Pressure-induced decomposition of binary lanthanum intermetallic compounds

We present a comprehensive study on structural and electronic properties of lanthanum intermetallic compounds (${M}_{x}{\mathrm{La}}_{y}$, $M=\text{Be}$, Mg, Al, Ga, In, Tl, Pb, and Bi) under high pressure. By using a swarm intelligence structure search method combined with first-principles calculations, pressure-induced phase transitions of ${M}_{x}{\mathrm{La}}_{y}$ were investigated, with several new structures predicted. A universal yet intriguing phenomenon was found; that is, all of these compounds will decompose into elemental solids at certain pressures, which is against the general intuition that extreme pressure always stabilizes and densifies materials. Mechanical analysis suggests that this anomalous behavior is associated to the elastic moduli and interatomic interaction in ${M}_{x}{\mathrm{La}}_{y}$, and their changes under extreme pressure. A low bulk modulus and larger atomic volume of La result in a smaller volume for the elemental mixture compared to their compound at high pressures, which leads to an energetically favorable $PV$ work and enthalpy for the elemental mixture. Furthermore, the external pressure tends to weaken the La-$M$ electrostatic interaction in compounds as evidenced by the reduced charge transfer between La and $M$, which in turn modifies the electronegativity of La and $M$ and destabilizes the compounds. Our results shed light on the high-pressure behaviors of La-based intermetallic compounds and provide important guidance for understanding other La-like intermetallic compounds at high pressures.