Intrinsic surface disorder in ice near the melting point

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Abstract
The crystalline perfection of the basal plane of ice Ih in thermodynamic equilibrium with its own vapour, is studied between -130 degrees C and -2 degrees C by means of 100 keV proton channelling. An intrinsically disordered surface region is found. The disorder extends into the bulk, and increases rapidly with temperature above -40 degrees C. A partial order is retained, however, at least up to -1.8 degrees C. The results are compatible with the postulated existence of large-amplitude surface vibrations. At -1.8 degrees C the square of the mean surface-vibration amplitude of oxygen is 3.3 times larger than the bulk value deduced from past x-ray measurements. The thickness t of an amorphous layer, which has the same dechannelling effect as the partially disordered region, increases with temperature as t(nm)=(94+or-17)-(54+or-14)lg(273-T). This is the functional dependence predicted by Fletcher's theory, and found experimentally by Volta effect and NMR measurements. The order of magnitude of the thickness near 0 degrees C agrees with the results of measurements of electrical and mechanical surface properties of ice, and with Weyl's original suggestion, but is ten times larger than the value estimated from NMR measurements and predicted by Fletcher.

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