We have performed a series of EPR measurements on M2SnCl6 (M = NH+4, CH3NH+3, and [(CH3)2NH2]+) crystals doped with the CrO2-4 ion and thermally treated at different temperatures. Via EPR, we have detected the Cr(VI)O2-4 to Cr(V) and/or Cr(III) thermal reductions. A Cr(V) species, in the form of CrO3-4, 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:CrO2-4 crystals either co-doped with one of the [(CH3)nNH4-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 CrO3-4 species, which is conventionally produced via either X- or γ-ray irradiation.
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