Structural Properties of Some Vacancy-Ordered Platinum Halide Perovskites

Abstract

The structural changes that accompany the dehydration of Na 2 PtX 6 •6H 2 O (X = Cl, Br) were studied using in situ variable temperature synchrotron X-ray diffraction. The two hexahydrates are isostructural, containing isolated PtX 6 octahedra separated by Na cations. Removal of the water results in the formation of the anhydrous vacancy ordered double perovskites Na 2 PtX 6 . The Na cation is too small for the cuboctahedron site of the parent cubic structure, resulting in cooperative tilting of the PtX 6 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 K 2 PtBr 6 and K 2 PtI 6 , 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: Fm3̅ m → P4/mnc → P2 1 /n. These phase transitions are associated with the onset of long-range cooperative tilting of the PtX 6 octahedra described using the Glazer tilt notation as a 0 a 0 a 0 → a 0 a 0 c + → a − a − c + .