T. Levola

Investigation of structural phase transitions of (CH3)4NCdCl3 by Brillouin scattering

Abstract The velocity and attenuation of the acoustic modes corresponding to the C11 and C33 elastic coefficients are investigated in (CH3)4NCdCl3 by Brillouin scattering. Above the order-disorder transition at 118 K softening of C11 and an additional weak anomaly near 150 K are found. A deformed step anomaly of C11 and an associated attenuation maksimum are observed at 400 K. The only anomaly of C33 occurs in this temperature region. Probably this transition is due to a change in the rotational motion of the tetramethyl ammonium molecules.

On the elastic properties of (N(CH3)4)2ZnCl4 in the incommensurate and the commensurate phases

Brillouin scattering measurements were made in the temperature range from 80K to 600K. From the data the velocities of the phonon modes corresponding to the elastic stiffness coefficients C11, C22 and C33 were determined. In the incommensurate phase between T1=297K and T2=280K, C22 and more clearly C33 shows an anomalous decrease with decreasing temperature. Between T1 and T=260K the Brillouin peaks are broadened attaining their maximum widths just above T2. These effects are interpreted by assuming a coupling between phonons and modulations of the incommensurate phase. The results suggest that the dominant contribution comes from the phase mode-acoustic mode interactions.

Acoustic Anomalies near the Phase-Transition of Uniaxial Ferroelectrics Tscc and Dtscc

Note: Laiho, R Univ Turku,Wihuri Phys Lab,Sf-20500 Turku 50,Finland Af Ioffe Phys Tech Inst,Leningrad 194021,Ussr Karl Marx Univ,Ddr-7010 Leipzig,Ger Dem RepP4713Times Cited:0Cited References Count:9 Reference LC-ARTICLE-1988-004 Record created on 2006-08-21, modified on 2017-11-27

Brillouin scattering study of phase transitions in T1Ins2

Abstract The temperature dependence of the elastic coefficients of T1InS 2 has been investigated by Brillouin scattering measurements. For longitudinal phonons propagating in the (001) plane the variation of ΔC ∼ ∼( T c − T ) was found below the ferroelectric transition point ( T c =189 K). For propagation perpendicular to the (001) plane ( C 33 mode) clear anomalies were observed at T =213 K and T =195 K. Between these temperatures the Brillouin peaks show broadening, which is attibuted to a coupling between the acoustic mode and an overdamped mode.

Brillouin scattering and linear birefringence investigations of (CnH2n + 1NH3)2MnCl4 with n = 1, 2

Abstract The structural phase transitions of the layered compounds (C n H 2 n +1 NH 3 ) 2 MnCl 4 with n = 1, 2 are investigated with the Brillouin scattering and the linear birefringence methods. The acoustic phonon modes propagating in the layers show clear anomalies near the phase transition temperatures. On the other hand, the phonon mode propagating perpendicular to the layers behaves rather regularly. The values of C ij are quoted for both compounds. They support the view that the lattice dynamics of these crystals is nearly three dimensional in nature. Compared with the existing ultrasonic and neutron scattering data the present results show very pronounced dispersion of the velocity of phonons. The birefringence experiments provide data on the various transition temperatures and on the critical exponents of the second-order phase transitions. In EAMC the magnetic birefringence allow us to detect the antiferromagnetic phase transition at low temperatures.

Dispersions of the optical Faraday rotation and of the refractive index in K2CuF4

Abstract The Faraday rotation of the transparent ferromagnet K 2 CuF 4 was measured between 250 and 2000 nm. The dispersion of the rotation near the high energy absorption edge was ascribed to the charge transfer transitions. From these data the Verdet coefficient was determined both in weak and in saturating external fields. Dispersion of the refractive index was measured at 300 and at 77 K.

Brillouin scattering investigation of excitations of glycerol above the liquid-glass transition

Abstract Brillouin scattering measurements made in glycerol show a linear temperature coefficient of the hypersound velocity in the glassy state, a break at Tg = 193 K and another linear part up to 280 K. The analysis of the data in the melt state suggests the presence of excitations, which have the formation energy δ = 1400·kB K. The results are compared with those obtained recently with a soliton model for the dynamics of disordered materials.

Birefringence and brillouin scattering at the structural phase transitions of (CnH2n+1NH3)2FeCl4 WITH n = 1;2

Abstract The phase transitions (PT) of the title compounds (MAFeC, EAFeC) are investigated by linear birefringence (LB), indicatrix rotation, Brillouin scattering and DSC. The main results are the detection of:(i) slight 1st-order nature of both THT-ORT PT, (ii) fluctuations at the TLT-ORT PT and anisotropic dispersion of TA modes in MAFeC, (iii) a double-peaked Cp anomaly at the II-III PT and a 2d Ising antiferro-magnetic (afm)cm peak at TN in EAFeC.

Brillouin scattering and crystal optical investigations of (C2H5NH3)2CuCl4

Abstract The structural phase transitions (SPT) of the layered compound (C2H5NH3)2CuCl4 are investigated by Brillouin scattering and by crystal optical methods, which allow to propose a revised phase sequence as follows: D174h (364 K) II (∼347 K) D152h (∼233 K) IV (39 K) V. It contains three phases (II, IV and V) with lower than orthorhombic symmetry. The newly detected low-temperature SPT are continuous, whereas those above RT are of first order. Anomalous softening and hysteresis of C11 and C22 is found in the vicinity of Tc2 ∼ 347 K. At Tc3 ∼ 233 K these coefficients show anomalies obeying ( T c 3 - T) 1 2 laws. Contributions due to 2d and 3d magnetic order are observed in the refractive indices around TN = 10.2 K.

On Lattice Excitations in the Modulated Structures of [N(CH3)4]2CuCl4

Brillouin spectroscopy has been used to investigate the acoustic excitations in [N(CH3)4]2CuCl4. This crystal is known to have successive transitions from the prototype phase (D2h16) to the incommensurate phase at T1 = 297 K, to the ferroelastic commensurate phase (C2h5) at T2 = 291 K and to another monoclinic phase at T3 = 263 K. Upon transition to the incommensurate phase no anomaly could be detected in the velocities of the longitudinal phonons propagating in the [100], [010] or [001] directions. Within the incommensurate phase the velocity of the mode propagating along [001] showed a decrease of about 1.2%. The Brillouin line width of this mode was found to depend on temperature both in the incommensurate and in the commensurate phases, attaining the maximum value near T2. The results are compared with the existing ultrasonic data and discussed by assuming the phason-phonon interaction in the incommensurate phase.