Variable-frequency EPR study of Mn2+-doped NH4Cl0.9I0.1 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 NH4Cl0.9I0.1 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 [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 Mn2+ spin-Hamiltonian parameters. The temperature variation of the linewidth and peak-to-peak intensities of the EPR lines indicate the presence of X-transitions in the mixed NH4Cl0.9I0.1 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 NH4Cl0.9I0.1 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. (C) 2003 Elsevier Science (USA). All rights reserved.