Polymorphic Behavior of Titania under Dynamic Loading

Abstract

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 a = 4.55 Å, b = 5.46 Å, and c = 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.

High-pressure polymorph of TiO2-II from the Xiuyan crater of China

“…TiO 2 -II in the Xiuyan crater mostly occurs as micrometer-sized or even much smaller particles. At atmospheric pressure, i  - reverts to rutile rapidly at 550°C [9]. The back transformation of i  - to rutile was not observed in our samples.…”

“…Shock-loading experiments at pressures from 20 to 100 GPa had succeeded in the transformation of rutile to TiO 2 -II [8][9][10]. The highest yield of i  - was achieved at a peak pressure of ~45 GPa [9,10].…”

“…The highest yield of i  - was achieved at a peak pressure of ~45 GPa [9,10]. A peak shock pressure between 35 and 43 GPa in the gneiss of the Xiuyan crater should be available for the transition from rutile to TiO 2 -II.…”

“…A total of 19 diffraction lines are collected from TiO 2 -II, in addition to the reflections from rutile and monazite ( Figure 7; Table 2). The powder X-ray diffraction data show that TiO 2 -II is isostructural with -PbO 2 -type TiO 2 , space group Pbcn, synthesized by shock experiments [8,9] and in situ high-pressure experiments using a diamond-anvil cell [21]. All reflections from TiO 2 -II can be indexed to -PbO 2 structure in space group Pbcn with lattice parameters of a=4.543(1) Å, b= 5.491(9) Å and c=4.895(2) Å; V=122.1(4) Å 3 , Z=4.…”

“…A number of static compression experiments succeed in the transition of rutile to TiO 2 -II at pressure range between 4 and 12 GPa and temperature between 400 and 1500°C [3][4][5][6][7]. Shock-loading experiments revealed that rutile transforms to i  - at peak shock pressure from 20 to 100 GPa [8][9][10].…”