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.
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