Measurements for PDF Analysis in Reflection Geometry

Introduction

Pair distribution function (PDF) analysis is a powerful technique for investigating the local structure of materials, irrespective of whether they are crystalline, amorphous, or liquid. PDF measurements, performed either at synchrotron facilities or on laboratory-based instruments, are typically carried out in transmission geometry (Debye-Scherrer geometry). Although PDF measurements can also be performed in reflection geometry (Bragg-Brentano geometry), this configuration is less commonly used.

The limited use of reflection geometry for PDF measurements can be attributed to several factors. First, hard X‑ray radiation from Mo or Ag anodes penetrates deeply into most materials, which may give rise to slight peak shifts caused by sample transparency effects. Second, background subtraction is more challenging, particularly when scattering from the sample holder contributes significantly to the measured signal. Third, when a constant divergence slit is used, the illuminated sample length varies with 2θ. If the sample is not homogeneous, this variation can lead to reduced counting statistics or inaccurate relative intensities.

Despite these challenges, reflection geometry can be advantageous for certain samples, particularly on laboratory-based systems. One key benefit is the reduced impact of X‑ray absorption. Even when very thin capillaries are used in transmission geometry, absorption effects can remain significant for materials with high molecular weight or high absorption coefficients. In contrast, reflection geometry restricts the penetration depth of the incident beam to a shallow region near the sample surface, thereby minimizing absorption-related intensity losses.

This application report demonstrates that, on XRDynamic 500, high-quality PDF measurements can also be successfully performed in reflection geometry for selected samples.