The authors thank A. Engwall for help with synthesis and E. Martinez for the helpful discussion. X.K., Z.P. and F.S. are grateful for the support from the US Department of Energy (DOE) (grant no. DE-SC0016270). The work at the Lawrence Livermore National Laboratory was performed under the auspices of DOE contract no. DE-AC52-07NA27344. J.G., M.F.B. and R.T.O. at the Ames Laboratory were supported by the DOE Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under contract no. DE-AC02-07CH11358. A.C. at the Los Alamos National Laboratory is under DOE contract no. DE-AC52-06NA253. Y.M.W. and D.Q. acknowledge the support of the Laboratory Directed Research and Development (LDRD) programme (grant no. 17-ERD-048) at Lawrence Livermore National Laboratory. J.M. acknowledges support from the National Science Foundation (grant no. DMR-1611342). The simulations in this research used resources of the National Energy Research Scientific Computing Centre, supported by DOE contract no. DE-AC02-05CH11231, and those of the Extreme Science and Engineering Discovery Environment, supported by National Science Foundation (grant no. ACI-1548562).