Free electron to electride transition in dense liquid potassium

• Published in: Nature physics Vol. 17; no. 8; pp. 955 - 960
• Main Authors: Zong, Hongxiang, Robinson, Victor Naden, Hermann, Andreas, Zhao, Long, Scandolo, Sandro, Ding, Xiangdong, Ackland, Graeme J.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.08.2021; Nature Publishing Group
• Subjects: 119/118; 639/638/563/979; 639/638/563/981; 639/766/119/1002; 639/766/119/2795; Abnormalities; Alkali metals; Anions; Atomic; Classical and Continuum Physics; Complex Systems; Condensed Matter Physics; Coordination numbers; Free electrons; Liquid phases; Liquid potassium; Mathematical and Computational Physics; Molecular; Optical and Plasma Physics; Phase diagrams; Physics; Physics and Astronomy; Potassium; Theoretical; Thermal expansion; Thermodynamic properties
• ISSN: 1745-2473; 1745-2481
• DOI: 10.1038/s41567-021-01244-w

At high pressures, simple metals such as potassium have a rich phase diagram including an insulating electride phase in which electrons have a localized, anionic character. Measurements in the liquid phase have shown a transition between two states, but experimental challenges have prevented detailed thermodynamic measurements. Using potassium as an example, we present numerical evidence that the liquid–liquid transition is a continuous transformation from free electron to electride behaviour. We show that the transformation manifests in anomalous diffusivity, thermal expansion, speed of sound, coordination number, reflectivity and heat capacity across a wide range of pressures. The abnormalities stem from a significant change in the local electronic and ionic structure. Although primarily a pressure-induced phenomenon, there is also a thermal expansion anomaly. By establishing the electride nature of the high-pressure liquid phase, we resolve the long-standing mystery of how a liquid can be denser than a close-packed solid. Our work is relevant for high-pressure thermodynamic properties of all alkali metal liquids. Alkali metals at high pressures have a liquid–liquid transition that is difficult to study in detail. Numerical calculations now suggest that the higher-pressure state is an electride liquid, in which electrons behave like localized anions.