COMPARISON BETWEEN CONVENTIONAL ANDTWO-DIMENSIONAL XRD

Bob B. He, Uwe Preckwinkel, and Kingsley L. Smith

Bruker Advanced X-ray SolutionsMadison, Wisconsin 53711-5373

ABSTRACTA comparison between the conventional Bragg-Brentano diffractometer and the two-dimensional(2D) diffractometer is made in terms of diffraction geometry, diffraction pattern, and variousapplications. A two-dimensional XRD system for combinatorial screening in transmission modeis introduced.

IINTRODUCTIONA two-dimensional X-ray diffraction (XRD2) system has both the capability of acquiringdiffraction patterns in 2D space simultaneously, and analyzing the 2D diffraction dataaccordingly [1-3]. In recent years, usage of two-dimensional (2D) detectors has dramaticallyincreased due to the advances in detector technology, point beam X-ray optics, and computingpower. The integrated data gives better intensity and statistics for phase identification andquantitative analysis, especially for those samples with texture, large grain size, or small quantity.2D detectors can collect diffraction data in a large 2θ range simultaneously without sample ordetector movement. This is extremely important for high-throughput diffraction screening [4].

Figure 1 is a comparison between the conventional diffractometer and the two-dimensionaldiffractometer. The diffraction data collection in the conventional diffractometer is confinedwithin a plane, here referred to as diffractometer plane. With a 2D detector, the measurablediffraction is no longer limited in the diffractometer plane. Instead, the whole or a large portionof the diffraction rings (as called Debye ring) can be measured simultaneously.

Figure 1. Comparison of the conventional XRD and the two-dimensional XRD.

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The conventional diffraction pattern, collected with either a scanning point detector or a linearPSD, is a plot of X-ray scattering intensity at different 2θ angles. Figure 2(a) shows theconventional diffraction pattern of corundum powder. Figure 2(b) shows the diffraction patterncollected with a 2D detector from the same corundum sample. The 2D diffraction patterncontains far more information then the conventional diffraction pattern for applications, such as:Phase ID; Percent Crystallinity; Particle Size and Shape; Texture; and Stress [5].

Corundum Powder Diffraction

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

20 25 30 35 40 45 50

Two Theta

Inte

nsity

(a) (b)Figure 2. The diffraction pattern of corundum powder: (a) the conventional diffraction pattern;

(b) the two-dimensional diffraction pattern.

LARGE GRAIN SIZE AND TEXTUREUnlike the conventional diffraction profile, the effects of large grain size and texture can beobserved directly from 2D diffraction patterns. Figure 3 shows an example. The large grain in aγ-TiAl sample gives a spotty diffraction ring (a), while a textured γ-TiAl sample shows theintensity variation. Reliable phase ID can be obtained from both samples by integration.

(a) (b)

Figure 3. 2D diffraction patterns from γ-TiAl alloys: (a) large grain size; (b) texture.

Copyright©JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46. 38 ISSN 1097-0002

PERCENT CRYSTALLINITYThe accuracy of measured percent crystallinity is dependent on the integrated diffraction profile.Since most crystallinity samples have preferred orientation, it is very difficult to have a consistentcrystallinity measurement with a conventional diffractometer. Figure 4 shows a 2D diffractionframe collected from an oriented polycrystalline sample. The diffraction is in transmission modewith the X-ray beam perpendicular to the plate sample surface. Figure 4(a) shows a diffractionprofile integrated from a horizontal region analogous to a profile collected with the conventionaldiffractometer. Only one crystalline peak can be observed from the profile. It means that thediffraction profile would look like the one in Figure 4(a) had a conventional diffractometer beenused for the data collection in the same sample orientation. It is also possible that a different peakor no peak is measured if the sample is loaded in another orientation. Figure 4(b) is thediffraction profile integrated from all parts of the 2D frame. A total of four crystalline peaks areobserved. Apparently, the percent crystallinity measured with the conventional diffractometer isnot consistent if the preferred orientation is not considered. While the sample orientation has noeffect on the full circle integrated diffraction profile from a 2D frame, an XRD2 system canmeasure percent crystallinity more accurately with consistent results.

(a) (b)Figure 4. 2D diffraction pattern from an oriented polycrystalline sample. (a) Diffraction profile

integrated from a horizontal region analogous to a profile collected with pointdetector. (b) Diffraction profile integrated from all parts of the 2D frame.

Copyright©JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46. 39 ISSN 1097-0002

INSTRUMENT BROADENINGConventional diffractometers use the Bragg-Brentano parafocusing geometry [6] as is show inFigure 5. A divergent beam from the X-ray tube passes first through the divergent slit, then hitsthe sample surface with an incident angle θ. The incident X-rays spread over the sample surfacewith various incident angles in the vicinity of θ. The area of irradiated region depends on theincident angle θ and beam divergence. The diffracted rays from the irradiated area leave thesample at an angle 2θ from the corresponding incident rays, pass through the anti-scatter slit, andfocus at the detector slit. The instrument broadening can be controlled by both slit size andscanning step.

Figure 5. A conventional diffractometer in Bragg-Brentano geometry

In an XRD2 system, the diffracted X-rays are measured simultaneously in a two-dimensionalrange so no slit or scanning step can be used to control the instrument broadening. The beam-spread over the sample surface can not be focused back to the detector. Figure 6 shows geometryof two-dimensional diffraction in reflection mode (a) and transmission mode (b). Defocusingeffect is observed with low incident angle over a flat sample surface in reflection modediffraction. In reflection mode, the diffracted beam in low 2θ angle is narrower than thediffracted beam in high 2θ angle. In transmission mode with the incident beam perpendicular tothe sample surface, no such a defocusing effect is observed.

(a) (b)Figure 6. Geometry of XRD2: (a) reflection mode; (b) transmission mode.

If one looks at the cross section on the diffractometer plane and forward diffraction (2θ<90°)only, the defocusing effect with reflection mode diffraction can be expressed as:

Copyright©JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46. 40 ISSN 1097-0002

Bb

= −sin( )sin2θ ω

ω(1)

where ω is the incident angle, b is the incident beam size and B is diffracted beam size. Thedefocusing with transmission mode with a perpendicular incident beam can be given as:

Bb

tb

= +cos sin2 2θ θ (2)

where t is the sample thickness. If the sample thickness t is negligible compared to the incidentbeam size b, we have:

Bb

= ≤cos2 1θ (3)

There should be no defocusing effect at all. Figure 7 is a comparison between reflection modeand transmission mode diffraction with data frames collected from corundum powder. With 5°incident angle (a), the reflection pattern shows severe peak broadening compared with nodefocusing in transmission mode pattern (b).

(a) (b)

Figure 7. Diffraction pattern from corundum: (a) reflection mode diffraction 5° incident angle,(b) transmission mode diffraction with perpendicular incident beam.

TWO-DIMENSIONAL XRD FOR COMBINATORIAL SCREENINGA two-dimensional diffraction system designed for XRD combinatorial screening in reflectionmode has been introduced previously [4]. In many combinatorial screening applications, such aspolymorphism study in pharmaceutical chemistry and catalysis development in the oil industry, atypical 2θ measuring range is from 2 to 60°. It is necessary to run the combinatorial XRDscreening in transmission mode in order to avoid the defocusing effect. A two-dimensionaldiffraction system is designed for XRD screening in transmission mode for various applications,including screening of material libraries for combinatorial chemistry. As is shown in Figure 8,the system is built on a vertical two-circle goniometer. An offset mounted XYZ translation stageyields space for X-ray source, optics, and X-ray detector, while it provides translations in X, Yand Z directions for material library scanning and sample alignment. A laser/video samplealignment system is mounted on the outer circle of the goniometer so that it can be driven away

Copyright©JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume 46. 41 ISSN 1097-0002

after alignment. An optional motorized beamstop has two positions, retracted position forloading, unloading and aligning sample, and extended position during diffraction and scatteringmeasurement. In the transmission mode X-ray diffraction measurement, the incident beam istypically perpendicular to the sample so the irradiated area on the specimen is limited to a sizecomparable to the X-ray beam size, allowing the X-ray beam concentrated to the intendedmeasuring area. In combinatorial screening applications, sample cells are located close to eachother. Therefore, The transmission mode diffraction can also avoid cross contamination betweenadjacent samples.

Figure 8. Transmission diffraction system for combinatorial screening.

CONCLUSIONThe two-dimensional diffraction provides far more information than the conventional one-dimensional diffraction. However, in order to utilize the extra information gained from 2Ddiffraction, distinct features of two-dimensional diffraction must be taken into account.

REFERENCES[1] Philip R. Rudolf and Brian G. Landes, Two-dimensional X-ray Diffraction and Scattering of

Microcrystalline and Polymeric Materials, Spectroscopy, 9(6), pp 22-33, July/August 1994.[2] S. N. Sulyanov, A. N. Popov and D. M. Kheiker, Using a Two-dimensional Detector for X-

ray Powder Diffractometry, J. Appl. Cryst. 27, pp 934-942, 1994.[3] B. B. He, U. Preckwinkel and K. L. Smith, Fundamentals of Two-dimensional X-ray

Diffraction (XRD2), Advances in X-ray Analysis, Vol. 43, Proceedings of the 48th AnnualDenver X-ray Conference, Steamboat Springs, Colorado, USA, 1999.

[4] B. B. He, et al, XRD Rapid Screening System for Combinatorial Chemistry, Advances in X-ray Analysis, Vol. 44, Proceedings of the 49th Annual Denver X-ray Conference, Denver,Colorado, USA, 2000.

[5] B. B. He, Introduction to 2D XRD, Bruker AXS Document # M86-E00055.[6] Ron Jenkins and Robert L. Snyder, Introduction to X-ray Powder Diffractometry, John Wiley

& Sons, New York, 1996.

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