Temperature-dependent structural studies of mullite-type Bi2Fe4O9

Abstract

We report on the temperature-dependent structural studies on the mullite-type Bi2Fe4O9 compound. The crystal structures were determined using both powder X-ray diffraction and single crystal neutron diffraction. The thermal expansion of the cell parameters from smallest to largest occurred in the order a<c<b; their expansion behavior was fit using either linear or nonlinear regression according to the region of choice. The anisotropic thermal expansion was monitored with respect to the expansion coefficients, anisotropy factor, and polyhedral geometry. At 900K an abrupt change of the anisotropy factor is observed which correlates with a slope in the BiO6 polyhedral volume. At the same temperature the FeO4 tetrahedral distortion starts to increase. The temperature-dependent stereochemically activity of the Bi3+ 6s2 lone electron pair was probed by the absolute value of eccentricity parameter. This parameter shows a significant drop at around 773K. Additionally, the average crystal size and micro-strain of the powder samples were studied. A healing of intrinsic defects is observed with respect to a decreasing micro-strain followed by crystal growth above about 900K. The temperature-dependent infrared spectra were described in three distinct zones. Deconvolution of the absorption features between 370cm−1 and 1000cm−1 required nine fitted bands, and the decomposed Pseudo-Voigt elements were assigned to Fe–O stretching, Fe–O–Fe bending and O–Fe–O bending vibrations. The shift of the observed modes was described with respect to temperature. An effective linewidth parameter Δcorr was determined by autocorrelation analysis between 740cm−1 and 900cm−1 for each temperature-dependent spectrum. The break of the slope of Δcorr at about 773K was interpreted as the change of some vibrational modes of the FeO4 tetrahedra. The intrinsic effects observed around 773K are responsible for extrinsic parameter changes found around 900K.