Synthesis and characterization of gamma-MnO2 samples with unusual structural parameters

gamma-MnO2 compounds are usually prepared by oxidative electrodeposition at high constant current density from boiling MnSO4 solutions. Their structur e is made of an intergrowth of ramsdellite and pyrolusite blocks (with a pe rcentage Pr (of rutile) of 40-50%) and of microtwinning (Mt) defects (with a percentage Mt > 40%). By varying the electrodeposition parameters, differ ent materials have been obtained. Syntheses at constant voltage or current density lead to samples with 35% less than or equal to Pr less than or equa l to 50% in which the Mn oxidation state can be modified by varying the ele ctrodeposition temperature or the current density. Whatever the temperature , a pH increase of the bulk solution from 1.8 to 3.5 favors smaller Pr valu es. A decrease of the current density leads to more oxidized and better cry stallized samples at 25 degrees C and smaller Mt values at 97 degrees C. Ne w gamma-MnO2 with never-reported (Pr, Mt) combinations can be prepared by s canning the voltage (or the current) between selected values. Under such co nditions Mt can be decreased by increasing the ratio between the total oxid ation and reduction quantities of electricity. Upon annealing at 250 degree s C of samples with Pr < 50%, new compounds were obtained with unusually sm all Pr values and/or unusually large Mt values for heat-treated manganese d ioxides. (C) 2000 The Electrochemical Society. S0013-4651(99)07-103-7. All rights reserved.