Synchrotron X-Ray Diffraction of Pyrolusite (Mno2) Andrutile (Tio2) During Heating to ~1000 °C

To compare thermal expansion behaviors in isomeric structures with different redox behaviors, we separately heated pyrolusite (Mn 4+ O 2 ) and rutile (Ti 4+ O 2 ) powders from 25 o C to ~1000 o C at a rate of 0.05 o C/sec and analyzed them by synchrotron X-ray diffraction and Rietveld refinement. Pyrolusite exhibited two reduction-induced phase transitions. Between 531 and 583 o C, pyrolusite transformed to bixbyite (Mn 3+2 O 3 ), and between 972 and 998 o C, bixbyite transitioned to hausmannite (Mn 2+ Mn 3+2 O 4 ). Rutile experienced no phase transitions over this same temperature interval. The refined bond lengths and angles for pyrolusite showed that the Mn 4+ coordination octahedron in pyrolusite became more distorted near the phase transition to bixbyite due to the approach of two coordinating oxygen atoms. Each Mn 4+ ion in pyrolusite has three 3 d electrons that can interact with the 2 p electrons of the surrounding O 2- ions to make π bonds, whereas the Ti 4+ of rutile has no 3 d electrons. Thus, π bonding between octahedral Mn 4+ and the surrounding O 2- 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 bonding between adjacent O anions presages the release of O 2 and the reduction of Mn 4+ to Mn 3+ . 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 provided