Fine Resolution Powder Diffractometer (FIREPOD)
The Fine Resolution Powder Diffractometer E9 (FIREPOD) is an angle-dispersive powder diffractometer optimized for a flat resolution function with a minimum width of the reflections at the 2Θ-region with the highest density of reflections. The monochromator is placed at a distance of 11 m from the reactor core, which allows for a large take-off angle at the monochromator. An evacuated beam tube and a sapphire single crystal filter reduce air scattering and epithermal neutrons. Neutron flux at the sample is increased by an adjustable vertically focusing Ge-monochromator. The detector consists of eight individual DENEX 3He 2D detectors with 300 x 300 mm active area each and a common radial collimator to reduce background noise. The individual detectors are arranged in a novel setup, at optimized, non-constant distances from the sample. Five of the individual detectors can be placed close to the sample in a high intensity conformation. Data collection with fixed detector position measures parts of the 2Θ-range with increased intensity and without loss in quality. Position-sensitive data integration of the Debye cones results in a strongly reduced asymmetry of the peaks. The 2D-data are directly accessible, allowing the early detection of preferred orientation or spottiness.
Depending on sample scattering power and volume and the resolution settings of the instrument, full powder diffraction patterns of a quality suitable for Rietveld refinement can be collected as fast as 30 minutes. With small 1 cm3 samples and high resolution, between 1 and 6 hours should be estimated, depending on the scattering power of the sample. Scans measuring only a selected angular with fixed detector position can be as fast as 10 minutes per step for good scatterers.
- Crystal structure determination
- Rietveld refinement
- Site occupation factors, e.g. of isoelectronic elements
- Determination of light atoms (e.g. H, Li)
- Rapid parameterized scans of selected angular regions of the diffraction pattern, e.g. temperature or magnetic field strength
- Non-destructive bulk phase analysis
|Collimation||α1: 10’ or 18’|
|Monochromator||Axially focusing, Risø design, 300 mm height |
19 Germanium composite plates with reflecting planes (311), (511), and (711)
(511) normal to crystal surface
Mosaicity FWHM = 17´
|Take off angle of monochromator||50° ≤ 2Θ ≤ 130° |
111.7(1)° is used by default
|Wave length||λ = 1.3084(2) Å from Ge(711)|
λ = 1.7977(1) Å from Ge(511)
λ = 2.8172(2) Å from Ge(311) & PG filter
|Flux||≈ 105 n/cm2s|
|Range of scattering angles||3° < 2Θ < 142°|
|Range of lattice spacing||25 Å < d < 0.7 Å from Ge(711) |
35 Å < d < 1.0 Å from Ge(511)
55 Å < d < 1.5 Å from Ge(311)
|d resolution||≈ 2·10-3|
|Sample size||1 cm3 - 5 cm3|
|Detector||Eight area detectors (300 mm x 300 mm) |
Oscillating radial collimator for background reduction
|Instrument options||Variable sample – detector distance for five of the individual area detectors|
|Sample environment||OS, OF, HTF, VM3, VM5, DEGAS, RTSC (room temperature 10-fold sample changer)|
Hints for sample preparation
For optimum results, when preparing samples for E9 the following points should be considered:
As always in powder diffraction, the sample should be a fine-grained, isotropic powder, without preferred orientation or large grains.
By default, samples will be packed in vanadium tubes of 6 or 8 mm diameter and 60 mm length, depending on available sample amount, absorption, and requirements of resolution vs. intensity.
For a given sample volume, a long & thin sample is preferable to a short & thick one. A higher sample diameter reduces the angular resolution, while sample length has no significant effect on the peak width. Up to a length of 40 mm the intensity increases linear, between 40 – 60 mm intensity still increases with the sample amount, but less than linear. If only small sample amounts are available, the sample length should thus be 40 mm.
A large sample combined with a high-resolution setting of the instrument is better than a small sample with a high-intensity setting. The loss in angular resolution going together with a given intensity gain is not as high for a sample diameter increase. Also, a larger sample increases the intensity and the peak-to-background ratio, while a high-intensity setting of the instrument increases intensity and background equally.
The default tubes are not air-tight. The sample will be in contact with air (ambient conditions) or vacuum (cryostats, furnaces, magnets). Sealed containers are available, but we need to know beforehand.
Vanadium or glass containers contribute considerably to the background, which is a problem especially for small and poor-scattering samples. Aluminium, steel or tantalum do not contribute to the background, but produce additional peaks in the diffraction pattern, which might overlap with important sample peaks. The best choice in tube material depends on your sample and question.
Vanadium is anodic, with quite strong reduction potential (E0 = -1.18 V). At high temperatures, samples containing less anodic metals (e.g. Bi, Pb, As, Sn, Hg) might be reduced. If the resulting metal amalgamates with vanadium, the sample container will be destroyed and the furnace contaminated. For such samples, high temperature experiments must be conducted using quartz glass, steel or tantalum as container material.
Sample grains showing ferromagnetism might reorient in an external magnetic field. Ask your local contact for the various ways to avoid this.
Some sample environments put limits on diameter and length of the sample container; check this if you want to use your own containers.
In case of possible problems, please talk to your local contacts well in advance about suitable options. This is especially important if the sample cannot be handled out in the open.