Journal of Optoelectronics and Advanced Materials Vol. 7, No. 1, February 2005, p. 537 - 540 OPTICAL INVESTIGATION OF A Pb 2 ScTaO 6 CRYSTAL WITH A ROOM TEMPERATURE PHASE TRANSITION Y. Sarov * , V. Marinova, M. Gospodinov a , S. Sainov Central Laboratory of Optical Storage and Processing of Information, Bulgarian Academy of Science, P.O. Box 95, 1113 Sofia, Bulgaria a Institute of Solid State Physics, Bulgarian Academy of Science 72, Tzarigradsko chausse Blvd. 1784 Sofia, Bulgaria The optical properties of a Pb 2 ScTaO 6 single crystal are investigated in the room temperature phase transition region. The measured refractometric data are used to determine the average temperature and the interval of the diffuse phase transition -15 ± 4 ˚ C. The dependence of the crystal's molar polarizability on temperature is determined from the refractometric results. (Received December 9, 2004; accepted January 26, 2005) Keywords: Ferroelectrics phase transition, Refractive index 1. Introduction Lead scandium tantalate Pb 2 ScTaO 6 (PST) single crystals are ferroelectric relaxors, with a high dielectric constant which changes smoothly with temperature and has a strong frequency dispersion. A diffusive dielectric response with broad maxima, high electrostriction coefficients and switchable pyroelectric-piezoelectric properties are the main characteristics of such ferroelectrics. PST exhibits the perovskite structure A(B' x B'' 1-x )O 3 , where A is Pb 2+ and B' - B'' are the low (Sc 3+ ) and high-(Ta 5+ ) valence cations, respectively. The arrangement of the two B-site cations plays an important role in determining the diffuseness of the ferroelectric phase transition. A structural paraelectric (PE) to ferroelectric (FE) diffuse phase transition between 263 and 298 K has been observed [1]. A PE-FE phase transition accompanied by the creation of ferroelectric domains of ordered Sc 3+ -Ta 5+ atoms with sizes from tenths to hundreds of nm, and a change from Fm3m (cubic) to R3m (rhombohedral) symmetry has been established using TEM and X-ray diffraction [2-3]. The particular PE-FE or disordered-ordered state transition temperature and ordering parameter depend on the growth conditions, the purity of the starting materials and the degree of structural order (the positions of Sc 3+ and Ta 5+ atoms in the crystal lattice). PST crystals find a wide range of applications as capacitor materials, pyroelectric detectors, ultrasonic and modern medical devices, as well as in information storage [4-7]. Furthermore, the diffuse phase transition close to room temperature allows easy PE-FE investigations and analysis of the nature of the physical properties. In this paper, we report the refractive index (RI) change during the PE-FE transition of a PST crystal. The measured RI data were used to determine the average temperature and the range of the diffuse phase transition. The result was confirmed by reflectance measurements, and by independent reference sources. The refractometric data were used to calculate the variation in the PST molar polarizability during the phase transition, using the Lorenz-Lorentz model. The unit cell restructuring during the PE-FE transition appears to explain the effect. * Corresponding author: ysarov@yahoo.com