Syntheses and structures of Li3ScF6 and high pressure LiScF4−, luminescence properties of LiScF4, a new phase in the system LiF–ScF3

Colorless single crystals of Li3ScF6 have been prepared by reacting the binary components LiF and ScF3 at 820 °C for 16 h in argon atmosphere. This complex fluoride is the only stable phase in the system LiF–ScF3 under ambient pressure. According to a structure refinement based on single crystal X-ray diffraction data it crystallizes in the centrosymmetric space group P3¯c1 with a=8.78.3(1)Å and c=9.518(1)Å. The new structure of Li3ScF6 is a filled variant of the Na2GeF6 type structure and can be described in terms of a hexagonal close packing of fluorine in which 2/3 of the octahedral holes are occupied by Sc and Li. High pressure/high temperature studies of the system LiF–ScF3 show that the new phase LiScF4 is formed at around 5.5 GPa and 575 °C. According to Rietveld refinements of powder X-ray diffraction data LiScF4 adopts the Scheelite type structure (space group I41/a) with a=4.980(1)Å and c=10.074(1)Å. A sample of LiScF4 doped with 1% Er exhibits an intense luminescence in the far IR region.