Crystal structure and thermal evolution of inedite alpha-Zr_2O(PO_4)_2 and alpha-Hf_2O(PO_4)_2

摘要

The structures of isotypic alpha-Zr_2O(PO_4)_2 (S.G. 121m, a = 10.2726(6), b = 6.5957(3), c = 10.0665(5) A, beta = 95.433(3) deg, V = 679.00(6) A~3, Z = 4) and alpha-Hf_2O(PO_4)_2 (S.G. 121 m, a = 10.2301(3), b = 6.5819(2), c = 10.0075(5) A, beta = 95.371 (1) deg, V = 670.87(3) A~3, Z = 4) have been resolved ab initio by Rietveld analysis. At variance with the beta-form, they show both MO_6 and MO__7 polyhedra (M = Zr, Hf), forming ribbons with the PO_4 tetrahedra. A lattice energy calculation explains why the a-form is less stable than the beta-form, thus making the alpha-beta transition irreversible. However, the substitution of smaller Hf~(IV) for Zr~(IV) allows to keep the a-framework up to 1500 deg C by stabilizing the MO_6 octahedra.