Crystal structure of K2Th(MoO4)3

The crystal structure has been determined for double potassium-thorium molybdate having composition K2Th(MoO4)3, Rsub(hkl)=11.6%, which is derived from the structure of scheelite, CaWO4. Monocrystals of K2Th(MoO4)3 are of the form of distorted tetragonal bipyramids; the crystals are negative, and biaxial, and have a large optical axis angle (approximately 70 deg), nsub(g)=1.743, nsub(p)=1.713. The results of the structural analysis of K2Th(MoO4)3 show that K, Th, Mo as well as O atoms are only slightly displaced from the ideal positions of metal and oxygen atoms in the scheelite structure. The tetrahedral enclosure of Mo atoms by O atoms is retained and K and Th atoms are located in the centers of octahedrons. However, these polyhedrons are somewhat distorted, and the distortion is less when the polyhedrons are located along the second-order axis. The Mo-O distances in MO2-tetrahedron are 1.78-1.81A; six Th-O distances in Th-octahedron are similar (2.28-2.42A) and two Th-O distances are somewhat greater than the others, being 2.84A. K and Th atoms are ordered and located in the same positions as Ca atoms in the CaWO4 structure and are in the centers of corresponding octahedrons, and Mo atoms are in the centers of somewhat distorted tetrahedrons that are isolated from each other