Four compounds of the Bi 4 Ti 3 O 12 /BiFeO 3 system with the formula Bi 2 Bi nϪ1 (Ti,Fe) n O 3nϩ3 , n ϭ 3, 4, 4.5 and 6 were studied using high-temperature X-ray powder diffraction and differential thermoanalysis methods. The crystal structure of the n ϭ 6 phase was refined by the Rietveld method. An unusual behaviour of thermal expansion attributed to an orthorhombic-to-tetragonal transformation was revealed. For all the compounds, the lattice parameter c vs temperature T dependence has three regions in the range of T ϭ 20 Ϫ750°C interpreted as (1) expansion of the initial orthorhombic phase, (2) a pronounced structure reconstruction to the tetragonal phase, (3) an expansion of the tetragonal phase. 1603The crystal structure of Bi 7 Ti 3 Fe 3 O 21 based on 6-layer-perovskite blocks is proposed from X-ray powder diffraction data. The Rietveld refinement of the structure in the orthorhombic space group F2mm with lattice parameters a ϭ 5.4699(3), b ϭ 5.4924(3), c ϭ 57.551(3) Å (R p ϭ 9.4, R wp ϭ 11.9, R exp ϭ 4.7, R B ϭ 4.4 %) shows that a distorted 6-layer model fits the data of Bi 7 Ti 3 Fe 3 O 21 .

The paper presents a brief review of the present state of high-temperature borate crystal chemistry. This review summarizes the results of high- and low-temperature single crystal X-ray diffraction studies for more than 10 borate structures and high-temperature powder Xray diffraction data for about 65 borates. Thermal behavior of their crystal structures, thermal expansion, polymorphic transitions and their relationship to borate glasses are presented. These studies allow to formulate the basic principles of high-temperature borate crystal chemistry and to reveal the regularities of thermal behavior of borates. On heating, the BO

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The new mineral saranchinaite, ideally Na2Cu(SO4)2, was found in sublimates of the Saranchinaitovaya fumarole, Naboko Scoria Cone, Tolbachik volcano, Kamchatka, Russia. Its discovery and study has enabled the characterization of the thermal decomposition of kröhnkite and provided an insight into the high-temperature behaviour of other kröhnkite-type materials. Saranchinaite is monoclinic, P21, a = 9.0109(5), b = 15.6355(8), c = 10.1507(5) Å, β = 107.079(2)°, V = 1367.06(12) Å3, Z = 8 and R1 = 0.03. Saranchinaite is a unique mineral in that two of its four independent Cu sites display a very unusual Cu2+ coordination environment with two weak Cu–O bonds of ~2.9–3.0 Å, resulting in [4+1+2] CuO7 polyhedra. Each of the Cu-centred polyhedra shares common corners with SO4 tetrahedra resulting in a [Cu4(SO4)8]8– framework with a complex channel system occupied by Na atoms. Saranchinaite is sensitive to moisture and transforms into kröhnkite within one week when exposed to open air at 87% relative humidity and 25°C. High-temperature X-ray diffraction studies were performed for both kröhnkite (from La Vendida mine, Antofagasta Region, Chile) and saranchinaite. During thermal expansion kröhnkite retains its strongly anisotropic character up to its full dehydration and the formation of saranchinaite at ~200°C, which then transforms back into kröhnkite after exposure to open air. The thermal expansion of saranchinaite is more complex than that of kröhnkite. Saranchinaite is stable up to 475°C with subsequent decomposition into tenorite CuO, thénardite Na2SO4 and unidentified phases.

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The paper summarizes results of high‐temperature single crystal X‐ray diffraction studies for six borate structures and high‐temperature powder X‐ray diffraction data for about 50 borates. The characteristics on thermal expansion of 50 borates of univalent, bivalent and trivalent metals of different dimensionality and anion structure are listed. The basic principles of high‐temperature borate crystal chemistry emerge from these studies and the regularities of borate thermal behavior are revealed. On heating the BO3 and BO4 polyhedra and rigid groups consisting of these polyhedra practically do not change their configuration and size but sometimes rotate like hinges exhibiting greatly anisotropic thermal expansion including linear negative expansion. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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