High-Resolution Specific Heat Measurements on MnCl2·4H2O near Its Néel Temperature

Specific heat measurements have been made on large single crystals of Mn${\mathrm{Cl}}_2$·4${\mathrm{H}}_2$O in the immediate neighborhood of its Néel temperature, with a temperature resolution of 2×${10}^{-6\mathrm{}}$°K. For $|1-\frac{T}{T_N}|=t<\mathrm{}{10}^{-2\mathrm{}}$ the results can be fitted by the relations: $C=A_{+}{t}^{-\alpha }+B_{+}$, for $T>\mathrm{}{T}_N$, and $C=A_{-}{log}_{10}t+B_{-}$, for $T<\mathrm{}{T}_N$, where $\alpha =0.35\mathrm{}\pm 0.02$. A rounding of the data occurred over approximately 100×${10}^{-6\mathrm{}}$°K near the peak. From these results, along with the recently reported thermal expansion coefficient measurements on Mn${\mathrm{Cl}}_2$·4${\mathrm{H}}_2$O, it is possible to calculate the dependence of the exchange constant on pressure at low pressures. The resulting value of $\frac{\left(\frac{1}{J}\right)dJ}{\mathrm{dP}}=9\mathrm{}\times {10}^{-6\mathrm{}}$ ${\mathrm{atm}}^{-1\mathrm{}}$ is about a factor of 2 smaller than the values for Fe${\mathrm{Cl}}_2$·2${\mathrm{H}}_2$O and Co${\mathrm{Cl}}_2$·2${\mathrm{H}}_2$O found by Narath and Schirber by direct measurement. The temperature dependence of the data is discussed in relation to the most recent three-dimensional Ising model calculations.