Synthesis and Characterization of γ-MnO[sub 2] Samples with Unusual Structural Parameters

Abstract

compounds are usually prepared by oxidative electrodeposition at high constant current density from boiling solutions. Their structure is made of an intergrowth of ramsdellite and pyrolusite blocks (with a percentage Pr (of rotile) of 40–50%) and of microtwinning (Mt) defects (with a percentage Mt > 40%). By varying the electrodeposition parameters, different materials have been obtained. Syntheses at constant voltage or current density lead to samples with 35% ≤ Pr ≤ 50% in which the Mn oxidation state can be modified by varying the electrodeposition temperature or the current density. Whatever the temperature, a pH increase of the bulk solution from 1.8 to 3.5 favors smaller Pr values. A decrease of the current density leads to more oxidized and better crystallized samples at 25°C and smaller Mt values at 97°C. New with never‐reported (Pr, Mt) combinations can be prepared by scanning the voltage (or the current) between selected values. Under such conditions Mt can be decreased by increasing the ratio between the total oxidation and reduction quantities of electricity. Upon annealing at 250°C of samples with Pr < 50%, new compounds were obtained with unusually small Pr values and/or unusually large Mt values for heat‐treated manganese dioxides. © 2000 The Electrochemical Society. All rights reserved.