Simulation study of polymorphism and diffusion anomaly for SiO2 and GeO2 liquid

We report molecular dynamic (MD) simulations of a silica and germania liquid. We show for the first time that the liquid models constructed of the BKS and Ocffner-Elliott potential are composed of D₄, D₅ and D₆ domains. These domains of different liquids (identical chemical composition but different density) are structurally identical. The liquid densification occurs gradually with variation in the volume of the D₄, D₅ and D₆ domains. The void radius distributions (VRD) have been computed and they are well fitted to a Gaussian form. Both the average void radii and width of the VRD decrease with pressure. The diffusivity of Si/Ge or O depends linearly on the fraction of TO_x (x = 4, 5 and 6; T is Ge or Si). The diffusion anomaly results from the very high mobility of silicon in the D₅ domain compared to domains D₄ and D₆.     

Interband optical transition strengths in SiO2, GeO2, SnO2 and TiO2

An analysis of interband optical transition strengths for a series of MO₂ oxides is presented which gives clear evidence for a direct proportionality between transition strengths (as measured by the dispersion energy) and nearest-neighbor cation coordination number. It is also found that neither the extent of O2p orbital delocalization nor the d-electron configuration has a strong influence on transition strengths suggestin that these strengths can be viewed, to a good approximation, as intrinsic properties of valence s,p-electrons which are largely independent of band structure details.     

Molecular dynamics analysis of impurity ions behavior in β″-Al2O3

The effects of foreign impurity ions in the conduction plane on the β″-Al₂O₃ lattice have been analyzed by molecular dynamics. As impurity ions, K and Ca ions were chosen and Na sites in the conduction plane were replaced with these ions. Results are summarized as follows: (1) The β″-Al₂O₃ lattice expands perpendicular to the conduction plane when K ions are doped; (2) Ca ions do not contribute to expansions of the β″-Al₂O₃ lattice; (3) both K and Ca ions reduce the mean square displacement of Na ions, which can be attributed to decreased Na ion diffusion.     

Fractional nonlinear diffusion equation,solutions and anomalous diffusion

We devote this work to investigate the solutions of a generalized diffusion equation which contains spatial fractional derivatives and nonlinear terms. The presence of external forces and absorbent terms is also considered. The solutions found here can have a compact or long tail behavior and, in particular, for the last case in the asymptotic limit, we relate these solutions to the Lévy or Tsallis distributions. In addition, from the results presented here a rich class of diffusive processes, including normal and anomalous ones, can be obtained.     

Stability of valence alternation pairs across the substoichiometric region at Ge/GeO2 interfaces

Through hybrid density functional calculations, we compare the Ge–Ge bond energy with the formation energy of a valence alternation pair as the O concentration varies across the Ge/GeO₂ interface. First, hole trapping energies are calculated for three atomistic models with different O concentrations: bulk Ge with isolated O atoms, amorphous GeO, and amorphous GeO₂ with an O vacancy. The reaction is then broken down in three steps involving the breaking of a Ge–Ge bond, charge transfer processes involving dangling bonds, and the formation of a threefold coordinated O atom. The energy of each elemental reaction is estimated through suitable model calculations. The charge transition levels resulting from this analysis agree with those obtained for the atomistic models. Our estimates indicate that hole trapping at low O concentrations occurs at no energy cost for p-type germanium owing to the formation of threefold-coordinated O atoms. Applied to n-type Ge, our analysis indicates that electron trapping in dangling bonds obtained from the breaking of Ge–Ge bonds is unfavorable. The formation energy of a valence alternation pair is evaluated and discussed in relation to previous results.     

AC-driven anomalous stochastic diffusion and chaotic transport in magnetic superlattices

We study regular and chaotic motion of the charge carriers in a two-dimensional electron gas subject to a spatially periodic magnetic field and to an AC electric field. We show how the interplay between the time-periodic electric and the spatially-periodic magnetic field leads to dynamical chaos and to fast stochastic diffusion of the electrons. The cases of a one-dimensional magnetic modulation with AC pumping and of a pure two-dimensional magnetic modulation are compared. We find the direct effect of anomalous diffusion and Lévy flights on the conductivity of a sample.     

Nature of intrinsic luminescence in the glasses of CaO–Ga2O3–GeO2 system

The intrinsic luminescence of glasses of the CaO–Ga₂O₃–GeO₂ system has been investigated. High chemical purity and optical quality glasses, both undoped and doped with transition and rare-earth ions with different compositions, were obtained by high-temperature synthesis. The influences of the basic glass composition, impurities (Cr³⁺, Mn²⁺, Eu²⁺, Nd³⁺, Ho³⁺, Er³⁺, and Ce³⁺) and different kinds of excitation, on the intrinsic luminescence of the CaO–Ga₂O₃–GeO₂ glasses were investigated. The nature and possible mechanisms of the intrinsic luminescence in glasses of this system are discussed. The proposed models of intrinsic luminescence are supported by electron spin resonance spectroscopy.     

Oxygen tracer diffusion in La1 − xSrxCoO3

Tracer diffusion of ¹⁸O in dense, polycrystalline La_1−xSr_xCoO₃ for x = 0.1 has been measured in the temperature range 400 to 600 °C and at 500 °C for x = 0.2 at an oxygen partial pressure of 1 × 10⁵ Pa. Depth profiles were obtained by secondary ion mass spectrometry. The diffusion coefficient for La_0.9Sr_0.1CoO₃ is given by D = (17–247) exp[(−232 ± 8 kJ/mole)/RT] cm²/s. This value is several orders of magnitude lower than D extrapolated from the results for x = 0.2 measured in the 700–900 °C temperature range. One possible explanation for the discrepancy is that the two measurements reflect different diffusion paths. As expected, La_0.8Sr_0.2CoO₃ exhibits a higher diffusivity at 500 °C than does La_0.9Sr_0.1CoO₃.     

The phase transition in VO2

The electronic d-states in VO₂ are investigated by a semi-empirical MO LCAO method. The calculations of clusters for the crystal structures above and below the phase transition point show that the energy gap is mainly formed by the crystal distortions. The energies calculated agree well with the ESCA experimental estimations and explain the large gap values obtained from the optical data.     

Magnetic phase transition in TbMn2Ge2

The crystal and magnetic stucture of TbMn₂Ge₂ are determined by neutron diffraction using a powder sample. The crystal structure of this compound is of the ThCr₂Si₂ type with small mixing of Mn and Ge atoms between 4(d) and 4(e) positions. At RT the antiferromagnetic collinear structure consist of a+?+? sequence of ferromagnetic layers of Mn atoms with the magnetic moment parallel to the c-axis. At 85 K, the ferromagnetic ordering within the Tb sublattice is observed. The magnetic moment (～7.7 μ_B) is parallel to the c-axis. At 4.2 K additional reflections are observed, which correspond to antiferromagnetic components in a monoclinic unit cell.     

Variable-order fractional differential operators in anomalous diffusion modeling

The purpose of this paper is to offer a unified discussion of variable-order differential operators in anomalous diffusion modeling. The characteristics of the new models, in contrast to constant-order fractional diffusion models, change with time, space, concentration or other independent quantities. We introduced a classification of variable-order fractional diffusion models based on the possible physical origins which prompt the variable-order. Some potential applications of the variable-order fractional diffusion models are also discussed.     

Lack of anomalous diffusion in linear translationally-invariant systems determined by only one initial condition

It is shown that as far as the linear diffusion equation meets both time- and space-translational invariance, the time dependence of a moment of degree α is a polynomial of degree at most equal to α, while all connected moments are at most linear functions of time. As a special case, the variance is an at most linear function of time.     

The effect of duration of diffusion on Ag diffusion coefficients in YBa2Cu3O7

The Ag diffusion in superconducting YBa₂Cu₃O₇ (YBaCuO) ceramic has been studied over the duration of the diffusion range 5-24 h in the temperature range 700-850 °C by the energy-dispersive X-ray fluorescence (EDXRF) technique. For the excitation of silver atoms, an annular Am-241 radioisotope source (50 mCi) emitting 59.543 keV photons was used. The temperature dependences of silver diffusion coefficients in grains (D₁) and over the grain boundaries in the range 700-850 °C (D₂) are described by the relations D₁=1.4×10⁻² exp[−(1.18±0.10)/kT] and D₂=3.1×10⁻⁴ exp[−(0.87±0.10)/kT].     

Structures and phase transition in Rb2NaHoF6

The phase transition at T_c=172(2) K and high as well as the low temperature structures at 205 K and 17 K were studied with a low temperature Guinier diffractometer and camera. A second order phase transition (Γ-condensation of the T_1g mode) lowers the symmetry from Fm3m at T>;T_c to I4/m at T<T_c. In the low temperature phase the octahedra are tilted (5.2 degrees at 17 K) and slightly stretched. The increase of the tilt angle and the linear increase of the tetragonal distortion suggests a mean field power law. This classical behaviour would agree with the assumption of a Jahn Teller splitting of the Ho³⁺ electronic state.     

Order-disorder phase transition in ZrV2D3.6

The deuterated C15-type Laves phase ZrV₂D_3.6 undergoes a structural phase transition near room temperature (T ≈ 325 K). In the cubic high-temperature phase the deuterium atoms are disordered over two types of tetrahedral interstices, the centres of which are 1.3 Å apart. In the tetragonal low-temperature phase the D atoms are ordered and occupy only the energetically more favourable interstices. The tetragonal structure is isotypic with the low-temperature phase of HfV₂D₄. The shortest D—D distance is 2.1 Å.     

Pressure induced phase transition in MnTe2

Energy dispersive high-pressure powder X-ray experiments have been performed for MnTe₂ up to a pressure of 20 GPa. MnTe₂ undergoes a discontinuous transformation from the cubic pyrite type structure to the orthorhombic marcasite type structure at 7.0±0.5 GPa upon increasing pressure. The transformation is accompanied by a large reduction in the specific volume (ΔV/V=0.18) which probably reflects different magnetic properties of the two modifications of MnTe₂.     

Pressure-induced phase transition in Sb2Te3

An anomalous change in electrical resistance with pressure was observed in crystalline Sb₂Te₃ around 80 kbar at room temperature. This anomaly was found to be closely related with a pressure-induced structural phase transition which is discovered by the present x-ray diffraction experiment.     

Dynamical heterogeneity and diffusion in high-density Al2O3·2SiO2 melts

Dynamical heterogeneity (DH) in high-density Al₂O₃·2SiO₂ melts has been studied in a model containing 3025 atoms via molecular dynamics (MD) simulation and at the fixed density of 4.0 g/cm³. Non-Gaussian parameter of atomic species in the system has been found and discussed. We found a clear evidence of the existence of DH in high-density Al₂O₃·2SiO₂, which has specific features differed from those observed in the lower-density one. The most mobile and immobile atoms in the system have a tendency to form clusters and temperature dependence of their mean cluster size was found. On the other hand, diffusion constant of atomic species in the system has been calculated at temperatures ranged from 3150 to 7000 K. Calculations show that at relatively not high temperatures, temperature dependence of diffusion constant shows an Arrhenius law and at higher temperatures it shows a power law: D∝(T−T_C)^γ. Diffusion data of high-density melts have been compared with those for the low-density ones. Diffusion mechanism in the system has been discussed via the temperature dependence of diffusion constant ratio and activation energy. And we found the existence of cooperative diffusion mechanism in the system.     

Surface reconstruction on stishovite SiO2,HfO2 and rutile TiO2 （001）

This paper systematically investigates the surface reconstruction processes and patterns on stishovite SiO 2,HfO 2 and rutile TiO 2 (001) by using classical molecular dynamics.It is found that these three surfaces relax instead of reconstruction at 0 K,and have little possibility to reconstruct below 40 K.Above 40 K,surface reconstructions take place as collective atomic motion which can be speeded by higher temperature or compressed strain.Several reconstruction patterns with approximate surface energies are found,and electrostatic potentials on them are also provided in comparison with possible microscopic results.     

Pressure-induced phase transition of BaH2 : Post Ni2In phase

Up to now a Ni₂In structure is a final step in the structural sequence of ionic AX₂ compounds under high pressure. Powder X-ray diffraction experiments on BaH₂ were performed at room temperature and high pressures up to 69 GPa. Successive phase transformations were observed to occur in two stages. The first was from the cotunnite to the Ni₂In structure at 2.5 GPa. The second transition commenced at pressures around 50 GPa and was completed at 65 GPa. At the transition the arrangement of a cation sublattice changes from an hcp to a simple hexagonal lattice. This is the first observation of the post Ni₂In phase.