Despite observations of massive methane release and geohazards associated with gas hydrate instability in nature, as well as ductile flow accompanying hydrate dissociation in artificial polycrystalline methane hydrates in the laboratory, the destabilising mechanisms of gas hydrates under deformation and their grain-boundary structures have not yet been elucidated at the molecular level. Here we report direct molecular dynamics simulations of the material instability of monocrystalline and polycrystalline methane hydrates under mechanical loading. The results show dislocation-free brittle failure in monocrystalline hydrates and an unexpected crossover from strengthening to weakening in polycrystals. Upon uniaxial depressurisation, strain-induced hydrate dissociation accompanied by grain-boundary decohesion and sliding destabilises the polycrystals. In contrast, upon compression, appreciable solid-state structural transformation dominates the response. These findings provide molecular insight not only into the metastable structures of grain boundaries, but also into unusual ductile flow with hydrate dissociation as observed during macroscopic compression experiments.

The silicate oligomerization reaction is key to sol–gel chemistry and zeolite synthesis. Numerous experimental and theoretical studies have been devoted to investigating the physical chemistry of silicate oligomers in the prenucleation stage of siliceous zeolite formation. Most of the previous quantum chemical computational work has used gas phase models or continuous solvent models for silica oligomerization. Here we apply Car–Parrinello molecular dynamics simulations with explicit inclusion of water molecules to investigate the reaction pathway for the anionic bond formation of siliceous oligomers. The rates of SiO–Si bond formation of linear or ring containing silicate oligomers become substantially enhanced, compared to gas phase results. The formation of 3-ring oligomer is more favorable than the formation of higher branched and ring silica oligomers.