Bond valence analysis of tetragonal zirconias

In tetragonal zirconia, the cation is coordinated by two interpenetrating t etrahedra of oxygen ions, implying two different cation-oxygen bond lengths . On substituting the different tetravalent ions Ge, Ti, Sn, and Ce into te tragonal ZrO2-2 mol% Y2O3, the mean value of the shorter cation-anion bond length varies linearly with the concentration of the substituent ion where the bond length increases or decreases depending on whether the substituted ion is larger or smaller than the zirconium ion it replaces. It is argued in this paper that the length of the longer bond is determined by the requi rement that the bond valence sum remains constant. In each case the length of the longer bond calculated on this basis is in good agreement with the m easured bond length (from neutron diffraction), and following small adjustm ents of the bond valence constants, excellent agreement is obtained. The re quirement for the bond valence sum evidently accounts for the physics of th e situation, and at the same time the available bond length data allow very precise determination of the bond valence constants of the different ions in the tetragonal zirconia environment, It is shown how these bond length c onsiderations provide an explanation for the variation with composition of oxygen position and lattice parameters in all of the materials considered. Among the interesting features accounted for by this analysis are the incre ase in cell volume occurring when Zr is replaced by the smaller Sn ion, and slight departures from Vegard's law observed in the substitution of Zr by Ti. (C) 1999 Academic Press.