  A series of electronic and structural transitions are predicted in molten
lithium from first principles. A new phase with tetrahedral local order
characteristic of $sp^3$ bonded materials and poor electrical conductivity is
found at pressures above 150 GPa and temperatures as high as 1000 K. Despite
the lack of covalent bonding, weakly bound tetrahedral clusters with finite
lifetimes are predicted to exist. The stabilization of this phase in lithium
involves a unique mechanism of strong electron localization in interstitial
regions and interactions among core electrons. The calculations provide
evidence for anomalous melting above 20 GPa, with a melting temperature
decreasing below 300 K, and point towards the existence of novel low-symmetry
crystalline phases.
