Structural, optical and Raman spectroscopy investigations on pure and Hf-Doped barium titanate

Abstract

Perovskite-type oxides with the chemical formula ABO3 have attracted much attention owing to their various unique properties and potential technological applications[1–4]. These oxide perovskites have great advantages as one tailor the properties of these materials such as its crystallographic, electronic, optical properties by means of doping at A and B site independently or simultaneously [5–8]. The most recent novel applications of perovskites are based on piezoelectric, ferroelectric, and related phenomena. The study of ferroelectricity in perovskite compounds has been a vast subject for investigation, not only because of its technical importance but also for understanding the physics of phase transitions[9,10]. The ferroelectric and piezoelectricity properties have mostly been observed in lead-based perovskite compounds, such as (PbLa)(ZrTi)O3(PLZT), Pb(Sc1/2Ta1/2)O3(PST), and Pb(Mg1/3Nb2/3)O3(PMN)[11–14]. However, these compositions bear a possible hazard due to the toxicity and volatility of lead, which has inspired the development of lead-free ceramics. Therefore, a lot of effort has been made towards investigating environmentally friendly ‘Pb-free’ ceramic materials. Doping impurity in ceramics is a common way of improving its performance. In recent years, BaTiO3 with isovalent substituted materials are found to be promising candidates for potential applications in the areas of microwave communications and optoelectronic devices due to its high dielectric constant and low loss characteristics[5,6,15,16]. Moreover, the physical properties of BaTiO3 are sensitive to variation in temperature, particle size, impurities, crystal structure, and defect densities[17,18]. The solid solution of BaTiO3 with any other suitable perovskite compounds enhance various exciting properties and device feasibility. It is very easy to dope respective materials at Ba and Ti sites independently or simultaneously. Extensive studies have been carried out for studying the effect of substitution on dielectric relaxation, electrical properties, the ferroelectric phase transition of BaTiO3[6,19– 21]. All these properties will depend on the type of dopant and the occupied site of the dopant in the BaTiO3 lattice. The dopant incorporation into A or B site of any perovskite depends upon the ionic radii difference between the dopant and the corresponding site. Because of the large difference in the ionic radii of Ba (1.47Å) and Ti (0.68Å), the relatively small ions go to the Ti site whereas the larger ions go to the Ba site. Dopant valency also alters the various physical and chemical properties of BaTiO3. Substituting material having the same valency as that of Ba and Ti ion does not create any vacancy in the system whereas doping material having different valency causes charge imbalance for maintaining electrical charge neutrality which requires the creation of vacancy on Ba/Ti site or in oxygen sublattice[6,20,22]. Based on the various applications different types of dopants are taken, for example, on transducer application A-site doping with Sr2+ is used to reduce Curie temperature below 1200C whereas Pb2+ is doped to vary Curie temperature above 1200C[23,24]. Also, to increase the stability range of the tetragonal phase Ca2+ is doped at A-site[15]. The reason behind decreasing the Curie temperature is to achieve a high permittivity value near room temperature. A large number of modified lead-free BaTiO3 based ceramics have been studied, due to their environmentally friendly properties, and some of them are potentially valuable materials.