Specific Heat of ${\mathrm{Fe}}_3$${\left(\mathrm{P}{\mathrm{O}}_4\right)}_2$·8${\mathrm{H}}_2$O

Specific-heat measurements have been made on a single crystal of ${\mathrm{Fe}}_3$ ${\left(\mathrm{P}{\mathrm{O}}_4\right)}_2$·8${\mathrm{H}}_2$O in the temperature range 1.8-25°K. Two $\lambda$-type anomalies were observed at 9.60° and 12.40°K and are associated with paramagnetic-antiferromagnetic transitions. The six iron ions per unit cell are not equivalent, giving rise to a two-step antiferromagnetic transition as observed in the two anomalies, and indicating a ratio of 2 for the entropy changes associated with the two sets of nonequivalent ions. The total entropy change associated with these transitions is 9.58 cal/mode deg, which differs by 1% from $3R\ln \mathrm{}(2S+1)\mathrm{}$, where $S=2\mathrm{}$ for ${\mathrm{Fe}}^{++}$, with approximately 27% of this entropy gained above 12.40°K. By using the simple molecular-field theory, an estimate of the magnitude of the exchange integral was obtained as well as the temperature dependence of the sublattice magnetization.