Complexity in the medium-range order of gallium as a polyvalent liquid metal

Simplicity in chemical composition does not always translate into simplicity in the structures and dynamics of liquids and solids. Some elementary liquid metals, such as gallium, show unusual behaviors in thermodynamic and transport properties as a result of their complex atomic structure and dynamics. In this work, we study the real-space atomic correlation function of liquid gallium by neutron scattering. In the pair-distribution function, there exist two kinds of medium-range order (MRO), characterized by oscillations beyond the first nearest neighbors. On the other hand, the first neighbor shell shows only one kind of bond. The two types of MRO are strongly overlapping in space and fluctuating in time. We propose that they are the basis for anomalous behavior of liquid gallium. This view challenges the current view that liquid gallium consists of fluctuating metallic and insulating domains. These findings shed new light on the interpretation of similar microscopic anomalies observed in other semi-metallic liquids.

💡 Research Summary

This paper investigates the origin of the anomalous thermodynamic and transport properties of liquid gallium, a polyvalent metal that exhibits unusual behavior despite its simple chemical composition. Using inelastic neutron scattering on the ARCS spectrometer at the Spallation Neutron Source, the authors measured the dynamic structure factor S(Q,E) of liquid gallium over a wide temperature range (310 K to 950 K). By integrating over energy transfer, they obtained the static structure factor S(Q) and, via Fourier transformation, the real‑space pair‑distribution function g(r).

The key observation is that the first peak of S(Q) is asymmetric, displaying a shoulder on the high‑Q side. Traditional interpretations attribute this shoulder to a coexistence of metallic and insulating (or covalent) local environments, implying two distinct nearest‑neighbor bond lengths. However, the corresponding g(r) shows a single, well‑defined first‑neighbor peak at ~2.8 Å with no splitting, indicating that only one type of short‑range bond exists in the measured liquid.

The authors demonstrate that the asymmetric shoulder in S(Q) does not arise from short‑range order (SRO) but from medium‑range order (MRO) that manifests as oscillations in g(r) beyond ~10 Å. They model the long‑range part of g(r) (r > 3.8 Å) with a sum of two damped sinusoidal terms:

G_MRO(r) − 1 ≈