Selective hydrothermal microwave synthesis of various manganese dioxide polymorphs

Hydrothermal microwave treatment of mixed solutions of potassium permanganate and hexamethylenetetramine within the pH range 0.5–6.9, resulted in various polymorphs of nanocrystalline manganese dioxide: α-MnO2 (cryptomelane), γ-MnO2 (nsutite), β-MnO2 (pyrolusite), and δ-MnO2 (birnessite). The pH values of the medium at which single-phase samples form were determined.