  The crystalline structures of lanthanum tetraboride, LaB4, under pressure are
investigated using a genetic algorithm method coupled with first-principles
calculations. We find two low-enthalpy phases for LaB4 as the thermodynamic
ground state up to 200 GPa: a phase previously observed in experiments (P4/mbm)
and a novel orthorhombic phase (Pnma-I). The P4/mbm phase transforms to the
Pnma-I phase at around 106 GPa, accompanied by breakage of the B-octahedron.
The P4/mbm phase of LaB4 exhibits a high hardness (30.5 GPa) at ambient
conditions, which is comparable to that of the hard material B4C. The hardness
of the two low-enthalpy phases increases with pressure. The electronic band
structures shows that all of the competitive phases (two stable and two
metastable phases) are metallic. Phonon calculations shows the P4/mbm and
Pnma-I phases are thermodynamically stable in their respective pressure ranges.
This elucidation of the phase transition behavior and hardness of LaB4 provides
new knowledge on the interesting high-pressure behaviors of the rare-earth
metal borides.
