Abstract. Electron paramagnetic resonance (EPR) measurements of natural barite BaSO4 have revealed the presence of PO~-and SO~ ionic radicals. For the latter a hyperfine structure from the 33S isotope has been detected and measured for the first time. The temperature dependence of the hyperfine interaction (HFI) constant of the PO22-and SO~ radicals has been investigated in the 100400 K range.
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