How to Read & Interpret an XRD Pattern | XRD Analyzer

For researchers in materials science and solid-state chemistry, an X-ray diffraction pattern is a structural fingerprint of a material. However, a raw diffraction scan is just a 1D line plot of intensity against the $2\theta$ scattering angle. Interpreting this data requires extracting structural parameters from the background, peak positions, and profiles. This article provides a step-by-step guide to reading and analyzing XRD patterns.

1. Understanding the Axes

The first step in reading an XRD pattern is understanding the axes of the plot:

X-Axis (2θ Angle): Represents the scattering angle (in degrees). The detector scans along this arc to measure diffracted X-rays. These positions are converted to interplanar spacings ($d$) using Bragg's Law.
Y-Axis (Intensity): Represents the count rate of diffracted photons (in counts or counts per second). Intensities depend on the atomic numbers and coordinates of the atoms in the unit cell.

Because raw counts depend on scan times and slit widths, intensities are often normalized relative to the strongest reflection (set to 100%) to simplify phase matching.

2. Analyzing the Baseline and Background

An XRD pattern consists of sharp diffraction reflections overlaid on a continuous background curve. This background is caused by several factors:

Air Scattering: X-ray interactions with air molecules create a continuous background, particularly at low angles (under 15° $2\theta$).
Sample Holder Scatter: Background contributions from sample holders, especially amorphous materials like glass slides.
Sample Fluorescence: Occurs when the X-ray energy matches an absorption edge of an element in the sample (e.g., using Copper radiation on Iron-rich samples). This fluorescence creates a high background that reduces the signal-to-noise ratio.

A broad, curved hump (amorphous halo) between 15° and 30° $2\theta$ indicates the presence of amorphous phases (like glass, polymers, or disordered structures) in the sample.

3. Identifying and Indexing Peak Positions

The angular positions of the peaks are determined entirely by the unit cell dimensions. Indexing is the process of assigning Miller indices ($hkl$) to each peak in the pattern.

Assigning these indices allows you to calculate the interplanar spacings ($d$) using:

d_hkl = λ / (2 sin(θ))

For cubic structures, the d-spacing is related to the lattice parameter ($a$) by:

d_hkl = a / √(h² + k² + l²)

By matching the d-spacings to these geometric equations, you can determine lattice parameters and identify unit cell geometries.

4. Phase Matching & Qualitative Analysis

Qualitative phase identification involves matching the peak positions and relative intensities of your sample to reference databases (like the ICDD or Crystallography Open Database libraries).

Each phase has a unique diffraction signature. If your sample is a mixture, its diffraction pattern will be a sum of the individual phase patterns. You can identify the phases present by matching peak coordinates to reference profiles.

5. Summary of the Interpretation Workflow

To interpret a raw diffraction scan:

Clean the Pattern: Subtract the background curve and filter out measurement noise.
Isolate Peak Locations: Determine peak centers ($2\theta$) and measure FWHMs.
Index the Reflections: Convert peak coordinates to d-spacings and assign Miller indices.
Identify Phases: Compare peak positions and intensities to reference database profiles.
Estimate Crystallite Size: Calculate domain sizes using the Scherrer equation.