High-pressure phase transition of Bi sub(2)Fe sub(4)O sub(9)

The high-pressure behaviour of Bi sub(2)Fe sub(4)O sub(9) 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 Bi sub(2)Ga sub(4)O sub(9). The lower phase transition pressure of Bi sub(2)Fe sub(4)O sub(9) with respect to that of Bi2Ga4O9 (16?GPa) confirms the previously proposed strorg influence of cation substitution on the high-pressure stability and the misfit of Ga super(3+) and Fe super(3+) 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 Gruneisen parameters were obtained from Raman-spectroscopic measurements.