The specific heats below 30 °K of lithium metal of various isotopic compositions and of sodium metal

Abstract

The specific heats of lithium metal of three different isotopic compositions (main components 99.99 at. % 7 Li, 92.7 at. % 7 Li ( ~ naturallithium) and 99.3 at. % 6 Li, respectively) and of sodium metal have been measured in the temperature range 3 to 30 °K. The 99.99 % 7 Li sample was also measured in the temperature range 0.4 to 1.5 °K. Within experimental error the differences in the lattice specific heats of the lithium samples are in accord with theoretical expectation. The values of the Debye temperatures in the low-temperature limit are 337.0±2*5 °K for 99.99% 7 Li, 344.0±2.5 °K for 92.7% 7 Li and 367.5±2.5 for 99.3% 6 Li. However, the temperature variation of the lattice specific heats of all the lithium samples is anomalous since the usual minimum in the Debye temperature as a function of temperature is not observed. Both lithium and sodium undergo a martensitic transformation on cooling to the temperature range of the present experiments. It was found that the amount of lithium transformed to the low-temperature phase is not increased appreciably by cooling below 20 °K, whereas the amount of sodium transformed is significantly increased on cooling below this temperature. After rapid cooling of the sodium sample to 3 °K, spontaneous heating is observed in the temperature range 4 to 16 °K ; the total heat emitted being about 0.05 cal/gatom . This is believed to correspond to annealing out of some of the defects introduced by the martensitic transformation. By thermally cycling the sodium sample up to 90 °K and back to 3 °K, the martensitic transformation was partially inhibited and the specific heats below 30 °K of the separated b.c.c. and h.c.p. phases were deduced, the low temperature limiting Debye temperatures being about 152.5 and 159 °K, respectively.