Phase transitions in K3AlF6

Phase transitions in the elpasolite-type K3AlF6 complex fluoride were investigated using differential scanning calorimetry, electron diffraction and X-ray powder diffraction. Three phase transitions were identified with critical temperatures image, image and image. The α-K3AlF6 phase is stable below T1 and crystallizes in a monoclinic unit cell with image, image, image, image (a=2ac−cc, b=4bc, c=ac+2cc; ac, bc, cc—the basic lattice vectors of the face-centered cubic elpasolite structure) and space group I2/a or Ia. The intermediate β phase exists only in very narrow temperature interval between T1 and T2. The γ polymorph is stable in the image temperature range and has an orthorhombic unit cell with image, image, image (a=3ac−3cc, b=2bc, c=ac+cc) at 250 °C and space group Fddd. Above T3 the cubic δ polymorph forms with image at 400 °C and space group image. The similarity between the K3AlF6 and K3MoO3F3 compounds is discussed.