Synthesis and Structural Characterization of Cs2Ga2Se5

In Situ X-ray Diffraction Study of the Thermal Decomposition of Selenogallates Cs2[Ga2(Se2)2- xSe2+ x] ( x = 0, 1, 2)

The selenogallates CsGaSe₃ and Cs₂Ga₂Se₅ release gaseous selenium upon heating. An in situ high-temperature X-ray powder diffraction analysis revealed a two-step degradation process from CsGaSe₃ to Cs₂Ga₂Se₅ and finally to CsGaSe₂. During each step, one Se₂^2- unit of the anionic chains in Cs₂[Ga₂(Se₂)_{2- x}Se_{2+ x}] ( x = 0, 1, 2) decomposes, and one equivalent of selenium is released. This thermal decomposition can be reverted by simple addition of elemental selenium and subsequent annealing of the samples below the decomposition temperature. The influence of the diselenide units in the anionic selenogallate chains on the optical properties and electronic structures was further studied by UV/vis diffuse reflectance spectroscopy and relativistic density functional theory calculations, revealing increasing optical band gaps with decreasing Se₂^2- content.

Polymorphism of CsGaS2– structural characterization of a new two-dimensional polymorph and study of the phase-transition kinetics

Anionic layers in CsGaS₂undergo a transition to infinite strands at high temperature and transfer back under pressure.

Synthesis, crystal and electronic structure of the new sodium chain sulfido cobaltates(II), Na3CoS3 and Na5[CoS2]2(Br)

The sulfido cobaltate(II) Na3CoS3 was synthesized from stoichiometric quantities of Na2S, elemental cobalt and sulfur at a maximum temperature of 1100°C. According to Na3CoS3=Na12[Co2S5(S2)][Co2S3(S2)] the orthorhombic structure of a new type (space group Cmc21, a=884.24(2), b=2177.38(5), c=1193.20(3) pm, Z=12, R1=0.0205) contains two different anions: i. dimers [Co2S5(S2)]8− of two edge-sharing [CoS4] tetrahedra with five sulfido and one η 1-disulfido ligand; ii. chains ∞ 1 [ Co 2 S 3 ( S 2 ) ] 4 − $_\infty ^1{[{\rm{C}}{{\rm{o}}_2}{{\rm{S}}_3}({{\rm{S}}_2})]^{4 - }}$ of [CoS4] tetrahedra connected via μ-sulfido (3×) besides μ-1,2-disulfido (1×) ligands. The second title compound, Na5[CoS2]2(Br), which has been likewise synthesized as a pure phase from stoichiometric quantities of Na2S, Co, S and NaBr, is isotypic to Na5[CoS2]2(S) (Na6PbO5 type; tetragonal, space group I4mm, a=914.58(7), c=625.59(5) pm; R1=0.0412). The structure contains linear chains ∞ 1 [ CoS 4 / 2 ] 2 − $_\infty ^1{[{\rm{Co}}{{\rm{S}}_{4/2}}]^{2 - }}$ running along the tetragonal c axis. In between, Br− ions are interspersed, which are coordinated by square pyramids of Na+ ions. The isotypic hydrogensulfide Na5[CoS2]2(SH) was obtained via the addition of NaSH. Several synthetic and structural arguments suggest that Na5[CoS2]2(SH) and the previously described pure sulfide are the same compound. The results of DFT band structure calculations (GGA+U, AFM spin ordering) are used to discuss and compare the chemical bonding in the new sulfido cobaltates(II) with that of the reference compound Na2[CoS2]. They also allow for obtaining insight into the superexchange path, which is responsible for the strong antiferromagnetic Co spin ordering along the chains.