Room Temperature Polymorphism in Metastable BiMnO₃ Prepared by High-Pressure Synthesis

Abstract

The BiMnO3 perovskite is a very interesting multiferroic material that, once synthesized at high pressure and high temperature, survives as a metastable phase at ambient conditions. We investigated ceramic samples prepared in different conditions (temperature, pressure, and composition), and the existence of polymorphism at room temperature was clearly evidenced by electron diffraction and high-resolution electron microscopy in all the samples. A new polymorph, characterized by a different distortion of the perovskite 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 segregated at the grain surface. Magnetic characterizations revealed for this second form a critical temperature of 107 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 higher energy polymorph, which kinetically converts in the usual phase once a sufficient temperature has been achieved.

Electric relaxation and Mn3+/Mn4+ charge transfer in Fe-doped Bi12MnO20–BiMn2O5 structural self-composite

“…When bismuth manganite ceramics is sintered at ambient air pressure, thermal decomposition occurs at *900 K. The appearance of such meta-stable phases, which exhibit different crystallographic symmetries, has been related to structural defects and internal stresses. Therefore, polymorph forms, BiMn n O m , have been detected and discussed in literature [7,8]. For instance, pure submicron BiMn 2 O 5 particles have been obtained by means of hydrothermal method [9].…”

Optical study of the vibrational and dielectric properties ofBiMnO3

“…However, both the detection of ferroelectricity, and the observation of such distortion have been long controversial, also for the difficulty to grow single crystals [15] of BMO, being it metastable at ambient pressure [14]. Indeed, the observation of ferroelectricity has been reported for thin films only [16,17], where moreover the results strongly depend on their thickness and oxygen stoichiometry [18].…”

Tunable Synthesis of Bismuth Ferrites with Various Morphologies

“…[3,4] More recently, the coexistence of ferroelectricity and magnetic order has been observed in the perovskite bismuth oxides with a noncentrosymmetric structure, such as BiFeO 3 and BiMnO 3 ; hence, they have attracted considerable attention. [5][6][7] The phenomenon of simultaneous ferroelectric and magnetic order, that is, multiferroic behavior, presents not only opportunities to study fundamental physics but also potential applications in information storage, spintronics, and sensors. [8,9] Some other bismuth ferrites are also important functional materials.…”