Ferroelectrics, 369:76–84, 2008 Copyright © Taylor & Francis Group, LLC ISSN: 0015-0193 print / 1563-5112 online DOI: 0.1080/00150190802375201 Thermal Diffusivity and Critical Behaviour of Uniaxial Ferroelectric Pb 5 Ge 3 O 11 A. OLEAGA, 1,∗ A. SALAZAR, 1 M. MASSOT, 1 A. MOLAK, 2 AND M. KORALEWSKI 3 1 Departamento F´ ısica Aplicada I, Escuela T´ ecnica Superior de Ingenier´ ıa, Universidad del Pa´ ıs Vasco, Alameda Urquijo s/n 48013-Bilbao, Spain 2 Institute of Physics, University of Silesia, ul. Uniwersytecka 4, 40-007 Katowice, Poland 3 Institute of Physics, Adam Mickiewicz University, ul. Umultowska 85, 61-614 Poznan, Poland Thermal diffusivity of uniaxial ferroelectric Pb 5 Ge 3 O 11 has been measured as a function of temperature using ac photopyroelectric calorimetry. The second order ferroelectric transition has been studied in order to ascertain the mechanisms which could explain the anomaly in specific heat through the study of the inverse of the thermal diffusivity. It has been shown that Pb 5 Ge 3 O 11 falls within the universality class of the mean-field model: the ferroelectric phase is very well explained within the framework of the Landau model, while in the paralectric phase a logarithmic correction must be taken into account, as predicted by renormalization group theory. Keywords Critical phenomena; ferroelectricity; phase transition; thermal diffusivity; thermodynamic properties 1. Introduction Lead germanate crystal Pb 5 Ge 3 O 11 is a uniaxial ferroelectric with interesting applications as pyroelectric material. It exhibits a second-order phase transition from the ferroelectric phase with trigonal group P3 to the paraelectric phase with a hexagonal space group P ¯ 6, at 450 K [1]. The structure consists of two layers arranged alternately along the c-axis in the Pb frame: a layer of GeO 4 tetrahedra and a layer of Ge 2 O 7 double tetrahedra [2, 3]. Different research groups have studied the dielectric, optical, pyroelectric, elastic or electro- mechanical properties [4–10]. On the other hand, thermal properties (such as specific heat, thermal conductivity, thermal diffusivity), have been scarcely studied [11]. There is agreement that the second order transition in this uniaxial ferroelectric is well described by the Landau theory [12, 13], but the critical behaviour of the transition has been studied so far only through spontaneous polarization [13, 14] and susceptibility data [5, 15, 16], with no attempts to obtain it through thermal properties such as specific heat or related magnitudes. The purpose of this work is to study the critical behaviour of the ferroelectric to paraelectric transition through the study of thermal diffusivity, using a high-resolution Received August 29, 2007; in final form February 25, 2008. ∗ Corresponding author. Tel +34946014008; Fax +34946013939. E-mail: alberto.oleaga@ehu.es [552]/76 Downloaded By: [Universidad Pablo de Olavide] At: 12:44 9 December 2008