  The interaction of coexisting structural instabilities in multiferroic
materials gives rise to intriguing coupling phenomena and extraordinarily rich
phase diagrams, both in bulk materials and strained thin films. Here we
investigate the multiferroic BiMnO3 with its peculiar 6s2 electrons and four
interacting mechanisms: electric polarity, octahedra tilts, magnetism, and
cooperative Jahn-Teller distortion. We have probed structural transitions under
high pressure by synchrotron x-ray diffraction and Raman spectroscopy up to 60
GPa. We show that BiMnO3 displays under pressure a rich sequence of five phases
with a great variety of structures and properties, including a metallic phase
above 53 GPa and, between 37 and 53 GPa, a strongly elongated monoclinic phase
that allows ferroelectricity, which contradicts the traditional expectation
that ferroelectricity vanishes under pressure. Between 7 and 37 GPa, the Pnma
structure remains remarkably stable but shows a reduction of the Jahn-Teller
distortion in a way that differs from the behavior observed in the archetypal
orthorhombic Jahn-Teller distorted perovskite LaMnO3.
