Metastable phase access and kinetic stability in synthesized hydride superconductors

Ascertain whether experimental synthetic techniques used to produce hydride superconductors access metastable phases, and determine the kinetic (thermal) stability at finite temperatures of metastable hydride structures predicted via density functional theory-based crystal structure prediction methods.

Background

The paper highlights limitations in current discovery pipelines for hydride superconductors, including metaheuristic crystal structure prediction (CSP) methods and uncertainties in experimental synthesis pathways. While many promising hydrides are identified computationally, the relationship between these predictions and experimentally realized phases—especially metastable ones—remains uncertain.

The authors explicitly note uncertainty regarding whether synthesis routes reach metastable configurations and whether computationally predicted metastable phases possess sufficient kinetic barriers to remain stable at finite temperatures. Resolving this would clarify the reliability and relevance of computational targets for experimental efforts.

References

Finally, it is not clear if the synthetic techniques access metastable phases and if the metastable systems that are computationally predicted are kinetically (thermally) stable at finite temperatures.

— Refining Tc Prediction in Hydrides via Symbolic-Regression-Enhanced Electron-Localization-Function-Based Descriptors  (2506.17456 - Belli et al., 20 Jun 2025) in Introduction

This remains an important open question that warrants further investigation. To further understand the decomposition observed in NMR measurements it would be desirable to perform combined XRD, Raman, and NMR measurements on samples synthesised via the same methods as in the NMR study, namely a laser-heated mixture of LaH$_3$ powder and ammonia borane. This would provide important insight into the synthesised crystal structures, reveal any sample heterogeneities and possible novel metastable phases, explore decomposition pathways and establish whether stabilisation requires a chemical equilibrium with excess molecular hydrogen in the sample chamber post laser heating.

— Synthesis and stability of high-$T_c$ LaH$_{10\pmδ}$ films at high pressures  (2608.18865 - Cross et al., 19 Aug 2026) in Section “Temporal stability of the crystal structure and superconductivity of LaH$_{10\pm\delta}$ films”