Structural, Electrical and Optical properties of rare earth Nd and Eu substitution in lead free Na0.5Bi0.5TiO3 based ferro/piezoelectric ceramics

Multi-functional materials possessing two or more desirable properties in a single entity have attracted broad interest in recent years. To this end, the utility of ferroelectric materials is now being explored. Various two-parameter coupling effects such as electro-mechanical interactions, electro-optical interaction, electro-magnetic coupling, and magneto-optical effects, have been observed and extensively studied. New coupling effects in materials that can realize multiple functions are desirable for developing multi-functional materials and devices. In this thesis we investigated the possibility of combining Piezoelectric and photoluminescence (PL) properties in lead-free ferroelectric systems. In multifunctional materials, tuning the structural sensitivity as well as photoluminescence efficiency by applying electrical fields has great attention due to desired applications in day to day life. Even though PZT (i.e., Pb (Zr, Ti)O3) is widely used owing to its superior piezoelectric properties, the prime concern is toxicity of lead (Pb) brought out urgent need in finding/developing new lead free materials as alternative to PZT. Among the several lead-free ferro/piezoelectric ceramics, Na0.5Bi0.5TiO3 (NBT) has emerged as a potential candidate to replace lead based piezoelectric materials. The lead-free exhibits a strong ferroelectric character with high remnant polarization (Pr=38μC/cm2) at room temperature. As per aforementioned advantages, has been chosen as a parent compound for the studies presented in this thesis. The tailoring of physical properties can be achieved by specific site substitution followed by solid solution approach. To achieve these properties, two optically-active rare earth ions Neodymium (Nd3+) and Europium (Eu3+) were selected as activators to substitute site specifically at Bi3+site as per ionic size criterion (i.e. ionic size difference is less than 15%). The optimally substituted compounds were expected to exhibit PL and as well as enhance the piezoelectric properties. Investigations of Nd3+ substituted NBT (NBNT) revealed enhanced properties upon substitution of 1 mol%, including best PL intensity, lower coercivity 50kV/cm, large viii piezoelectric coefficient d33 of 110pC/N and higher off-resonance figure of merit ( ) for energy harvesting by and respectively in comparison with un-substituted NBT. For Eu3+ substituted NBT (NBET) the optimal substitution was for 2.5 mol%. The optimized NBET coercivity (Ec) is 42kV/cm, bipolar strain 0.15(%) as well as optimized luminescent character turned to be a cross over composition with respect to all the property correlation. The results presented here convey a simple material system, yet, with elegant multifunctional properties. Therefore, the optimally substituted NBTs displayed convincing reasons to be a potential multifunctional material exhibiting simultaneous, ferroelectric, and luminescent properties for energy harvesting properties. Considering NBNT is a ferroelectric ceramic, it subjected to electric field during the routine device operation making it vital to understand the effect of applied electric field on the PL. Upon electrical poling, PL is observed to have quenched in the intensity compared to the respective unpoled samples. We elucidate this observation qualitatively by invoking the Judd–Ofelt theory. It is well known that the Nd3+ emission is extremely sensitive to the local host symmetry. The important factors affecting the local environments of Nd3+ in NBT pertain to fluctuations in the Na: Bi ratio from 1:1 as a result of substitution, different chemical ordering at the local scale and disorder associated with atomic displacements. With the solid matrix with fixed lattice position for Na and Bi, the external electric field is expected to induce long-range order in the polar displacements of the atoms. The XRD investigation reveals the electrical poling, indeed, induces structural order and the NBNT transform from the lower symmetry ( ) to a higher symmetry ( ) resulting in the change in the local host environment and consequently quenching PL intensity. One of the major shortcomings for NBT as well as RE-substituted NBT is its large coercivity (EC 60 kV/cm), high dielectric loss and high conductivity. These issues were addressed with the help of specific site substitution and fabricating solid solutions (like rare earth-doped NBT-BT) with higher symmetry matrix to minimization of Ec and conductivity which is useful for the storage device applications a solid solution approach is adopted for enhancing above structural, electrical and luminescence properties to explore in different fields of applications, such as energy storage and electro caloric effects. The substitution of Nd3+ in NBT-BT induces relaxor future more prominently; this feature is more favourable to explore ix different fields of applications such as energy storage and electro caloric effect. Here we obtained the substitution of Nd3+ in NBT-BT induce the energy storage density 1.5 J/cm3 with efficiency 93% and the induce temperature change T=1.5K from electro caloric effect was observed