Département de physique

Dielectric properties of perovskites with polar disorder (K1-xLix TaO3 and Pb[Mg1/3Nb2/3]O3) and of SrTiO3 films

Christen, Hans-Martin ; Châtelain, André (Dir.)

Thèse Ecole polytechnique fédérale de Lausanne EPFL : 1994 ; no 1235.

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    Dielectric properties of perovskites are studied both to gain insight into fundamental aspects of the freezing of polar disorder, and to investigate their technological potential. The dynamics of freezing of polar disorder is investigated in single crystals of Li-doped potassium tantalate (K1-xLixTaO3) and lead magnesium niobate (pb[Mg1/3Nb2/3]O3). For the mixed crystal K1-xLixTaO3, the dielectric permittivity ε (ω) is measured at various temperatures and for frequencies ω/2π ranging from 2 × 10-4 to 109 Hz. These results are complemented by measurements of field-induced pyroelectric currents and polarization vs field loops. Further information is gained from microscopic model calculations of the Li-Li interaction energy. The present study reveals that this material can neither be described by a spin-glass model nor as a ferroelectric. In particular, the polarization of previously reported nanometer-sized clusters cannot be switched by an applied electric field. The analysis of the numerical results of the Li-Li interaction energy reveals the importance of elastic interactions. Nearest-neighbor effects are observed in the shape of the ε (ω) curves. Finally, the freezing of disorder is interpreted as a consequence of hierarchical constraints. For the "prototype" relaxor Pb(Mg1/3Nb2/3)O3, a comparison between new dielectric results and published data provides evidence of two relaxation branches. Phenomenologically, they bear strong similarities to those of glass-forming polymers and are thus labelled α- and β-type relaxations. The α branch is responsible for the characteristic peak in the temperature dependence of the permittivity. The parameters describing this relaxation show no anomalies or extrema in the investigated temperature range, and the peak is seen to be of purely dynamic origin, rather than being related to a structural phase transition. This confirms earlier claims that Pb(Mg1/3Nb2/3)O3 is nontransforming in the absence of an applied electric field. Motivated by a technological interest in highly polarizable dielectric layers, epitaxial SrTiO3 films with thicknesses between 50 and 330 nm are studied using Mg/SrTiO3/-SrTiO3:Nb heterostructures. Their low-temperature dielectric properties differ from those of single-crystal SrTiO3 samples, but can be understood in the framework of a simple model built for this purpose. This approach takes into consideration the non-linear polarization vs field dependence of SrTiO3 and assumes strong charge trapping at the film/substrate interface. The present films are characterized by a high permittivity: ε ≈ 100 is reproducibly obtained at 4.2 K, corresponding to capacitances of the order of 10 nF/mm2. Large charge densities (80 mC/m2) can be induced by applied voltages as low as ± 5 V.