Electron paramagnetic resonance investigation of the Cr(VI) to Cr(V) and/or Cr(III) thermal reductions in single crystals of CrO^2-₄-doped M₂SnCl₆ (M = NH⁺₄, K⁺, CH₃NH⁺₃, and [(CH₃)₂NH₂]⁺)

We have performed a series of EPR measurements on M₂SnCl₆ (M = NH⁺₄, CH₃NH⁺₃, and [(CH₃)₂NH₂]⁺) crystals doped with the CrO^2-₄ ion and thermally treated at different temperatures. Via EPR, we have detected the Cr(VI)O^2-₄ to Cr(V) and/or Cr(III) thermal reductions. A Cr(V) species, in the form of CrO^3-₄, is thermally produced in (NH₄)₂SnCl₆ and in K₂SnCl₆ co-doped with NH⁺₄. We tentatively assign the thermally produced Cr(III) species as isolated Cr³⁺ ions. We have also studied in some detail the Cr(VI) to Cr(III) thermal reduction in K₂SnCl₆:CrO^2-₄ crystals either co-doped with one of the [(CH₃)_nNH_4-n]⁺ ions (n = 0-4) or coated with CH₃NH₂ · HCl or NH₄Cl. The experimental results indicate that it is the hydrogens of the NH groups which are responsible for the observed reduction. We have analyzed the magnetic symmetries and the spin Hamiltonian parameters of the thermally produced Cr(V) and Cr(III) species. These species can be controllably and repeatably produced via thermal treatments. This thermal method provides an attractive and easily accessible method for the production of the CrO^3-₄ species, which is conventionally produced via either X- or γ-ray irradiation.