Fe-doping in Ba0.75Pb0.25TiO3
Ferroelectric perovskite oxides including BaTiO3 (BT), PbTiO3 (PT), (Ba,Sr)TiO3 (BST), and Pb(Zr,Ti)O3 (PZT) are extremely important materials for their applicability in high dielectric materials, transducers, actuators, transistors, high-performance capacitors, sensors, and memory application. Coexistence and coupling of ferroelectricity with magnetism has become an extremely important domain of research in the last two decades which have led the field towards exploring a class of materials called multiferroics (materials that possess two ferroic orders; particularly ferroelectricity and ferromagnetism). Multiferroics have a wide range of electrical and electromagnetic applications similar to simple ferroelectrics and are used in random access memories, multiple-stage memories, spintronics. Theoretically, ferromagnetism and ferroelectricity are mutually exclusive phenomena. This is due to the requirement of transition metal d-electrons for presence of magnetism, while on the other hand, their presence results in reduction of the much needed lattice distortion (c/a) required for ferroelectricity. Thus, multiferroicity was believed to be an impossible phenomenon and remained unexplored for decades. Only a few decades ago, it was realized that structural or electronic driving force could generate coexisting ferroelectric and magnetic order in a single lattice.
There are only a few naturally occurring materials that exhibit multiferroicity. BiFeO3 is one of the most discussed multiferroic materials. However, it has low electrical resistivity which limits the ferroelectric order. New multiferroic materials have been explored in the last two decades. Strong ferroelectric materials, like PT, BT, etc., have been doped with elements that possess magnetic properties to achieve multiferroic properties. However, due to its toxic nature Pb has been banned from applications nowadays. Hence, purely Pb-based materials do not find proper utilization. However, the covalent bonding between Pb and O is a source of strong structural distortion in a perovskite material which may withstand considerable amounts of magnetic element doping for introduction of magnetism yet preserve the ferroelectric properties. Elimination of such a strong source reduces the possibility of achieving new effective multiferroic materials. In BT, the covalent bonding between Ba and O is not as strong as the Pb-O bond in PT. Hence, a small percentage of Fe leads to the dominance of the hexagonal phase. Such Fe-doping has revealed an introduction of the ferromagnetism but restricted the ferroelectric ordering at room temperature.
A magnetoelectric coupling was reported in Ba and Fe-substitute PT at room temperature. In a previous report, it was shown that the ferroelectric properties of BT can be enhanced considerably by replacing 25% Ba by Pb. With a deliberation of introducing ferromagnetism in the Ba0.75Pb0.25TiO3 lattice, substitution of B-site Ti atoms by a magnetic Fe atom is being attempted in this present work as a continuation to the previous work. A possible Fe-Fe exchange interaction is discussed to analyze the ferromagnetic behavior.
Ba0.75Pb0.25Ti1-xZrxO3
Piezoelectric materials are intensively used in sensors and actuators due to their extremely high electromechanical coupling coefficients and an ability to work under high mechanical stress. Compared with other materials for sensing and actuation, piezoelectric materials can work at high frequencies and temperatures, are chemically stable, and can be scaled down for use in micro and nano-electromechanical systems. Over the past few decades the most widely used piezoelectric materials have been the PbTiO3–PbZrO3 (PZT)-based systems, which exhibit good piezoelectricity. However, PZT is now facing a global restriction for its Pb toxicity.
As a typical ferroelectric perovskite, BaTiO3 (BT), has been extensively studied in the electronic industry and used as a passive component in capacitors. Barium titanate is the most common ferroelectric oxide in the perovskite ABO3 structure. BaTiO3 perovskite is an important ferroelectric which undergoes the paraelectric–ferroelectric (cubic to tetragonal symmetry) phase transition at 393 K. The phase further changes with lowering temperature to orthorhombic (278 K) and rhombohedral (183 K). Insulating BaTiO3 is widely used as a capacitor because of its high dielectric constant. To increase the tunability of dielectric constant and to reduce dielectric loss, ZrO2 was used as an addition. BaZrTiO3 (BZT) is found to be an effective candidate to replace the BaSrTiO3 (BST), which is a high dielectric material with low loss factor and one of the promising candidates for dynamic random access memory. The phase transition behavior in tetragonal to cubic phase transition temperature (Tm) changes rapidly with Zr concentration. With incorporation of Zr, a broad transition is observed in dielectric constant-temperature graph. Further increase in Zr content leads two phase transitions to coincide on each other and a single phase transition exists.
In BT, all the low temperature phases are ferroelectric, whereas the high temperature cubic phase is paraelectric. The value of the piezoelectric charge coefficient (d33) increases near phase transition temperature. Zr is also known for decreasing the tetragonal to cubic phase (Tc) temperature. As a result, at higher Zr concentration, the transition temperature falls down which restricts many industrial applications. Pb is one of the elements known for increasing Tc in perovskite. In BT, 25% Pb incorporation shifts the Tc at ~250 °C. In present study, the parent material is chosen to be Ba0.75Pb0.25TiO3, where the Pb% is reduced by a factor of 300% as compared to PZT based materials. Zr concentration is varied and the corresponding structural, dielectric and ferroelectric properties have been studied.
Electrical behavior of the materials including ac conductivity is also studied at various temperatures for all the samples. An analytical description of the variation of ac conductivity at different temperatures is also vigorously discussed. The contribution from grain and grain boundary are also defined at different temperatures for different resistive materials.