1. Centre for Science at Extreme Conditions and School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3FD, U.K.
2. Center for High Pressure Science and Technology Advanced Research, Shanghai 100094, P. R. China
3. Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, U.K.
4. Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
5. Key Laboratory of Materials Physics, Institute of Solid State Physics, Hefei 230031, P. R. China