Ferroelastic transition triggered by an ordering mechanism in [(CH3)(4)N]SCN

Differential scanning calorimetry on powders and x-ray diffraction on singl e crystals of (CH3)(4)NSCN reveal a cell-doubling cubic-to-orthorhumbic (O- h(1)-->D-2h(13)) ferroelastic transition at 181 degrees C. This transformat ion is interpreted as a two-step mechanism in which an incomplete ordering of the SCN groups triggers the ferroelastic deformation, ia an intermediate tetragonal or orthorhombic phase. Diffuse scattering and diffraction patte rns under stress illustrate the theoretical argumentation. [S0163-1819(99)1 4421-7].