Pressure- and temperature-induced M3 phonon softening in ReO3

We have investigated the temperature and pressure variation of the M-3 phonon mode of ReO3 by inelastic neutron scattering on a single crystal. The energy of the M-3 phonon mode decreases with decreasing temperature contrary to the normal hardening behavior. The energy of this mode also decreases with increasing pressure and tends to zero at about 3.5 kbar at T=150 K. The M-3 mode softening drives the pressure-induced phase transition in ReO3. The temperature-induced softening of the M-3 mode on lowering the temperature can be understood from the known negative thermal-expansion behavior and also from the fact that the high-pressure phase can also be approached at ambient pressure by cooling. In order to understand the M-3 phonon softening in ReO3 we have done lattice-dynamical calculations that support the qualitative arguments given above.