Atomic structure of the binary icosahedral Yb-Cd quasicrystal

Icosahedral quasicrystals (i-QCs) are long-range ordered solids that show non-crystallographic symmetries such as five-fold rotations. Their detailed atomic structures are still far from completely understood, because most stable i-QCs form as ternary alloys suffering from chemical disorder. Here, we present the first detailed structure solution of i-YbCd(5.7), one of the very few stable binary i-QCs, by means of X-ray structure determination. Three building units with unique atomic decorations arrange quasiperiodically and fill the space. These also serve as building units in the periodic approximant crystals. The structure is not only chemically feasible, but also provides a seamless structural understanding of the i-YbCd(5.7) phase and its series of related i-QCs and approximant crystals, revealing hierarchic features that are of considerable physical interest.     

Lattice dynamics and the compression curves of cobalt

The phonon dispersion relations along symmetry directions in cobalt have been obtained using a nearest neighbor central force model for hcp lattices. The third-order elastic (TOE) constants obtained in this model have been used to determine the changes in the lattice parameters due to high pressure, using Thurston's extrapolation formula. The variation of volume with pressure has been calculated using Murnaghan's logarithmic equation. The agreement of the calculated values with the experimental shock wave data up to 1.2 Mbar pressure is remarkably good.Supported in part by the Council of Scientific and Industrial Research, Government of India, through the grant of a Research Fellowship to A.R.     

Lattice dynamics and molecular dynamics simulation of complex materials

In this article we briefly review the lattice dynamics and molecular dynamics simulation techniques, as used for complex ionic and molecular solids, and demonstrate a number of applications through examples of our work. These computational studies, along with experiments, have provided microscopic insight into the structure and dynamics, phase transitions and thermodynamical properties of a variety of materials including fullerene, high temperature superconducting oxides and geological minerals as a function of pressure and temperature. The computational techniques also allow the study of the structures and dynamics associated with disorder, defects, surfaces, interfaces etc.     

Lattice dynamics of solid Ne

Previously derived interatomic potentials for Ne are used to examined selected solid state properties. The self-consistent phonon harmonic approximation with suitable corrections for cubic anharmonic terms is used to calculate phonon dispersion curves at 5 K anda=4.454 Å. The isochoric temperature shifts between 5 and 22 K of phonons propagating in the [100] direction are evaluated using two approximations for the phonon self-energy. ThepV isotherm at 4.2 K and the volume dependence of the elastic constants and the zero-temperature Debye theta from the heat capacity are also calculated. There is reasonable agreement with experiment at low temperatures but near the melting point there is a real discrepancy with recent Brillouin scattering data and reasons for this are discussed.     

Lattice dynamics and thermal expansion of lutetium

Dispersion relations, the lattice specific heat, third-order elastic constants, and the temperature variation of the lattice volume Grüneisen function of lutetium are determined on the basis of Keating's method. The lattice heat capacity is calculated using the frequency distribution function g(), and compared with the experimental values of Gerstein et al. The third-order elastic (TOE) constants are calculated using two anharmonic parameters. The low- and high-temperature limits _L and _H of the lattice thermal expansion are evaluated. The variation of lattice volume Grüneisen function with tempera- ture is presented. The pressure dependence of the lattice parameters and the compression ratio V/V ₀ of Lu are calculated using the theoretical TOE constants and Thurston's extrapolation formula.     

Lattice dynamics in VO2 near the metal-insulator transition

We present a comprehensive experimental study of the lattice dynamics in the vicinity of the metal-insulator transition in VO₂ by means of combined use of Raman spectroscopy and ultrasonic microscopy. Single crystalline samples of high quality particularly allow quite a complete determination of all Raman modes in the insulating phase at the Γ-point and the observation of an optical soft mode. In addition, the elastic behavior has been successfully investigated by measuring the propagation velocity of ultrasonic surface waves microscopically excited in various crystal directions. Our study reveals a striking coincidence of strong lattice softening attributable to certain acoustic branches and the occurrence of the optical soft mode, which precedes the metal-insulator transition over more than 100 K on approaching the critical temperature.     

Lattice dynamics of Ga1−xAlxAs studied by ab initio calculations

We have calculated the phonon dispersion and phonon density of states of superlattices of Ga_1−xAl_xAs for x=0.0, 0.25, 0.75 and 1.0 using the ab initio method within the supercell approach and calculating the Hellmann–Feynman forces. A noticeable difference of force constants between Ga–As and Al–As atomic pairs has been found. In any case, Al atoms vibrate in well-separated high-frequency optic modes.     

Lattice dynamics of (Ba/K)BiO3

The Lattice dynamics of Ba_.6K_.4BiO₃ was investigated by inelastic neutron scattering on a superconducting single crystal (T _c =26 K (midpoint)). At low frequencies the dispersion curves are very similar to those observed in BaPb_.75Bi_.25O₃. Differences were found in the bond bending vibrations of the BiO₆ octahedra which indicate that the binding in the K-doped compound is more ionic. Rather anomalous features were observed in the high frequency Bi-O bond stretching vibrations which resemble those observed in the high T _c cuprates La_1.85Sr_.15CuO₄ and YBa₂Cu₃O₇. The observed frequency shifts are interpreted as the consequence from a strong electron phonon coupling. The data are compared to the results obtained on non superconducting Ba_.98K_.02BiO₃.     

Effect of cooling rate on the formation of metastable icosahedral quasicrystal phase in rapidly solidified Al-8.2 at % Mn alloy

The microstructure of rapidly solidified Al-8.2 at % Mn alloy was analysed by X-ray diffraction, transmission electron microscopy and energy-dispersive analysis of X-rays and the effect of cooling rate on the formation of the metastable icosahedral quasicrystal phase (IQP) was investigated. The formation of IQP was found to be sensitive to the cooling rate in a rapidly solidified alloy of a certain composition. A lower critical cooling rate at which metastable IQP starts to appear and an upper critical cooling rate at which IQP suppresses completely the stable crystalline phase exist. The fact that the amount and the manganese concentration of IQP change non-linearly with the cooling rate suggests that there is an optimum cooling rate at which both the amount of IQP and its solute concentration reach maximum values in an alloy of a certain composition.     

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.     

The role of defects in the crystal/quasicrystal transformation

The transformation between quasicrystals and related crystals, the so-called approximant phases, appears as a major point in the understanding of quasicrystal stability. Structural defects of approximant phases seem to be involved in the mechanism of crystal/quasicrystal transformation. Theoretical works as well as observations are supporting this point of view. We here report observations made by transmission electron microscopy on two systems which provide relevant examples for two types of mechanisms. In both cases, the approximant defects are identified as antiphase boundaries. In the first system (Al-Li-Cu), the transformation is due to a progressive organization of the approximant phase defects. In the second system (Fe-Cr-Mo), the vertex of intersecting defects exhibits key structural feature for the transformation in a quasicrystal.     

正二十面体金刚石的形成及其计算机模拟

采用热丝化学气相沉积(HFCVD)法,以甲烷和氢气为反应气体,在YGl3(WC-13%Co)硬质合金基体上制备了金刚石膜.膜中存在大量五重对称结构的正二十面体金刚石晶粒(IDC).当尺寸较小时晶粒为较完整的正二十面体形状,尺寸达微米级后晶粒为带有"沟槽"或"凹坑"的变种正二十面体形状.研究了IDC的形成机制,并进行了计算机模拟.结果表明:对于四面体立方结构来说,IDc的{111}孪晶面与正常{111}孪晶面相比存在2.87°的差异,孪晶面两侧原子几何位置失配,使其成为畸变孪晶面,这种畸变孪晶面导致IDC晶粒存在"凹坑"和"沟槽";IDC的晶核是正十二面体烷(C20H20).     

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.     

Lattice dynamics and specific heat of Al-Si and Al-Ge solid solutions

A simplified treatment has been proposed to study quantitatively the lattice dynamics of Al-Si and Al-Ge alloy systems by solid-solutioning under pressure. The volume and electron density effects on the lattice dynamics of the pure constituents aluminium, silicon or germanium are considered, and the phonon dispersion relations of the local and band modes were obtained. Then, the concentration dependence of the local and band mode frequencies were calculated for the Al_1–x Si _x and Al_1–x Ge _x systems. Using the local and band mode frequencies, the lattice specific heat at constant volume was determined theoretically, and results obtained for the temperature-dependent specific heat of matrix aluminium were found to be in good agreement with the experimental data. The concentration dependence of the specific heat could then be predicted quantitatively for Al_1–x Si _x and Al_1–x Ge _x alloy systems.     

Lattice dynamics,thermal expansion,and bulk modulus of ruthenium

The lattice dynamics, third-order elastic (TOE) constants, and the temperature variation of the volume Grüneisen function of ruthenium have been calculated using the nearest neighbor central interaction model for hexagonal metals proposed by Srinivasan and Ramji Rao. The TOE constants have been employed to calculate the low-temperature limit of the lattice thermal expansion, the Anderson-Grüneisen (AG) parameter , and the second Grüneisen constantq of ruthenium. has the value 2.79 for ruthenium. The high-temperature limit has the value 3.31, which agrees well with the experimental value 3.25 obtained from thermal expansion and specific heat data of ruthenium. Anderson's theory has been used to explain the temperature dependence of the bulk modulus of ruthenium up to 923 K and has been compared with the experimental values obtained from the elastic constant data of Fisher and Dever. The variation of the lattice parameters of ruthenium with hydrostatic pressure up to 400 kbar has been calculated from its TOE constant data using the extrapolation formula of Thurston and has been compared with the experimental results of Clendenen and Drickamer. The fit is remarkably good.     

Conservation integrals of any quasicrystal and application

By using direct calculation of the gradient and divergence of the Lagrangian of any quasicrystal, its dynamic conservation integrals are derived. These conservation integrals can be reduced to J- and M-integrals for plane and antiplane problems, which are calculated around the tip of an interfacial crack of antiplane sliding mode between a crystal and a quasicrystal.     

Atomic structure and vibrational properties of icosahedral α-boron and B4C boron carbide

The Raman and infrared spectra of α-rhombohedral boron B₁₂ and of B₄C boron carbide have been determined by accurate first-principles calculations based on density-functional perturbation theory. Our results account for all the features observed experimentally, including the characteristic Raman-active mode around 530 cm⁻¹, which is attributed to the libration of the icosahedra. A comparison of the calculated vibrational spectra with experimental data allows the first unambiguous determination of the atomic structure of B₄C. Analysis of our data shows that the high bulk moduli of α-rhombohedral boron and of B₄C boron carbide – 220 and 240 GPa, respectively – are mainly determined by the stiff intramolecular bonding within each icosahedron. This finding is at variance with the current interpretation of recent neutron diffraction data on B₄C in terms of a postulated larger stiffness of the intermolecular bonds in icosahedral solids (inverted molecular compressibility). Our results show that icosahedral boron-rich solids should be considered as members of a new class of covalently bonded materials.