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

The new ALn2CuTi2O9 (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. CaLn2CuTi2O9 samples are isostructural, crystallizing at room temperature in an orthorhombic cell with space group Pbnm (ITA No. 62); and BaLn2CuTi2O9 samples in the tetragonal I4/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/TiO6 octahedra tilting. By comparing the room temperature symmetry structure with an archetype (Pm3¯m, ITA, No. 221) phase of Pbnm or I4/mcm   symmetry, two distortion modes R4+ and M3+ were found to be dominant for the orthorhombic phase; whereas for tetragonal phase there is a unique active mode, R4+. They correspond to Cu/TiO6 octahedron tilt modes, commonly found in many perovskite type materials. The high temperature study for CaLa2CuTi2O9, CaNd2CuTi2O9 and CaSm2CuTi2O9 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.

► Crystal structure of CaLn2CuTi2O9 (Pr, Nd, Sm) and BaLn2CuTi2O9 (Ln = La, Pr, Nd) is determined.
► Room-temperature orthorhombic and tetragonal structural determination by symmetry-mode analysis.
► Identification of the irreducible representations participating in the symmetry breaking for pure crystallographic purposes.