Why does the sample holder matter so much in powder XRD?

The holder is part of the optical system whether you want it to be or not. If it does not suit the sample, it can add background, produce holder peaks, or change the measured peak intensity ratios. That can lead to wrong conclusions in both qualitative and quantitative work. The holder should be chosen based on the sample’s composition, absorption, physical form, and air sensitivity.

In powder XRD, what is the best approach for low-absorption powders such as carbon or many organic materials?

These materials allow X-rays to penetrate more deeply, so the beam can reach the holder and scatter from it. That is why glass holders can produce a broad halo around 2θ = 20–30°. A shallow holder, especially around 0.2 mm depth, helps reduce peak broadening and asymmetry. A Si zero-background holder is often a better option when low background is important.

What particle size is usually best for powder XRD?

A practical target is about 10 μm. If particles are much larger, the diffraction rings become spotty and the measured intensity ratios become less reliable because too few crystallites are sampled. If particles are ground too finely, especially to around 3 μm or below, peak broadening increases and peak-top intensity decreases because of small crystallite size and grinding effects.

How can preferred orientation be reduced in powder XRD?

One effective method is transmission measurement using a capillary or films, because the preparation pressure is lower than in a typical pressed reflection sample. Sample spinning helps further by averaging out orientation and coarse-grain effects. In reflection geometry, back-loading into a ring-shaped holder can also help because the sample surface is not smoothed from the top, which reduces orientation bias and improves reproducibility.

Why do X-ray diffraction peaks shift even when the sample composition has not changed?

A common reason is sample height error, also called eccentricity. In reflection geometry, the sample surface must sit at the goniometer’s rotation center. If the surface is too low, peaks shift to lower angles; if it is too high, peaks shift to higher angles. The effect is strongest at low 2θ and becomes more serious when using a smaller goniometer radius.

How should the scan range for a powder XRD measurement be chosen?

A quick preliminary scan is the safest starting point. The final range should begin slightly below the lowest-angle diffraction peak so no real peaks are missed. For many materials, measuring farther into the high-angle region improves reliability because more peaks are included. As a general guide, upper limits around 2θ = 60° are often suitable for organics, while inorganics often benefit from going to about 2θ = 90°.

How should step size and scan speed for an XRD measurement be set?

Step size should be small enough to preserve peak shape but not so tiny that counting statistics become noisy. A good rule is to use about one-seventh to one-tenth of the FWHM of the sharpest peak seen in the preliminary scan. Scan speed should then be chosen so the strongest peak reaches at least about 3,000 counts for qualitative work and around 10,000 to 20,000 counts for quantitative work. Slower scans can also reveal weak peaks that disappear in fast scans.

How can you prepare samples and collect powder XRD data for air-sensitive materials?

They should be prepared and sealed in an air-tight holder under inert atmosphere, typically in a glove box. That prevents reaction with oxygen or moisture during measurement. A knife-edge holder can lower background in the low-angle region, but it also reduces peak intensity above about 2θ = 60°, so the holder design should match the intended scan range.

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Powder X-ray Diffraction Basic Course - Third Installment: Sample Preparation and Measurement Conditions to Obtain High-quality Data

Masashi Omori

Summer 2021 Volume 37, No. 2 , 21-25

In the second installment of the powder X-ray diffraction (PXRD) basic course, how to select instrument configurations to obtain high-quality data was described. This third installment provides information on how to prepare samples and determine the best measurement conditions to obtain high-quality data. Regarding sample preparations, the type of sample holders, the effect of grain size of the sample, and the impact of the eccentricity and asperity of the sample surface must be taken into consideration.

In most PXRD measurements, a glass sample holder is usually filled with sample. However, other appropriate sample holders should sometimes be selected based on the chemical properties, composition, and shape of the sample, which will allow you to obtain high-quality data with high resolution and low background. Also, the impact of preferred orientation can sometimes be suppressed. Appropriate optics should also be used for the selected sample holder. The following chapters and sections will describe the details of sample preparation and measurement conditions, along with a few tricks and traps.

Highlights

The choice of sample holder can change background, introduce extra peaks, and distort intensity ratios enough to affect phase identification and quantification.
For low-absorption materials such as carbon or many organics, shallow holders and zero-background holders help reduce penetration-related peak distortion and holder scattering.
Powder that is too coarse gives unstable, orientation-biased intensities; powder that is too finely ground broadens peaks and lowers peak-top intensity. Around 10 μm is a practical target.
In Bragg-Brentano geometry, sample surface flatness and correct height are critical because eccentricity shifts peak positions, especially at low angles.
A fast preliminary scan is the best way to set the final scan range, step size, and speed so weak peaks are not missed and counting statistics are strong enough

Summary

Good powder XRD data depends just as much on sample preparation as on the instrument itself. The holder should match the sample type, because the wrong holder can add background or even create unwanted diffraction peaks. Light-element powders often need shallow or zero-background holders, coarse or strongly oriented powders often benefit from transmission setups or sample spinning, and air-sensitive materials need sealed holders loaded under inert conditions.

Particle size matters a lot. A powder that is too coarse can give misleading peak intensities because too few crystallites are contributing to the pattern. A powder that is too fine can also be a problem because over-grinding broadens peaks and reduces intensity. A reasonably fine, uniform powder is usually best.

Sample geometry matters too. For reflection measurements, the sample surface should be flat and positioned at the correct height. If it sits too high or too low, the diffraction peaks shift, which can make the pattern look wrong even when the material is correct.

A sensible workflow is to run a quick scan first, then use that result to choose the final 2θ range, step size, and scan speed. Step size should be small enough to reproduce the peak shape, and scan speed should be slow enough to collect enough counts to clearly show weak peaks.

Frequently asked questions

: The holder is part of the optical system whether you want it to be or not. If it does not suit the sample, it can add background, produce holder peaks, or change the measured peak intensity ratios. That can lead to wrong conclusions in both qualitative and quantitative work. The holder should be chosen based on the sample’s composition, absorption, physical form, and air sensitivity.
: These materials allow X-rays to penetrate more deeply, so the beam can reach the holder and scatter from it. That is why glass holders can produce a broad halo around 2θ = 20–30°. A shallow holder, especially around 0.2 mm depth, helps reduce peak broadening and asymmetry. A Si zero-background holder is often a better option when low background is important.
: A practical target is about 10 μm. If particles are much larger, the diffraction rings become spotty and the measured intensity ratios become less reliable because too few crystallites are sampled. If particles are ground too finely, especially to around 3 μm or below, peak broadening increases and peak-top intensity decreases because of small crystallite size and grinding effects.
: One effective method is transmission measurement using a capillary or films, because the preparation pressure is lower than in a typical pressed reflection sample. Sample spinning helps further by averaging out orientation and coarse-grain effects. In reflection geometry, back-loading into a ring-shaped holder can also help because the sample surface is not smoothed from the top, which reduces orientation bias and improves reproducibility.
: A common reason is sample height error, also called eccentricity. In reflection geometry, the sample surface must sit at the goniometer’s rotation center. If the surface is too low, peaks shift to lower angles; if it is too high, peaks shift to higher angles. The effect is strongest at low 2θ and becomes more serious when using a smaller goniometer radius.
: A quick preliminary scan is the safest starting point. The final range should begin slightly below the lowest-angle diffraction peak so no real peaks are missed. For many materials, measuring farther into the high-angle region improves reliability because more peaks are included. As a general guide, upper limits around 2θ = 60° are often suitable for organics, while inorganics often benefit from going to about 2θ = 90°.
: Step size should be small enough to preserve peak shape but not so tiny that counting statistics become noisy. A good rule is to use about one-seventh to one-tenth of the FWHM of the sharpest peak seen in the preliminary scan. Scan speed should then be chosen so the strongest peak reaches at least about 3,000 counts for qualitative work and around 10,000 to 20,000 counts for quantitative work. Slower scans can also reveal weak peaks that disappear in fast scans.
: They should be prepared and sealed in an air-tight holder under inert atmosphere, typically in a glove box. That prevents reaction with oxygen or moisture during measurement. A knife-edge holder can lower background in the low-angle region, but it also reduces peak intensity above about 2θ = 60°, so the holder design should match the intended scan range.