Comparison of Laboratory and Energy-Dispersive Diffraction Data

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Comparison of Laboratory and Energy-Dispersive Diffraction Data

A direct comparison of angle- and energy-dispersive diffraction data is not straightforward because the experimental conditions and objectives are usually quite different in the two cases. Data from both types of experiment may be plotted as a function of 1/d (as shown below), but care should be taken in making comparisons by this method. In particular, the ability to resolve peaks depends on the choice of wavelength in the angle-dispersive case and on the choice of detector angle for the energy-dispersive case. One solution is to compare data sets in which the data have been collected over a near identical range of d spacings. Thus an energy-dispersive instrument with the detector set to, say, 6° and a maximum detector energy of, say, 150 keV, will measure d spacings down to 0.790 Å: this is equivalent to measuring with an angle-dispersive diffractometer to an angle of 160° using λ = 1.55 Å.

The first data set shown below is of the standard yttria sample measured by energy-dispersive diffraction with the detector at 6° (as shown earlier), but with the data plotted in 1/d. You will notice that the intensity of the peaks is strongly influenced by the wavelength spectrum of the incident white X-ray beam and by absorption effects; the intensity of the peaks with large d spacings is near zero due to the fact that they were measured with relatively low-energy X-rays that have been absorbed by the sample, while the intensity of the peaks with small d spacings is reduced by the fact that the flux of incident X-rays is much reduced for the highest energies measured by the detector.

For comparison, a laboratory data set on the same sample is shown directly above collected with Cu Kα₁ radiation (λ = 1.540 Å) to a scattering angle of 160°. Given that the peaks are much sharper in the laboratory data, you may wonder about the value of energy-dispersive methods. One big difference is sample penetration, which allows the energy-dispersive experiment to be sensitive to the bulk, while the angle-dispersive case involves only a small penetration below the surface.

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