Structural, optical, dielectric and magneto-dielectric studies on Fe doped LaGaO3

Abstract

Multifunctional materials are of very high research interest due to their potential applications and interesting physics. LaGaO3 (LGO) has been established as a prototype fuel cell material having perovskite structure. It is also a good substrate material for the growth of superconducting thin films. LGO exhibits room temperature (RT) orthorhombic crystal symmetry with pnma space group and it undergoes through various structural phase transformations under different temperature and pressure conditions. Moreover, this perovskite exhibits different interesting properties with the doping of Mn ions. However, transition metal (TM) ion doped LGO compound may have different potential scopes which have not been explored so far. For example - it may have the potential to be a good candidate for optoelectronic devices and it may also display magneto-dielectric (MD) properties near room temperature as it exhibits- (i) temperature (almost up to 400K) and frequency independent dielectric properties (dielectric permittivity ε‟ and tanδ) and (ii) structural phase transition well above the room temperature (418 K). Keeping this all in view, in present work, a series of Fe doped LGO samples i.e., LaGa1-xFexO3 (LGFO); 0 ≤ x ≤1 is prepared by solid-state reaction route. The purity of structural phase of all prepared polycrystalline LGFO samples has been validated (considering orthorhombic symmetry with pnma space group) through x-ray diffraction (XRD) experiments by refining the XRD data with the help of Fullprof Rietveld refinement package.Vegard‟s law has been verified for the present LGFO series and corresponding „bowing‟ parameter has been calculated by using refined lattice parameters and the band gap of LGFO samples respectively. The band gap for all presently studied samples is determined through diffuse reflectance spectroscopy (DRS) based on the formalism of Kubelka-Munk function and Tauc‟s relation. LGFO sample with x=0.3 exhibits a noticeable MD effect (i.e. change in the value of dielectric constant due to the application of magnetic field). The trends of changes observed in dielectric constant (ε‟) and tanδ due to the application of magnetic field suggest that the observed MD effect should be an intrinsic property of these samples. In order to estimate the intrinsic and resistive contributions of observed RT MDeffect, dc MR (using four probe geometry) and frequency dependent magneto resistance measurements by means of impedance spectroscopy (MRIS) along with the iodometric titration and XANES measurements has been carried out at RT. Present MRIS analysis suggests that at frequencies corresponding to grain contribution (≥106 Hz for present samples), the observed MD phenomenon appears to be an intrinsic property of presently studied samples whereas at lower probing frequencies (<106 Hz) the observed change appears to be MR (considering frequency dependent resistance) dominated possibly due the coexistence Fe3+ and Fe4+. The coexistence of Fe3+ and Fe4+ in LaGa0.7Fe0.3O3 has been validated through iodometric titration and Fe K-edge XANES measurements. Present study reveals the possibility of RTMD coupling in LGFO (x=0.3).