A structural study of the perovskite series Na 0.75Ln0.25Ti 0.5Nb 0.5O 3

Abstract

The ternary stoichiometric perovskite compounds, Na 0.75 Ln 0.25Ti 0.5Nb 0.5O 3 ( Ln = La , Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm) are intermediate members of the NaNbO 3–Na 0.5 Ln 0.5TiO 3 solid solution series. The compounds were synthesized by standard ceramic methods at 1300 °C followed by annealing at 800 °C and quenching to ambient conditions. Rietveld analysis of the powder X-ray diffraction patterns shows that the compounds with Ln ranging from Pr to Tm adopt the orthorhombic space group Pbnm ( a ≈ b ≈ √ 2 a p ; c ≈ 2 a p ; Z = 4 ) and the GdFeO 3 structure. In contrast, Na 0.75La 0.25Ti 0.5Nb 0.5O 3 adopts the orthorhombic space group Cmcm ( a ≈ b ≈ c ≈ 2 a p ; Z = 4 ). All cations located at the A- and B-sites are disordered in these compounds. The unit cell parameters and cell volumes of the compounds decrease regularly with increasing atomic number of the Ln cation. The Pbnm compounds with Ln from Sm to Tm have A-site cations in eight-fold coordination. A-site cations in the Pr and Nd compounds are considered to be in ten-fold coordination. Analysis of the crystal chemistry of the Pbnm compounds shows that B-site cations enter the second coordination sphere of the A-site cations for compounds with Ln from Tb to Tm as the A–B intercation distances are less than the maximum A– IIO(2) bond lengths. The [111] tilt angles of the (Ti,Nb)O 6 polyhedra in the Pbnm compounds increase with increasing atomic number from 11.1° to 15.8° and are less than those observed in lanthanide orthoferrite and orthoscandate perovskites. These data are considered as relevant to the sequestration of lanthanide fission products in perovskite and the structure of lanthanide-bearing perovskite-structured minerals.