V4- AN AB-INITIO PERTURBED ION STUDY( DOPING INTO SIO2, ZRO2 AND ZRSIO4 STRUCTURES )

An ab initio perturbed ion study using X-ray diffraction data has been carried out for ZrSiO4 (zircon), ZrO2 (monoclinic zirconia, baddeleyi te), and SiO2 (alpha-cristobalite) crystal lattice structures. The dif ferent substitutions of V4+ for Zr4+ and Si4+ occurring in these host lattices have been analyzed. Geometry optimizations have been performe d with the aim of determining the relative stability, cell parameters, and force constants of radial displacement associated with the local relaxation for pure and doped structures. Numerical results are confro nted against experimental data and compared with previous results. The geometrical call parameters of different structures obtained by compu ter simulation and the results of the X-ray diffraction studies agree with previous experimental data. For the zircon lattice, the substitut ion of V4+ for Zr4+ at an eightfold-coordinated site is energetically favorable while the substitution of V4+ for Si4+ at a fourfold-coordin ated site is unstable. For ZrO2, the substitution of V4+ for Zr4+ is e nergetically favorable while the substitution of V4+ for Si4+ in SiO2 is energetically unfavorable. There is less sensitive influence of the crystal lattice parameters for substitutions occurring at the eightfo ld-coordinated ion site in ZrSiO4 and SiO2 structures. The doping proc ess produces a decrease of force constant (k) values associated with t he breathing fundamental vibrational made for all structures. The k as sociated with the radial displacement in dodecahedral substitution in the ZrSiO4 structure is especially high. The force constants for this movement in tetrahedral substitution in the ZrSiO4, ZrO2, and SiO2 str uctures have a lower value. The differences between ionic radii report ed by Shannon and Prewitt of the species concerned in the doping proce ss are not capable of explaining the relaxation of crystal lattice par ameters in the ZrO2 and SiO2 structures. (C) 1993 John Wiley & Sons, I nc.