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

Abstract

The light gray selenogallate CsGaSe 2 -mC64 was obtained by reaction of stoichiometric amounts of CsN 3 , 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) . 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. ■ INTRODUCTIONGroup 13 chalcogenometallates containing alkali metal cations crystallize in a large variety of different structures.1 Multinary chalcogenometallates are important materials for technical applications due to their semiconducting properties.2,3 Most of the crystal structures of the known phases in the ternary systems of alkali metal, triel, and chalcogen consist of linked MQ 4 -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. , and consist of i n fi n i t e a n i o n i c c h a i n s 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, 9 yielding information on the crystallization behavior and activation energy. , and Z = 4 for CsGaSe 2 -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, 10 implemented in Jana2006, 11 and refined to R 1 = 0.0547 and wR 2 = 0.0668 for CsGaSe 2 -

Influence of Alkali Metal Substitution on the Phase Transition Behavior of CsGaQ2 (Q = S, Se)

“…The low-temperature polymorph CsGaQ 2 -mC64 [6,7], as well as KGaQ 2 [8,9] and RbGaQ 2 (Q = S, Se) [10,11], crystallize in the TlGaSe 2 structure type ( Figure 1, Table 1 610 °C), we decided to explore the possibilities of the formation of solid solutions of these compounds with RbGaQ2 and KGaQ2. As the compounds KGaQ2 [8,9], RbGaQ2 [10,11] and the low-temperature phases CsGaQ2-mC64 [6,7] (Q = S, Se) all crystallize isotypically in the TlGaSe2 structure type, the formation of solid solutions Cs1−xMxGaQ2-mC64 (M = K, Rb; Q = S, Se; x = 0-1) should be possible. CsGaTe2 [12] also crystallizes in the TlGaSe2 structure type, but no phase transition occurs upon heating of this compound.…”

“…The low-temperature polymorph CsGaQ2-mC64 [6,7], as well as KGaQ2 [8,9] and RbGaQ2 (Q = S, Se) [10,11], crystallize in the TlGaSe2 structure type ( Figure 1, Table 1 showing: (a) the stacking of the anionic layers in the TlGaSe 2 structure type; (b) the supertetrahedral building blocks Ga 4 Q 10 of these layers; (c) coordination polyhedra of the alkali metal sites; (d) the arrangement of the anionic chains in the KFeS 2 structure type; (e) the SiS 2 analogous anionic chain; and (f) the coordination of the cesium site. The high-temperature polymorphs CsGaQ 2 -mC16 (Q = S, Se) crystallize in the KFeS 2 structure type ( Figure 1).…”

“…Several different annealing temperatures and times were tested to optimize the syntheses. The best quality for these solid solution samples was achieved by annealing the pellets at 550 • C (slightly below the phase transition temperatures of pure CsGaQ 2 -mC64 [6,7]) for seven days. This step was repeated two more times in order to ensure a maximum intermixture of the starting phases.…”

“…Tables S1 and S2 list all the observed Raman shifts in all of the solid solution series, as well as an assignment of the vibrational modes. The slight blue shift of all vibrations in the spectra of the lighter alkali metals indicate slightly stronger bonds Ga-Q, corresponding to the smaller distances d(Ga-Q) in the rubidium and potassium compounds [6][7][8][9][10][11]. In the solid solutions of Cs1−xRbxGaS2-mC64 (x = …”

“…The dominating structure type among these solids is the TlGaSe 2 structure type [3] [4] and KFeS 2 [5] structure types, respectively. Only the high-temperature polymorphs of CsGaS 2 [6] and CsGaSe 2 [7] crystallize in the latter type, but no analogous phase transition is known for the lighter homologous compounds. This behavior likely results from the higher degree of spatial separation necessary for the formation of one-dimensional structures, which can only be realized by the larger cesium cations.…”