HIGH-RESOLUTION SPECIFIC HEAT MEASUREMENTS ON MNCL2.4H2O NEAR ITS NEEL TEMPERATURE

Specific heat measurements have been made on large single crystals of MnCl2•4H2O in the immediate neighborhood of its Néel temperature, with a temperature resolution of 2×10-6A°K. For 1-TTN=t<10-2 the results can be fitted by the relations: C=A+t-α+B+, for T>TN, and C=A-log10t+B-, for T<TN, where α=0.35±0.02. A rounding of the data occurred over approximately 100×10-6A°K near the peak. From these results, along with the recently reported thermal expansion coefficient measurements on MnCl2•4H2O, it is possible to calculate the dependence of the exchange constant on pressure at low pressures. The resulting value of (1J)dJdP=9×10-6 atm-1 is about a factor of 2 smaller than the values for FeCl2•2H2O and CoCl2•2H2O 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. © 1969 The American Physical Society.