Ronald Miletich

Crystal structures of arsenic(III)-oxide intercalation compounds with ammonium chloride (NH₄Cl∙As₂O₃∙¹/₂H₂O), ammonium bromide (NH₄Br∙2As₂O₃), and ammonium iodide (NH₄I∙2As₂O₃) have been determined under high pressure up to 12, 15 and 11 GPa, respectively. No phase transitions have been observed for the investigated compounds. The compression of arsenic∙∙∙halogen secondary bonds, expressed as penetration indices of the bonds, has been shown to be a linear function of unit-cell volume ratio V/V₀, where V₀ is unit-cell volume at ambient pressure, with similar slopes as the compression of arsenic∙∙∙oxygen secondary bonds. The behavior of arsenic coordination number, expressed as first-order valence entropy coordination number, at high pressures and the stereoactivity of arsenic lone electron pairs in the studied intercalates are the same as in arsenic(III)-oxide polymorphs – the former decreases linearly with V/V₀, while the latter remains unchanged. The high-pressure study lends further support to the fact that the nature of arsenic∙∙∙halogen and arsenic∙∙∙oxygen secondary bonds is the same.

Synthetic single crystals of K[(Cu^II,Mn^II,Mn^III)₂(TeO₃)₃]·2H₂O were obtained from an overconcentrated alkaline aqueous solution in the system K₂O-MnO-CuO-TeO₂ under hydrothermal conditions at T ≤ 220 °C. Subeuhedral single crystals have been investigated by means of single-crystal X-ray diffraction. The crystal structure of this new zemannite-type representative adopts a monoclinic twofold superstructure. The doubling of the unit-cell volume is accompanied by a hexagonal-to-monoclinic symmetry reduction, resulting in threefold twinning with individual crystal domains following the space group symmetry P2₁. Refinements of site-occupation factors and the evaluation of bond valences suggest a distribution of di- and trivalent cations at the octahedrally coordinated M sites with a ratio (Cu^II + Mn^II):Mn^III approximating 1:1. Based on arguments about the cation sizes and the individual bond valence sums, a distribution of Cu^II_1–xMn^III_x and Mn^II_x Mn^III_1–x at each two of the four M sites can be assumed with x between ~ 0.14 and ~ 0.50. The K⁺ cations and H₂O molecules inside the channels are located off the central channel axis. In contrast to most other known zemannite-type phases, the extra-framework atoms show full occupancies and are not disordered. The distribution of the channel contents supports the anisotropic deformation of the surrounding framework, which follows the local symmetry reduction as required for the Jahn-Teller distortion of the octahedrally coordinated M sites within the framework. The arrangement of the deformed channels can be understood as the origin of the existing superstructure.

Monohydrate sulfate kieserites (M²⁺SO₄·H₂O) and their solid solutions are essential constituents on the surface of Mars and most likely also on Galilean icy moons in our solar system. Phase stabilities of end-member representatives (M²⁺= Mg, Fe, Co, Ni) have been examined crystallographically using single-crystal X-ray diffraction at 1 bar and temperatures down to 15 K, by means of applying open He cryojet techniques at in-house laboratory instrumentation. All four representative phases show a comparable, highly anisotropic thermal expansion behavior with a remarkable negative thermal expansion along the monoclinicbaxis and a pronounced anisotropic expansion perpendicular to it. The lattice changes down to 15 K correspond to an `inverse thermal pressure' of approximately 0.7 GPa, which is far below the critical pressures of transition under hydrostatic compression (P_c≥ 2.40 GPa). Consequently, no equivalent structural phase transition was observed for any compound, and neither dehydration nor rearrangements of the hydrogen bonding schemes have been observed. TheM²⁺SO₄·H₂O (M²⁺= Mg, Fe, Co, Ni) end-member phases preserve the kieserite-typeC2/csymmetry; hydrogen bonds and other structural details were found to vary smoothly down to the lowest experimental temperature. These findings serve as an important basis for the assignment of sulfate-related signals in remote-sensing data obtained from orbiters at celestial bodies, as well as for thermodynamic considerations and modeling of properties of kieserite-type sulfate monohydrates relevant to extraterrestrial sulfate associations at very low temperatures.

This note reports a study of the coating of a crystal with `ice' at temperatures below 45 K during single-crystal in-house diffraction experiments when using an open-flow helium cryostat. The `ice' consists mainly of crystalline oxygen and nitrogen. This suggests completely different techniques for avoiding this type of icing compared with water icing. With appropriate choices of crystal mount, crystal position with respect to the nozzle and gas flow conditions, it is possible to avoid detectable condensation. However, sometimes this cannot be achieved in practice (poor diffraction from a smaller crystal, necessity of positioning the crystal in certain orientations to achieve desired data completeness, need to reduce helium consumption etc.). The problem of icing seems to be less common for powder experiments where the laminar gas flow is parallel to the capillary containing the sample, and for synchrotron experiments where the sample is comparatively small and almost continuously rotated, which facilitates the ice covering being removed by the gas flow. This last technique can in principle also be applied to single-crystal X-ray diffraction using laboratory diffractometers – periodic rapid rotation of the crystal can help to minimize any icing, but this technique will not work when the condensation rate is comparable to or faster than one frame of data collection. The coating around a sample crystal reduces the quality of the diffraction data, and the temperature at the sample below the coating may differ significantly from that at the cryostat nozzle reported by the instrument.

The pressure effect on the carbonation behavior of CaO as model compound is studied under mild hydrothermal conditions, as relevant to sustainable geological CO₂ sequestration and for potential utilization in thermochemical energy storage. Reaction yields are determined experimentally by means of in situ powder X‐ray diffraction using CaO powder samples in a controlled reaction with CO₂ under gas pressures between 1.0 and 5.0 MPa and at temperatures between 298 and 373 K. The results show a two‐step conversion of CaO to CaCO₃, involving Ca(OH)₂ as a reactive intermediate, with differing influences of the microstructures on the individual reaction sub‐steps. A kinetic evaluation of the experimental data delivers a high rate‐enhancing effect of temperature on the hydration reaction, whereas the CaCO₃ formation is strongly dependent on the available CO₂ gas pressure. With this systematic investigation the optimal pressure and temperature conditions for this reaction system can be determined delivering a contribution to a sustainable climate and energy management.

The thermochemical energy-storage material couple CuSO₄/[Cu(NH₃)₄]SO₄ combines full reversibility, application in a medium temperature interval (3 uptake. In order to limit this volume work, as well as enhance the thermal conductivity of the solid material, several composites of anhydrous CuSO₄ with inorganic inert support materials were prepared and characterized with regard to their energy storage density, reversibility of the storage reaction, thermal conductivity, and particle morphology. The best thermochemical energy storage properties were obtained for a 10:1 CuSO₄-sepiolite composite, combining an attractive energy storage density with slightly improved thermal conductivity and decreased bulk volume work compared to the pure salt.