High-pressure phase transitions of LaF3 and CeF3

The effect of high pressure on the structure of LaF3 (tysonite), CeF3, and nonstoichiometric LaF3-based solid solutions (rhombohedral system, P (3) over bar c1, Z = 6) is discussed. In the systems MF2-LaF3 with M = Pb and Sr, fluorite-like M1-xLaxF2+x solid solutions (Fm3m, Z = 4) are obtained in the composition range 0.5 less than or equal to x less than or equal to 0.75 by quenching from 1500-1700 K at p = 10 GPa. By extrapolating the composition dependences of the lattice parameter for these cubic solid solutions, the lattice parameter of cubic LaF3 is estimated at a = 5.86 +/- 0.02 Angstrom. High-pressure x-ray diffraction studies indicate that CeF3 undergoes a first-order phase transition similar to that revealed recently in LaF3. At p = 20.6 GPa, the lattice parameters of orthorhombic CeF3 (Cmma, Z = 8) are a = 8.062 +/- 0.006 Angstrom, b = 8.457 +/- 0.009 Angstrom, and c = 5.091 +/- 0.006 Angstrom. The transition is accompanied by a 7.3% decrease in volume. The high-pressures structure of LaF3 and CeF3 can be thought of as a distorted modification of the structure of a hypothetical fluorite-like trifluoride. The lattice parameters of the orthorhombic and cubic cells are related by a(ortho) similar to b(ortho) similar to root2a(cubic) and c(ortho) similar to a(cubic).