Anomalous temperature behavior of hypersonic acoustic phonons in a lysozyme crystal

The behavior of acoustic phonons in tetragonal lysozyme crystals is studied using Brillouin scattering over a temperature range from 298 to 330 K. Around 307 K, anomalies in the temperature dependence of the velocity and integral intensity of a quasi-transverse phonon and in the velocity of a quasi-longitudinal phonon are detected. The experimental results are discussed in terms of the current concepts of the dynamics of proteins. The observed anomalies in the behavior of hypersonic acoustic phonons are indicative of a structural phase transition occurring in a lysozyme crystal at 307 K.     

高压下γ'-Fe4N晶态合金的声子稳定性与磁性

利用基于密度泛函理论的第一性原理研究了高压下有序晶态γ’-Fe₄N合金的晶格动力学稳定性与磁性. 对比没有考虑磁性的γ’-Fe₄N的声子谱, 得出压力小于1 GPa时, 自发磁化诱导了铁磁相γ’-Fe₄N基态晶格动力学稳定. 压力在1.03-31.5 GPa时, Σ线上的点(0.37, 0.37, 0)、对称点X和M 上相继出现了声子谱软化现象. 压力在31.5-60.8 GPa时, 压致效应与自发磁化对诸原子的作用达到了稳定平衡, 表现出了声子谱稳定. 压力大于61.3 GPa时, 随着压力的增大压力诱导体系动力学不稳定性越强. 通过软模相变理论对于γ’-Fe₄N, 在10 GPa下的声学支声子的M点处软化现象的处理, 发现了动力学稳定的高压新相P2/m-Fe₄N. 压力小于1 GPa时高压新相P2/m-Fe₄N 是热力学稳定的相, 且磁矩与γ’-Fe₄N的磁矩几乎相同. 2.9-19 GPa时, P2/m相的焓比γ’相的焓小, 基态结构更稳定. 大于20 GPa时, 两相磁矩几乎相同.     

Thermal conductivity of NbSe3

The thermal conductivity, κ, of NbSe₃ has been measured by novel self-heating techniques that allowed the electric field dependence of κ to also be measured. Measurements were made from 35 K to room temperature. Above the charge density wave transitions, the phonon thermal conductivity is 4–7 times the electron thermal conductivity, and it rises smoothly below the transitions, indicating that phonon-phonon scattering predominates. Phonon mean free parths have been estimated at 187 A° at 60 K and 60 A° at 150 K. No clear anomalies were observed at the phase transitions, giving upper limits to changes in the phonon mean free path. No field dependence of κ was observed.     

Simulation of α-Zr structural stability under pressure using the molecular dynamics method

The structural stability of α-Zr was studied using the molecular dynamics method in wide temperature and pressure ranges. The interatomic interaction was described by a pair potential calculated within the Animalu pseudopotential model. The potential parameters were selected using α-Zr phonon spectra. The features in the dynamics of the α-β and α-ω phase transitions at various temperatures and pressures were considered. The calculated hysteresis of forward and backward phase transitions and its pressure and temperature dependences are discussed. The data obtained were used to plot phase equilibrium lines in the P-T phase diagram.     

Distortion of alpha-uranium structure in praseodymium metal to 311 GPA

It is generally observed that the rare earth metals adapt an orthorhombic alpha-uranium (α-U) structure at high pressures following the delocalization of 4f shell under compression. We examine the stability of the α-U structure in praseodymium metal at ultrahigh pressures of 313 GPa (volume compression V/V ₀?=?0.343) in a diamond anvil cell at room temperature. X-ray diffraction data show a transformation from the α-U structure to a primitive orthorhombic P2₁2₁2₁ phase at 147±5 GPa, which is characterized by the anisotropic compressibility of various crystallographic axes. This anisotropic compressibility leads to an interesting situation when the b-axis and the c-axis of the orthorhombic phase become nearly equal above 260 GPa and the structure can be regarded as a pseudo-tetragonal phase. Our present study shows that the 4f band metal Pr does not adapt a body centred tetragonal phase as predicted by theory, but instead novel crystallographic phases are observed at extreme compressions. The present results have a broader impact on the stability of the α-U phase in a variety of f-band systems at high pressures.     

Raman scattering study of the phase sequence in A2BX4 halides

Raman spectra of Cs₂ZnI₄ single crystal have been measured in different scattering orientations covering the successive phase transitions down to 68 K. Two second order anomalies at around ice temperature (270 K) and at ～ 92 K are observed. The anomaly around ice temperature exhibits the features of a normal-incommensurate transition evidenced by softening of a low-lying lattice phonon whose frequency decreases as the transition temperature is approached from below whereas the latter one appears to arise from usual structural distortions.     

Infrared spectra and lattice vibrations of the spin-chain compound LiCuVO 4

The infrared spectra of the one-dimensional antiferromagnet LiCuVO₄ are measured in the frequency range from 10 cm^-1 to 10 000 cm^-1 and at temperatures from 2 K to 300 K, for the electric field vector E of the radiation polarized either along the a- or along the b-crystallographic directions. For each polarization six infrared active phonon modes are observed in accordance with factor group analysis of the crystal structure of LiCuVO₄. The theoretical group analysis of the possible spinel low-symmetry phases is performed within the framework of Landau's theory of phase transitions. The parameters of several phonon lines show noticeable anomalies around 150 K where the magnetic correlations appear in the copper chains, which may indicate a finite interaction between the phonon and the magnon subsystems in LiCuVO₄. Received 19 February 2001 and Received in final form 26 June 2001     

Raman spectroscopy of CaSnO3 at high temperature: a highly quasi-harmonic perovskite

Calcium stannate perovskite (CaSnO(3)) has been studied by Raman spectroscopy at two excitation wavelengths (514.5 and 632.8 nm). No phase transition was observed. Rather, the thermal evolution of the Raman lines showed a high degree of harmonicity with small Grüneisen parameters and thermal line broadening following Γ=Acothθ/T, where the quantum temperature θ is determined by the phonon branch without further coupling with other degrees of freedom. The geometrical nature of phonon lines has been identified. High-temperature powder x-ray diffraction measurements provide thermal expansion coefficients of α(x)=13.9 × 10(-6) K(-1), α(y)=2.7 × 10(-6) K(-1), α(z)=14.3 × 10(-6) K(-1). The strongly quasi-harmonic behaviour observed and the lack of any indication of instability with respect to the post-perovskite structure points to the strongly first-order character of the reported perovskite to post-perovskite phase transition in this material, which appears to behave as a very good analogue to MgSiO(3) in the Earth's interior.     

Phase transition and thermodynamic properties of Sr under high pressure

We have performed the first-principles and classical molecular dynamics simulations to investigate the phase diagram and thermodynamic properties of Sr under high pressure and temperature. The obtained solid phase diagram of Sr, based on the quasi-harmonic approximation (QHA), is greatly supported by the available experimental data under low pressure. From the coexistence-phase molecular dynamics simulations, we also obtained the high-pressure melting curve of Sr which shows good agreement with the experiment. While, the experimentally observed β-Sn structure of Sr-III was found to be mechanically unstable according to our phonon dispersion calculations and evolutionary algorithm structure searches. We find that α-U phase (space group Cmcm) is energetically favorable and is the good candidate of Sr-III.     

Low frequency excitations in multiferroic MnWO(4)

Dynamic anomalies have been found in the magnetically ordered phases of multiferroic MnWO(4) using polarized Raman scattering. Strong phonon damping is observed for several B(g) modes within the ferroelectric phase and has been attributed to spin-phonon interactions. Moreover, a new low frequency excitation was detected near 33?cm(-1) that grows in intensity on cooling into the antiferromagnetic phases. It is argued that this signal is most probably due to a two-magnon process.     

Novel superconductivity in metallic SnH(4) under high pressure

From first-principles calculations, a high-pressure metallic phase of SnH(4) with a novel layered structure intercalated by "H(2)" units is revealed. This structure is stable at pressure between 70 and 160 GPa. A remarkable feature of this structure is the presence of soft modes in the phonon band structure induced by Fermi surface nesting and Kohn anomalies that lead to very strong electron-phonon coupling. The application of the Allen-Dynes modified McMillan equation with the calculated electron-phonon coupling parameter lambda shows that a superconducting critical temperature close to 80 K can be achieved at 120 GPa.     

Electronically driven phonon anomalies in transition metal carbides

Large maxima in the generalized susceptibility of χ(q) of TaC calculated from its APW band structure are found to correlate very well with observed soft phonon modes. Such maxima are not found for HfC where phonon anomalies are not observed.     

Pressure-Stabilized New Phase of CaN_4

We propose a new CaN_4 high pressure structure with the P2_1/m space group.The P2_1/m-CaN_4 structure is constituted by the infinite armchair N-chain.The dynamical stability and mechanical stability are verified by the calculations of phonon dispersion curves and elastic constants.The enthalpy difference calculation shows that the P2_1/m phase is more stable than the reported P4_12_12 phase.The advantaged properties of P2_1/m-CaN_4,such as high nitrogen content(58.3%) and low polymerization pressure(18.3 GPa),allow it to be a potential high energy materlal.Band structure calculation shows that the P2_1/m-CaN_4 structure is a metallic phase.The nonpolar covalent single N-N bond is a sigma bond.The charge transfer between the Ca and N atoms results in an ionic bond interaction.     

Lattice dynamics of solid xenon under pressure

We use density-functional perturbation theory to obtain the phonon spectrum of fcc xenon under pressure. Thermodynamic properties obtained within the quasiharmonic approximation are in fair to good agreement with experiment at zero pressure. The transition pressure from the fcc to hcp phase is predicted to occur at 5 GPa. The fcc structure is found to be dynamically stable up to a pressure of 100 GPa, beyond which the phonon modes at the X and L symmetry points soften. We attribute the observed sluggish kinetics of the fcc-hcp transition to the small energy difference between the phases as well as to the high dynamical stability of the fcc phase.     

Exchange interaction effects on the optical properties of LuMnO3

We have measured the optical conductivity of single crystal LuMnO3 from 10 to 45000 cm(-1) at temperatures between 4 and 300 K. A symmetry allowed on-site Mn d-d transition near 1.7 eV is observed to blueshift ( approximately 0.1 eV) in the antiferromagnetic state due to Mn-Mn superexchange interactions. Similar anomalies are observed in the temperature dependence of the TO phonon frequencies which arise from spin-phonon interaction. We find that the known anomaly in the temperature dependence of the quasistatic dielectric constant epsilon(0) below T(N) approximately 90 K is overwhelmingly dominated by the phonon contributions.     

Stability of the bulk phase of layered ZnO

Recently a novel phase of ZnO has been synthesized which is analogous to α-boron nitride, although more three dimensional, and consists of planar hexagonal sheets of ZnO. Examining the dynamic stability of the structure, we find unstable phonon modes over a considerable part of the Brillouin zone. Local-density approximation (LDA) and generalized gradient approximation level calculations have usually been able to predict the structural stability of s-p bonded systems. The failure in the present case is a surprise and is traced to the self-interaction error which incorrectly locates the localized Zn d states in the valence band of ZnO. Correcting for this with a Hubbard-like U on the Zn d states, the optimized structure is predicted to be stable. This highlights the fact that the large bond length contraction that one finds in going from sp(3)- to sp(2)-type bonding results in an increased necessity to correct for self-interaction errors.     

Crystal structures of dense lithium: a metal-semiconductor-metal transition

Ab initio random structure searching and single-crystal x-ray diffraction have been used to determine the full structures of three phases of lithium, recently discovered at low temperature above 60 GPa. A structure with C2mb symmetry, calculated to be a poor metal, is proposed for the oC88 phase (60-65 GPa). The oC40 phase (65-95 GPa) is found to have a lowest-enthalpy structure with C2cb symmetry, in excellent agreement with the x-ray data. It is calculated to be a semiconductor with a band gap of ～1 eV at 90 GPa. oC24, stable above 95 GPa, has the space group Cmca, and refined atomic coordinates are in excellent agreement with previous calculations.     

Evidence of a phase transition in a new type of halogenated niobium tetraselenide (NbSe4)xI

Temperature dependences of the dc electrical resistance and far-infrared reflectance spectra are reported in a new type of halogenated niobium tetraselenide (NbSe₄)_xI. In the dc resistance measurement, successive resistance anomalies were observed between 270 K and 90 K. Far-infrared reflectance spectra clearly show that several new phonon modes appear below 260 K. These features evidently show the existence of at least one structural phase transition.     

Lattice dynamics in ferromagnetic invar alloys

The lattice dynamical properties of the Invar alloys Fe₆₅Ni₃₅ and Fe₇₂Pt₂₈ are discussed. The experiments on the lattice vibration by inelastic neutron scattering have shown an apparent correlation of phonon anomalies with the ferromagnetic long range order. The elastic softening below the Curie temperature is attributed to the dynamical aspects, at least for the [ςς0]TA₁ mode. The lattice dynamical features are derived from the electronic structure of the Invar alloys. They are consistent with the theoretical model that Invar characters are closely related to the detailed band structure of the d electrons near the Fermi level. The present studies indicate significant contributions of phonon anomalies to the Invar problem.     

Structural stability and phase transitions in K8Si46 clathrate under high pressure

The structural stability of type-I K8Si46 clathrate has been investigated at high pressure by synchrotron x-ray diffraction. In contrast to that observed in the Na-doped structure-II analogue [A. San-Miguel, Phys. Rev. Lett. 83, 5290 (1999)]], no phase separation into the beta-Sn Si structure was identified at 11 GPa. Instead, K8Si46 is found to undergo a transition to an isostructural positional disordered phase at around 15 GPa. Ab initio phonon band structure calculations reveal a novel phenomenon of phonon instabilities of K atoms in the large cavities is responsible for this transition. Above 32 GPa, the new structure transforms into an amorphous phase.