Ramsdellite is one of two naturally occurring polymorphs of MnO₂, and both natural and synthetic varieties are important battery materials. Ramsdellite has the diaspore structure, consisting of double chains of Mn-O octahedra that share corners to form a framework with rectangular-shaped tunnels. Post and Ross (1989) showed that most natural ramsdellite samples contain a second isostructural phase with unit-cell parameters intermediate between those of ramsdellite and groutite (MnOOH).

We have refined the structure of the intermediate phase using the Rietveld method and synchrotron X-ray diffraction data that were collected at Beamline X7B, National Synchrotron Light Source, Brookhaven National Laboratory using a Mar345 imaging plate. The refinement shows longer mean Mn-O distances (1.93 Å) relative to ramsdellite (1.89 Å), and Jahn-Teller type distortion of the Mn-O octahedra. Both observations indicate substitution of Mn³⁺ for about one-third of the Mn⁴⁺, presumably with coupled replacement of O^2- with OH^-. Infra-red spectra from several samples confirm that the amount of OH^- is approximately proportional to the fraction of the intermediate phase.

Temperature-resolved synchrotron diffraction experiments were performed for groutite and a sample with roughly equal amounts of ramsdellite and the intermediate phase. The fraction of the intermediate phase increased from ~50% to ~60% up to about 275 ºC, and then decreased, until by 310 ºC all of the intermediate phase had transformed to ramsdellite. A mass spectrometer monitoring effluents from the heated sample showed a water loss concurrent with the disappearance of the intermediate phase. From 310 to 450 ºC, the ramsdellite gradually tranformed to pyrolusite, but at the maximum temperature both phases remained. The groutite transformed to hausmannite and pyrolusite between approximately 250 and 290 ºC. Between 400 and 460 ºC the pyrolusite and hausmannite transformed to bixbyite.