High-pressure phase transition of Bi2Fe4O9.

The high-pressure behaviour of Bi2Fe4O9 was analysed by in?situ powder and single-crystal x-ray diffraction and Raman spectroscopy. Pressures up to 34.3(8)?GPa were generated using the diamond anvil cell technique. A reversible phase transition is observed at approximately 6.89(6)?GPa and the high-pressure structure is stable up to 26.3(1)?GPa. At higher pressures the onset of amorphization is observed. The crystal structures were refined from single-crystal data at ambient pressure and pressures of 4.49(2), 6.46(2), 7.26(2) and 9.4(1)?GPa. The high-pressure structure is isotypic to the high-pressure structure of Bi2Ga4O9. The lower phase transition pressure of Bi2Fe4O9 with respect to that of Bi2Ga4O9 (16?GPa) confirms the previously proposed strong influence of cation substitution on the high-pressure stability and the misfit of Ga3+ and Fe3+ in tetrahedral coordination at high pressure. A fit of a second-order Birch?Murnaghan equation of state to the p?V data results in K0?=?74(3)?GPa for the low-pressure phase and K0?=?79(2)?GPa for the high-pressure phase. The mode Gr?neisen parameters were obtained from Raman-spectroscopic measurements.