Abstract: Polycrystalline samples of nominal composition BiFe0.5Co0.5O3 were prepared by solid state reaction, nitrate decomposition as well as gel-combustion procedures, and characterized by powder XRD studies. The XRD studies indicated formation of sillenite type cubic (I23) phase in all the cases while having few additional weak peaks accountable to spinel type phase. It was observed that the sample prepared by gel-combustion show better crystallinity and phase purity compared to other methods. Powder neutron diffraction studies carried out at ambient and 7 K on the sample prepared by gel-combustion indicated that the studied sample has coexisting Fe3O4 and Co3O4 along with the sillenite type phase with composition Bi25FeO40. Additionally, the ferromagnetism observed in the sample is attributed to the ferrimagnetic spinel phase. Further the lattice contraction on lowering temperature or increasing pressure was concluded from the low temperature neutron diffraction studies and in situ high pressure XRD studies in DAC using synchrotron radiation source. The lattice shows only minor (-1 %) decrease in unit cell volume while cooling from ambient to 7 K, whereas it shows an appreciable contraction (- 16 %) on increasing the pressure up to 21.5 GPa. The coefficient of average volume thermal expansion in the temperature range of RT to 973 K is found to be 47.1 × 10-6 K-1. Also, the sillenite phase shows stability up to 21 GPa, the maximum pressure of this study and does not show any pressure induced amorphization as commonly observed in Bi2O3 and its other related compounds. The analyses of the pressure evolution of unit cell parameters by using a 3rd order Birch-Murnaghan EOS indicated the bulk modulus (B0) of 55(4) GPa and its pressure derivative (B0') 8.4(7), and linear modulus (M0) 122(18) GPa and its pressure derivative (M0') 47(9). The observed low value of bulk modulus is in accordance with its loosely packed structure with active lone pair on Bi3+.

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