Estimation of effective ionic radii in highly defective perovskite-type oxides from experimental data

Effective ionic radii in crystals with higher concentrations of defects may considerably differ from tabulated values. For a number of perovskite-type oxides A(1-a)A(a)'B1-bBb'O-3 not equalx (A, A'=rare earth, earth alkaline, B, B'=Al, Ga, In, Zr, Ce, Cr, Mn, Fe, Co, Mg) a calculation mode for avera ge ionic radii of each sub-lattice is proposed on the basis of experimental ly determined oxygen stoichiometries (vacancy concentrations) and unit cell volumes. The effect of the vacancies on the lattice expansion is considere d. A two-dimensional radius diagram combined with Goldschmidt's tolerance f actors resulting from the effective radii represents the structure modifica tions of non-defective and cation vacant perovskite-type oxides. For anion vacant perovskite-type oxides a three-dimensional diagram with the axes r(A )-r(B)-r(O) was constructed. For more than 50 compositions of oxides the sp ecific free volumes of the unit cell are correlated with the t-factors calc ulated from the effective ionic radii. Independently on structural modifica tions of the perovskite-type and of the defect-type a linear relation betwe en V-f,V-s and t was found. The approaching character of the calculation sc heme is discussed, and the results are evaluated with regard to the gradual ly improved calculation modes of t-factors. (C) 2001 Elsevier Science B.V. All rights reserved.