Variable-frequency EPR study of Mn²⁺ -doped NH₄Cl_0.9I_0.1 single crystal at 9.6, 36, and 249.9 GHz: Structural phase transition

Multifrequency electron paramagnetic resonance studies on the Mn²⁺ impurity ion in a mixed single crystal NH₄Cl_0.9I_0.1 were carried out at 9.62 (X-band) in the range 120-295 K, at 35.87 (Q-band) at 77 and 295 K, and at 249.9 GHz (far-infrared band) at 253 K. The high-field EPR spectra at 249.9 GHz are well into the high-field limit leading to a considerable simplification of the spectra and their interpretation. Three magnetically inequivalent, but physically equivalent, Mn²⁺ ions with their respective magnetic Z-axes oriented along the crystallographic [100], [010], [001] axes were observed. Simultaneous fitting of EPR line positions observed at X-, Q-, and far infra-red bands was performed using a least-squares procedure and matrix diagonalization to estimate accurately the Mn²⁺ spin-Hamiltonian parameters. The temperature variation of the linewidth and peak-to-peak intensities of the EPR lines indicate the presence of λ-transitions in the mixed NH₄Cl_0.9I_0.1 crystal at 242 and 228 K consistent with those observed in the pure NH₄Cl and NH₄I crystals, respectively. A superposition-model analysis of the spin-Hamiltonian parameters reveals that the local environment of the Mn²⁺ ion is considerably reorganized to produce axially symmetric crystal fields about the respective Z-axes of the three magnetically inequivalent ions as a consequence of the vacancy created due to charge-compensation when the divalent Mn²⁺ ion substitutes for a monovalent NH₄⁺ ion in the NH₄Cl_0.9I_0.1 crystal. This reorganization is almost the same as that observed in NH₄Cl and NH₄I single crystals, although the latter two are characterized by different, simple cubic and face-centered cubic, structures.