| Summary: | The literature data and the results obtained by the authors on the study of the structure and properties of a series of polycrystalline and single-crystal samples of pure and Mg-doped oxymolybdates <i>Ln</i><sub>2</sub>MoO<sub>6</sub> (<i>Ln</i> = La, Pr, Nd) are analyzed. Presumably, the high-temperature phase <i>I</i>4<sub>1</sub>/<i>acd</i> of Nd<sub>2</sub>MoO<sub>6</sub> single crystals is retained at room temperature. The reason for the loss of the center of symmetry in the structures of La<sub>2</sub>MoO<sub>6</sub> and Pr<sub>2</sub>MoO<sub>6</sub> and the transition to the space group <i>I</i><inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover accent="true"><mn>4</mn><mo stretchy="true">¯</mo></mover></mrow></semantics></math></inline-formula>c2 is the displacement of oxygen atoms along the twofold diagonal axes. In all structures, Mg cations are localized near the positions of the Mo atoms, and the splitting of the positions of the atoms of rare-earth elements is found. Thermogravimetric studies, as well as infrared spectroscopy data for hydrated samples of <i>Ln</i><sub>2</sub>MoO<sub>6</sub> (<i>Ln</i> = La, Pr, Nd), pure and with an impurity of Mg, confirm their hygroscopic properties.
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