Electron paramagnetic resonance characterization and electron spin relaxation of manganate ion in glassy alkaline LiCl solution and doped into Cs2SO4

Abstract

Manganate ion, MnO₄^2-, has important roles in catalysis and potential roles in water treatment. EPR spectra of MnO₄^2- in a glassy alkaline solution of concentrated LiCl at X-band and Q-band at 80 K exhibit g₁ = 1.9776 ± 0.001, g₂ = 1.9677 ± 0.001, g₃ = 1.9560 ± 0.001 and A₁ = 182 ± 9, A₂ = 275 ± 15, and A₃ = 400 ± 15 MHz. In Cs₂SO₄ the spectra were simulated with 1.908 ± 0.001, g₂ = 1.909 ± 0.001, g₃ = 1.937 ± 0.001 and A₁ = 90 ± 20, A₂ = 100 ± 20, and A₃ = 400 ± 15 MHz. Simulations required large distributions in A values which suggests that hyperfine splittings are sensitive to differences in geometry. Continuous wave spectra are observable at 80 K in glassy alkaline LiCl, but only up to about 20 K in Cs₂SO₄. In glassy alkaline LiCl electron spin relaxation was measured at X-band using spin echo and inversion recovery from 4.2 to 60 K. T_m is 4.6 μs at 4.2 K and decreases at higher temperatures as it becomes driven by T₁. T₁ decreases from ca. 34 ms at 4.2 K to ca. 240 ns at 60 K. T_m and T₁ in Cs₂SO₄ are too short to measure by electron spin echo. The distorted tetrahedral geometry of MnO₄^2- results in faster relaxation than for other 3d¹ spin systems that have square pyramidal (C_4v) or distorted octahedral geometries.

Keywords: Electron spin relaxation; Glassy alkaline LiCl; Manganate ion.