| 1. |
- Aleksandrovskii, A.N., et al.
(författare)
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On the polyamorphism of fullerite-based orientational glasses.
- 2005
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Ingår i: Low Temperature Physics. - : AIP Publishing. - 1063-777X .- 1090-6517. ; 31:5, s. 429-444
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Tidskriftsartikel (refereegranskat)abstract
- The dilatometric investigation in the temperature range of 2–28 K shows that a first-orderpolyamorphous transition occurs in the orientational glasses based on C60 doped with H2, D2 andXe. A polyamorphous transition was also detected in C60 doped with Kr and He. It is observed thatthe hysteresis of thermal expansion caused by the polyamorphous transition (and, hence, the transitiontemperature) is essentially dependent on the type of doping gas. Both positive and negativecontributions to the thermal expansion were observed in the low-temperature phase of the glasses.The relaxation time of the negative contribution occurs to be much longer than that of the positivecontribution. The positive contribution is found to be due to phonon and libron modes, whilst thenegative contribution is attributed to tunneling states of the C60 molecules. The characteristictime of the phase transformation from the low-T phase to the high-T phase has been found for theC60–H2 system at 12 K. A theoretical model is proposed to interpret these observed phenomena.The theoretical model proposed, includes a consideration of the nature of polyamorphism inglasses, as well as the thermodynamics and kinetics of the transition. A model of noninteractingtunneling states is used to explain the negative contribution to the thermal expansion. The experimentaldata obtained is considered within the framework of the theoretical model. From the theoreticalmodel the order of magnitude of the polyamorphous transition temperature has been estimated.It is found that the late stage of the polyamorphous transformation is described well by theKolmogorov law with an exponent of n = 1. At this stage of the transformation, the two-dimensionalphase boundary moves along the normal, and the nucleation is not important.
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| 2. |
- Araujo, Carlos Moyses, et al.
(författare)
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Pressure-induced structural phase transition in NaBH4
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 72:5, s. 054125-
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Tidskriftsartikel (refereegranskat)abstract
- We present a combined experimental and theoretical study of the technologically important NaBH4 compound under high pressure. Using Raman spectroscopy at room temperature, we have found that NaBH4 undergoes a structural phase transformation starting at 10.0 GPa with the pure high-pressure phase being established above 15.0 GPa. In order to compare the Raman data recorded under high pressure with the low-temperature tetragonal phase of NaBH4, we have also performed a cooling experiment. The known order-disorder transition from the fcc to the tetragonal structure was then observed. However, the new high pressure phase does not correspond to this low-temperature structure. Using first-principle calculations based on the density functional theory, we show that the high-pressure phase corresponds to the alpha-LiAlH4–type structure. We have found a good agreement between the measured and calculated transition pressures. Additionally, we present the electronic structure of both the fcc and the high-pressure phases.
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| 3. |
- Buga, Sergei G., et al.
(författare)
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Electrical properties of 3D-polymeric crystalline and disordered C60 and C70 fullerites.
- 2005
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Ingår i: Diamond and Related Materials vol. 14, special Issue (3-7). - : Elsevier B.V.. ; , s. 896-901
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Konferensbidrag (refereegranskat)abstract
- Crystalline and disordered 3D-polymeric C60 and C70 with different lattice parameters and with densities in the range of 2.2÷3.25 g cm−1 were obtained by high-pressure-high-temperature treatment at P=11.5÷15 GPa, T=670÷1800 K. Their d.c. electrical conductivity was investigated at temperatures 30÷350 K. The variable range hopping mechanism dominates in all samples up to room temperature. A temperature activated conductivity via delocalized states was observed at T>160÷260 K in crystalline C60 samples and in some samples with disordered structure. The evaluated band gap value is Eg=0.28÷0.54 eV for different crystalline structures and 0.06÷0.25 eV for disordered ones
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| 4. |
- Chorro, Matthieu, et al.
(författare)
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Discriminated structural behaviour of C60 and C70 peapods under extreme conditions
- 2007
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Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 79:5, s. 56003-
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Tidskriftsartikel (refereegranskat)abstract
- We studied structural changes in C60 and C70 peapods—i.e. C60 and C70 chains inserted inside single-walled carbon nanotubes—when submitted to high pressure and temperature conditions. X-ray diffraction experiments showed that while C60 molecules polymerize inside nanotubes at pressures and temperatures consistent with polymerization in bulk C60 phases, no polymerization is observed in C70 peapods, even at pressures and temperatures above the threshold for polymerization in bulk C70. Tubular confinement in the nanotube container prevents the monomers to rearrange in the way needed for polymerization. This result testifies for the strong influence of confinement on the behaviour of C70.
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| 5. |
- Dmitriev, Vladimir, et al.
(författare)
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Pressure-temperature phase diagram of LiBH4: Synchrotron x-ray diffraction experiments and theoretical analysis
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 77:17
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Tidskriftsartikel (refereegranskat)abstract
- An in situ combined high-temperature high-pressure synchrotron radiation diffraction study has been carried out on LiBH4. The phase diagram of LiBH4 is mapped to 10 GPa and 500 K, and four phases are identified. The corresponding structural distortions are analyzed in terms of symmetry-breaking atomic position shifts and anion ordering. Group-theoretical and crystal-chemical considerations reveal a nontrivial layered structure of LiBH4. The layers and their deformations define the structural stability of the observed phases.
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| 6. |
- Dolbin, Alexander Vitoldovich, et al.
(författare)
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Influence of dissolved oxygen on the thermal expansion and polyamorphism of fullerite C60
- 2007
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Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 33:5, s. 465-471
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Tidskriftsartikel (refereegranskat)abstract
- The temperature dependence of the coefficient of linear thermal expansion alpha of O-2-C-60 solutions with 20% and 80% filling of the octahedral cavities with oxygen is investigated in the temperature interval 2.2-24 K. Hysteresis of alpha(T) is observed, attesting to the coexistence of two orientational glasses in these solutions. A comparison of the behavior of these glasses is made. The characteristic times for reorientation of the C-60 molecules and for the phase transformations in the solutions are determined. When the temperature of the O-2-C-60 solution with 20 mol.% oxygen is increased to 450 degrees C, a chemical interaction of the oxygen with the C-60 molecules is manifested. It proves possible to separate the influences of the chemical and physical sorption of oxygen on the thermal expansion of polycrystalline fullerite C-60.
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| 7. |
- Dolbin, Alexander V., et al.
(författare)
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Radial thermal expansion of pure and Xe-saturated bundles of single-walled carbon nanotubes at low temperatures
- 2009
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Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 35:6, s. 484-490
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Tidskriftsartikel (refereegranskat)abstract
- The radial thermal expansion coefficient alphar of pure and Xe-saturated bundles of single-walled carbon nanotubes (CNTs) is measured in the interval 2.2–120 K. The coefficient is positive above T=5.5 K and negative at lower temperatures. The experiment was done using a low-temperature capacitance dilatometer with a sensitivity of 2×10−9 cm, and the sample was prepared by compacting a CNT powder in such a way that the pressure applied oriented the nanotube axes perpendicular to the axis of the cylindrical sample. The data show that individual nanotubes have a negative thermal expansion, while the solid compacted material has a positive expansion coefficient due to expansion of the intertube volume in the bundles. Doping the nanotubes with Xe caused a sharp increase in the magnitude of alphar in the whole range of temperatures used and gave rise to a peak in the dependence alphar(T) in the interval 50–65 K. A subsequent decrease in the Xe concentration lowered the peak considerably but had little effect on the thermal expansion coefficient of the sample outside the region of the peak. The features revealed are explained qualitatively.
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| 8. |
- Dolbin, Alexander V, et al.
(författare)
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Radial thermal expansion of single-walled carbon nanotube bundles at low temperatures.
- 2008
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Ingår i: Fizika Nizkikh Temperatur. - : AIP Publishing. - 0132-6414. ; 34:8, s. 860-862
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Tidskriftsartikel (refereegranskat)abstract
- For the first time, the linear coefficient of radial thermal expansion is measured on a system of carbon single-walled nanotube (SWNT) bundles at low temperatures (2.2–120 K). The measurements are performed using a dilatometer with a sensitivity of 2×10−9 cm. A cylindrical sample 7 mm high and 10 mm in diameter was obtained by compressing powder. The resulting bundles of nanotubes are oriented perpendicular to the sample axis. The starting powder consisted of over 90% SWNTs with outer diameter 1.1 nm, the length varying in the range 5–30 µm. A change of sign of the radial thermal expansion coefficient at 5.5 K is observed.
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| 9. |
- Dolbin, Alexander V., et al.
(författare)
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Specific features of thermal expansion and polyamorphism in CH4 – C60 solutions at low temperatures
- 2007
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Ingår i: Low Temperature Physics. - : AIP Publishing. - 1063-777X .- 1090-6517. ; 33:12, s. 1068-1072
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Tidskriftsartikel (refereegranskat)abstract
- The temperature dependence of the linear thermal expansion coefficient alpha(T) is investigated in the temperature range of 2.5 to 23 K for two different CH4-C-60 solutions in which CH4 molecules occupy 24 and 50% of the octahedral interstitial sites of the C-60 lattice. In both cases, alpha(T) exhibits hysteresis, suggesting the existence of two types of orientational glass associated with these solutions. The temperature of the first-order phase transition between these two glasses is estimated, and the behavior of these two glasses is compared. The characteristic times of thermalization tau(1), reorientation of the C-60 molecules tau(2), and of the phase transformation between the glasses tau('), are estimated for these solutions. Both the temperature dependence of alpha(T) and the characteristic thermalization time tau(1) are found to have features near the phase transition temperature, and an explanation is put forward to explain these observed features.
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| 10. |
- Dolbin, Alexander V., et al.
(författare)
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The effect of sorbed hydrogen on low-temperature radial thermal expansion of single-walled carbon nanotube bundles
- 2009
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Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 35:12, s. 939-943
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Tidskriftsartikel (refereegranskat)abstract
- The effect of a normal H2 impurity upon the radial thermal expansion ar of single-walled carbon nanotube (SWNT) bundles has been investigated in the interval T = 2.2–27 K using the dilatometric method. It is found that H2 saturation of SWNT bundles causes a shift of the temperature interval of the negative thermal expansion towards lower (as compared to pure carbon nanotubes) temperatures and a sharp increase in the magnitude of ar in the whole range of temperatures investigated. The low temperature desorption of H2 from a powder consisting of bundles of SWNTs, open and closed at the ends, has been investigated.
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