Singlehyphen;crystal and polycrystalline specimens (including powder compacts of various densities) of titanium dioxide have been subjected to shockhyphen;wave pressures in the 150ndash;1000ndash;kbar range. Hugoniot measurements have disclosed a phase transition commencing below 200 kbar, and xhyphen;ray diffraction studies of specimens recovered after shocking to various pressures above 150 kbar have shown the presence of an orthorhombic phase with theagr;hyphen;PbO2structure. Calculated lattice parameters areathinsp;equals;thinsp;4.55 Aring;,bthinsp;equals;thinsp;5.46 Aring;, andcthinsp;equals;thinsp;4.92 Aring;, 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 oxygenhyphen;titanium coordination) that dynamic measurements indicate is formed under shock. Yields of theagr;hyphen;PbO2phase as high as about 90percnt; have been obtained from 001hyphen; and 111hyphen;oriented rutile crystals shocked to 450 kbar. At atmospheric pressure this phase can be retained indefinitely at temperatures below 340deg;C, but reverts to rutile rapidly at 550deg;C. An activation energy of 66thinsp;plusmn;thinsp;5 kcal/ghyphen;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 100ndash;200 Aring;, and crystallites obtained from initially singlehyphen;crystal rutile exhibit preferred orientation. Shocked specimens of rutile and of the orthorhombic phase have ESR spectra that exhibit surfacehyphen;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.
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