Structural analysis of AgxCu1-xI (0 <= x <= 0.5) by solid Cu-63 NMR and X-ray diffraction methods

Cu-63 NMR and X-ray diffraction methods have been used to investigate the t emperature dependence of the nuclear chemical shifts and the diffraction pa tterns, respectively, for mixed crystal AgxCu1-xI (0 < x < 0.5). We indicat e from the X-ray diffraction method that the lattice constants increase gra dually a little with the temperature, except for the range of the gamma-to- alpha' phase transition. For NMR experiments, the chemical shifts show a li near dependence of temperature to the high-field in the lower range of abou t 150-350 K, and the shift increases nonlinearly with the higher temperatur e (350-500 K). The high-held shifts above 350 K seem to depend on the catio n disorder which occurs with the fact that a part of the tetrahedral cation s move to the qausi-stable interstitial octahedral sites. The observed high -field shifts are explained by Cu chemical shielding calculations of TdCuI4 3- and OhCuI65- species, as obtained by the finite perturbation theory in a n ab initio GAUSSIAN 94 program using double-zeta basis set for Cu and I at oms. In the case of the nonlinear temperature dependence of the shift, we i ntroduce the shift model as proposed by Negita and coworkers: The shift is also due to the cation which moves to the qausi-stable interstitial octahed ral sites. (C) 2000 Elsevier Science B.V. All rights reserved.