Question

The ideal perovskite (ABO3) structure consists of AOs closest packed layers (ordered Am cations and O2-anions) with B cations in ¼ of the octahedral holes. What pairs of charges are possible for A and B (there is more than one combination of charges)? For each pair, say which cation will likely sit in the closest packed layers and which will likely sit in the holes. Without specific examples of ions (e.g., Ca, Ti+), base your answer around the fact that in general, the more positive the charge, the smaller the cation. Finally which of these pairs would you expect to adopt an ideal cubic perovskite structure, versus a distorted one, where the coordination numbers are changed from those in the ideal perovskite structure. 4.

Answer 1

A pervoskite unit cell is as follows: - AO_3 rightarrow closest pack layrs (Face centred cubic) Two possible combination (a) & (b) are possible for two different cations of A^m + & B^n + When charges will be + 2 for A and + 4 for B then A will sit in the closest pack layer (larger cation). B will sit in the hole (smaller cation). Refer the above image \r\n A^2 + B^4 + O_3 type pervoskite sturcture has ideal cubic structure as the mentioned diagram When the charges are equal i.e. + 3 for both cations A & B, then the pervoskite will be distorted. (e.g. LaTiO_3 for reference). As the charges are equal for both cations, the effective charge (Z_eff) will decide the radius of cation The cation which will have lowest radii will occupy the hole. This pair of cations will form distorted pervoskite structure (A^3 + & B^3 +)