A variable temperature EPR study of Mn2+-doped NH4Cl0.9I0.1 single crystal at 170Â GHz: Zero-field splitting parameter and its absolute sign

EPR measurements have been carried out on a single crystal of Mn2+-doped NH4Cl0.9I0.1 at 170-GHz in the temperature range of 312-4.2 K. The spectra have been analyzed (i) to estimate the spin-Hamiltonian parameters; (ii) to study the temperature variation of the zero-field splitting (ZFS) parameter; (iii) to confirm the negative absolute sign of the ZFS parameter unequivocally from the temperature-dependent relative intensities of hyperfine sextets at temperatures below 10 K; and (iv) to detect the occurrence of a structural phase transition at 4.35 K from the change in the structure of the EPR lines with temperature below 10 K.