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Crystal structure and Raman spectral studies of BaSO4—PbSO4 solid solution

Abstract Natural hokutolite, (Ba,Pb)SO4, is a radioactive sulfate mineral occurring in hot-spring deposit. Unlike the limited PbSO4 content observed in natural specimens, the present study reveals that (Ba,Pb)SO4 crystals can be grown with compositions covering the whole solid solution series of PbSO4 and BaSO4. X-ray measurements show that the unit cell parameters gradually increase with increasing BaSO4 content. The nonlinear behavior of the unit cell dimensions with composition shows that a positive deviation from linear variation exists in the b-axis direction and negative deviations for a- and c-axes. It is evident that the variations in the M-O bond lengths result in the significantly negative deviations of all unit cell parameters from Vegard’s rule around 70 mol% BaSO4. Hence, the abnormally negative deviations of all unit cell parameters from Vegard’s rule are primarily attributed to the discontinuities in the mean M-O bond lengths. In addition, the existence of structure gap in natural hokutolite samples reported may be ascribed to the same reason. Raman bands in these synthetic crystals show monotonous changes in ν1 frequency from anglesite to barite. It is inferred that the slight decrease in ν1 frequency for crystals with BaSO4 content being smaller than 20 mol% may be attributed to the net effect of the S—O force constants, intertetrahedral O—O force constants, and distortions of SO4 tetrahedra. The present results also indicate that the positional disordering along the solid solution series is responsible for the Raman line broadening, showing the local maximum in the line width plot occurs at about 50 mol% substitution. This work also demonstrates that variations of Raman shift of ν1 band as well as its line width are essentially related to the random Pb—Ba substitution in our synthetic crystals.