Zn 28x Cu x )V 2 O 7 solid-solutions were synthesized by heating to 10003C, cooling to 7503C, and then quenching. Powder-di4raction patterns and the single-crystal structure re5nements of (Cu 0.56 Zn 1.44 )V 2 O 7 , (Cu 1.0 Zn 1.0 )V 2 O 7 , and (Cu 1.53 Zn 0.47 )V 2 O 7 show that the solid-solution series between -Zn 2 V 2 O 7 and -Cu 2 V 2 O 7 is complete and that there is no phase transition. With substitution of Zn by Cu 2؉ , 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 (MO 5 ) square pyramid (M ؍ Zn, Cu 2؉ ) and from rotation of the vanadate tetrahedra. In order to form the elongated squarepyramid that must accompany increasing substitution of Zn by Cu 2؉ , the apical M}O bond length increases and decreases its incident bond valence at M 2؉ . The resulting bond-valence de5cit is compensated by shortening of the equatorial M}O bond lengths. The response of the -Zn 2 V 2 O 7 / -Cu 2 V 2 O 7 framework to the strain produced by elongation of the square pyramid involves coupled clockwise and counter clockwise rotations of the (VO 4 ) 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 (VO 4 ) tetrahedra.