Crystal chemical studies of the 5f-series of elements. IV. The crystal structure of Ca(UO₂)O₂and 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., α = 36° 2′ ± 1′, Sr(UO2)O2: a = 6.53 ± 0.03 kX., α = 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 R{ 3}m with 1 U in (0, 0, 0), 1 Ca (or Sr) in ({1 2}, {1 2}, {1 2}), 2 OI in ±(u, u, u), 2 OII 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—OI = 1.91 ± 0.10 A. Each uranium atom forms six weaker bonds with U—OII = 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.