Crystal structures of two hydrous sodium potassium molybdates: Na3K(MoO4)2(H2O)9 and NaK(MoO4)(H2O).

Single-crystals of the hydrated sodium potassium orthomolybdate phases Na3K(MoO4)2(H2O)9, tris-odium potassium bis-(orthomolybdate) nona-hydrate, and NaK(MoO4)(H2O), sodium potassium orthomolybdate monohydrate, were obtained as by-products in a flux growth experiment aiming on the synthesis of silicates from the quaternary system Na2O-K2O-CaO-SiO2. The asymmetric unit of Na3K(MoO4)2(H2O)9 (space group P63/m, Z = 2) comprises one Na+ atom and one water mol-ecule on a mirror plane, one K+ atom on a 6 axis, one Mo6+ cation and one O atom on a threefold rotation axis, and one O atom and one water mol-ecule in general sites. The crystal structure is built from isolated MoO4 tetra-hedra, which are linked by common corners to trimers containing three face-sharing Na(H2O)4O2 octa-hedra. Linkage between the heteropolyhedral groups is provided by potassium cations, which are coordinated by six water mol-ecules in form of trigonal prisms. The resulting layers are located at z = 1/4 and z = 3/4, respectively. Intra- and inter-layer hydrogen bonds between the water mol-ecules and the oxygen atoms of the MoO4 tetra-hedra consolidate the structure. The backbone of the crystal structure of NaK(MoO4)(H2O) (space group P212121, Z = 4) is made from chains of edge-sharing Na(H2O)2O4 octa-hedra running parallel [100], which are decorated by MoO4 tetra-hedra via corner sharing. Neighbouring heteropolyhedral chains are linked by additional potassium cations, which are coordinated by seven oxygen ligands and one water mol-ecule. Moreover, intra- and inter-chain hydrogen bonds exist.