Acoustic absorption in superionic PbF2 across transition temperature

In the present paper, we report a theoretical calculation of temperature variation of the ultrasonic absorption coefficient of PbF₂. The absorption can be explained by the TLS model, with parameters which simultaneously reproduce the specific heat, thermal expansion, bulk modulus and ionic conductivity. Our results agree well with recent experimental reports of ultrasonic attenuation in PbF₂.     

Atomistic simulation of the transition from atomistic to macroscopic cratering

Using large-scale atomistic simulations, we show that the macroscopic cratering behavior emerges for projectile impacts on Au at projectile sizes between 1000 and 10000 Au atoms at impact velocities comparable to typical meteoroid velocities. In this size regime, we detect a compression of material in Au nanoparticle impacts similar to that observed for hypervelocity macroscopic impacts. The simulated crater volumes agree with the values calculated using the macroscopic crater size scaling law, in spite of a downwards extrapolation over more than 15 orders of magnitude in terms of the impactor volume. The result demonstrates that atomistic simulations can be used as a tool to understand the strength properties of materials in cases where only continuum models have been possible before.     

X-ray study of the superionic phase transition in CuCrS2

The temperature disorder of copper ions in the CuCrS₂ superionic conductor was studied by X-ray analysis. It is shown that the average square deviations of atoms sharply increase and become comparable with the distance between neighbouring tetrahedral sites when CuCrS₂ transforms into the superionic phase. It is also established that the direction of maximum deviation of atoms is close to the direction connecting the tetrahedral positions.     

单轴应变驱动铁bcc—hcp相转变的微观模拟

用分子动力学方法模拟了沿〈001〉晶向应变加载和卸载情况下单晶铁中体心立方(bcc)与六方密排(hcp)结构的相互转变,分析了相变的可逆性和微结构演化特征.微观应力的变化显示样品具有超弹性性质,而温度变化表明在相变和逆相变过程中均出现放热现象.相变起始于爆发式均匀形核,晶核由块状颗粒迅速生长为沿{011}晶面的片状分层结构; 而卸载逆相变则从形核开始就呈现片状形态,且相界面晶面指数与加载相变完全一致,表现出形态记忆效应.在两hcp晶核生长的交界面易形成面心立方(fcc)堆垛层错. fcc通过在hcp晶粒内     

Atomistic simulation of fcc-bcc phase transition in single crystal Al under uniform compression

By molecular dynamics simulations employing an embedded atom model potential,we investigate the fcc-to-bcc phase transition in single crystal Al,caused by uniform compression.Results show that the fcc structure is unstable when the pressure is over 250 GPa,in reasonable agreement with the calculated value through the density functional theory.The morphology evolution of the structural transition and the corresponding transition mechanism are analysed in detail.The bcc (011) planes are transited from the fcc (11) plane and the (11) plane.We suggest that the transition mechanism consists mainly of compression,shear,slid and rotation of the lattice.In addition,our radial distribution function analysis explicitly indicates the phase transition of Al from fcc phase to bcc structure.     

Molecular dynamics simulation of the superionic conductor BaF2

Molecular dynamics simulations of the BaF₂ fluoride crystal were carried out over a wide range of temperatures in order to study structural and transport characteristics in the low-temperature, the high-temperature superionic, and the molten state. The experimental temperature dependence of the lattice constant was taken into account. A sharp change in total energy of the system in the vicinity of T=1200 K indicates a phase transition to the high-temperature state with a transition energy U=(18.8±0.2) kJ/mol which is close to the value of the latent heat Q=18.36 kJ/mol obtained experimentally at 1275 K. Calculation of the radial distribution functions g(r) shows that in the high-temperature superionic state the F^– sublattice is already disordered while the Ba²⁺ sublattice stays regularly ordered, which keeps the crystal in the solid state. In the low-temperature state both sublattices are regularly ordered, and in the molten state both sublattices are disordered. The calculated diffusion constants of F^– in the superionic state is about 10^–9m²/s which is a typical value for superionic conductors. The temperature dependence of the diffusion constant obeys the Arrhenius equation. Higher statistical moments of the trajectories are used to characterise the type of ion movement.     

Atomistic simulation of fccben bcc phase transition in single crystal Al under uniform compression

By molecular dynamics simulations employing an embedded atom model potential, we investigate the fcc-to-bcc phase transition in single crystal Al, caused by uniform compression. Results show that the fcc structure is unstable when the pressure is over 250 GPa, in reasonable agreement with the calculated value through the density functional theory. The morphology evolution of the structural transition and the corresponding transition mechanism are analysed in detail. The bcc (011) planes are transited from the fcc (111) plane and the (111) plane. We suggest that the transition mechanism consists mainly of compression, shear, slid and rotation of the lattice. In addition, our radial distribution function analysis explicitly indicates the phase transition of Al from fcc phase to bcc structure.     

The superionic phase transition of fluorite-type crystals

The diffuse phase transition of fluorite-type superionic conductors will be studied in terms of the Bragg-Williams and the quasichemical approximation. The results are compared with experimental data taken from the literature. Improvements of the above-mentioned approximations, taking care of the long-range interactions, have been suggested. Good agreement between our theoretical results and experiment has been found. In addition, we have studied the effect of doping with trivalent cations. In agreement with experimental observations we find that the superionic phase transition shifts to lower temperatures with increasing concentrations of trivalent ions.     

Atomistic simulation of aging and rejuvenation in glasses

Slow structural relaxation ("aging") observed in many atomic, molecular, and polymeric glasses substantially alters their stress-strain relations and can produce a distinctive yield point. Using Monte Carlo simulation for a binary Lennard-Jones mixture, we have observed these phenomena and their cooling-rate dependences for the first time in an atomistic model system. We also observe that aging effects can be reversed by plastic deformation ("rejuvenation"), whereby the system is expelled from the vicinity of deep minima in its potential energy surface.     

Atomistic and lattice model of a grain boundary defaceting phase transition

Recent calculations have shown that grain boundary (GB) stress is too small to stabilize finite GB facets, suggesting that the existing theory of GB defaceting phase transitions is incomplete. We perform molecular dynamics calculations, which show a reversible phase transition at approximately 400 K with a concerted shuffle of two atoms at the facet junction as the elementary excitation. Based on this excitation we formulate an appropriate lattice model, perform Monte Carlo simulations, and establish an analytical relationship between the elementary excitation energy and the transition temperature.     

Atomistic simulation of CO2 solubility in poly(ethylene oxide) oligomers

We have performed atomistic molecular dynamics simulations coupled with thermodynamic integration to obtain the excess chemical potential and pressure-composition phase diagrams for CO₂ in poly(ethylene oxide) oligomers. Poly(ethylene oxide) dimethyl ether, CH₃O(CH₂CH₂O)_nCH₃ (PEO for short) is a widely applied physical solvent that forms the major organic constituent of a class of novel nanoparticle-based absorbents. Good predictions were obtained for pressure–composition–density relations for CO₂ + PEO oligomers (2 ≤ n ≤ 12), using the Potoff force field for PEO [J. Chem. Phys. 136, 044514 (2012)] together with the TraPPE model for CO₂ [AIChE J. 47, 1676 (2001)]. Water effects on Henry’s constant of CO₂ in PEO have also been investigated. Addition of modest amounts of water in PEO produces a relatively small increase in Henry’s constant. Dependence of the calculated Henry’s constant on the weight percentage of water falls on a temperature-dependent master curve, irrespective of PEO chain length.     

Static lattice simulation of structure and transport in superionic conductors

We describe the scope, methodology and achievements of static lattice simulation methods in the study of fast-ion conductors. We discuss first the type of conduction mechanism to which simulations may be applied; we contrast the information available from static with that from dynamics simulations. The techniques and potentials used by the calculations are then described. Applications to β-Al₂O₃, Li₃N and fluorite structured compounds are then considered.     

Atomistic simulation of slow grain boundary motion

Existing atomistic simulation techniques to study grain boundary motion are usually limited to either high velocities or temperatures and are difficult to compare to realistic experimental conditions. Here we introduce an adapted simulation method that can access boundary velocities in the experimental range and extract mobilities in the zero driving force limit at temperatures as low as ～0.2T(m) (T(m) is the melting point). The method reveals three mechanistic regimes of boundary mobility at zero net velocity depending on the system temperature.     

Atomistic simulation of the point defects in B2-type MoTa alloy

The formation and migration mechanisms of three different point defects (mono-vacancy, anti-site defect and interstitial atom) in B₂-type MoTa alloy have been investigated by combining molecular dynamics (MD) simulation with modified analytic embedded-atom method (MAEAM). From minimization of the formation energy, we find that the anti-site defects Mo_Ta and Ta_Mo are easier to form than Mo and Ta mono-vacancies, while Mo and Ta interstitial atoms are difficult to form in the alloy. In six migration mechanisms of Mo and Ta mono-vacancies, one nearest-neighbor jump (1NNJ) is the most favorable due to its lowest activation and migration energies, but it will cause a disorder in the alloy. One next-nearest-neighbor jump (1NNNJ) and one third-nearest-neighbor jump (1TNNJ) can maintain the ordered property of the alloy but require higher activation and migration energies, so the 1NNNJ and 1TNNJ should be replaced by straight [1 0 0] six nearest-neighbor cyclic jumps (S[1 0 0]6NNCJ) or bent [1 0 0] six nearest-neighbor cyclic jumps (B[1 0 0]6NNCJ) and [1 1 0] six nearest-neighbor cyclic jumps ([1 1 0]6NNCJ), respectively. Although the migrations of Mo and Ta interstitial atoms need much lower energy than Mo and Ta mono-vacancies, they are not main migration mechanisms due to difficult to form in the alloy.     

Atomistic simulation of Mn-site substitution in multiferroic h-YMnO3

Atomistic simulation has been performed to systematically investigate the Mn-site doping of h-YMnO(3) (hexagonal yttrium manganese oxide). It is found that tetravalent dopants are the most energetically favorable for incorporation into a crystal lattice. For divalent dopants, hole compensation is the more likely charge compensation mechanism, whereas for dopants with mixed valence, the divalent state is the energetically preferred form. Structural and local polarization changes caused by Mn-site doping are also investigated. The tilting angle of the MnO(5) trigonal bipyramid is suppressed for all of the dopants investigated, with the reduction dependent on the dopant ion radius, while the influence on buckling is closely related to the valence of the dopants. Our results reveal that both electrostatic and size effects play important roles in the ferroelectric polarization of h-YMnO(3).     

Ion dynamics and transition to the superionic state in β-AgI

By measuring the nuclear spin-lattice relaxation time of ¹²⁷I cooperative effects in Ag diffusion have been found which point to the necessity to include mutual cation interaction in the diffusion theory. An anomalous increase of the relaxation rate within 5 K just below the transition into the superionic state is taken as dynamical evidence for a discontinuity in the number of diffusing Ag ions, i.e. for sublattice melting.     

Atomistic simulation of radiation damage to carbon nanotube

Damaging carbon nanotube upon energetic irradiation has been modeled with molecular-dynamics simulations. The angular dependence of the threshold energy of the primary knock-on atom (PKA) escaping from the tube is investigated in the initial PKA directions spanning half space. The average value of the threshold energy is calculated to be 19.3 eV. The simulations provided a detailed picture of the damaging processes, in which four mechanisms were revealed. The interactions between carbon atoms are described with the Tersoff mode modified to match a screened Coulomb potential at short range.     

Microwave conductivity evidence for order-disorder transition in the superionic conductor β-eucryptite

The impedance of LiAlSiO₄ (β-eucryptite) at 9 GHz shows a marked decrease in activation energy as the temperature is raised above 450°C, where X-ray data indicate an order-disorder transition.     

Orientational disorder,glass/crystal transition and superionic conductivity in nasicon

The relations between the orientational disorder of SiO₄(PO₄) tetrahedra and fast sodium diffusion in superionic NASICON have been studied by conductivity (complex impedance method), DSC, X-ray powder difraction and vibrational spectroscopy (IR and Raman). Sol-gel routes allow to obtain pure glassy NASICON (Na_1+xZr₂Si_xP_3-xO₁₂x≅2) in the 500−700°C temperature range. Tetragonal zirconia nucleates above 700°C and disappears at about 900°C when the isolated tetrahedra framework is formed: a high orientational static disorder of tetrahedra exists and the symmetry is rhombohedral at all studied temperatures (20−600 K). Thermal treatment above 1100°C induces a drastic decrease of the static orientational disorder and nucleation of monoclinic zirconia. The resulting compound exhibits a monoclinic symmetry at R.T. and three phase transitions, two diffuse at about 60 K and 520 K and the 423 K monoclinic-rhombohedral transition associated with the superionic conducting state. An increase in dynamic disorder (broad Rayleigh wing up to 500 cm^-1 is simultaneously observed. The lower the static disorder at low temperature, the higher the dynamic orientational disorder and the phase transitions, and the lower the activation energy of conductivity at high temperature.