X-ray diffraction is a structural analysis technique that is often used to characterize materials as a function of environment, temperature and other external conditions. Whenever materials undergo thermal cycles (at low temperatures or high temperatures), phase transitions invariably take place and the crystal structures of these materials differ considerably resulting in their different properties.
Using the real-time detector technology of the Thermo Scientific™ ARL™ EQUINOX 100, rapid XRD measurements can be performed to analyze structural phase transitions of materials at high temperature.
Dynamic studies at high temperature can now be performed more quickly and easily, thanks to the recent availability of a compact high-temperature chamber, developed for integrating within a bench-top XRD instrument and combined with a concurrent full pattern XRD capability.
In order to demonstrate this latest cost-effective solution for Material Scientists, this article provides an example of such an analysis with ARL EQUINOX 100 combined with Anton-Parr BTS500 chamber
The Thermo Scientific™ ARL™ EQUINOX Series includes a variety of XRD instruments that range from simple and easy-to-use bench-top systems for regular analysis to more sophisticated high performance, floor-standing, research-grade systems for investigative laboratories. A custom-made micro-focus tube (50 W for Cu or 15W for Co) is used by the ARL EQUINOX 100 which eliminates the need for an external water chiller. The same unit can be transported into the field or between laboratories without the need for any special infrastructure.
Compared to other diffractometers, the ARL EQUINOX 100 provides extremely fast data collection times because of its unique curved position sensitive detector (CPS) that can measure all diffraction peaks simultaneously AND IN REAL TIME.
When the ARL EQUINOX 100 is coupled with Anton Paar BTS500 high temperature chamber, possibly the fastest bench-top XRD for dynamic studies was obtained. Under low power requirements (less than 250 Watts) and in a very small form factor, chemical reactions or phase transitions can be studied easily and with complete computer control of both the high temperature chamber and the ARL EQUINOX 100 itself.
In order to prove the performance of this system, a quick experiment was conducted, less than 3 hours from start to 3D result plot.
Figure 1. BTS500 chamber into the ARL EQUINOX 100.
For these studies, RbNO3 powder, as obtained from supplier (alfa aesar, 13496, 99% purity) was used in a BTS500 nickel sample cup. Next, Symphonix software was used to prepare a batch run, as follows:
- A 2 °C/minute heating ramp was used on the furnace to reach a temperature of 350 °C
- A cycle of 2 minute acquisitions was started
This means that due to the curved detector, that permits to get a full XRD pattern over 110° all the time, a new pattern covering the entire angular range is obtained every 2 minutes. Without doing any dwell, a new pattern like this is obtained every 4 °C.
On the ARL EQUINOX 100, a copper minisource was combined with SmartOptics, running at 36 W power (40 kV, 0.9 mA). During the entire experiment, there was a live display of the on-going full XRD pattern.
Figure 2. Typical RbNO3 pattern obtained in 2 minutes at room temperature.
Rubidium nitrate exhibits a number of phase transitions between room temperature and 310 °C, the melting temperature. If all the patterns obtained are plotted from room temperature up to 350 °C, the following picture is obtained (the 20 to 70° 2θ region was zoomed in on, which is more interesting, but the full angular range is always available with a curved detector).
Figure 3. Rubidium nitrate patterns obtained from temperature up to 350 °C.
It is evident that during the entire experiment, the pattern changes a number of times. Rubidium nitrate has a trigonal structure at room temperature, up to around 160 °C, and then changes rapidly to another structure, which is reported as being cubic.
Figure 4. Trigonal structure of rubidium nitrate at temperature around 160 °C.
A closer look at around 165 °C revealed that the tiny peaks between the higher ones disappear around 170 °C.
Figure 5. Rubidium nitrate pattern saved at 181 °C.
The phase transitions demonstrated here occur at higher temperature as described in the literature, but since no dwells are being done around the phase transition temperature, and the fact that the sample is heated from the bottom and with the X-rays the top surface can be seen, it takes time before we effectively see it in the diffraction pattern.
The experiment performed here should be regarded as a fast screening prior to repeating the measurements with relatively slower speed around the phase transition temperature.
A second phase transition is observed at a higher temperature, around 250 °C in the experiment (while reported at 219 °C in the literature). It is very obvious since the switch was made from cubic to rhombohedral.
Figure 6. Rubidium nitrate patterns at around 250 °C.
Figure 7. Rubidium nitrate pattern saved at 260 °C.
With the ARL EQUINOX 100 equipped with a BTS500 high temperature chamber, the 4 known structures of RbNO3 were observed in less than 3 hours and with just 10 mouse clicks. The subsequent step would be to ramp rapidly around the approximately known phase transition temperatures and record the sample slowly every degree or so, to have a nice view at exact temperatures of phase transitions.
Figure 8. Rubidium nitrate pattern saved at various temperatures.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers.
For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers.