EPR and Optical Measurements of Highly Doped SrLaGa3O7:Co, V Single Crystals

Abstract

SrLaGa3O7:Co, V single crystals were investigated by room-temperature optical spectroscopy and the X-band electron paramagnetic resonance (EPR) technique in the temperature range of 3–300 K. The optical absorption spectrum reveals the presence of vanadium V3+ and V4+ ions at tetrahedral coordinated sites and at least two C3 centers of Co2+. The optical conclusions are confirmed by the EPR results where the existence of paramagnetic centers arising from Co2+ and V4+ ions has been found. The resonance positions of the EPR lines as a function of the rotating angle suggest that two nonequivalent cobalt centers arise in the SrLaGa3O7:Co, V crystal structure. V4+ centers reveal a well-resolved hyperfine structure (I = 7/2), whereas Co2+ EPR signal has no clear hyperfine lines. Both types of ions are affected by the axial crystal symmetry environment with the symmetry axis parallel to the c crystallographic direction. The EPR signal integral intensity as a function of temperature indicates significant antiferromagnetic interactions between V4+ ions, whereas Co2+ paramagnetic centers revealed no significant interactions. Additionally, as the temperature increased between 24 and 80 K, a very wide resonance line was observed in the EPR spectra. We have attributed this line to V3+ ions at tetrahedral positions. Since the resonance signal is very complex, we focused on analyzing of Co2+ lines observed in the ab-plane, where the EPR signal does not change as the crystal rotates. The estimated spin Hamiltonian parameters of Co2+ centers (S = 3/2, I = 7/2) are the following: geff = 2.91, D = 87 mT, A⊥ = 1.5 mT, A∥ = 10 mT (type center 1), and geff = 3.35, D = 108 mT, A⊥ = 4.5 mT, A∥ = 17 mT (type center 2). Both centers have no rhombic distortion of a crystal field (E = 0).