Structural Properties of Multiferroic BiFeO₃ under Hydrostatic Pressure

High-pressure structural properties of multiferroic perovskite-type BiFeO 3 have been investigated by high-resolution synchrotron X-ray powder diffraction at room temperature up to 9.7 GPa. BiFeO 3 shows rather complicated structural behavior. The ambient-pressure ferroelectric R 3 c phase transforms to an orthorhombic phase OI at around 4 GPa during compression. The OI phase is characterized by a superstructure √2 a p × 3√2 a p × a p, where a p is the parameter of the cubic perovskite subcell ( a = 5.4939(4) Å, b = 16.6896(9) Å, c = 3.8728(2) Å at 4.9 GPa). The OI phase transforms to an orthorhombic phase OII at around 7 GPa. The OII phase is characterized by a superstructure √2 a p × 3√2 a p × 2 a p ( a = 5.5021(3) Å, b = 16.2439(11) Å, c = 7.6960(4) Å at 9.7 GPa). During decompression, significant hysteretic behavior was found. The OII phase was stable down to about 3 GPa. The OII phase then transforms to an orthorhombic phase OIII that is characterized by a superstructure √2 a p × 2√2 a p × 2 a p ( a = 5.5617(6) Å, b = 11.2153(10) Å, c = 7.7788(7) Å at 2.2 GPa). The R 3 c phase appeared below about 1 GPa; however, even at ambient pressure, traces of the OIII phase remained. The OIII phase seems to be isostructural with antiferroelectric PbZrO 3 (space group Pbam ).