Peculiar High-Pressure Behavior of BiMnO₃

High-pressure structural properties of perovskite-type BiMnO₃ have been investigated by synchrotron X-ray powder diffraction at room temperature. A new monoclinic phase having P2₁/c symmetry was found between about 1.5 and 5.5 GPa. Above 8 GPa, the orthorhombic GdFeO₃-type phase (space group Pnma) is stable. The crystal structure of BiMnO₃ at 8.6 GPa and room temperature was investigated (a = 5.5132(3) Å, b = 7.5752(3) Å, c = 5.4535(3) Å). The orthorhombic phase of BiMnO₃ has an orbital order similar to LaMnO₃ but with a different arrangement of orbitals in the ac plane. High-pressure room-temperature behavior of BiMnO₃ differs from high-temperature behavior at ambient pressure in comparison with BiCrO₃ and BiScO₃. These findings may open new directions in investigation of BiMnO₃.