STRUCTURAL REFINEMENT OF BaxSr1-xSO4 USING X-RAY POWDER DIFFRACTION DATA

Various mixtures of barite (BaSO4) and celestine (SrSO4) powders were prepared and fused at a temperature of 1000C for 10 hours to form barium-strontium sulfate [BaxSr1-xSO4 (x=0 to 0.85)]. X-ray powder diffraction (XRD) data of the fused barium-strontium sulfate powders were measured using a powder diffractometer with a copper X-ray tube. The XRD data were refined using Rietveld analysis with generalized spherical harmonic description for preferred orientation correction. The results showed that the structural refinement parameters agree reasonably well with the single-crystal XRD data. Also, the Rietveld refinement provided reasonable Rietveld fits for the fused barium-strontium sulfate powders. Moreover, the refined unit-cell parameters increase linearly as the x-composition increases for all XRD data sets. INTRODUCTION The minerals barite (BaSO4) and celestine (SrSO4) are important inorganic chemical products. These minerals have been widely used as raw materials in high temperature solid lubricants for drilling purposes. Feely et al. (1987) indicated that barite and celestine, as well as intermediate composition solid solutions (BaxSr1−xSO4; x=0 to 1), can form from seawater as a result of hydrothermal processes. The solid solution barium-strontium sulfate, which occurs in a wide variety of rock formations from Archean to present times, has received considerable interest with regard to deposition of barite and celestine minerals in hydrothermal and industrial aspects (Putniss et al., 1992). Subsequently, Li et al. (2009) indicated that synthetic BaxSr1−xSO4 (x=0 to 1) nano-materials crystallized in the orthorhombic structure and exhibited an ellipsoidal morphology with an average size of 800 Å, and that the cell parameters increased with increasing x-value. However, they did not report the crystal structure parameters derived from their XRD data. The form of the barium-strontium sulfate [BaxSr1−xSO4 (x=0 to 1)] solid solution series over the entire composition range was described by Bostrom et al. (1967) and Brower (1973). Additionally, Bostrom et al. (1967) and Burkhard (1973) indicated that the cell parameters vary nonlinearly with x, which is contrary to the results reported by Sabine and Young (1954). Moreover, Hanor (1968) showed that the large majority of naturally occurring samples have compositions within 10 mole percent of the pure end members. Subsequently, Goldish (1989) used XRD data sets of Ba0.75Sr0. 25SO4, Ba0.50Sr0. 50SO4 and Ba0.25Sr0. 75SO4 to determine the compositional shifts in d-spacings and cell-parameters along with the variation of intensities. The results showed that the crystal structure is in the orthorhombic with the space group of Pbnm (62), and cell parameters are (a=7.0786Å, b=8.7331 Å and c=5.4422Å) for Ba0.75Sr0. 25SO4; (a=7.0061Å, b=8.6039Å and c=5.4214Å) for Ba0.50Sr0. 50SO4; and (a=6.9357Å, b=84774Å, and b=5.3946Å) for Ba0.25Sr0. 75SO4. However, the atomic positions and isotropic displacement coefficients derived from the all XRD data sets were not reported. 0 Copyright ©JCPDS-International Centre for Diffraction Data 2011 ISSN 1097-0002 1 Advances in X-ray Analysis, Volume 54