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Structural Deformation of Sm@C88 under High Pressure.

Cui J, Yao M, Yang H, Liu Z, Ma F, Li Q, Liu R, Zou B, Cui T, Liu Z, Sundqvist B, Liu B - Sci Rep (2015)

Bottom Line: Pressure induced a significant reduction of the band gap of the crystal.Both effects decrease the band gap of the sample.The carbon cage deforms significantly above 7 GPa, from spherical to a peanut-like shape and collapses at 18 GPa.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory of Superhard Materials, Jilin University, No. 2699 Qianjin Street, Changchun 130012, P.R. China.

ABSTRACT
We have studied the structural transformation of Sm@C88 under pressure up to 18 GPa by infrared spectroscopy combined with theoretical simulations. The infrared-active vibrational modes of Sm@C88 at ambient conditions have been assigned for the first time. Pressure-induced blue and red shifts of the corresponding vibrational modes indicate an anisotropic deformation of the carbon cage upon compression. We propose that the carbon cage changes from ellipsoidal to approximately spherical around 7 GPa. A smaller deformation of the carbon bonds in the area close to the Sm atom in the cage suggests that the trapped Sm atom plays a role in minimizing the compression of the adjacent bonds. Pressure induced a significant reduction of the band gap of the crystal. The HOMO-LUMO gap of the Sm@C88 molecule decreases remarkably at 7 GPa as the carbon cage is deformed. Also, compression enhances intermolecular interactions and causes a widening of the energy bands. Both effects decrease the band gap of the sample. The carbon cage deforms significantly above 7 GPa, from spherical to a peanut-like shape and collapses at 18 GPa.

No MeSH data available.


Related in: MedlinePlus

(a) The mid-IR region shows the absorption edge under high pressure. (b) The band gap as a function of pressure. Inset is the plots of (αhν)2 versus hν at ambient pressure and 13.9 GPa. (c) IR reflectivity spectra of the sample under high pressure.
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f5: (a) The mid-IR region shows the absorption edge under high pressure. (b) The band gap as a function of pressure. Inset is the plots of (αhν)2 versus hν at ambient pressure and 13.9 GPa. (c) IR reflectivity spectra of the sample under high pressure.

Mentions: High-pressure absorption spectra shown in Fig. 5(a) display clear onset of a broad hump which corresponds to the absorption edge of the sample. This absorption edge gradually shifts to lower energy with increasing pressure and completely moves into FIR region at 13.9 GPa. The indirect band gap Eg of the sample can be estimated from the x-axis intercept by extrapolating the linear portion of the (αhν)2 versus hν plot to α = 0 (inset of Fig. 5(b)), where α and hν are the absorption coefficient and the incident photon energy. We plotted the band gap as a function of pressure in Fig. 5(b). The band gap decreases almost linearly from 0.73 eV at ambient pressure to 0.16 eV at 13.9 GPa with a pressure coefficient of −0.043 ± 0.001 eVGPa−1. It is worth mentioning that the band gaps of solid C7026 and C6027 decrease as pressure increases, to a value of 1.4 eV before the molecular collapse, but never reach such low value as observed in our Sm@C88. To determine whether the material experiences a metallization transformation, we measured the high-pressure infrared reflectivity spectra of the material as shown in Fig. 5(c). The reflectivity of the sample gradually increased with pressure, which is consistent with the result of the absorption spectra of the sample. However, neither an abrupt increase in reflectivity nor a Drude-Lorentz oscillator model feature in the reflectivity spectra was observed. That suggests that the band gap of the material only decreases to a small value, rather than closes.


Structural Deformation of Sm@C88 under High Pressure.

Cui J, Yao M, Yang H, Liu Z, Ma F, Li Q, Liu R, Zou B, Cui T, Liu Z, Sundqvist B, Liu B - Sci Rep (2015)

(a) The mid-IR region shows the absorption edge under high pressure. (b) The band gap as a function of pressure. Inset is the plots of (αhν)2 versus hν at ambient pressure and 13.9 GPa. (c) IR reflectivity spectra of the sample under high pressure.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4548219&req=5

f5: (a) The mid-IR region shows the absorption edge under high pressure. (b) The band gap as a function of pressure. Inset is the plots of (αhν)2 versus hν at ambient pressure and 13.9 GPa. (c) IR reflectivity spectra of the sample under high pressure.
Mentions: High-pressure absorption spectra shown in Fig. 5(a) display clear onset of a broad hump which corresponds to the absorption edge of the sample. This absorption edge gradually shifts to lower energy with increasing pressure and completely moves into FIR region at 13.9 GPa. The indirect band gap Eg of the sample can be estimated from the x-axis intercept by extrapolating the linear portion of the (αhν)2 versus hν plot to α = 0 (inset of Fig. 5(b)), where α and hν are the absorption coefficient and the incident photon energy. We plotted the band gap as a function of pressure in Fig. 5(b). The band gap decreases almost linearly from 0.73 eV at ambient pressure to 0.16 eV at 13.9 GPa with a pressure coefficient of −0.043 ± 0.001 eVGPa−1. It is worth mentioning that the band gaps of solid C7026 and C6027 decrease as pressure increases, to a value of 1.4 eV before the molecular collapse, but never reach such low value as observed in our Sm@C88. To determine whether the material experiences a metallization transformation, we measured the high-pressure infrared reflectivity spectra of the material as shown in Fig. 5(c). The reflectivity of the sample gradually increased with pressure, which is consistent with the result of the absorption spectra of the sample. However, neither an abrupt increase in reflectivity nor a Drude-Lorentz oscillator model feature in the reflectivity spectra was observed. That suggests that the band gap of the material only decreases to a small value, rather than closes.

Bottom Line: Pressure induced a significant reduction of the band gap of the crystal.Both effects decrease the band gap of the sample.The carbon cage deforms significantly above 7 GPa, from spherical to a peanut-like shape and collapses at 18 GPa.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory of Superhard Materials, Jilin University, No. 2699 Qianjin Street, Changchun 130012, P.R. China.

ABSTRACT
We have studied the structural transformation of Sm@C88 under pressure up to 18 GPa by infrared spectroscopy combined with theoretical simulations. The infrared-active vibrational modes of Sm@C88 at ambient conditions have been assigned for the first time. Pressure-induced blue and red shifts of the corresponding vibrational modes indicate an anisotropic deformation of the carbon cage upon compression. We propose that the carbon cage changes from ellipsoidal to approximately spherical around 7 GPa. A smaller deformation of the carbon bonds in the area close to the Sm atom in the cage suggests that the trapped Sm atom plays a role in minimizing the compression of the adjacent bonds. Pressure induced a significant reduction of the band gap of the crystal. The HOMO-LUMO gap of the Sm@C88 molecule decreases remarkably at 7 GPa as the carbon cage is deformed. Also, compression enhances intermolecular interactions and causes a widening of the energy bands. Both effects decrease the band gap of the sample. The carbon cage deforms significantly above 7 GPa, from spherical to a peanut-like shape and collapses at 18 GPa.

No MeSH data available.


Related in: MedlinePlus