Th₄(PO₄)₄P₂O₇, an Original Ultralow Expansion Material

The thermal expansion of the thorium phosphate diphosphate has been studied from 20 to 800 °C by dilatometry and high-temperature X-ray diffraction. Both experiments indicated an ultralow expansion, with a mean global coefficient of 1.9 × 10 - 6 °C - 1 over the considered range, but the phenomenon appeared to be highly anisotropic. A comparison with the known families of low- or negative-expansion compounds allowed us to conclude that Th 4 (PO 4 ) 4 P 2 O 7 did not belong to any one of them. The Rietveld refinements of the high-temperature XRD patterns revealed no structural change but slight atomic displacements; the quasicompact packing following the c axis yields a normal expansion in this direction, but the distortion of the structural units leads to a very slight contraction of parameter b up to 300 °C, while their expansion following the a axis is almost damped by the tunnels of the oxygen framework. This somewhat complex structural mechanism appears to be generated by the thorium−thorium Coulombic repulsions.