Phase transition and enhanced hardness of LaB4 under pressure

TL;DR

This study investigates the phase transitions and hardness of LaB4 under high pressure, revealing a new orthorhombic phase, phase transformation at 106 GPa, and high hardness comparable to B4C, with implications for high-pressure material behavior.

Contribution

The paper identifies a novel orthorhombic phase of LaB4 and details its phase transition behavior and hardness evolution under pressure, expanding understanding of rare-earth borides.

Findings

LaB4 has two low-enthalpy phases up to 200 GPa.

The P4/mbm phase transforms to Pnma-I at 106 GPa.

LaB4 exhibits high hardness (~30.5 GPa) at ambient conditions.

Abstract

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.