Structure stability in the simple element sodium under pressure

The simple alkali metal Na that crystallizes in a body-centred cubic structure at ambient pressure exhibits a wealth of complex phases at extreme conditions as found by experimental studies. The analysis of the mechanism of stabilization of some of these phases, namely, the low-temperature Sm-type phase and the high-pressure cI 16 and oP 8 phases, shows that they satisfy the criteria for the Hume-Rothery mechanism. These phases appear to be stabilized due to a formation of numerous planes in a Brillouin–Jones zone in the vicinity of the Fermi sphere of Na, which leads to the reduction of the overall electronic energy. For the oP 8 phase, this mechanism seems to work if one assumes that Na becomes a divalent metal at this density. The oP 8 phase of Na is analysed in comparison with the MnP-type oP 8 phases known in binary compounds, as well as in relation to the NiAs-type hP 4 structure.