Interconversion of One-Dimensional Thiogallates Cs₂[Ga₂(S₂)_2–<i>x</i>S_2+<i>x</i>] (<i>x</i> = 0, 1, 2) by Using High-Temperature Decomposition and Polysulfide-Flux Reactions

The potential of cesium polysulfide-flux reactions for the synthesis of chalcogenogallates was investigated by using X-ray diffraction and Raman spectroscopy. An investigation of possible factors influencing the product formation revealed that only the polysulfide content <i>x</i> in the Cs₂S_<i>x</i> melts has an influence on the crystalline reaction product. From sulfur-rich melts (<i>x</i> > 7), CsGaS₃ is formed, whereas sulfur-poor melts (<i>x</i> < 7) lead to the formation of Cs₂Ga₂S₅. <i>In situ</i> investigations using high-temperature Raman spectroscopy revealed that the crystallization of these solids takes place upon cooling of the melts. Upon heating, CsGaS₃ and Cs₂Ga₂S₅ release gaseous sulfur due to the degradation of S₂^2– units. This decomposition of CsGaS₃ to Cs₂Ga₂S₅ and finally to CsGaS₂-<i>mC</i>16 was further studied <i>in situ</i> by using high-temperature X-ray powder diffraction. A combination of the polysulfide reaction route and the high-temperature decomposition leads to the possibility of the directed interconversion of these thiogallates. The presence of disulfide units in the anionic substructures of these thiogallates has a significant influence on the electronic band structures and their optical properties. This influence was studied by using UV/vis-diffuse reflectance spectroscopy and DFT simulations, revealing a trend of smaller band gaps with increasing S₂^2– content

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-<i>mC</i>64 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 <i>C</i>2/<i>c</i> (No. 15) with <i>a</i> = 11.043(2) Å, <i>b</i> = 11.015(4) Å, <i>c</i> = 16.810(2) Å, β = 99.49(1) °, <i>V</i> = 2016.7(8) ų, and <i>Z</i> = 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 high-temperature phase CsGaSe₂-<i>mC</i>16 also crystallizes in the monoclinic space group <i>C</i>2/<i>c</i> (No. 15) with <i>a</i> = 7.651(3) Å, <i>b</i> = 12.552(4) Å, <i>c</i> = 6.170(3) Å, β = 113.62(4)°, <i>V</i> = 542.9(5) ų, and <i>Z</i> = 4 (powder data, ambient temperature). The crystal structure of the high-temperature phase consists of SiS₂ analogous chains _∞¹[GaSe₂^–]. <i>In situ</i> 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

In Situ X‑ray Diffraction Study of the Thermal Decomposition of Selenogallates Cs₂[Ga₂(Se₂)_2–<i>x</i>Se_2+<i>x</i>] (<i>x</i> = 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–<i>x</i>Se_2+<i>x</i>] (<i>x</i> = 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

Na₂TeS₃, Na₂TeSe₃-<i>mP</i>24, and Na₂TeSe₃-<i>mC</i>48: Crystal Structures and Optical and Electrical Properties of Sodium Chalcogenidotellurates(IV)

Pure samples of Na₂TeS₃ and Na₂TeSe₃ were synthesized by the reactions of stoichiometric amounts of the elements Na, Te, and Q (Q = S, Se) in the ratio 2:1:3. Both compounds are highly air- and moisture-sensitive. The crystal structures were determined by single-crystal X-ray diffraction. Yellow Na₂TeS₃ crystallizes in the space group <i>P</i>2₁/<i>c</i>. Na₂TeSe₃ exists in a low-temperature modification (Na₂TeSe₃-<i>mP</i>24, space group <i>P</i>2₁/<i>c</i>) and a high-temperature modification (Na₂TeSe₃-<i>mC</i>48, space group <i>C</i>2/<i>c</i>); both modifications are red. Density functional theory calculations confirmed the coexistence of both modifications of Na₂TeSe₃ because they are very close in energy (Δ<i>E</i> = 0.18 kJ mol^–1). To the contrary, hypothetic Na₂TeS₃-<i>mC</i>48 is significantly less favored (Δ<i>E</i> = 1.8 kJ mol^–1) than the primitive modification. Na₂TeS₃ and Na₂TeSe₃-<i>mP</i>24 are isotypic to Li₂TeS₃, whereas Na₂TeSe₃-<i>mC</i>48 crystallizes in its own structure type, which was first described by Eisenmann and Zagler. The title compounds have two common structure motifs. Trigonal TeQ₃ pyramids form layers, and the Na atoms are surrounded by a distorted octahedral environment of chalcogen atoms. Raman spectra are dominated by the vibration modes of the TeQ₃ units. The activation energies of the total conductivity of the title compounds range between 0.68 eV (Na₂TeS₃) and 1.1 eV (Na₂TeSe₃). Direct principal band gaps of 1.20 and 1.72 eV were calculated for Na₂TeSe₃ and Na₂TeS₃, respectively. The optical band gaps are in the range from 1.38 eV for Li₂TeSe₃ to 2.35 eV for Na₂TeS₃

Synthesis and Crystal Structure Determination of Ag₉FeS_4.1Te_1.9, the First Example of an Iron Containing Argyrodite

Ag₉FeS_4.1Te_1.9 was prepared by solid state synthesis from stoichiometric amounts of the elements at 873 K. The compound forms gray crystals which are stable against air and moisture. The crystal structure was determined by X-ray diffraction from selected single crystals. Ag₉FeS_4.1Te_1.9 crystallizes in the space group <i>F</i>4̅3<i>m</i>, <i>a</i> = 11.0415(7) Å, <i>V</i> = 1346.1(1) ų, and <i>Z</i> = 4 (powder data at 293 K). The compound shows a reversible phase transition upon cooling to the space group <i>P</i>2₁3, <i>a</i> = 11.0213(1) Å, <i>V</i> = 1338.75(2) ų, and <i>Z</i> = 4 (single crystal data at 200 K). The title compound is the first example of an iron containing argyrodite-type material with Fe³⁺ located in tetrahedral sites. Silver atoms are disordered at room temperature which was taken into account by nonharmonic refinement of the silver positions. The refinement converged to <i>R</i>₁ = 3.51% and <i>wR</i>₂ = 10.66% for the room temperature measurement and to <i>R</i>₁ = 1.55% and <i>wR</i>₂= 5.23% for the 200 K data set (all data). Impedance measurements were performed in the temperature range from 323 to 473 K. Ionic conductivity values are 1.81 × 10^–2 S cm^–1 at 323 K and 1.41 × 10^–1 S cm^–1 at 468 K. The activation energy is 0.19 eV from 323 to 423 K and 0.06 eV from 393 to 473 K. DTA measurements reveal congruent melting at 907 K. A phase transition temperature of 232 K with an enthalpy of 7.9 kJ/mol was determined by DSC measurements. ⁵⁷Fe Mössbauer spectra show one signal at 298 K and a doublet at 78 K, indicating Fe³⁺ and structural distortions upon cooling the samples. Hyperfine field splitting of iron is observed at 5 K. Measurements of the molar susceptibility revealed that the compound is paramagnetic down to a Néel temperature of <i>T</i>_N = 22.1(5) K. Antiferromagnetic ordering is observed at lower temperatures

Crystallization of Mixed Alkaline-Earth Carbonates in Silica Solutions at High pH

The ability of silica to influence the mineralization of alkaline-earth carbonates is an outstanding example for the formation of biomimetic structures in the absence of any organic matter. Under suitable conditions, silica-stabilized carbonate nanocrystals can spontaneously self-assemble into hierarchical materials with complex morphologies, commonly referred to as “silica biomorphs”. However, growth of these crystal aggregates has largely been restricted to the higher homologues in the alkaline-earth series, i.e., SrCO₃ and BaCO₃, while corresponding architectures of the much more relevant calcium carbonate are quite difficult to realize. To systematically address this problem, we have crystallized metal carbonates in the presence of silica at high pH, using barium and strontium chloride solutions that contained increasing molar fractions of Ca²⁺. The resulting materials were analyzed with respect to their composition, structure, and crystallography. The obtained data demonstrate that the growth process is already strongly affected by small amounts of calcium. Indeed, morphologies typically observed for SrCO₃ and BaCO₃ remained absent above certain thresholds of added Ca²⁺. Instead, globular and hemispherical structures were generated, owing to fractal branching of carbonate crystals as a consequence of poisoning by silica. These alterations in the growth behavior are ascribed to relatively strong interactions of hard calcium ions with silicate species in solution, shifting their speciation toward higher oligomers and even inducing partial coagulation. This notion is confirmed by additional experiments at increased ionic strength. Our results further demonstrate that the observed hemispherical particles exhibit distinct polymorphism, with orthorhombic solid solutions (aragonite-type (Sr,Ca)­CO₃ and (Ba,Ca)­CO₃) being formed at lower Ca²⁺ contents, whereas Sr²⁺/Ba²⁺-substituted calcite prevails at higher Ca²⁺ fractions. In the case of Ba²⁺/Ca²⁺ mixtures, there is moreover an intermediate range where virtually identical morphologies were confirmed to be Ba²⁺-doped vaterite. These findings extend the variety of structures and compositions accessible in these simple systems, and may explain difficulties previously encountered in attempts to prepare CaCO₃ biomorphs at standard conditions