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) Ln₂TiO₅ 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 ~1.5% whereas both a and b by ~6% from Dy₂TiO₅ to La₂TiO₅. The mean Ti–O bond length only increases by ~1% with increasing radius of the Ln cation from Gd to La, primarily due to expansion of the pair of Ti–O₃ bonds to opposite corners of the Ti–O₅ square based pyramid polyhedra. For Dy₂TiO₅ and Tb₂TiO₅, a significant variation in Ti–O₁ and Ti–O₄ bond lengths results in an increased deformation of the Ti–O₅ 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. - Graphical abstract: Figure: The crystallographic study of a systematic series of compounds with nominal stoichiometry Ln₂TiO₅ (with Ln representing La, Pr, Nd, Sm, Gd, Tb and Dy) and orthorhombic, Pnma, symmetry shows changes in cell parameters which fit a linear trend. However, bond lengths are shown to deviate from trend with compounds containing the smaller, heavier lanthanides. - Highlights: • First fabrication and crystallographic refinement of compound Pr₂TiO₅. • First systematic study of the crystallography, using S-XRD, for Ln₂TiO₅ series. • Cation to anion bonding trends and valence states are investigated. • The densities and band-gaps of the series are experimentally determined