Ab initio calculations based on the Density Functional Theory are used to show that the Debye frequency is a linear function of density to a high accuracy for several elemental solids at pressures (at least) up to 360 GPa. This implies that the ratio of density over the (Debye-frequency-based) vibrational Grüneisen parameter is a linear function of density in this region. Numerical data from first principles calculations for several systems at temperatures up to 2000K suggest that this is also true for the thermal Grüneisen parameter in the same range of pressure. Our analytical form of the vibrational Grüneisen parameter is applied to an implementation of the Lindemann's melting criterion to obtain a simple extrapolation formula for the melting temperatures of materials at higher densities. This prediction is tested against available experimental and numerical data for several elemental solids.
$δ$-AlOOH ($δ$) is a high-pressure hydrous phase that participates in the deep geological water cycle. At 0 GPa, $δ$ has asymmetric hydrogen bonds (H-bonds). Under pressure, it exhibits H-bond disordering, tunneling, and finally, H-bond symmetrization at ~18 GPa. This study investigates these 300 K pressure-induced state changes in $δ$ with ab initio calculations. H-bond disordering in $δ$ was modeled using supercell multi-configuration quasiharmonic calculations. We examine: (a) energy barriers for proton jumps, (b) the pressure dependence of phonon frequencies, (c) 300 K compressibility, (d)…
Based on ab initio molecular dynamics simulations, we determined the melting curve of magnesium (Mg) up to ~460 GPa using the solid-liquid coexistence method. Between ~30 and 100 GPa, our melting curve is noticeably lower than those from static experiments, but is in good agreement with recent shock experiments. Up to ~450 GPa, our melting curve is generally consistent with the melting points from first-principles calculations using the small-cell coexistence method. We found that, at high pressures of a few hundred GPa, due to the strong softening of interatomic interactions in the liquid pha…
Here we have used the density functional theory (DFT) with the GGA-PBE approximation to investigate the structural, mechanical, electronic, hardness, thermal, superconductivity and optoelectronic properties under pressure for monoclinic (P21/m) and orthorhombic (Pmn21) structures of MgVH6. We have studied optical properties of P21/m phase at 0 GPa and Pmn21 phase at 100 GPa only (considering phase stability). Both of the phases of MgVH6 are thermodynamically stable. P21/m phase is mechanically stable but Pmn21 is mechanically unstable in our calculations for the pressures considered. Monoclini…
Recent experimental work shows that FePS$_3$ undergoes phase transitions from $C2/m$ ($β\sim107^{\circ}$) to $C2/m$ ($β\sim90^{\circ}$) at $6$ GPa and then to metallic $P\bar{3}1m$ at $14$ GPa, with the magnetic ordering wave vector turning from $k=(01\frac{1}{2})$ to $k=(010)$ at $2$ GPa and to short-range magnetic order accompanying the insulator-metal transition. By preserving the magnetic point groups in $ab \ initio$ calculations we report the following: (1) We successfully reproduce the first magnetic structure transition at $1.2$ GPa and briefly discuss the influence of the Hubbard U pa…
Polymeric nitrogen with single bonds can be created from the molecular form at high pressure and due to large energy difference between triple and single bonds it is interesting as energetic material. Its structure and properties are, however, still not well understood. We studied amorphous nitrogen by \textit{ab initio} simulations, employing molecular dynamics and evolutionary algorithms. Amorphous nitrogen was prepared at pressure of 120 GPa by quenching from hot liquid, by pressure-induced amorphization of molecular crystal and by evolutionary search. All three amorphous forms were found t…
We use first principles molecular dynamics simulations coupled to the thermodynamic integration method to study the hcp-bcc transition and melting of beryllium up to a pressure of 1600~GPa. We derive the melting line by equating solid and liquid Gibbs free energies, and represent it by a Simon Glatzel fit $T_m= 1564~\text{K} (1 + P/(15.6032 ~\text{GPa}))^{0.383}$, which is in good agreement with previous two-phase simulations below 6000~K. We also derive the hcp-bcc solid-solid phase boundary and show the quasiharmonic approximation underestimates the stability of the hcp structure, predicting…
The structural phase behaviors of pure zirconium metal under compressions up to $160$ GPa at room temperature are investigated from the perspective of ensemble theory where the partition function is solved by our recently proposed method with \emph{ab initio} precision. The derived Gibbs free energy is employed as the very criterion to determine phase transitions and the calculated transition pressures of the $α\rightarrowω\rightarrowβ$ are $6.93$ and $24.83$ GPa respectively, the former one of which is so far the only theoretical result agreeing with multiple experimental measurements to our …
Ab initio molecular dynamic method within the framework of density functional theory is applied to analyze the structural and electronic properties of crystalline molecular hydrogen at temperature 100\,K. Pressure, pair correlation function and band structure are calculated. The crossover of molecular crystalline hydrogen from the state of a semiconductor to a semimetallic and metallic state is observed upon compression in the pressure range of 302-626\,GPa. At pressures below 361\,GPa, the molecular crystal with the C2/c structure is a semiconductor with an indirect gap. In the pressure range…
It is a long-time pursuit of computations with \emph{ab initio} precision of thermal contributions to phase behaviors of condensed matters under extreme conditions. In this work, the pressure induced structural phase transitions of crystalline aluminum up to $600$ GPa at room temperature are investigated based on the criterion of Gibbs free energy derived directly from the partition function that formulated in the ensemble theory with the interatomic interactions characterized by density functional theory computations. The transition pressures of the FCC$\rightarrow$HCP$\rightarrow$BCC phase t…
The experimental study of the CO$_2$ phase diagram is hampered by strong kinetic effects leading to wide regions of metastability and to large uncertainties in the location of phase boundaries. Here we determine the CO$_2$ phase boundaries by means of {\it ab initio} calculations of the Gibbs free energy of several molecular and non-molecular solid phases of CO$_2$. Temperature effects are included in the quasi-harmonic approximation. Contrary to previous results, we find that the boundary between non-molecular phases and phase V has a positive slope and starts at 21.5 GPa at $T$ = 0 K. A trip…
High-pressure x-ray diffraction and ab initio study of Ni2Mo3N, Pd2Mo3N, Pt2Mo3N, Co3Mo3N, and Fe3Mo3N: Two families of ultra-incompressible bimetallic interstitial nitrides
本文用高压X射线衍射研究了Ni_2 Mo_3 N、Co_3 Mo_3 N和Fe_3 Mo_3 N的结构稳定性。我们还报告从头计算这些化合物，Pd 2 Mo 3 N和Pt 2 Mo 3 N的高压性能的建模。我们已经发现，氮化物保持稳定的环境压力立方结构至少高达50 GPa，并确定其状态方程。它们都具有大于300 GPa的体积模量。单晶弹性常数已被计算，以量化的刚度的研究氮化物。我们发现它们应该具有与立方尖晶石氮化物如γ-Si 3 N4类似的维氏硬度
Combined X-ray diffraction, electrical resistivity, and $ab$ $initio$ study of (TMTTF)$_2$PF$_6$ under pressure: implications to the unified phase diagram
通过联合 X 射线衍射、电阻率测量与从头计算，揭示了 (TMTTF)₂PF₆ 在压力下关联效应减弱、二聚化消失，并阐明了电荷与自旋有序的不同压力响应，为统一相图提供依据。
Rhombohedral dense forms of carbon, rh-C2 (or hexagonal h-C6), and boron nitride, rh-BN (or hexagonal h-B3N3), are derived from rhombohedral 3R graphite based on original crystal chemistry scheme backed with full cell geometry optimization to minimal energy ground state computations within the quantum density functional theory. Considering throughout hexagonal settings featuring extended lattices, the calculation of the hexagonal set of elastic constants, provide results of large bulk moduli i.e. B0(rh-C2) = 438 GPa close to that of diamond, and B0(rh-BN) = 369 GPa close to that of cubic BN. T…
The extremely large magnetoresistance (XMR) in nonmagnetic semimetals has inspired growing interest owing to both intriguing physics and potential applications. We report results of synchrotron X-ray diffraction (SXRD) and electrical transport measurements on TaAs2 under pressure up to ~ 37 GPa, which revealed an anisotropic compression of the unit cell, formation of unusual As-As bonds above 9.5 GPa, and enhancement of metallicity. Interestingly, the MR of TaAs2 under pressure changed gently, which at 1.7 GPa is 96.6% and at 36.6 GPa is still 36.7%. The almost robust MR under pressure could b…
New ultra-hard rhombohedral B2N2 and BC2N - or hexagonal B6N6 and B3C6N3 - are derived from 3R graphite based on crystal chemistry rationale schematizing a mechanism for 2D => 3D transformation. Full unconstrained geometry optimizations leading to ground state energy structures and energy derived quantities as energy-volume equation of states (EOS) were based on computations within the density functional theory (DFT) with generalized gradient approximation (GGA) for exchange-correlation (XC) effects. The new binary and ternary phases are characterized by tetrahedral stacking alike diamond, vis…
Quasi-hydrostatic compression of rhombohedral boron monosulfide (r-BS) has been studied up to 50 GPa at room temperature using diamond-anvil cells and angle-dispersive synchrotron X-ray diffraction. A fit of the experimental P-V data to the Vinet equation of state yields bulk modulus of 42.2(1.4) GPa and its first pressure derivative of 7.6(2) that are in excellent agreement with our ab initio calculations. Formation of a new high-pressure phase of boron monosulfide (hp-BS) has been observed above 35 GPa. According to ab initio evolutionary crystal structure predictions combined with Rietveld …
We have performed ab-initio molecular dynamics simulations to elucidate the mechanism of the phase transition at high pressure from hexagonal graphite (HG) to hexagonal diamond (HD) or to cubic diamond (CD). The transition from HG to HD is found to occur swiftly in very small time of 0.2 ps, with large cooperative displacements of all the atoms. We observe that alternate layers of atoms in HG slide in opposite directions by (1/3, 1/6, 0) and (-1/3, -1/6, 0), respectively, which is about 0.7 Å along the pm[2, 1, 0] direction, while simultaneously puckering by about pm0.25 Å perpendicular to the…
Hydrogen-rich superhydrides are believed to be very promising high-Tc superconductors as they are expected to mimic characteristics of metallic hydrogen. Recent experiments discovered superhydrides at very high pressures, e.g. FeH5 at 130 GPa and LaH10 at 170 GPa. With the motivation of discovering new hydrogen-rich high-Tc superconductors at lowest possible pressure, here we report the prediction and experimental synthesis of cerium superhydride CeH9 below 100 GPa in the laser-heated diamond anvil cell. Ab-initio calculations were carried to evaluate the detailed chemistry of the Ce-H system …
Density-functional theory forces, stresses and energies comprise a database from which the optimal parameters of a spline-based empirical potential combining Stillinger-Weber and modified embedded-atom forms are determined. Accuracy of the potential is demonstrated by predictions of ideal shear, stacking fault, vacancy migration, elastic constants and phonons all between 0 and 100 GPa. Consistency with existing models and experiments is demonstrated by application to screw dislocation core structure and deformation twinning in a tungsten nanorod. Lastly, the potential is used to study the high…
The high-pressure melting diagram of iron is a vital ingredient for the geodynamic modeling of planetary interiors. Nonetheless, available data for molten iron show an alarming discrepancy. Herein, we propose an efficient one-phase approach to capture the solid-liquid transition of iron under extreme conditions. Our basic idea is to extend the statistical moment method to determine the density of iron in the TPa region. On that basis, we adapt the work-heat equivalence principle to appropriately link equation-of-state parameters with melting properties. This strategy allows explaining cutting-…
We report the XRD and Raman scattering measurements in combination with DFT calculations that reveal the formation of beryllium polyhydride Be4H8(H2)2 by laser heating Be/H2 mixture to above 1700 K at pressures between 5 GPa and 8 GPa. The Be4H8(H2)2 crystallizes in a P6_3/mmc structure and consists of corner-sharing BeH4 tetrahedrons and H2 molecules that are in an interstitial site. The Be4H8(H2)2 is stable at least to 14 GPa on compression and stable down to 4 GPa at room temperature. Our ab-initio calculations suggest that the Be4H8(H2)2 is a meta-stable phase of Be-H system.
In this work, we synthesize and investigate lower technetium hydrides at pressures up to 45 GPa using the synchrotron X-ray diffraction, reflectance spectroscopy, and ab initio calculations. In the Tc-H system, the hydrogen content in TcH$_x$ phases increases when the pressure rises, and at 27 GPa we found a new hexagonal (hcp) nonstoichiometric hydride TcH$_{1.3}$. The formation of technetium hydrides is also confirmed by the emergence of a new reflective band at 450-600 nm in the reflectance spectra of TcH$_x$ samples synthesized at 45 GPa. On the basis of the theoretical analysis, we propos…
Computational searches for iron oxides at high pressures
我们已经使用密度泛函理论方法和从头算随机结构搜索（AIRSS）的方法来预测在高压下的铁和氧的混合物的稳定结构和化学计量。在100、350和500 GPa的压力下进行了12种不同化学计量的搜索，其中涉及32，000多个结构的弛豫。我们发现，Fe $_2$ O $_3$和FeO $_2$是唯一稳定的相分解在100 GPa，而在350和500 GPa的几个化学计量被发现是稳定的或非常接近稳定。本文报道了一种新的具有$P2_12_12_1$对称性的Fe $_2$ O $_3$相的结构，它在$\sim$ 233 GPa以上的压力下比已知的Rh $_2$ O $_3$（II）相更稳定。我们还报道了两种新的FeO结构，具有$Pnma$和$R\bar{3}m$对称性，它们分别在195-285 GPa和285-500 GPa范围内是稳定的，以及两种新的Fe $_3$ O $_4$结构，具有$Pca2_1$和$P2_1/c$对称性，它们分别在100-340 GPa和340-500 GPa范围内是稳定的。最后，我们报道了两种新的具有$P4_2/n$和$P\bar{3}m1$对称性的Fe $_4$ O $_5$结构，它们分别在100-231 GPa和231-500 GPa范围内是稳定的。我们发现Fe $_3$ O $_4$和Fe $_4$ O $_5$的新结构在其各自的稳定压力范围内具有比它们的已知结构更…
Atomic and electronic structures of Cu2H and CuH have been investigated by high pressure NMR spectroscopy, X-ray diffraction and ab-initio calculations. Metallic Cu2H was synthesized at a pressure of 40 GPa, and semi-metallic CuH at 90 GPa, found stable up to 160 GPa. Experiments and computations suggest the formation of a metallic 1H-sublattice as well as a high 1H mobility of ~10-7 cm2/s in Cu2H. Comparison of Cu2H and FeH data suggests that deviations from Fermi gas behavior, formation of conductive hydrogen networks, and high 1H mobility could be common features of metal hydrides.
We study the stability of Li-O compounds as a function of pressure, with Li ion battery applications and fundamental chemical interest in mind. Using the ab initio evolutionary algorithm, we predict stability of novel compounds LiO4, Li5O3 and Li6O under pressure. LiO4, formed at the pressure of just 6 GPa, can be seen as ε-O8 accepting two electrons from two Li atoms. This phase is superconducting, with Tc up to 12.2 K at 10 GPa. This is remarkable, because elemental oxygen becomes superconducting at much higher pressure (96 GPa) and has much lower Tc (<0.6 K), and suggests that chemical allo…
The physicochemical behavior of elements and compounds is heavily altered by high pressure. The occurrence of pressure-induced reactions and phase transitions can be revealed by crystal structure prediction approaches. In this work, we explore the C-H-O phase diagram up to 400 GPa exploiting an evolutionary algorithm for crystal structure predictions along with ab initio calculations. Besides uncovering new stable polymorphs of high-pressure elements and known molecules, we predicted the formation of new compounds. A 2CH4:3H2 inclusion compound forms at low pressure and remains stable up to 21…
