ESR Studies of Cu²⁺ in NH₄Cl Single Crystals between 4.2 and 453 °K. I. Spin‐Hamiltonian Parameters and Lattice Constant for NH₄Cl

ESR spectra from two different Cu 2+ with [100] tetragonal symmetry were observed in NH 4 Cl single crystals. The spin‐Hamiltonian parameters for both centres were determined in the temperature range 4.2 to 453 °K where NH 4 Cl has the CsCl structure. The g ‐values and hyperfine constants for both centres show a marked anomaly in the vicinity of the λ‐point and significant variations over the whole temperature range. Both centres have essentially a |3 z 2 – r 2 〉 ground state, but with evidence of vibrational admixtures of the | x 2 – y 2 〉 state. In view of possible future calculations of the Cu 2+ spin polarization and radial wave function for the ground state, the lattice parameter of NH 4 Cl and NH 4 Cl:Cu was measured in the temperature range 105 to 453 °K.