Unusual crystalline and polycrystalline structures in methane hydrates.

We present previously unreported crystalline and polycrystalline structures for methane hydrates obtained at relatively low pressures. These structures, which contain unusual cages with 12 pentagonal faces and 3 hexagonal faces, were observed during the atomistic simulations of the crystal growth of sI and sII methane hydrates. These 51263 cages have a significant impact on the structure of the resulting crystal and could explain several experimental observations regarding in-situ transformations between sI and sII hydrates. We document a previously unidentified structure of methane hydrates which we designate structure sK. Additionally, we predict a polycrystalline structure consisting of this new hydrate and sI and suggest a mechanism for the formation of a polycrystalline structure consisting of sequences of sI and sII hydrates.