First principles calculations are employed to investigate both type I and II silicon clathrates. The phonon and infrared spectra of both types of clathrate are obtained. We find a localized vibrational mode of Si46 whose frequency is 375.0 cm -1 , where the vibration in the mode localizes in the hexagonal chain. The heat capacity of both clathrates is the same as that of the diamond phase Si (d -Si ) . When the temperature is lower than 100 K, the Debye temperatures of the clathrates are higher than that of d -Si ; however, the Debye temperatures of both clathrates at high temperature (≫100 K ) are lower than that of the d -Si . The mean free paths (λ) and thermal conductivities (κ) of the clathrates are larger than those of d -Si at low temperature. The Seebeck coefficients (S) of the clathrates are higher than that of d -Si in the temperature interval 300–1000 K; however, both clathrates exhibit a lower value of σ/τ when compared to the d -Si .
[Show abstract][Hide abstract] ABSTRACT: We employ state-of-the-art first-principles calculations based on density-functional theory and density-functional perturbation theory to investigate relevant physical properties and phase diagram of the free guest type-I (X-46) and type-II (X-34) carbon clathrates. Their properties and those of silicon and germanium diamonds, and clathrates have been computed and compared within the same approach. We briefly present and discuss their structural, cohesive, and electronic properties. In particular, we present different results about electronic properties of carbon clathrates. From the symmetry analysis of electronic states around the band gap, we deduce their optical properties, and we forecast the effects of hypothetical-doped elements on their electronic band gap. We then report first-principles calculations of vibrational, thermodynamical, and elastic properties. Whereas vibrational properties of Si and Ge systems can be linked through their atomic weight ratio, we show that the vibrational properties of carbon structures differ strongly. Raman and infrared spectra of all clathrates are also calculated and compared. The effects of pressure and temperature on thermodynamical properties (heat capacity, entropy, thermal expansion, etc.) within static and quasiharmonic approximations are investigated. It is shown that thermodynamical properties of carbon clathrates and diamond present a similar evolution up to high pressures (100 GPa) and over a large range of temperatures ([0, 1500] K). Then we deduce the equilibrium phase diagram (P,T) of C-2/C-34/C-46. We conclude the paper with a presentation of elastic properties computed from acoustic slopes.
[Show abstract][Hide abstract] ABSTRACT: Density functional theory can be used to interpret and predict spectroscopic properties of solid-state materials. The relevant computational solutions are usually available in disparate DFT codes, so that it is difficult to use a consistent approach for analyzing various spectroscopic features of a given material. We review the latest developments that are aimed to provide a collection of analytical tools within one DFT package, CASTEP. The applications covered include core-level EELS, solid-state NMR, optical properties, IR and Raman spectroscopy. We present also results of the EELS analysis of NbO and Nb2O5 that show the first published example of CASTEP spectra from d-states. Raman activities calculated for a test set of small molecules and the convergence requirements for such calculations are discussed.
[Show abstract][Hide abstract] ABSTRACT: We present the results of vibrational and thermal properties for small diameter silicon nanowires (Si-NWs) from first principles calculations. Phonon spectrums of the Si-NWs are obtained based on the density functional perturbation theory. We found that heat-carrying acoustic branches exhibit “bending,” which results from the strong interaction between acoustic and no-zero-frequency flexural modes. The bending of acoustic branches implies that the phonon group velocity (V = dω/dq) of Si-NWs is less than that of corresponding bulk silicon. Therefore, a lower lattice thermal conductivity of Si-NWs can be caused by the bending of acoustic phonon. In comparison with bulk silicon, optical branches of Si-NWs exhibit “blueshift,” which is due to the high frequency vibration of silicon atoms at the edge of Si-NWs. From the obtained phonon spectrums, specific heat is calculated. The specific heat of Si-NWs is also lower than that of bulk silicon crystal. The reduction in the specific heat is due to the small magnitude of vibration density of states of low frequency phonons. In the temperature range from 100 to 1000 K, the Debye temperatures are obtained. We found that the Debye temperature of the Si-NWs is much higher than that in the corresponding bulk silicon. Especially, Debye temperature of tetrahedral Si-NW is nearly twice higher than that of bulk silicon. From the temperature dependence of Hamholtz free energy of Si-NWs, we find that the cagelike Si-NWs have higher thermal stability than the tetrahedral Si-NW.
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