| Abstract
| - The BiMnO3 perovskite is a very interesting multiferroic material that, once synthesized at high pressureand high temperature, survives as a metastable phase at ambient conditions. We investigated ceramicsamples prepared in different conditions (temperature, pressure, and composition), and the existence ofpolymorphism at room temperature was clearly evidenced by electron diffraction and high-resolutionelectron microscopy in all the samples. A new polymorph, characterized by a different distortion of theperovskite basic cell, was found to coexist as a minor phase with the well-known C2 monoclinic form.The new polymorph, which can be described by a triclinic (pseudorhombohedral) superstructure with a= 13.62 Å, b = 13.66 Å, c = 13.66 Å, α = 110.0°, β = 108.8°, and γ = 108.8°, is mostly segregatedat the grain surface. Magnetic characterizations revealed for this second form a critical temperature of107 K, a few degrees above the ferromagnetic transition of the monoclinic C2 form measured at 99 K.The new phase disappears by reheating the samples at ambient pressure, suggesting the idea of a higherenergy polymorph, which kinetically converts in the usual phase once a sufficient temperature has beenachieved.
- The existence of polymorphism at room temperature in BiMnO3 samples, prepared by high-pressure synthesis, was evidenced by electron diffraction and high-resolution electron microscopy. A different perovskite form, characterized by a magnetic critical temperature of 107 K, was found to segregate at the grain surface of the well-known monoclinic polymorph.
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