Crystal chemistry of perovskite-type compounds in the tausonite-loparite series, (Sr_(1-2x)Na_xLa_x)TiO_3

摘要

Perovskite-type compounds in the series tausonite-loparite, (Sr_(1-2x)Na_xLa_x)TiO_3, were synthesized by solid-state reaction (final heating at 1200-1300 ℃), 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 Pm(3-bar)m, a ≈ 3.905 - 3.902 A, Z = 1). Intermediate compounds in the range 0.15 ≤ x ≤ 0.35 crystallize with tetragonal symmetry (14/mcm, a ≈ (2a_p)~(1/2), c ≈ (2a_p)~(1/2), Z = 4) derived from the cubic aristotype by antiphase rotation of the TiO_6 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 Sr~(2+) by the comparatively smaller Na~(1+) and La~(3+) cations. A further transition from the tetragonal to rhombohedral symmetry (R(3-bar)c, a ≈ (2a_p)~(1/2), c ≈ 23~(1/2)a_p, 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_(1-2x)Na_xLa_x)TiO_3, the unit-cell dimensions decrease, and the degree of structural distortion increases with x.