Temperature-dependent structural studies of mullite-type Bi2Fe4O9

Abstract We report on the temperature-dependent structural studies on the mullite-type Bi 2 Fe 4 O 9 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 900 K an abrupt change of the anisotropy factor is observed which correlates with a slope in the BiO 6 polyhedral volume. At the same temperature the FeO 4 tetrahedral distortion starts to increase. The temperature-dependent stereochemically activity of the Bi 3+ 6 s 2 lone electron pair was probed by the absolute value of eccentricity parameter. This parameter shows a significant drop at around 773 K. 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 900 K. The temperature-dependent infrared spectra were described in three distinct zones. Deconvolution of the absorption features between 370 cm −1 and 1000 cm −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 740 cm −1 and 900 cm −1 for each temperature-dependent spectrum. The break of the slope of Δcorr at about 773 K was interpreted as the change of some vibrational modes of the FeO 4 tetrahedra. The intrinsic effects observed around 773 K are responsible for extrinsic parameter changes found around 900 K.