Microstructural, magnetic, dielectric, and optical studies of La-doped Sr2Fe0.5Hf1.5O6 double perovskite oxides

Abstract

Herein, we report on the structural, magnetic, dielectric, and optical properties of La-doped Sr_2-xLa_xFe_0.5Hf_1.5O₆ (SLFHO, 0.0 ≤ x ≤ 2.0) double perovskite oxides synthesized by solid-state reaction method. X-ray powder diffraction data and their Rietveld refinement reveal that the SLFHO powders crystallize in an orthorhombic structure with the Pnma space group. The SLFHO powders exhibit spherical morphology with an average particle size of 160 nm, as shown from SEM images. Their molar ratios of the Sr:La:Fe:Hf elements were determined as 1.77:0.50:0.50:1.95 by energy dispersive X-ray spectroscopy data, approaching their nominal stoichiometry. X-ray photoelectron spectra verified that Sr²⁺, La³⁺, Fe³⁺, and Hf⁴⁺ (Hf^x+) ions existed in the SLFHO powders, and oxygen existed in species lattice oxygen and adsorbed oxygen, respectively. The SLFHO powders exhibit ferromagnetic behavior at 2 and 300 K, respectively, and at 2 K the saturated magnetization was 5.66 emu/g (or 0.60 μ_B/f.u.). Magnetic transition temperature, T_C was determined to be 867 K. Dielectric measurements demonstrated a strong frequency-dependent dielectric behavior observed in the SLFHO ceramics and a relaxor-like dielectric behavior appearing in the temperature range of 200–600 K. Such relaxor-like dielectric behavior is ascribed to the dielectric response of the singly-ionized oxygen vacancies ($V_O^{·})\mathrm{associated}\mathrm{with}$ an activation energy of 0.54 eV. Room temperature optical properties of the SLFHO powders examined by using UV–vis diffuse reflectance spectroscopy exhibit high optical absorption in this wavelength region, and a wide indirect optical bandgap (E_g = 3.39 eV) was estimated using the Tauc's relation from the diffuse reflectance spectra. The SLFHO double perovskite oxides display high-temperature ferrimagnetism, relaxor-like dielectric behavior, and wide optical bandgap, making them potential to be employed in the fields of high-temperature spintronics and magnetic semiconductor devices.