Nuclear resonant inelastic x-ray scattering on quartz structured 57FePO4 as a function of pressure, up to 8 GPa reveals hardening of the low-energy phonons under applied pressures up to 1.5 GPa, followed by a large softening at 1.8 GPa upon approaching the phase transition pressure of ~2 GPa. The pressure-induced phase transitions in quartz-structured compounds have been predicted to be related to a soft phonon mode at the Brillouin-zone boundary (1/3, 1/3, 0) and to the break-down of the Born-stability criteria. Our results provide the first experimental evidence of this predicted phonon softening.
@article{arxiv.0903.1550,
title = {Phonon Softening and Pressure-Induced Phase Transitions in Quartz Structured Compound, FePO4},
author = {R. Mittal and R. P. Hermann and M. Angst and S. L. Chaplot and E. E. Alp and J. Zhao and W. Sturhahn and F. Hatert},
journal= {arXiv preprint arXiv:0903.1550},
year = {2009}
}