Lattice defects in plastic organic crystals. Part 8.—Self-diffusion and the isotope mass effect in single crystals of 2,2′,3,3′-tetramethylbutane

Abstract

A radiotracer study has been made of self-diffusion in the rotator crystalline phase of 2,2′,3,3′-tetramethylbutane. Self-diffusion at temperatures <356 K can be described by an equation of the form: D_t/m² s^–1= 2.2 exp –[85.7/RT]. At temperatures <356 K the measured diffusion coefficients are higher than predicted by this expression and indicate the possibility of lattice premelting. Isotope mass effect measurements yield a value for the mass factor E^ab_t= 0.52±0.02 at temperatures < 353 K. This figure, which is greater than that recently measured for cyclohexane, is interpretable in terms of a slightly disordered lattice vacancy. At higher temperatures, E^ab_t < 0.52 which confirms that there is an increasing disorder in the lattice as the melting point is approached. The tracer diffusion coefficients are in excellent agreement with values obtained from n.m.r. line-narrowing studies on the same material. This agreement is presumed to be a consequence of the relatively ordered nature of the point defect.