Synchrotron X-ray diffraction of pyrolusite (MnO2) and rutile (TiO2) during heating to ∼1000 °C

Abstract

To compare thermal expansion behaviors in isomorphic structures with different redox behaviors, we separately heated pyrolusite (Mn4+O2) and rutile (Ti4+O2) powders from 25 to ∼1000 °C at a rate of 0.05 °C/s at ambient pressure and analyzed them using synchrotron X-ray diffraction and Rietveld refinement. The pyrolusite exhibited two reduction-induced phase decompositions. Between 531 and 583 °C, pyrolusite decomposed to bixbyite (Mn3+2O3), and between 972 and 998 °C, bixbyite decomposed to hausmannite (Mn2+Mn3+2O4). The rutile experienced no phase decompositions or transitions over this same temperature interval.The refined bond lengths and angles for pyrolusite showed that the Mn4+ coordination octahedron became more distorted near the phase decomposition to bixbyite due to the approach of two coordinating oxygen atoms. The 3d electrons of each Mn4+ ion in pyrolusite make π bonds with the 2p electrons of the surrounding O2− ions, whereas the Ti4+ of rutile has no 3d electrons. Thus, π bonding between octahedral Mn4+ and the surrounding O2− anions 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 decomposition, suggesting that at high temperature, increased π bonding between adjacent O anions precedes the release of O2 and the reduction of Mn4+ to Mn3+. Analogous behavior was observed for bixbyite before its reductive phase decomposition to hausmannite. In contrast, no anomalous changes in the O–O distances occurred for rutile.Our X-ray diffraction analyses provided accurate thermal expansion coefficients for these materials over a broader temperature range than reported in previous studies. The Mn–O bond lengths in pyrolusite were shorter and stronger than the Ti–O bonds of rutile; for example, at 60 °C, <Mn–O> bond distances in pyrolusite refined to 1.8854(6) Å whereas the <Ti–O> bond lengths of rutile were 1.9575(4) Å. Accordingly, the axial and volumetric thermal expansion coefficients refined for rutile were greater than those for pyrolusite. The axial coefficients of thermal expansion (CTE) for pyrolusite were 96% (a-axis) and 69% (c-axis) of the corresponding values for rutile, and the volumetric CTE for pyrolusite was 85% of that for rutile.