Crystal chemistry of perovskite-type compounds in the tausonite-loparite series, (Sr_{1−2 x} Na _x La _x )TiO₃

Abstract

Perovskite-type compounds in the series tausonite-loparite, (Sr_{1−2 x} Na _x La _x )TiO₃, were synthesized by solid-state reaction (final heating at 1200–1300 °C), and studied using “conventional” and synchrotron X-ray powder diffractometry. The structures of intermediate compositions were determined using the Rietveld profile refinement method. In the compositional range 0 ≤x ≤ 0.1, the series comprises perovskites characterized by an undistorted cubic structure (space group Pm3¯m, a ≈ 3.905–3.902 Å, Z = 1). Intermediate compounds in the range 0.15 ≤ x ≤ 0.35 crystallize with tetragonal symmetry (I4/mcm, a ≈ , c ≈ , Z = 4) derived from the cubic aristotype by antiphase rotation of the TiO₆ octahedra about a fourfold axis. The angle of rotation estimated from the positional parameters of oxygen atoms ranges from 2.5(7)° to 5.5(4)°. The cubic-to-tetragonal transition arises from substitution of Sr²⁺ by the comparatively smaller Na¹⁺ and La³⁺ cations. A further transition from the tetragonal to rhombohedral symmetry (R3¯c, a ≈ , c ≈ 2, Z = 6) occurs between x = 0.35 and 0.40, and apparently does not involve formation of perovskite with an intermediate two-tilt structure (Imma). The rhombohedral structure is characterized by a multicomponent octahedral tilt about a threefold axis ranging in magnitude from 6.5(2)° to 7.7(2)°. In the series (Sr_{1−2 x} Na _x La _x )TiO₃, the unit-cell dimensions decrease, and the degree of structural distortion increases with x.