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

Abstract The structures of isotypic α -Zr 2 O(PO 4 ) 2 (S.G. I 2/ m , a = 10.2726 ( 6 ) , b = 6.5957 ( 3 ) , c = 10.0665 ( 5 ) A , β = 95.433 ( 3 ) ° , V = 679.00 ( 6 ) A 3 , Z = 4 ) and α -Hf 2 O(PO 4 ) 2 (S.G. I 2/ m , a = 10.2301 ( 3 ) , b = 6.5819 ( 2 ) , c = 10.0075 ( 5 ) A , β = 95.371 ( 1 ) ° , V = 670.87 ( 3 ) A 3 , Z = 4 ) have been resolved ab initio by Rietveld analysis. At variance with the β -form, they show both M O 6 and M O 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 M O 6 octahedra.