Transformations of structural defects in heated model clay precursors by ESR spectroscopy

Abstract

In this paper we discuss the broad ESR spectrum at g = 2, which we have observed in model clay compounds exposed to different conditions: (a) aged in the solid form in a closed test tube; (b) heated in an oven in the presence of oxygen; and (c) evacuated at room temperature in order to remove oxygen. Ageing experiments in which the ESR line finally disappeared led us to conclude that its origin is not an Fe 3+ impurity. The fact that an ESR line at g = 2 was not observed in single crystals of Na 4Al 2(OH) 2(C 2O 4) 4 · 6H 2O indicated that the line arises from some kind of structural defect. The fact that the line appeared upon evacuation of Al 3(OH) 7C 2O 4 · 3H 2O suggested that it might be related to structural defects at the surface of model clay compounds. As the line does not saturate easily, one might suggest that it is related to the oxygen atoms of the clay surface.