Soliton microdynamics of heat conduction in plutonium and uranium in the temperature range of martensitic phase transitions

The microdynamics of large-amplitude nonlinear lattice vibrations of plutonium and uranium has been investigated at high reactor temperatures in the ranges of martensitic phase transitions. Solutions of nonlinear dynamic equations have been obtained using the Lennard-Jones interatomic potential for the soliton generation and the energy transfer by solitons between the crystal boundaries in the shells. The synchronism of the soliton trajectories and peaks of energy fluxes demonstrates an analog of the shot effect. The temperature dependences of thermal conductivity coefficients are consistent with the experiment and exhibit local maxima in the ranges of phase transitions. A spectral analysis has revealed that the dominant heat transfer is provided by rarefaction solitons. The martensitic phase transitions are accompanied by a reorganization of the spectral density in the phase plane with a sharp increase in the high-frequency range. The spectral density has maxima of the quasi-biphonon type. The obtained data in dimensionless form, apart from plutonium and uranium, can be used for other monatomic crystals. The specific features of the thermal conductivity and microdynamics of the formation of vacancies and pores in crystals without shells have been discussed.     

Soliton and multiphonon microdynamics of thermal conductivity of plutonium and uranium in the temperature range of martensitic phase transitions

The microdynamics of nonlinear crystal lattice vibrations of Pu and U at temperatures of martensitic phase transitions has been studied. Using the Lenard-Jones potential, the dynamic equations are solved when the energy is transferred by solitons. The synchronism of solitons and the energy flux peaks demonstrates the staccato-effect. The temperature dependences of the thermal conductivity have maxima at the phase transitions. A spectral analysis shows that the main heat transfer is provided by rarefaction solitons. Upon the martensitic transition, the spectral density is transformed with “ignition” of the high-frequency region. The spectral density demonstrates maxima of the quasi-biphonon type.     

Self-diffusion of oxygen in superstoichiomertric uranium dioxide in the range of the superion phase transition

The self-diffusion of oxygen in the superion transition range (1300–3000 K) of superstoichiometric uranium dioxide UO_{2 + x} is studied by the method of molecular dynamics using the pair interaction potential recovered from data for the thermal expansion of the UO₂ lattice. It is shown that three portions can be distinguished in the temperature dependence of the coefficient of oxygen self-diffusion in UO_{2 + x} , lnD = f(1/T), for all the compositions studied (x = 0, 0.008, and 0.030). These portions, each being described by the Arrhenius relationship, correspond to the crystalline, transition, and superion states of UO_{2 + x} . At low temperatures (1300–1820 K), the activation energies of oxygen diffusion for the above compositions are, respectively, 2.66 ± 0.44, 1.33 ± 0.10, and 1.00 ± 0.09 eV. At the beginning of the transition region, these activation energies rise to 3.40 ± 0.11, 2.24 ± 0.10, and 1.66 ± 0.60 eV. In the superion state, the activation energy of oxygen diffusion for all the compositions is the same, 1.25 ± 0.15 eV, within the error limit. As the oxygen content in UO_{2 + x} grows, the phase transition temperature decreases considerably and may reach 1600 K at x = 0.2. Comparison with experimental data for the low-temperature oxygen diffusion coefficient and with the data of UO₂ simulation using graphic processors shows good agreement of the results. By comparing the concentration dependences of the oxygen diffusion coefficient that are obtained by magnetic dynamics simulation with experimental data, it is shown that quantitative calculation of these dependences in the case of UO_{2 + x} can be carried out only for compositions with x < 0.03 if the given type of potential is used.     

Phase transitions in uranium dioxide at high pressures and temperatures

The melting curve is plotted for uranium dioxide with fluorite structure in a pressure range from ?2.5 to +100 GPa. This curve has a peak at the point 3348 K, 6 GPa, and has a negative derivative at high pressures. The pressure corresponding to a polymorphic transition of uranium dioxide (37 GPa) at a temperature of 1015 K is determined. The slope of the equilibrium curve of the polymorphic transition in UO₂ in the temperature range 300–1000 K is ? 56 K/GPa.     

Entropy-driven phase transitions with influence of the field-dependent diffusion coefficient

We present a comprehensive study of phase transitions in a single-field reaction-diffusion stochastic systems with a field-dependent mobility of a power-law form and internal fluctuations. Using variational principles and mean-field theory we have shown that the noise can sustain spatial patterns and leads to phase transitions type of “order-disorder”. These phase transitions can be critical and non-critical in character. Our theoretical results are verified by computer simulations.     

Pressure and temperature induced phase transitions in LiInSe2

Abstract

At 4.1 GPa LiInSe₂ transforms from the β-NaFeO₂ - type structure to the NaCl-type structure LiInSe₂-hpI (cubic; Fm3m; a=546.4(3)pm, Z=2, D _x =5.75g/cm³; 4.1GPa) which remains metastable at normal conditions. Heating to 210°C at 1.8 GPa causes ordering of the cations and a phase transition from LiInSe₂-hpI to the α-NaFeO₂ - type structure LiInSe₂-hpII (rhombohedral; R3m; a=393.4pm, c=1919.7pm, Z=3, D _x =5.53g/cm³; 1.8GPa). Heating to 210°C at 0.27 GPa results in a phase transformation from LiInSe₂-hpII to the chalcopyrite-type phase LiInSe₂-hpIII (tetragonal; 142d; a=580.7(8)pm, c=1181.0(31)pm, Z=4, D _x =4.66g/cm³; 0.27 GPa).     

Influence of temperature and anisotropic pressure on the phase transitions in alpha-cristobalite

The role of temperature and anisotropy of the applied load in the pressure-induced transformations of alpha-cristobalite is investigated by means of first principles molecular dynamics combined with the metadynamics algorithm for the study of solid-solid phase transitions. We reproduce the transition to alpha-PbO2 as found in experiments and we observe that the transition paths are qualitatively different and yield different products when a nonhydrostatic load is applied, giving rise to a new class of metastable structures with mixed tetrahedral and octahedral coordination.     

氧空穴导致二氧化钒低温相带隙变窄

具有一定能量的光照导致低温绝缘二氧化钒(VO_2)发生绝缘体金属转变.本文通过密度泛函理论的Heyd-Scuseria-Ernzerhof杂化泛函方法对含氧空穴的低温绝缘VO_2非磁M1相进行第一性原理研究.研究发现,含氧空穴的M1的晶格参数几乎不变,但氧空穴附近的长的V—V键长却变短了.进一步研究发现,尽管纯的非磁M1的带隙是0.68 eV,但含O1和O2位的氧空穴非磁M1带隙分别为0.23 eV和0.20 eV,同时含有O1和O2位氧空穴非磁M1带隙为0.15 eV,这很好地解释了实验结果.     

Chemical potentials and high temperature phase transitions in electroweak theory

The phenomena of electroweak phase transitions in the early universe depend crucially on the inclusion of the charge corresponding to the spontaneously broken symmetry, which is usually taken as classically conserved. This paper is devoted to the study of the electroweak phase transitions at high temperatures and, correspondingly, to the controversy in the literature concerning the question of the inclusion of the weak neutral charge in the standard model, for the investigation of the phase transition ofW-boson condensation and symmetry restoration in the early universe. Several arguments, general and perturbative, are given against the introduction of the weak neutral charge conservation in the nonsymmetric phase. As a consequence of the above, theW-condensation should occur even at low lepton densities and the symmetry restoration critical curve is independent of the lepton density. The use of different gauges such as the renormalizable and the unitary ones is also discussed.

     

Multiple phase transitions in rare earth tetraborides at low temperature

We report the temperature dependence of the magnetic susceptibility of single crystals of PrB₄, GdB₄, TbB₄, HoB₄ and TmB₄, both parallel and perpendicular to the tetragonal c-axis. We also present low temperature resistance measurements on crytals of GdB₄ through TmB₄. Two magnetic phase transitions are found for TbB₄, DyB₄, HoB₄ and TmB₄. For the latter two compounds, the lower transitions appear to be first order. For HoB₄, we have measured the low temperature specific heat. The lower transition in TbB₄ and HoB₄ is rapidly depressed upon dilution with YB₄.     

Long-time correlations in a model of structural phase transitions with infinite range interaction

The long-time value of the displacement-displacement correlation function is calculated within the ⁴-lattice model used to simulate structural phase transitions. Its temperature dependence and dispersion is discussed for infinite range interaction. Special attent is devoted to the natural explanation of a central peak in the scattering function using nonvanishing long-time correlations.     

Quadrupolar waves in uranium dioxide

In presence of active orbital degrees of freedom, elementary excitations around a broken-symmetry state may include multipolar waves, but none of these exotic dispersive excitation branches has ever been identified. We show that quadrupolar waves constitute a major component of the dynamics of uranium dioxide in its magnetoquadrupolar ordered phase, and that many unexplained features in existing inelastic neutron scattering data, including a whole excitation branch, are associated with these propagating quadrupolar fluctuations. Our model permits us to separate the role of Jahn-Teller and superexchange mechanisms as sources of quadrupolar interactions.     

Crystal-field excitations in uranium dioxide

The energy levels of the configuration f² in an eight-fold cubic crystal field (CF) have been calculated, and the results are used to explain the experimental spectrum of UO₂. The fourth-order CF potential turns out to be much smaller than usually assumed for this compound. This has an effect of reducing the J-mixing in the wavefunctions, particularly in the case of the ground state wavefunction. In spite of the strength of the CF, the ground state ³H₄T₂ is found to be modified only slightly be the J-mixing effect; it consists of 89.4% ³He₄, and the remaining eleven components make up the rest. Very good correlation is obtained between the experimental and simulated energy-level schemes. The predominance of ³H₄ in the ground state consequently increases the value of the calculated effective magnetic moment. The results are compared with our previous predictions about the system, and relevant conclusions drawn in the light of experimental data now available.     

High temperature phase transitions in the crystal [4]of p-trimethylammoniumbenzenesulfonate zwitterion

Crystals of p-trimethylammoniumbenzenesulfonate (ZWT) have been obtained from the thermally induced soli d-state reaction of methyl p-dimethylaminobenzenesulfonate (MSE) or by recrystallization in aqueous solution. The characterization of four crystalline phases has been performed by calorimetric, birefringence, X-ray and Raman scattering studies. An unknown phase transition at room temperature yielding a new crystallographic phase has been found by calorimetric measurements. ZWT crystalline samples obtained from the solid state reaction are disordered as compared to ZWT single crystals obtained by growing from aqueous solution. This disorder is related to different arrangements of the phenyl ring orientations. Received 21 October 1998 and Received in final form 14 April 1999     

39K NMR study of the low temperature phase transitions in KLiSO4

The³⁹K NMR spectra, spin-spin (T ₂) and spin-lattice (T ₁) relaxation times of KLiSO₄ have been measured in the temperature range from 300 K to 90 K. The temperature dependence of the³⁹K (I=3/2) NMR spectra demonstrates the occurrence of a first order phase transition atT _c1=217 K which occurs without a change in the K⁺ site symmetry and another first order transition atT _c2=190 K which is connected with a lowering of the K⁺ site symmetry and the formation of three kinds of ferroelastic domains. From the angular dependence of the second order quadrupole shifts of the³⁹K NMR 1/2–1/2 transitions the electric-field gradient (EFG) tensors at the potassium sites were determined at 290 K, 204 K and 180 K. The symmetry of the ferroelastic phase is monoclinic and not orthorhombic as the K⁺ EFG tensors are tilted away from thec-axis belowT _c2. TheT ₁ data further show the freezing in of the slow reorientational motion 10^–8 s with decreasing temperature from 300 to 90 K.On leave from: J. Stefan Institute, E. Kardelj University of Ljubljana, Ljubljana, Yugoslavia     

Mode coupling approximation in a model of structural phase transitions with infinite range interaction

The long-time value of the displacement-displacement correlation function and the mean square displacements are calculated within the ⁴-lattice model with infinite range interaction by use of a mode coupling approximation. Their temperature dependence is compared with exact results. While the mean square displacements (zero decoupling times) agree with exact results except the low temperature range for weak coupling, the long time correlations from MCA (infinite decoupling times) are wrong. Even a separation of the displacement function into two parts representing the two different time scales is of no use since the distribution function doesn't separate.