Photoacoustic investigations on thermal anisotropy in urea l-malic acid single crystals

Laser-induced photoacoustic technique in the closed cell configuration has been employed to study the anisotropy in thermal parameters of urea l-malic acid, a nonlinear optical crystal. Thermal diffusivity values of the specimen along different crystallographic axes are evaluated from the measured amplitude and phase spectra of photoacoustic signal using one-dimensional model of Rosencwaig and Gersho. Thermal conductivity of the crystal along different axes is calculated using the measured thermal diffusivity and specific heat values. The measured thermal parameters are correlated with mechanical hardness as well as the bonding scheme of the crystal. Analyses of the results show that there is considerable anisotropy in heat diffusion parameters along different crystal orientations. Results are explained on the basis of crystallographic structure and the phonon-assisted heat diffusion mechanism in crystals.