, Volume 27, Issue 8, pp 583-589

Crystal chemistry of perovskite-type compounds in the tausonite-loparite series, (Sr1−2 x Na x La x )TiO3

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Abstract

Perovskite-type compounds in the series tausonite-loparite, (Sr1−2 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 Pmm, 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 (Rc, 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 (Sr1−2 x Na x La x )TiO3, the unit-cell dimensions decrease, and the degree of structural distortion increases with x.

Received: 12 July 1999 / Accepted: 1 March 2000