Sound velocities, and mechanical and electronic properties of the intermetallic compound CeAl2 at high pressure

Compressional $({V}_{P})$ and shear $({V}_{S})$ wave velocities of polycrystalline $\mathrm{Ce}{\mathrm{Al}}_{2}$ have been measured up to 12.5 GPa at room temperature by ultrasonic interferometry in a multi-anvil apparatus. Both ${V}_{P}$ and ${V}_{S}$ show a weak pressure dependence up to 7.5 GPa, and then a remarkable discontinuity is observed which supports the earlier report about a volume collapse in this material. An anomaly has also been found in the pressure derivative of resistance $dR/dP$ at around $P=7.5$ GPa. Based on the experimental data, the high-pressure behaviors of the bulk and shear moduli are investigated. Complementary to the measured data, the single-crystal elastic constants are computed by first-principles calculations. Both the pressure-dependent ${C}_{11}$ and ${C}_{44}$ exhibit a change of slope at about $P=7.5$ GPa. At low pressure, the observed flat curve for the shear modulus may be due to ${C}_{44}$ which shows negligible pressure dependence. At higher pressure, ${C}_{44}$ softens more rapidly and the mechanical instability occurs at $P>15$ GPa. The calculated electronic densities of states show that sharp peaks near the Fermi level are mainly originated from the $4f$ and $d$ states, which may be responsible for the Ce-Ce covalent bond. High pressure leads to a broadening of $4f$ band and an increase of the $d/f$ hybridization.