Determining the Small Angle Diffraction Capabilities of XRD

The calculation of reflections at low angles is an essential component of multiple procedures that involve compounds with extensive structural properties. These compounds have gained attention within various sectors, including polymer research and production, catalyst development and certain mining processes. As it comprises well-aligned fatty acid chains, constituting a complex network (c.f., Figure 1), Agbehenate has become a commonly utilized reference compound for low angle diffraction experiments.

Figure 1: Projection of the crystalllographic structure of Ag-behenate along the b-axis.

Instrument

The Thermo Scientific™ ARL™ EQUINOX 100 uses a bespoke Cu (50 W) or Co (15 W) micro-focus X-ray tube fitted with mirror optics. The minimal power the unit consumes, which negates the requirement for an external water chiller, enables it to be totally portable. This optimal portability similarly enables inter-laboratory transportation, without the requirement for any specific infrastructure.

The ARL EQUINOX 100 offers particularly rapid data acquisition rates in comparison to competing diffractometers as a result of its distinctive curved position sensitive detector (CPS). This enables the instrument to measure all diffraction peaks simultaneously, in real time, and is thus ideally suited for both reflection and transmission calculations (Figure 2).

Figure 2: ARL EQUINOX 100 X-ray diffractometer.

Experimental

A specimen of Ag-behenate was inserted into a glass capillary (0.5 mm diameter; 10 min), and measurements were taken in both transmission mode, utilizing Cu Ka radiation, and in reflection mode (3 mm depth; 5 min), utilizing Co Ka radiation.

Results

The implementation of transmission mode enables the explicit resolution of the [001] reflection of Ag-behenate at 1.51°2θ (d = 58.35 Å), up to the [009] reflection at 13.65°2θ (d = 6.48 Å), with an anticipated FWHM of ~0.25°2θ (c.f., Figure 3). Comparatively, reflection mode (1° incidence angle) yields similar results, but whereas the anticipated FWHM is ~0.35°2θ, the measurement time has decreased by a factor of two (c.f., Figure 4).

Figure 3: Measurement of Ag-behenate in transmission mode (0.5 mm capillary, 10 min measurement time).

Figure 4: Measurement of Ag-behenate in reflection mode (3 mm cup, 5 min measurement time).

Conclusion

The ARL EQUINOX 100 provides effective apparatus to calculate reflections at low angles, down to ~1.5°2θ, as demonstrated in the case of Ag-behenate, which therefore enables the examination of polymers for phase determination.

Moreover, the instrument permits assignment of the structure of mesostructured compounds, which are often utilized as catalysts or substrates. The structure directly influences the catalytic activity of such compounds.

Author: Dr. Simon Welzmiller, XRD Application Specialist

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers.

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