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 La2Li0.5M0.53+O4 (M = Cu, Ni, or Co) or La2-xSrxCu1-yLiyO4 (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, ΔHX, arguments are presented for the existence of a maximum value of ΔHX 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 CuO1+δ component of ternary copper oxides of metals such as La, Ba, Li, etc. which have ΔHO close to the maximum value. We then find that the value of ΔH(Cu)O in La2Li0.5M0.53+O4 (∼400 kJ/g at O) is very large relative to that in CuO (∼165 kJ/g at O) and suggests an ionic Cu3+−O2- linkage (metal oxidation). In La2-xSrxCuO4 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 La2Li0.5M0.5O4 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 La2-xSrxCu1-yLiyO4 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.