STRUCTURAL RELATIONS IN MIXED OXIDES CUXZN1-XNB2O6

Crystal structures of monoclinic CuNb2O6 and mixed niobates CuxZn1-xNb 2O6, with x = 0.85 and 0.36, respectively, have been solved by neutron powder and X-ray single-crystal diffraction. Space groups and cell di mensions are P2(1)/c, a = 5.0064(1) Angstrom, b = 14.1733(3) Angstrom, c = 5.7615(1) Angstrom, beta = 91.672(1)degrees for monoclinic CuNb2O 6; P2(1)/c, a = 5.0070(1) Angstrom, b = 14.1706(2) Angstrom, c = 5.754 7(1) Angstrom, and beta = 91.451(1)degrees for Cu0.85Zn0.15Nb2O(6); an d Pbcn, a = 14.187(5) Angstrom, b = 5.730(2) Angstrom, and c = 5.031(2 ) Angstrom for Cu0.36Zn0.64Nb2O6. The crystal structure of orthorhombi c CuNb2O6 was confirmed by neutron powder and X-ray single-crystal dif fraction. The orientations of the O-Cu-O axes, elongated due to Jahn-T eller distortion, differ in a characteristic way for orthorhombic CuNb 2O6 on one hand and monoclinic CuNb2O6 and the monoclinic mixed oxides on the other hand. For Cu0.36Zn0.64Nb2O6 no elongated axes were found by diffraction methods. A thermally or stress-induced, irreversible t ransformation from monoclinic to orthorhombic CuNb2O6 has been observe d. (C) 1995 Academic Press, Inc.