Heat capacity of iron(II) and cobalt hexafluorosilicates from 8 to 298 K

Abstract

The heat capacity, C_p, of the salts FeSiF₆· 6H₂O and CoSiF₆· 6H₂O has been measured from ca. 8 to ca. 298 K. Below ca. 35 K C_p of the iron compound is anomalously high compared with C_p of the cobalt salt and of the four other fluorosilicates for which C_p has been determined. This anomaly is a Schottky effect which arises from the triplet nature of the ground state of the Fe(H₂O)²⁺₆ ion in the fluorosilicate. The measured C_p values of the iron salt are shown to be quantitatively consistent with the information about the triplet ground state derived from magnetic susceptibility studies, which therefore supports the conclusion reached from the magnetic work that the octahedral cation in the salt FeSiF₆· 6H₂O is distorted by being compressed along the crystal axis.

Both the iron and the cobalt salt have a transition in the range covered by the heat-capacity measurements which is essentially first order. For the iron compound T_t= 224.2 K and ΔS_t= 3.76 J K^–1 mol^–1, while for the cobalt salt T_t= 264.8 K and ΔS_t= 5.25 J K^–1 mol^–1.