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-Gruneisen parameters are compared with the experiments, and Zallen's vibrational scaling law has been successfully investigated.

The structure of ternary, charge-transfer crystals of anthracene, phenanthrene, and tetracyanobenzene, AxPh1−xTCNB, in a wide range of concentrations has been studied. We have focused on the problem of substitutional and orientational disorder of anthracene and phenanthrene molecules and its influence on the orientational instability. The crystal structure determination has been performed at room temperature and 120 K, and the refinement procedures allowed us to determine the orientational disorder for different concentrations. The phase diagram for the orientational phase transition with symmetry change, C2/m to P21/a, has been determined and a critical concentration (xc=0.74) was found. The disorder and the phase diagram has been successfully modeled by a minimization of the crystal energy using the ‘‘split molecule’’ concept and the virtual crystal approximation to mimic both orientational and substitutional disorders. The ternary crystals, their structure, disorder, and phase transitions are well understood in terms of a competition between orientational disorder of anthracene molecules and local stresses created by the substitutional disorder (with statistical orientations) of phenanthrene molecules. A possibility for an existence of molecular orientational glassy state is suggested for concentrations below the critical one.

Anthracene-TCNB is a weak charge-transfer molecular crystal which presents a second order phase transition. At atmospheric pressure, a soft mode has been observed by inelastic neutron scattering in the high temperature phase. Under hydrostatic pressure, the transition remains continuous whereas a drastic change occurs in the dynamical regime. At 3 kbar, at the superlattice point, the soft mode-type scattering is indeed replaced by a quasielastic response. This feature indicates a continuous crossover from a displacive regime to an order-disorder regime for a modest pressure of 1.5 kbar.

Raman scattering studies of the low-energy excitations in crystals of weak charge transfer complexes A-TCNB and N-TCNB are presented. The results are discussed in relation to a character of orientational disorder and expected mechanism of lattice instabilities in the measured systems.

Direct observations of the soft mode in A-TCNB by inelastic neutron scattering in association with Raman scattering are presented. These measurements indicate a complex dynamic regime which seems to be intermediate between displacive and order-disorder types.

Brillouin-scattering experiments performed at room temperature indicate unambiguous relations between the crystalline structure and the optical and elastic properties of this charge-transfer crystal. The largest refractive index and the large longitudinal sound velocity are found along the molecular long axis direction, which is [201\ifmmode\bar\else\textasciimacron\fi{}]. All the elastic constants are determined, and the anisotropy of the derived linear compressibility is found in reasonably good agreement with the thermal-expansion anisotropy. Two kinds of elastic anomalies have been recorded at low temperatures and analyzed as being mainly due to an interaction that occurs along the [201\ifmmode\bar\else\textasciimacron\fi{}] direction. Elastic anomalies of relaxational character as large as 22% have been recorded, and a strong temperature dispersion allows the determination, in the low-temperature phase, of the order-parameter relaxation time 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ (${\mathit{T}}_{\mathit{c}}$-T${)}^{\mathrm{\ensuremath{-}}1}$ s ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$.

Earlier experimental results have been considerably strengthened by Raman scattering from crystalline sym-C6Cl3F3. Measurements have been made as a function of temperature at ambient pressure, and as a function of pressure at room temperature. Apart from a new first-order phase transition observed at 62 kbar but as yet uncharacterised, the crystal structure maintains the P63/m space group symmetry. Anomalies in phonon frequencies and line-widths at 296 K indicate a phase transition, which is 'isostructural' order-disorder. Similar features are seen at 15 kbar, suggesting that the phase transition is of the same nature as that observed at 296 K. The half-width of the phonon associated with dynamic disorder is found by fitting to be Gamma 12/(T)=0.032+0.0061 T+3400 exp(- Delta Ea/kT)cm-1, with Delta Ea=35 kJ mol-1. Although in-plane 120 degrees molecular reorientational jumps may play a role, this may not be a major role at room temperature due to a rather high activation energy. The full understanding of the results requires there to be another minor activation process that also obeys Ag symmetry. The variation in the crystal structure parameters and the external mode frequencies with pressure up to 60 kbar is calculated using a 6-exp atom-atom potential with the rigid-body approximation. The molecular orientation changes by only 0.5 degrees between 0 and 60 kbar. The calculated external mode frequencies agree with Raman experiment up to 60 kbar within 5-15%. A molecular dynamics simulation for 300 K indicates a residence time between reorientations greater than 500 ps, giving a quasi-elastic Lorentzian broadening of less than 0.01 cm-1.

ChemInformVolume 20, Issue 24 Physical Organic Chemistry ChemInform Abstract: Inelastic Neutron Scattering Determination of Phonon Dispersion Curves in the Molecular Crystal sym-C6F3Cl3. M. T. DOVE, M. T. DOVE Dep. Earth Sci., Univ. Cambridge, Cambridge CB2 3EQ, UKSearch for more papers by this authorB. M. POWELL, B. M. POWELL Dep. Earth Sci., Univ. Cambridge, Cambridge CB2 3EQ, UKSearch for more papers by this authorG. S. PAWLEY, G. S. PAWLEY Dep. Earth Sci., Univ. Cambridge, Cambridge CB2 3EQ, UKSearch for more papers by this authorS. L. CHAPLOT, S. L. CHAPLOT Dep. Earth Sci., Univ. Cambridge, Cambridge CB2 3EQ, UKSearch for more papers by this authorA. MIERZEJEWSKI, A. MIERZEJEWSKI Dep. Earth Sci., Univ. Cambridge, Cambridge CB2 3EQ, UKSearch for more papers by this author M. T. DOVE, M. T. DOVE Dep. Earth Sci., Univ. Cambridge, Cambridge CB2 3EQ, UKSearch for more papers by this authorB. M. POWELL, B. M. POWELL Dep. Earth Sci., Univ. Cambridge, Cambridge CB2 3EQ, UKSearch for more papers by this authorG. S. PAWLEY, G. S. PAWLEY Dep. Earth Sci., Univ. Cambridge, Cambridge CB2 3EQ, UKSearch for more papers by this authorS. L. CHAPLOT, S. L. CHAPLOT Dep. Earth Sci., Univ. Cambridge, Cambridge CB2 3EQ, UKSearch for more papers by this authorA. MIERZEJEWSKI, A. MIERZEJEWSKI Dep. Earth Sci., Univ. Cambridge, Cambridge CB2 3EQ, UKSearch for more papers by this author First published: June 13, 1989 https://doi.org/10.1002/chin.198924056AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume20, Issue24June 13, 1989 RelatedInformation

C 10 H 2 N 4 (TCNB) cristallise dans P2 1/a avec affinement jusqu'a 0,039. C 14 H 10 •C 10 H 2 N 4 (A-TCNB) cristallise dans C2/m (a 294 et 225 K) ou P2 1/a (a 65 K). De meme pour C 10 H 8 •C 10 H 2 N 4 (N-TCNB)

Phonon dispersion curves have been obtained along the Δ, Σ, and T directions in a single crystal of sym-C6F3Cl3 at 5 K by inelastic neutron scattering measurements. These have been interpreted within the framework of rigid-molecule lattice dynamics. A model intermolecular potential reproduces the overall behavior, but there remains scope for improvement in the model. Other models based on transferable potentials are found to be less satisfactory.

The generalized susceptibility approach has been used to study temperature dependent librational dynamics and orientational instability in anthracene-tetracyanobenzene crystal.

We present results of polarized, temperature-dependent Raman spectra measurements on A-TCNB which is a charge-transfer crystal. A soft-mode and some other anomalies in the spectra have been observed. The results suggest the interpretation of the orientational phase transition as a transformation which takes place between a temperature Ts corresponding to a displacive regime in the (ab) crystal planes and another temperature Tc corresponding to an order-disorder regime along the c direction. The interval Ts−Tc is estimated to a few degrees and it is believed that this could be an observation of the temperature interval expected for highly anisotropic systems.

Ultrasonic wave velocity, Raman spectra and electrical measurements have been made in samarium phosphate glasses. Both the bulk and the shear moduli decrease under hydrostatic pressure: these glasses show the extraordinary property of becoming easier to compress as pressure is increased. This is in marked contrast to the normal behaviour under pressure of other phosphate glasses. Raman spectroscopy shows that the structures of the samarium phosphate glasses are similar to those of other phosphate glasses. The unusual pressure-induced behaviour is consistent with valence instability of the samarium ions. The temperature dependence of the electrical conductivity is consistent with the small polaron mechanism.

Samarium phosphate glasses in the composition range 5 mol% to 25 mol% Sm2O3 have been prepared. Raman spectra have been characterized in terms of structural components and their resemblance to those of other phosphate glasses shows that the samarium-modified glasses have similar structural features to those of other vitreous phosphates. The compositional dependences of the elastic constants and their hydrostatic pressure derivatives have been measured at room temperature. An increase in samarium content in a decrease of both C11 and the bulk modulus, Bs, to minimum values at a composition of about 20 mol% Sm2O3. Both C11 and C44 decrease with application of hydrostatic pressure: these glasses become easier to compress as pressure is increased. Such acoustic mode softening behaviour is not typical of other phosphate glasses. Visual examination in a diamond cell up to about 100 kbar does not reveal a structural or valence change associated with this vibrational instability. The vibrational anharmonicity and the pressure induced acoustic mode softening are discussed in terms of the Grüneisen approach. It is concluded that the anomalous elastic behaviour under pressure may arise from valence instability of the samarium ions.

Phonon dispersion relations have been determined using neutron coherent inelastic scattering in the monoclinic phase of tetracyanoethylene supercooled to 5K, for 24 low-frequency branches comprising 12 external and 12 internal vibrations along the symmetry directions a*, b* and c*. Theoretical predictions have been made using an extension of the external mode formalism in which only the seven low-frequency internal vibrations (one just out of experimental range) of the free molecule are considered along with the external vibrations. The various experimental results from neutron, Raman and infrared spectroscopy agree satisfactorily with the theoretical predictions. Appreciable dispersion and Davydov splitting are found for the 'internal' phonons and the eigenvectors in general contain both internal and external character. Model improvements are thought to be possible only by the introduction of Coulomb interactions between these highly polarisable molecules. The dispersion curves show no evidence of phonon instability which may be related to the solid state phase transition.