Fine structure due to magnetic dipole coupling in the low temperature EPR spectrum of Rb2Cu(SO4)2⋅6H2O

Abstract

A study of the EPR spectrum of single crystals of Rb2Cu(SO4)2⋅6H2O over the temperature range 290 to 115 K is reported. Below ∼240 K unusual fine structure occurs for certain orientations of the magnetic field H, consisting of eight equally spaced lines when H is approximately along the z molecular axis and five equally spaced lines when H lies along the y molecular axis of the Cu(H2O)62+ ion. The fine structure may be explained largely in terms of a magnetic dipole coupling with four neighboring copper (II) ions, each of which produces an approximately equal splitting of the EPR lines at the central copper (II) ion; the five line pattern occurs when the hyperfine interaction with the copper nucleus is small, while the eight line pattern results from a hyperfine interaction which is essentially equal to the magnetic dipole interactions (these each being ∼120 G). Spectra were simulated by considering the point magnetic-dipole interactions with the ten nearest Cu(H2O)62+ ions using the expression K̂=g1g2β2 (1–3 cos2ϑ)/r3, where g1 and g2 are the g values of the two interacting ions, r is the distance between them, ϑ is the angle between the internuclear vector and the magnetic field, and β is the Bohr magneton. The resulting line shapes are in reasonable agreement with experiment for most orientations of the magnetic field in the (001) and (010) crystal planes. The agreement is significantly improved if the interaction with the two nearest copper (II) ions is ∼20% greater than that calculated using the simple magnetic dipole model, and it is shown that this is consistent with the unpaired electron density being represented as a quadrupole, as is required by the shape of the dx2y2 ground state orbital, rather than as a point charge. Further improvement occurs if a weak exchange coupling of J=+30 and J=−15 G takes place with the nearest and next-nearest copper ions, respectively, and if an anisotropic component to the background half-width is introduced.