金属铜铁矿型 PdCoO₂ 在压力下的反常压缩性和电子稳定性
Anomalous Compressibility and Electronic Robustness of Metallic Delafossite PdCoO$_2$ under Pressure
AI总结：
研究金属铜铁矿型 PdCoO₂ 在压力下的特性，采用高压单晶 X 射线衍射结合第一性原理计算，发现其晶格有异常各向异性压缩，化学键网络稳健，压力下晶体结构和金属态得以保持，电导率表现出相应特性。
AI中文摘要：
层状铜铁矿型 PdCoO₂ 是一种特殊的氧化物金属，其超高电导率源于 Pd 衍生的近自由电子带。本文采用高压单晶 X 射线衍射结合第一性原理计算，研究了其在高达 10 GPa 压力下的结构、键合和电子演化。PdCoO₂ 在整个研究压力范围内保持菱面体 R-3m 结构，无结构相变。晶格呈现异常各向异性压缩，面内 a 轴收缩比堆叠 c 轴更强。尽管有这种反常压缩性，但局部 Pd-O 和 Co-O 配位环境变化很小，表明键合框架非常刚性。晶体轨道哈密顿布居分析显示现有键合相互作用仅略有增强，没有出现不稳定的 Pd 相关反键态。与这些发现一致，Pd 衍生的金属带和准二维费米面在压缩下基本不变。玻尔兹曼输运计算进一步表明，面内电导率几乎与压力无关，而面外电导率适度下降，导致输运各向异性略有增加。这些结果表明，PdCoO₂ 的反常压缩性源于其稳健的化学键网络，该网络在压力下保持晶体结构和高导电的 Pd 衍生金属态。
英文摘要：
The layered delafossite PdCoO$_2$ is an exceptional oxide metal whose ultrahigh conductivity arises from a Pd-derived nearly-free-electron band. Here, high-pressure single-crystal X-ray diffraction combined with first-principles calculations is used to investigate its structural, bonding, and electronic evolution up to 10 GPa. PdCoO$_2$ retains the rhombohedral R-3m structure throughout the investigated pressure range, with no structural phase transition. The lattice exhibits an unusual anisotropic compression, with the in-plane a-axis contracting more strongly than the stacking c-axis, opposite to the behavior of most layered materials. Despite this anomalous compressibility, only minor changes are observed in the local Pd-O and Co-O coordination environments, indicating a remarkably rigid bonding framework. Crystal orbital Hamilton population analysis reveals only subtle strengthening of the existing bonding interactions, without the emergence of destabilizing Pd-related antibonding states. Consistent with these findings, the Pd-derived metallic band and quasi-two-dimensional Fermi surface remain essentially unchanged under compression. Boltzmann transport calculations further show that the in-plane conductivity is nearly pressure-independent, whereas the out-of-plane conductivity decreases modestly, resulting in a slight increase in transport anisotropy. These results demonstrate that the anomalous compressibility of PdCoO$_2$ originates from its robust chemical bonding network, which preserves both the crystal structure and the highly conductive Pd-derived metallic state under pressure.
