Heat capacity of the two-dimensional ferromagnet bis(monomethylammonium)tetrachlorochromate(II) from 10 to 302 K

Abstract

The heat capacity C_p of the S= 2 layer ferromagnet (CH₃NH₃)₂CrCl₄ has been measured from 10 to 302 K. Although the salt at room temperature has an orthorhombic structure in which, by comparison with similar compounds, some disorder might be expected, there is no sign of any phase transition in the temperature range covered by the C_p measurements. Between ca. 100 and 210 K, however, there is a broad anomaly in the heat capacity. On the basis of a comparison of the heat capacity of the chromium compound with that of the corresponding manganese salt, it is suggested that this anomaly results from the development in the chromium compound of magnetic disorder, since the “extra” entropy so gained is approximately R ln 5, the expected magnetic entropy. On this interpretation, the magnetic entropy is acquired in two stages, the first of these being the breakdown of three-dimensional magnetic order at T_c(41 K). Values of the maximum in the magnetic heat capacity, and the temperature at which it occurs, are compared with those of S=½ and S= layer Heisenberg ferromagnets. There are features in the thermodynamic properties which present problems that cannot be satisfactorily resolved with the structural information at present available.