Crystallization and Anisotropic Hyperfine Field Fluctuation in Double Perovskite A₂FeMoO₆ (A=Ba, Sr, Ca)

Abstract

The double perovskite oxides A₂FeMoO₆ (A=Ba, Sr. Ca) have been studied by Mössbauer technique, neutron and x-ray diffraction. The single phases of the polycrystalline A₂FeMoO₆ powders have been prepared by a solid-state reaction method. Their crystal structures are cubic for A=Ba, tetragonal for A=Sr, and monoclinic for A=Ca, respectively. In Sr₂FeMoO₆, the unit-cell parameters increase linearly with increasing the temperature, and the crystal symmetry changes into cubic in the paramagnetic phase (above T _C ). Mössbauer spectra measurements of the A₂FeMoO₆ powder samples have been taken at various temperatures ranging from 18 to 450 K. The Curie temperatures are 345 K for Ba₂FeMoO₆, 425 K for Sr₂FeMoO₆, and 350 K. for Ca₂FeMoO₆, respectively. As the temperature increases toward the Curie temperature, Mössbauer spectra show the line broadening and the difference between 1, 6 and 3, 4 line-width because of anisotropic hyperfine field fluctuation. In Sr₂FeMoO₆, the anisotropic field fluctuation of +H(P ₊=0.85) was great than −H(P ₋=0.15). We also calculated frequency factor and anisotropy energy with values of 9.8 Γ/ħ and 149.6 erg/cm³ for Sr₂FeMoO₆, respectively, using the relatively accurate data for T=260 K which is associated with the large line broadening. The anisotropy energy increase with decreasing ions radius of A-site atom (Ba, Sr, Ca).