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

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 Ln 2 CuO 4 , Sr 2 CuO 2 Cl 2 , Sr 2 CuO 3 and Ca 2 CuO 3 ) or 90 degrees Cu-O-Cu interactions (such as Y 2 Cu 2 O 5 or BaCuO 2 ) as well as those in which the Cu 2+ ions are isolated (such as Y 2 BaCuO 5 , La 1.8 Ba 1.2 Cu 0.9 O 4.8 and Bi 2 CuO 4 ). 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 Ca 2 CuO 3 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 Y 2 BaCuO 5 as well as for La 1.8 Ba 1.2 Cu 0.9 O 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. Bi 2 CuO 4 also shows a similar behaviour. In contrast, for Y 2 Cu 2 O 5 , 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 CuO 4 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 CuO 4 6- clusters the magnetic interactions seem to involve only the states which have mainly anionic character.