Structural phase transition, mechanics, and thermodynamics of heavy fermion metal UPt3 under pressure†

Crystal structures, electronic structures, mechanics, and thermodynamics of the heavy fermion superconductor UPt₃ under a pressure of up to 300 GPa have been investigated by a particle swarm optimization structure prediction method together with detailed first-principles calculations. A pressure-induced structural phase transition (P_T) is predicted at 155.9 GPa, where the hexagonal crystal structure with the space group P6₃/mmc transforms into an orthorhombic structure with the space group Cmmm. The molar volume of UPt₃ drops about 2.52% at 155.9 GPa, while the distance between the first-nearest neighbor of U atoms (d_U–U) decreases, implying a switch from the heavy electronic states to the weakly correlated electronic states. The metal nature is well retained upon the phase transition and upon further compression to 300 GPa. Phonon dispersions and elastic constants are used to confirm the dynamical and mechanical stability of both phases under different pressures. The bulk modulus B, shear modulus G, and Young's modulus E of the Cmmm are all higher than those of the P6₃/mmc, indicating enhanced mechanical properties of the Cmmm phase at the same pressure. The highest phonon vibration frequency increases with pressure, suggesting strengthened atom–atom interactions. Thermodynamic properties, evaluated using the quasi-harmonic approximation (QHA), reveal that the P6₃/mmc phase remains stable in the 0–155.9 GPa range, while the Cmmm phase emerges under higher pressures. Our results provide theoretical insights into the pressure-driven phase transition of UPt₃ and provide its detailed electronic, phononic, mechanical, and thermodynamic properties under external pressure.