General Information

Modern diffraction equipment produces very accurate diffraction patterns from polycrystalline specimens. However, to get the most out of the data, it is still necessary to minimize all non-desirable scattering from reaching the detector during the experiment. The two largest contributors to the background are the air in the beam path and the specimen support. The air contribution may be minimized by evacuating the beam path or by using helium. The support contribution may be minimized by using specimen holders made from single crystals cut so no Bragg diffraction occurs. Quartz and silicon are the most favorable materials for this purpose.

Definitions

Low-background Specimen Supports

The Gem Dugout supplies quartz and silicon plates in many configurations. The quartz is cut 6° from (0001). Silicon is usually cut parallel to (510). Both these orientations contribute no Bragg peaks in the normal Bragg-Brentano diffractometer. A (511) cut silicon plate may also be used for a specimen support, but it has the 511 peak at 96°2theta with CuKalpha radiation. Silicon (511) plates 0.5 mm thick are available because an inexpensive supply was obtained several years ago. Quartz plates should not be heated above about 200°C because of the phase change that occurs at 572°C. Silicon can be heated, but it will eventually show some oxidation and reaction with the specimens.

A Problem with Quartz Plates

Quartz is not recommended where the specimen support is used in a spinner. Some quartz plates in certain diffractometers will allow the forbidden 0001, 0002 reflections to occur. This effect is due to double diffraction from the highly perfect crystal structure of the quartz. The effect depends on the optics of the diffractometer. Consequently, not all quartz plates perform perfectly in rotating specimen devices. Silicon is the preferred material for specimen spinners.

Alignment Specimens

Alignment of diffractometers is usually checked with permanently mounted standard specimens. The most commonly used types are polycrystalline silicon and quartz. The Gem Dugout makes a silicon/epoxy composite using a stock of calibrated silicon whose lattice parameter is 5.4301 ±0.0001Å. This material is not identical to SRM-640 and is not certified to the accuracy quoted for SRM-640. Knowing the expected diffraction angles, this material is very suitable for checking diffractometers and adding to specimens where the NIST certification is not needed. The quartz used in the permanent specimens is a natural material that is also used for sharpening stones, hence is is usually called "soft novaculite". This material is used by all diffraction equipment manufacturers for checking intensity response and for checking alignment.

Reference Materials

The Gem Dugout does supply a few reference materials in powdered form. The silicon used in the epoxy composites is available.

Twice Polishing of some Quartz Plates

During manufacture of some quartz plates surface marks develop from handling. Although these marks have no effect on the use as XRD supports, some users prefer a scratch free surface. To meet this goal many supports are polished twice.

A Comment on Cavities

The use of a cavity specimen mount does allow the use of small amounts of sample confined to the center of the support. However, the user must weigh the effect of compacting the specimen into a small area with depth to spreading the specimen over the full active area of the X-ray beam. If the sample is composed of a low Z material, then the beam will penetrate into the depth of the specimen. The penetration will show up in the diffractometer trace as a specimen displacement to lower diffraction angles. If the sample is composed of a high Z material, only the surface grains of the specimen are seen by the X-ray beam, and the more efficent use of the sample would be to spread it on the surface of a non cavity slide. There will be a minimal sample displacement error with the thin specimen, so that the measured anfles will be more representative of the true values.