"Iron-Deficient" YIG?

In a recent letter to this TRANSACTIONS [1], Guyot, Cagan, and Merceron claim to have measured the initial permeabilities, at different constant temperatures, as a function of frequency, of "iron-deficient" yttrium iron garnet (YIG). They attribute the nature of their results to the presence of Fe⁴⁺ions in their samples, which provide a means of valency interaction. The method of preparation of the polycrystalline material on which these measurements were made is given in another letter [2] by the same authors. This author wishes to point out that this method, taken at face value, will not produce the material they claim. In fact, it is very unlikely that an "iron-deficient" YIG, which might be written Y3Fe5-δO12, with measurable δ, can be produced at all.