Crystal Structure Refinements of the Lead(II) Oxoarsenates(V) Pb<sub>2</sub>As<sub>2</sub>O<sub>7</sub>, Pb(H<sub>2</sub>AsO<sub>4</sub>)<sub>2</sub>, Pb<sub>5</sub>(AsO<sub>4</sub>)<sub>3</sub>OH and NaPb<sub>4</sub>(AsO<sub>4</sub>)<sub>3</sub> from Single-Crystal X-ray Data
Single-crystals of lead(II) oxoarsenates(V) were grown from the melt (Pb<sub>2</sub>As<sub>2</sub>O<sub>7</sub>), from solution (Pb(H<sub>2</sub>AsO<sub>4</sub>)<sub>2</sub> and Pb<sub>5</sub>(AsO<sub>4</sub...
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| Formato: | article |
| Lenguaje: | EN |
| Publicado: |
MDPI AG
2021
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| Materias: | |
| Acceso en línea: | https://doaj.org/article/b0a4407510794afb923e23e1c36d2e0a |
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| Sumario: | Single-crystals of lead(II) oxoarsenates(V) were grown from the melt (Pb<sub>2</sub>As<sub>2</sub>O<sub>7</sub>), from solution (Pb(H<sub>2</sub>AsO<sub>4</sub>)<sub>2</sub> and Pb<sub>5</sub>(AsO<sub>4</sub>)<sub>3</sub>OH), and under hydrothermal conditions (NaPb<sub>4</sub>(AsO<sub>4</sub>)<sub>3</sub>). Crystal structure refinements from single-crystal X-ray diffraction data revealed isotypism for both Pb<sub>2</sub>As<sub>2</sub>O<sub>7</sub> and Pb(H<sub>2</sub>AsO<sub>4</sub>)<sub>2</sub> with the corresponding barium and phosphate phases. A quantitative comparison of the crystal structures showed a high similarity for the isotypic <i>M</i><sub>2</sub><i>X</i><sub>2</sub>O<sub>7</sub> structures (<i>M</i> = Pb, Ba; <i>X</i> = As, P), whereas for the <i>M</i>(H<sub>2</sub><i>X</i>O<sub>4</sub>)<sub>2</sub> structures only the pair Pb(H<sub>2</sub>AsO<sub>4</sub>)<sub>2</sub> and Pb(H<sub>2</sub>PO<sub>4</sub>)<sub>2</sub> is similar, but not Ba(H<sub>2</sub>AsO<sub>4</sub>)<sub>2</sub>. Pb<sub>5</sub>(AsO<sub>4</sub>)<sub>3</sub>OH adopts the apatite structure type in space group <i>P</i>6<sub>3</sub>/<i>m</i>, with the hydroxyl group disordered around Wyckoff position 2 <i>b</i> (0, 0, 0) in the channels of the structure. NaPb<sub>4</sub>(AsO<sub>4</sub>)<sub>3</sub> represents a lacunar apatite with two of the three metal positions occupationally disordered by Pb and Na. In contrast to a previous X-ray powder study of NaPb<sub>4</sub>(AsO<sub>4</sub>)<sub>3</sub> that reported an apatite-type structure in space group <i>P</i>6<sub>3</sub>/<i>m</i>, the current single-crystal data clearly revealed a symmetry reduction to space group <i>P</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>. Hence, NaPb<sub>4</sub>(AsO<sub>4</sub>)<sub>3</sub> is the first lacunar apatite that comprises only tetrahedral anions and adopts the belovite structure type. |
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