Phonon dispersion of the external and low-frequency internal vibrations in monoclinic tetracyanoethylene at 5K

Phase transition between cubic and monoclinic polymorphs of the tetracyanoethylene crystal: the role of temperature and kinetics

“…11 Here we concentrate on understanding the fundamental properties that dictate the phase behavior of crystalline TCNE, with an outlook to study the more complex materials mentioned above. To date, four polymorphic forms of TCNE have been reported, [12][13][14][15][16][17][18][19][20][21][22] where only the monoclinic and cubic polymorphs have ever been resolved. These different crystal symmetries are accessible via variations in the temperature, pressure, and crystallization conditions.…”

“…The temperature-induced phase transition is first order, appearing as an irreversible (down a Department of Chemistry, The Pennsylvania State University, FayetteThe Eberly Campus, 1 University Dr, Uniontown, PA, 15401, USA. E-mail: bxs54@psu.edu to 4 K) cubic to monoclinic evolution at temperatures above 318 K. [17][18][19]23 The pressure-induced phase transitions have also been extensively investigated, [6][7][8]21,22,[24][25][26] where the monoclinic phase converts to an intermediate 'disordered' metastable phase when pressurized between 2.0-6.3 GPa. Upon release of pressure, the intermediate phase is followed by conversion to either a) the cubic phase (provided sufficient time under high pressure) or b) the original monoclinic phase.…”

On the anomalous vibron-phonon dispersion in solid tetracyanoethene, C2(CN)4

“…We have also simulated the rigid C-C^N model of Chaplot et al (1983) by making all stretch force constants and the C-C = N bending force constant infinitely high and adapting the remaining v f f constants to obtain practically the same frequencies as Chaplot et al (1983). The eigenvectors are completely determined by symmetry in this model, but some of the frequencies differ considerably from those obtained in the complete v f f model (see table 1).…”

“…The calculated free molecule frequencies are replaced by experimental values, as far as available (see table 1). The intermolecular potential is represented by an atom -atom potential V(r) = e x p ( -C /y r) -A {]r~^ + q■lq]r~x with the same parameters A Vp B¡j and C,y as used by Chaplot et al (1983). The electrostatic interactions have been added, however, and the fractional atomic charges q{ have been determined by the following procedure.…”

“…Neutron scattering experiments (Chaplot et al 1983) were performed on t c n e in the monoclinic phase, space group P2 Jn, supercooled to 5 K. The two molecules in the unit cell are centred at (0, 0, 0) and (a/2, b/2, c/2), respectively, and they are related by a screw axis 2 1 along the b direction and a glide plane parallel to the ac plane. We use the same convention as Chaplot etal (1983) for displaying the phonon and vibron dispersion curves along the a*, b* and c* directions of the Brillouin zone and defining the symmetry of the normal modes with respect to the screw axis and the glide plane.…”