IN SITU HIGH-PRESSURE SYNCHROTRON X-RAY POWDER DIFFRACTION STUDY OF TUNNEL MANGANESE OXIDE MINERALS

Microporous Mn oxides are valued for their catalytic, ion exchange, electrochemical, and adsorption properties. Of particular interest have been Mn-oxide catalysts with large tunnel structures; they exhibit a range of tunnel shapes and sizes that offer potential for complementary catalytic or cation-exchange applications. We used synchrotron powder X-ray diffraction to study structural changes with increasing pressure for three tunnel Mn oxide minerals. Experiments were performed using a Diamond Anvil Cell (DAC) at beamline X7A at the National Synchrotron Light Source, Brookhaven National Laboratory. Powdered samples of hollandite [(Ba_.75,Pb_.16Na_.10K_.04)(Mn,Fe)₈O₁₆], romanechite (Ba_.66Mn₅O₁₀ · 1.34H₂O), and todorokite [(Mg_.45,Na_.42,Ca_.15)Mn₆O₁₂ · 4H₂O] were, in turn, loaded into the DAC at ambient pressure and room temperature along with a few small ruby chips. A mixture of methanol:ethanol:water was used as a pressure transmitting fluid. The pressure at the sample was measured using the shift in the R1 emission line of the included ruby chips. Data were collected with a gas-proportional position-sensitive detector and radiation wavelength of 0.6839(1) Å. Powder X-ray diffraction data were measured at pressure increments of 0.5 - 1.0 GPa between ambient pressure and 7 GPa; the samples were equilibrated for about 15 minutes or more at each measured pressure. A second set of measurements was made for each sample as the pressure was released. Unit-cell parameters were determined by whole pattern fitting using the LeBail method. The hollandite structure (I2/m) has tunnels with square cross-sections measuring 2 octahedra on an edge (2x2), and as pressure increased to ~3 GPa, the a-axis lengthened, b and c and unit-cell volume decreased, and b increased. Above ~3 GPa, a started to decrease, and the slopes of the changes for the other parameters changed, suggesting a phase transition at about 3 GPa. Romanechite (C2/m) has 2x3 tunnels, and with increasing pressure all axial parameters and volume decreased and b increased, exhibiting a slight change in slope at ~2 GPa. For the todorokite structure (P2/m), with 3x3 tunnels, the c-axis increased slightly below 1 GPa, but at higher pressures all of the axial parameters and unit-cell volume decreased and b increased, consistent with a steady collapse of the tunnels.