Results from crystal structural analyses of CuSb 2 O 6 with the trirutile structure, which transforms from the phase (space group P2 1 /n) to the -phase (space group P4 2 /nmn) at 380 K, are reported. While extensive twinning prevents the single crystal structure determination of the modification, the phase reveals compressed CuO 6 polyhedra with Cu-O spacings of 202.6 pm (2؋) and 206.6 pm (4؋). From the spectroscopic investigation (EPR, optical) of mixed crystals Zn(Mg) 1؊x Cu x Sb 2 O 6 with dependence on x and temperature it is deduced that the CuO 6 polyhedra are compressed (spacings 197 pm (2؋) and 208.5 pm (4؋)) for x(0.5 but transform to elongated entities at larger Cu 2؉ concentrations (spacings 200.4 pm (2؋), 201.2 pm (2؋), and 212.0 pm (2؋) from neutron diffraction powder analysis (3)). Evidence for anisotropic -contributions to the Cu-O bond is presented. A detailed analysis of the ground state potential surface in terms of a vibronic Jahn-Teller coupling model in the presence of a host site strain is given for the two alternative CuO 6 geometries. The Cu-O spacings in -CuSb 2 O 6 are explained as resulting from those in the phase by a dynamic averaging process (201.2 pm (2؋), 212.0 pm (2؋)P 206.6 pm (4؋) above 380 K).