Heat Capacity and Entropy of NiSiF6·6H2O from 0.35° to 4.2°K with Magnetic Fields 0–90 kG Perpendicular to the c axis. The Use of 3He Gas Conduction in Calorimetry

Abstract

The heat capacity of nickel fluosilicate hexahydrate has been measured over the range 0.35–4.2°K in stabilized magnetic fields of 0, 1, 5, 10, 20, 40, 60 and 90 kG. The field was perpendicular to the crystallographic c axis of the 3.5‐cm‐diam single crystal. The entropy changes in NiSiF₆·6H₂O have been accurately tabulated over the ranges 0.35°—4.2°K and 0–90 kG, by combining the heat‐capacity data and the change in temperature with field on isentropics. The temperature was measured by means of a carbon thermometer which was also used to introduce calorimetric heat. The total electronic entropy removed to magnetic saturation was Rln3. The substance gave no indication of hysteresis at any field or temperature. At the higher fields and lower temperatures it was necessary to consider a small entropy change due to nuclear magnetization of the hydrogen and fluorine atoms. As a practical rule it has been shown that a 2 mtorr pressure of ³He, which is sufficient to cause liquefaction near 0.3°K, is acceptable in a calorimeter over the range 0.4°—4°K, provided that the ratio of sample surface to volume is small, as, e.g., with a very large spherical single crystal.