Variable-frequency EPR study of Mn 2+-doped NH 4Cl 0.9I 0.1 single crystal at 9.6, 36, and 249.9 GHz: structural phase transition

Abstract

Multifrequency electron paramagnetic resonance studies on the Mn 2+ impurity ion in a mixed single crystal NH 4Cl 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, Mn 2+ ions with their respective magnetic Z-axes oriented along the crystallographic [1 0 0], [0 1 0], [0 0 1] 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 Mn 2+ 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 NH 4Cl 0.9I 0.1 crystal at 242 and 228 K consistent with those observed in the pure NH 4Cl and NH 4I crystals, respectively. A superposition-model analysis of the spin-Hamiltonian parameters reveals that the local environment of the Mn 2+ 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 Mn 2+ ion substitutes for a monovalent NH 4 + ion in the NH 4Cl 0.9I 0.1 crystal. This reorganization is almost the same as that observed in NH 4Cl and NH 4I single crystals, although the latter two are characterized by different, simple cubic and face-centered cubic, structures.