Soft Mode Anomalies in the Perovskite Relaxor Pb(Mg1/3Nb2/3) O3

摘要

Neutron inelastic scattering measurements of the polar TO phonon mode in thecubic relaxor Pb(Mg(sub 1/3)Nb(sub 2/3))O(sub 3), at room temperature, reveal anomalous behavior similar to that recently observed in Pb(Zn(sub 1/3)Nb(sub 2/3))(sub 0.92)Ti(sub 0.08)O(sub 3) in which the optic branch appears to drop precipitously into the acoustic branch at a finite value of the momentum transfer q = 0.2 (angstrom)(sub (minus)1), measured from the zone center. By contrast, a recent neutron study indicates that PMN exhibits a normal TO phonon dispersion at 800 K. The authors speculate this behavior is common to all relaxor materials, and is the result of the presence of nanometer-scale polarized domains in the crystal that form below a temperature T(sub d), which effectively prevent the propagation of long wavelength (q = 0) phonons.