Use of the thermal lens effect at TC= 855K in Ba2NaNb5O15to measure order parameter evolution

Abstract
By measuring the beam divergence θ (T) as a function of temperature in the recently discovered thermal lens effect in ferroelectrics (J. F. Scott et al., Phys. Rev. B, in press, 1989), we have been able to determine preliminary values of the critical exponent β. We find that 2β = 0.50 ± 0.05 in agreement with mean field predictions. We also find an unexpected deviation from mean field within ∼ 5K of TC. This is probably due to defect mechanisms and agrees quantitatively with birefringence results elsewhere (E. Salje, U. Bismayer, and M. Jansen, J. Phys. C20, 3613 (1987); I. G. Wood and A. M. Glazer, J. Appl. Cryst. 13, 217 (1980)).