1National Lab of Superhard Materials, Jilin University, Changchun 130012, People's Republic of China
2Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5E2
3Department of Geology, Moscow State University, 119992 Moscow, Russia
4Department of Materials, Laboratory of Crystallography, ETH Zurich, HCI G 515, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland