The crystal structures of α and γ SrUO₄

Abstract

On rhombohedral strontium monouranate which is metastable below ca. 770°C (α-phase) and stable above ca. 1230°C (γ-phase) in air, x-ray powder diffraction study was made with the samples of three different compositions, i.e. SrUO_3.948(1), SrUO_3.597(1) and SrUO_3.777(1). Their structures were essentially identical with that of Zachariasen (1948) on SrUO₄ with space group R[unk]m, Z = 1; 1 U in (000), 1 Sr in (½½½),2 O(1) in ±(uuu) and 2 O(2) in ± (vvv), and no more discrepancies were observed in the cell and the oxygen parameters between α and γ phases than those resulted from the oxygen non-stoichiometry: SrUO_3.948(1)a = 6.551(3) Å, α = 34.82(2)°, u = 0.112(2), v = 0.357(2) SrUO_3.597(1)a = 6.587(3) Å, α = 35.30(2)°, u = 0.110(2), v = 0.340(2) SrUO_3.777<1)a = 6.542(3) Å, α = 35.54(2)°, u = 0.113(2), v = 0.340(2). In oxygen deficient crystals, oxygen vacancies were found to be formed in O(2) position but not in O(1). The U–O(2) bond length 2.30(1) ∼ 2.31(1) Å is reasonably consistent with the other alkali and alkaline-earth uranates, whereas the U–O(1) bond length lies in the range 2.04(4) ∼ 2.08(4) Å which is longer than the usual uranyl bond length of 1.8 ∼ 1.9 Å.