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

Abstract

Abstract The crystal structures of flux-grown Li2(MoO4), LiY(MoO4)2 and LiNd(MoO4)2 were solved and refined from single-crystal intensity data collected with a four-circle diffractometer (MoK αX-radiation, CCD area detector, room temperature). Li2(MoO4) is isostructural with phenacite, Be2(SiO4) and has space group R3̅, a = 14.330(2), c = 9.584(2) Å, 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 Å (Li1) and 1.967 Å (Li2), and 1.764 Å (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 Li2(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 I41/a and Z = 2 (R1 = 1.7 and 2.7 %, re-spectively). The first has a = 5.148(1), c = 11.173(2) Å, V = 296.11(10) Å3, the latter a = 5.243(1), c = 11.440(2) Å, V = 314.47(10) Å3. 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 Å, respectively. In both compounds the unique MoO4 tetrahedron has four identical Mo–O bonds with lengths of 1.779 Å. 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.