Physics Department - Structural and Melting Anomalies of Highly Compressed. Sodium

Abstract
Pressure and temperature are two fundamental thermodynamic variables that can be used to manipulate the properties of materials. Advances in synchrotron-based experimental methodologies, together with the development of efficient computational electronic-structure algorithms and software, have greatly expanded our knowledge in this rapidly evolving field. Recent efforts have led to the prediction and experimental validation of a broad spectrum of novel phenomena, including structural phase transitions and superconductivity in dense metallic hydrides. Nevertheless, a unifying framework that elucidates the underlying structure–property relationships remains to be fully established.

An example is that alkali and alkaline earth elements undergo a series of structural transformations under compression, typically progressing from cubic phases to exotic complex frameworks and even to incommensurate structures. Notably, sodium transforms into a wide band gap insulator at extremely high pressures. In addition, the melting behaviour of Group I and II elemental solids deviates significantly from that of most materials, for which the melting temperature generally increases with pressure. In contrast, these elements exhibit a non-monotonic melting curve: the melting temperature initially rises, then decreases to a minimum, and subsequently increases again. The underlying mechanisms and electronic origins of these anomalous behaviours remain incompletely understood.

A distinctive feature of compressed alkali and alkaline earth metals is the presence of non-nuclear maxima (NNMs), which are regions of localized electron density found in the interstitial spaces of the crystal lattice, often associated with electride behaviour. Chemically, this phenomenon results from the hybridization of valence orbitals with unoccupied orbitals of higher angular momentum. This presentation explores the properties of NNMs through Bader’s Quantum Theory of Atoms in Molecules (QTAIM). The analysis reveals that NNMs are crucial for stabilizing the crystalline structure. Furthermore, the average electron density associated with NNMs is found to be related to the densities of the solid and liquid phases, serving as a crucial factor in determining the melting behaviour of these highly.compressed alkali and alkaline earth elements