Environment dependence of metal or ligand oxidation in copper oxide systems: evidence from heats of formation and ⁷Li solid state NMR studies

Abstract

The stability of the Cu³⁺ ion in an oxide matrix such as La₂Li_0.5M_0.53³⁺O₄ (M = Cu, Ni, or Co) or La_2-xSr_xCu_1-yLi_yO₄ (x = 0 or 0.15, y = 0 or 0.025) is examined. Heats of formation (as measured by solution calorimetry) and ⁷Li NMR have been used for this purpose. From the systematic nature of the reported heats of formation of binary compounds analyzed per ligand X ion, ΔH_X, arguments are presented for the existence of a maximum value of ΔH_X corresponding to a maximum ionicity (~660 kJ/g at X). A scheme is then proposed to extract the heat of formation per oxygen, ΔH(Cu)_O, in the CuO_1+δ component of ternary copper oxides of metals such as La, Ba, Li, etc. which have ΔH_O close to the maximum value. We then find that the value of δH(Cu)_O in La₂Li_0.5M_0.5³⁺O₄ (~400 kJ/g at O) is very large relative to that in CuO (~165 kJ/g at O) and suggests an ionic Cu³⁺-O^2- linkage (metal oxidation). In La_2-xSr_xCuO₄ the low value of ΔH(Cu)_O is consistent with considerable O → Cu charge transfer and creation of holes on oxygen. These conclusions are supported by ⁷Li NMR studies which probe the local environment in the CuO matrix. Advantage is taken of the paramagnetic shifts of the resonance frequency in such anisotropic systems and the dual principal axes (quadrupolar and magnetic dipolar) interaction tensors to understand the ⁷Li NMR. The main results of the NMR study are that in La₂Li_0.5M_0.5O₄ systems the quadrupolar splitting of the ⁷Li is nearly constant for all M ions indicating a similar LiO₆ environment. When coupled with the ΔH(M)_O data, this implies that there is little O → M charge transfer. In La_2-xSr_xCu_1-yLi_yO₄ on the other hand the ⁷Li NMR shows three kinds of environments for the Li ions despite there being only a single crystallographic site. We suggest that at least one of these environments is due to the creation of holes on oxygen.