Electron spin resonance evidence for the role of copper ions in interlayer coupling in layered copper oxycarbonates

Abstract

Electron spin resonance (ESR) studies have been carried out on the layered copper oxycarbonates (alternating layers of CuO₂ and CO^2-₃) with the nominal composition Ba₃SrCu₂O₆.2CO₂ and their reaction products with ammonium slats. The ESR signals typical of copper ions in square-planar geometry with a slight orthorhombic distortion presumably arise from the copper ions being incorporated in the carbonate layer. A simple grinding procedure at room temperature using (NH⁺₄)_nX^n- (X=F^-, (NO₃)^-, (VO₃)^-, (HPO₄)^2-) incorporates X^n- anions in the structure with the liberation of ammonia. The environment of the cop per ion in the carbonate layer is thus changed without altering the structure. The g₃ values are most affected by this reaction showing that the filled e_g orbitals are oriented in the plane of the carbonate layer. The half-filled e_g orbitals (transforming s d_x²-y² orbitals) are oriented out of the plane. This suggests that the copper ions in the carbonate layers are probably involved in introducing magnetic coupling between the CuO₂ layers.