Abstract
A general expression for the lattice viscosity in doped displacive ferroelectrics, in the presence of an external electric field, is obtained using the double-time thermal Green's-function technique. The mass and force-constant changes between the impurity and host-lattice atoms are taken into account in the Silverman Hamiltonian augmented with higher-order anharmonic and electric moment terms. The defect-dependent, electric-field-dependent, and anharmonic contributions to the lattice viscosity are discussed separately. It is shown that the lattice viscosity tensor, which is the sum of two terms arising from acoustical and optical phonons, can be further separated into diagonal and nondiagonal parts. The nondiagonal contribution vanishes in the absence of defects. The frequency and temperature dependences of the viscosity tensor are also discussed.
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