Competing Hydrogenation Pathways to Metastable CaH${}_6$ Revealed by Machine-Learning-Potential Molecular Dynamics

Abstract

The synthesis of the high-$T_c$ superhydride CaH${}_6$ has stimulated significant interest in understanding synthesis pathways for metastable hydrides. However, the microscopic mechanisms governing such hydrogenation reactions remain poorly understood. Here, we show that machine-learning potential molecular dynamics (MLP-MD) simulations can reproduce and distinguish competing reaction pathways leading to metastable and stable hydrides. By simulating hydrogenation reactions at CaH${}_2$/H${}_2$ and CaH${}_4$/H${}_2$ interfaces, we identify two distinct pathways that produce clathrate-type CaH${}_6$ and A15-type CaH${}_{5.75}$, respectively. CaH${}_{5.75}$ lies on the convex hull but requires extensive Ca sublattice rearrangement and therefore forms only at elevated temperatures. In contrast, CaH${}_6$ becomes kinetically accessible when CaH${}_2$ is used as the precursor. The crystallographic compatibility between the Ca sublattice of CaH${}_2$ and the bcc framework of CaH${}_6$ enables a martensitic-like topotactic transformation that bypasses the reconstructive pathway leading to CaH${}_{5.75}$. These results reveal how precursor structure and thermodynamic stability compete to determine superhydride formation pathways and demonstrate that machine-learning molecular dynamics can directly capture the kinetic selection of metastable phases in reactive materials systems.

5 figures with supporting information

References in corpus (19)

• DeePMD-kit: A deep learning package for many-body potential energy representation and molecular dynamics
• Superconductivity at 250 K in lanthanum hydride under high pressures
• Evidence for superconductivity above 260 K in lanthanum superhydride at megabar pressures
• Synthesis of clathrate cerium superhydride CeH9 below 100 GPa with atomic hydrogen sublattice
• Superconductive "sodalite"-like clathrate calcium hydride at high pressures
• DeePMD-kit v2: A software package for Deep Potential models
• Crystal Structure of 200 K-Superconducting Phase of Sulfur Hydride System
• Anomalous high-temperature superconductivity in YH${}_6$
• Superconductivity above 200 K Observed in Superhydrides of Calcium
• Towards high-throughput superconductor discovery via machine learning
• Evidence for supercritical behavior of high-pressure liquid hydrogen
• Prediction of high-Tc superconductivity in ternary lanthanum borohydrides
• Feasible route to high-temperature ambient-pressure hydride superconductivity
• Tuning Chemical Precompression: Theoretical Design and Crystal Chemistry of Novel Hydrides in the Quest for Warm and Light Superconductivity at Ambient Pressures
• Search for ambient superconductivity in the Lu-N-H system
• Novel Strongly Correlated Europium Superhydrides
• High-Pressure Synthesis of Seven Lanthanum Hydrides with a Significant Variability of Hydrogen Content
• Developments and Further Applications of Ephemeral Data Derived Potentials
• High Pressure and Temperature Neural Network Reactive Force Field for Energetic Materials