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

Abstract

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-MnO 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 (Mn/Mn). The high density of Jahn-Teller active Mn cations breaks the inversion symmetry in α-MnO, thereby altering the atomic environment inducing distortion in the basic MnO 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 is an outcome of the combined effects of orbital ordering (OO) of the e electron in Mn and charge ordering (CO) of Mn and Mn 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.

The effects of Fe-doping on MnO2: phase transitions, defect structures and its influence on electrical properties

“…By incorporating K + ions into the tunnel, a state of charge neutrality is maintained in the structure by reducing Mn 4+ to Mn 3+ . 48 This confirms the structural distortion in the Mn 1− x Fe x O 2 sample with x = 0. Moreover, small amounts of Fe element are confirmed in the EDS table.…”

Tuning the structural stability and spin-glass behavior in α-MnO2nanotubes by Sn ion doping

“…This result suggests that, due to the radius of Sn 4+ (0.69 Å) being larger than that of Mn 4+ (0.53 Å), the inter-spacing expands with the replacement of Mn by Sn. 25 Refinements of the XRD data by least squares method using the tetragonal system led to the lattice parameters a = 9.820( 1 Considering that the cryptomelane-type compound consists of edge-sharing double-chain MnO 6 structural units, and forms square cross-section tunnels with a diameter of 0.46 nm, 26 a small amount of Sn 4+ ions should not induce an increase in the lattice parameters when the Sn 4+ ions locate in the tunnels. Hence, Sn ions are introduced into the lattice position to replace the Mn ions instead of the tunnels.…”

“…This result suggests that, due to the radius of Sn 4+ (0.69 Å) being larger than that of Mn 4+ (0.53 Å), the inter-spacing expands with the replacement of Mn by Sn. 25 Refinements of the XRD data by least squares method using the tetragonal system led to the lattice parameters a = 9.820(1) Å, c = 2.855(2) Å for SN0, a = 9.824(2) Å, c = 2.856(3) Å for SN1, a = 9.828(3) Å, c = 2.856(7) Å for SN2, and a = 9.830(1) Å c = 2.857(3) Å for SN3. The lattice expansion of Sn-doped α-MnO 2 varies mainly along the a - and b -axes, and remains almost unchanged in the c direction.…”

Insights into the roles of superficial lattice oxygen in formaldehyde oxidation on birnessite

“…40 The Raman band observed at 630-640 cm −1 corresponds to the stretching of the O-Mn-O bonds, which is perpendicular to the chains of O-Mn-O-Mn-O in the basal plane of the MnO 6 octahedra. [41][42][43] The peaks of this Raman mode over MnO 2 , MnO 2 -1K, MnO 2 -3K, and MnO 2 -5K are at 639, 637, 633, and 633 cm −1 , respectively. The slight red shift of the Raman mode indicates the weakening of the Mn-O bond strength along the interlayer direction of the birnessite structure after K + introduction, which improves oxygen mobility.…”