THEORETICAL-STUDY OF LATTICE STABILITY AND SELECTIVE DOPING EFFECTS OF V4+ AND TB4+ IN THE ZRGEO4 LATTICE

A theoretical study on lattice stability and local geometry distortions produced by the presence of V4+ and Tb4+ ions in a ZrGeO4 lattice has been carried out by means of the ab initio perturbed ion method. Slater type orbitals for representing atomic centers and large clusters ranging from 122 to 135 atoms have been used. The energy required to substitute Zr4+ Or Ge4+ for V4+ is Similar in spite of the negative defect reaction energy calculated for V4+ entering into the dodecahedral site. The Tb4+ doping into the eightfold coordinated site requires half the energy needed for Tb4+ entering into the tetrahedral site. Positive defect reaction energies have been calculated for the substitution of Zr4+ Or Ge4+ for Tb4+. The doping process produces a decrease of force constant and vibrational frequency Values associated with the approximate breathing normal vibrational mode for the two host sites.