Electron paramagnetic resonance and phase transition in (NH4)2SeO4:Mn2+ single crystals

Abstract

The electron paramagnetic resonance of Mn2+ in ammonium selenate has been studied at X-band and at various temperatures between 77 and 395 K. The spectrum consists of a set of 30 lines corresponding to single type of Mn2+ environment. The principal y axis is along the b axis and the principal z axis makes an angle 54°±2° with the c axis in the ac plane. Around 390 K the spectrum shows an anomalous change in line shapes followed by a complete collapse of the spectrum which is found to be reversible. This has been discussed in terms of a possible first-order phase transition. The spectra at 77 K and at 300 K (including fine structure ΔM=±1 and Δm=0 hyperfine structure) are satisfactorily described by a spin Hamiltonian appropriate for Mn2+ in a rhombic crystal field. The parameters are determined at 77 and 300 K. Forbidden hyperfine transitions Δm=±1 and Δm=±2 have been observed. From the unequal separations between Δm=±1 doublets in M=+1/2→−1/2 transition, the values of the axial and rhombic parts of the quadrupole coupling constants are determined. The observed doublet separations of forbidden hyperfine transitions in different electronic transitions are in fairly good agreement with those calculated. The observed angular variation of the intensities of the allowed and forbidden hyperfine transitions is found to be in good agreement with theory.