Ab initio study of metastable occupation of tetrahedral sites in palladium hydrides and its impact on superconductivity

Abstract

A recent experimental work on palladium hydrides suggested that metastable structures with hydrogen atoms occupying tetrahedral sites could lead to superconductivity above 50 K, a huge increase compared to the 9 K critical temperature of the stable structure with all hydrogen atoms occupying octahedral sites. By generating many structures with hydrogen atoms randomly occupying the octahedral and tetrahedral sites of the face-centered cubic lattice and calculating their energy at different theoretical levels from first principles, we determine that metastable structures with partial or full occupation of tetrahedral sites are possible, even when the ionic quantum zero-point energy and anharmonicity are included in the calculations. Anharmonicity is crucial in palladium hydrides when hydrogen atoms occupy octahedral sites and, in fact, makes the structure with full octahedral occupation the ground state. Despite the metastable existence of structures with full or partial tetrahedral sites occupation, the superconducting critical temperature is reduced with the number of tetrahedral sites occupied. Our calculations discard that the occupation of tetrahedral sites can increase the critical temperature in palladium hydrides.

Advances in highly hydrided palladium

“…In a broad sense, insights into the fundamentals of palladium hydrides especially those with unusual H/Pd ratios would be instructive to revealing their latent properties and functionalities that can be exploited towards other research fields. As a typical example, available studies have manifested the potential of highly hydrided palladium in hightemperature conductivity (Meninno and Errea, 2023).…”

Ab initio study of the structural, vibrational, and optical properties of potential parent structures of nitrogen-doped lutetium hydride

“…[38]). This compound is therefore strongly anharmonic, as it happens for other fcc metals with H atoms in octahedral sites [30,31,41]. The spectral function calculation reveals there might be an instability around the X point of the Brillouin zone.…”

“…We find that the anharmonicity of phonon modes plays a large role in determining the critical temperature, as it changes by a 50% depending on the method used to calculate the Eliashberg spectral function α 2 F (ω). The modes that contribute significantly to α 2 F (ω) are the octahedral modes, as it happens in palladium hydrides [41], which explains the existence of superconductivity in LuH 3 and not in LuH 2 .…”

Quantifying the Nonadiabaticity Strength Constant in Recently Discovered Highly Compressed Superconductors

“…First-principles calculations [12][13][14][15][16][17][18][19][20][21][70][71][72][73][74][75][76][77][78][79] are essential tools in the quest for roomtemperature superconductivity (they were used [76] to explain the experimental results [80] for one of the most difficult-to-explain hydride cases, AlH 3 ), and the primary calculated parameter in these calculations is the transition temperature, T c . In addition, another difficult-to-explain hydride should be mentioned, which is LiPdH x [46].…”