Crystal structure and thermal evolution of inedite α-Zr2O(PO4)2 and α-Hf2O(PO4)2

The structures of isotypic α-Zr2O(PO4)2 (S.G. I2/m  , a=10.2726(6)a=10.2726(6), b=6.5957(3)b=6.5957(3), c=10.0665(5) Åc=10.0665(5) Å, β=95.433(3)°β=95.433(3)°, V=679.00(6) Å3V=679.00(6) Å3, Z=4Z=4) and α-Hf2O(PO4)2 (S.G. I2/m  , a=10.2301(3)a=10.2301(3), b=6.5819(2)b=6.5819(2), c=10.0075(5) Åc=10.0075(5) Å, β=95.371(1)°β=95.371(1)°, V=670.87(3) Å3V=670.87(3) Å3, Z=4Z=4) have been resolved ab initio by Rietveld analysis. At variance with the β-form, they show both MO6 and MO7 polyhedra (M=ZrM=Zr, Hf), forming ribbons with the PO4 tetrahedra. A lattice energy calculation explains why the α-form is less stable than the β  -form, thus making the α–βα–β transition irreversible. However, the substitution of smaller HfIV for ZrIV allows to keep the α-framework up to 1500 °C by stabilizing the MO6 octahedra.

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