Understanding controversies in the α-ω and ω-ß phase transformations of zirconium from nonhydrostatic thermodynamics.

Significant debate has been noted in the α-ω and ω-ß phase transformations of zirconium. The initial pressure of the α-to-ω transformation at room temperature has been reported to vary from 0.25 to 7.0 GPa, while the hydrostatic transformation is believed to occur at approximately 2.2 GPa. Shear stress is commonly considered as a key factor leading to the discrepancy. However, the principal mechanisms previously proposed concluded that the phase transformation pressure would be decreased in the presence of shear stress. The experimental results of the α-ω transformation in zirconium are contrary to this conclusion. In the ω-ß phase diagram of zirconium, the dT/dP along the phase boundary near the α-ω-ß triple-point was reported to be either positive or negative, but no theoretical explanation, especially a quantitative one, has been proposed. This article aimed to quantitatively investigate and explain the controversies reported in the α-ω and ω-ß phase transformations of zirconium by applying a new nonhydrostatic thermodynamic formalism for solid medium, which has recently been proposed and is capable of quantitatively estimating the impact of shear stress on phase transformations in solids.

Unexpected coordination number and phase diagram of niobium diselenide under compression.

We discovered several new energetically competitive structures of NbSe2 using the multi-algorithm collaborative (MAC) crystal structure prediction algorithm combined with the density functional theory. It was found that the coordination number of Nb in NbSe2 is increased from 6 to 7, and then to 8 with increasing pressure. Furthermore, it was unexpected that an Se atom would be squeezed to the center of a cage formed by 12 other Se atoms and then have 12-fold coordination when the pressure was increased to 130.4 GPa. The 12-coordination metalloid atom has never been discovered in other transition metal dichalcogenides. The new C2/m, I4/mmm, and P4/mmm NbSe2 were verified to be stable under both dynamically and mechanically stabile conditions. It is especially noteworthy that the new C2/m-NbSe2 was predicted to be potentially synthesized at high pressure and recovered under ambient conditions. A detailed high-pressure and high-temperature phase diagram was constructed based on the quasi-harmonic approximation up to 200 GPa, and the synthesis conditions of different new NbSe2 materials were also analyzed. All the discoveries in this study will guide the future synthesis of new NbSe2 materials at specific pressure and under temperature conditions and also help to further understand other transition metal dichalcogenides.

Ab Initio Study of Ionized Water Radical Cation (H2O)8+ in Combination with the Particle Swarm Optimization Method.

The structures of cationic water clusters (H2O)8+ have been globally explored by the particle swarm optimization method in combination with quantum chemical calculations. Geometry optimization and vibrational analysis for the 15 most interesting clusters were computed at the MP2/aug-cc-pVDZ level and infrared spectrum calculation at MPW1K/6-311++G** level. Special attention was paid to the relationships between their configurations and energies. Both MP2 and B3LYP-D3 calculations revealed that the cage-like structure is the most stable, which is different from a five-membered ring lowest energy structure but agrees well with a cage-like structure in the literature. Furthermore, our obtained cage-like structure is more stable by 0.87 and 1.23 kcal/mol than the previously reported structures at MP2 and B3LYP-D3 levels, respectively. Interestingly, on the basis of their relative Gibbs free energies and the temperature dependence of populations, the cage-like structure predominates only at very low temperatures, and the most dominating species transforms into a newfound four-membered ring structure from 100 to 400 K, which can contribute greatly to the experimental infrared spectrum. By topological analysis and reduced density gradient analysis, we also investigated the structural characteristics and bonding strengths of these water cluster radical cations.

Predicted low thermal conductivities in antimony films and the role of chemical functionalization.

Chemical functionalization is an effective means of tuning the electronic and crystal structure of a two-dimensional material, but very little is known regarding the correlation between thermal transport and chemical functionalization. Based on the first-principles calculation and an iterative solution of the Boltzmann transport equation, we find that antimonene is a potential excellent thermal material with relatively low thermal conductivity k, and furthermore, chemical functionalization can make this value of k decrease greatly. More interestingly, the origin of the reduction in k is not the anharmonic interaction but the harmonic interaction from the depressed phonon spectrum mechanism, and for some chemical functional atom in halogen, flat modes appearing in the low frequency range play also a key factor in the reduction of k by significantly increasing the three-phonon scattering channels. Our work provides a new view to adjust thermal transport which can benefit thermal material design, and analyzes the reduction mechanism in k from the chemical functionalization of antimonene.

Ab initio investigation of structure, stability, thermal behavior, bonding, and infrared spectra of ionized water cluster (H2O)6.

The low-lying isomers of cationic water cluster (H2O)6+ have been globally explored by using particle swarm optimization algorithm in conjunction with quantum chemical calculations. Compared with previous results, our searching method covers a wide range of structural isomers of (H2O)6+ and therefore turns out to be more effective. With these local minima, geometry optimization and vibrational analysis are performed for the most interesting clusters at second-order Møller-Plesset (MP2)/aug-cc-pVDZ level, and their energies are further refined at MP2/aug-cc-pVTZ and coupled-cluster theory with single, double, and perturbative triple excitations/aug-cc-pVDZ level. The interaction energies using the complete basis set limits at MP2 level are also reported. The relationships between their structure arrangement and their energies are discussed. Based on the results of thermal simulation, structural change from a four-numbered ring to a tree-like structure occurs at T ≈ 45 K, and the relative population of six lowest-free-energy isomers is found to exceed 4% at some point within the studied temperature range. Studies reveal that, among these six isomers, two new-found isomers constitute 10% of isomer population at 180 K, and the experimental spectra can be better explained with inclusions of the two isomers. The molecular orbitals for six representative cationic water clusters are also studied. Through topological and reduced density gradient analysis, we investigated the structural characteristics and the bonding strengths of these water cluster radical cations.

Shock melting method to determine melting curve by molecular dynamics: Cu, Pd, and Al.

A melting simulation method, the shock melting (SM) method, is proposed and proved to be able to determine the melting curves of materials accurately and efficiently. The SM method, which is based on the multi-scale shock technique, determines melting curves by preheating and/or prepressurizing materials before shock. This strategy was extensively verified using both classical and ab initio molecular dynamics (MD). First, the SM method yielded the same satisfactory melting curve of Cu with only 360 atoms using classical MD, compared to the results from the Z-method and the two-phase coexistence method. Then, it also produced a satisfactory melting curve of Pd with only 756 atoms. Finally, the SM method combined with ab initio MD cheaply achieved a good melting curve of Al with only 180 atoms, which agrees well with the experimental data and the calculated results from other methods. It turned out that the SM method is an alternative efficient method for calculating the melting curves of materials.

Study of the thermodynamic properties of CeO2 from ab initio calculations: the effect of phonon-phonon interaction.

The thermodynamic properties of CeO2 have been reevaluated by a simple but accurate scheme. All our calculations are based on the self-consistent ab initio lattice dynamical (SCAILD) method that goes beyond the quasiharmonic approximation. Through this method, the effects of phonon-phonon interactions are included. The obtained thermodynamic properties and phonon dispersion relations are in good agreement with experimental data when considering the correction of phonon-phonon interaction. We find that the correction of phonon-phonon interaction is equally important and should not be neglected. At last, by comparing with quasiharmonic approximation, the present scheme based on SCAILD method is probably more suitable for high temperature systems.

Ab initio investigation of the lower energy candidate structures for (H2O)5⁺ water cluster.

The particle swarm optimization method in conjunction with density functional calculations is used to search the lower energy structures for the cationic water clusters (H2O)5(+). Geometry optimization, vibrational analysis, and infrared spectrum calculation are performed for the most interesting clusters at the MP2/aug-cc-pVDZ level. The relationships between their structural arrangements and their energies are discussed. According to their relative Gibbs free energies, their energy order is determined and four lowest energy isomers are found to have a relative population surpassing 1% below 350 K. Studies reveal that, among these four isomers, one new cluster found here also contributes a lot to the experimental infrared spectrum. Based on topological analysis and reduced density gradient analysis, some meaningful points are found by studying the structural characteristics and the bonding strengths of these cationic water clusters: in the first solvation shell, the central H3O(+) motifs may have a stronger interaction with the OH radical than with the water molecules. The interaction in the second solvation shell may also be stronger than that in the first solvation shell, which is opposite to our intuition.

Size-dependent melting and coalescence of tungsten nanoclusters via molecular dynamics simulation.

We obtained the melting temperatures of the W nanoclusters with diameters in the range of 2.5-5.0 nm which manifest the good linear fitting to the size of nanoclusters (N(-1/3)). Four different initial configurations at each size produce nearly the same melting points, with the maximum discrepancies less than 40 K. The extrapolated bulk melting point 4210 K is lower than the simulated bulk value 4520 K. Surface premelting is detected by density profiles, deformation parameters and bond orientational order parameters. Moreover, by dividing particles into surface and subsurface layers, we analyzed the different behaviors of the inner and outer shell atoms during melting in detail. During coalescence of W nanoclusters (W(N) + W(N)→ W(2N)), the shape change is along the path of peanut → rod-like → spherical → liquid structure. The obtained melting points from W(2N) are in good agreement with those from W(N) + W(N), indicating that melting temperatures are mainly relevant to the number of atoms, and nearly not affected by the different surface areas in nanoclusters.

Density functional theory investigation of the phonon instability, thermal equation of state and melting curve of Mo.

The phonon instability and thermal equation of state of Mo are extensively investigated using density functional theory. The calculated phonon dispersion curves agree well with experiments. Under compression, we captured a large softening in the transverse acoustic (TA) branches of body-centred cubic Mo. When the pressure is raised to 716 GPa, the frequencies along Γ-N in the TA branches soften to imaginary frequencies, indicating structural instability. For face-centred cubic Mo, the phonon calculations predicted the stability by promoting the frequencies from imaginary to real. Within quasi-harmonic approximation, we predicted the thermal equation of state and some other properties including the thermal expansion coefficient α, product αK(T), heat capacity C(V), entropy S, Grüneisen parameter γ and Debye temperature Θ(D). The melting curves of Mo were also obtained successfully.

Lattice dynamics and thermodynamics of molybdenum from first-principles calculations.

We calculated the phase transition, elastic constants, full phonon dispersion curves, and thermal properties of molybdenum (Mo) for a wide range of pressures using density functional theory. Mo is stable in the body-centered-cubic (bcc) structure up to 703 +/- 19 GPa and then transforms to the face-centered close-packed (fcc) structure at zero temperature. Under high temperature and pressure, the fcc phase of Mo is more stable than the bcc phase. The calculated phonon dispersion curves accord excellently with experiments. Under pressure, we captured a large softening along H-P in the TA branches. When the volume is compressed to 7.69 A(3), the frequencies along H-P in the TA branches soften to imaginary frequencies, indicating a structural instability. When the pressure increases, the phonon calculations on the fcc Mo predict the stability by promoting the frequencies along Gamma to X and Gamma to L symmetry lines from imaginary to real. The thermal equation of state was also investigated. From the thermal expansion coefficient and the heat capacity, we found that the quasiharmonic approximation was valid only up to about melting point at zero pressure. However, under pressure, the validity can be extended to a much higher temperature.

Ab initio refinement of the thermal equation of state for bcc tantalum: the effect of bonding on anharmonicity.

We report a detailed ab initio study for body-centered-cubic (bcc) Ta within the framework of the quasiharmonic approximation (QHA) to refine its thermal equation of state and thermodynamic properties. Based on the excellent agreement of our calculated phonon dispersion curve with experiment, the accurate thermal equations of state and thermodynamic properties are well reproduced. The thermal equation of state (EOS) and EOS parameters are considerably improved in our work compared with previous results by others. Furthermore, at high temperatures, the excellent agreement of our obtained thermal expansion and Hugoniot curves with experiments greatly verifies the validity of the quasiharmonic approximation at higher temperatures. It is known that pressure suppresses the vibrations of atoms from their equilibrium positions, i.e. the bondings among atoms are strengthened by pressure; for the same temperature, anharmonicity becomes less important at high pressure. Thus the highest valid temperature of the QHA can be reasonably extended to the larger range.

Self-consistent fluid variational theory for the dissociation of dense nitrogen.

The self-consistent fluid variational theory is used to calculate the pressure dissociation of dense nitrogen at high temperatures. The accurate high-pressure and high-temperature effective pair potentials are adopted to describe the intermolecular interactions, which are made to consider molecular dissociation. This paper focuses on a mixture of nitrogen atoms and molecules and is devoted to the study of the phenomenon of pressure dissociation at finite temperature. The equation of state and dissociation degree are calculated from the free-energy functions in the range of temperature of 2000-15 000 K and density of 0.2-3.0 gcm(3), which can be compared with other approaches and experiments.

[Wavelengths and oscillator strengths of transitions 4s2-4s4p,4s4p-4s4d in ions from RbVIII--NbXII].

In this paper, we have calculated the energy levels of configurations 4s2,4s4p,4s4d in ions from RbVIII-NbXII by HXR method. By analyzing variation of differences between the experimental and theoretical energies with Zc along the zinc isoelectronic sequence, we put forward a new formula. Using this formula and the FORTRAN program designed by us, we calculated the energy levels of the configurations mentioned above by generalized-lests-square-fit (GLSF) method. Also, the wavelengths and the corresponding oscillator strengths of transitions 4s2-4s4p,4s4p-4s4d are given here too.