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[SMP27-P01] Correlation field splitting of OH stretching vibrations in phase A
Keywords:dense hydrous magnesium silicates, phase A, Correlation field splitting, OH stretching vibration, vibrational coupling
Phase A (Mg7Si2O8(OH)6) was discovered by Ringwood and Major (1967), along with phase B and phase C. In these phases, two OH species are present and they have a similar local structure with hydrogen bonding to a common oxygen. For phase A, Liu et al. (1997) performed high-pressure in-situ Raman measurements and reported that two OH stretching bands gradually approach each other up to 18 GPa but do not intersect and move apart thereafter. They interpreted this behavior as a phase transition. In this study, I consider this behavior of phase A as a pressure-tuned correlation field splitting and try to reproduce it from vibrational calculations.
The vibrational calculations were performed using the phonon code of Qunatum Espresso. The most effective way to determine the effect of correlation field splitting is to uncouple vibration by diluting with isotopes. In the case of this system, calculations were performed with the two H sites replaced by D to obtain the effect of isotope dilution. So calculations for systems without D and with H1 or H2 or both replaced by D were performed every 1 GPa up to 25 GPa. The pseudopotential of D used is exactly the same as that of H, but the mass is doubled.
In phase A, two OH bands are seen in Raman, but vibrational calculations show that they consist of A, E1, and E2 modes, respectively. In Raman, the contribution of the A mode is dominant. In terms of atomic displacements during the OH stretching modes, at ambient pressure, the low-frequency band has a large displacement of H1 of OH1 bond, and the high-frequency band has a large displacement of OH2, which is consistent with the previous studies. At 13 GPa, however, the situation changes and the contributions of OH1 and OH2 are almost the same in both bands. This indicates that the two modes are mixing due to strong OH coupling. At 25 GPa, the displacement of the low frequency band is now dominated by OH2 and that of the high frequency band is dominated by OH1, in reversal of the ambient pressure. In other words, OH1 and OH2 are interchanged. The obtained OH stretching frequency variation with pressure (see Figure a) roughly reproduces the results of Liu et al. (1997). The calculation with D yields the OH frequency uncoupled from the OH vibration. In this case (Figure b), the two OH bands change monotonically and intersect at about 13 GPa. This result shows that the behavior of OH in phase A is strongly influenced by the correlation field splitting. Furthermore, these analyses show that the correlation field splitting increases with pressure.
The calculations for phase C show that it is affected by strong correlation field splitting even at ambient pressure, although it does not behave like phase A because the two OH species are not significantly different as in phase A. The experimental observation of two OH bands that slowly separate with pressure (Liu et al., 2002) can be explained by an increase in correlation field splitting with pressure. Although phase B has not been calculated yet, the similarity of the structure suggests that it is similar to phase C.
Although correlation field splitting has been generally overlooked, it is in fact important and should be taken into account in vibrational spectroscopic analysis. As shown here, its effects can be revealed in vibrational calculations by dilution with isotopes.
References:
Liu et al. (1997) J. Phys. Chem. Solids, 58, 2023-2030
Liu et al. (2002) Eur. J. Min., 14, 15-23
Ringwood & Major (1967) Earth Planet. Sci. Lett., 2, 130-133.
The vibrational calculations were performed using the phonon code of Qunatum Espresso. The most effective way to determine the effect of correlation field splitting is to uncouple vibration by diluting with isotopes. In the case of this system, calculations were performed with the two H sites replaced by D to obtain the effect of isotope dilution. So calculations for systems without D and with H1 or H2 or both replaced by D were performed every 1 GPa up to 25 GPa. The pseudopotential of D used is exactly the same as that of H, but the mass is doubled.
In phase A, two OH bands are seen in Raman, but vibrational calculations show that they consist of A, E1, and E2 modes, respectively. In Raman, the contribution of the A mode is dominant. In terms of atomic displacements during the OH stretching modes, at ambient pressure, the low-frequency band has a large displacement of H1 of OH1 bond, and the high-frequency band has a large displacement of OH2, which is consistent with the previous studies. At 13 GPa, however, the situation changes and the contributions of OH1 and OH2 are almost the same in both bands. This indicates that the two modes are mixing due to strong OH coupling. At 25 GPa, the displacement of the low frequency band is now dominated by OH2 and that of the high frequency band is dominated by OH1, in reversal of the ambient pressure. In other words, OH1 and OH2 are interchanged. The obtained OH stretching frequency variation with pressure (see Figure a) roughly reproduces the results of Liu et al. (1997). The calculation with D yields the OH frequency uncoupled from the OH vibration. In this case (Figure b), the two OH bands change monotonically and intersect at about 13 GPa. This result shows that the behavior of OH in phase A is strongly influenced by the correlation field splitting. Furthermore, these analyses show that the correlation field splitting increases with pressure.
The calculations for phase C show that it is affected by strong correlation field splitting even at ambient pressure, although it does not behave like phase A because the two OH species are not significantly different as in phase A. The experimental observation of two OH bands that slowly separate with pressure (Liu et al., 2002) can be explained by an increase in correlation field splitting with pressure. Although phase B has not been calculated yet, the similarity of the structure suggests that it is similar to phase C.
Although correlation field splitting has been generally overlooked, it is in fact important and should be taken into account in vibrational spectroscopic analysis. As shown here, its effects can be revealed in vibrational calculations by dilution with isotopes.
References:
Liu et al. (1997) J. Phys. Chem. Solids, 58, 2023-2030
Liu et al. (2002) Eur. J. Min., 14, 15-23
Ringwood & Major (1967) Earth Planet. Sci. Lett., 2, 130-133.