Jahn–Teller distortion and thermal expansion anisotropy: temperature-dependent behavior of lindgrenite, Cu3(MoO4)2(OH)2, szenicsite, Cu3(MoO4)(OH)4, and cupromolybdite, Cu3O(MoO4)2

Abstract

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.