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Structure and Phase Transition of [(CH2

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Structure and Phase Transition of [(CH

2

OH)

3

CNH

3

]

2

SiF

6

B. Kosturek, Z. Czapla, and A. Wa´skowskaa

Institute of Experimental Physics, Wrocław University, M. Born Sq. 9, 50-204 Wrocław, Poland

aInstitute of Low Temperature and Structure Research, Polish Academy of Sciences, Ok´olna 2, 50-422 Wrocław, Poland

Reprint requests to Prof. Z. C.; E-mail: czapla@ifd.uni.wroc.pl Z. Naturforsch. 58a, 121 – 125 (2003); received August 5, 2002

Single crystals of (TRIS)2SiF6were grown and characterised by X-ray analysis, differential scan- ning calorimetry (DSC) and optical investigations. They were bond to be trigonal, space group P¯3, with the unit cell dimensions a=7.699(1), c=7.818(2) ˚A. The SiF26−anions, located in large cav- ities formed by hydrogen bonded cations, are strongly disordered at room temperature. The DSC measurements revealed a first-order phase transition at TC≈177 K with a hysteresis of 4 K. The na- ture of the transition was confirmed by a sharp increase of the linear birefringence below TC. Optical observations under a polarizing microscope showed a domain structure of the low temperature phase, characteristic of ferroelastic materials.

Key words: Phase Transition; X-ray Crystal Structure; Birefringence Domain Structure.

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