Raman scattering study of monoclinic and cubic tetracyanoethylene under high pressures

Abstract

Raman scattering spectra of the cubic and monoclinic phases of tetracyanoethylene in the external-mode region have been investigated as a function of hydrostatic pressure in a diamond-anvil cell. There is evidence of a possible transition from the cubic phase to a new unknown phase at 13 kbar. The intensities of the Raman spectra are found to vanish at high pressures. Calculation of phonon frequencies as a function of pressure has been carried out in both the cubic and monoclinic phases using atom-atom potentials with a semi-rigid molecule model which takes account of both the external vibrations and those internal vibrations up to 300 cm -1. The calculated frequencies and mode-Gr/ineisen parameters are compared with the experiments, and Zallen's vibrational scaling law has been successfully investigated.

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

“…Finally, high pressure polymerization reactions of TCNE at room temperature have been observed when starting from either the monoclinic or cubic phases. [6][7][8]21,22,[24][25][26] The most recent investigations indicate that when using no pressure medium, the monoclinic phase completely polymerizes above 6.4 GPa while the cubic phase is stable up to 14 GPa. 22 Interestingly, the polymerized material is a form of amorphous carbon with 52% less nitrogen than crystalline TCNE, and the C : N ratio obtained from neutron/X-ray diffraction was 7 : 1 for the polymerized product compared to 3 : 2 for the original material.…”

“…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.…”

Pressure-induced transformations and high-pressure behaviour in cyanoadamantane plastic crystal

“…Figures 5 and 6 display two different (ordered or disordered) crystal-to-amorphous transitions leading to a quasi-similar description of the amorphous state. Pressure-induced amorphizations have been observed in many other compounds [15][16][17][18][19][20][21][22][23][24][25][26][27], among which are several recognized to be typical molecular crystals [15][16][17][18][19][20][21]. However, the evidence for two different routes to a single amorphous state is unusual.…”

Pressure and temperature studies of Raman scattering of the molecular crystal sym-C6Cl3F3

“…Once the potential function parameters have been chosen, it is straightforward to calculate the structure and dynamics as a function of pressure for zero temperature [ 111; the method has also been adopted in more recent work [18]. First the structure is found by minimising the total of the potential and pressure energy ( p V ) with respect to the structure variables, the lattice parameters and molecular coordinates, keeping the space group symmetry unchanged.…”