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

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.