Critical heat capacity of NH4Br and NH4BrxCl1−x single crystals

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Abstract
A high‐resolution ac calorimeter has been used to measure the heat capacity of NH4Br, NH4Br0.5lCl0.49, NH4Br0.26Cl0.74 and NH4Br0.04Cl0.96 single crystals over the range of 200–300 K. The behavior of Cp near the order–disorder transition in these crystals and in NH4Cl and ND4Cl is analyzed to obtain the critical exponent α′ in both the ’’ferro’’‐ordered and ’’antiferro’’‐ordered phases. In the ferro regime (NH4Cl, ND4Cl, and NH4Br0.04Cl0.96), the transition is weakly first order but quasitricritical behavior (α′?0.5) is observed. In the antiferro regime, the data for NH4Br and NH4Br0.26Cl0.74 (but not NH4Br0.51Cl0.49) can be well represented using α′=0.20 but may also be consistent with a crossover from Gaussian to cubic (d=3, n=3) critical behavior. The effects of inhomogeneous internal strains on Cp and the order parameter are discussed.