High-pressure polymorphic transformation of rutile to α-PbO₂-type TiO₂ at {0 1 1}_R twin boundaries

The presence of nano-scale lamellae of the α-PbO₂-type polymorph of TiO₂ sandwiched between twinned rutile inclusions in jadeite has been confirmed by electron diffraction and high-resolution transmission electron microscopy, backed up by image simulation techniques, from ultrahigh-pressure jadeite quartzite at Shuanghe in the Dabie Mountains, China. The crystal structure is orthorhombic with lattice parameters a = 4.58 Å, b = 5.42 Å, c = 5.02 Å and space group Pbcn. A three-dimensional structural model has been constructed for the rutile to α-PbO₂-type TiO₂ phase transformation based on high-resolution electron microscopic images. Computer image simulation and structural model analysis reveal that rutile {0 1 1}_R twin interface is a basic structural unit of α-PbO₂-type TiO₂. Nucleation of α-PbO₂-type TiO₂ lamellae 1-2 nm thick is caused by the displacement of one half of the titanium cations within the {0 1 1}_R twin slab. This displacement reduces the Ti-O-Ti distance and is favored by high pressure.