The low-temperature diffusivity of He in crystals of 2 types was investigated by monitoring desorption from crystals which had been previously saturated in the gas phase. One crystal type was cubic in shape and was obtained by the evaporation of aqueous solutions. The other crystals were obtained in the form of plates which were cleaved from a crystal that had been grown by using the Kyropoulos method. A negative enthalpy for He solubility in the cubic crystals suggested that gas solution occurred at crystal defects. The results for the cubic crystals, at temperatures ranging from 100 to 400C, could be described by:

D (cm2/s) = 3 x 10-5 exp[-9.4(kcal/mol)/RT]

while those for the plate crystals, at temperatures ranging from 100 to 400C, could be described by:

D (cm2/s) = 2.5 x 10-6 exp[-3.1(kcal/mol)/RT]

A.Y.Kupryazhkin, P.V.Volobuev, P.E.Suetin: Zhurnal Fizicheskoi Khimii, 1975, 49[2], 546-7

Table 16

Isotope Effect for 22Na and 24Na

in KBr Single Crystals