GSA Connects 2022 meeting in Denver, Colorado

Paper No. 167-2
Presentation Time: 9:00 AM-1:00 PM


CHUNG, Dongyoun1, HEANEY, Peter1, POST, Jeff2, STUBBS, Joanne3 and ENG, Peter J.2, (1)Dept. of Geosciences, Pennsylvania State University, 540 Deike Bldg, University Park, PA 16802, (2)Center for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637, (3)Center for Advanced Radiation Sources, The University of Chicago, Lemont, IL 60439

To compare thermal expansion behaviors in isomeric structures with different redox behaviors, we separately heated pyrolusite (Mn4+O2) and rutile (Ti4+O2) powders from 25 oC to ~1000 oC at a rate of 0.05 oC/sec and analyzed them by synchrotron X-ray diffraction and Rietveld refinement. Pyrolusite exhibited two reduction-induced phase transitions. Between 531 and 583 oC, pyrolusite transformed to bixbyite (Mn3+2O3), and between 972 and 998 oC, bixbyite transitioned to hausmannite (Mn2+Mn3+2O4). Rutile experienced no phase transitions over this same temperature interval.

The refined bond lengths and angles for pyrolusite showed that the Mn4+ coordination octahedron in pyrolusite became more distorted near the phase transition to bixbyite due to the approach of two coordinating oxygen atoms. Each Mn4+ ion in pyrolusite has three 3d electrons that can interact with the 2p electrons of the surrounding O2- ions to make π bonds, whereas the Ti4+ of rutile has no 3d electrons. Thus, π bonding between octahedral Mn4+ and the surrounding O2- cations increases the strength of the Mn-O bonds of pyrolusite relative to the Ti-O bonds of rutile. However, we observed a small decrease in O-O distances in pyrolusite before the transition, suggesting that at high temperature, increased p bonding between adjacent O anions presages the release of O2 and the reduction of Mn4+ to Mn3+. Analogous behavior was observed for bixbyite before its reductive phase transition to hausmannite. In contrast, no anomalous changes in the O-O distances occurred for rutile.

Our XRD analyses also have provided accurate thermal expansion coefficients for these materials over a broader temperature range than reported in previous studies. Mn-O bond lengths in pyrolusite were shorter and stronger than the Ti-O bonds of rutile; for example, at 60 oC, <Mn-O> bond distances in pyrolusite refined to 1.8837(5) Å whereas the <Ti-O> bond lengths of rutile were 1.9554(4) Å. Accordingly, the axial and volumetric thermal expansion coefficients refined for rutile were greater than those for pyrolusite. The axial CTEs for pyrolusite were 97% (a-axis) and 68% (c-axis) of the corresponding values for rutile, and the volumetric CTE for pyrolusite was 85% of that for rutile.