THE GEOMETRY AND ELECTRONIC-STRUCTURE OF THE CU2-TYPE COMPOUNDS ZN(MG)(1-X)CUXSB2O6 AND THE DIMORPHISM OF CUSB2O6 - A SOLID-STATE AND EPR STUDY( POLYHEDRA IN TRIRUTILE)

Results from crystal structural analyses of CuSb2O6 with the trirutile structure, which transforms from the beta phase (space group P2(1)/n) to the alpha-phase (space group P4(2)/nmn) at 380 K, are reported. Wh ile extensive twinning prevents the single crystal structure determina tion of the beta modification, the alpha phase reveals compressed CuO6 polyhedra with Cu-O spacings of 202.6 pm (2x) and 206.6 pm (4x). From the spectroscopic investigation (EPR, optical) of mixed crystals Zn(M g)(1-x)CuxSb2O6 with dependence on x and temperature it is deduced tha t the CuO6 polyhedra are compressed (spacings congruent to 197 pm (2x) and congruent to 208.5 pm (4x)) for x < 0.5 but transform to elongate d entities at larger Cu2+ concentrations (spacings 200.4 pm (2x), 201. 2 pm (2x), and 212.0 pm (2x) from neutron diffraction powder analysis (3)). Evidence for anisotropic pi-contributions to the Cu-O bond is pr esented. A detailed analysis of the ground state potential surface in terms of a vibronic Jahn-Teller coupling model in the presence of a ho st site strain is given for the two alternative CuO6 geometries. The C u-O spacings in alpha-CuSb2O6 are explained as resulting from those in the beta phase by a dynamic averaging process (201.2 pm (2x), 212.0 p m (2x) --> 206.6 pm (4x) above 380 K). (C) 1997 Academic Press.