STRUCTURAL REFINEMENT OF Ba x Sr 1-x SO 4 USING X-RAY POWDER DIFFRACTION DATA

Transcription

1 01 STRUCTURAL REFINEMENT OF Ba x Sr 1-x SO 4 USING X-RAY POWDER DIFFRACTION DATA Husin Sitepu and Syed R. Zaidi Research and Development Center, Saudi Aramco, PO Box 62, Dhahran 31311, Saudi Arabia ABSTRACT Various mixtures of barite (BaSO 4 ) and celestine (SrSO 4 ) powders were prepared and fused at a temperature of 1000 C for 10 hours to form barium-strontium sulfate [Ba x Sr 1-x SO 4 (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 (BaSO 4 ) and celestine (SrSO 4 ) 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 (Ba x Sr 1 x SO 4 ; 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 Ba x Sr 1 x SO 4 (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 [Ba x Sr 1 x SO 4 (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 Ba 0.75 Sr 0.25 SO 4, Ba 0.50 Sr 0.50 SO 4 and Ba 0.25 Sr 0.75 SO 4 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= Å and c=5.4422å) for Ba 0.75 Sr 0.25 SO 4 ; (a=7.0061å, b=8.6039å and c=5.4214å) for Ba 0.50 Sr 0.50 SO 4 ; and (a=6.9357å, b=84774å, and b=5.3946å) for Ba 0.25 Sr 0.75 SO 4. However, the atomic positions and isotropic displacement coefficients derived from the all XRD data sets were not reported.

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 12 In the present study, the XRD data of fused (1000 C,10h) Ba x Sr 1-x SO 4 (x=0 to 0.85) powders were measured using a powder diffractometer with a copper X-ray tube. Subsequently, the crystal structure parameters of all XRD data sets were refined using the Rietveld method with the generalized spherical harmonic description (Von Dreele, 1997; Sitepu, 2002; Sitepu, O Connor and Li, 2005; Sitepu, 2007, 2008 & 2009) for preferred orientation correction. Rietveld analysis adjusts the refinable parameters until the best fit of the entire calculated pattern to the entire measured pattern is achieved. Additionally, the refined atomic parameters should agree well with the structure derived from single-crystal XRD data. EXPERIMENTAL The starting barite (BaSO 4 ) and celestine (SrSO 4 ) powders were analytical reagents which were supplied by Fisher Scientific Co. (USA). The powders were considered to be excellent for this study because high-quality crystal structures (Jacobsen et al., 1998) have been reported. Six samples with compositions Ba 0.85 Sr 0.15 SO 4, Ba 0.65 Sr 0.35 SO 4, Ba 0.60 Sr 0.40 SO 4, Ba 0.55 Sr 0.45 SO 4, Ba 0.35 Sr 0.65 SO 4, and Ba 0.17 Sr 0.82 SO 4 were prepared with a great care. The BaSO 4 and SrSO 4 powders were mixed homogenously and fused at a temperature of 1000 C for 10 hours. The fused barium-strontium sulfate powders were manually ground in an agate mortar and a pestle for several minutes to achieve fine particle size. Then, the powder was mounted into the XRD sample holder by back pressing. The continuous-scanned patterns were measured with a PANalytical X PERT Pro XRD diffractometer equipped with a sample spinner, X Celerator position sensitive detector (PSD) and focusing incident beam monochromator tuned to transmit only the CuK 1 component of CuK doublet. Data were collected from 10 to in step size of 0.01 using a scan rate of 1 degree per minute. All of the samples were spun during the data collection to improve particle counting statistics; thus the intensities are not dominated by a small number of crystallites (Sitepu et al., 2001 & 2005). Table 1 shows XRD pattern measurement condition. Name Instrument Radiation Optics Specimen Detector Table 1. XRD pattern measurement condition Description PANalytical X PERT PRO MPD Copper-anode tube operated at 45 kv and 40 ma Wavelength: Cu K 1 = Å Bragg Brentano, measuring circle diameter = 480 mm Divergence slit type: fixed with divergence slit size= Irradiated length = 10 mm & Specimen length = 10 mm Distance Focus to Divergence Slit = 100 mm Holder: circular format, diameter = 16 mm Rotation on for all measurements PSD X Celerator PSD mode: scanning; PSD length in 2 = 2.12 Acquisition Angular range in 2 : Step size: 0.01 Scan rate: 1 degree per minute

4 23 The Rietveld refinements were performed with the GSAS software package (Larson and Von Dreele, 2000). The refinement strategy was similar to that described by Sitepu, O Connor and Li (2005) and Sitepu (2009). The structural model used for the barium-strontium sulfate singlecrystal XRD data was described by Jacobsen et al. (1998) and the International Center for Diffraction Data (ICDD), Powder Diffraction File (PDF) numbers and The refined parameters were phase scale factors and the background component of the patterns with an eighteen-parameter linear interpolation function (i.e. function type 7 in GSAS), lattice parameters, the instrument zero-point 2 o (off-set in the 2 scale of goniometer), the Lorentzian and the Gaussian terms of a pseudo-voigt profile function and anisotropic strain parameters (Stephens, 1999), structural parameters (i.e. x,y,z and frac) and isotropic thermal parameters (Uiso). After the preliminary refinement without preferred orientation correction had converged, the generalized spherical harmonic coefficients (Von Dreele, 1997; Sitepu, 2002; Sitepu, O Connor and Li, 2005; Sitepu, 2007, 2008 & 2009) were then included. The default sample texture symmetry was chosen to be cylindrical (or fiber texture). Twelve-order harmonics were selected following preliminary calculations using fourteen orders that yielded the same figure-of merit and goodness of fit-index. RESULTS AND DISCUSSION Table 2 depicts the structural parameters of the fused (1000 C,10h) barium-strontium sulfate [Ba x Sr 1-x SO 4 (x=0 to 0.85)] obtained from Rietveld refinement with generalized spherical harmonic description. The number in parentheses gives the estimated standard uncertainty for the least significant figure of the parameter. The refined structural parameters agreed quite satisfactorily with the corresponding single-crystal results (Jacobsen et al., 1998). Additionally, the refined site occupancies agree reasonably well with the measured x-composition for Ba x Sr 1-x SO 4. However, the chemical analysis of the final powders has not yet been determined. The goodness of fit index ( 2 ) ranges from to 1.956, the weighted crystallographic R- factors (R WP ) range from 4.17 to 7.97, and the structure factor [R(F 2 )] ranges from 6.42 to when the Rietveld refinements were carried out using generalized spherical harmonic description. The texture index increases as the x-composition increases. The values range from to 1.435, which indicates that the fused (1000 C,10h) barium-strontium sulfate powders have substantial texture. However, the relationship between the morphology and nature of texture of the final powders has not yet been determined. Figure 1 shows the agreement between the calculated and measured XRD patterns, for the fused (1000 C,10h) barium-strontium sulfate (Ba 0.35 Sr 0.65 SO 4 ) powders, following Rietveld refinement with generalized spherical harmonic description for preferred orientation correction. Thus, the generalized spherical harmonic description should be used for correction of the preferred orientation in XRD analysis for crystal structure refinement (Sitepu et al., 2005). Figure 2 shows the variation of the refined unit-cell parameters a, b and c, and volume V with x- chemical compositions from 0 to 0.85 for the fused (1000 C,10h) barium-strontium sulfate [Ba x Sr 1-x SO 4 ] powders. The standard deviations, which were taken from the estimated standard uncertainty for the least significant figure of the parameter produced by Rietveld refinement, are smaller than the symbols. The results show that the refined unit-cell parameters increase linearly as the x-chemical composition increases for all XRD data sets, which agree reasonably well with

5 34 the results reported by Goldish (1989) and Li et al. (2009). The gradients of the regression plot for all XRD data sets are positive. Table 2. Summary of refined structure parameters for the (1000 C,10h) barium-strontium sulfate [Ba x Sr 1-x SO 4 (x=0 to 0.85)] obtained from Rietveld refinement. The space group used was Pbnm (No. 62). Parameters This work Single-crystal x-composition for Ba x Sr 1-x SO 4 powders XRD data (Jacobsen et al., 1998) Ba(x,y,f,¼) x (27) (27) (5) (4) (2) y (16) (17) (25) (25) (2) Frac 0.194(14) 0.304(14) 0.59(4) 0.598(29) 0.99 U (10) (9) (7) (14) 0.011(4) Sr(x,y,¼) x (27) (27) (5) (4) (2) y (16) (17) (25) (25) (2) Frac 0.806(14) 0.696(14) 0.41(4) 0.402(29) 0.01 U (10) (9) (7) (14) 0.011(4) S(x,y,¾) x (7) (7) (11) (12) (9) y (5) (5) (11) (8) (7) U (19) (20) (25) (34) 0.009(9) O1(x,y,¾) x (11) (13) (22) (22) (4) y (9) (11) (19) (18) (3) U (21) (22) (31) 0.023(4) 0.023(5) O2(x,y,¾) x (15) (16) (29) (28) (3) y (14) (16) (26) (27) (2) U (21) (22) (31) 0.023(4) 0.018(6) O3(x,y,z) x (11) (13) (22) (25) (2) y (6) (8) (13) (12) (2) z (10) (11) (27) (19) (2) U (21) (22) (31) 0.023(40) 0.013(2)

6 45 Table 2. (Continue) Parameters This work Single-crystal x-composition for Ba x Sr 1-x SO 4 powders XRD data (Jacobsen et al., 1998) Ba(x,y,f,¼) x (34) (29) (17) (2) y (19) (18) (12) (2) Frac 0.651(34) 0.836(24) U (9) (12) (5) 0.011(4) Sr(x,y,¼) x (34) (29) (2) y (19) (18) (2) Frac 0.349(34) 0.164(24) 0.01 U (9) (12) 0.011(4) S(x,y,¾) x (8) (9) (6) (9) y (8) (7) (5) (7) U (19) (26) (12) 0.009(9) O1(x,y,¾) x (18) (19) (14) (4) y (17) (15) (12) (3) U (27) (32) (17) 0.023(5) O2(x,y,¾) x (26) (23) (16) (3) y (21) (22) (15) (2) U (27) (32) (17) 0.018(6) O3(x,y,z) x (15) (18) (11) (2) y (12) (10) (8) (2) z (22) (15) (17) (2) U (27) (32) (17) 0.013(2)

7 Copyright JCPDS-International Centre for Diffraction Data 2011 ISSN (b) (a) Figure 1. The agreement between the calculated and measured XRD patterns for (a) Ba0.55Sr0.45SO4 and (b) Ba0.17Sr0.83SO4 powders following Rietveld refinement with generalized spherical harmonic description for preferred orientation correction. (a) (b) (c) (d) Figure 2. Variation in the refined unit-cell parameters along (a) a-axis, (b) b-axis, (c) c-axis, and (d) volume with x-composition for BaxSr1-xSO4 powders. Linear regressions are shown for all XRD data sets. Error bars are smaller than the symbols.

8 67 CONCLUSIONS The following conclusion can be drawn: The refined structural parameters obtained from Rietveld refinement agreed reasonably well with the single-crystal XRD data results. The refined unit-cell parameters increase linearly as the x-chemical composition increases for all XRD data sets. ACKNOWLEDGEMENTS The authors would like to acknowledge Saudi Aramco for giving permission to publish the results. Mr Yazeed Al-Dukhayyil, Mr Abdulelah Al-Naser, Mr Noqtan Al-Yami and Mr Mohammad Al-Qarni are acknowledged for their help in the study. The authors are grateful for constructive comments from the anonymous reviewers; Dr Thomas N. Blanton of the AXA Editor-in-Chief, and Professor James Kaduk of Illinois Institute of Technology that helped to improve the manuscript. REFERENCES Bostrom, K., Frazer, J., and Blankenburg, J. (1967). Subsolidus phase relations and lattice constants in the system BaSO 4 -SrSO 4 -PbSO 4, Ark Mineral. Geol. 4, Brower, E. (1973). Synthesis of barite, celestite, and barium strontium sulfate solid solution crystals, Geochim. Gosmocilim. Acta. 37, Burkhard, A. (1973). Optical and X-ray investigations in the system BaSO 4 -SrSO 4, Schweiz. Mineral. Petrog. Mill. 53, Feely, R.A., Gammon, R.H., Taft, B.A., Pullen, P.E., Waterman, L.S., Conway, T.J., Gendron, J. F., and Wisegarver, D. P. (1987). Distribution of chemical tracers in the eastern equatorial Pacific during and after the El Nifio/Southern Oscillation Event, J. Geophys. Res. 92, Goldish, E. (1989). X-ray diffraction analysis of barium-strontium sulfate (Barite-Celestite) solid solutions, Powder Diffr. 4, Hanor, J.S. (1968). Frequency distribution of compositions in the barite-celestite series, Am. Mineral. 53, Jacobsen, S.D., Smyth, J.R., Swope, R.J., and Downs, R. (1998). Rigid-body character of the SO 4 groups in celestine, anglesite and barite, Canadian Min. 36, (1998). Larson, A.C. and Von Dreele, R.B. (2000). General Structure Analysis System (GSAS) (Report LAUR ). Los Alamos, New Mexico: Los Alamos National Laboratory.

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