R. S. Katiyar

We report on extensive experimental studies on thin film, single crystal, and ceramics of multiferroic bismuth ferrite $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ using differential thermal analysis, high-temperature polarized light microscopy, high-temperature and polarized Raman spectroscopy, high-temperature x-ray diffraction, dc conductivity, optical absorption and reflectivity, and domain imaging, and show that epitaxial (001) thin films of $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ are clearly monoclinic at room temperature, in agreement with recent synchrotron studies but in disagreement with all other earlier reported results. We report an orthorhombic order-disorder $\ensuremath{\beta}$ phase between 820 and 925 $(\ifmmode\pm\else\textpm\fi{}5)\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, and establish the existence range of the cubic $\ensuremath{\gamma}$ phase between 925 $(\ifmmode\pm\else\textpm\fi{}5)$ and 933 $(\ifmmode\pm\else\textpm\fi{}5)\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, contrary to all recent reports. We also report the refined ${\mathrm{Bi}}_{2}{\mathrm{O}}_{3}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ phase diagram. The phase transition sequence rhombohedral-orthorhombic-cubic in bulk [monoclinic-orthorhombic-cubic in $(001)\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ thin film] differs distinctly from that of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$. The transition to the cubic $\ensuremath{\gamma}$ phase causes an abrupt collapse of the band gap toward zero (insulator-metal transition) at the orthorhombic-cubic $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ transition around $930\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Our band structure models, high-temperature dc resistivity, and light absorption and reflectivity measurements are consistent with this metal-insulator transition.

Polycrystalline samples of Bi0.90La0.10(Fe1−xMnx)O3 (x=0, 0.05, 0.10, 0.15, and 0.20) were prepared using a novel mechanical activation followed by a conventional solid-state reaction technique. The formation of the desired materials was confirmed using x-ray diffraction. The electrical and magnetic properties of the materials were investigated at different Mn concentrations as a function of temperature. Both dielectric constant and loss tangent increased with the increase in Mn content in the system. The grain and grain boundary contributions have been estimated using impedance spectroscopy analysis. Both grain and grain boundary conductivity increased with a rise in temperature for all Mn concentrations. The value of activation energy for both grain and grain boundary is nearly the same, and decreased with an increase in Mn concentration. There is a systematic increase in the value of magnetization on increasing Mn concentration.

The coexistence of the magnetic and the electrical properties in lanthanum (La)-modified bismuth ferrite (Bi1−xLaxFeO3, x=0.05, 0.1, 0.15, and 0.2) ceramics was studied and compared with those of bismuth ferrite (BiFeO3). The presence of a small secondary phase of BiFeO3 (arises due to excess Bi2O3) was removed on La substitution at the Bi site, as observed in x-ray diffraction (XRD). The effect of La substitution on dielectric constant, loss tangent, and remnant polarization of the samples was studied in a wide range of temperature (77–400K) and frequency (1kHz–1MHz). The variation of magnetization, coercive field, and exchange bias with temperature (2–300K) and La concentration were investigated. These changes in the magnetic parameters with La doping along with those of the electron magnetic resonance parameters measured at 300K and 9.28GHz are understood in terms of increase in the magnetic anisotropy and magnetization. These results also show that stabilization of crystal structure and nonuniformity in spin cycloid structure by La substitution enhances the multiferroic properties of BiFeO3.

Plasticized polymer nanocomposite electrolytes (PPNCEs) based on Poly (ethylene oxide) + NaI with dodecyl amine modified montmorillonite (DMMT) as the filler and Poly (ethylene glycol) as the plasticizer were prepared by a self-designed tape caster. The effect of plasticization on structural, microstructure, thermal and electrical properties of the PPNCEs were investigated. The changes in the structural and microstructural properties of the materials were investigated by XRD and SEM techniques. Differential scanning calorimetry (DSC) technique was used to study the thermal parameters (i.e., glass transition temperature (Tg) and crystalline melting temperature (Tm) of the nancomposites. Complex impedance analysis was used to calculate the bulk resistance of the composites. The typical complex impedance spectrum of the samples comprises of a compressed semicircle in the high frequency region (due to the bulk properties) followed by a tail (spike) in the lower frequency region indicating the double layer response at the electrode/sample interface. The maximum conductivity of PPNCE was found to be 1.05x10-6 for x=50% of plasticizer (at 400C). The effect of plasticizer on the structural and physical properties of polymer nanocomposites was well correlated.