The crystal structures of phenacite-type Li 2 (MoO 4 ), and scheelite-type LiY(MoO 4 ) 2 and LiNd(MoO 4 ) 2

✍ Scribed by Kolitsch, U.

• Book ID: 118151786
• Publisher: Oldenbourg Wissenschaftsverlag
• Year: 2001
• Tongue: English
• Weight: 135 KB
• Volume: 216
• Category: Article
• ISSN: 2194-4946
• DOI: 10.1524/zkri.216.8.449.20358

✦ Synopsis

Abstract

The crystal structures of flux-grown Li~2~(MoO~4~), LiY(MoO~4~)~2~ and LiNd(MoO~4~)~2~ were solved and refined from single-crystal intensity data collected with a four-circle diffractometer (Mo__K__
~α~X-radiation, CCD area detector, room temperature). Li~2~(MoO~4~) is isostructural with phenacite, Be~2~(SiO~4~) and has space group __R__3̅, 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 LiO~4~ and fairly regular MoO~4~ 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 Li~2~(MoO~4~) in the range from 103 to 293 K gave no evidence of a phase transition, thus contradicting recent literature reports. Both LiY(MoO~4~)~2~ and LiNd(MoO~4~)~2~ crystallise in the scheelite-type structure, with space group __I__4~1~/a and Z = 2 (R__1 = 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) Å^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 Å, respectively. In both compounds the unique MoO~4~ tetrahedron has four identical Mo–O bonds with lengths of 1.779 Å. The stoichiometry of Li__RE(MoO~4~)~2~ (RE = rare-earth element) compounds is discussed and the relation to structure types of other MRE(XO~4~)~2~ (M = alkali metal, X = Mo, W) compounds is briefly addressed.