Study of the modifications of manganese dioxide

Abstract

P ast work on the modificat ion s of manganese dioxide of in terest in dry-cell manu facture is r evie wed. New X -ray da ta, at bo th room and elevaLcd te mperat ures, combined ' with differen t ial hea t ing curves lead to t he co ncl usion that five type of m anga nese dioxide exi t :(1) well-crystallized p yrolu site ; (2) gamm a m a nganese di ox ide, a p oorly crystalli zed pyrol usite; (3) ramsd ellite ; (4) cryp tomelane, a form co n taining esse nt ial p otassium or sodi ulll.; a nd (5) delta m angan ese dioxide, belie ved to be a p oorly crystalli ze d cryptom ela ne. The high-t empera ture X-r ay diffraction da ta indicate d t he phase cha nges t h at ca use th e heatingcur ve effects. A new cr ystal fo rm of man gan osic oxide (Mn30 4), stable a bove 1,170 0 C, w~s fo und t o be cubic of spinel structure. Fincness dcten n in aLion by b oth t hc ni t rogen a dsorpt ion and Lhe X-ray line broadening m e thod s were m ad e on selected sam p les.

Thermal analysis of manganese dioxide in controlled atmospheres

“…This is due principally to the loss of water and other adsorbed species, although loss of oxygen probably occurs, since manganese dioxide is non-stoichiometric [3,8,9] and its composition varies with oxygen partial pressure and temperature. Either superimposed on this endotherm or almost immediately after it is a very small exotherm, whose peak temperature varies slightly with the oxygen partial pressure (Table 1, reaction a).…”

“…There are various forms of manganese dioxide [1,8] which probably does not exist as stoichiometric MnO~ [3]. X-ray powder traces correspond with the form described as 7-MnO2 (ASTM card 4-0779).…”

“…There is disagreement, however, about the temperature of the transformation from Mn304 to MnO. Several workers [2][3][4] have reported an endothermic peak, on DTA, between 1160 ~ and 1230 ~ which is reversible on cooling and interpret it as due to the polymorphic transformation from tetragonal fl-hausmannite (Mn304) to the cubic y-form. According to Dollimore [5,61 this reaction is due to the formation of MnO from MnzO4.…”

Synthesis of potassium manganese oxides under hydrothermal conditions

“…SEVERAL minerals containing potassium and manganese as the main constituent elements are known in nature. In relation with the natural occurrences and geneses, laboratory preparations of these materials have been undertaken (McMurdie and Golovato, 1948; Cole et al, 1947; Dubois, 1936; Pauling et al, 196o) but few attempts at the synthesis of related substances have been made under hydrothermal conditions, although some minerals such as cryptomelane or coronadite have often been found in hydrothermal ore deposits (Roy, I968; Hewett, 197I ). The authors tried a series of experiments on hydrothermal syntheses using a solution of potassium permanganate as starting material.…”

Stabilization of cubic structure in Mn-doped hafnia

“…Here we selected MnO 2 as starting material due to its good chemical stability at ambient conditions. As heated in Ar MnO 2 easily decomposes to Mn 2 O 3 with a formal valence of Mn +3 accompanied with an oxygen loss [17]. Therefore, the weight loss and the endothermic peak at close to 600 in the cubic phase, but the total sample weight still is larger than that before annealing due to the increased oxygen concentration.…”