Phase transition in the ternary fluorides Li3AlF6 and Li3CrF6

The phase transitions of two ternary fluorides have been reinvestigated by thermal and X-ray diffraction analyses. Four phase transitions have been detected at T-1 =483 +/- 5 K, T-2= 548 +/- 5 K, T-3 = 765 +/- 5 K and T-4= 867 +/- 5 K for Li3AlF6. Only two transformations occurred at T-1 = 550 +/- 5 K and T-2 = 660 +/- 10 K for Li3CrF6 Dielectric and optical measurements performed respectively on ceramics and single crystals revealed a ferroelectric behaviour coupling partially with ferroelastic coupling for the two investigated fluorides.