Thermal stability of MnO₂ polymorphs

This study reports thermal stability of α, β, γ, δ, and λ-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: β ​> ​α ​> ​γ ​> ​δ ​≈ ​λ. The β-MnO₂ phase (1 ​× ​1 tunnel, rutile structure), the most stable form between the MnO₂ polymorphs, maintains the structure up to 500 ​°C until oxygen release causes phase transformation to Mn₂O₃. The α-MnO₂ phase (2 ​× ​2 and 1 ​× ​1 tunnel structure) exhibits high thermal stability comparable to the β phase, despite its large vacant 2 ​× ​2 tunnel. The γ-MnO₂ phase (mixed microdomain structure of 2 ​× ​1 tunnel and 1 ​× ​1 tunnel) shows a stepwise transformation into β-MnO₂ from 400 ​°C after structure relaxation. The δ-MnO₂ phase (layered structure) is easily destabilized by extracting interlayer K ions, resultingly, to lose the interlayer periodicity below 200 ​°C, while each layer itself is preserved up to ∼500 ​°C. The λ-MnO₂ phase (defect spinel structure) is thermally unstable and transforms into β-MnO₂ around 250 ​°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.