Pretransitional dynamics of the structural phase transition in anthracene-TCNB: A comparison of Raman-scattering and inelastic-neutron-scattering experiments

Abstract

We report on the study of the pretransitional dynamics of the structural phase transition in anthracene-TCNB by Raman scattering and inelastic neutron scattering at atmospheric pressure. This mixed-stack charge-transfer molecular crystal presents an antiferrodistorsive structural phase transition ͑SPT͒ corresponding to the reorientation of the anthracene molecules in their molecular plane. The dynamical regime of this SPT appears to be mainly displacive. However, our measurements clearly show different behavior of soft mode parameters in both phases. This particularity could be interpreted in terms of an intermediate regime between displacive and order-disorder regimes as was proposed in a coupled double-well potential model ͓S. Aubry, J. Chem. Phys. 64, 3392 ͑1976͔͒. An intense central peak is also found in the neutron spectra just above the transition temperature. It presents a critical behavior in Q space clearly different from the soft-mode one.

Direct evidence of weakly dispersed and strongly anharmonic optical phonons in hybrid perovskites

“…The prefactor of DHOs is the phonon population factor. This full scattering function, I(Q, ω), is next convoluted by the 4D spectrometer resolution function of the instrument as explained in [68], and used to fit the experimental data. The presence of several phonon lines in a given phonon bundle would have the effect to increase the damping when fitted by a DHO due to the energy resolution function.…”

Pseudospin-phonon pretransitional dynamics in lead halide hybrid perovskites

“…In constrast, our inelastic neutron experiments clearly indicate no soft phonon at the phase transition. It should be stressed that these measurements directly probe in single crystals the phonon spectral response at M-or R-points in the cubic phases above the transition temperature, where phonon softening is to be expected in a displacive-type phase transition [7,9,41] and where the critical fluctuations associated to the phase transition should occur. The absence of soft mode at the tetragonal-cubic transition was also reported in the chlorine materials MAPbCl 3 [26].…”

“…The latter around zero energy represents two possible contributions. On the one hand, it primarily describes the elastic incoherent scattering which is generally not negligible in organic compounds [41]. On the other hand, it can also correspond to a possible unresolved central peak (CP) which is typically observed at structural phase transition [7,41].…”

“…On the one hand, it primarily describes the elastic incoherent scattering which is generally not negligible in organic compounds [41]. On the other hand, it can also correspond to a possible unresolved central peak (CP) which is typically observed at structural phase transition [7,41]. These responses stand on a constant background accounting for the tail of the modes constituting bundle (a).…”

“…These experimental observations recall phase transitions in molecular crystals and in plastic crystals. In the former, a pretransitionnal dynamical regime directly related to the coupling between organic molecules has a character ranging from displacive to order-disorder which can be interpreted within a coupled double-well potential model [39][40][41]. More, structural phase transitions in molecular crystals with a full or partial displacive character, may exhibit a quasi-elastic central peak, which has been tentatively attributed to defects or impurities in many inorganic systems [42].…”