The magnetic properties of some paramagnetic double sulphates at low temperatures

In a previous paper† an account was given of a series of measurements on compounds of ferrous iron, nickel and cobalt at low temperatures. One of the objects of that investigation was to determine whether the Δ in the Weiss law χ (T + Δ) = C, which is followed by these substances at not too Iow temperatures, is purely a function of the “ magnetic dilution " of the compound, as seemed probable from earlier measurements. It was shown that the problem is not so simple as it was thought to be at first, the run of the Δ's in general not corresponding exactly to that of the dilutions in the series of compounds—anhydrous sulphate, hydrated sulphate and ammonium double sulphate. Another point of interest was the relatively large value of Δ (== 22) in the case of cobalt ammonium sulphate, notwithstanding the great magnetic dilution of this substance. The investigation has now been extended by the measurement of the susceptibilities of the following substances : Cobalt potassium sulphate, cobalt rubidium sulphate and manganese ammonium sulphate. These compounds were investigated with the following objects in view. The former two substances, together with cobalt ammonium sulphate, form a series of isomorphous compounds which differ from one another only in the monovalent ion in the diamagnetic part of the molecule and in the actual dimensions of the unit cells of their structures. They would thus seem to be very suitable to employ in an endeavour to determine whether in a series of compounds of exactly similar constitution, in which only the relative dimensions of the unit cells vary from member to member, the Δ is definitely a function of the mean distance apart of the paramagnetic atoms in the crystal,i. e ., of the dilution of the substance. Furthermore, since cobalt ammonium sulphate and cobalt rubidium sulphate are almost exactly isostructural, it would be expected that the Δ's for the two compounds would be nearly equal unless the diamagnetic part of the molecule exerts an influence on the paramagnetic atom other than that of a mere diluent.