An EXAFS study of interionic distances in complex lanthanide oxides

When the smaller vanadium ion is substituted for niobium in NdNbO₄, the volume of the unit cell is almost unchanged; in the lanthanum and cerium counterparts the volume actually increases. In order to understand these phenomena we have employed the technique of extended x-ray absorption fine structure (EXAFS) to determine all of the near neighbor anion-cation distances in the series of mixed-B-ion oxides, NdNb_1?xV_xO₄, including the end-member compounds, NdNbO₄ and NdVO₄. We find that the average Nb-O, V-O and Nd-O interionic distances in the mixed compounds closely resemble the distances exhibited in the corresponding end-member compounds, but there are large variances associated with these averages. We take this to mean that, while crystal symmetry is not broken by the substitution of the smaller V⁵⁺ ion, there are substantial local distortions in bond lengths and angles in order to meet the ion-size requirements of immediate neighbors.