| Summary: | Three new mixed metal fluoride hydrates, <i>M</i><sup>2+</sup>AlF<sub>5</sub>(H<sub>2</sub>O)<sub>7</sub> (<i>M</i><sup>2+</sup> = Fe<sup>2+</sup>, Co<sup>2+</sup>, or Ni<sup>2+</sup>), were synthesized and characterized. The crystals of <i>M</i><sup>2+</sup>AlF<sub>5</sub>(H<sub>2</sub>O)<sub>7</sub> were obtained using a hydrothermal method with a CF<sub>3</sub>COOH aqueous solution. The crystal structures displayed polymorphisms in <i>C</i>2/<i>m</i> (No. 12) or <i>P</i>-1 (No. 2) space groups, depending on temperature variations. The observed polymorphisms in <i>M</i><sup>2+</sup>AlF<sub>5</sub>(H<sub>2</sub>O)<sub>7</sub> are associated with changes in the bonding environment of [<i>M</i>(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup> and [AlF<sub>5</sub>(H<sub>2</sub>O)]<sup>2−</sup> octahedra, along with changes in hydrogen bonds and unit cell volumes. Infrared spectra and thermogravimetric analyses confirmed the presence of water molecules. The ultraviolet–visible spectra of <i>M</i><sup>2+</sup>AlF<sub>5</sub>(H<sub>2</sub>O)<sub>7</sub> revealed distinctive absorption bands dependent on the [<i>M</i>(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup> complex. This work provides a detailed account of the synthetic procedure, crystal structures, and spectroscopic characterization of <i>M</i><sup>2+</sup>AlF<sub>5</sub>(H<sub>2</sub>O)<sub>7</sub>.
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