The crystal structures of phenacite-type Li2(MoO4), and scheelite-type LiY(MoO4)2 and LiNd(MoO4)2

The crystal structures of flux-grown Li-2(MoO4), LiY(MoO4)(2) and LiNd(MOO4)(2) were solved and refined from single-crystal intensity data collected with a four-circle diffractometer (MoKalpha X-radiation, CCD area detector, room temperature). Li-2(MoO4) is isostructural with phenacite. Be-2(SiO4) and has space group R (3) over bar a = 14.330(2), c = 9.584(2) A, Z = 18 (R1 = 1.6%). The structure consists of a three-dimensional network of corner-linked, slightly distorted LiO4 and fairly regular MoO4 tetrahedra. Average Li-O and Mo-O distances are 1.965 Angstrom (Lil) and 1.967 Angstrom (Li2), and 1.764 Angstrom (Mo). The atomic arrangement is characterised by a narrow open channel along the three-fold axis. Comparisons are drawn with other phenacite-type compounds. Low-temperature single-crystal X-ray studies of Li-2(MoO4) in the range from 103 to 293 K gave no evidence of a phase transition, thus contradicting recent literature reports. Both LiY(MoO4)(2) and LiNd(MoO4)(2) crystallise in the scheelite-type structure, with space group I4(1)/a and Z = 2 (R1 = 1.7 and 2.7%, respectively). The first has a = 5.148(1), c = 11.173(2) Angstrom, V = 296.11(10) Angstrom (3), the latter a = 5.243(1), c = 11.440(2) Angstrom, V = 314.47(10) Angstrom (3). Both Li and Y/Nd are completely disordered on a jointly occupied site. Mean (Y,Li)- and (Nd,Li)-O distances are 2.40 and 2.47 Angstrom, respectively. In both compounds the unique MoO4 tetrahedron has four identical Mo-O bonds with lengths of 1.779 Angstrom. The stoichiometry of LiRE(MoO4)(2) (RE = rare-earth element) compounds is discussed and the relation to structure types of other MRE(XO4)(2) (M = alkali metal, X = Mo, W) compounds is briefly addressed.