Abstract Palladium hydride was discovered more than 150 years ago and remains one of the most-studied interstitial metal hydrides because of the richness of its physical behaviours, which include ordered phases and anomalous properties at temperatures below 100 K, a superabundant-vacancy (SAV) phase with stoichiometry Pd3H4 formed at high temperature and pressure, and quenching of the enhanced Pauli paramagnetism of palladium. One of the most fascinating properties of palladium hydride is superconductivity at about 10 K without external pressure, in contrast to the newly-discovered polyhydride room-temperature superconductors that require megabar pressures. Moreover, the superconductivity exhibits an inverse isotope effect. Remarkably, modern first-principles approaches are unable to accurately predict the superconducting transition temperature by calculating the electron–phonon coupling constant within Migdal-Eliashberg theory. Anharmonicity of the hydrogen site potential is a key factor and poses a great challenge, since most theoretical approaches are based on the harmonic approximation. This review focuses on the electron and phonon band structures that underpin all such calculations, with palladium as a reference point. While the electron band structures of palladium and its monohydride are uncontroversial, the phonon band structure of palladium hydride in particular is problematic, with a realistic treatment of anharmonicity required – and largely yet to be achieved – to reproduce the results of inelastic neutron scattering experiments. In addition to the monohydride and SAV phases, possible higher hydrides are surveyed and the origin of the famous “50-K” anomaly in specific heat and other physical properties is critically reviewed.

中文翻译：

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钯和氢化钯的电子和声子能带结构：综述

摘要 氢化钯在 150 多年前被发现，由于其丰富的物理行为，包括有序相和温度低于 100 K 的异常性质，一种超富空位 (SAV)，它仍然是研究最多的间隙金属氢化物之一。在高温和高压下形成具有化学计量的 Pd3H4 相，并淬火钯的增强的泡利顺磁性。与新发现的需要兆巴压力的多氢化物室温超导体形成对比，氢化钯最迷人的特性之一是在约 10 K 时在没有外部压力的情况下具有超导性。此外，超导表现出逆同位素效应。值得注意的是，现代第一性原理方法无法通过计算 Migdal-Eliashberg 理论中的电子-声子耦合常数来准确预测超导转变温度。氢位势的非谐性是一个关键因素，也是一个巨大的挑战，因为大多数理论方法都是基于谐波近似的。本综述重点关注支持所有此类计算的电子和声子能带结构，并以钯为参考点。虽然钯及其一氢化物的电子能带结构没有争议，但氢化钯的声子能带结构尤其存在问题，需要对非谐性进行现实处理——而且很大程度上尚未实现——以重现非弹性中子散射实验的结果。