Low temperature heat capacities and thermal properties of DyAl2, ErAl2 and LuAl2

The heat capacities of the compounds DyAl₂, ErAl₂ and LuAl₂ were measured in an adiabatic calorimeter from approximately 5 to 300 K. The compounds DyAl₂ and ErAl₂ show C_P anomalies at 58.0 and 10.2 K, respectively, which are attributed to the destruction of magnetic order. In order to separate the crystal field and magnetic contributions from the measured heat capacities, it was necessary to evaluate the lattice heat capacity. The lattice term, C_L was obtained from the C_P data of LuAl₂ by a method of interpolation which gave values of C_L for an arbitrary R Al₂ compound. Using this “interpolated lattice blank”, excess entropies associated with the crystal field and magnetic terms were computed throughout the series. These values are quite close to R In (2J + 1). The results also indicate that, for the compounds studied, the degeneracy of the lowest ground state is completely lifted. In addition, the magnetic contribution to the heat capacity of the magnetically ordered R A1₂ phases was found to exhibit an exponential dependence below the temperature corresponding to the spin wave energy gap and a T^{$\text{3}\text{2}$} dependence above this temperature.Detailed calculations were performed to characterize the influence of cubic crystal field in ErAl₂ on the ⁴I_{$\text{15}\text{2}$} ground state multiplet of the Er³⁺ ion. It is concluded that the magnetic ordering in ErAl₂ takes place within the Γ₈³ quartet state. Smoothed values of heat capacity, entropy and related thermodynamic functions are tabulated.