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Abstract

This work presents experimental studies of ordering phenomena, electromechanical and electrocaloric properties in single crystal, ceramic and solid solution relaxor ferroelectrics and single crystal ferroelectric materials. The main aim of investigations is to prove the existence of a liquid-vapor critical point in these systems and to verify its impact on physical properties, such as electromechanical and electrocaloric response.
In the first part, the study of relaxor ferroelectric single crystal Pb(Mg1/3Nb2/3)O3 (PMN) oriented along the [110] direction performed via polarization and calorimetric measurements is presented. Motivated by the long-standing unresolved enigma of the relaxor ferroelectric ground state, a high-resolution heat capacity and polarization experiments were performed in the vicinity of the field-induced ferroelectric phase transition in the relaxor ferroelectric single crystal PMN. It is shown that the discontinuous evolution of the polarization as a function of the electric field or temperature is a consequence of a true first-order transition from a glassy to ferroelectric state, which is accompanied by an excess heat capacity anomaly and released latent heat. It is also shown that in a zero field, there is no ferroelectric phase transition in bulk PMN at any temperature, indicating that the nonergodic dipolar glass phase persists down to the lowest temperatures. Furthermore, the detailed study of the latent heat reveals that the first-order phase transition line ends in the liquid-vapor critical point. In addition, the presented calorimetric results are shown to be in a good agreement with the upgraded version of the spherical random bond random field model developed by R. Pirc. The presence of the critical point is tested also in a relaxor ceramic Pb(1-x)Lax(ZryTi(1-y))(1-x/4)O3 (PLZT) with relaxor composition 9/65/35 and ferroelectric single crystal BaTiO3 oriented in [001] direction. The heterogeneity, typical for the ceramic material, plays an important rule in the observed smeared position of the critical point. On the other hand, a cubic to tetragonal first-order phase transition line in BaTiO3 ferroelectric clearly ends in a sharply defined critical point.
The second part of the work is devoted to the influence of the critical point on electromechanical and electrocaloric response in solid solution Pb(Mg1/3Nb2/3)O3-0.26PbTiO3 (PMN-0.26PT) oriented in [100] direction and ferroelectric single crystal BaTiO3 oriented in [001] direction. Special attention is paid to the mechanism responsible for the enhancement of the piezoelectric coefficient in PMN-0.26PT and BaTiO3, material with and without morphotropic phase boundary, respectively. The results obtained in both materials suggest that the proximity of the critical point is a driving mechanism for the enhancement of piezoelectric responses and not the presence of the morphotropic phase boundary. In addition, the influence of the proximity of critical point on the electrocaloric (EC) response in the BaTiO3 ferroelectric single crystal was investigated. In particular, the temperature change related to the released latent heat at the first-order cubic to tetragonal phase transition and the temperature change related to the continuous variation of the polarization due to the applied electric field were studied. The experimental results show the shift of EC responsivity maximum away from the critical point. It is suggested that the shift of the EC responsivity maximum is directly related to the ratio between two electrocaloric contributions, i.e., the relatively large amount of the released latent heat at the paraelectric to ferroelectric phase transition and the contribution related to the continuous variation of the polarization. The amount of the released latent heat decreases fast when approaching the critical point and accordingly the corresponding electrocaloric contribution cannot be compensated by the electrocaloric contribution stemming from continuous variations of the polarization. This results in decreasing of the EC responsivity when approaching the critical point and consequently in the shift of the EC responsivity maximum away from the critical point.
The main purpose of the doctoral dissertation is to better understand critical physical mechanisms responsible for the enhancement of certain physical properties of advanced ferroelectric and relaxor materials which are important for the engineering of new high technology devices, such as novel advanced sensors, actuators, temperature controlling elements, ultrasound generators etc.