What is crystallinity?

Crystallinity is the proportion of a material that exists in an ordered crystal structure. A fully amorphous material would have 0% crystallinity, while a material with more ordered regions has a higher value. In PXRD, it is estimated from the relative contributions of crystalline peaks and the amorphous halo.

How does XRD distinguish crystalline and amorphous material?

Crystalline regions produce sharp, well-defined diffraction peaks because their atomic arrangement is periodic. Amorphous regions do not have long-range order, so they produce a much broader scattering feature, usually called a halo. Crystallinity analysis works by separating these two contributions within the same diffraction pattern.

Is a reference sample required to measure crystallinity this way?

No. This approach can be used without pure crystalline and pure amorphous reference materials. Instead, the measured pattern is fitted into crystalline peaks and an amorphous halo. That only works well when the sample meets key assumptions, such as having the same chemical composition in the crystalline and amorphous portions and a halo that can be reasonably identified.

Why can crystallinity results vary from one analyst to another?

The main reason is that the result depends on how the background and amorphous halo are modeled. When the halo is weak or overlaps heavily with crystalline peaks, different fitting choices can shift the calculated crystallinity. Reproducibility improves when analysts use fixed fitting conditions, smooth background models, and consistent parameter settings.

Why is the sample holder important?

The holder can add its own diffraction or halo-like scattering. Glass holders are especially problematic because they can contribute a broad feature in the same angular region where the sample’s amorphous halo may appear. For low-absorbing organic materials, X-rays can penetrate deeply enough that holder signals become more noticeable, so zero-background holders are often preferred.

What makes oriented samples harder to analyze?

In oriented materials such as polymer fibers or films, diffraction intensity depends strongly on sample orientation. That means peak intensities may not be reproducible in a standard setup, even if the sample composition is the same. Transmission methods and 2D-WAXS are useful because they collect a broader portion of the diffraction rings and reduce errors caused by orientation effects.

How can reproducibility be improved?

Good reproducibility starts with high-quality data: proper slit settings, reduced air scatter, and subtraction of holder background all help. During analysis, it helps to choose stable background regions, keep the background curve smooth, and fix halo parameters when the halo is hard to see. Using templates for multiple datasets also makes results more consistent across samples.

What do the example results in this paper show?

The example results show that the method can distinguish meaningful differences among real materials. POM was measured at 68% crystallinity, a polypropylene fiber at 50%, HDPE at 72%, and LDPE at 49%. These values illustrate that materials with different structures and processing histories can show clearly different crystalline fractions.

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Powder X-ray Diffraction Basic Course - Eighth Installment: Crystallinity

Kasumi Kihara

Summer 2025 Volume 41, No. 2 , 11-15

In the eighth lecture of the Powder X-ray Diffraction Basic Course, we will describe “Crystallinity.” Crystallinity is a parameter that indicates the degree of crystallinity of a sample, such as a pharmaceutical or polymeric material, and can be estimated from a powder X-ray diffraction (PXRD) pattern. It is defined as the ratio of the crystalline phase to the total weight (crystalline phase+amorphous phase). The percent crystallinity (reported as %crystallinity) is evaluated from the difference in profiles between crystalline and amorphous phases. In this paper, we describe a method for calculating the crystallinity by decomposing peaks using a profile fitting method. This method is performed by separating diffraction peaks from the crystalline phase and haloes due to scattering from the amorphous phase, and using the integrated intensity obtained by profile fitting. This method does not require pure crystalline and amorphous materials. However, the results can be influenced by the analyst’s subjectivity, as the crystallinity varies depending on how the halo is estimated. High reproducibility values can be obtained independent of the analyst by carefully selecting the parameters that determine the background estimation and the peak shape of halo.

Highlights

Crystallinity is the fraction of a sample that is crystalline rather than amorphous.
In PXRD, sharp peaks come from crystalline regions and a broad halo comes from amorphous regions.
Crystallinity can be calculated by separating peak intensity from halo intensity using profile fitting.
The biggest challenge is getting a consistent background and halo estimate, especially for highly crystalline samples.
Measurement setup matters: holder choice, slit settings, and sample orientation can change the result.

Summary

Crystallinity tells you how much of a material has an ordered crystal structure and how much is disordered, or amorphous. In X-ray diffraction, crystalline material gives sharp diffraction peaks, while amorphous material produces a broad hump called a halo. By separating those two parts of the pattern, you can estimate the percent crystallinity of the sample.

The basic calculation compares the scattering from the crystalline part with the total scattering from both crystalline and amorphous parts. In practice, this is done with profile fitting rather than by just looking at peak heights. That makes the method useful even when crystalline peaks and the amorphous halo overlap, and it can be done without pure crystalline and pure amorphous reference materials.

Accurate results depend heavily on measurement quality and analysis choices. Background scattering from air or the sample holder can interfere with the pattern, and preferred orientation in materials such as fibers or films can distort peak intensities. Reproducibility improves when the background is treated carefully, halo-shape parameters are constrained when needed, and the same fitting approach is applied consistently across samples.

Examples show how this works in real materials: POM was measured at 68% crystallinity, a polypropylene fiber at 50%, HDPE at 72%, and LDPE at 49%.

Frequently asked questions

: Crystallinity is the proportion of a material that exists in an ordered crystal structure. A fully amorphous material would have 0% crystallinity, while a material with more ordered regions has a higher value. In PXRD, it is estimated from the relative contributions of crystalline peaks and the amorphous halo.
: Crystalline regions produce sharp, well-defined diffraction peaks because their atomic arrangement is periodic. Amorphous regions do not have long-range order, so they produce a much broader scattering feature, usually called a halo. Crystallinity analysis works by separating these two contributions within the same diffraction pattern.
: No. This approach can be used without pure crystalline and pure amorphous reference materials. Instead, the measured pattern is fitted into crystalline peaks and an amorphous halo. That only works well when the sample meets key assumptions, such as having the same chemical composition in the crystalline and amorphous portions and a halo that can be reasonably identified.
: The main reason is that the result depends on how the background and amorphous halo are modeled. When the halo is weak or overlaps heavily with crystalline peaks, different fitting choices can shift the calculated crystallinity. Reproducibility improves when analysts use fixed fitting conditions, smooth background models, and consistent parameter settings.
: The holder can add its own diffraction or halo-like scattering. Glass holders are especially problematic because they can contribute a broad feature in the same angular region where the sample’s amorphous halo may appear. For low-absorbing organic materials, X-rays can penetrate deeply enough that holder signals become more noticeable, so zero-background holders are often preferred.
: In oriented materials such as polymer fibers or films, diffraction intensity depends strongly on sample orientation. That means peak intensities may not be reproducible in a standard setup, even if the sample composition is the same. Transmission methods and 2D-WAXS are useful because they collect a broader portion of the diffraction rings and reduce errors caused by orientation effects.
: Good reproducibility starts with high-quality data: proper slit settings, reduced air scatter, and subtraction of holder background all help. During analysis, it helps to choose stable background regions, keep the background curve smooth, and fix halo parameters when the halo is hard to see. Using templates for multiple datasets also makes results more consistent across samples.
: The example results show that the method can distinguish meaningful differences among real materials. POM was measured at 68% crystallinity, a polypropylene fiber at 50%, HDPE at 72%, and LDPE at 49%. These values illustrate that materials with different structures and processing histories can show clearly different crystalline fractions.