Crystal chemistry of the orthorhombic Ln2TiO5 compounds with Ln=La, Pr, Nd, Sm, Gd, Tb and Dy

The crystal structures of seven samples of orthorhombic (Pnma) Ln(2)TiO(5) compounds with Ln=La, Pr, Nd, Sm, Gd, Tb and Dy were refined by Rietveld analysis of synchrotron X-ray powder diffraction (S-XRD) data. With increasing size of the lanthanide cation, the lattice parameters increase systematically: c by only similar to 1.5% whereas both a and b by similar to 6% from Dy2TiO5 to La2TiO5. The mean Ti-O bond length only increases by similar to 1% with increasing radius of the Ln cation from Gd to La, primarily due to expansion of the pair of Ti-O-3 bonds to opposite corners of the Ti-O-5 square based pyramid polyhedra. For Dy2TiO5 and Tb2TiO5, a significant variation in Ti-O-1 and Ti-O-4 bond lengths results in an increased deformation of the Ti-O-5 base. The particular configuration consists of large rhombic shaped tunnels and smaller triangular tunnels along the b axis, which have implications for defect formation and migration caused by radiation damage or the ionic conductivity. Crown Copyright (C) 2015 Published by Elsevier Inc. All rights reserved.