Neutron scattering study at high pressure of the structural phase transition in paratellurite

The dispersion relation for the transverse acoustic phonon mode propagating along (110) polarized along (110) in TeO 2 has been measured at P=1 atm, P c ( approximately=9.0 kbar), and 2P c where P c is the tetragonal-to-orthorhombic structural transition pressure. Complete softening of the mode at P c is restricted to the q to 0 limit while at finite wavevectors there is a nearly constant energy shift of the dispersion curve of approximately=-0.1 meV between 1 atm and P c and approximately=+0.25 meV between P c and 2P c . Measurements of the order parameter are consistent with a mean field of a pressure-induced elastic instability.