Investigations on the spin Hamiltonian parameters and the local structures for the orthorhombic [CuX4(H2O)2]2− centers in NH4X (X = Cl, Br)

The spin Hamiltonian parameters (the g factors and the hyperfine structure constants) and the local structures for the orthorhombic [CuX4(H2O)2]2− clusters in NH4X (X = Cl, Br) are theoretically investigated from the perturbation formulas of these parameters for a 3d9 ion in orthorhombically compressed octahedra. The related molecular orbital coefficients are quantitatively determined from the cluster approach in a uniform way. The studied defect centers are attributed to the impurity Cu2+ occupying the interstitial site associated with two adjacent water molecules along the [1 0 0] (or C2) axis. The ligand octahedra are found to suffer the relative planar bond length variations of about 0.19 and 0.20 Å along X and Y axes for X = Cl and Br, respectively, due to the Jahn–Teller effect. The calculated spin Hamiltonian parameters based on the above local structures show reasonable agreement with the observed values for both the centers. The validity of the results is also discussed.