SYNCHROTRON X-RAY DIFFRACTION OF PYROLUSITE (MNO2) AND RUTILE (TIO2) DURING HEATING TO ~1000 °C

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 O^2- ions to make π bonds, whereas the Ti⁴⁺ of rutile has no 3d electrons. Thus, π bonding between octahedral Mn⁴⁺ 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 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.