Elastic constants of lithium thallium tartrate measured by Brillouin scattering

All components of the elastic stiffness tensor of lithium thallium tartrate at room temperature have been measured by Brillouin scattering techniques using two different scattering configurations. Refractive indices have been determined independently by optical measurement. The results are compared with previous work on lithium thallium tartrate and lithium ammonium tartrate.     

Elastic constants and softening of acoustic modes in A2MX6-crystals observed by Brillouin scattering

Room-temperature Brillouin-scattering measurements of the longitudinal and transverse sound velocities for the cubic crystals K₂SnCl₆, K₂ReCl₆, (NH₄)₂SnCl₆ and (NH₄)₂SiF₆ are presented. The elastic constantsc ₁₁,c ₁₂ andc ₄₄ are determined from the angular variation of the Brillouin line shifts. Furthermore, for K₂SnCl₆ the sound velocities in the [100]- and [111]-direction are investigated as a function of temperature, 256K≦T≦330K. A strong softening of thec ₁₁–c ₁₂ mode is observed in the para-distortive phase as the temperature approaches the phase transition temperatureT _c1=262K from above, whilec ₄₄ shows no anomaly within the experimental error. These acoustic anomalies are interpreted theoretically by a mechanism based on the coupling of the elastic strain field to the fluctuations of the soft-mode coordinates.     

Elastic constants in RbI,determined by inelastic neutron scattering

Long-wavelength acoustic phonons have been studied in the whole (100)-plane of RbI at 295 K by means of inelastic neutron scattering. The raw data have been corrected for resolution effects taking into account the curvature of the dispersion surface and variations of the mode eigenvectors. The shifts of the neutron groups due to these resolution effects are discussed in detail. The analysis of the experimental results gives for the zero sound elastic constantsc ₁₁=28.15±0.5,c ₁₂=3.7±0.5 andc ₄₄=2.85±0.1 10¹⁰ dyn/cm². A comparison with first sound elastic constants taken from ultrasonic measurements yields significant differences between the high and low frequency elastic constantsc ₁₁ andc=(c ₁₁–c ₁₂)/2. The differences calculated from recent theories are in agreement with the experimental results for the elastic constantsc ₁₁,c ₄₄ andc, but not forc ₁₂. The dispersion surface in the (100)-plane is also shown for some out-of-symmetry data and compared with ultrasonic data as well as with theoretical results.Research supported by BMFT     

Study of the elastic stiffness constants of thiourea by Brillouin scattering

All the elastic stiffness constants of thiourea have been measured at room temperature. The value of C₆₆(0.7 × 10¹⁰dynes/cm²) is very low. The C₁₁ and C₃₃ constants exhibit discontinuities at 169 and 202 K. The longitudinal acoustic phonons propagating in the c direction are very strongly damped in the vicinity of 202 K.     

Surface acoustic waves and elastic constants of InN epilayers determined by Brillouin scattering

The surface acoustic wave velocity in InN has been experimentally determined by means of Brillouin scattering experiments on c ‐ and m ‐face epilayers. From simulations based on the Green's function formalism we determine the shear elastic constants c₆₆ and c₄₄ and propose a complete set of elastic constants for wurtzite InN. The analysis of the sagittal and azimuthal dependence of the surface acoustic wave velocity indicates a slightly different elastic behavior of the m ‐face sample that basically affects the c₄₄ elastic constant. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Elastic and quasielastic neutron scattering studies in KBr:KCN mixed crystals

Neutron scattering studies in (KBr)_1–x(KCN)_x mixed crystals are presented utilizing powder diffraction, single crystal diffraction and time-of-flight techniques. Forx>0.6 (KBr)_1–x(KCN)_x crystals exhibit ferroelastic and ferroelectric low-temperature phases. Crystals withx<0.6 undergo transitions into an orientational glass state. Here we present a detailed phase diagram including new results for x=0.85 and x=0.65. For the latter system a stable rhombohedral low-temperature phase has been detected where the orientational disorder of the plastic phase is only partly removed and quadrupolar relaxations between three body diagonals are still possible. From the powder diffraction experiments we determined further the concentration dependence of the static Debye-Waller factors which can be explained by an interplay of the rotation-translation and the rotation-random strain coupling. With single crystal diffraction techniques we studied the diffuse scattered intensities which are directly related to the order parameter of the glass state. The temperature dependence of the quasielastic intensities near the critical concentration shows a strong increase forT<110K indicative for a freezing-in of shear fluctuations which is a characteristic feature of a non-ergodic instability. This phenomenon appears for ordering (x=0.65) and for non-ordering, glassy compounds. A further anomaly in (KBr)_0.43(KCN)_0.57 at 75 K is interpreted in terms of a residual elastic ordering process. With high-resolution time-of-flight techniques we analysed the dynamic structure factor for x=0.57. We demonstrate that the central peak consists of a static and a dynamic component. The results are compared with mode coupling theories which describe the glass transition in supercooled liquids.     

Elastic constants for NaCl and KCl solids at high temperatures

In the present study we derive new relationships to predict the temperature dependence of elastic constants. Proposed relationships are applied to study elastic constants of NaCl and KCl. A linear relationship between thermal expansivity and elastic constants at high temperatures is found to exhibit for solids. The extrapolated data on elastic constants in very high temperature region obtained in the present study are useful to understand the thermoelastic properties of NaCl and KCl. A close agreement between theory and experiment reveals the validity of the formulations used.     

Luminescence spectra of lead-doped NaCl,KCl and KBr

The luminescence spectra of lead-doped NaCl, KCl and KBr have been systematically investigated. Special attention has been paid to the effects of concentration and thermal history of the crystals. In the three systems, the emission spectra for A and C band excitation consists mainly of two well-defined emission bands whose energy separation is ～0.7 eV. It has been concluded that none of the bands can be attributed to a single type of lead center but are both typical of Pb²⁺ luminescence. In fact, their behavior can be correlated with that found for most monovalent ions and interpreted in a similar way. The excitation spectra for the two emissions have shown that the A-band is complex. One of the components appearing in very low doped and quenched samples is ascribed to dipoles, whereas additional side bands are attributed to complexes or small aggregates involving Pb²⁺ ions.     

BGO晶体的布里渊散射与弹性系数测量

ＢＧＯ晶体的布里渊散射与弹性系数测量刘金全＊陈浩＊刘玉龙（中国科学院物理研究所北京１０００８０）（＊北京大学北京１００８７１）肖季驹（香港城市大学物理及材料科学系）ＥｌａｓｔｉｃＣｏｎｓｔａｎｔｓｏｆＢＧＯｂｙＢｒｉｌｏｕｉｎＳｃａｎｔｅｒｉｎｇＬｉ...     

Elastic and inelastic scattering of slow positrons from a LiF(100) surface

The intensity profile for the elastic specular reflection of 5–100 eV positrons from a LiF(100) surface (ang1e of incidence 45°) has been measured using a simple time-of-flight spectrometer. The profile exhibits strong maxima below 25 eV and a smaller peak at 57 eV. Positron energy loss spectra have also been measured for a range of incident energies by retarding field analysis of the scattered beam. The mean energy loss appears to increase with increasing incident beam energy. Both the elastic and inelastic results are compared with similar data for slow-electron scattering obtained with the same apparatus.     

Detecting submerged objects by Brillouin scattering

The principle of detecting submerged objects by Brillouin scattering is analyzed. It is based on testing the frequency shift rather than the intensity of the scattered signal. Experiments based on the principle are carried out. An actual detecting system for practical applications is designed, and the experimental results obtained by using the system are given. The experimental results show that the method of Brillouin scattering possesses several advantages compared to the method of testing the signal intensity. The conclusion is that Brillouin scattering is a good method for detecting submerged objects. PACS 42.68.Wt; 42.79.Qx; 78.35.+c     

Determination of elastic, piezoelectric, and dielectric constants of an R:BaTiO3 single crystal by Brillouin scattering

From the sound velocity measured using the Brillouin scattering technique, the elastic, piezoelectric, and dielectric constants of a high-quality monodomain tetragonal Rh：BaTiO3 single crystal are determined at room temperature. The elastic constants are in fairly good agreement with those of the BaTiO3 single crystal, measured previously by Brillouin scattering and the low-frequency equivalent circuit methods. However, their electromeehanical properties are significantly different. Based on the sound propagation equations and these results, the directional dependence of the compressional modulus and the shear modulus of Rh：BaTiO3 in the （010） plane is investigated. Some properties of sound propagation and electromechanical coupling in the crystal are discussed.     

Elastic constants of solid krypton at T = 77 K determined by inelastic neutron scattering

Long-wavelength acoustic phonons have been studied for each of the [ζ00]T, [ζ00]L, [ζζ0]L and [ζζ0]T₁ branches in solid Kr at T = 77 K by means of inelastic neutron scattering utilizing ‘cold neutrons’ as they are available in the long-wave length tail of the pile spectrum. The raw data have been corrected for resolution effects taking into account the curvature of the dispersion surface and variation of mode eigenvectors. It has turned out, that this yields appreciable shifts of the raw data. The results of our experiment give c₁₁ = 4·25 ± 0·10, c₄₄ = 2·04 ± 0·03, c₁₂ = 2·82 ± 0·12 and a value for B = (c₁₁ + 2c₁₂)/3 = 3·30 ± 0·09 × 10¹⁰ dyne/cm². Available thermodynamic data for Kr gives a derived value for B^ad = 2·58 ± 0·06 × 10¹⁰ dyne/cm² indicating a large difference between zero sound and first sound in solid Kr at high temperatures.     

Brillouin scattering and the CMB

Brillouin scattering of photons off the density fluctuations in a fluid is potentially important for cosmology. We derive the Brillouin spectral distortion of blackbody radiation, and discuss the possible implications for the cosmic microwave background. The thermal Sunyaev-Zeldovich effect is slightly modified by Brillouin distortion, but only at very long wavelengths.