A structural study of the perovskite series Ca 1−xNa xTi 1−xTa xO 3

Abstract

Compounds in the solid solution series Ca 1− x Na x Ti 1− x Ta x O 3 were synthesized at 1300 °C, followed by annealing at 850 °C or 800 °C with quenching and/or slow cooling to room temperature. Rietveld refinement of their powder X-ray diffraction patterns show that all compounds are single-phase ternary perovskites which adopt the space group Pbnm ( a≈b≈√2 a p; c≈2 a p; Z=4) at ambient conditions. The unit cell parameters and cell volumes of the compounds increase regularly with increasing values of x. The coordination of the A-site cations changes throughout the series from eight for CaTiO 3 to nine for NaTaO 3. Compounds with 0⩽ x ⩽0.4 have A-site cations in eight fold coordination, whereas the coordination of those with 0.4< x<0.9 is ambiguous. Analysis of the crystal chemistry of the compounds shows that the change in coordination at x=0.4 is related to the departure of the B-site cations from the second coordination sphere of the A-site cations, as in compounds with x>0.4 the A– IIO distances become less than the A–B intercation distances. Contemporaneous with these coordination changes, the tilt angles of the BO 6 polyhedra decrease with increasing values of x. This solid solution series is unusual in that these structural and coordination changes occur regardless that Goldschmidt tolerance factors remain essentially constant at approximately 0.89, and observed tolerance factors, assuming eight fold coordination of the A-site cations, range only from 0.91 to 0.93 (0⩽ x⩽0.8).