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

¹ Department of Mineral Sciences, Smithsonian Institution, Washington, D.C. 20560-0119, U.S.A.
² Department of Geosciences, 309 Deike, Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A.

Correspondence: ^* E-mail: post.jeffrey{at}nmnh.si.edu

The crystal structure of ramsdellite, MnO₂, 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 O_1.5(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 Mn⁴⁺ and O^2– in ramsdellite by Mn³⁺ 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 450 K. At higher temperatures, the unit-cell volume gradually decreased, primarily as a result of a decrease in c.

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