PREFACE

In recent years there has been an increasing interest in optically excited thermal processes, known as photothermal effects, which use lasers as optical heat sources. The photothermal effect in a material is a consequence of the deposition of heat in the sample following absorption of a light beam and subsequent thermal deexcitaions, or other nonthermal deexcitation processes, which result in the indirect heating of the sample. If the excitation is modulated, the corresponding temperature variations developed in the sample gives rise to a variety of different effects and \

most directly, to periodic temperature variations of the sample, which constitute the basis for a distinct experimental technique, known as photopyroelectric (PPE) effect. The PPE technique involves measurement of the temperature variations in the sample due to absorption of radiation, by placing a pyroclectric detector in thermal contact with the sample. The PPE technique is the only technique based on the direct detection of photothermal heating, and therefore has a number of advantages over other detection schemes involving secondary mechanisms (like photoacoustic technique). A PPE experiment is rather simple to design and the materials that can be investigated range from \veakly absorbing solids like thin films or liquids, to strongly absorbing solids and liquids or highly diffusing materials. Even though much progress has been made on both theoretical and experimental fronts, enough efforts have not been made to exploit the advantages of the photopyroelectric technique in the measurement of thermal parameters, like thermal diffusivity (a), thermal effusivity (e), thermal conductivity (K) and heat capacity (cp ) of solid samples. The technique is particularly useful to study thermal

properties of samples undergoing transitions such as ferroclectric phase transitions. metal-insulator transitions etc. In this thesis we present the results of our systematic investigation of the variations in the thermal parameters across phase transitions in selected systems employing photopyroclectric technique. Para-ferroelectric phase transitions are always associated with atomic rearrangements or structural changes. The change of structure during a phase transition can occur in two distinct ways. Firstly, there are transitions where the atoms of a solid reconstruct a new lattice as in the case of an amorphous solid changing to a crystalline state. Secondly, there are transitions where a regular lattice is distorted slightly without disrupting the linkage of the network. This can occur as a result of small displacements in the lattice position of single atoms or groups. or the ordering of molecules among various equivalent positions. Most of the ferroelectric phase transitions belong to the second group. Since all the phase transitions involve configurational changes, one can identify a physical quantity that is characteristic of the new ordered configuration. Such a concept of order parameter was introduced by Landau. In a ferroelcctrc transition the order parameter is the spontaneous polarization and he expanded the Gibbs free energy in powers of the order parameter in the vicinity of a transition where the value of order parameter is very small. Accordingly, a transition is said to be of the same order as the derivative of the Gibb's function, which shows a discontinuous change at the transition. If there is a discontinuity in quantities sllch as volume and entropy, which are first order derivatives of Gibb's function. the transition is said to be of first order. If specific heat. compressibility, thermal expansion etc. are quantities undergoing discontinuity. which arc second order

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derivatives of Gibb's free energy function. the transition is said to be second order. Our aim has been to study the variations of heat capacity and thermal conductivity of selected ferroelcctrics and analyze the nature of phase transitions associated with

it. Mixed valence perovskites. with the general formula R1.\ArMnOJ with (R = La. Nd or Pr and A = Ba. Ca. Sr or Pb) have been the materials of intense experimental and theoretical studies over the past few years. These materials show colossal magneto resistance (CMR) in samples with 0.2 < x < 0.5. In such a doping region. resistivity exhibits a peak at temperature T = Tp. the metal-insulator transition temperature. The system exhibits metallic characteristics with dp / dT > 0 below

7~}

(where p is the resistivity of the sample) and insulating characteristics with dp; £IT