The structural behaviour of LaF(3) at high pressures.

We have performed in situ X-ray diffraction, Raman scattering, pseudosymmetry analysis and quantum mechanical atomistic simulations of the structure and behaviour of LaF(3) at high pressure, up to and exceeding the phase transition pressure reported for the I4/mmm structure and Cmma symmetry proposed previously. We observe that the structure of LaF(3)-II is best described as being of anti-Cu(3)Ti-type (oP8, Pmmn, SG59), which is closely related to the I4/mmm structure obtained by simulation, through notional distortion, and is evidently similar to the Cmma symmetry by comparison of published diffraction data. We demonstrate that the models are also related to each other, and can be derived through pseudosymmetry searches. LaF(3)-II does not undergo further phase transitions before at least 60 GPa, and none are expected before 1 Mbar. The similarity between the anti-Cu(3)Ti-type and the I4/mmm structure models and our in situ diffraction data, supports the transition mechanisms derived from atomistic model simulations for a generic REF(3) transition to the post-tysonite phase (where RE = La, Ce, Pr, Nd).