Volume 83, Number 3, August 2008
Article Number 39001
Number of page(s) 6
Section Geophysics, Astronomy and Astrophysics
Published online 28 July 2008
EPL, 83 (2008) 39001
DOI: 10.1209/0295-5075/83/39001

Origin of the change of the electrical and optical properties in shocked Al2O3 and prediction of an increase in electrical conductivity in MgSiO3 at pressure-temperature conditions of the Earth's D'' layer

L. He1, 2, M. J. Tang2, Y. Fang2 and F. Q. Jing1, 3

1  Institute of High Pressure Physics, Southwest Jiaotong University - Chengdu 610031, PRC
2  Institute of Solid State Physics and College of Physics and Electronic Engineering, Sichuan Normal University - Chengdu 610066, PRC
3  Laboratory for Shock Wave and Detonation Physics Research, Institute of Fluid Physics, China Academy of Engineering Physics - P. O. Box 919-111, Mianyang 621900, PRC

received 5 February 2008; accepted in final form 20 June 2008; published August 2008
published online 28 July 2008

Shock-wave experiments of Al2O3 indicated the onset of an increase in electrical conductivity and observed the optical transparency loss at ~130 GPa. Here, based on first-principles calculations, we determine the pressure dependence of the band gap of perfect Al2O3 to 220 GPa, and investigate the optical absorption of Al2O3 without and with oxygen and aluminum vacancies within 220 GPa. Our results indicate that: 1) the onset of the conductivity increase is attributed to a band-gap decrease due to the Rh2O3(II)-CaIrO3 transition at ~130 GPa and ~ 1500 K; 2) this transition is not responsible for the transparency loss, but heterogeneous absorption in the visible-light region, induced by the +2 charge oxygen vacancy, should be a source of this phenomenon. The calculations of perfect MgSiO3, a material analogous to Al2O3, suggest that a perovskite to post-povskite transition in MgSiO3 at ~125 GPa and ~2500 K also yields a band-gap reduction. This causes an increase in electrical conductivity in MgSiO3 at pressure-temperature conditions of the Earth's D'' layer.

91.60.Gf - High-pressure behavior.
62.50.-p - High-pressure effects in solids and liquids.
71.15.Mb - Density functional theory, local density approximation, gradient and other corrections.

© EPLA 2008