Thermal stability of MnO2 polymorphs

This study reports thermal stability of alpha, beta, gamma, delta, and lambda-type MnO2 polymorphs investigated using differential scanning calorimetry (DSC) and in-situ high-temperature X-ray diffraction measurement (HTXRD). These experiments revealed that the thermal stability of the polymorphs is in the following order: beta > alpha > gamma > delta approximate to lambda. The beta-MnO2 phase (1 x 1 tunnel, rutile structure), the most stable form between the MnO2 polymorphs, maintains the structure up to 500 degrees C until oxygen release causes phase transformation to Mn2O3. The alpha-MnO2 phase (2 x 2 and 1 x 1 tunnel structure) exhibits high thermal stability comparable to the beta phase, despite its large vacant 2 x 2 tunnel. The gamma-MnO2 phase (mixed microdomain structure of 2 x 1 tunnel and 1 x 1 tunnel) shows a stepwise transformation into beta-MnO2 from 400 degrees C after structure relaxation. The delta-MnO2 phase (layered structure) is easily destabilized by extracting interlayer K ions, resultingly, to lose the interlayer periodicity below 200 degrees C, while each layer itself is preserved up to similar to 500 degrees C. The lambda-MnO2 phase (defect spinel structure) is thermally unstable and transforms into beta-MnO2 around 250 degrees C, and subsequently decomposes to Mn2O3 above the temperature. The thermal stability of MnO2 polymorphs is discussed in terms of the bonding environment of oxide ions.