Optical Measurements of Diffusivities in Incommensurate Barium Sodium Niobate

Three different optical techniques have been used for measuring diffusivities in Ba2NaNb5O15: The original method of Burkhart and Rice (1977) yielded a2 thermal diffusivity of 0.02 cm2/s at 373K in the quasi-commensurate phase; dynamic central mode measurements of Oliver et al. (1988, 1989) yield a value 20x larger near 565K, at the hypothesized lq-2q transition in the middle of the incommensurate phase from 54 3-582K; and two quite different dif fusivities are measured at 855K=TC -a thermal diffusivity of 0.03 cm2/s from a new thermal lens effect recently discovered (Scott et al., 1989), and a much larger value of 1.2 cm2/s from dynamic central mode light scattering. This shows that at both TC and TI at least two separate diffusion processes occur; the light scattering studies are sensitive to the faster process, which at 565K may be related to the roughening transition of anti-phase boundaries originally predicted by Rice, Whitehouse and Littlewood (1981).