High cubicity of D2O ice inside spherical nanopores of MIL-101(Cr) framework: a neutron diffraction study

Abstract

Although cubic ice (ice I_c) 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_h). This is because pristine ice I_c is not formed in simple laboratory conditions. Ice I_c 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_c. We demonstrate the evolution of almost pure ice I_c 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 D₂O. It is observed that at temperatures below 230 K a fraction of liquid D₂O 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_c formed under simple conditions inside the spherical pores of a Cr-based MOF. It is also observed that upon increasing the temperature, i_ce I_c remains stable until its melting point, without being transformed into ice I_h. This observation is in contrast to our previous observation of ice structure in the 2D cylindrical nanopores of MCM-41, where H₂O ice after creeping out from the cylindrical channel was seen to be dominated by hexagonal shape. In the present study, the D₂O 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_c with 100% cubicity under simple laboratory conditions, which will help in exploring the microphysics of ice cloud formation in the upper atmosphere.