Crystal structure and thermal evolution of inedite α-Zr 2O(PO 4) 2 and α-Hf 2O(PO 4) 2

Abstract

The structures of isotypic α-Zr 2O(PO 4) 2 (S.G. I2/ m, a = 10.2726 ( 6 ) , b = 6.5957 ( 3 ) , c = 10.0665 ( 5 ) Å , β = 95.433 ( 3 ) ° , V = 679.00 ( 6 ) Å 3 , Z = 4 ) and α-Hf 2O(PO 4) 2 (S.G. I2/ m, a = 10.2301 ( 3 ) , b = 6.5819 ( 2 ) , c = 10.0075 ( 5 ) Å , β = 95.371 ( 1 ) ° , V = 670.87 ( 3 ) Å 3 , Z = 4 ) have been resolved ab initio by Rietveld analysis. At variance with the β-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 α-form is less stable than the β-form, thus making the α – β transition irreversible. However, the substitution of smaller Hf IV for Zr IV allows to keep the α-framework up to 1500 °C by stabilizing the MO 6 octahedra.