Possible metal-metal phase transitions, particularly in LiC₆

We study the hypothetical state of affairs arising if the "interlayer" electronic state of a donor graphite intercalate happened to cross the Fermi level, as a function of temperature, or possibly of other external parameters. Some aspects of the ensuing metal-metal transition are outlined according to two separate model situations. The first is one where the transition occurs simply via thermal anharmonic expansion of the c-axis. In this case, no proper phase transition is expected. In a second model, an accompanying structural deformation with loss of inversion symmetry at the donor site is considered. The possibility that one such transition could account for the observed anomalies of LiC₆ around 190 K is discussed.