Effect of Isotopic Composition on Electrical Resistance of Lithium

Abstract

The electrical resistance of solid metallic lithium containing varying proportions of lithium-6 and lithium-7 was measured between 4.2\ifmmode^\circ\else\textdegree\fi{} and 295\ifmmode^\circ\else\textdegree\fi{}K. For the isotopically pure substances, the main features of the behavior agree with the predictions of the Bloch-Gr\uneisen law, the characteristic temperature being inversely proportional to the square root of the mass. The deviation in the details is just that found for most other metals. For the isotopically impure substances, the behavior of the resistance as a function of temperature can be described just as that of an isotopically pure substance with a mass dependent on the isotopic composition. In fact, the temperature dependence of resistance for all compositions, including the pure isotopes, can be represented as a universal curve by use of appropriate scaling factors depending on average isotopic mass. From the present data the arithmetic mean cannot be preferred over the harmonic mean isotopic mass, and hence one cannot decide between certain theoretical proposals concerned with the effect of isotopic impurities on lattice vibration spectra. On the other hand, it appears definite that no need exists to invoke a scattering mechanism that looks upon isotopes as introducing imperfections in the lattice; rather, their presence merely modifies the lattice vibration spectrum. The effect of the martensitic transition at low temperatures is barely, if at all, discernible.