Heat Capacity of CoCl2·2H2O between 7 and 120°K and its Anomaly Associated with Magnetic Transition

A small adiabatic calorimeter is described, which is capable of measuring the solid heat capacities between 5° and 300°K continuously. The heat capacities of CoCl 2 ·2H 2 O were measured between 7° and 120°K. A lambda-shaped anomaly in the heat capacity, due apparently to a transition between antiferromagnetic and paramagnetic states, occurs below 20°K with the peak at 17.20°K. An analysis of the experimental heat capacity curve was made in the light of the series expansion method through help of the experimental heat capacities of CoCl 2 ·6H 2 O at the low temperatures in a semi-empirical way. This result gives the exchange interactions | J |/ k =10.8°K in the direction of the chain of the coordinated Co 2+ ions and | J '|/ k =2.03°K between Co 2+ ions in the neighboring chains. The total entropy of the transition is 1.39 cal/deg mole and about 37% of this is acquired in the temperature region above 17.20°K.