Crystal Structure and Temperature-Induced Phase Transitions in the New Copper Vanadate Cs2Cu2V4O12Br2

The new compound Cs2Cu2V4O12Br2 was synthesized by the chemical vapor transport reaction method. Structural data obtained by single-crystal X-ray diffraction in the temperature range 100–700 K revealed three successive (with decreasing temperature) structural phase transitions: from the high-temperature aristotype structure I4/mmm (>550 K) to the polymorph P4/mnc (550–340 K), then to P4/m (340–300 K), and finally to the low-temperature phase I4/m (<300 K). The crystal structure of the new compound is based upon the Cu2V4O120 layers, consisting of four-membered rings of corner-sharing vanadate tetrahedra linked by CuO4 squares. Analysis of the structural evolution with increasing temperature shows that the entire sequence of phase transitions is governed by the rotation of the V4O124− rings about the z axis.