The specific heats below 320°K of potassium, rubidium and caesium

Abstract

The specific heat of potassium has been measured in the range 0·4 to 26°K and the specific heats of rubidium and caesium in the range 0·4 to 320°K. Previously reported specific heat anomalies in the range 100 to 300°K were not confirmed. The θ c 0 and γ values were estimated as 90·6 +1·4 -0·3 °K and 497 ± 20 μ cal degK -2 gatom -1 for potassium, as 55·6 ± 0·5°K and 576 +70 -40 μ cal degK -2 gatom -1 for rubidium and as 38·4 ± 0·6°K and 764 ± 250 μ cal degK -2 gatom -1 for caesium. A slow release of energy (~ 1 μ cal s -1 gatom -1 ), dependent on thermal history, was observed from rubidium and caesium in the region of 4°K and may correspond to the annealing out of defects introduced by plastic strain on cooling. Positive anharmonic contributions to the specific heat are evident at high temperatures and an additional contribution to the specific heat, of the form (e - E / RT /T 2 ), becomes apparent from about 50°K below the melting point and may be identified with the thermal generation of lattice vacancies. The melting point of pure rubidium is estimated as 312·65 ± 0·01°K and the latent heat of fusion as 524·3 ± 1·0 cal/gatom. For caesium the melting point is 301·55 ± 0·01°K and, with some assumptions, the latent heat is 498·9 ± 0·5 cal/gatom. For both metals the specific heat of the liquid decreases with increasing temperature.