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Thermal behavior of three copper molybdates, lindgrenite, Cu3(MoO4)2(OH)2, szenicsite Cu3(MoO4)(OH)4, and cupromolybdite, Cu3O(MoO4)2, has been studied by high-temperature X-ray powder diffraction in the temperature range of 25–900 °C. The decomposition of lindgrenite and formation of cupromolybdite were observed at T > 325 °C according to the reaction: Cu3(MoO4)2(OH)2 → Cu3O(MoO4)2 + H2O. Szenicsite decomposes to cupromolybdite and tenorite at T > 350 °C according to the reaction: 2Cu3(MoO4)(OH)4 → Cu3O(MoO4)2 + 3CuO + 2H2↑. Cupromolybdite is stable up to 825 °C. The thermal expansion of lindgrenite, szenicsite and cupromolybdite is strongly anisotropic; the values of αmax/αmin are 5.6, 11.0 and 4.3, respectively. The maximal/minimal thermal expansion refers to the dominant direction of long/short Cu–O bonds owing to the distortion of CuO6 octahedral geometry due to the Jahn–Teller effect. For all three Cu molybdates, there are more or less aligned orientations of Jahn–Teller-distorted (CuO6) octahedra, which allows calculating the average 〈Cu–O〉 bond lengths along particular crystallographic directions. The 〈Cu–O〉 values observed are found to be in good correlation with the observed anisotropy of thermal expansion. The additional O2− anions in cupromolybdite act as coordination centers that form four strong Cu–O bonds each. These bonds propagate along the direction of the minimal thermal expansion, thus showing that the anion-centered structural motifs in cupromolybdite are the strongest after the Mo6+-centered (MoO4) tetrahedra. [ABSTRACT FROM AUTHOR]

Thermal expansion of Ba silicates with tetrahedrally coordinated Si atoms in the temperature range of 25–1100 °C had been studied by high-temperature X-ray powder diffraction. The volume thermal expansion coefficients (TECs) are in the range 41–50×10 −6 °C −1 with an average value of 〈 α V 〉 = 45 × 10 − 6 ° C − 1 . In the structures with chain and layered silicate anions, thermal expansion is anisotropic: the direction of maximal TEC is parallel to the extension of the zweier chains of silicate tetrahedra, which are strained owing to the interactions with Ba 2+ . The strain is released during thermal expansion due to the increasing effective size of Ba 2+ induced by thermal vibrations. Information-theoretic analysis of the structural and topological complexities of Ba silicates indicates that their structural complexity is a function of the topological complexity of their silicate anions. The latter displays a non-linear behaviour with increasing SiO 2 content (=the increasing degree of polymerization and increasing dimensionality): it starts from simple topologies, reaches a maximum at topologies of intermediate complexity, and ends up at simple topologies again. The specificity of the interactions of Ba 2+ with the silicate anions results in higher complexity of high-temperature α -BaSi 2 O 5 compared to that of low-temperature β -BaSi 2 O 5 . This uncommon behaviour may be explained by the vibrational advantages provided by flatter and more complex silicate layers in the α -phase, which overcome negative differences in configurational entropies of the two modifications apparent in the differences of their structural Shannon information. [ABSTRACT FROM AUTHOR]

Thermal behavior of three copper molybdates, lindgrenite, Cu3(MoO4)2(OH)2, szenicsite Cu3(MoO4)(OH)4, and cupromolybdite, Cu3O(MoO4)2, has been studied by high-temperature X-ray powder diffraction in the temperature range of 25–900 °C. The decomposition of lindgrenite and formation of cupromolybdite were observed at T > 325 °C according to the reaction: Cu3(MoO4)2(OH)2 → Cu3O(MoO4)2 + H2O. Szenicsite decomposes to cupromolybdite and tenorite at T > 350 °C according to the reaction: 2Cu3(MoO4)(OH)4 → Cu3O(MoO4)2 + 3CuO + 2H2↑. Cupromolybdite is stable up to 825 °C. The thermal expansion of lindgrenite, szenicsite and cupromolybdite is strongly anisotropic; the values of αmax/αmin are 5.6, 11.0 and 4.3, respectively. The maximal/minimal thermal expansion refers to the dominant direction of long/short Cu–O bonds owing to the distortion of CuO6 octahedral geometry due to the Jahn–Teller effect. For all three Cu molybdates, there are more or less aligned orientations of Jahn–Teller-distorted (CuO6) octahedra, which allows calculating the average 〈Cu–O〉 bond lengths along particular crystallographic directions. The 〈Cu–O〉 values observed are found to be in good correlation with the observed anisotropy of thermal expansion. The additional O2− anions in cupromolybdite act as coordination centers that form four strong Cu–O bonds each. These bonds propagate along the direction of the minimal thermal expansion, thus showing that the anion-centered structural motifs in cupromolybdite are the strongest after the Mo6+-centered (MoO4) tetrahedra. [ABSTRACT FROM AUTHOR]