Solid Solutions of Lead Metaniobate-Stabilization of the Ferroelectric Polymorph and the Effect on the Lattice Parameters, Dielectric, Ferroelectric, and Piezoelectric Properties

Ferroelectric orthorhombic lead metaniobate (PbNb2O6) is known to be metastable with respect to the thermodynamically stable nonferroelectric rhombohedral polymorph. The high-temperature tetragonal to low temperature rhombohedral phase transition is reconstructive and thereby sluggish; ferroelectric PbNb2O6 is obtained by quenching from the stable phase field of the tetragonal polymorph. We report on the stabilization of the ferroelectric tungsten bronze polymorphs of PbNb2O6 by minor chemical substitution in the series [(1 - x) PbNb2O6-xBiTiNbO(6)], [(1 - x)PbNb2O6-xNa(0.5)Bi(0.5)Nb(2)O(6)], [(1 - x)PbNb2O6-xK(0.5)Bi(0.5)Nb(2)O(6)], and [(1 - x)PbNb2O6-xCaTiO(3)]. The high- temperature tungsten bronze polymorph is entropy stabilized with respect to the stable rhombohedral polymorph, and we propose that the tungsten bronze is further entropy stabilized by chemical substitution, reducing the transition temperature of the rhombohedral polymorph and further disfavoring the kinetics of the undesired phase transition. Optimized solid-state synthesis and processing to obtain dense ceramics were developed for the solid solutions, and the dielectric, ferroelectric, and piezoelectric properties of the PbNb2O6 solid solutions are reported. Curie temperature is suppressed with chemical substitution in all the systems. Lattice cell parameters display systematic variation with composition, reducing the molar volume, and the lattice parameter ratio 2b/a with increasing degree of substitution, reflecting a suppression of the polarization along the (010) direction due to chemical substitution. The piezoelectric properties improved with increasing substitution level probably due to the ease of poling of the materials with lower T-c. However, some improvements seen with 2% CaTiO3 were not accompanied by T-c decrease.

Pulsed laser deposition of (SrBa)Nb2O6 thin films and their properties

Anna Infortuna

Strontium barium niobate (SrxBa1-xNb2O6, shortly SBN) is a solid solution system with tetragonal tungsten bronze crystal structure. It exhibits a ferroelectric phase with only one polar axis and a transition temperature depending on the Sr/Ba ratio. This thesis studies the growth of SBN thin films, in order to obtain a ferroelectric thin film model system for the study of 180° domain switching close to the ferroelectric phase transition. SBN has been chosen not only because uniaxial, but because it is possible to tune the temperature of phase transition varying the Sr/Ba ratio. It is thus possible to study electric properties of the phase transition close to room temperature, where conduction phenomena related to defects are normally lower. Thin films of SBN were grown by Pulsed Laser Deposition in a vacuum system constructed during this thesis work. For the formulation of a model it is desirable to have a system as close as possible to a single crystal. Therefore deposition parameters have been optimized to grow SBN thin films with the polar axis oriented perpendicular to the film plane. The films are integrated in parallel plate capacitor structures, in order to measure dielectric properties. The stability of the ferroelectric phase is found to be sensitive to impurities; contaminants with concentration below 1% induce a lowering of the transition temperature of about 100°C. Strain as well has influence on the phase transition; in the case of substrate with thermal expansion coefficients different from the SBN ones, the SBN film is under stress. A 0.6% tensile strain induces a lowering of the transition temperature of about 100°C, this is the case of silicon single crystal substrate. Grown on strontium titanate single crystals (STO), whose thermo-mechanical properties are close to the one of SBN, the films exhibit a phase transition temperature compatible with the one measured on single crystals. The obtained films suffer from leakage that is reduced by thermal treatment in oxygen rich atmosphere. The oxygen vacancies, created during the deposition process, are considered to be responsible of leakage. Vacancies recovery upon annealing does not eliminate the problem, and measurement of polarization at room temperature is difficult. However, piezoelectric measurements performed on individual grains, with atomic force microscope, prove that these have ferroelectric properties; by these experiment has been proved that it is possible to switch the polarization. Films with a disordered structure of grains are much less leaky than the ones with columnar grains spanning the whole cross section. These observations lead to the conclusion that conduction goes trough the grain boundaries. On STO single crystals, SBN grows epitaxial in Volmer-Weber mode. With a suitable preparation of the substrate surface, it is possible to induce the growth of films with different orientations of the polar axis: in the film plane, perpendicular to it or inclined of 45°. A model is proposed, explaining the epitaxial relation with the substrate on the basis of the perovskite kernel contained in the unit cell of the tetragonal tungsten bronze structure. The film grows from the nucleation of the perovskite structure that rules the orientation of the film; the high temperature at which the substrate is kept during the deposition, assure the necessary mobility for the arriving atoms to organize in the SBN structure around these nuclei. The study of literature data allows to state that such a model is valid not only for substrates with perovskite structure, but can be extended to substrates with a cubic crystal structure, like magnesium oxide.

EPFL2005

Ferroelectric Lead Metaniobate and its Solid Solutions: Solid State Synthesis and Characterization

Nava Setter

Orthorhombic Lead metaniobate (PbNb2O6) exhibits ferroelectric properties and has a Curie temperature, TC of 570 degrees C. This work reports on investigations made on ferroelectric Lead metaniobate, (PN) and its solid solutions with bismuth and titanium Pb(1-X)BiX(Nb(1-X/2)TiX/2)(2)O-6, (PBTN), bismuth and sodium Pb(1-X)BiX/2NaX/2Nb2O6, (PBNN), bismuth and potassium Pb(1-X)BiX/2KX/2Nb2O6,(PBKN), and calcium and titanium (1-x)PbNb2O6-xCaTiO(2), (PCTN). The ferroelectric polymorph was obtained for all the solid solutions without quenching, and the optimized sintering temperature was reduced with increasing substitution level except in the system PCTN. The lattice parameters of the solid solutions were found to systematically vary with composition. Curie temperature was reduced with increasing substitution level in all the series. Piezoelectric charge constant d33 increases with composition of the solid solutions.

Ieee2012

Films minces relaxeur-ferroélectriques à base de Pb(Mg1/3NB2/3)O3

Zian Kighelman

Relaxor Pb(Mg1/3Nb2/3)O3(PMN) and its solid solutions with ferroelectric PbTiO3 (PT) are of considerable interest from both the applications and the scientific point of view. In the past, many attempts were made to prepare and study the properties of these material in thin film form. However, due to difficulties in the preparation of pure phase films with high PMN content, there exists very little knowledge on the properties of these important materials. It is the goal of this thesis to prepare PMN and PMN-PT films without second any phases, to study in detail their structure, dielectric and electromechanical properties, and to see how these properties compare with those of the bulk materials. The studies were carried out along three major directions: PMN, 0.9PMN-0.1PT and PT chemical solution precursor syntheses, thin films preparation, dielectric and electromechanical properties characterization and their interpretation. The synthesis of precursors used for the preparation of PMN, 0.9PMN-0.1PT and PT films is reported in detail. Careful control and optimization of the precursors was important to obtain thin films without second any phases. The presence of new compounds and reaction mechanisms were established in the course of developing reproducible and stable precursors solutions. Crystallization of pure PMN and 09PMN-0.1PT films requires high temperatures (∼800°C) and lead excess in the precursor solutions making processing difficult. The correct choice of seeding layers that favor perovskite phase nucleation and growth minimizing substrate instabilities at high temperatures was essential. This approach allowed us to find a narrow processing window that lead to films without pyrochlore and other second any phases. Important PMN and 09PMN-0.1PT processing parameters are reported. It is shown that crystallographic orientation and microstructure could be modified by controlling the perovskite phase nucleation and growth with different seeding layers (TiO2 and PbTiO3). PMN film epitaxial growth on conductive SrTiO3 single crystals allowed an original transmission electron microscopy study. The results showed that PMN in thin film and bulk forms shows the same structural characteristics (morphology and size of polar regions). However, temperature dependence of unit cell parameters of PMN and 09PMN-0.1PT films seems to be qualitatively different from these obtained in bulk materials. PMN and 0.9PMN-0.1PT thin films show typical relaxor characteristics (dielectric permittivity dependence on frequency and temperature, presence of polar regions, superstructure, ferroelectric hysteresis at low temperature) however significant differences exist between films and bulk materials. Most of these differences (low permittivity, presence of the self-polarization) are at least in part related to the properties of the substrate used, as well as the conditions of films preparation. The presented results clearly show the influence of the measuring signal and processing parameters (AC and DC filed amplitude, sintering temperature, substrate quality) on the dielectric response of the films. Due to the small thickness of the films, the non-linearity of the dielectric properties of PMN and 0.9PMN-0.1PT could be studied over a field range never before used in the characterization of bulk samples. The choice of measuring conditions (field strength) can significantly alter the dielectric behavior of the films, and the use of large fields can explain certain results published in the literature. By investigating the AC and DC field dependence of the dielectric permittivity for oriented films it was possible to show that the decrease of the permittivity with the field, at strong fields, is consistent with the model of coalescence of neighboring polar regions. For the first time, the electromechanical properties of PMN and 09PMN-0.1PT films were characterized in details. Longitudinal electrostrictive (M and Q) and induced piezoelectric (d33) coefficients were determined for both compositions. Compared to bulk materials, it seems that thin film properties are reduced selectively. The reduction of dielectric permittivity and, consequently, the electrostrictive coefficient M is large, whereas the value of the Q coefficient is close to that in bulk materials. The preparation, dielectric, electrostrictive and piezoelectric properties of PbTiO3 films, as end member of the (1-x)PMN-(x)PT binary system, were characterized in detail. Stresses in the plane of the film, which are due to the clamping of the film by the substrate, are most likely responsible for the decrease in the temperature of the dielectric permittivity maximum. It was found that clamping of the film by the substrate does not have a strong influence on the value of the electrostrictive coefficients. The contribution of domain-walls displacement to dielectric and piezoelectric properties was investigated in detail and found to be non-negligible.