High cubicity of DO ice inside spherical nanopores of MIL-101(Cr) framework: a neutron diffraction study.

Although cubic ice (ice I) is considered to be an important phase of water that impacts ice cloud formation in the Earth's upper atmosphere, its properties have not been studied to the same extent as those of hexagonal ice (ice I). This is because pristine ice I is not formed in simple laboratory conditions. Ice I formed in ambient conditions has a stacking disordered array of both hexagonal and cubic-structured hydrogen-bonded water molecules. It is therefore an active area of research to find ways of developing stacking disorder-free pure ice I. We demonstrate the evolution of almost pure ice I structure within the spherical nanopores of a hydrostable Cr-based metal-organic framework MIL-101(Cr) with an average pore size of 1 nm by low-temperature neutron diffraction study on DO. It is observed that at temperatures below 230 K a fraction of liquid DO transforms into ice and more than 94% of ice crystals evolved inside the pore are cubic in shape. This is a significantly high fraction of ice I formed under simple conditions inside the spherical pores of a Cr-based MOF. It is also observed that upon increasing the temperature, ie I remains stable until its melting point, without being transformed into ice I. This observation is in contrast to our previous observation of ice structure in the 2D cylindrical nanopores of MCM-41, where HO ice after creeping out from the cylindrical channel was seen to be dominated by hexagonal shape. In the present study, the DO molecules were confined into well-defined spherical nanopores, which hindered the growth of crystals above a certain size, thus minimizing the stacking disordered array. Nanoconfinement of water inside uniform spherical pores is therefore a promising method for the evolution of a significantly large fraction of cubic ice by minimizing the stacking disorder. This finding may open up the possibility of forming ice I with 100% cubicity under simple laboratory conditions, which will help in exploring the microphysics of ice cloud formation in the upper atmosphere.