ANISOTROPIC THERMAL-LENS EFFECT IN FERROELECTRIC BA2NANB5O15 AT TC

We have discovered a novel effect in barium sodium niobate at its Curie temperature Tc=853 K, in which the large positive temperature derivative of the c-axis index of refraction associated with the ferroelectric phase transition, dnc/dT=+1.4×10-3 K-1, produces a convex (positive) thermal lens of typical focal length 3 cm. This is 30 times stronger than the isotropic thermal-lens effect found in fluids by Gordon et al., and of the opposite sign. This effect potentially provides an accurate way of measuring dn/dT very near phase-transition temperatures, which may be of interest in studies of critical phenomena associated with structural phase transitions in solids. We find 2=0.58±0.05 in the present study. Temporal effects are found to be in accord with the thermal-focusing theories for fluids and yield the thermal diffusivity at Tc accurately as well. © 1990 The American Physical Society.