X-ray Diffractometry

XRD – Options & Accessories

FAST HIGH-TEMPERATURE MEASUREMENT WITH THE MYTHEN 1K STRIPDETECTOR

provides the ideal equipment for powder diffractometer systems to observe and evaluate transitions in nonambient environments. With the ultrafast MYTHEN 1K Stripdetector manufactured by DECTRIS very rapid measurements can be perfomed. Highest quality data is obtained with the capillary furnaces for powder investigations at high temperatures. The furnaces can be mounted on every STADI P or MP diffractometer (moving circles or fixed stage).

The data collection and evaluation is easily performed with 's WinXPOW software, providing full operation control and data visualization.

Ammonia nitrate NH4NO3 · x H2O has been used for the experiment. The measurement has been performed with 0.75 °C per step and an exposure time of 10 sec per step. The data collection has lasted only 30 min to complete, covering a data range of 25° 2θ and a temperature range 120 °C as shown below.

The MYTHEN 1K detector has been placed at the distance with an inherent resolution of 0.02° 2θ. The several well known phase transitions of NH4NO3 · x H2O have been clearly observed.

LOW TEMPERATURE MEASUREMENTS WITH OXFORD CRYOSYSTEMS COBRA

Setup

Powder Samples of KCN and NH4Br were measured using capillaries (0.3mm, 0.5mm TYPE 14) in Debye-Scherrer setup on a STOE STADI P ESSENTIALS diffractometer equipped with pure Cu Kα1 radiation and a 320µ DECTRIS MYTHEN2 1K 450. The temperature was controlled by an Oxford Cryosystems Cobra closed-cycle He cryostat (c.f. Figure 1).

Results

A first phase transition in KCN (Fm`3m à Immm) is reported to appear at 168 K (~ -105°C) and a second (Immm à Pmmn) at 83 K (~-190°C). [[1]] KCN was measured between 293 K and 83 K (20°C to -190°C) in 2°C steps. Figure 2 shows a Guinier-Plot between -105°C and -115°C (168K – 158K) where a phase transition at -113°C (160K) is clearly visible. The difference of 8K between the transition temperature reported in [1] and the measurement are in an expected range for such a setup and temperature region. See Figure 2 and 3.

NH4Br exhibits two phase transitions at 235K (-38°C; Pm`3m à P4/nmm) and at 107K (-166°C; P4/nmm à P`43m)[[2]], which are both clearly visible in the diffraction patterns (c.f. Figure 3, left). The 2nd phase transition is characterized only by a small shift in the 2q position of the reflections (c.f. Figure 3, right). The expected and measured temperature of the 1st phase transition match very well, whereas there is a deviation of ~11°C at the 2nd phase transition.

Conclusion

The combination of an Oxford Cryosystems Cobra with an STOE STADI P ESSENTIALS diffractometer is a very sophisticated setup, which enables one to measure high resolution powder diffraction patterns in a temperature range from 400 K to ~80 K. The samples chosen here cannot be seen as standard samples in terms of absolute temperature but are comparable to regular laboratory samples. Due to the high resolution of the STOE STADI P diffractometer it is possible to identify the phase transition of NH4Br at 107K, which is only due to a small shift in the 2q position of the reflections (c.f. Figure 3, right).

[[1]] D. Fontaine, C.R. Acad. Sci Set. 1975, B281, 443.
[[2]] N. G. Personage, L. A. K. Staveley, Disorder in crystals 1978, Clarendon Press, Oxford.