Brillouin spectroscopy of acoustic phonon dispersion in Ba2NaNb5O15 at its incommensurate transition

Barium sodium niobate, Ba2NaNb5O15, is known to undergo an incommensurate phase transition at TI ≈ 300°C from a tetragonal structure to an orthorhombic one, with the incommensurate direction along the orthorhombic [100], i.e. tetragonal [110], axis. Sound velocity anomalies were reported earlier for ultrasonic frequencies and for right-angle Brillouin scattering. We have found strong dispersion in the orthorhombic elastic coefficients C11(ω) and C22(ω) near TI, using near-forward and near-backward Brillouin scattering. The frequency region 4-63 GHz was probed. The data exhibit two characteristic relaxation times; of these, one is on a time scale between 10 MHz and 1 GHz and is inaccessible to our measurements, whereas the second satisfies [image omitted] with τo = 1.7 × 10-13 sec, and τ1 = 5.2 × 10-12 sec, and D = 1.3 cm2/sec. These relaxation times are evaluated from sound velocity dispersion ΔV(ω)/V and linewidths Ω(ω, T). A frequency dependent increase in linewidth (maximum increase is 300%) is observed near TI.