Variable-Temperature Transmission-Mode Powder Diffraction
While the Department's Diffraction Facility is primarily a single-crystal facility, we also offer transmission-mode (in capillaries or kapton loops) powder diffraction at temperatures from 100K to 500K. Transmission-mode powder diffractomery is particularly interesting when only small amounts of sample are available, because one only needs to fill a tube with an inner diameter of 0.64 mm about 5 mm high, which corresponds to approximately 1.6 cubic millimeters of tightly packed sample. With very careful sample preparation one can further reduce the amount of sample needed to approximately 1 cubic millimeter.
The quality of the powder data collected in our facility is generally high and on several occasion was good enough for Rietveld refinement.
If you are interested, feel free to download the sample preparation document (by Dr. Stacey Smith). You need to bring your own standards if you want to collect a corundum or other reference diffractogram.
Advantages of transmission-mode powder diffraction
(1) As mentioned above, the most striking advantage of transmission-mode powder diffraction is the circumstance that very small amounts of sample are sufficient.
(2) Another strong advantage of our setup is the aspect of variable temperatures (from 100K to 500K) which allows to study phase transitions and other temperature dependent properties of materials easily and quickly. This is the only setup on MIT campus that allows for collecting powder diffraction data in this temparature range.
(3) Experiment times are typically shorter when compared to conventional powder diffractometers.
(3) Our instrument is equipped with an area detector. This means that we are integrating a large area, which translates into a superior counting statistic when compared to collecting powder data in reflection mode and using a point or linear detector.
(4) Transmission-mode powder diffraction is generally good for low density samples (less issues with sample transparency and possibly higher achievable resolution).
(5) Mounting powder samples in capillaries or kapton loops leads to reduced issues with preferred orientation (likely still there on challenging samples, but not as severe as top loading in Bragg-Brentano mode).
(6) Transmission-mode powder diffraction can be used for determination of pole figures for texture, quick comparison of single-crystal result to bulk sample, phase ID on small samples and even collecting of powder data for Rietveld analysis. And all that at temperatures between 100K and 500K.
Disadvantages of transmission-mode powder diffraction
(1) Generally, sample preparation is somewhat more difficult, especially when capillaries are used (oil in loop is easier).
(2) The different detector technology may result in somewhat reduced resolution but we haven't conducted any systematic comparisons.
(3) Possibly there may be issues with inorganic compounds due to high electron density and lack of sample penetration (thinner capillaries can be used to counteract this).
(4) Somewhat broader peaks (see point 2 of this list) and higher background may make indexing difficult.
(5) May have lower intensity than reflection mode experiment.