| Summary: | Rare-earth sodium- and fluorine-substituted molybdates with the Na<sub>0.65</sub>La<sub>4.35</sub>Mo<sub>3</sub>O<sub>15.81±δ</sub>F<sub>0.07±ε</sub> composition were synthesized for the first time as single crystals. An accurate X-ray diffraction analysis of three samples at room temperature showed that the obtained crystals are isostructural to undoped cubic compounds of the <i>Ln</i><sub>5</sub>Mo<sub>3</sub>O<sub>16+δ</sub> family (space group <i>Pn</i><inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mover accent="true"><mn>3</mn><mo>¯</mo></mover></semantics></math></inline-formula><i>n</i>). Sodium cations partially occupy one of the two lanthanum positions and center the more distorted LaO<sub>8</sub> polyhedra, while fluorine anions occupy over-stoichiometric oxygen positions in the vast interstices (cavities) of the structure. The partial substitution of fluorine atoms for oxygen ones affects the transport characteristics of compounds by activating the oxygen atoms in the anionic subsystem due to the effect of supplementary electrostatic repulsion between anions of different types.
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