Mechanism of the low thermal expansion in α-Hf2O(PO4)2 and its zirconium analog

The thermal expansion of recently characterized α-Zr2O(PO4)2 and α-Hf2O(PO4)2 is found to be very low (respectively 2.6 and 2.9×10−6 K−1, 20–900 °C). High-temperature X-ray diffraction and Rietveld analysis allowed to identify a dual contraction mechanism, involving a classical ring deformation and the rocking of bridging oxygens.