Temperature-induced changes in the EPR spectrum of the magnetic center in kaolin

Studies of kaolin have revealed an effect characterized by an unusual temperature-induced change of the EPR spectrum of the Fe³⁺ ion, which is the magnetic probe in kaolin-clay. At low temperature (T = 4.2 K) a resonance line with an effective g-value g₁ = 4.13 ± 0.16 is observed. At high temperature (T = 288 K) one observes a resonance line with the effective g-value g₂ = 2.15 ± 0.1. The transition from the low- to high-temperature spectrum is gradual and it is accompanied by a redistribution of the absorption intensity. The observed properties of the temperature dependence of the EPR spectrum are characteristic of systems with a multiminimum potential.