Heat capacity and thermodynamic functions of Mn xFe 1− xCl 2 · 4H 2O, x = 0.51

Abstract

The heat capacity of Mn x Fe 1− x Cl 2 · 4H 2O for x = 0.51 ± 0.005 has been experimentally determined from 1.4 to 300°K. The smoothed heat capacity and thermodynamic functions ( H 0 T  H 0 10) and ( S 0 T  S 0 10) are reported over the temperature range 10 to 300 K. The value of S 0 10 is given based on theoretical fits to the experimental data below 10 K. The error in the tabulated values arising from the heat capacity data above 10 K is thought to be less than 1%. Mn x Fe 1− x Cl 2 · 4H 2O, x = 0.51 is isostructural with MnCl 2 · 4H 2O, but exhibits a Schottky anomaly similar to FeCl 2 · 4H 2O. Separating the lattice and Schottky heat capacities yields a T 3 lattice constant of 1.413 ± 0.028 · 10 −3 J/mole-K 4 and a Schottky doublet-triplet mean separation of 18.0 ± 0.5 K. The lattice constant is, within the quoted uncertainty, identical to the T 3 lattice constant of MnCl 2 · 4H 2O. The Schottky level separation, however, is significantly greater than the 7.6 K separation found in FeCl 2 · 4H 2O. This is presumably due to the change in chloride coordination from trans to cis in the present case. A cooperative transition just below the extent of the present data is thought to involve both the 5 2 spin Mn 2+ and the quasi- 1 2 spin Fe 2+ ions.