Structural Properties of Some Vacancy-Ordered Platinum Halide Perovskites

The structural changes that accompany the dehydration of Na₂PtX₆·6H₂O (X = Cl, Br) were studied using in situ variable temperature synchrotron X-ray diffraction. The two hexahydrates are isostructural, containing isolated PtX₆ octahedra separated by Na cations. Removal of the water results in the formation of the anhydrous vacancy ordered double perovskites Na₂PtX₆. The Na cation is too small for the cuboctahedron site of the parent cubic structure, resulting in cooperative tilting of the PtX₆ octahedra and lowering of the symmetry. Replacing Na with a larger alkali metal (K, Rb, or Cs) invariably enabled the isolation of the anhydrous hexahalide, and we found no evidence that these readily hydrated. For all cations, other than Na, it was possible to observe the archetypical cubic structure, although for the two potassium salts K₂PtBr₆ and K₂PtI₆, this was only observed above a critical temperature of 175 and 460 K, respectively. As these two samples were cooled, symmetry lowering was observed, yielding a tetragonal structure initially and ultimately a monoclinic structure: Fm3̅m → P4/mnc → P2₁/n. These phase transitions are associated with the onset of long-range cooperative tilting of the PtX₆ octahedra described using the Glazer tilt notation as a⁰a⁰a⁰ → a⁰a⁰c⁺ → a^-a^-c⁺.