Best Practice of Capillary Transmission XRD

Achieving high-quality XRD data involves optimizing several factors:

• High background: This can be caused by sample fluorescence (e.g., iron content with copper radiation) or the sample holder (e.g., glass substrate). Solutions include changing the detector mode to suppress fluorescence, using a different X-ray source (e.g., cobalt for iron-rich samples), or using low-background sample holders.
• High noise & low intensity: Often due to short counting times or insufficient sample. Decrease scan speed (longer measurement time), optimize step size for better statistics, or use a larger capillary if sample amount allows.
• Poor reproducibility: Frequently linked to bad sample preparation, such as large grains, lack of sample spinning, or moisture absorption. Proper grinding, spinning the capillary during measurement, and sealing capillaries for air/moisture-sensitive samples are crucial.
• Bad resolution (Broad peaks): Can result from large step sizes, wide incident slits, or inherently poor sample crystallinity (small crystallite size). Adjusting step size and closing incident slits can improve resolution, but improving sample crystallinity through synthesis is also important. Aim for at least 3,000 counts for phase identification and 10,000 counts for quantification in the strongest peaks, with 5-10 data points across the full width at half maximum.

Ultimately, the goal is a diffractogram with high intensity, low background, excellent reproducibility, and sharp, well-resolved peaks.

Detector technology has significantly evolved, impacting XRD measurements:

• 0D detectors (Scintillator): These older detectors provide no position information on their surface, requiring a physical slit to be placed before them to achieve resolution. They essentially count X-rays at a single point as the detector scans.
• 1D detectors (Stripe): Representing an improvement, these detectors have multiple stripes (e.g., 75 or 100 µm wide), acting like multiple independent detectors. They provide position information in one direction, eliminating the need for a physical slit and offering better efficiency and speed.
• 2D detectors (Pixel): The latest advancement, 2D pixel detectors provide information in both scanning (2θ) and "beta" directions, allowing visualization of diffraction rings. Pixel sizes (e.g., 75 or 100 µm) influence resolution. Beyond smaller pixel size, energy resolution is a critical feature of advanced 2D detectors (e.g., 340 eV). High energy resolution dramatically improves the peak-to-background ratio by effectively suppressing fluorescence signals from the sample (e.g., iron with copper radiation), leading to cleaner data without needing to switch X-ray sources, thus saving cost and experimental time. The position of 2D detectors can also affect resolution; installing them vertically can provide higher 2θ angular range but might introduce defocusing effects if not properly corrected by software.

Effective sample preparation is key for high-quality capillary transmission XRD:

• Particle size: Grind the powder sample to a very fine particle size, ideally around 5-10 µm, to ensure homogeneous packing and minimize preferred orientation. Be cautious not to over-grind, as this can induce stress or alter sample phases. Sieving can help achieve a consistent particle size distribution.
• Filling the capillary: Simple vibration methods, like using an ultrasound toothbrush, can aid in uniformly loading the powder into the capillary. Avoid using metallic parts to push down stuck particles, as large grains can lead to strong, incorrect peaks and poor reproducibility. Remove any large grains and re-grind if necessary.
• Capillary diameter and material: The choice of capillary diameter depends on the X-ray radiation and the sample's atomic number (Z). For low-Z elements with copper radiation, a 0.3-0.4 mm capillary is suitable. For heavier elements or hard radiation like molybdenum, smaller capillaries (e.g., 0.3 mm) might be needed to allow X-rays to pass through, or larger ones (e.g., 0.5 mm) with moly radiation for stronger signals. Borosilicate glass capillaries are generally cheaper and suitable for most applications unless specific temperature or chemical resistance properties are required.
• Sealing: For air or moisture-sensitive samples, sealing the capillary is crucial. While play-doh or plasticine can prevent sample dropping, for true environmental control, glue or specific sealing methods are required. The type of glue should not interfere with the measurement or the sample holder's fit.

Specialized attachments and alignment procedures significantly improve the ease and quality of capillary transmission XRD:

• Standard capillary attachment (Goniometer head): This traditional attachment allows for precise alignment of the capillary and enables spinning (up to 120 RPM) to average crystallite orientations. It can also be combined with cryostats for temperature-dependent studies. However, manual alignment with a microscope can be time-consuming.
• Innovative capillary attachment (Pre-aligned holder): Newer, more user-friendly attachments feature pre-aligned capillary holders, eliminating the need for extensive manual alignment. The capillary is fixed horizontally, but still rotates. While they might require slightly longer capillaries, they can accommodate smaller capillaries with specific holders.
• Automated Z-alignment: Modern XRD software often includes automated Z-alignment routines using X-rays (not lasers) to precisely position the capillary at the correct height, simplifying the setup process for both standard and innovative attachments. This ensures the X-ray beam consistently hits the center of the capillary for optimal data collection.

Bragg's Law is a fundamental principle in X-ray Diffraction, described by the equation: nλ = 2d sinθ.

• n: An integer (order of diffraction).
• λ (lambda): Wavelength of the incident X-rays.
• d: Spacing between atomic planes in a crystal lattice (lattice plane spacing).
• θ (theta): Angle of incidence of the X-rays with the crystal planes.

This law explains how X-rays constructively interfere when they encounter the ordered atomic planes within a crystalline material. When Bragg's Law is satisfied, a maximum in X-ray intensity (a "peak") is observed in the diffractogram. By analyzing the positions (2θ angles) and intensities of these diffraction peaks from a powder sample in a capillary, researchers can extract a wealth of information:

• Crystal structure: The specific arrangement of atoms within the unit cell.
• Phase identification: Identifying the different crystalline compounds (phases) present in the sample.
• Lattice parameters: Determining the dimensions of the unit cell.
• Crystallinity: Assessing the ratio of crystalline to amorphous content, which indicates the quality of the sample.
• Compositional ratio: Quantifying the relative amounts of different phases in a mixture.

In essence, Bragg's Law is the mathematical foundation that allows us to interpret the diffraction patterns obtained from capillary transmission XRD to understand the atomic and molecular structure of materials.

For reliable results in capillary transmission XRD, specific peak intensity targets are recommended, typically referring to peak height (not counts per second):

• Phase identification: To confidently identify the different crystalline phases present in a sample, the main peaks should reach at least 3,000 counts. This threshold ensures sufficient statistical reliability for pattern matching.
• Quantification: For accurate quantification of the relative amounts of different phases within a sample, a higher intensity is required. The main peaks should ideally reach at least 10,000 counts. This provides the necessary statistical precision for quantitative analysis methods like Rietveld refinement.

To achieve these intensity levels, researchers often need to adjust the scan speed and step size. For instance, decreasing the scan speed and optimizing the step size will increase the measurement time, allowing more X-rays to be collected and thus boosting peak intensity and improving the signal-to-noise ratio. It is also recommended to have 5 to 10 data points above the full width at half maximum of the strongest reflections to ensure accurate peak shape analysis.