Evidence that Amorphous Water below 160 K Is Not a Fragile Liquid

We have examined transport mechanisms in amorphous solid water (ASW) by studying thermal desorption of layered nanoscale films of CCl4 and labeled ASW. The interlayer mixing observed near T ∼ 150−160 K is inconsistent with a mechanism involving diffusion through a dense phase. Instead, intermixing occurs via vapor-phase transport through an interconnected porous network created within the film during crystallization. As a consequence, the self-diffusivity of ASW is significantly smaller than previously thought, indicating that water undergoes either a glass transition or a fragile-to-strong transition at a temperature above 160 K.