Marc Schlosser

Synthesis, structural characterization, and physical properties of Cs2Ga2S5, and redetermination of the crystal structure of Cs2S6.

The reaction of CsN3 with GaS and S at elevated temperatures results in Cs2Ga2S5. Its crystal structure was determined from single-crystal X-ray diffraction data. The colorless solid crystallizes in space group C2/c (no. 15) with V = 1073.3(4) Å(3) and Z = 4. Cs2Ga2S5 is the first compound that features one-dimensional chains ∞(1)([Ga2S3(S2)(2-)] of edge- and corner-sharing GaS4 tetrahedra. The vibrational band of the S2(2-) units at 493 cm(-1) was revealed by Raman spectroscopy. Cs2Ga2S5 has a wide bandgap of about 3.26 eV. The thermal decomposition of CsN3 yields elemental Cs, which reacts with sulfur to provide Cs2S6 as an intermediate product. The crystal structure of Cs2S6 was redetermined from selected single crystals. The red compound crystallizes in space group P1 with V = 488.99(8) Å(3) and Z = 2. Cs2S6 consists of S6(2-) polysulfide chains and two Cs positions with coordination numbers of 10 and 11, respectively. Results of DFT calculations on Cs2Ga2S5 are in good agreement with the experimental crystal structure and Raman data. The analysis of the chemical bonding behavior revealed completely ionic bonds for Cs, whereas Ga-S and S-S form polarized and fully covalent bonds, respectively. HOMO and LUMO are centered at the S2 units.

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

CsGaS2-mC64 was obtained by reaction of CsN3 with stoichiometric amounts of Ga2S3 and S at elevated temperatures. The crystal structure of the air- and moisture stable compound was determined from single-crystal X-ray diffraction data. The colourless solid crystallizes in the monoclinic space group C2/c (no. 15) with the lattice parameters a = 10.5718(6) A, b = 10.5708(6) A, c = 16.0847(8) A, β = 99.445(4)°, V = 1773.1(2) A3, and Z = 16. The compound crystallizes in the TlGaSe2 structure type and features anionic layers 2∞[Ga4S84−] consisting of corner-sharing Ga4S10 supertetrahedra. At temperatures above 600 °C an irreversible phase-transition to CsGaS2-mC16 occurs. The phase-transition kinetics were studied using in situ high-temperature X-ray powder diffraction techniques. This transition can only be reversed by using high pressures (>5 GPa at 500 °C). The compound was further characterized using Raman- and diffuse reflectance spectroscopy. Chemical bonding was analysed by DFT calculations.

Preparation of Magnesium, Cobalt and Nickel Ferrite Nanoparticles from Metal Oxides using Deep Eutectic Solvents

Natural deep eutectic solvents (DESs) dissolve simple metal oxides and are used as a reaction medium to synthesize spinel-type ferrite nanoparticles MFe2 O4 (M=Mg, Zn, Co, Ni). The best results for phase-pure spinel ferrites are obtained with the DES consisting of choline chloride (ChCl) and maleic acid. By employing DESs, the reactions proceed at much lower temperatures than usual for the respective solid-phase reactions of the metal oxides and at the same temperatures as synthesis with comparable calcination processes using metal salts. The method therefore reduces the overall required energy for the nanoparticle synthesis. Thermogravimetric analysis shows that the thermolysis process of the eutectic melts in air occurs in one major step. The phase-pure spinel-type ferrite particles are thoroughly characterized by X-ray diffraction, diffuse-reflectance UV/Vis spectroscopy, and scanning electron microscopy. The properties of the obtained nanoparticles are shown to be comparable to those obtained by other methods, illustrating the potential of natural DESs for processing metal oxides.

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 Se22- unit of the anionic chains in Cs2[Ga2(Se2)2- xSe2+ 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 Se22- content.

The crystal structure of Cs2S2O3·H2O

Abstract A reinvestigation of the alkali metal thiosulfates has led to the new phase Cs2S2O3·H2O. At first cesium thiosulfate monohydrate was obtained as a byproduct of the synthesis of Cs4In2S5. Further investigations were carried out using the traditional synthesis reported by J. Meyer and H. Eggeling. Cs2S2O3·H2O crystallizes in transparent, colorless needles. The crystal structure of the title compound was determined by single crystal X-ray diffraction at room temperature: space group C2/m (No. 12), unit cell dimensions: a = 11.229(4), b = 5.851(2), c = 11.260(5) Å, β = 95.89(2)°, with Z = 4 and a cell volume of V = 735.9(5) Å3. The positions of all atoms including the hydrogen atoms were located in the structure refinement. Cs2S2O3·H2O is isotypic with Rb2S2O3·H2O. Isolated tetrahedra [S2O3]2− are coordinated by the alkali metal cations, and in addition they serve as acceptors for hydrogen bonding. For both Cs atoms the shortest distances are observed to oxygen atoms of the S2O32− anions whereas the terminating sulfur atom has its shortest contacts to the water hydrogen atoms. Thus, an extended hydrogen bonding network is formed. The title compound has also been characterized by IR spectroscopy. IR spectroscopy reveals the vibrational bands of the water molecules at 3385 cm−1. They show a red shift in the OH stretching and bending modes as compared to free water. This is due both to the S···H hydrogen bonding and to the coordination of H2O molecules to the cesium atoms.

New Approach to Diffuse Scattering in Complex Chalcogenides

Earth abundant materials and low-cost fabrication are utmost criteria for future photovoltaic technologies [1]. Mixed main-group transition-metal chalcogenides fulfill these requirements, they are prospective important thermoelectric materials and useful for thin-film solar cells [2]. As an example, Cu3BiS3 thin films have shown promising structural and optical properties for special use in photovoltaics [3,4]. Despite exhibiting these promising material characteristics, thin film photovoltaic cells based on Cu3BiS3 have not yet reached the device level. This is mainly because the fundamental physical properties of Cu3BiS3 are not yet well understood, and the quality of the films is not optimized [5]. Open-ended questions also exist along the lines of Cu3BiSe3. A characteristic structure-related phenomenon not yet understood are circular diffuse scattering features in electron and X-ray diffraction. They are found not only for Cu3BiSe3 but for instance in La0.70Al0.14I0.86 [6] and K2In12Se19 [7], too.