Crystal chemistry of perovskite-type compounds in the tausonite-loparite series, (Sr1-2xNaxLax)TiO3

Perovskite-type compounds in the series tausonite-loparite, (Sr1-2xNaxLax)T iO3, were synthesized by solid-state reaction (final heating at 1200-1300 d egrees C), and studied using "conventional" and synchrotron X-ray powder di ffractometry. The structures of intermediate compositions were determined u sing the Rietveld profile refinement method. In the compositional range 0 l ess than or equal to x less than or equal to 0.1, the series comprises pero vskites characterized by an undistorted cubic structure (space group Pm (3) over bar m, a approximate to 3.905 3.902 Angstrom, Z = 1). Intermediate co mpounds in the range 0.15 less than or equal to x less than or equal to 0.3 5 crystallize with tetragonal symmetry (I4/mcm, a approximate to root 2a(p) , c approximate to root 2a(p), Z = 4) derived from the cubic aristotype by antiphase rotation of the TiO6 octahedra about a fourfold axis. The angle o f rotation estimated from the positional parameters of oxygen atoms ranges from 2.5(7)degrees to 5.5(4)degrees. The cubic-to-tetragonal transition ari ses from substitution of Sr2+ by the comparatively smaller Na1+ and La3+ ca tions. A further transition from the tetragonal to rhombohedral symmetry (R (3) over bar c, a approximate to root 2a(p), c approximate to 2 root 3a(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 til t about a threefold axis ranging in magnitude from 6.5(2)degrees to 7.7(2)d egrees. In the series (Sr1-2xNaxLax)TiO3, the unit-cell dimensions decrease , and the degree of structural distortion increases with x.