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

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.