Structural Characterization of the β-Cu 2V 2O 7- α-Zn 2V 2O 7 Solid Solution

(Zn_2-xCu_x)V₂O₇ solid-solutions were synthesized by heating to 1000°C, cooling to 750°C, and then quenching. Powder-diffraction patterns and the single-crystal structure refinements of (Cu_0.56Zn_1.44)V₂O₇, (Cu_1.0Zn_1.0)V₂O₇, and (Cu_1.53Zn_0.47)V₂O₇ show that the solid-solution series between α-Zn₂V₂O₇ and β-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 α-Zn₂V₂O₇/β-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.