High-pressure polymorphic transformation of rutile to alpha-PbO2-type TiO2 at {011}R twin boundaries.
The presence of nano-scale lamellae of the alpha-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 A, b=5.42 A, c=5.02 A and space group Pbcn. A three-dimensional structural model has been constructed for the rutile to alpha-PbO2-type TiO2 phase transformation based on high-resolution electron microscopic images. Computer image simulation and structural model analysis reveal that rutile {011}R twin interface is a basic structural unit of alpha-PbO2-type TiO2. Nucleation of alpha-PbO2-type TiO2 lamellae 1-2 nm thick is caused by the displacement of one half of the titanium cations within the {011}R twin slab. This displacement reduces the Ti-O-Ti distance and is favored by high pressure.
Meng
D W
DW
Faculty of Materials Science and Chemical Engineering, China University of Geosciences, Wuhan 430074, China.
Wu
X L
XL
Sun
F
F
Huang
L W
LW
Liu
F
F
Han
Y J
YJ
Zheng
J P
JP
Meng
X
X
Mason
R
R