Experiments on the Zero‐Field Splitting of Trivalent Gadolinium

The zero‐field resonance frequencies of Gd3+ and their dependence on temperature and hydrostatic pressure have been measured on a set of nine isomorphic crystals having the zircon structure. It is shown that the volume effect on the quadrupole term b is much smaller than that produced by the change in local anisotropy of the gadolinium ion. The variety of data obtained fits qualitatively into this interpretation in terms of the dominating role of the quadrupole term. A qualitative microscopic model is suggested correlating the unit cell dimensions to the local anisotropy of the central ion.