Four New Dysprosium and Neodymium Octamolybdate Hydrates: Assembly of RE2(Mo8O27) Sheets and Topotactic Transformations

• Published in: Inorganic chemistry 2010-07, Vol.49 (13), p.6005-6012
• Main Authors: Kuang, Xiaojun, Evans, Ivana R
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic100646e

Four new Dy and Nd hydrated octamolybdate materials have been prepared: [Dy2(H2O)12](Mo8O27)·8H2O, [Dy2(H2O)6](Mo8O27), [Nd2(H2O)12](Mo8O27)·6H2O, and [Nd2(H2O)6]Mo8O27·3H2O. They adopt one known and three new structure types, which we have determined ab initio from powder X-ray diffraction data. The four compounds contain the same basic structural building block, in the form of RE2Mo8O27 (RE = Dy, Nd) chains, which are arranged differently for the two rare earths in the fully hydrated precursor materials. [Dy2(H2O)12](Mo8O27)·8H2O comprises isolated Dy2Mo8O27 chains assembled in a zigzag manner along the a axis with interspersed crystallized water molecules, which differs from the parallel arrangements of isolated Nd2Mo8O27 chains in [Nd2(H2O)12](Mo8O27)·6H2O. Partial dehydration of the two precursors leads to new phases [Dy2(H2O)6](Mo8O27) and [Nd2(H2O)6]Mo8O27·3H2O, respectively, prior to complete dehydration, which leads to initially amorphous and eventually crystalline rare-earth molybdenum mixed metal oxides. Both initial transformations occur topotactically. Partial dehydration of the Dy phase condenses the assembly of the Dy2Mo8O27 chains principally along the a axis, leading to a 3-dimensional (3D) framework, with each Dy bridging two (Mo8O27)6− sheets in the product. The partial dehydration of the Nd precursor condenses the assembly of Nd2Mo8O27 chains along the [110] axis, leading to a 3D framework where each Nd bridges three (Mo8O27)6− units. Both new dysprosium octamolybdates adopt noncentrosymmetric polar structures in space group P21 and are second harmonic generation (SHG) active.