The elastic behaviour of ferroelastic Sb5O7I polytype 2MC studied by ultrasonic experiments and Brillouin scattering

The temperature dependence of the elastic functions of the improper ferroelastic polytype 2MC-Sb5O7I has been investigated in the temperature range from room temperature to well above the structural phase transition atT c=481 K. The stiffnessesc(c*c*),c(a,a),c 22 andc(c*a) show a considerable softening up to 20% aroundT c whereasc 44 remains unaffected by the phase transition. The experimental results are discussed considering cubic and quartic anharmonic coupling between two components of the zone boundary order parameter and elastic waves

Influence of solvent viscosity on the photoisomerization of a novel trans-stilbene derivative with hindered single bond torsion.

Fluorescence life-time measurements were performed at elevated pressure and temperature to examine the viscosity dependence of the rate of photoinduced isomerization, kiso, in a stilbene derivative in which the single bond torsion is hindered by the introduction of CH2 bridges. The results obtained with n-hexane and methylcyclohexane as solvents (0.2 mPa s<η<2 mPa s) could not be fitted satisfactorily on the basis of Kramers' expression. A good global fit was, however, achieved when applying the empirical relationship kiso=C·(η/mPa s)−a·exp(−EA/RT). The value for a drops from about 0.30 in n-hexane to about 0.15 in methylcyclohexane thus pointing to solvent specific contributions to the friction. The derived activation energies EA are about 12.8 and 12.5 kJ mol−1 in n-hexane and methylcyclohexane, respectively. &nbsp