The D8 DISCOVER system, integrated with the INCOATEC IµSHigh Brilliance Microfocus source, is a new X-ray diffraction (XRD) solution that is suitable for characterizing multipurpose modern materials research. This article illustrates the use of the system in a high resolution powder diffraction configuration to perform phase identification (Phase ID) and quantitative Rietveld analysis, through the analysis of polycrystalline materials or powders in reflection geometry.
This article reports the signal to background ratio, and the resolution acquired from the SRM 1976b standard corundum sample, in addition to results from real-world experiments such as; retained austenite quantification, the Rietveld refinement of geological samples, and measurement of standard pharmaceutical materials.
Figure 1 shows the configuration to perform powder diffraction, and details of the configuration are presented in Table 1. The IµSHigh Brilliance, integrated with the MONTEL-P optic, generates a brilliant primary beam with a spot size diameter of 1 mm, which can be optimized for maximum resolution (0.2 mm slit) or maximum flux on the sample (1 mm slit) using a linear slit in the beam path. For tiny probes the collimator can be exchanged, which results in the smallest probe size measuring 50 µm. During measurement the “rastering” of larger samples is done to enhance particle statistics. This is achieved by employing the motorized XY translation of the Centric Eulerian Cradle (CEC) stage so that the diffraction patterns highly represent the present phases. Here the rotation (Phi) and tilt (Psi) of the CEC stage can be combined with the XY raster motion to enable all possible orientations of the crystalline materials to the beam, and to reduce the impact of a preferred orientation to produce a randomized powder pattern from samples of different sizes and morphologies. Identical to the classic Gandalfi camera configuration, this technique cannot be carried out with line focused beams because of the defocusing effects that occur when the sample is tilted.
Figure 1. D8 DISCOVER with IµSHigh Brilliance and LYNXEYE XE
Table 1. Typical powder instrument setup for the D8 DISCOVER with IµSHigh Brilliance
||IìSHigh Brilliance Microfocus (Cu)
||0.1 – 1.0 mm
||Centric Eulerian Cradle (CEC)
Measurement of NIST SRM 1976b
Figure 2 illustrates a combined scan of the NIST standard reference material (SRM) 1976b (NIST SRM 1976b), which consists of a sintered plate of corundum that is determined at an angle of 20°-90° 2è, and step size of 0.01° at the rate of 1 second per step for a period of 2 hours. Resolution of the instrument (defined by the minimum Full Width at Half Maximum, FWHM) measured from the (104) reflection is less than 0.04° 2è. Importantly, the peaks at low and high angles do not display asymmetric broadening caused by axial divergence, even during the absence of axial soller slits on the primary or secondary side. This is because the integrated MONTEL-P optic produces a strong axial collimation. The peak to background ratio significantly impacts the sensitivity during weak peaks and trace phases. For the (104) reflection, this is around ~300. For this peak to background ratio, reflections of small relative intensities, such as (211) with a rating of <1% intensity found in the powder diffraction database, are clearly differentiated from the background.
Figure 2. Measurement of NIST 1976b with the D8 DISCOVER with IµSHigh Brilliance and LYNXEYE XE. The (104) reflection at 35° 2q has FWHM less than 0.04° 2q, while the < 1% relative intensity (211) reflection at 60° 2q demonstrates the exceptional Peak to Background
Measurement of Retained Austenite
A common method of powder diffraction employed in metallurgical industries is the quantification of the retained austenite in steel. Austenite remains in a high temperature phase, and may be trapped in the material because of the kinetic limitations of processing. A very high quantity of retained austenite results in dimensional instability, caused by a gradual conversion of austenite to ferrite. Figure 3a displays a scan of a steel specimen consisting of about 5% retained austenite. The enhanced energy discrimination of the LYNXEYE XE detector ensures the complete elimination of the iron fluorescence from the Cu radiation, enabling a lower background. Mapping of the retained austenite over the surface can be ensured by choosing the probe size of 0.5 mm by 1 mm.
Figure 3a. Measurement of a steel specimen for retained austenite quantification with the D8 DISCOVER with IµSHigh Brillianceand LYNXEYE XE. Background subtraction has not been performed
The global retained austenite content can also be measured by moving the sample in a raster motion using the CEC stage. The quantitative Rietveld refinement carried out in DIFFRAC.TOPAS of the austenite content is presented in Figure 3b.
Figure 3b. Rietveld refinement of the retained austenite content using DIFFRAC.TOPAS.
Measurement of Pharmaceutical Materials
Pharmaceutical industries commonly employ powder diffraction method to perform polymorph analysis, to ensure the phase purity of active ingredients. As the unit cell of an organic pharmaceutical material is bigger, most of the peaks are formed at a low angle, with typical scan ranges of 3°-40° 2è. Scanned images of 1 mg samples of Ibuprofen, Sucrose, and Acetaminophen are illustrated in Figure 4. The low divergence and small size of the beams make the background stay very low when the scan approaches 0° 2è, enabling the identification of phases at very low angles.
Figure 4. Measurement of various pharmaceutical materials with the D8 DISCOVER with IµSHigh Brilliance and LYNXEYE XE.
Measurement of Geological Materials
Quantitative mineralogy of geological samples can also be performed using powder diffraction. For reliable results fluorescence elimination and superior instrument resolution are mandatory, as the samples usually include iron and numerous crystallographic phases. Phase identification in DIFFRAC.EVA of 1 mg of a standard geological sample is shown in Figure 5a.
Figure 5a. Measurement of a geological sample with the D8 DISCOVER with IµSHigh Brilliance Microfocus source and LYNXEYE XE. Phase identification was performed in DIFFRAC.EVA
Narrow and well separated peaks are found from each phase, suggesting superior instrument resolution and a very low background even in the presence of a Pyrite phase. Good coupling of a small sample amount to the beam size of the IµSHigh Brilliance provides data comparable to data produced by a large sample, treated under conventional divergent beam geometry. The quantitative Rietveld fit of the data produced in DIFFRAC.TOPAS is presented in Figure 5b.
Figure 5b. Retveld Refinement of the geological sample using DIFFRAC.TOPA.
The instrument functions without any anomalies, such as axial divergence that can cause asymmetric peaks. This positive outcome can be attributed to the very low axial divergence of the primary beam, ensured by the MONTEL-P optic. The quantitative fitting process is highly simplified, as the number of parameters that need to be refined to account for the peak shapes is reduced.
The D8 DISCOVER, with IµSHigh Brilliance and LYNXEYE XE detector, were successfully employed to quantify powder samples for different industrial applications. The formation of a beam with immensely high brilliance and low divergence, using the IµSHigh Brilliance in combination with the MONTEL-P optic, results in a suitable configuration to measure samples with very small dimensions of 1 mg. The CEC stage enables rastering of tiny probes over large areas in the X and Y planes, increasing particle statistics and eliminating the preferred orientation effects by adjusting the rotation and tilt of the sample.
The 192 channels and improved electronic energy discrimination of the LYNXEYE XE position sensitive detector enable performing scans with unprecedented peak to background and signal to noise ratios. The D8 DISCOVER allows traditional powder diffraction analysis without the restrictions posed by the standard large area powder diffraction measurement geometries.
This information has been sourced, reviewed and adapted from materials provided by Bruker AXS Inc.
For more information on this source, please visit Bruker AXS Inc.