Der Strukturtyp des Perowskits (CaTiO3)

KRb2(NO3)2Cl: a new birefringent crystal exhibiting a perovskite-related framework and a short cutoff edge

The combination of π-conjugated groups [NO3] and Cl-centered polyhedra generates a new birefringent crystal with a perovskite-related framework, KRb2(NO3)2Cl, which is the first alkali metal nitrate chloride synthesized by a mild hydrothermal method. It crystallizes in the orthorhombic space group pbam (no. 55). In addition, KRb2(NO3)2Cl crystals with dimensions up to 7 × 1.5 × 1 mm3 were grown. Notably, KRb2(NO3)2Cl has a short UV cut-off edge (below 228 nm) and a significantly enhanced birefringence (Δn = 0.084 at 1064 nm). Theoretical calculations indicate that the birefringence enhancement mainly derives from π-conjugated [NO3] plane triangles.

Evaluation of Calcium Aluminate Slags and Pig Irons Produced from the Smelting-Reduction of Diasporic Bauxite

This work evaluates the characteristics of calcium aluminate slag and pig iron samples obtained from the smelting of calcined and reduced diasporic bauxite ore. The study is conducted in the Pedersen process framework, which is a method to produce alumina from low-grade resources. Parameters such as the effect of crucible type, lime addition, and atmospheric conditions are studied considering the characteristics of the product pig irons and calcium aluminate slags for further uses. The behavior of the bauxite and distribution of the species between slag and metal was assessed based on the applied analytical techniques and thermodynamic calculations. Iron was reduced and separated from the slags in the presence of carbon (graphite crucible) for both the reduced and calcined bauxite. Si and Ti were mainly concentrated in the slags. Iron was separated from the slag in the absence of carbon (alumina crucible) for the H₂-reduced bauxite. The results show that slags with increased lime additions are composed mainly of 5CaO.Al₂O₃ and CaO.Al₂O₃, that are considered highly leachable compounds. An optimum CaO/Al₂O₃ mass ratio of 1.12 was suggested. The presence of O₂ and/or OH^- in the furnace atmosphere will result in the formation of 12CaO.7Al₂O₃.

Probing cation and vacancy ordering in the dry and hydrated yttrium-substituted BaSnO3 perovskite by NMR spectroscopy and first principles calculations: implications for proton mobility

Hydrated BaSn(1-x)Y(x)O(3-x/2) is a protonic conductor that, unlike many other related perovskites, shows high conductivity even at high substitution levels. A joint multinuclear NMR spectroscopy and density functional theory (total energy and GIPAW NMR calculations) investigation of BaSn(1-x)Y(x)O(3-x/2) (0.10 ≤ x ≤ 0.50) was performed to investigate cation ordering and the location of the oxygen vacancies in the dry material. The DFT energetics show that Y doping on the Sn site is favored over doping on the Ba site. The (119)Sn chemical shifts are sensitive to the number of neighboring Sn and Y cations, an experimental observation that is supported by the GIPAW calculations and that allows clustering to be monitored: Y substitution on the Sn sublattice is close to random up to x = 0.20, while at higher substitution levels, Y-O-Y linkages are avoided, leading, at x = 0.50, to strict Y-O-Sn alternation of B-site cations. These results are confirmed by the absence of a "Y-O-Y" (17)O resonance and supported by the (17)O NMR shift calculations. Although resonances due to six-coordinate Y cations were observed by (89)Y NMR, the agreement between the experimental and calculated shifts was poor. Five-coordinate Sn and Y sites (i.e., sites next to the vacancy) were observed by (119)Sn and (89)Y NMR, respectively, these sites disappearing on hydration. More five-coordinated Sn than five-coordinated Y sites are seen, even at x = 0.50, which is ascribed to the presence of residual Sn-O-Sn defects in the cation-ordered material and their ability to accommodate O vacancies. High-temperature (119)Sn NMR reveals that the O ions are mobile above 400 °C, oxygen mobility being required to hydrate these materials. The high protonic mobility, even in the high Y-content materials, is ascribed to the Y-O-Sn cation ordering, which prevents proton trapping on the more basic Y-O-Y sites.

Temperature changes in the crystal structure of barium titanium oxide

Barium titanium oxide, which is tetragonal at room temperature, changes about 120° C to a cubic structure. This change has been followed in detail by means of X-ray powder photo­graphs taken in a 19 cm. powder camera at intervals of a few degrees over a range covering the transition point. The unit cell, which contains the formula number of atoms, retains its identity throughout the transition, and the atomic parameters are unaltered. The change is simply in the axial lengths, and these vary continuously with the temperature, though not linearly, the varia­tion becoming more rapid near the transition point. While the linear expansion coefficients along and perpendicular to the tetrad axis are large and of opposite sign, the volume expan­sion coefficient is small and positive. There is no discontinuous change either of linear spacing or of volume detectable at the transition point, but there is a sharp discontinuity in the linear expansion coefficients, and a marked increase in the volume expansion coefficient which is probably, though not certainly, discontinuous. The transition suggests a typical λ-point change. The specific heat has not been deter­mined, but the thermal expansion curve has the characteristic λ shape. Co-existence of cubic and tetragonal structures, in proportions depending on the temperature, occurs over a range of some degrees near the transition point, and is attributed to the effect of local stresses in facilitating or hindering a change between two structures whose energy difference is very small in this temperature range. Below room temperature, observations made down to -183° C suggest that the structure may have a second transition point somewhere below this and become cubic again, the change being of the same nature as that at 120° C. It is argued that the room-temperature structure can only be explained by the existence of directed bonds, and that the breaking of these bonds with increasing temperature is respon­sible for the 120°C transition. The low-temperature transition is explained by postulating a more complete set of bonds, probably an octahedral complex, which partially breaks down at this temperature to give the square formation observed in the room-temperature structure. The possible nature of the directed bonds is discussed qualitatively. The condition which makes possible the formation of such bonds is likely to be the abnormal volume available to the Ti atom, which is due to the effect of the large Ba ion in forcing apart the oxygen lattice. The directed bond system will only contribute a small part to the attractive energy of the lattice, which is mainly ionic in character. The hypothesis that directed bonds exist, whatever their origin, is used for a tentative explanation of anomalous variations of intensity of the X-ray lines observed at temperatures near the transition point.