, ambient temperature). The crystal structure of the high-temperature phase consists of SiS2 analogous chains ∞ 1 [GaSe2 −]. In situ high-temperature X-ray diffraction experiments were performed to study this phase transition. The crystallization kinetics of the phase transitions were studied using Johnson−Mehl−Avrami−Kolmogorov (JMAK) theory for isothermal crystallization processes. The activation energy of the phase transition was determined using the Arrhenius equation. Furthermore, the compound was studied by vibrational and diffuse reflectance spectroscopy. ■ INTRODUCTION Group 13 chalcogenometallates containing alkali metal cations crystallize in a large variety of different structures. Multinary chalcogenometallates are important materials for technical applications due to their semiconducting properties. Most of the crystal structures of the known phases in the ternary systems of alkali metal, triel, and chalcogen consist of linked MQ4-tetrahedra (M = Al, Ga, In; Q = S, Se, Te) embedded in a cationic surrounding of alkali metal ions. The resulting anionic substructures composed of these connected tetrahedra range from discrete tetrahedra to more complex one-, two-, and threedimensional networks. All previously known compounds in the system cesium, gallium, and selenium contain infinite onedimensional selenogallate anions. The phases Cs6Ga2Se6, 4 Cs8Ga4Se10, 4 and Cs10Ga6Se14 4 feature edge-sharing dimeric, tetrameric, and hexameric selenogallate units. Cs2Ga2Se5 5 and CsGaSe3 6 contain polychalcogenide units Se2 2−, and consist of i nfin i t e an i on i c ch a i n s ∞ 1 [Ga 2S e 3 (S e 2 ) 2− ] and ∞ 1 [Ga2Se2(Se2)2 2−], respectively. Prior studies by Deiseroth et al. revealed that two polymorphs of CsGaSe2 might exist; however, no crystal structures (besides comparisons of powder diffraction patterns) or details of the phase transition were reported. Recently we reported on some details of the crystal structures and the phase transition temperatures of CsGaSe2, 8 which we will now discuss in more detail. Furthermore, the progress of the phase transition is observed using in situ hightemperature X-ray diffraction. The kinetics of the isothermal phase transitions are studied using Johnson−Mehl−Avrami− Kolmogorov (JMAK) theory, yielding information on the crystallization behavior and activation energy. ■ RESULTS AND DISCUSSION Single-Crystal Structure Determination. CsGaSe2 forms air and moisture sensitive colorless crystals. The crystal structures were determined from single-crystal X-ray diffraction data collected at 123 K. Both solids crystallize in the monoclinic space group C2/c (No. 15) with a = 11.0011(6) Å, b = 11.0007(6) Å, c = 16.7390(8) Å, β = 99.465(4)°, V = 1998.2(2) Å, and Z = 16 for CsGaSe2-mC64, and a = 7.6458(3) Å, b = 12.5337(5) Å, c = 6.1634(3) Å, β = 113.716(4)°, V = 540.76(4) Å, and Z = 4 for CsGaSe2-mC16. The change of Z by a factor of 4 comes along with a change of the unit cell volume by almost a factor of 4; that is, the structural changes are accompanied by a change of the density. The crystal structures were solved by charge flipping methods using SUPERFLIP, implemented in Jana2006, and refined to R1 = 0.0547 and wR2 = 0.0668 for CsGaSe2Received: April 7, 2016 Revised: May 19, 2016 Published: May 24, 2016 Article pubs.acs.org/crystal © 2016 American Chemical Society 3983 DOI: 10.1021/acs.cgd.6b00532 Cryst. Growth Des. 2016, 16, 3983−3992 mC64 and R1 = 0.0271 and wR2 = 0.0484 for CsGaSe2-mC16 by full-matrix least-squares methods using Jana2006. Both polymorphs exhibited twinning. The twin matrices were analyzed using TwinRotMat of the PLATON software package. Details on the final crystallographic data and details of the structure solution and refinement are listed in Tables1−3. Anisotropic displacement parameters, interatomic distances, and angles can be found in the Supporting Information (Table S1−S4). The low-temperature phase CsGaSe2-mC64 crystallizes isotypic to many related compounds MTQ2 (M = Na, K, Rb, Cs, Tl; T = Al, Ga, In; Q = S, Se, Te) in the TlGaSe2 structure type. The crystal structure features anionic layers ∞ 2 [Ga4Se8 4−], consisting of four corner-sharing Ga4Se10 supertetrahedra, stacked along [001] and separated by Cs atoms (Figure 1). Both independent trivalent gallium sites have a slightly distorted tetrahedral coordination formed by four Se atoms. The interatomic distances range from d(Ga−Se) = 2.384(1) Å to d(Ga−Se) = 2.444(1) Å with a mean bond length of d̅(Ga−Se) = 2.413(1) Å. The angles ∠(Se−Ga−Se) range from 105.85(4)° to 114.08(3)°. Both distances and angles are in good agreement with comparable compounds. The bond lengths differ slightly from the sum of the ionic radii, d(Ga3+−Se2−) ∼ Table 1. Crystallographic Data for CsGaSe2-mC64 and CsGaSe2-mC16 CsGaSe2-mC64 CsGaSe2-mC16 Formula weight/g·mol−1 360.553 Color, shape, crystal size/mm Colorless plate, 0.12 × 0.04 × 0.02 Colorless rod, 0.29 × 0.04 × 0.02 Crystal system, space group Monoclinic, C2/c (No. 15) Lattice constants from single crystals a/Å 11.0011(6) 7.6458(3) b/Å 11.0007(6) 12.5337(5) c/Å 16.7390(8) 6.1634(3) β/Å 99.465(4) 113.716(4) Volume V/Å 1998.2(2) 540.76(4) Number of formula units Z 16 4 Calculated density ρcalc/g·cm −3 4.794 4.427 Temperature T/°C −150(1) Diffractometer Rigaku Supernova Wavelength λ/Å 0.71073 Absorption coeff. μ(Mo Kα)/mm−1 27.069 25.002 θ range of data collection/deg 3.39−29.1 3.25−29.03 Index range −14 ≤ h ≤ 14 −10 ≤ h ≤ 10 −14 ≤ k ≤ 14 −16 ≤ k ≤ 17 −22 ≤ l ≤ 22 −8 ≤ l ≤ 8 Absorption correction Analytical, CrysAlisPro No. of reflections collected 23138 10383 Independent reflections 2597 1021