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 T1=132°C, T2=153°C and T3=306°C. The α-K3AlF6 phase is stable below T1 and crystallizes in a monoclinic unit cell with a=18.8588(2)Å, b=34.0278(2)Å, c=18.9231(1)Å, β=90.453(1)° (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 T2<T<T3 temperature range and has an orthorhombic unit cell with a=36.1229(6)Å, b=17.1114(3)Å, c=12.0502(3)Å (a=3ac-3cc, b=2bc, c=a c+cc) at 250 °C and space group Fddd. Above T 3 the cubic δ polymorph forms with ac=8.5786(4)Å at 400 °C and space group Fm3̄m. The similarity between the K 3AlF6 and K3MoO3F3 compounds is discussed.
- Electron diffraction
- Phase transition