Crystal Structure of Piezoelectric Ferromagnetic Gallium Iron Oxide

Abstract

The crystal structure of Ga2—xFexO3, with 0.7≤x≤1.4, has been determined at 298°K by x-ray diffraction. Ga2—xFexO3 crystallizes in the orthorhombic system, space group Pc21n, with lattice constants a = 8.7512±0.0008, b = 9.3993±0.0003, c = 5.0806±0.0002 Å and eight formula weights in the unit cell. The complete scattering pattern was measured with PEXRAD to (sinθ)/λ≤1.02 Å−1. One gallium atom in the asymmetric unit is in a tetrahedral site, the other gallium and the two iron atoms are in octahedral sites. For x>1, the excess iron partly occupies the octahedral gallium site: the iron sites are to a smaller extent occupied by gallium. The gallium tetrahedron is very nearly regular, with Gatetr−0 = 1.846±0.005 Å. The gallium octahedron is irregular, with average Gaoct−0 = 2.026±0.004 Å; the iron octahedra are somewhat more irregular, with average Feoct−0 = 2.036±0.004 and 2.049±0.004 Å in the two crystallographically independent octahedra. The oxygen atoms pack in pseudohexagonal layers with a double-hexagonal close-packed repeat. The absolute configuration has been determined by the anomalous scattering: the positive sense of the polar b-axis is essentially parallel to an 0→Gatetr bond. The thermal vibrations in the crystal are isotropic, with a characteristic temperature Θ of 534°K. The magnetic spin structure has been deduced, and an atomic mechanism postulated for the piezoelectric effect, in terms of the crystal structure.

Structural, magnetic and vibrational properties of multiferroic GaFeO3 at high pressure

“…Calculated lattice parameters at ambient conditions are in agreement with that one found in previous studies [13,14,18,20 density functional theory within the local density approximation (LDA) [18] and somewhat less than the value of B 0 = 230(4) GPa, found for stoichiometric GaFeO 3 in [20]. The value of the bulk modulus pressure derivative was found to be B' = 4.0(5).…”

“…According to different experiments on x-ray [13] and neutron scattering [11], mӧssbauer spectroscopy [14] site disorder between Fe/Ga positions was observed in GaFeO 3 . It becomes possible due to nearly close ionic radii of Fe 3+ (0.645 Å) and Ga 3+ (0.62 Å) in octahedral coordination [15].…”

Indicators of accuracy in structure factor measurement

“…The angle 0max = 45 o is the maximum value in the list of GaFeOa structure factors, using Mo K~ radiation (Abrahams, Reddy & Bernstein, 1965). The constant B was given N set R 1 0.0258 2 0.0257 3 0.0249 Table 3.…”

“…The model necessary for such simulation was chosen to consist of the atomic position coordinates and isotropic temperature factors experimentally determined for GaFeO3 and listed in Table 3 of Abrahams, Reddy & Bernstein (1965). The form factors and dispersion corrections given in that paper were regarded as free from error (EF).…”

“…The resulting set of Fracas were then used, in a leastsquares minimization procedure, to refine the coordinates of an initial model in which each metal atom was randomly displaced by 0.05 .~ from the 'correct' position (Table 3 of Abrahams, Reddy & Bernstein, 1965) and each oxygen atom was randomly displaced by 0.10/~. The metal atom isotropic temperature factors were initially set equal to 0.3 A 2, the oxygen atom temperature factors to 0.7/~2.…”