The phase diagrams of beryllium and magnesium oxide at megabar pressures

Author(s):  
Jizhou Wu ◽  
Felipe J González-Cataldo ◽  
Francois Soubiran ◽  
Burkhard Militzer

Abstract We perform ab initio simulations of beryllium (Be) and magnesium oxide (MgO) at megabar pressures and compare their structural and thermodynamic properties. We make a detailed comparison of our two recently derived phase diagrams of Be [Wu et al., Phys. Rev. B 104, 014103 (2021)] and MgO [Soubiran and Militzer, Phys. Rev. Lett. 125, 175701 (2020)] using the thermodynamic integration technique, as they exhibit striking similarities regarding their shape. We explore whether the Lindemann criterion can explain the melting temperatures of these materials through the calculation of the Debye temperature at high pressure. From our free energy calculations, we obtained a melting curve for Be that is well represented by the fit Tm(P) = 1564K*[1 + P/(15.8037 GPa)]^0.414 , and a melting line of MgO, which can be well reproduced by the fit Tm(P) = 3010K*(1 + P/a)^(1/c) with a = 10.5797 GPa and c = 2.8683 for the B1 phase and a = 26.1163 GPa and c = 2.2426 for the B2 phase. Both materials exhibit negative Clapeyron slopes on the boundaries between the two solid phases that are strongly affected by anharmonic effects, which also influences the location of the solid-solid-liquid triple point. We find that the quasi-harmonic approximation underestimates the stability range of the low-pressure phases, namely hcp for Be and B1 for MgO. We also compute the phonon dispersion relations at low and high pressure for each of the phases of these materials, and also explore how the phonon density of states is modified by temperature. Finally, we derive secondary shock Hugoniot curves in addition to the principal Hugoniot curve for both materials, and study their offsets in pressure between solid and liquid branches.

2014 ◽  
Vol 70 (a1) ◽  
pp. C894-C894
Author(s):  
Masakazu Matsumoto ◽  
Kazuhiro Himoto ◽  
Kenji Mochizuki ◽  
Hideki Tanaka

Water distributes ubiquitously among the solar system and outer space in a wide variety of solid forms, i.e. more than ten kinds of crystalline ice, two types of amorphous ice, and clathrate hydrates. These polymorphs often play crucial roles in the planetary geology. Diversity of the stable ices and hydrates also suggests the existence of the various kinds of stable and metastable phases yet to be discovered [1]. Computer simulations and the theoretical treatments are useful to explore them. In this talk, we introduce the phase transitions of ice VII, which is one of the highest-pressure ice phases. The melting curve of ice VII to high-pressure liquid water has not been settled by experiments. We have proposed the intervention of a plastic phase of ice (plastic ice) between ice VII and liquid water, based on molecular dynamics (MD) simulations and the free energy calculations [2], which enables to account for large gaps among the various experimental curves of ice VII. In plastic ice, the water molecules are fixed at the lattice points, while they rotate freely. Interestingly, our additional survey by large-scale MD simulations elucidates that the phase transition between ice VII and plastic ice is first-order at low pressure as it was already predicted, while it is found to be second-order at higher pressures, where a tricritical point joins these phase boundaries together [3]. The critical fluctuations may give a clue for determining the phase boundary experimentally. We also argue about the phase transition dynamics of liquid water to ice VII at their direct phase boundary where metastable plastic ice phase plays an important role.


2018 ◽  
Vol 32 (16) ◽  
pp. 1850200
Author(s):  
Guo Zhang ◽  
Yu-Xin Zhao ◽  
Jun Zhu ◽  
Yan-Jun Hao ◽  
Lin Zhang

The structure of ZrB2 under high-pressure was predicted by Particle Swarm Optimization method (CALYPSO). We investigated the structure stability, phonon dispersion curve, elasticity, electronic structure and thermodynamic properties of ZrB2 under high-pressure and high-temperature via first-principles calculations. It maintained the hexagonal structure when the pressure lowers below 600 GPa at 0 K, which is confirmed by the calculated phonon dispersion curve. Studies indicate that the elastic modulus and Poisson’s ratio increase monotonically with pressure, as supported by some theoretical and experimental evidences. Calculated anisotropic factors demonstrate that compression and shear isotropy of ZrB2 weakens as the pressure increases. Using the quasi-harmonic approximation Debye model, the Debye temperature, sound velocity, expansion coefficient, thermal capacity under the high-temperature and pressure were also predicted.


RSC Advances ◽  
2019 ◽  
Vol 9 (7) ◽  
pp. 3577-3581 ◽  
Author(s):  
Nursultan Sagatov ◽  
Pavel N. Gavryushkin ◽  
Talgat M. Inerbaev ◽  
Konstantin D. Litasov

We carried out ab initio calculations on the crystal structure prediction and determination of P–T diagrams within the quasi-harmonic approximation for Fe7N3 and Fe7C3.


1998 ◽  
Vol 43 (3) ◽  
pp. 362-366 ◽  
Author(s):  
P. Marteau ◽  
P. Tobaly ◽  
V. Ruffier-Meray ◽  
J. C. de Hemptinne

2021 ◽  
Vol 104 (10) ◽  
Author(s):  
Hao Song ◽  
Zihan Zhang ◽  
Mingyang Du ◽  
Qiwen Jiang ◽  
Defang Duan ◽  
...  
Keyword(s):  

1991 ◽  
Vol 94 (5) ◽  
pp. 3835-3838 ◽  
Author(s):  
W. L. Vos ◽  
J. A. Schouten ◽  
D. A. Young ◽  
M. Ross
Keyword(s):  

2020 ◽  
Author(s):  
Christopher R. Taylor ◽  
Matthew T. Mulvee ◽  
Domonkos S. Perenyi ◽  
Michael R. Probert ◽  
Graeme Day ◽  
...  

<div> <p>We combine state-of-the-art computational crystal structure prediction (CSP) techniques with a wide range of experimental crystallization methods to understand and explore crystal structure in pharmaceuticals and minimize the risk of unanticipated late-appearing polymorphs. Initially, we demonstrate the power of CSP to rationalize the difficulty in obtaining polymorphs of the well-known pharmaceutical isoniazid and show that CSP provides the structure of the recently discovered, but unsolved, Form III of this drug despite there being only a single known form for almost 70 years. More dramatically, our blind CSP study predicts a significant risk of polymorphism for the related iproniazid. Employing a wide variety of experimental techniques, including high-pressure experiments, we experimentally obtained the first three known non-solvated crystal forms of iproniazid, all of which were successfully predicted in the CSP procedure. We demonstrate the power of CSP methods and free energy calculations to rationalize the observed elusiveness of the third form of iproniazid, the success of high-pressure experiments in obtaining it, and the ability of our synergistic computational-experimental approach to “de-risk” solid form landscapes.</p> </div>


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