On the Polymorphism of Cu2V2O7: Synthesis and Crystal Structure of δ-Cu2V2O7, a New Polymorph

Single crystals of the new modification of copper pyrovanadate, delta-Cu2V2O7, were prepared using the chemical vapor transport reaction method. The crystal structure (monoclinic, P2(1)/n, a = 5.0679(3), b = 11.4222(7), c = 9.4462(6) & Aring;, beta = 97.100(6)degrees, V = 542.61(6) & Aring;(3), Z = 4) was solved by direct methods and refined to R-1 = 0.029 for 1818 independent observed reflections. The crystal structure contains two Cu sites: the Cu1 site in [4 + 2]-octahedral coordination and the Cu2 site in [4 + 1]-tetragonal pyramidal coordination. There are two V5+ sites, both tetrahedrally coordinated by O atoms. Two adjacent V1O(4) and V2O(4) tetrahedra share the O4 atom to form a V2O7 dimer. The crystal structure of delta-Cu2V2O7 can be described as based upon layers of V2O7 dimers of tetrahedra parallel to the (001) plane and interlined by chains of the edge-sharing Cu1O(6) and Cu2O(5) polyhedra running parallel to the a axis and arranged in the layers parallel to the (001) plane. The crystal chemical analysis of the three other known Cu2V2O7 polymorphs indicates that, by analogy with delta-Cu2V2O7, they are based upon layers of V2O7 groups interlinked by layers consisting of chains of CuOn coordination polyhedra (n = 5, 6). The crystal structures of the Cu2V2O7 polymorphs can be classified according to the mutual relations between the Cu-O chains, on the one hand, and the V2O7 groups, on the other hand. The analysis of the literature data and physical density values suggests that, at ambient pressure, alpha- and beta-Cu2V2O7 are the low- and high-temperature polymorphs, respectively, with the phase transition point at 706-710 degrees C. The beta-phase (ziesite) may form metastably under temperatures below 560 degrees C and, under heating, transform into the stable alpha-phase (blossite) at 605 degrees C. The delta- and gamma-polymorphs have the highest densities and most probably are the high-pressure phases. The structural complexity relations among the polymorphs correspond to the sequence alpha = beta < gamma < delta; i.e., the delta phase described herein possesses the highest complexity, which supports the hypothesis about its stability under high-pressure conditions.