A structural study of ternary lanthanide orthoscandate perovskites

Ternary lanthanide scandates (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho) have been synthesized at ambient pressure. Their structure has been investigated at room temperature by Rietveld analysis of powder X-ray diffraction data. The Ln-scandates are orthorhombic perovskites, adopting space group Pbnm (# 62), a approximate to b approximate to root2a(p), c approximate to 2a(p), Z = 4. Heavy lanthanides (Er-Lu), and Y do not form perovskites at ambient conditions. Compositionally driven phase transitions were not observed. The unit-cell parameters decrease with increasing ScO6 octahedron rotation and atomic number of the Ln cation. In common with lanthanide orthoferrites, the uniform structural evolution is interrupted at the middle-heavy part of the lanthanide sequence. This is probably due to an interplay between: (i) enlargement of the ScO6 octahedra relative to BO6 in other perovskites (e.g., FeO6 in GdFeO3); (ii) reduction in size of the first coordination sphere of Ln(3+) coincident with the lanthanide contraction; (iii) coincident expansion of the second coordination sphere due to screening effects of O(1)1 on O(1)2, and entry of Sc to the lanthanide coordination sphere; (iv) complex mixing between oxygen and lanthanide lanthanide f- and scandium d-orbitals. In the series studied, Ln(3+) are in eight-fold coordination (tetragonal antiprism), and are considerably displaced from the center of the LnO(8) polyhedron along [001]. Evolution of the crystallochemical characteristics through the Ln orthoscandate series is complex due to both the antipathetic distortions of A- and B-site coordination polyhedra and interaction of the orbitals of oxygen, Ln and Sc. Empirically obtained limits of Goldschmidt and observed (viii)t(o) tolerance factors for ternary LnBO(3) compounds adopting the Pbnm structure are 0.795 and 0.841, respectively. (C) 2004 Elsevier Inc. All rights reserved.