A. Yoshihara

Light scattering study of the phase transition in sym‐ triazine

The ferroelastic phase transition in sym‐triazine C3N3H3 has been investigated by means of Brillouin scattering and correlation spectroscopy. Soft TA modes are observed by Brillouin scattering but a well‐defined central peak could not be found. Near the transition in the high temperature phase a 50% increase in Rayleigh intensity is found. No correlation function was observed in the time domain between 1 and 10−6 s. These results are discussed by a Landau mean field theory and dynamical rotation–translation coupling theory.

Brillouin and Rayleigh studies of urea single crystals

Brillouin and Rayleigh scattering techniques have been employed to study the elastic behavior of single crystals of urea over the temperature range 15–300 K. An extensive investigation of the room temperature elastic constants is presented yielding a complete evaluation of their anistropy. The temperature dependence of these elastic modes along the [001] and near the [110] directions is followed to 15 K; no evidence of a structural phase transition has been found. Rayleigh scattering experiments reveal a correlation time in the ms range and an intensity behavior that are temperature dependent but show no anomalies as a function of temperature for a number of directions in the crystal. It is found that scattering intensity and correlation time are caused by the modulation and driving of defects by an external periodic applied stress pulse. Neither the correlation time nor the intensity behavior supports the conclusion that urea undergoes a structural second order phase transition in the temperature range 1...

On the phase transition in benzil

The phase transition in crystalline benzil [(C6H5CO)2] at 84 K is investigated through Brillouin scattering. The major experimental findings are two transverse acoustic phonon modes exhibiting softening near the phase transition, a longitudinal acoustic mode that is temperature sensitive, and the ratio of the Rayleigh peak intensity to the soft mode intensity behaves anomalously. These results are discussed using a theory developed for an elastic phase transition in sym‐triazine (C3N3H3). It is found that bilinear‐coupling terms involving strains and the order parameter can explain transverse mode behavior but not that of the longitudinal acoustic mode.

Critical behavior in annealed and unannealed crystals of benzil

Simultaneous Brillouin‐correlation light scattering results obtained under an external applied stress pulse are presented for annealed and unannealed (as grown) crystals of benzil. Critical fluctuations are characterized for the LA a‐axis mode governed by the elastic constant c11 and are shown to contribute to this elastic anomaly in the high temperature phase. Both a central peak intensity anomaly and a critical relaxation time anomaly are characterized for unannealed crystals. These effects disappear in well‐annealed crystals and without the external stress pulse even in as grown crystals. The critical relaxation behavior in benzil is coupled to the external stress compression wave generated by the mechanical refrigerator used to cool the sample. The latter interaction and relaxation anomaly are observed in polarized scattered light with the stress applied along the b (or a) axis as required for a c11 coupling. These observations are qualitively analyzed employing the theory of anelastic solids with hig...

An equation of motion approach to ferroelasticity in sym-triazine

A description of the phase transition in sym-triazine based on an Onsager equations of motion approach is presented. Using a hamiltonian which includes the full dynamics (kinetic and potential energies) of the coupled rotations and translations, Greens functions and correlation functions for librational and acoustic modes are obtained. The method employed here is similar to pseudospin-phonon theories in which spin coordinates are higher order rotational variables such as RxRy and (Rx 2) - Ry 2). Calculations for phonon groups in the small osciallator limit are presented and compared with neutron scattering data between 300 K and 205K (T c ∼ 198 K). Good agreement is obtained outside the immediate region of the phase transition (T > 205 K).

Structural transition and elastic anomalies in s-TRIAZINE, C3N3H3

Abstract Dynamical behavior of the coupled system of molecular orientations and acoustic phonons is investigated in order to provide a description of the structural phase transition in s-triazine, s-triazine molecular reorientations are in the fast relaxation limit. A softening of the acoustic phonon frequencies is found as T → T c.

Brillouin scattering studies of the successive phase transitions in (CH3NH3)2FeCl4

Brillouin scattering spectra are reported and analyzed for the layered perovskite compound (CH3NH3)2FeCl4. Particular attention has been given to successive phase transitions in this system: D174h ↔ D182h ⇄ D164h. It is found that the elastic constant c66 is the major contributor to instabilities in this system. A two‐dimensional order parameter at the Brillouin zone boundary X point is introduced into the Landau free‐energy to account for this sequence of transitions. An additional secondary instability (one‐dimensional order parameter) at the Z point is used to generate the D182h ⇄ D164h phase transition: inclusion of the two order parameters circumvents the need for postulating a D102h phase between the D182h and D164h phases and for higher order temperature dependent terms in the free energy expansion. The D174h ↔ D182h phase transition is characterized by a strong Landau–Khalatnikov contribution to c66 and a large dynamical critical behavior.

Critical fluctuations at the phase transition in benzil

New Brillouin scattering data are presented for benzil single crystals near the phase transition at 83.5 K. These data demonstrate that for the c11 governed longitudinal acoustic (LA) mode at ∼15 GHz, critical fluctuations are quite large near the phase transition and dominate the behavior of this mode within +40 K of the transition. These observations are analyzed in terms of four contributing soft modes; an optical soft mode and two transverse acoustic (TA) soft modes at the zone center and a zone boundary M‐point soft mode. It is argued that the zone boundary mode is the major contributor to the width and elastic constant anomalies of the LA mode. Calculations of these properties support these conclusions. Critical exponents are evaluated for Δc11 and ΔΓa, the critical contributions to the elastic constant and width of the c11 governed LA mode, based on the experimental data.

Brillouin and Rayleigh scattering studies of the phase transition in chloranil

The nonferroic (constant rotational symmetry) phase transition at ∼93 K in chloranil (C6Cl4O2) is investigated by Brillouin and Rayleigh light scattering. The phase transition is driven by a zone boundary soft mode at the (00 1/2) point which, through various coupling mechanisms, can affect the zone center strains. The major conclusions of these investigations are: (1) the LA‐phonon mode propagating along the [010*] direction exhibits a step function anomaly at Tc with an associated transition region in the low temperature phase and a linewidth anomaly. The frequency of this LA mode in the transition region decreases below its low temperature phase value near Tc; (2) the lowest frequency QTA phonon propagating around [010*] has a negative temperature coefficient in the high temperature phase which goes to zero in the low temperature phase; (3) external stress induced relaxation processes can be observed by correlation scattering spectroscopy which are highly polarization, experimental scattering geometry,...

Reply to ‘‘Comment on ‘Light scattering study of the phase transition in sym‐triazine’ ’’

Rae’s1 comments on the author’s paper concerning the phase transition in sym‐triazine2 are answered. (AIP)