High temperature ferroelectric behaviour in α-MnO₂ nanorods realised through enriched oxygen vacancy induced...

Nanostructuring followed by incorporation of defect induced non-stoichiometry is an emerging field of prominence due to its capacity to introduce unprecedented properties in materials with potential applications. In this work, crystalline α-MnO₂ nanorods are synthesised using a facile co-precipitation method to exhibit ferroelectric behaviour for the first time. The evolution mechanism of the nanorods is investigated using XRD, HRTEM and FTIR spectra, while their thermal stability is probed using TGA/DTA. The novel properties observed are the result of structural rearrangements sparked by electrons in mixed valence cations (Mn³⁺/Mn⁴⁺). The high density of Jahn-Teller active Mn³⁺ cations breaks the inversion symmetry in α-MnO₂, thereby altering the atomic environment inducing distortion in the basic MnO₆ octahedra. Since variable temperature XRD analysis confirms the phase stability of the crystal structure up to very high temperatures, the ferroelectric phase exhibited by the material below T_c is an outcome of the combined effects of orbital ordering (OO) of the e_g electron in Mn³⁺ and charge ordering (CO) of Mn³⁺ and Mn⁴⁺ cations. This is confirmed by DSC analysis. The breakdown of the ferroelectric nature is identified to originate as a result of octahedral tilting as suggested by temperature-dependent Raman studies. Magnetic and electrical transport studies provide additional evidence of a CO ferroelectric phase as they predict the existence of double-exchange hopping conduction and surface ferromagnetism in the sample.