An X-ray diffraction (XRD) spectrum provides a detailed look into the atomic and molecular structure of a crystalline material. It serves as a unique “fingerprint,” as no two crystalline substances produce the same diffraction pattern, making it a powerful tool for material identification. Its primary purpose is to reveal the arrangement of atoms within a substance. By analyzing this pattern, scientists can determine what the material is and gain insights into its physical properties.
The Generation of an XRD Spectrum
The creation of an XRD spectrum begins inside an instrument called a diffractometer, which has three primary components: an X-ray source, a sample holder, and a detector. The process starts at the source, where a heated filament produces electrons. These electrons are accelerated by a high voltage and bombard a metal target, causing it to emit a beam of X-rays with a specific wavelength.
This focused beam is then directed at the material being analyzed, often a finely ground powder. When the X-rays strike the sample, they interact with the electrons of the atoms and scatter. While most of these scattered waves cancel out, in specific directions they align perfectly. This phenomenon, known as constructive interference, generates a new, strong beam that exits the sample.
To capture these diffracted beams, the detector moves in a precise arc around the sample, recording the intensity of X-rays at every angle. The instrument plots this intensity versus the detection angle to create the spectrum, ensuring all diffraction conditions for the sample’s crystals are measured.
Anatomy of an XRD Spectrum Plot
The resulting XRD spectrum is a two-dimensional plot representing the material’s structural information. The horizontal axis (X-axis) represents the detector angle, labeled as 2θ (two-theta), which is the angle between the incident and diffracted X-ray beams. The vertical axis (Y-axis) measures the intensity of the detected X-rays, often in units of “counts.”
The main features of a spectrum are the sharp, high-intensity spikes called peaks. Each peak corresponds to a specific 2θ angle where constructive interference occurred, meaning atomic planes were oriented to strongly diffract the X-ray beam toward the detector. The collection of these peaks forms the characteristic pattern of the material.
Beneath these peaks lies a relatively flat line known as the background. This signal arises from sources like X-ray scatter in the air, the sample holder, and incoherent scattering from within the sample. Its presence is a normal part of an XRD measurement.
A visual distinction revealed by the plot is the difference between a crystalline and an amorphous material. A crystalline substance, with its highly ordered arrangement of atoms, produces a pattern with sharp, well-defined peaks. In contrast, an amorphous material lacks this long-range atomic order and generates a broad hump across the spectrum.
Decoding the Information in a Spectrum
Once a spectrum is generated, the position, intensity, and shape of its peaks can be decoded to reveal specific properties. The position of each peak along the 2-theta axis is directly related to the distance between the layers of atoms in the crystal. This relationship is described by Bragg’s Law, which connects the diffraction angle, X-ray wavelength, and spacing between atomic planes. This unique set of spacings allows for phase identification by comparing the pattern to databases of known materials.
The intensity of the peaks provides information about the arrangement and types of atoms within the crystal’s unit cell. Since electrons in atoms scatter the X-rays, atoms with more electrons will scatter more strongly. Analyzing the relative intensities helps confirm the material’s identity and understand how atoms are positioned.
The shape of the peaks, specifically their width, offers insights into the material’s microstructure. Narrow, sharp peaks are indicative of a well-ordered, large crystal. In contrast, broader peaks suggest that the crystalline domains are very small, a common feature in nanomaterials, or that there is strain within the crystal lattice.