Crystal Structure and Temperature-Induced Phase Transitions in the New Copper Vanadate Cs2Cu2[V4O12]Br2

The new compound Cs2Cu2[V4O12]Br2 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 Cu2[V4O12]0 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 [V4O12]4− rings about the z axis.