Incommensurately modulated ZrO 2− xF 2 x: 0.698 ≤ x ≤ 0.714

Abstract

The compound ZrO 2− x F 2 x has been studied by X-ray powder diffraction and transmission electron microscopy. It was previously described as a series of anion-deficient, α-UO 3-type “line phases,” with compositions in between resulting from unit cell scale intergrowths of the line phases. More correctly ZrO 2− x F 2 x is described as an incommensurately modulated structure with composition-dependent primary modulation wave-vector, q = x b∗. The underlying orthorhombically distorted α-UO 3-type parent structure has space group symmetry Cmmm ( a ≈ 6.44, b ≈ 3.83, c ≈ 4.07 Å). Characteristic extinction conditions imply a superspace group symmetry of P: Cmmm: s, −1,1. The previously reported crystal structure of Zr 7O 9F 10 is Fourier decomposed in these terms. Crystal chemical reasons are proposed for the very limited composition range, 0.698 ≤ x ≤ 0.714, observed for the title compound.