A structural study of the CaLn2CuTi2O9 (Ln=Pr, Nd, Sm) and BaLn2CuTi2O9 (Ln=La, Pr, Nd) triple perovskite series

The new ALn 2 CuTi 2 O 9 ( A = Ca, Ba; Ln = La, Pr, Nd, Sm) series have been synthesized and the structures have been characterized by X-ray and neutron powder diffraction techniques. The whole series show a cationic disordered distribution over the A - and B -sites of the perovskite structure. Ca Ln 2 CuTi 2 O 9 samples are isostructural, crystallizing at room temperature in an orthorhombic cell with space group Pbnm (ITA No. 62); and Ba Ln 2 CuTi 2 O 9 samples in the tetragonal I 4/ mcm (ITA No. 140) space group. The homovalent substitution of a larger lanthanide cation leads to an increase in the unite-cell volume, but a decrease in the Cu/TiO 6 octahedra tilting. By comparing the room temperature symmetry structure with an archetype ( Pm 3 ¯ m , ITA, No. 221) phase of Pbnm or I 4/ mcm symmetry, two distortion modes R 4 + and M 3 + were found to be dominant for the orthorhombic phase; whereas for tetragonal phase there is a unique active mode, R 4 + . They correspond to Cu/TiO 6 octahedron tilt modes, commonly found in many perovskite type materials. The high temperature study for CaLa 2 CuTi 2 O 9 , CaNd 2 CuTi 2 O 9 and CaSm 2 CuTi 2 O 9 materials (300–1670 K) showed that the room temperature Pbnm orthorhombic symmetry seems to be stable up to 1475 K; and irreversible phase transition is observed above that temperature.