Empyrean, X'Pert3 Powder
Non-ambient diffraction is the general term used to describe XRD experiments performed under atmospheric conditions that are different from those normally existing in a laboratory environment. Non-ambient diffraction experiments give information about changes in the sample as a function of temperature, relative humidity, pressure or gas phase composition. Under altered atmospheric conditions, samples may show thermal expansion of the lattice, crystallographic phase transitions, or may react chemically with the gas phase. XRD is a non-destructive technique that provides unambiguous information about rearrangements in the crystal structure under changing environmental conditions.
Accurate determination of peak positions and peak shifts is difficult under changing non-ambient conditions. Especially with the para- focusing geometry, which is normally preferred because it yields high-quality diffractograms in just a few minutes per temperature; instrumental artifacts like macroscopic sample height variations induce additional peak shifts. This application tutorial describes how to get rid of this effect.
In collaboration with Anton Paar we developed the automatic height compensation mechanism to eliminate the sample displacement effect in non- ambient XRD experiments on both Empyrean and X’Pert3 Powder diffractometers. The height compensation mechanism adjusts the sample height as a function of temperature. Adjustment is automatic, software-controlled and reproducible. As a result, accurate peak positions are achieved, even in para-focusing geometry.
Non-ambient X-ray diffraction - an important methodology
Studying materials in non-ambient conditions is important for many industries and research topics. The combination of X-ray diffraction (XRD) and a controlled environment gives direct information about the behavior of materials under varying temperatures and atmospheres. Unlike complementary techniques, such as differential scanning calorimetry, XRD not only shows that something changes, it also shows what is happening.
Anton Paar non-ambient chamber on the programmable z-stage mounted on the Empyrean diffractometer
High-temperature non-ambient chambers fall into two categories: direct heaters, which make use of a heating strip or a Peltier element, and indirect heaters in which the sample is surrounded by heating elements. Indirect heaters are the preferred choice for high-temperature non-ambient XRD as they offer much better temperature homogeneity and accuracy compared to strip heaters. Furthermore, indirect heaters make sample handling easier, facilitating sample spinning and allowing a wide choice of sample carrier material. A disadvantage of the indirect heating method is, however, that the chamber is much slower: it takes some time before changes in heating power reach the sample. Furthermore, the maximum temperature that can be achieved using an indirect heater is lower than with a direct heater. The choice of non-ambient chamber depends on the analytical problem to be solved.
On the left: one side heat transfer through contact surface with heating/cooling device On the right: uniform heat transfer from surrounding heater
The problem of thermal expansion of the sample holder
In chambers that make use of an indirect heater, the sample is often mounted on a rod, which is inside the heated environment. This rod will expand as a function of temperature. In non-ambient XRD research with Bragg-Brentano para-focusing geometry, this thermal expansion of the sample holder causes an additional peak shift.
Sample holder of the HTK 1200N. The sample itself is put in or on the blue part on top.
Classical geometry (Bragg-Brentano): errors in peak position when the sample moves out of its ideal plane. The schematic drawings show the situation with a receiving slit; the same effect occurs with a line detector (X’Celerator or PIXcel).
Solving the problem - possibilities
There are several methods for addressing the problem of thermal expansion of the sample holder:
- Ignore sample height variations
- Correct known sample displacements in the analysis software
- Add a standard with known thermal expansion to the unknown sample
- Use parallel beam geometry instead of Bragg-Brentano geometry
- Actively correct sample displacement during measurement
The first two options are not satisfactory for studies of temperature-induced structural changes, while the third can cause unwanted reactions.
The next method (shown at the right) is the use of parallel beam geometry instead of Bragg-Brentano. Although parallel beam geometry is insensitive to peak shifts resulting from sample displacements it does have several disadvantages: it places a higher demand on the size and random orientation of the grains; it cannot be combined with a line detector and is therefore much slower; and good angular resolution is difficult to obtain.
Parallel beam geometry: a change in height causes the diffracted beam to pass through another part of the collimator. The angle, however, stays unchanged.
Instrumentation and method
Non-ambient XRD experiments were conducted using the X’Pert3 Powder diffractometer equipped with an Anton Paar HTK 1200N chamber on a motorized z-stage.
Diffractograms on alumina powder, from room temperature to 1200 °C were recorded. Three configurations were used:
- Bragg-Brentano para-focusing geometry: no active compensation
- Parallel beam geometry: no active compensation
- Bragg-Brentano para-focusing geometry: active compensation
In the Bragg-Brentano geometry, a line detector (X’Celerator) was used. Scan range, step size and total scan time are given in the table.
The solution: Active height compensation
Active correction of sample displacement during measurement is a novel method for tackling thermal expansion of sample holders. By using the automatic height compensation mechanism, the height of the sample can be corrected automatically when the temperature is varied. As a result, a fast line detector can be used. This delivers results more than ten times faster than the best that can be achieved using parallel beam geometry, with much better resolution.
The effectiveness of the automatic height compensation mechanism for non-ambient XRD experimentation has been tested experimentally.
The X-ray diffraction experiments were performed on an X’Pert3 Powder system equipped with the Anton Paar HTK1200 non-ambient chamber on a motorized z-stage.
This figure shows the peak shift as a function of the temperature. The total shift, however, is a combination of lattice expansion and sample holder expansion.
With parallel beam geometry, the second cause of the shift (sample holder expansion) is intrinsically absent. The peak shift is now much larger, and shows that in the previous figure, the two effects were partially cancelling each other. In other words: with parallel beam geometry one can see the lattice expansion directly.
Note however, that the data collection time has increased with a factor of 10, when compared to the previous figure. Furthermore resolution is greatly reduced because the contributions from the individual Cu Kα1 and Kα2 lines are no longer visible. By using Bragg-Brentano para-focusing geometry together with the Automatic Height Compensator, one gets the best of both: high-speed data collection and good peak resolution are now combined with accurate peak position
This figure shows a comparison of the peak positions obtained in the diffractograms using all three configurations. It clearly demonstrates that Bragg-Brentano para-focusing geometry, together with the height compensator, eliminates the sample displacement effect.
With the automatic height compensation mechanism the sample displacement effect can be eliminated allowing accurate data to be obtained using Bragg-Brentano geometry. This is achieved in a shorter time and with higher angular resolution than with parallel beam geometry. Automatic height compensation is therefore an invaluable tool for non-ambient XRD applications.