Lattice dynamics of disordered perovskite Pb(Zn1/3Nb2/3)O3

Inelastic-neutron-scattering experiments were performed to investigate the phonon-dispersion relations in the cubic relaxor Pb(Zn1/3Nb2/3)O3. The transverse acoustic (TA) phonon branches have flat and isotropic dispersion curves. The zone-boundary [ζζ0]-TA2 phonon energy is approximately 6 meV, considerably lower than the values reported for simple perovskites. The low-lying transverse optical (TO) branches drop sharply with decreasing ζ, similar to the behavior observed for the complicated system Pb(Zn1/3Nb2/3)0.92Ti0.08O3. The constant-Q scans reveal the absence of the well-defined TO phonon spectra in the range 0≤ζ≤0.1, in contrast with the sharp TA spectra. The TA, TA2 and TO phonon energies at ζ=0.1 and 0.2 decrease with increasing temperature up to 673 K. The phonon hardening toward the transition temperature contradicts the usual soft mode behavior in simple perovskites. (author)