Ce_2[MoO_5][MoO_4] and Ce_5[MoO_4]_8: Two New Cerium Oxomolybdates, EachExh ibiting a Special Structural Feature

摘要

Ce_2[MoO_5][MoO_4] crystallizes monoclinically in space group P2_1/c (a = 2241.98(9), b = 712.95(3), c = 2044.19(8) pm, β = 116.936(2)?, Z = 16). Besides eight crystallographically different Ce~(3+) cations with coordination numbers ranging from seven plus one to ten, its structure contains two isolated oxomolybdate(VI) species, namely [MoO4]~(2?) tetrahedra and [MoO5]~(4?) trigonal bipyramids and square pyramids. The oxygen polyhedra around the Ce~(3+) cations build up a three-dimensional ~3 _∞{[Ce_2O_9]~(12?)} network by vertex-, edge-, and face-connections with the Mo~(6+) cations residing in tetrahedral, square-pyramidal, and trigonal-bipyramidal voids therein. Ce_5[MoO_4]_8 also crystallizes monoclinically in space group P2_1/c (a = 1133.14(5), b = 1704.96(7), c = 1406.21(6) pm, β = 90.749(2)?, Z = 4). In its structure eight isolated [MoO4]~(2?) tetrahedra are present, which display an overall negative charge of ?16. To maintain electroneutrality, mixed-valency of the five crystallographically independent cerium cations must be considered. A thorough bond-valence analysis reveals cerium at the Ce1 site as tetravalent, while the other four cerium cations prove to be trivalent. The polyhedra around all cerium cations form chains with a V-shaped profile along [100] via vertex-, edge-, and face-connections to result in a sawblade pattern. These chains are interlocked into sheets parallel to the (001) plane and linked by the Mo~(6+) cations to form a three-dimensional network.