Materials Science and Engineering, B25 (1994) 133-134 133 Thermal diffusivity of BaLiF 3 crystals M. Duarte, M. M. F. Vieira* and S. L. Baldochi lnstituto de Pesquisas Energ~ticas e Nucleares, Caixa Postal 11049, CEP 05422-970, Sdo Paulo (Brazil) (Received June 15, 1993) Abstract BaLiF3 crystals doped with transition metal ions are potential new laser active media. Thermal diffusivity of a pure BaLiF3 crystal was determined at room temperature using the photoacoustic two-beam phase lag method, which consists in measuring the relative phase-lag for the photoacoustic signal between rear and front surface illumination at a single modulation frequency. 1. Introduction The thermal diffusivity a is the parameter that pro- vides the rate at which heat flows through a medium. Knowledge of this parameter is very important in crystal growth, laser engineering and photoacoustic spectroscopy studies. Thermal diffusivity is related to thermal conductivity K through the equation a = K~ (pC), where p is the density and C is the thermal capacity at constant pressure. In this way it is possible to determine the thermal conductivity from the thermal diffusivity once p and C are known. In this paper we report on the determination of the thermal diffusivity value for pure BaLiF 3 crystals at room temperature. BaLiF 3 crystals are important materials since, when doped with transition metal ions, they are a potential new laser host. BaLiF 3 crystals are an "inverse" perovskite with cubic structure (space group Ohl-Pm3m), where the monovalent ion Li ÷ is at the center of six F- octhaedrons, whereas the Ba 2÷ divalent ions are in the 12-fold environment site, resulting in a different crystal field interaction from the classic perovskite structure [1]. The recent interest in this material is due to its expected tunable laser action in the infrared region at room temperature. 2. Methodology The applied methodology (photoacoustic two beam phase-lag method [2, 3]) involves measuring the rela- tive phase-lag between rear and front crystal surface *Author to whom correspondence should be addressed. 0921-5107/94/$7.00 SSD10921-5107(94)01054-L illumination at a single modulation frequency using conventional photoacoustic spectroscopy. Photo- acoustic signal generation is described using the Rosencwaig and Gersho (RG) thermal diffusion model [4, 5], giving the phase-lag A~ as a function of the sample thickness I and the thermal diffusion length/a: tan A~ = tan(l//a)tanh(l//a) ( 1 ) with/a--(2~-/to) where to is the illumination modula- tion frequency. The restrictions of the applied model are to consider that all the incident light is absorbed at the surface, and to assume that the heat flux into the surrounding air is negligible. For this reason optically opaque absorbing layers have to be provided, by thin film deposition for example. 3. Experimental details and results The pure BaLiF 3 crystal [6] was grown by the Czochralski method. The sample studied was 1.0 x 1.0 x 0.03 cm 3. The sample thickness was chosen in order to ensure that the thermal oscillations due to rear and front illumination are of the same order. A Kr laser with 0.5 W power, modulated at 40 Hz, was used as the excitation light. The photoacoustic cell employed was of the gas-microphone type (Fig. 1), with a commercial electret microphone, whose output was measured using a lock-in amplifier. The sample had both faces coated with silver paint, with a layer thick- ness of the order of 60/am, to ensure that all the inci- dent light would be absorbed at the crystal surface. As the thermal diffusivity for silver paint is considerably © 1994 - Elsevier Science S.A. All fights reserved