Structural and physical properties of Sr-based 3d-5d double perovskites of Sr2Fe0.5Hf1.5O6-δ oxides

Sr-based 3d-5d double perovskite Sr2Fe0.5Hf1.5O6-delta (SFHO) oxide powders were synthesized via the solid-state reaction method, and their structural, dielectric, magnetic, optical, and transport properties were investigated. Structural investigations revealed the SFHO powders crystallized in an orthorhombic crystal structure with Pnma space group. Scanning electron microscopy images demonstrated the relatively homogeneous distributions of spherical SFHO powders, and x-ray energy dispersive spectra gave out the molar ratio of Sr:Fe:Hf equal to 1.94:0.5:1.87, close to the nominal value. XPS spectra confirmed the existence of Sr2+, Fe3+, Hf4+ ions in the SFHO powders, and oxygen appeared in the forms of lattice oxygen and adsorbed oxygen species, respectively. The SFHO ceramics exhibited a relaxor-like dielectric behavior. Two dielectric abnormities appeared around 518 and 257 K, which were ascribed to the dielectric response of the oxygen vacancies (<($)double over dot>(O)) with activation energy E-a = 0.93 eV and thermal excitation of the electrons with E-a = similar to 0.02 eV bound weakly to the <($)double over dot>(O), respectively. Ferromagnetic behavior was observed in the SFHO powders at 300 and 2 K, and the saturated magnetization at 2 K was 0.67 mu(B)/f.u. Magnetic transition temperature, T-C was determined to be 638 K from the temperature dependence of the magnetization under the zero-field coolingmode. Aspin glass-like behaviorwas observed around 390 K, which was confirmed by the frequency-dependent alternating current (ac) susceptibility measurements. The SFHO powders exhibit a butterflylike magnetoresistance (MR)-H plot at 2 K, and the MR (2 K, 7 T) is found to be -2.73% owing to the intergranular magneto-tunneling effect. Temperature dependence of the resistivity of the SFHO ceramics displayed a semiconducting behavior, and the electrical transport properties of the SFHO ceramics were investigated by Mott's variable-range-hopping model, thermally activated semiconductor conductivity model, and the adiabatic small polarons hopping model, respectively. UV-vis diffuse-reflectance spectra of the SFHO powders demonstrated a direct optical bandgap of E-g = 2.62 eV, which was contributed from the electron transfer from the O(2p) orbital to Fe(3d)/Hf(5d) orbitals. A combination of high-temperature ferrimagnetism and semiconducting nature of the SHFO powders makes them attractive for spintronics and photovoltaics.