A very low-temperature calorimeter with a (3He + 4He) dilution refrigerator. The heat capacity of trans-bis(ethylenediamine)-bis(isothiocyanato)nickel(II)

Article abstract of DOI:10.1016/0021-9614(87)90005-X

A calorimeter with a (3He + 4He) dilution refrigerator capable of measuring heat capacity between 20 mK and 25 K has been constructed.The calorimeter works under three operational modes in accordance with the temperature region suitable for heat-capacity measurements: the isoperibol low-temperature mode covers the range below 200 mK, the adiabatic high-temperature mode is used in the range from 0.2 to 25 K, and the isoperibol third mode is adopted when the thermal relaxation time of a sample is long.The accuracy of the calorimeter has been determined indirectly by comparing heat capacities of a common sample with those measured by a 3He calorimeter whose accuracy has been established on the basis of the 1965 Calorimetry Conference Copper Standard.The heat capacity of a paramagnetic nickel complex (en = H2NCH2CH2NH2) has been measured between 0.3 and 20 K.A Schottky anomaly centred around 3.3 K was detected.This anomaly was well accounted for in terms of the zero-field splitting of a single nickel ion arising from the crystalline-field anisotropy.The best fit between theory and experiment was obtained for D/k = 8.98 K and /E//k = 1.62 K (or /E//D = 0.18) where D and E are the uni- and bi-axial crystalline-field anisotropy parameters, respectively, and k is the Boltzmann constant.The heat-capacity anomaly can likewise be accounted for in terms of different set of the parameters (D/k = -6.92 K and /E//k = 3.67 K or /E//D = 0.53) because both the sets bring about identical splitting with respect to the spin energy-level.