Electron paramagnetic resonance studies of some ternary oxides of copper (II)

Abstract

The electron paramagnetic resonance (EPR) of ternary oxides of Cu(II) has been studied between 4.2 and 300 K. The systems include those with 180 degrees Cu-O-Cu interactions (such as Ln₂CuO₄, Sr₂CuO₂C_l2, Sr₂CuO₃ and Ca₂CuO₃) or 90 degrees Cu-O-Cu interactions (such as Y₂Cu₂O₅ or BaCuO₂) as well as those in which the Cu²⁺ ions are isolated (such as Y₂BaCuO₅, La_1.8Ba_1.2Cu_0.9O_4.8 and Bi₂CuO₄). The change in the EPR susceptibility as a function of temperature is compared with that of the DC magnetic susceptibility. Compounds with extended 180 degrees Cu-O-Cu interactions which have a low susceptibility also do not give EPR signals below room temperature. For compounds such as Ca₂CuO₃ with one-dimensional 180 degrees Cu-O-Cu interactions a weak EPR signal is found the temperature dependence of which is very different from that of the DC susceptibility. For Y₂BaCuO₅ as well as for La_1.8Ba_1.2Cu_0.9O_4.8 the EPR susceptibility as well as its temperature variation are comparable with those of the static susceptibility near room temperature but very different at low temperatures. Bi₂CuO₄ also shows a similar behaviour. In contrast, for Y₂Cu₂O₅, in which the copper ions have a very distorted nonsquare-planar configuration, the EPR and the static susceptibility show very similar temperature dependences. In general, compounds in which the copper ions have a square-planar geometry give no EPR signal in the ground state (0 K) while those with a distortion from square-planar geometry do give a signal. The results are analysed in the light of recent MS X_alpha calculations on CuO₄^6- square-planar clusters with various Cu-O distances as well as distortions. It is suggested that in square-planar geometry the ground state has an unpaired electron in anionic orbitals which is EPR inactive. Competing interactions from other cations, an increase in Cu-O distance or distortions from square-planar geometry stabilise another state which has considerably more Cu 3d character. These states are EPR active. Both these states, however, are magnetic. For isolated CuO₄^6- clusters the magnetic interactions seem to involve only the states which have mainly anionic character.