Spin-lattice relaxation in the rotating frame and extreme slowing-down at the antiferroelectric transition in copper formate tetrahydrate

The effect on the temperature behavior of the spin-lattice relaxation rates in laboratory and rotating frames in presence of extreme slowing-down of the critical fluctuations in an Ising-type system is discussed. Proton spin-lattice relaxation measurements of T₁ and T_1? in water-deuterated copper formate tetrahydrate are presented. The data shown that the anomalous behavior of the proton T_1? in the neighbourhood of the antiferroelectric phase transition recently observed by Zumer and Pir? in the ordinary crystal cannot be ascribed to the critical slowing-down of the water molecules. A possible interpretation on the basis of a mechanism of creation and annihilation of paramagnetic excitons is discussed.