Negative thermal expansion in Th2O(PO4)2

High temperature X ray diffraction performed on recently discovered orthorhombic Th2O(PO4)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 Zr2O(PO4)2 (+1.5 × 10−6 °C−1) and U2O(PO4)2 (−1.4 × 10−6 °C−1), those observed in Th2O(PO4)2 are particularly intense because of the high ionic radius of tetravalent thorium.