Brillouin-scattering investigation of a LaP5O14 crystal with periodic ferroelastic-domain structures.

Brillouin scattering has been used to study the elastic properties of a rare-earth pentaphosphate crystal ${\mathrm{LaP}}_{5}$${\mathrm{O}}_{14}$ that has periodic arrays of ferroelastic domains. Measurements of sound velocities for longitudinal and transverse acoustic waves showed that these velocities are lower than those in the ordinary polydomained crystal, due to the presence of coherent strains throughout whole crystal. In order to study the influence of the periodic ferroelastic domains on acoustic-wave propagation, we consider an infinite periodic ferroelastic-domain structure, with two types of a-type domains designated by basic elements A and B, respectively, and have successively determined the effective elastic tensor, specifically for ${\mathrm{LaP}}_{5}$${\mathrm{O}}_{14}$, in the long-wavelength regime. The theoretical results show that the softening of the sound velocity should be attributed to the effect of the periodic ferroelastic domains on the elastic constants.