Vergasovaite to cupromolybdite topotactic transformation with crystal shape preservation

Abstract

Thermal behavior of vergasovaite, ideally Cu 3 O(SO 4 )(MoO 4 ), and its synthetic analogue has been studied by high-temperature single crystal X-ray diffraction in the temperature range of 300-1100 K. According to the EMPA results, the empirical formulas are (Cu 2.36 Zn 0.61 ) ∑=2.97 O[(Mo 0.91 S 0.08 V 0.04 ) ∑=1.03 O 4 ](SO 4 ) for vergasovaite and Cu 2.97 O[(Mo 0.92 S 0.09 ) ∑=1.01 O 4 ](SO 4 ) for its synthetic analogue. The mineral is stable until 950±15 K; at 975 K, the unit-cell parameters and volume increase abruptly due to topotactic transformation of vergasovaite into cupromolybdite, Cu 3 O(MoO 4 ) 2 . The transformation is accompanied by loss of sulfur (and excess copper) without destruction of the crystal. The thermal expansion of the vergasovaite structure is strongly anisotropic, being minimal along the [O 2 Cu 6 ] 8+ chains comprised of vertex-sharing OCu 4 tetrahedra. The peculiarities of the thermal expansion of vergasovaite can be explained by the anisotropy of bond length evolution in the Cu1O 6 and Cu3O 6 octahedra and flexibility of the S-O-Cu and Mo-O-Cu bond angles. Synthetic Zn-and V-free analogue demonstrates negative thermal expansion at 425-625 K and melts at as low temperature as 700 K with no indication of transformation or recrystallization at least below 1200 K.