X-ray structure refinements and strain analysis of substituted cubic lead pyrochlores Pb₂(M_2-yPb_y)O_7-δ(0.0<y<0.8; M=Nb or Ta)

Abstract

The phase diagrams in the PbO-Nb₂O₅ system and the PbO-Ta₂O₅ system depict pyrochlore structure at certain molar ratios. Compositions Pb₂Nb_1.51Pb_0.49O_6.30 (1), Pb₂Ta_1.4Pb_0.6O_6.21 (2), and Pb₂Ta_1.25Pb_0.75O_6.57 (3) belonging to this family, are refined in the cubic space group Fd3^-m (Z=8; lattice parameter a=10.762(1), 10.744(1), 10.757(5) Å, respectively) using the Rietveld refinement approach. The analyses suggest that the B-site is partially occupied by Pb leading to the general formula Pb₂(M_2-yPby)O_7-δ (0.0<y<0.8; M=Nb or Ta). There is an overall broadening observed in the X-ray peak widths in 1, 2, and 3 compared to the Pb-deficient parent phases. It is observed that the X-ray peak widths of 2 is broad, while 3 displays narrow peak widths. It is found via strain analysis that the line broadening observed correlates with the strain in the lattice.