Deviations of the specific heats of the alkali metals from harmonic behaviour. A possible dependence on isotopic composition of the anharmonic contribution to specific heat

Abstract

Abstract An analysis of the ‘high temperature‘ specific heats of all the alkali metals shows that positive departures from harmonic lattice theory predictions occur at temperatures in the range θ/3 to θ, suggesting the onset of appreciable anharmonic contributions to the specific heat in this temperature range. A further positive anomaly is observed starting at temperatures of about 50 °K below the melting point (except possibly in lithium) and is believed to correspond to the thermal generation of lattice vacancies. Recent measurements have shown that at low temperatures the specific heats of alloys of the two isotopes of lithium are intermediate between the specific heats of the separated isotopes. However, as the temperature is increased the specific heats of the alloys increase more rapidly than do the specific heats of the separated isotopes. As a result by 300 °K (where the lattice constants of the alloys and pure isotopes are probably all the same) the specific heats of alloys containing more than about 50 per cent 7Li exceed the specific heat of pure 7Li (which is itself greater than the specific heat of pure 6Li). These results suggest that the anharmonic contribution to the specific heat may depend on isotopic composition. However, the observed effect is rather small (less than 1 per cent of the total specific heat) and an independent set of measurements is desirable to confirm its existence and magnitude.