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 2^nd 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 1^st phase transition match very well, whereas there is a deviation of ~11°C at the 2^nd 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.

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