Heat-capacity measurements on manganese dibromide tetrahydrate near its Néel temperature

Heat-capacity measurements on very small single crystals of manganese dibromide tetrahydrate have been made using the ac or oscillating-temperature method. These measurements show rounding in a temperature range of about 1 mK at the Néel temperature (2.12 K). We believe this rounding is due to chemical impurities. Outside this region the data may be fitted by a power-law singularity, within the experimental error, with 0.08≤α≤0.23, -0.10≤α′≤0.03, and 2.1195≤TN≤2.1203 K. The errors in α, α′, and TN were obtained from a χ2 test for 90% probability that the true values lie within these limits. Comparison is made to the heat-capacity measurements by Dixon and Rives on MnCl2•2H2O, which is nearly isostructural to MnBr2•4H2O. The heat capacity in the critical region of partially deuterated crystals with 1% deuterium was measured to obtain the effect of this impurity on the rounding. No significant effect was observed. © 1973 The American Physical Society.