Polymorphic Behavior of Titania under Dynamic Loading

Single‐crystal and polycrystalline specimens (including powder compacts of various densities) of titanium dioxide have been subjected to shock‐wave pressures in the 150–1000–kbar range. Hugoniot measurements have disclosed a phase transition commencing below 200 kbar, and x‐ray diffraction studies of specimens recovered after shocking to various pressures above 150 kbar have shown the presence of an orthorhombic phase with the α‐PbO₂ structure. Calculated lattice parameters are $\text{a\hspace{0.17em}=\hspace{0.17em}4.55 Å}$ , $\text{b\hspace{0.17em}=\hspace{0.17em}5.46 Å}$ , and $\text{c\hspace{0.17em}=\hspace{0.17em}4.92 Å}$ , which correspond to a crystal density of 4.34 g/cm³. The orthorhombic phase appears to result on release of pressure from a considerably denser phase of TiO₂ (postulated to have approximately 8:4 oxygen‐titanium coordination) that dynamic measurements indicate is formed under shock. Yields of the α‐PbO₂ phase as high as about 90% have been obtained from [001]‐ and [111]‐oriented rutile crystals shocked to 450 kbar. At atmospheric pressure this phase can be retained indefinitely at temperatures below 340°C, but reverts to rutile rapidly at 550°C. An activation energy of 66 ± 5 kcal/g‐mole has been computed for reversion of specimens containing predominantly the orthorhombic phase but also containing some nuclei of rutile. The average crystallite size of shocked material is typically as small as 100–200 Å, and crystallites obtained from initially single‐crystal rutile exhibit preferred orientation. Shocked specimens of rutile and of the orthorhombic phase have ESR spectra that exhibit surface‐sensitive behavior, but which do not appear to be dependent upon the relative amounts of rutile and orthorhombic phase present. Petrographic and electron microscopy observations of shocked material are also reported.