Heat Capacity and Entropy of CuCl2 and CrCl2 from 11° to 300°K. Magnetic Ordering in Linear Chain Crystals

Abstract

The heat capacities of CuCl2 and CrCl2 have been measured between 11° and 300°K. The contributions to the entropy and heat capacity arising from the ordering of the magnetic moments are evaluated. Maxima in the heat capacity are found at 23.91±0.1°K for CuCl2 and 16.06±0.05°K for CrCl2. Both compounds show gradual maxima in the magnetic contribution to the heat capacity at higher temperatures. These are interpreted as short-range one-dimensional ordering arising from relatively strong antiferromagnetic coupling within the chains of atoms which make up these crystals. At lower temperatures relatively weak interactions between atoms in different chains cause the development of long-range order. By use of the Ising model to describe interactions within a chain and a molecular field to describe the secondary interactions, an approximate theoretical treatment is given. Smooth values of the heat capacity, entropy, enthalpy, and free energy are tabulated at selected temperatures. The values of the entropy and enthalpy at 298.15°K are: CuCl2, S° = 25.83±0.05 cal deg—1 mole—1, H°–H0° = 3581±7 cal mole—1; CrCl2, S° = 27.56±0.05 cal deg—1 mole—1, H°–H0° = 3593±7 cal mole—1. By use of equilibrium data of Doerner and of Sano the standard heat of formation of CrCl2 is calculated to be —94.52±0.4 kcal.