Neutron scattering study of acoustic phonon softening in BiVO4

Abstract

Inelastic neutron scattering has been used to study the transverse acoustic (TA) phonon propagating on the (0,0,1) plane polarized on this plane in BiVO${}_{4}$ above the ferroelastic phase transition at $T$${}_{c}$$=528$ K. The energies of the TA phonon dispersion curve along $[0.7\ensuremath{\xi},\ensuremath{\xi},0]$ are slightly lower than those along $[\ensuremath{\xi},\ensuremath{\xi},0]$, indicating that the acoustic symmetry direction deviates from the high-symmetry $[\ensuremath{\xi},\ensuremath{\xi},0]$ direction. The $[0.7\ensuremath{\xi},\ensuremath{\xi},0]$ TA branch softens in a limited range $\ensuremath{\xi}\ensuremath{\lesssim}0.2$ as the temperature approaches $T$${}_{c}$ from above. The gradual decrease in the initial slope of the $[0.7\ensuremath{\xi},\ensuremath{\xi},0]$ TA branch is quite different from the complete softening of the TA mode reported from Brillouin scattering measurements. A temperature-dependent central peak is associated with the $[0.7\ensuremath{\xi},\ensuremath{\xi},0]$ TA mode. Furthermore, an elastic diffuse streak along $[1,\overline{1},0]$ exists up to 878 K. These precursor effects are discussed in connection with the TA phonon softening.