A Straightforward and Effective Procedure to Test for Preferred

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A Straightforward and Effective Procedure to Test for Preferred Orientation in Polycrystalline Samples Prior to Structure Determination from Powder Diffraction Data

CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 5 705-710

Eugene Y. Cheung, Kenneth D. M. Harris,* and Bruce M. Foxman† School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom Received May 16, 2003

ABSTRACT: If a polycrystalline sample exhibits preferred orientation, the relative peak intensities in the powder diffraction pattern deviate from the intrinsic relative diffraction intensities, and can cause major difficulties in attempts to determine structural information from the powder diffraction pattern. To circumvent this problem, we report a straightforward and effective test that we use routinely as a screening procedure to ensure that polycrystalline samples are free of preferred orientation before recording high quality powder diffraction data for use in structure determination calculations. 1. Introduction In recent years, there has been considerable interest among solid state and materials chemists in structure determination from powder diffraction data,1-6 both in exploiting new opportunities for ab initio structure solution and in the widespread application of Rietveld refinement. While advances are continually being made in the techniques for carrying out each stage of the structure determination process, it is crucial to recall that successful structure determination requires experimental powder diffraction data of sufficiently high quality for the material of interest. Relevant issues in this regard include resolution, monochromaticity, definition of peak shape and peak width functions, and preferred orientation. In this paper, we focus on preferred orientation, and discuss a simple test that we carry out routinely in our laboratory as a screening procedure to ensure that powder samples are free of preferred orientation (within experimental detection levels) before undertaking structure determination calculations. Preferred orientation arises when the crystallites in a powder are oriented preferentially in certain directions, such that there is a nonrandom distribution of crystallite orientations. If a sample exhibits preferred orientation, the relative peak intensities in the experimental powder diffraction pattern deviate from the intrinsic relative intensities of the diffraction maxima, which can cause major difficulties in attempts to determine structural information from the powder diffraction pattern. Preferred orientation can be particularly severe when the crystal morphology is strongly anisotropic (e.g., long needles or flat plates). Although some powder diffractometers attempt to overcome the effects of preferred orientation by rotating the sample during data collection, rotation about a single axis * To whom correspondence should be addressed. Telephone: +44121-414-7474; fax: +44-121-414-7473; e-mail: K.D.M.Harris@ bham.ac.uk. † Permanent address: Department of Chemistry, Brandeis University, Waltham, MA 02454-9110.

(usually the capillary axis of a capillary sample holder, or the normal to the plate of a flat-plate sample holder) does not guarantee complete averaging of crystal orientations, which would require isotropic rotation of the sample. Clearly, the existence of preferred orientation can severely limit the potential for determining reliable structural information from the powder diffraction pattern, although the implications of preferred orientation are somewhat different for the structure solution and structure refinement stages of structure determination. Structure solution starts with essentially no knowledge of the correct structural model, whereas structure refinement starts with a structural model that is at least approximately correct. Most Rietveld refinement codes allow parameters describing preferred orientation7,8 to be refined together with the structural parameters and profile parameters, and satisfactory refinement can often be achieved using powder diffraction data with appreciable preferred orientation. However, in structure solution, preferred orientation imposes more severe limitations, as it implies establishing an approximately correct structural model from scratch, using measured diffraction intensities that deviate, as a result of preferred orientation, from the true relative intensities. In general, structure solution from powder diffraction data has a good chance of success only when there is no significant preferred orientation in the sample, although direct-space structure solution techniques6 are probably more robust than traditional techniques for data affected by preferred orientation2,9,10 (presumably because some amount of structural knowledge is input directly into the calculation through the use of a structural fragment). Other approaches11,12 to address the problem of preferred orientation during structure solution have included the detection of preferred orientation by mathematical means and the application of corrections based on statistical analysis of extracted intensities. In another approach for structure solution, preferred orientation has actually been

10.1021/cg0340796 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/10/2003

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Figure 1. Powder X-ray diffraction patterns for sample X of galvinoxyl, recorded at ambient temperature for the same sample packing: (a) powder diffraction photograph (STOE Reciprocal Lattice Explorer), (b) powder diffraction pattern recorded (Siemens D5000 diffractometer) with no sample rotation, (c) powder diffraction pattern recorded (Siemens D5000 diffractometer) with sample rotation. In panels b and c, the powder diffraction pattern calculated from the known structure of galvinoxyl is also shown, and the lower trace shows the difference between the calculated and experimental powder diffraction patterns.

exploited13 by studying several different powder diffraction patterns recorded for a (deliberately) textured sample. Although significant progress has been made in these areas of research, the most reliable approach for tackling the issue of preferred orientation in structure solution is to establish secure procedures for recording experimental powder diffraction data that are known to be free of preferred orientation. In our research, we adopt a strategy, described in this paper, in which we screen samples to ensure that they are free of preferred

orientation before measuring high quality powder diffraction data for use in structure determination. 2. Methodology Our test for preferred orientation is carried out by assessing the uniformity of the intensity distribution around the powder diffraction rings in a Debye-Scherrer type of experiment. This test can be carried out using any X-ray diffraction apparatus in which an appropriate detector (e.g., photographic film, image plate, CCD, etc.) can be placed perpendicular to the incident X-ray beam. In practice, we use a STOE Reciprocal Lattice Explorer14 with the flat photographic film positioned

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Figure 2. Powder X-ray diffraction patterns for sample Y of galvinoxyl, recorded at ambient temperature for the same sample packing: (a) powder diffraction photograph (STOE Reciprocal Lattice Explorer), (b) powder diffraction pattern recorded (Siemens D5000 diffractometer) with no sample rotation, (c) powder diffraction pattern recorded (Siemens D5000 diffractometer) with sample rotation. In panels b and c, the powder diffraction pattern calculated from the known structure of galvinoxyl is also shown, and the lower trace shows the difference between the calculated and experimental powder diffraction patterns. perpendicular to the incident X-ray beam, although any normal precession camera can be set up in the same way. The powder sample (loaded in a capillary or pressed between pieces of tape, for subsequent use in our transmission powder diffractometer) is placed in the position normally occupied by the single crystal. The photographic film is inspected to assess the uniformity of the intensity distribution around the powder diffraction ringssuniform intensity distribution around each ring indicates that the sample is free of preferred orientation. If the sample is found to exhibit preferred orientation, various methods may be employed to re-prepare the sample, including repacking in the sample holder, regrinding, recrystallization or (ultimately) mixing the powder sample with an amorphous material. When a sample preparation is obtained that is free

of preferred orientation, the sample is used directly (in the same sample holder, without repacking) on our powder diffractometer to record a high quality powder diffraction pattern for use in structure determination.

3. Results and Discussion To illustrate our method, Figures 1-3 each show the powder diffraction photograph [panel a] recorded on the STOE Reciprocal Lattice Explorer14 and two powder diffraction patterns recorded [panels b and c] on a Siemens D5000 diffractometer15 for a given packing of a powder sample of galvinoxyl. The three different

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Figure 3. Powder X-ray diffraction patterns for sample Z of galvinoxyl, recorded at ambient temperature for the same sample packing: (a) powder diffraction photograph (STOE Reciprocal Lattice Explorer), (b) powder diffraction pattern recorded (Siemens D5000 diffractometer) with no sample rotation, (c) powder diffraction pattern recorded (Siemens D5000 diffractometer) with sample rotation. In panels b and c, the powder diffraction pattern calculated from the known structure of galvinoxyl is also shown, and the lower trace shows the difference between the calculated and experimental powder diffraction patterns.

sample packings (denoted X, Y, and Z) used in Figures 1-3, respectively, differed in the levels of grinding prior to loading the sample between two pieces of tape. In each of Figures 1-3, panels a and b involved no sample rotation, whereas panel c involved sample rotation about an axis perpendicular to the sample tape, thus giving some averaging of the effects of preferred orientation. It is clear that sample X (Figure 1) has severe preferred orientation, giving a severely nonuniform intensity distribution around each ring in the powder diffraction photograph [Figure 1a], and substantial

discrepancies between the experimental and calculated16 powder diffraction patterns [Figure 1b,c]. In contrast, sample Z (Figure 3) has no significant preferred orientation, as evident from the uniform intensity distribution around each ring in the powder diffraction photograph [Figure 3a] and the good agreement (with essentially zero difference trace) between the experimental and calculated powder diffraction patterns [Figures 3b,c]. Although sample Y (Figure 2) exhibits only a relatively small degree of preferred orientation [with comparatively small differences between experimental and calculated powder diffraction patterns

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Figure 4. Result from Rietveld refinement on the powder X-ray diffraction pattern recorded (with sample rotation) for sample X of galvinoxyl, with refinement of a preferred orientation parameter.

Figure 5. Result from Rietveld refinement on the powder X-ray diffraction pattern recorded (with sample rotation) for sample Y of galvinoxyl, with refinement of a preferred orientation parameter.

(Figure 2b,c)], the preferred orientation is nevertheless detectable from the nonuniform intensity distributions around each ring in the powder diffraction photograph [Figure 2a]. Of the three different sample preparations, only sample Z would be considered acceptably free of preferred orientation for use in structure determination. For samples X and Y, sample rotation gives rise to some averaging of the effects of the preferred orientation, but does not completely eliminate it [compare Figure 1, panel b with c, and Figure 2, panel b with c]. Next we assess whether the preferred orientation in samples X and Y can be handled within the context of Rietveld refinement calculations (using the GSAS program17) by refinement of a preferred orientation parameter, using the powder diffraction patterns (recorded with sample rotation) in Figures 1c and 2c. In these calculations, the fractional coordinates and atomic displacement parameters were fixed at those in the published structure of galvinoxyl,18 whereas the unit cell parameters, zero-point error, profile parameters (peak shape, peak width, and asymmetry) and overall scale factor were refined together with the preferred orientation parameter (preferred orientation direction [110]) according to the March-Dollase method.7 The final refined value of the preferred orientation parameter was 1.50 for sample X and 1.04 for sample Y, reflecting the greater extent of preferred orientation in sample X. As shown in Figures 4 (sample X) and 5 (sample Y), refinement of the preferred orientation parameter leads to improved fit between experimental and calculated powder diffraction patterns [sample X: Rwp ) 0.156 (Figure 1c) without and Rwp ) 0.112 (Figure 4) with refinement of preferred orientation parameter; sample Y: Rwp ) 0.088 (Figure 2c) without and Rwp ) 0.057 (Figure 5) with refinement of preferred orientation parameter]. Nevertheless, the fits obtained with refinement of a preferred orientation parameter for samples X and Y are not as good as the fit between the experimental and calculated powder diffraction patterns for sample Z [Rwp ) 0.029 (Figure 3c) with no preferred orientation parameter]. Thus, the Rietveld refinement of highest quality is obtained using the experimental powder diffraction pattern that is free of preferred orientation, rather than by using experimental data that

are affected by preferred orientation and correcting for the preferred orientation during the Rietveld refinement. 4. Concluding Remarks In principle, any X-ray diffraction apparatus that allows the powder diffraction rings (or sections of these rings) to be measured may be used as the basis of the test for preferred orientation described here. The technique is not in any way specific to the particular device (STOE Reciprocal Lattice Explorer) employed in our work, and we have also used an image-plate single crystal X-ray diffractometer (Rigaku R-Axis II) in this type of application. In view of the additional challenges introduced into the structure determination process when the experimental powder diffraction data are affected by preferred orientation, we recommend that the simple test described in this paper should be applied routinely before recording powder diffraction data for use in such applications. Acknowledgment. We are grateful to EPSRC (general support to K.D.M.H.), Purdue Pharma LP (postdoctoral fellowship to E.Y.C.), and the National Science Foundation (Grant No. DMR-0089257 to B.M.F.) for supporting this research, to Professor K. Awaga (University of Nagoya) for providing the sample of galvinoxyl used in this work, and to Dr B.M. Kariuki for helpful discussions. References (1) Cheetham, A. K.; Wilkinson, A. P. Angew. Chem., Int. Ed. Engl. 1992, 31, 1557. (2) Harris, K. D. M.; Tremayne, M. Chem. Mater. 1996, 8, 2554. (3) Langford, J. I.; Loue¨r, D. Rep. Prog. Phys. 1996, 59, 131. (4) Poojary, D. M.; Clearfield, A. Acc. Chem. Res. 1997, 30, 414. (5) Meden, A. Croat. Chem. Acta 1998, 71, 615. (6) Harris, K. D. M.; Tremayne, M.; Kariuki, B. M. Angew. Chem., Int. Ed. 2001, 40, 1626. (7) Dollase, W. A. J. Appl. Crystallogr. 1986, 19, 267. (8) Toraya, H. J. Appl. Crystallogr. 1986, 49, 440. (9) Tremayne, M.; Kariuki, B. M.; Harris, K. D. M., manuscript in preparation. (10) Aakeroy, C. B.; Beatty, A. M.; Tremayne, M.; Rowe, D. M.; Seaton, C. C. Cryst. Growth Des. 2001, 1, 377. (11) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl. Crystallogr. 1994, 27, 1045.

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(12) Peschar, R.; Schenk, H.; Capkova´, P. J. Appl. Crystallogr. 1995, 28, 127. (13) Wessels, T.; Baerlocher, C.; McCusker, L. B. Science 1999, 284, 477. (14) STOE Reciprocal Lattice Explorer: flat film cassette placed in the position normally occupied for precession photography and positioned perpendicular to the incident X-ray beam (with µ ) 0°); Ni-filtered CuKR radiation; generator settings, 40 kV, 20 mA; sample-to-film distance, 37 mm; typical exposure time, 45 min. Note that this instrument is normally used to record single-crystal X-ray diffraction photographs of either precession or de Jong-Bouman type. (15) Siemens D5000 diffractometer: transmission geometry; primary Ge-monochromated CuKR1 radiation; linear position-sensitive detector covering 8° in 2θ; step size ∆2θ ) 0.0194°.

Cheung et al. (16) The comparison of experimental and calculated powder diffraction patterns in Figures 1b,c, 2b,c, 3b,c was made within the GSAS program [17] by fixing the fractional coordinates and atomic displacement parameters at those in the published structure of galvinoxyl [18] (determined from single-crystal X-ray diffraction at ambient temperature) and carrying out refinement of unit cell parameters, zero-point error, profile parameters (peak shape, peak width, and asymmetry) and overall scale factor. (17) Larson, A. C.; Von Dreele, R. B. Los Alamos Laboratory Rep. No. LA-UR-86-748, 1987. (18) Williams, D. E. Mol. Phys. 1969, 16, 141.

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