ESR Studies of Cu2+ in NH4Cl Single Crystals between 4.2 and 453 °K. II. Theoretical Analysis: Vibrational Admixtures, Spin Polarization, and 〈r−3〉 for Cu2+ Ions

The temperature dependence of the spin‐Hamiltonian parameters for two Cu2+ centres in NH4Cl single crystals, which were reported in the previous paper (I), is analysed in detail. The results show that, apart from zero point vibrations, higher lattice modes also contribute to the admixture of the |x2 – y2〉 state into the |3 z2 – r2〉 ground state for these centres. For one of the centres there is also evidence that vibrations and reorientations of a single nearest neighbour NH4+ ion contribute to the admixture. From the analysis, the temperature dependence of 〈r−3〉3z2 – r2, a crystal field splitting parameter and the contact contribution to the hyperfine field at the Cu2+ nucleus is obtained. All these quantities vary strongly with temperature and show sudden changes at the λ‐point ( −30.5 °C) for NH4Cl. The temperature dependence of the contact term in the hyperfine interaction can be qualitatively understood in terms of core spin polarization by taking into account the observed changes in the radial distribution of the 3d ground state. The analysis gives further support for the models proposed for the two Cu2+ centres.