Lattice dynamic properties of BaSi2 and BaGe2 from first principle calculations

First principle calculations have been performed to study the lattice vibration of BaX₂ (X = Si, Ge). A rigid-unit vibrational mode has been observed, and this mode confines and scatters acoustic phonon modes, leading to a low thermal conductivity. Their stability is analyzed from the calculations of thermodynamic properties.     

Vibrational modes and phase transition of Si-Ge solid solution

We present the simplified treatment where the lattice vibrations of Si or Ge atoms in the Si-Ge solid solution are replaced with that of pure Si or Ge crystal at lattice constants of the alloy. Considering the volume effect on the force constants of the pure constituent, we obtain the phonon dispersion curves of the local and band modes for Si_0.91Ge_0.09 and Si_0.11Ge_0.89 systems and the concentration x-dependence of the local and band modes frequencies in the Si_1?xGe_x solid solutions. Then, from the calculation of the effective mode Grüneisen parameter γ_i for the average phonon modes in the Si_1?xGe_x systems, we obtain the predominant correlation between TA mode Grüneisen parameter γ^X_TA at the point X and the phase transition pressure P_t, and the softening of TA modes is related to the pressure-induced phase transition of the Si-Ge solid solution.     

Temperature dependent photoluminescence processes in ZnO thin films grown on sapphire by pulsed laser deposition

Temperature dependence of the photoluminescence (PL) transitions in the range of 10–300 K was studied for ZnO thin films grown on sapphire by pulsed laser deposition. The low temperature PL spectra were dominated by recombination of donor bound excitons (B_X) and their phonon replicas. With increasing temperature, free exciton (F_X) PL and the associated LO phonon replicas increased in intensity at the expense of their bound counterparts. The B_X peak with line width of ∼6 meV at 10 K exhibited thermal activation energy of ∼17 meV, consistent with the exciton-defect binding energy. The separation between the F_X and B_X peak positions was found to reduce with increasing temperature, which was attributed to the transformation of B_X into the shallower donor bound exciton complexes at consecutive lower energy states with increasing temperature, which are possible in ZnO. The energy separation between F_X peak and its corresponding 1-LO phonon replica showed stronger dependence on temperature than that of 2-LO phonon replica. However, their bound counterparts did not exhibit this behavior. The observed temperature dependence of the energy separation between the free exciton and it is LO phonon replicas are explained by considering the kinetic energy of free exciton. The observed PL transitions and their temperature dependence are consistent with observations made with bulk ZnO crystals implying high crystalline and optical quality of the grown films.     

First-principles calculation of the phonon frequencies in γ Fe

The total energy, the equilibrium lattice constant, and the bulk modulus of the fcc phase of iron have been calculated by the full-potential LMTO method. The use of a generalized gradient approximation in the calculation of the electronic structure and lattice properties of γ-Fe is discussed. Next, the transverse phonon frequency at the W point of the Brillouin zone of a fcc lattice is calculated by the “frozen-phonon” method in the harmonic approximation. A local minimum has been found in the curve of the variation of the total energy of the system as a function of the amplitude of the atomic displacements corresponding to the chosen normal mode. To take account of anharmonic effects, a pseudoharmonic approximation is used and an effective potential that approximates the curve of the variation of the total energy of the system and depends on the temperature via the correlation function of the mean-square displacement of atoms from their equilibrium positions is constructed. The theoretical temperature dependence of the effective frequency of the phonon mode responsible for the structural phase transition corresponds qualitatively to the experimentally observed dependence. A new interpretation is given for the structural phase transition as a transition of the corresponding phonon mode from the excited to the ground state. Fiz. Tverd. Tela (St. Petersburg) 39, 171–175 (January 1997)     

The phonon dispersion relations in a disordered Ni1-xPtx system

The phonon dispersion relations of disordered alloys Ni_1-xPt_x were measured for five specimens covering the full range of atomic concentration by means of neutron inelastic scattering. At the resonant frequencies well separated double peak structures were observed in the phonon spectra. For the specimens with $\text{x ≧ 0.30}$, the energy splitting of the phonon dispersion curve for the L-branch takes place at a higher frequency than that for the T-branch. The maximum phonon energy decreases with increasing Pt concentration but deviates from the theoretical prediction based on the mass defect CPA model. For x = 0.95 the localized mode cannot be observed as a well defined peak and does not seem to be split off from the main phonon band.     

Soft phonon mode at the martensitic phase transformation of AuCuZn2

The lattice dynamical property of AuCuZn₂ has been investigated by means of inelastic neutron scattering technique in connection with its martensitic phase transformation. The temperature dependent soft phonon was observed transformation. The temperature dependent soft phonon was observed around $\text{q =}\text{2}\text{3}\text{[110]}$ of TA₂ phonon branch. We have also found a premartensitic metastable phase, in which new satellite reflections at (H± $\text{2}\text{3}$, K ? $\text{2}\text{3}$, 0) have been observed around fundamental reflections with H + K = 4n. The atomic displacements of the soft phonon mode correspond to the atomic arrangement of the premartensitic phase.     

The physical process of melting and freezing

It is suggested that at the melting temperature the wavelength of the average thermal phonon vibration is equal to or is a harmonic of the distance separating the atomic layers in the crystal. This resonance between the phonon and lattice vibrations equals out the energy of the vibrating atoms in the surface layer. If this “uniform” energy is higher than the energy corresponding to the metastable transition state then all the surface atoms lose its position stability. In order to make the jump to the next potential well energy is required to overcome on the viscous resistance of the liquid. If this energy, latent heat of fusion, is supplied then the atomic/molecular sheet or platelets from the surface are detached and melting occurs. The proposed model is consistent with all of the characteristic features of melting and freezing. Equations calculating the average phonon wavelength and the corresponding lattice distance at the melting temperature are derived from fundamental thermodynamic relationships. The required thermodynamic parameters are determined from experiments of the nine selected highly symmetrical solids. The calculated wavelengths of the phonon vibration are equal to or is a harmonic of the $d$-spacing of the atomic/molecular layers in agreement with theory.     

Phonons and electron-phonon interactions in rare-gas crystals at high pressures

The lattice dynamics of compressed rare-gas crystals is theoretically investigated within the ab initio approach in the framework of the Tolpygo model, which explicitly allows for the deformation of electron shells. The deformation of the electron shells is associated with the retardation of the electron response and treated as a nonadiabaticity (the electron-phonon interaction). This approach and the ab initio short-range repulsive potentials are used to construct the dynamic matrix, which makes it possible to calculate the phonon frequencies and the electron-phonon interaction of crystals in the series Ne-Xe at any point of the Brillouin zone. The contributions of the long-range Coulomb and van der Waals forces to the dynamic matrix are the structure sums that depend only on the lattice type. The structure sums for the face-centered cubic lattice are calculated using the Ewald and Emersleben methods, as well as the direct summation over the vectors of the face-centered cubic lattice. The use of 20 spheres in the last case provides an accuracy of no less than four significant figures. An analysis of the role played by the phonon-electron interaction at five points of high symmetry in the Brillouin zone (X, L, U, K, W) at high pressures demonstrates that not only the longitudinal phonon modes (at the points X and L) but also the transverse phonon modes (at the points U, K, and W) are softened. The inclusion of the electron-phonon interaction at the point X improves agreement between the theoretical and experimental phonon frequencies for the argon crystal.     

Investigation of the vibrational properties of cubic yttria-stabilized zirconia: A combined experimental and theoretical study

A combined experimental and theoretical investigation into the vibrational properties of cubic 8–9 mol% yttria-stabilized zirconia (YSZ) is presented. Measurements of acoustic phonon dispersion curves have been obtained from inelastic neutron scattering investigations using a triple axis spectrometer, as well as calculations of the vibrational density-of-states (vDOS) using density-functional theory. The present measurements agree closely with, and extend, previously published results. The phonons become broader and decrease in intensity as the Brillouin zone boundary is approached, particularly in the Γ–Δ–X direction. Interestingly, there is evidence of a previously unreported low energy phonon band (8–9 meV) in the Γ–Σ–X direction, which could possibly be related to the stabilization (by yttria doping) of the imaginary mode of cubic ZrO₂ about the X-point. Compared to pure cubic ZrO₂, the vDOS of YSZ are broader and extend to higher frequency. Furthermore, the prominent Zr-related feature in the vDOS of c-ZrO₂ at ≈14 meV is shifted to higher energy in the vDOS of YSZ. This behavior is consistent with the measured dispersion bands (first acoustic branch in the Γ–X direction, about the X-point) of YSZ which is higher in energy by a similar amount relative to that of c-ZrO₂, thus providing support for the structural model considered.     

First-principles study on the structural,electronic and optical properties of BiOX (X=Cl,Br, I) crystals

In order to investigate systematically the structural, electronic and optical properties of bismuth oxyhalides BiOX (X=Cl, Br, I) semiconductors, the lattice constants, structural characteristics, band structures, densities of states, atomic charge populations and optical properties of BiOX crystals have been calculated using first-principles based on DFT. The calculated indirect band gaps of BiOCl, BiOBr and BiOI crystals are 2.50, 2.10 and 1.59 eV, respectively. The analysis of densities of states and atomic charge populations for BiOX crystals indicates that, (a) the valance band maximum is mainly contributed to O 2p and X np states and the Bi 6p states dominate the conduction band minimum; (b) the contribution of X ns states obviously increases with the increase of X atomic numbers, and the dispersive energy level becomes more and more significant and (c) the sequence of covalent bonding strength between atoms is Bi–O >Bi–I>Bi–Br>Bi–Cl. In addition, the calculated absorption edges of the absorption coefficients I(ω) for BiOCl, BiOBr and BiOI crystals are 355, 448 and 645 nm, respectively, which agree well with our experimental measurements of 376, 442 and 628 nm and the previous reported results of 370, 440 and 670 nm.     

Ab initio calculation of the structure and optical properties of lead oxyhalides Pb<Subscript>3</Subscript>O<Subscript>2</Subscript><Emphasis Type="Italic">X</Emphasis><Subscript>2</Subscript> (<Emphasis Type="Italic">X</Emphasis> = Cl,Br, I)

Using ab initio methods, we obtained information on the crystal structure, calculated the phonon spectra, and determined the optical properties of oxyhalides Pb₃O₂Br₂ and Pb₃O₂I₂. A compound of Pb₃O₂Br₂ is synthesized, and its phonon spectra are recorded. The experimentally observed vibrational bands are assigned. A comparative analysis of the crystal lattice parameters, the phonon spectra, and the anisotropy parameters of lead oxyhalides Pb₃O₂ X ₂ (X = Cl, Br, I) is performed.     

Comparison of the molecular cluster model with the phonon model for Jahn—Teller active impurities in crystals

For comparing the molecular cluster model with the phonon model in describing the Jahn-Teller interaction for magnetic impurities in crystals, the Jahn-Teller energy and the Ham reduction factors are expressed in terms of the phonon Green's function of the host crystal for an orbital triplet coupled to E_g modes of vibrations. Numerical results for the case of MgO lattice have been obtained using the phonon Green's function derived from the breathing shell model.     

Study on the oscillatory behaviour of the lattice parameter in ternary iron–nitrogen compounds

The structural properties of the XFe₃N (X=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) cubic ternary iron based nitrides as well as the preferential occupation site of X in the structure were studied using Full Potential Linearized Augmented Plane Wave method, within the Density Functional Theory formalism, Wien2k code, the exchange-correlation potential described with the Perdew–Burke–Ernzerhof expression, based in the Local Spin Density Approximation and Generalized Gradient Approximation. According the calculations, the Sc, Co, Ni, Cu and Zn, atoms preferred the corner sites of the cubes, while Ti, V, Cr and Mn occupy the centre of the faces of the equilibrium structures. The equilibrium structure lattice parameters have an oscillatory behaviour with the atomic number of X, with decreasing amplitude as the atomic number of X increases. This trend do not correlated with the atomic radii of X.     

纳米晶体线的晶格动力学格林函数及晶格振动

通过求解差分方程,推导了纳米晶体线的晶格动力学格林函数,分析了其晶格振动,并推导了声子数表象中的原子位移及晶格振动哈密顿公式.研究结果表明,纳米晶体线的晶格振动能带分裂为一系列的子带,格波只能沿纳米晶体线的纵向传播,沿纳米晶体线的横截面只存在驻波.     

Pressure induced lattice anomalies in pnictides

Hydrostatic pressure Raman and synchrotron XRD measurements at room temperature have been carried out on a series of NdFeAsO_1−xF_y (Nd1111) oxypnictides in order to investigate pressure-induced lattice modifications. The synchrotron XRD data indicate that there is an increased deviation of the lattice constants from smooth pressure dependence in the superconducting compound, in close agreement with the results from SmFeAsO_1−xF_x (Sm1111), although the effect is less pronounced in Nd1111. As in Sm1111 the hydrostatic pressure Raman measurements show that the A_1g mode of the rare earth atom deviates from the linear pressure dependence. Anomalous pressure dependence and a hysteresis is observed in the phonon width of the phonon modes. The calculated Grüneisen parameter for the Nd phonon is very similar to the corresponding value for SmFeAsO_1−xF_x compound and it does not vary with substitution. For the As mode it has a lower value indicating a stiffer phonon with the increased ion size. In connection with the XRD measurements the Raman data show a sudden increase of the pressure-induced lattice anomalies close to doping where the compounds become superconducting.     

Thermal expansion coefficient of GaAs and InP

The temperature dependence of the thermal expansion for GaAs and InP is investigated theoretically using the experimental pressure derivatives of elastic stiffness constants and phonon frequencies. The linear correlation between the transverse acoustical mode Grüneisen parameter γ^X_TA and the metallic transion pressure P_t obtained by Weinstein is not satisfied for GaAs and InP, but the observed thermal expansion of GaAs is well reproduced. In addition, the linear expansion coefficient of InP is predicted theoretically as a function of temperature. Then, the phonon dispersion curves of GaAs and InP at their covalent-metallic transition pressures are quantitatively shown.     

Additional anisotropy of electron bands in quasi-1d organic conductors (TMTSF)2X; (CH)x and related compounds due to the small polaron's effects

The strong anisotropy of the small polaron narrowing of electron bands in quasi-1d organic conductors (TMTSF)₂X; (CH)_x and in related compounds due to the anisotropy of a crystal lattice and of a phonon spectrum is proved. The large difference between the experimental and numerical results for electron band-widths in these substances and the extremely large anisotropy of electron bands ～ 10⁶, been found in (CH)_x, are interpreted in terms of this effect.     

Polarized Raman scattering from oriented single crystals of A2BX4 halides (A = Cs,B = Zn,X = Cl,Br, I)

Raman spectra of isostructural tetrahalozincates viz. Cs₂ZnI₄, Cs₂ZnBr₄ and Cs₂ZnCl₄ single crystals have been measured in various scattering orientations using polarized laser excitation. The lattice dynamics of the crystals is probed down to 70 K in order to cover the successive phase sequences. Except Cs₂ZnI₄, none of the other salts shows the characteristic commensurate-incommensurate features of lattice instabilities. The unit cell dynamics is understood in terms of 84 phonon branches originating from various internal modes of tetrahedral ZnX²⁻₄ group and external lattice modes. The enormous splittings, responsible for a complete loss of degeneracy in the internal modes for all the three salts, suggest a considerable distortion particularly at low temperatures in the tetrahedral geometry of the ZnX²⁻₄ group.     

Ab initio simulations on the atomic and electronic structure of single-walled BN nanotubes and nanoarches

To simulate the perfect single-walled boron nitride nanotubes and nanoarches with armchair- and zigzag-type chiralities and uniform diameter of ∼5 nm, we have constructed their one-dimensional (1D) periodic models. In this study, we have compared the calculated properties of nanotubes with those for both hexagonal and cubic phases of bulk: bond lengths, binding energies per B-N bond, effective atomic charges as well as parameters of total and projected one-electron densities of states. For both phases of BN bulk, we have additionally verified their lattice constants. In the density functional theory (DFT), calculations performed using formalism of the localized Gaussian-type atomic functions as implemented in the CRYSTAL-06 code we have applied Hamiltonians containing either PWGGA or hybrid (DFT+HF) B3PW exchange-correlation functionals. After calculation of Hessian matrix for the optimized structures of BN bulk (both phases) and nanotubes (both chiralities) using the CRYSTAL code we have estimated their normal phonon modes within the harmonic approximation. Applying both atomistic and continuum models we have calculated the elastic energies and moduli for SW BN nanoarches. Our calculations clearly show a reproducibility of the atomic structure, effective charges and total energy, as well as phonon and elastic properties when using either PWGGA or hybrid B3PW Hamiltonians. On other hand, there is a high sensitivity of the discrete energy spectra parameters (including band gap) to the choice of the first principles approach (the hybrid method reproduce them noticeably better).     

Raman scattering from GaAs-InxGa1−xAs strained-layer superllatices

Measurements of Raman scattering were performed on GaAs-In_xGa_1?xAs strained-layer superlattices, grown by molecular beam epitaxy, with lattice periods ranging from 30 ～ 250 Å and In concentrations x, 0.22 and 0.37. Only one GaAs-like longitudinal optical phonon peak was observed in each strained-layer superlattice, in contrast to the well-known result that two peaks were observed in GaAs-Al_xGa_1?xAs superlattices. The GaAs-like phonon frequencies shifted from those of bulk GaAs to those of bulk In_xGa_1?xAs alloys as the ratio of the one-layer thickness of In_xGa_1?xAs to the lattice period increases from zero to one. We conclude that the GaAs-like phonon mode is a uniform mode of the whole strained-layer superlattice and the phonon frequency is determined by the averaged In concentration.