DIELECTRIC-DISPERSION OF FERROELECTRIC CERAMICS AND SINGLE-CRYSTALS AT MICROWAVE-FREQUENCIES

The dielectric constant of ferroelectric ceramics shows strong Debye-l ike relaxation at GHz frequencies. This relaxation is caused by sound generation inside the crystallites. The ferroelastic domain walls whic h vibrate in the electric ac field are very effective shear wave trans ducers. The shear wave emission has a maximum when the wavelength of t he sound wave is comparable to the width of the domains. Above this fr equency the domain walls can no longer vibrate due to inertia and do n ot contribute to the dielectric constant. The observed dependence of t he microwave dispersion on temperature, doping and grain size of the f erroelectric ceramic is in good agreement with the proposed mechanism of the emission of acoustic shear waves.