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 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)Åa=18.8588(2)Å, b=34.0278(2)Åb=34.0278(2)Å, c=18.9231(1)Åc=18.9231(1)Å, β=90.453(1)°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

The monoclinic α-K3AlF6 phase is stable below 132 °C (a=18.8588(2)Åa=18.8588(2)Å, b=34.0278(2)Åb=34.0278(2)Å, c=18.9231(1)Åc=18.9231(1)Å, β=90.453(1)°β=90.453(1)°, SG I2/a or Ia). The β phase exists in very narrow temperature interval between 132 and 153 °C. The orthorhombic γ polymorph is stable between 153 °C and 306 °C (a=36.1229(6)Åa=36.1229(6)Å, b=17.1114(3)Åb=17.1114(3)Å, c=12.0502(3)Åc=12.0502(3)Å, S.G. Fddd). Above 306 °C the cubic δ   polymorph forms with a=8.5786(4)Åa=8.5786(4)Å and SG Fm3¯m.Figure optionsDownload as PowerPoint slide