Crystal structure and thermal evolution of inedite α-Zr2O(PO4)2 and α-Hf2O(pO4)2

The structures of isotypic alpha-Zr2O(PO4)2 (S.G. beta/m, a=10.2726(6), b=6.5957(3), c=10.0665(5)A, beta=95.433(3) deg , V=679.00(6)A3, Z=4) and alpha-Hf2O(PO4)2 (S.G. beta/m, a=10.2301(3), b=6.5819(2), c=10.0075(5)A, beta=95.371(1) deg , V=670.87(3)A3, Z=4) have been resolved ab initio by Rietveld analysis. At variance with the beta-form, they show both MO6 and MO7 polyhedra (M=Zr, Hf), forming ribbons with the PO4 tetrahedra. A lattice energy calculation explains why the alpha-form is less stable than the beta-form, thus making the alpha-beta transition irreversible. However, the substitution of smaller HfIV for ZrIV allows to keep the alpha-framework up to 1500 deg C by stabilizing the MO6 octahedra.