Neutron Scattering Study of Phonon Dispersion Relations along [110] in TlCl

The phonon dispersion relations of TlCl along the [110] direction have been studied by inelastic neutron scattering at 80 K, partly at 293 K. The energies of four zone-boundary phonons at the \(M(1/2\), \(1/2\), 0) point associated with the indirect band-gap transition between the X (0, 0, \(1/2)\) and \(R(1/2\), \(1/2\), \(1/2)\) points were determined. The softening of the TA 2 branch ( e //[001]) was found around the zone-boundary. The zone-boundary phonon softens from 1.75 meV (293 K) to 1.28 meV (4.5 K). The mechanism of this phenomenon is discussed in connection with the CsCl–TlI or CsCl–NaCl type transitions which can be accomplished by a shear of the (110) planes corresponding to the TA 2 mode in the CsCl structure.