A calorimetric study of CoxNi1−xCl2⋅6H2O

Abstract

Using a standard He4 adiabatic calorimeter, we have measured the specific heats of a series of mixed crystals with chemical formula CoxNi1−xCl2⋅6H2O. All the samples studied show fairly sharp λ-type transitions in the liquid He4 region. The Néel temperatures vary from 2.43 °K to 4.47 °K for x ranging from 0.89 to 0.13. We find that the critical behaviors of the mixed crystals can be explained by assuming that the Ni and Co spins behave like Ising spins randomly distributed on a square lattice. The TN vs concentration diagram can be reasonably well accounted for by (a) the annealed Ising model, (b) the annealed Bethe-Peierls-Weiss (BPW) model and to a lesser extent, (c) the quenched BPW model. The experimental specific heats have also been compared with theoretical predictions. In all cases, the annealed Ising model fits the data the best near and above the critical temperatures. For high Ni concentrations, the quenched BPW model fits the data better than the annealed BPW model but predicts critical temperatures which are too high. We present a quantitative argument to support the view that an exact solution of the quenched Ising model is unlikely to differ significantly from the annealed Ising model.