EPR of transition metals in fluoroaluminate glasses

Abstract

The results of EPR study of fluoroaluminate glasses (FAG) in the AlF 3-YF 3-RF 2 system (where R = Mg, Ca, Sr, Ba) doped with small amounts of transition metals (Ti, V, Mn, Fe, Co, Cu) are presented. The EPR spectrum of Ti 3+ ions in FAG is similar to that reported for many glasses and can be described by g 1 = 1.96 and g 2 = 1.91 at 77 K. The vanadium-containing FAG exhibit EPR spectrum only when V is introduced as vanadium oxides; this spectrum belongs to VO 2+ ions and can be described by an axial spin-Hamiltonian with parameters (determined by fitting the experimental and simulated spectra): g ∥ = 1.949; g ⊥ = 1.980; A ∥ = 163 × 10 −4 cm −1; A ⊥ = 63 × 10 −4 cm −1. An X-band spectrum of Mn 2+ ions consists of a well-resolved sextet near g = 2.0 and weak background and g ≅ 4.3 resonances. The hyperfine splitting of the g ≅ 2.0 resonance measured at the Q-band is A = 98.5±0.5 Oe. Concentration dependence of the EPR spectrum permits one to assume a more uniform distribution of Mn 2+ ions in FAG than in HMFG where these ions are believed to be clustered. Iron is present in FAG predominately as Fe 2+ ions. A small portion of Fe 3+ ions gives EPR spectrum which is similar to that in oxide glasses but very different from the spectrum observed for Fe 3+ in HMFG. By contrast with fluoroberyllate glasses for which EPR signals of Co 2+ have been observed at 77 K the broad single line centered near g ∼ 4.3 is observed for Co 2+ in FAG only at 4.2 K. The EPR spectra of FAG containing copper are superpositions of the spectra of isolated Cu 2+ ions and the almost isotropic line belonging presumably to crystalline occlusions which undergo cooperative Jahn-Teller effect. The spectrum of Cu 2+ ions dissolved in FAG exhibits superhyperfine structure with a splitting of about 50 Oe from the interaction of unpaired electrons of Cu 2+ ions with 19F nuclei.