The influence of composition on optical properties of ferroelectric potassium lithium niobate single crystals

Homogeneous crack-free Potassium Lithium Niobate (KLN) single crystals with different Li content were grown up by the TSSG technique. Lattice vibration spectra of these samples were investigated using Raman spectroscopy. The results showed that characteristic Raman spectra of the [NbO₆]⁷⁻ octahedral ions were strikingly influenced by the Li ions. Thus, the symmetric bend vibration mode ν₅ of the KLN sample with higher Li content was split into three Raman peaks, while the sample with lower Li content displayed a single peak, supporting that the bend vibration modes of the [NbO₆]⁷⁻ octahedrons were obviously perturbed by Li ions occupying C-sites. Enhanced Raman peak intensities of the KLN sample post-annealed at 900 °C for 24 h evidenced that content of defects in KLN crystal might be reduced. The transmittance spectra showed that the crystal with higher Li content had better transmittance intensity.     

Nonequilibrium molecular dynamics of liquid crystals

During the last 15 years, noneyuilibrium molecular dynamics (NEMD) has been successfully applied to study transport phenomena in fluids that are isotropic at equilibrium. A natural extension is therefore to study liquid crystals, which are anisotropic al equilibrium. The lower symmetry of these systems means that the linear transport coefficients are considerably more complicated than in an isotropic system. Part of the reason for this is that there are crosscouplings between tensors of different rank and parity. Such couplings arc symmetry-forbidden in isotropic phases. In this paper. we review some of fundamental theoretical results we have derived concerning the rheology of liquid crystals. report NEMD simulations of thermal conductivity and shear viscosity of liquid crystals, and present NEMD simulations of shear cessation phenomena. All of the NEMD results are presented for a model liquid crystal fluid which is a modification of the Gay-Borne fluid. The results obtained are in qualitative agreement with experimental measurements on liquid crystal systems.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Deformation rates effects in softwoods: Crack dynamics with lattice fracture modelling

Present study contributes towards understanding crack toughness against the intrinsic deformation rate sensitivity. A methodology for characterizing fracture dependence in softwoods through experimental and numerical analysis has been developed. Time-dependence was found to be the characterising parameter. Image analysis of fracture data acquired with high-speed camera showed that the crack speed histories are stochastic and erratic. In the higher rate range, crack dynamics is characterized as episodic and locally heterogeneous, with irregular jumps and arrests. Critical crack propagation speed at the highest rate tested of 200 mm/min was found to be between 0.7 m/s and 4 m/s (14.3 km/h). Fracture toughness decreased at both slow static and high loading rates, with the mean maximum at 1 mm/min, which is a static deformation rate specific to short-term standard tests. At 200 mm/min deformation rate, inertial effects suggested dynamic fracture response. Explanations of loading rates effects relate to the micro-processes in the fracture process zone (FPZ) and fracture mechanisms, which are simulated with discrete lattice fracture model (LFM). The model included viscous bi-linear stress relaxation into the softening relation and random stochastic finite element properties. Novel characterisation of softwoods is crucial for sensible numerical modeling in seismic structural situations.     

Computer simulation of deformation and fracture of small crystals by molecular dynamics method

Body-centred cubic iron whiskers having [100] and [110] axes were pulled in a molecular dynamics simulation using a supercomputer. The upper yield stress close to the theoretical strength was found. Above the upper yield stress, phase transformation was observed; at the same time the stress was greatly reduced. A new possible mechanism of twinning is proposed. The whiskers were pulled until they had broken into two pieces. Copper small crystals with and without a notch were sheared. It was observed that the edge dislocations were created at the surface and moved through and escaped from the crystals. Copper small single crystals with a notch were pulled. A half-dislocation was created near the tip of the notch. Sharp yield stress was observed. In medium deformation dislocations on different slip planes were created. Due to the cutting of dislocations the tensile stress increased.     

Pressure effects on the Raman spectra of 3-deutero-ammonia,lattica dynamics and vibrons up to 40 GPa

High-pressure Raman studies up to 40 GPa have been made on solid ND₃ at room temperature. Features of external and internal modes are compared to that of solid NH₃ and mode assignments are proposed in solid phase IV (p3.8 GPa). In contrast with NH₃, which transforms to a cubie phase at 15 GPa, solid IV ND₃ was observed to be stable up to 40 GPa, the upper limit of the present investigation. It is concluded that bond symmetrization in ND₃ will occur above that of NH₃.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Temperature dependence of microhardness in mixed crystals

The temperature dependence of microhardness of mixed KCl-KBr crystals has been studied using the Kataoka and Yamada theory which considers plastic deformation and internal stress field due to elastic interactions in mixed crystals. The microhardness showed systematic variation with temperature in both the components and the mixed crystals and the probable reasons for this variation are indicated.     

Energy band theory and the lattice dynamics of rare gas crystals

The results of a simple local-density energy band model (X) are used to carry through a calculation of the phonon dispersion relations in fcc Ne and Ar. Adiabatic phonons, calculated from the computed X total energy surface, are perturbed by the Fröhlich Hamiltonian with electron-phonon matrix elements calculated from the X energy bands. The use of low-order perturbation theory gives results in fair comparison with observed values, but shows the general scheme to be feasible. Except for questions of the exact physical content of the X model itself, the calculation is entirely ab initio, and is believed to be the first such achieved in insulators.Work supported by Research Corporation and National Science Foundation, and based in part on the Ph.D. thesis of J. P. Worth.     

Effect of Uniaxial Pressure on the Isotope Effect on the Spectrum of Hamiltonians with Electron–Phonon Coupling

We have studied the effect of a change in the equilibrium nearest neighbour distances on the dynamics of charge and ions on a three-site cluster, identified with an O–Cu–O cluster present in high temperature superconductors. We consider a model Hamiltonian that contains an electronic part represented by a single band Hubbard model with onsite electronic correlations and a phononic part consisting of non-interacting Raman and infrared active phonons. The electron–phonon coupling is introduced through the change in interatomic distances generated by Coulomb repulsion between charges at neighbouring sites. For intermediate and strong values of the electron–phonon coupling, this model generates correlated electron–ion motion, i.e., polaron formation. In order to simulate the effect of change in the equilibrium nearest neighbour distances, we assume that the main effect such a change is a variation of the intersite electron hopping probability, t. We, therefore, studied the excitation spectrum of this model and the local lattice distortion in the Cu–O bond length as a function of t. We also studied the effect of a change in t in the polaron tunnelling energy when we use different oxygen isotopic masses, i.e., O¹⁶ and O¹⁸. We find that as function of t, the isotopic shift does not show a monotonic behaviour, as it does as a function of the electron–phonon coupling constant. It exhibits a minimum for values of t for which the electron–phonon coupling generates local lattice distortions with magnitudes similar to those observed experimentally in high-temperature superconductors. This observation could be related with the observed maximum on T _c as a function of the microstrain of the Cu–O bonds (Sanna et al. in Int. J. Mod. Phys. B 14(29–31), 2000; Bianconi et al. in J. Phys.: Condens. Matter 12:10655, 2000; Agrestini et al. in J. Phys. A: Math. Gen. 36:9133, 2003).     

硒化镉(CdSe)单晶体的变温霍尔效应研究

通过变温（20～300K）霍尔效应测量,研究了CdSe单晶体的电阻率ρ（T）、载流子浓度n（T）、霍尔系数RH（T）和霍尔迁移率μH（T）的温度依赖关系.实验结果表明CdSe单晶体的导电类型总为n型,且它的电阻率与载流子浓度的温度依赖关系与n-Si单晶类似.通过拟合禁带宽度约为1.7eV.本文还进一步研究了本征区、饱和区、弱电离区内电子浓度的变化和霍尔因子γ随温度变化关系,并由此计算出杂质电离能（24.7meV）与补偿度（23.7%）.上述结果表明CdSe单晶体具有优良的电学特性,是制作室温核辐射探测器的理想材料.     

Simulation of ionic crystals and calculation of electrostatic potentials

By assuming complete ionicity, an efficient algorithm was performed for the evaluation of the electrostatic interactions. There are no symmetry restrictions for the application of this algorithm. The lattice self-potentials for distinct ion positions and the Madelung constant were computed for a wide variety of ionic crystals. The simulation was carried out using the conventional Ewald method. This work highlights the significance of the Coulomb potential in lattice effects and the existence of differences in potentials for complicated compounds. Finally, we remark that the general framework presented in this paper is more appropriate to study structural phase transitions.     

Lattice vibrations of materials for lithium rechargeable batteries. VI: Ordered spinels

Raman scattering spectra have been investigated to evaluate the local structure of lithiated oxides used as electrode materials for lithium-ion batteries. We report the analysis of the vibrational spectra of ordered spinel phases including the partially delithiated λ-Li_0.5Mn₂O₄ ( SG), the partial charge-ordered LiMn₂O₄ orthorhombic form (Fddd SG) and the LiNi_0.5Mn_1.5O₄ substituted oxide (P4₁32 SG). Analysis of spectroscopic data is performed using the classical factor group theory and the vibration features are compared with those of the ordered lithium ferrite -LiFe₅O₈ and the normal spinels LiMn₂O₄ and LiNi_0.5Mn_1.5O₄ (Fd3m SG), and the inverse spinel LiNiVO₄.     

NMR study of the slow dynamics and local structure in lyotropic micellar liquid crystals

The first experimental evidence of director fluctuations in a micellar lyotropic liquid crystal, studied by ¹H spin-lattice relaxation rate, is reported. The system is constituted by the ternary mixture: potassium laurate/1-decanol/water. With the aim to test the slow dynamics, the experiment has been performed over a broad range of Larmor frequencies (2 × 10³− 6.6 × 10⁶ Hz), using fast field-cycling NMR relaxometry. The results evidence that in the nematic and isotropic mesophases and the poliphasyc region, director fluctuations are responsible for the spin-lattice relaxation dispersion in the low Larmor frequency range (<10⁵ Hz). By increasing the intermicellar water content, a crossover was found between a quasi-isotropic 3D director fluctuations behavior to a 2D pseudo-lamellar undulation fluctuations. In order to explain the higher frequency range (>10⁵ Hz), two relaxation mechanisms are assigned: i) molecular reorientation by translational diffusion on the micellar surface, and ii) molecular exchange between the micelle and the bulk. Received: 30 March 2000     

Phonon spectra of eulytite crystals Bi4M3O12 (M = Ge,Si): ab initio study

In this paper we present the results of ab initio DFT calculation of phonon spectra for bismuth ortho-germanate Bi₄Ge₃O₁₂ and bismuth ortho-silicate Bi₄Si₃O₁₂ crystals, in the center of the first Brillouin zone. First, the geometry optimization was performed using the analytical energy gradients, with respect to atomic coordinates and unit cell parameters. Vibrational frequencies and normal modes were calculated within the harmonic approximation by diagonalizing the mass-weighted Hessian matrix. The IR and Raman spectra of both crystals were simulated with the periodic ab initio Crystal 09 code and B3LYP hybrid functional and the two sets of Transverse-Optical and Longitudinal-Optical frequencies are generated, together with their intensities. Also, the influence of isotopic substitution for Bi, Ge and O in phonon modes and the picture with values of frequencies shift in each mode by isotopic substitution were calculated. The obtained results are discussed and the comparision between the computed spectra and experimental data is quite satisfactory, which justifies the model and simulation scheme used for the title systems.     

纳米氧化铝的晶型及粒度对其红外光谱的影响

利用硫酸铝铵热解法,通过控制焙烧温度,制备了不同晶型和粒度的纳米Al2O3.XRD物相分析表明,焙烧至900℃可得到纯γ-Al2O3,1200℃发生相转变,生成α-Al2O3.用Scherrer公式计算得到了各样品的晶粒度.对所制备的纳米Al2O3的红外光谱进行了详细研究.结果表明,不同晶型的纳米Al2O3具有不同的红外光谱特征,因此,红外光谱可以作为一种定性判断Al2O3是否发生了相转变的辅助手段.实验中发现所制备的纳米Al2O3的红外光谱存在吸收峰的蓝移和宽化,对出现此现象的原因进行了分析讨论.最后,对纳米金属氧化物材料出现谱移现象的原因进行了归纳总结.     

光子晶体及其制备方法研究进展

光子晶体是具有光子能带及能隙的一类新型材料,具有奇特的性质和广阔的应用前景。本文简述了光子晶体的主要特征,重点阐述了三维有序结构光子晶体的制作方法,并介绍了调节光子带隙的手段。     

Study of the structure and lattice vibrations of Ce6Rh30Si19 and A6Pt30Si19 (A = Th, U)

We present an atomistic study on the phase stability, site preference and lattice constants of the rare earth or actinium intermetallics Ce₆Rh₃₀Si₁₉ and A₆Pt₃₀Si₁₉ (A = Th, U). The calculated preferential occupation site of one Si atom is found to be the 2b site in Ce₆Rh₃₀Si₁₉ and A₆Pt₃₀Si₁₉. The Si atom is located at the sites 2b with occupancy close to 50%. Our calculated lattice constants agree with a report in the literature. The total and partial phonon densities of states are evaluated first for these complicated defect structures. The analysis of the inverted potentials explains qualitatively the contributions of different atoms to the vibrational modes.     

Lattice dynamics of Mg2Si and Mg2Ge compounds from first-principles calculations

We report first-principles calculations of the structure, elastic properties, lattice dynamics and some thermodynamic properties of Mg₂Si and Mg₂Ge compounds. The fully relaxed structure parameters and elastic stiffness constants of Mg₂Si and Mg₂Ge compounds are in good agreement with previous experimental data. The linear response method is applied to determine the phonon dispersion relations, phonon density of states, and Born effective charge. The computed thermodynamic properties such as vibrational entropy, constant-volume specific heat, and Debye temperature coincide with previous experimental data.     

Crystal growth and spectroscopic studies of novel Yb-doped K5Nd(MoO4)4 single crystals

K₅Nd(MoO₄)₄ crystals with different Yb³⁺ concentrations were grown using Czochralski technique. Room-temperature absorption spectra were recorded and assigned on the basis of Dieke's diagram for Nd³⁺ ion; the standard Judd-Ofelt theory has been used to analyze the spectra. Increase of Yb³⁺ concentration leads to variation of the corresponding Judd-Ofelt intensity parameters. Significant contribution of the Yb↔Nd energy transfer into the formation of the Nd³⁺ absorption spectra causes the observed changes. After illumination of the crystals with CW Nd:YAG laser changes of the birefringence for the laser line of CW He-Ne laser at 633 nm were studied. Birefringence changes show good correlation with the content of Yb ions.     

Lattice dynamics of layered ferroelectric semiconductor compound TlGaSe2

We present the first-principles calculation of the lattice dynamics of the TlGaSe₂ ternary semiconductor having highly anisotropic crystal structure. Calculations have been performed using open-source code ABINIT on the basis of the density functional perturbation theory within the plane-wave pseudopotential approach. Results on the frequencies of phonon modes in the centre of Brilloin zone and the dispersion of transverse shear acoustic branch of the phonon spectra agree well with the experimental data on Raman scattering, infrared reflectivity and ultrasound wave propagation in TlGaSe₂. The calculated and experimental temperature dependencies of heat capacity are in a good agreement up to the room temperature. Along the layer, the low-frequency acoustic branch displays the bending wave behavior which is characteristic of the layer crystal structures.