Journal of Applied Crystallography

Volume 46, Part 3 (June 2013)

laboratory notes

J. Appl. Cryst. (2013). 46, 823-825    [ doi:10.1107/S0021889813005955 ]

A cautionary tale on the use of GE-7031 varnish: low-temperature thermal expansion studies of ScF3

C. R. Morelock, M. R. Suchomel and A. P. Wilkinson

Abstract: GE-7031 varnish, a commonly used low-temperature adhesive and electrical insulator owing to its high thermal conductivity and mechanical strength at low temperatures, was used as a sample matrix for low-temperature powder X-ray diffraction measurements of the negative thermal expansion (NTE) material ScF3. When ScF3 powder was mixed with GE-7031 varnish, an unexpected cubic to rhombohedral phase transition in the ScF3 sample was observed at ~50 K, and it exhibited smaller low-temperature unit-cell volumes than samples without the varnish matrix. Experimental observations and quantitative estimates suggest that these anomalies are the result of stress induced by a thermal expansion mismatch between the varnish matrix (large positive coefficient of thermal expansion, CTE) and ScF3 (quite large negative CTE). The use of GE-7031 varnish as a sample matrix for low-temperature measurements should be approached with caution if a large thermal expansion mismatch is expected.

Keywords: GE-7031 varnish; scandium trifluoride; low-temperature X-ray diffraction; powder X-ray diffraction; negative thermal expansion; composites.

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