Critical heat capacity of NH4Br and NH4BrxCl1−x single crystals

Abstract

A high‐resolution ac calorimeter has been used to measure the heat capacity of NH₄Br, NH₄Br_0.5lCl_0.49, NH₄Br_0.26Cl_0.74 and NH₄Br_0.04Cl_0.96 single crystals over the range of 200–300 K. The behavior of C_p 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 NH₄Br_0.04Cl_0.96), the transition is weakly first order but quasitricritical behavior (α′?0.5) is observed. In the antiferro regime, the data for NH₄Br and NH₄Br_0.26Cl_0.74 (but not NH₄Br_0.51Cl_0.49) 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 C_p and the order parameter are discussed.