A Calorimetric Study of Ammonium, Rubidium, and Potassium Hexafluophosphate

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Abstract
Measurements have been made of the heat capacities of ammonium, rubidium, and potassium hexafluophosphate from ∼ 20°K to ∼ 300°K. The heat capacity curve of the ammonium salt shows two anomalous regions, and an order-disorder change also occurs in the rubidium salt. The potassium salt, however, undergoes a first-order transition with a large entropy change. The heat capacity of the ammonium and rubidium salts in the neighbourhood of room temperature (but not that of the potassium salt) is altered by cooling to low temperatures. In certain ranges of temperature it was unusually difficult with the rubidium salt to obtain reproducible heat capacity values. The results show that the rotation of the ammonium ion is not completely free, but they are consistent with almost free rotation in one degree of freedom and partially restricted rotation in the other two. The possible significance of the entropy changes of the various transitions is briefly discussed.

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