[Paper Review] Mechanisms of alkali ionic transport in amorphous oxyhalides solid state conductors

Luca Binci, KyuJung Jun+2 more|arXiv (Cornell University)|Jan 10, 2026

Machine Learning in Materials Science0 citations

TL;DR

The paper uses a tuned machine-learning interatomic potential to study diffusion mechanisms in amorphous AMX2.5O0.75 oxyhalides, revealing universal diffusion behavior dominated by uncorrelated self-diffusion with minimal alkali–polyanion coupling and oxygen content as a key diffusion bottleneck.

ABSTRACT

Amorphous oxyhalides have attracted significant attention due to their relatively high ionic conductivity ($>$1 mS cm$^{-1}$), excellent chemical stability, mechanical softness, and facile synthesis routes via standard solid-state reactions. These materials exhibit an ionic conductivity that is almost independent of the underlying chemistry, in stark contrast to what occurs in crystalline conductors. In this work, we employ an accurately fine-tuned machine learning interatomic potential to construct large-scale molecular dynamics trajectories encompassing hundreds of nanoseconds to obtain statistically converged transport properties. We find that the amorphous state consists of chain fragments of metal-anion tetrahedra of various lenght. By analyzing the residence time of alkali cations migrating around tetrahedrally-coordinated trivalent metal ions, we find that oxygen anions on the metal-anion tetrahedra limit alkali diffusion. By computing the full Einstein expression of the ionic conductivity, we demonstrate that the alkali transference number of these materials is strongly influenced by distinct-particles correlations, while at the same time they are characterized by an alkali Haven ratio close to one, implying that ionic transport is largely dictated by uncorrelated self-diffusion. Finally, by extending this analysis to chemical compositions $AMX_{2.5} extsf{O}_{0.75}$, spanning different alkaline ($A$ = Li, Na, K), metallic ($M$ = Al, Ga, In), and halogen ($X$ = Cl, Br, I) species, we clarify why the diffusion properties of these materials remain largely insensitive to variations in atomic chemistry.

Motivation & Objective

• Explain the microscopic diffusion mechanism of alkali ions in amorphous AMX2.5O0.75 oxyhalides.
• Assess how alkali diffusion depends on alkali size, trivalent metal, and halide chemistry.
• Quantify residence times and activation energies for diffusion in different local environments.
• Evaluate alkali transference numbers and Haven ratios using different theoretical approximations.
• Identify structural features that govern the near-chemical-independence of conductivity across compositions.

Proposed method

• Build amorphous structures via melt-and-quench for AMX2.5O0.75 compositions.
• Train and fine-tune CHGNet machine-learning interatomic potential on DFT data.
• Run large-scale MD trajectories (hundreds of ns, ~1300 atoms) to sample diffusion.
• Compute diffusion coefficients from mean square displacement and extract activation energies.
• Perform residence-time analysis using P(t) to link diffusion to specific Al coordination environments.
• Evaluate ionic conductivity via Einstein (Green-Kubo) formalism and compare NE, cluster NE (cNE), and exact expressions.
• Analyze displacement-displacement correlation functions to assess inter- and intra-particle correlations.
• Analyze alkali transference numbers and Haven ratios across the AMX2.5O0.75 family.

Figure 1: (a–c) Radial distribution function (RDF) $g_{A-B}(r)$ (left scale, solid line) and coordination number $N_{A-B}(r)$ (right scale, dashed line) as a function of the distance of the alkali $A=$ Li (a), Na (b), K (c) from the $B$ atom ( $B=$ O [pink], Cl [purple], Al [light blue]). (d) Snapsh

Experimental results

Research questions

• RQ1 How does alkali diffusion in amorphous oxyhalides depend on alkali size (Li, Na, K) and local coordination around trivalent metals (Al, Ga, In) with Cl/O ligands?
• RQ2 Why is the ionic conductivity in amorphous oxyhalides largely insensitive to underlying chemistry across A, M, X substitutions?
• RQ3 What are the activation energies and residence times for alkali diffusion in different tetrahedral environments (e.g., Cl4, Cl3O1, Cl2O2, Cl1O3)?
• RQ4 What is the role of interparticle correlations in determining transference numbers and Haven ratios in these materials?
• RQ5 Can the diffusion mechanism be described as uncorrelated self-diffusion of alkali ions through extended M–X–O tetrahedral complexes?

Key findings

• All studied amorphous oxyhalides form extended complexes of metal–anion tetrahedra bridged by oxygen, through which alkali ions diffuse.
• Oxygen content within the metal–anion tetrahedra slows alkali diffusion; Cl4 coordination yields faster escape than oxygen-rich environments.
• Activation energies from residence-time analysis (εa) broadly match those from diffusion coefficients (Ea), 0.2–0.4 eV range, with Cl3O1 environments being most influential.
• Haven ratio for alkalis is close to unity across the series, indicating diffusion dominated by uncorrelated self-diffusion rather than strong alkali–alkali correlations.
• Full Einstein conductivity (σE) lies between NE and cNE estimates, showing inter-particle correlations affect the macroscopic conductivity but largely cancel to yield similar transference across compositions.
• Transference numbers from NE are significantly underestimated; cNE improves agreement but still deviates from σE, highlighting the importance of including cross-correlations.
• Diffusion properties are largely insensitive to chemistry (A, M, X substitutions) due to the universal diffusion mechanism within the amorphous tetrahedral network.
• Li, Na, and K show similar transference numbers when full correlations are included, suggesting a universal transport mechanism in these amorphous oxyhalides.

Figure 2: (a) Mean square displacement (MSD) of Li atoms in LiAlCl 2.5 O 0.75 at different temperatures: 500 K (turquoise), 600 K (blue), 700 K (pink) and 800 K (purple). (b) MSD of the different species (Li [purple], Al [pink], Cl [blue], O [turquoise]) of LiAlCl 2.5 O 0.75 ; the insets report the