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.