We use the spin polarized pseudofunction(PSF) method with the local-spin-density approximation (LSDA) to calculate the electronic structures and the total and partial charge densities (TDOS, PDOS) of bulk Ba1-xSrxTiO3 (BSTO) with x = 0, 0.25, 0.5, 0.75, and 1. We find that the calculated direct energy gap Eg bows up as a function of the Sr concentration, which is related to the bowling downward of the Ti-O bond length inferred from the experimental lattice constants. The calculated partial densities of unoccupied O-p derived states (PDOS) agree well with the features observed in the O K-edge XANES spectra of BaTiO3 and SrTiO3. The calculated total densities of valence-band states (TDOS) and the valence-band widths for BaTiO3 and SrTiO3 agree well with the UPS spectra.

Solid-state cooling based on the electrocaloric effect might be a promising alternative to vapor-compressed refrigeration, not only for its increased efficiency but also for its role in preventing the emission of hazardous gases. The electrocaloric effect (ECE) refers to the reversible adiabatic temperature change that occurs in polar materials when an external electric field is applied or varied. In ferroelectric materials, the ECE is particularly pronounced at the transition temperature between the ferroelectric and paraelectric phases. It was shown recently that ferroelectric thin films in general exhibit excellent electrocaloric properties due to their capacity to withstand high electric fields, which typically results in an increase in the adiabatic temperature change. Therefore, the major aim of this thesis was to study environmentally friendly lead-free compounds for their feasibility as electrocaloric active layers in epitaxial film architectures prepared by pulsed laser deposition. Reports in literature on bulk materials suggest that Na0.5Bi0.5TiO3 (NBTO) compounds may be suitable for electrocaloric cooling. Therefore, the growth of epitaxial NBTO-based thin films was studied, which helps to study the correlation between composition, microstructure, and functional properties of this material. Epitaxial films were deposited on different single crystalline substrates applying a thin epitaxial La0.5Sr0.5CoO3 layer as the bottom electrode for subsequent electric measurements. Structural investigation by X-ray diffraction revealed an undisturbed epitaxial growth on LaAlO3, whereas a significantly smaller temperature window for epitaxy was found on YAlO3. The differences might be explained by the lattice misfit resulting in a higher defect density of the intermediate buffer layer on YAlO3. For all samples, a columnar structure with additional pores was found leading to substantial surface roughness. Dielectric measurements revealed significantly decreased permittivity values and increased losses at elevated temperatures if compared to bulk samples. While polarization loops at -100 °C indicated a distinct ferroelectric behavior, ambient temperature data revealed significant resistive contributions due to high leakage currents. As a result, it was not possible to determine the electrocaloric properties for all NBTO-based thin films deposited with the indirect method. In the second part of the thesis, the correlation between structural properties and the electrocaloric effect was investigated in lead-free epitaxial Ba1-xSrxTiO3 (BSTO) thin films. Here, BSTO thin films with Sr contents ranging from x = 0 to x = 0.3 were deposited on SrRuO3 buffered SrTiO3 single crystalline substrates. X-ray diffraction analysis verified a pure epitaxial growth for all Sr concentrations and film thicknesses indicating a larger tetragonal distortion if compared to the bulk material. Dense layers with a low surface roughness were found in microstructural studies. Temperature and frequency-dependent dielectric measurements indicate a diffuse phase transition for all samples, where thicker films showed larger permittivity values. The temperature of maximum permittivity decreases as Sr concentration increases. Polarization curves demonstrate a relaxor-like behavior, particularly above room-temperature. The adiabatic temperature change due to the ECE was determined with the indirect method showing |ΔT| values of up to 2.9 K for an electric field change of 750 kV cm-1.

Unlike the binary compound, where the simple charge transfer between cation and anion, or the ternary compound, which is composed of two binary compound semiconductors with a common cation or anion and whose electronic structures usually can be derived from those of the two constituent binary compounds with some modifications, the electronic property of quaternary compound is quiet complicated and interesting because of its complex charge transfer due to the electronegativity differences of its composed atoms. In this thesis, the first-principles pseudofunction (PSF) method and the first-principles molecular dynamics (MD) method are used to investigate the complicated variations of the electronic properties of three kinds of quaternary compounds, namely titanates [Ba1-xSrxTiO3 (BSTO) and Pb1-xSrxTiO3 (PSTO)], manganites [La1-xSrxMnO3 (LSMO) and La1-xCaxMnO3 (LCMO)], and (SiC)1-x(AlN)x. First, for BSTO and PSTO titanates, the first-principles calculation results and O K-edge x-ray absorption near edge structure (XANES) measurements are used to study their electronic structures. Because the valence band maximum (VBM) and conduction band minimum (CBM) are composed of O-p and Ti-d partial densities of states (PDOS), respectively, the bowing upward of calculated band gaps are related to the bowing downward of the Ti-O bond lengths for both of BSTO and PSTO, though for PSTO Pb-p PDOS also contributes to states near CBM. The substitutions of Sr by Ba in BSTO and by Pb in PSTO are quiet different, and it is because Pb atom has two extra valence electrons and a larger electronegativity than other cations. Second, we provide a new interpretation of the insulator-like to metal-like and anti-ferromagnetic to ferromagnetic transitions with Sr and Ca doping concentrations of La1-xSrxMnO3 and La1-xCaxMnO3, which is based on the variations of the Sr and Ca induced delocalization of the Mn majority-spin eg subband and the lowering of the Mn minority-spin t2g subband down to the Fermi energy (EF). Moreover, this study also suggests that the magnetic properties of manganites result from a detailed balancing between the O-mediated super-exchange mechanism that favors anti-ferromagnetism and the delocalized-state mediated Mn-spin coupling that favors ferromagnetism. Third, for (SiC)1-x(AlN)x superlattice, where SiC and AlN layers arranged alternatively along a common c-axis, our analysis shows subtle charge transfer among Si, C, Al and N ions and the band gap is not linear but bows downwards with respect to x. The calculated results suggest that the direct band gap of (SiC)1-x(AlN)x can be tuned over a wide range from 2.97eV to 6.28eV. Thus, (SiC)1-x(AlN)x is potentially useful for optoelectronic applications. It can be inferred from the calculated electronic properties of the above three kinds of quaternary compounds, the subtle charge transfer is because of differing electronegativities of constituent atoms, especially cations, and the relative valence state of the dopant with respect to the host cation. The subtle charge transfer also influences the magnetic properties of these hole-doped manganites. Moreover, the quaternary compounds have four kinds of atoms with different electronegativities and relative orbital energies, the complicated competition and balancing between the occupation of orbitals and charge transfer render the electronic properties of these material unable to be predicted from constituent binary oxides/semiconductors or even ternary compounds. For example, even though Pb substitutes Sr only in Pb1-xSrxTiO3, the effective charges of Ti and O are significantly altered.

Relaxor ferroelectrics have been a subject of intense research owing to their interesting physical properties such as high dielectric constant and giant electro-striction. Unlike the conventional lead based relaxors, the relaxors belonging to Aurivillius family of oxides have received much less attention because of the poor understanding of the origin of the relaxor behavior and high processing temperatures involved. In the present investigations, an attempt has been made to understand the origin of relaxor behavior of the materials belonging to Aurivillius family of oxides. The structure and relaxor behavior of BaBi2Nb2O9 (BBN) has been established via the XRD, electron diffraction and dielectric spectroscopy. The results are compared with that of a normal ferroelectric like SrBi2Nb2O9 belonging to the same family as well with that of a conventional relaxor like PMN. The results indicate that the dielectric behavior of BBN is significantly different from that of the conventional relaxors like BBN with very slow broadening of relaxation times and was attributed to the absence of significant polar ordering. To substantiate the existing understanding, studies have been carried out by adopting different strategies such as B-site and A-site cationic substitutions and texturing of the ceramics. Vanadium doping on B-site was found to decrease the sintering temperatures significantly. Aliovalent La3+ doping was found to affect the dielectric behavior strongly with substantial decrease of the freezing temperature and dielectric constants which shows that the relaxor behavior of BBN is highly sensitive to A-site order-disorder. The (00l) textured ceramic of pure and vanadium doped BBN was fabricated via a simple melt-quenching technique and was found to exhibit a significant dielectric and pyroelectric anisotropy. A new class of relaxor compositions (K0.5La0.5Bi2Nb2O9 & K0.5La0.5Bi2Ta2O9) have been synthesized and characterized. These new compounds exhibited interesting physical properties which are akin to that of the conventional lead based relaxors. The presence of superlattice reflections in the electron diffractin patterns recorded on these compounds establish the presence of polar nano regions of significant size. These relaxor crystallites at nano/micro level embedded in a glass matrix have been found to be very promising from their physical properties view point.