Self-Diffusion in liquid alkali metals

Abstract

Abstract A recently developed capillary method has been employed to study tracer self-diffusion in liquid Na (102 to 284°C) and K (97 to 284°C). The results may be expressed by D=D 0 exp(-Q/RT), where 103 D oNa = 0.86 (+0.09, −0.08) (cm2/sec), Q Na=2.22 ±0.08 (kcal/mole), 103 D OK = 0.76 (+0.06, −0.05), and Q K = 2.02 ± 0.07. The reexamination of electrotransport data for Rb (60 to 230°C) yielded the self-diffusion data 103 D ORb = 0.66 (+0.11, −0.09) and Q Rb = 1.98 ± 0.16. For liquid Li, the results of two investigations (195 to 440°C) have been combined, yielding 103 D OL1 = 1.44 (+ 0.07, −0.06) and Q L1 = 2.87 ± 0.07 For all four metals the empirical relation Q ⋍ 2.5 R[ttilde] is found to apply ([ttilde] being the mean exp. temp). Also well obeyed is a nearly linear dependence on (T −0.72 T m) (where T m is the m.p.). Further, the self-diffusion coefficients at the respective melting points can all be expressed by , where M is atomic mass, V mol atomic volume and β = 0.11. Three alternative empirical equations are presented, each about equally well representing the experimental self-diffusion data in Li, Na, K, and Rb. The diffusion characteristic of cesium, as predicted from these regularities, is found to obey, as do those of the four other alkali metals, a modified Stokes-Einstein type relation connecting diffusion with viscosity. Accordingly, self-diffusion parameters may be computed also for Cs, 103 D oCs = 0.48 and Q Cs = 1.86 kcal/mole.