Calculation of the turnover in the vibrational frequencies of solid hydrogen at high pressures

Scheerboom, M. I. M.; Schouten, J.A.

doi:10.1103/physrevb.53.r14705

Cited by 9 publications

(6 citation statements)

References 21 publications

(51 reference statements)

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“…Furthermore, compression also results in a significant (unexplained) intensity enhancement of the n 1 guest vibrons. The situation is made more complex through the recent result that the turnover of the Raman and infrared vibron in solid H 2 need not require charge transfer or the onset of bond weakening [22]. Finally, the isotopic spectra are shown to be of great importance for the identification of different site symmetries not easily detected in x-ray diffraction.…”

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Vibrational Dynamics of Isotopically Dilute Nitrogen to 104 GPa

Olijnyk

Jephcoat

1999

Phys. Rev. Lett.

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Raman studies to 104 GPa of a 14 N 2 host with dilute 15 N 2 and 14 N 15 N isotopic species indicate an increase in vibrational coupling with pressure. The lowest frequency branch of the isotopic species 15 N 2 reaches a plateau around 100 GPa suggesting the onset of bond weakening for molecules located on particular sites. An unusual intensity increase is observed for the isotopic n 1 guest modes. The phase transitions above 20 GPa appear to involve an increase in the number of inequivalent sites, with possibly subtle changes in the orientational behavior of particular molecules in the pressure range 25 to 50 GPa not readily revealed in recent x-ray diffraction studies. PACS numbers: 62.50. + p, 78.30.Hv The P 2 T -phase diagram of solid nitrogen appears to be well established up to pressures near ϳ20 GPa [1-10]. At higher pressures the behavior of solid nitrogen is not well understood: Above 25 GPa Raman spectra [8,11] are not compatible with the rhombohedral R3c lattice as suggested by x-ray diffraction studies [5][6][7]; the theoretically predicted tetragonal lattice in this pressure range [12] is in disagreement with experiment; and calculated pressurevolume relations using gas-phase intermolecular potentials for nitrogen [13] significantly depart above 12 GPa from accurate equations of state [6]. A further open question concerns the mechanism responsible for the decrease in frequency of the lower frequency Raman-active vibron above 60 GPa in nitrogen [11,14], that has been discussed in terms of weakening of the N-N bonds under compression as a precursor of molecular dissociation. Indeed theory predicts the molecular to nonmolecular transition well below 100 GPa [15][16][17], but this is in apparent disagreement with the experimental observation of molecular nitrogen to 180 GPa [11,14] though this discrepancy might be due to the existence of a high-energy barrier associated with molecular dissociation [16,17].These results demonstrate that we lack a sufficient theoretical description of the interaction potentials and a quantitative understanding of the vibrational dynamics at high pressures. Raman studies of nitrogen isotopic mixtures have provided a measure of the intermolecular and intramolecular interactions, but have been limited to moderate pressures [8]. We report Raman measurements at room temperature up to 104 GPa for the internal modes of 14 N 2 as host, and for the isotopic species 15 N 2 and 14 N 15 N as dilute (guest) molecules. Shifts and splittings of the internal modes in the solid state result from the local anisotropy of the crystalline field (site-group interactions) as well as from the coupling of identical vibrations of adjacent molecules in the unit cell (variously known as factor-group interactions, and dynamic, vibrational, or resonance coupling). At low concentrations, the isotopically substituted molecules are isolated in the host matrix and the coupling effect is switched off for these guest isotopic species because of the frequency difference with the corresponding vibrations of...

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Vibrational Dynamics of Isotopically Dilute Nitrogen to 104 GPa

Olijnyk

Jephcoat

1999

Phys. Rev. Lett.

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“…In fact, the D 2 Raman frequency (scaled by √ 2) is roughly 200 cm −1 higher than that of H 2 . To first order in h, quantum corrections to the frequency are known to scale as the inverse of the particle's mass, no matter what the potential is [29]. Thus, using the experimental H 2 and D 2 frequencies, we obtain empirical but accurate quantum corrections to the classical values following Ref.…”

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Solid Molecular Hydrogen: The Broken Symmetry Phase

et al. 1997

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By performing constant-pressure variable-cell ab initio molecular dynamics simulations we find a quadrupolar orthorhombic structure of Pca2 1 symmetry for the broken symmetry phase (phase II) of solid H 2 at T 0 and P 110 150 GPa. We present results for the equation of state, lattice parameters, and vibronic frequencies, in very good agreement with experimental observations. Anharmonic quantum corrections to the vibrational frequencies are estimated using available data on H 2 and D 2 . We assign the observed modes to specific symmetry representations. [S0031-9007(97)02938-4] PACS numbers: 62.50.+p, 64.30.+t, The quest for the structure of the high-pressure phases of hydrogen is a long standing one. Early predictions of an insulator-metal transition [1] led to a large body of experimental and theoretical work during the past 60 years. Metallization was not found as promptly as initially expected [2] but a rich phase diagram emerged. The current picture of the low and room-temperature phase diagram up to ϳ230 GPa is essentially based on the optical studies performed in diamond anvil cell (DAC) devices during the past decade [3,4].There is a consensus that hydrogen exhibits at least three different solid molecular phases. (1) At low pressures (,110 GPa for para-H 2 ) the centers of the H 2 molecules crystallize into an hcp lattice, but zero-point motion overcomes rotational energy barriers, leading to a free-rotator phase (phase I).(2) Between 110 and 150 GPa intermolecular interactions freeze the molecular rotations into an ordered broken-symmetry phase (BSP, or phase II). (3) Above ϳ150 GPa, a third phase (H-A, or phase III) is attained, whose structure is unknown. Here we focus specifically on phase II. Optical measurements in phase II indicate the presence of two, possibly three, infrared (IR) active modes, in constrast to phase I, where only one mode is observed. Raman spectra show a single peak, at a frequency ϳ10% lower than that of the IR modes. A consistent picture of the structural and dynamical properties of phase II is still lacking [5].On the theoretical side, most of the existing work consists of static total energy calculations within the local density approximation (LDA). Zero-point energy (ZPE) of the protons has been in a few cases included a posteriori based on frozen phonon calculations [6,7]. Hexagonal close packed structures with two and four molecules per unit cell appear to be the strongest candidates for the ground structure of phase II, but the relative orientation of the molecules is uncertain [8][9][10]. Cubic structures were also suggested [9]. This uncertainty persists, due to the incomplete optimization of the lattice parameters and the a priori selection of a particular space group symmetry in static calculations. Ab initio molecular dynamics simulations, which do not rely on the choice of a specific space group, have also been reported [11,12]. However, these earlier attempts were hampered by a fixed simulation cell and a poor Brillouin zone (BZ) sampling.We report extensive ab init...

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“…A similar frequency shift is seen in the Raman spectrum of the hydrogen compounds. This suggests that the effect is caused by cou-pling between hydrogen molecules weakening as they are separated by the chemically inert elements [14][15][16] . A simple classical molecular potential with nearest neighbour interactions has shown that this effect is of the right order of magnitude to explain the behaviour in Argon-hydrogen mixtures 14,17 .…”

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Localization effects on the vibron shifts in helium-hydrogen mixtures

2020

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The vibrational frequency of hydrogen molecules has been observed to increase strongly with He concentration in helium hydrogen fluid mixtures. This has been associated with He-H interactions, either directly through chemical bonding 1 , or indirectly through increased local pressure 2 . Here, we demonstrate that the increase in the Raman frequency of the hydrogen molecule vibron is due to the number of H2 molecules participating in the mode. There is no chemical bonding between He and H2, helium acts only to separate the molecules. The variety of possible environments for H2 gives rise to many Raman active modes, which causes broadening the vibron band. As the Raman active modes tend to be the lower frequency vibrons, these effects work together to produce the majority of the shift seen in experiment. We used Density Functional Theory (DFT) methods in both solid and fluid phases to demonstrate this effect. DFT also reveals that the pressure in these H2-He mixture is primarily due to quantum nuclear effects, again the weak chemical bonding makes it a secondary effect.

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Calculation of the turnover in the vibrational frequencies of solid hydrogen at high pressures

Cited by 9 publications

References 21 publications

Vibrational Dynamics of Isotopically Dilute Nitrogen to 104 GPa

Vibrational Dynamics of Isotopically Dilute Nitrogen to 104 GPa

Solid Molecular Hydrogen: The Broken Symmetry Phase

Localization effects on the vibron shifts in helium-hydrogen mixtures

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