EPR and ENDOR of the PO 4 2? radical in anhydrite, celestite and barite

Abstract

Abstract. Electron paramagnetic resonance (EPR) andelectron nuclear double resonance (ENDOR) low temperature measurements of natural anhydrite CaSO4, celestite SrSO4 and barite BaSO 4 have revealed the presence of PO 2-radical. The principal values of the g tensor and the A tensor [MHz] of hyperfine interaction (HFI) are found to be gxx=2.0124, gyy=2.0159, g~z=2.0098, Axx= 82.1, Ayy--81.4, A~z=77.2 in CaSO4, g~=2.0250, gyy=2.0070, g~z=2.0131, A~=84.8, Ayy=82.7, Az~ =90.5 in SrSO4, gx~=2.0302, gyy=2.0079, g~z=2.0135, A~ = 85.2, Ayy --82.3, A~ = 90.0 in BaSO4. The principal axes Axe, Ayy and A~= are parallel to the crystallographic axes a, b, c in all three matrices. In anhydrite the principal g axes coincide with the A axes. In celestite and barite such coincidence is found to be only along the b axis (a low symmetry effect). The coherence effect of splitting of ENDOR lines by a strong microwave field has been detected. From the analysis of the relative intensities of the ENDOR lines of the PO~-radical in CaSO4 the relation of probabilities of cross-relaxation processes W~/W~ = 3.5 are evaluated.