Crystal structure and phase transitions in R2TeO6 (R = La, Pr, Nd, Tb, Ho, Er, Tm, Lu) oxides: A neutron diffraction study

Abstract

Eight members of the R2TeO6 family for the neutron non-absorbing rare-earths (R = La, Pr, Nd, Tb, Ho, Er, Tm, Lu) have been synthesized by solid-state reactions as pure specimens and structurally characterized by neutron powder diffraction (NPD), allowing for the precise determination of the oxygen positions and thermal displacements in the presence of heavy rare-earth and tellurium atoms. For the large rare-earth cations (from La3+ to Tm3+) the crystal structure adopts the Nd2WO6 type, defined in the orthorhombic P212121 space group, containing two types of crystallographically independent rare earth atoms, R1 and R2, sevenfold coordinated with oxygen atoms; the tellurium atoms form distorted TeO6 octahedra, sharing corners and edges with both kinds of R1O7 and R2O7 polyhedra. For Lu2TeO6 a morphotropic phase transition is observed, and this compound crystallizes in the trigonal P321 space group, with two RO6 and two independent TeO6 coordination polyhedra, also sharing vertices. For Yb2TeO6 a mixture of orthorhombic and trigonal phases is identified. A bond valence study, confirming that R is trivalent and Te is hexavalent in the whole series, helps to understand the driving force for the morphotropic transition to a more stable crystal structure where Lu3+ adopts a lower coordination index. Keywords: Phosphors, La2TeO6, Pr2TeO6, Nd2TeO6, Tb2TeO5, Ho2TeO6