CRITICAL-REGION THERMAL EXPANSION IN MNCL2.4H2O AND MNBR2.4H2O

The Néel-point anomaly in the linear thermal-expansion coefficients along three mutually perpendicular directions in MnCl2-4H2O and MnBr2-4H2O has been observed. Using the three-terminal capacitance technique, length changes were measured between 0.4 and 4.2 K with a resolution of approximately 0.02 Å under the best conditions. Temperature resolution was about 4μK, with points taken at 40-μK intervals near the transition. In MnCl2-4H2O, the linear expansion coefficient along the a axis, αa, is negative, while αb and αc' are positive. All three linear expansion coefficients are negative in MnBr2-4H2O. The temperature dependences of the thermal-expansion coefficients for a given salt are similar but not identical, in agreement with theoretical expectations. On a reduced temperature scale, the temperature dependences of the thermal-expansion coefficients for the two salts are also similar. Near the transition (|T-TN|TN>0.1), the results can be fitted by the relations: α=A-+B-1n(1-TTN,), T>TN and α=A++B+(TTN-1)-p, T<TN where p=0.30±0.06 in MnCl2-4H2O and p=0.42±0.08 in MnBr2-4H2O. Using recent high-resolution critical-region specific-heat results for MnCl2-4H2O, we calculate the dependence of the transition temperature on applied stress for this salt. © 1969 The American Physical Society.