Crystal chemical studies of the 5f-series of elements. IV. The crystal structure of Ca(UO₂)O₂ and Sr(UO₂)O₂

Ca(UO₂)O₂ and Sr(UO₂)O₂ are rhombohedral and isostructural. The unit cell containing one molecule has dimensions: Ca(UO₂)O₂: a = 6.254 ± 0.001 kX., = 36° 2' ± 1', Sr(UO₂)O₂: a = 6.53 ± 0.03 kX., = 35° 32' ± 20'. The calculated density is 7.45 g.cm.^-3 for the calcium compound and 7.62 g.cm.^-3 for the strontium compound. The space group is Rm with 1 U in (0, 0, 0), 1 Ca (or Sr) in (, , ), 2 O_I in ±(u, u, u), 2 O_II in ±(v, v, v). For the calcium compound u = 0.109 ± 0.006, v = 0.361 ± 0.006. The structure contains collinear uranyl groups with U-O_I = 1.91 ± 0.10 A. Each uranium atom forms six weaker bonds with U-O_II = 2.29 A. A calcium or strontium atom is bonded to eight oxygen atoms with Ca-O = 2.45 A., Sr-O = 2.58 A.