Abstract
New mixed-cation fluorides KNa2HfF7 (Pmn21, a = 8.213(2) Å, b = 6.126(2) Å, c = 5.979(2) Å, Z = 2) and KNa2(Hf,Zr)F7 (refined composition KNa2Hf0.44(1)Zr0.56(1)F7; Pmn21, a = 8.2210(8) Å, b = 6.1288(6) Å, c = 5.979(1) Å, Z = 2) together with isostructural KNa2ZrF7 (Pmn21, a = 8.221(2) Å, b = 6.118(2) Å, c = 5.994(2) Å, Z = 2) have been synthesised in aqueous solutions and structurally characterised at room temperature by powder and single-crystal X-ray diffraction. The solid-solid phase transformations have been revealed during the differential scanning calorimetry experiments and further confirmed by temperature-dependent X-ray powder diffraction. It has been found that mixed-cation phases KNa2ZrF7, KNa2HfF7 and KNa2(Hf,Zr)F7 undergo irreversible phase transformations, which can be explained by their decomposition over 673 K. XPS analysis confirmed the presence of exclusively terminated fluorines around the transition metals and the absence of oxygen impurities in the crystal structures of compounds.
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