Structural Characterization of the beta-Cu2V2O7–alpha-Zn2V2O7 Solid Solution

(Zn2-xCux)V2O7 solid-solutions were synthesized by heating to 1000°C, cooling to 750°C, and then quenching. Powderdiffraction patterns and the single-crystal structure refinements of (Cu0.56Zn1.44)V2O7, (Cu1.0Zn1.0)V2O7, and (Cu1.53Zn0.47) V2O7 show that the solid-solution series between alpha-Zn2V2O7 and beta-Cu2V2O7 is complete and that there is no phase transition. With substitution of Zn by Cu2+, a and b lattice constants increase and decrease, respectively, while c lattice constant and cell volume (V) do not change. This results from elongation of the (MO5) square pyramid (M=Zn, Cu2+) and from rotation of the vanadate tetrahedra. In order to form the elongated square-pyramid that must accompany increasing substitution of Zn by Cu2+, the apical M–O bond length increases and decreases its incident bond valence at M2+. The resulting bond-valence deficit is compensated by shortening of the equatorial M–O bond lengths. The response of the alphaZn2V2O7/beta-Cu2V2O7 framework to the strain produced by elongation of the square pyramid involves coupled clockwise and counter clockwise rotations of the (VO4) tetrahedra, accounting for the constant cell volumes and increase and decrease of the a and b lattice constants, respectively. This cooperative response is possible because there are no symmetry restrictions on the rotation of the (VO4) tetrahedra.