Negative thermal expansion in Th 2O(PO 4) 2

Abstract

High temperature X ray diffraction performed on recently discovered orthorhombic Th 2O(PO 4) 2 shows a continuous linear thermal contraction (−1.6 × 10 −6 °C −1) in 20–600 °C range and a near-zero expansion at higher temperatures resulting from a dual structural deformation involving oxygen oscillations and inter-cations repulsions. Although similar mechanisms were observed in isotypic Zr 2O(PO 4) 2 (+1.5 × 10 −6 °C −1) and U 2O(PO 4) 2 (−1.4 × 10 −6 °C −1), those observed in Th 2O(PO 4) 2 are particularly intense because of the high ionic radius of tetravalent thorium.