Neutron and synchrotron X-ray diffraction study of structures and dehydration behaviors of ramsdellite and groutellite

The crystal structure of ramsdellite, MnO (sub 2) , was refined using time-of-flight powder neutron diffraction data and the Rietveld method in order to assess the effects of reduction in cathodic battery materials. For the first time, we present a refined structure for "groutellite," a heretofore poorly characterized phase with ideal formula O1.5(OH)0.5. "Groutellite" is generated synthetically as an intermediate compound during the reduction of ramsdellite to groutite (MnOOH), and it also occurs as an intergrowth in certain natural specimens of ramsdellite. The Jahn-Teller distortions in "groutellite" are confined to the a-c plane, and they result in a 6.8% unit-cell volume increase relative to ramsdellite. The Mn-O bond lengths refined for "groutellite" are consistent with the replacement of half of the Mn (super 4+) and O2- in ramsdellite by Mn (super 3+) and (OH)-, respectively. In addition, the high-temperature behaviors of ramsdellite and "groutellite" were investigated by temperature-resolved synchrotron powder X-ray diffraction from 298 to 720 K. Rietveld refinements revealed a gradual thermal expansion of the groutellite structure to approximately 450 K. At higher temperatures, the unit-cell volume gradually decreased, primarily as a result of a decrease in c.