Ferroelastic Phase Transition in the Urea Inclusion Compounds: N-Heptadecane/Urea and N-Nonadecane/Urea

Author: Rubio-Peña L.   Breczewski T.   Bocanegra E. H.  

Publisher: Taylor & Francis Ltd

ISSN: 0015-0193

Source: Ferroelectrics, Vol.269, Iss.1, 2002-01, pp. : 171-176

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Abstract

Decreasing the temperature, the urea inclusion compounds with n-alkanes guest molecules exhibit the phase transitions. The symmetry superspace group of low temperature phase is not yet established. A difficulty arises due to the large number of domains generated at this phase transition. Therefore, the domain structure in the low temperature phase and the phase boundaries of the n-nonadecane/urea and n-heptadecane/urea crystals has been studied by direct observation under polarized light in the temperature range 293 K - 100 K. The phase transitions at 159 K in the urea/n-heptadecane crystals and at 158 K in urea/n-nonadecane has been observed. By applying an external, compressive and unidirectional mechanical stress, the ferroelastic character of the domains has been confirmed. The observed switching process among the different domains can be understood as a mechanism that minimizes the elastic energy.