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In this paper, the phonon spectra of ZrSe 2 , HfS 2 , and HfS 2 /ZrSe 2 heterojunctions are calculated based on the density-functional perturbation method for biaxial tensile strains to study their stability and thermal properties. It was found that monolayers of ZrSe 2 and HfS 2 should not be subjected to > 8% biaxial tensile strain, and HfS 2 and HfS 2 /ZrSe 2 heterojunctions should not be subjected to > 6%. By looking at the graphs of enthalpy, entropy, free energy, and heat capacity under strain for the three systems, it is found that the effect on heat capacity is greater after destabilization in the monolayer system and after destabilization in the heterojunction structure, it is greater for the free energy. Enthalpy, entropy and free energy all increase continuously with increasing temperature, and heat capacity increases continuously with the rising temperature at temperatures < 200 K and tends to remain constant when the temperature is >200 K. Fig. 1 (a and b) Geometric structures of monolayers of HfS 2 and ZrSe 2. (c) The structure of the HfS 2 /ZrSe 2 heterojunction. [Display omitted] In this paper, the phonon spectra of ZrSe 2 , HfS 2 , and HfS 2 /ZrSe 2 heterojunctions are calculated based on the density-functional perturbation method for biaxial tensile strains to study their stability and thermal properties. It was found that monolayers of ZrSe 2 and HfS 2 should not be subjected to >8 % biaxial tensile strain, and HfS 2 /ZrSe 2 heterojunctions should not be subjected to >6 %. By looking at the graphs of enthalpy, entropy, free energy, and heat capacity under strain for the three systems, it is found that the effect on heat capacity is greater after destabilization in the monolayer system and after destabilization in the heterojunction structure, it is greater for the free energy. Enthalpy, entropy and free energy all increase continuously with increasing temperature, and heat capacity increases continuously with the rising temperature at temperatures <200 K and tends to remain constant when the temperature is >200 K. [ABSTRACT FROM AUTHOR]

Based on first-principles calculations with generalized gradient approximation as exchange-correlation functional, the structural, electronic, mechanical, thermal, and phonon properties of III-phosphide binary compounds, namely BP, AlP, GaP, and InP, with cubic zincblende structure have been investigated. The calculations were performed in the framework of density functional theory and density functional perturbation theory (DFPT) implemented in the Quantum ESPRESSO package. The results obtained for the structural and electronic properties are in good agreement with available theoretical and experimental results. The results of our electronic calculations indicate semiconducting properties for these binary compounds. Furthermore, the frequency bandgaps and phonon density of states were also investigated. The computed mechanical constants predict that BP, AlP, GaP, and InP are elastically stable. Finally, we determined the heat capacity and entropy for these binary compounds within a quasiharmonic Debye model using DFPT for comparison. [ABSTRACT FROM AUTHOR]

The study of vibrational characteristics of chains of rare gas atoms adsorbed in the grooves between nanotubes in nanobundles is reduced to the analyses of the phonon spectrum and the vibrational characteristics of linear chains of atoms in an external field. Atoms in the chain have three degrees of freedom. The analytical expressions for the vibrational characteristics of the atoms in the chain, depending on the ratio between the interatomic distance in the chain r and the equilibrium distance between atoms in the chain $$r_0$$ , are obtained. It is shown that at $$r

To measure a heat capacity of a single carbon nanotube is impossible nowadays and experiments are performed with the nanotubes bundles. However, the existing theories of carbon nanotubes heat capacity do not take into account an interaction between an individual nanotube and its environment. Here we show that due to this fact the known theories overestimate the heat capacity in the ultra-low temperature limit. In this region the main contribution to heat capacity of a single free nanotube is caused by the bending phonon mode with a quadratic dispersion. At T =1 K different known models of the phonon dynamics lead to the theoretical specific heat which is at least 10 times greater than the experimentally measured value. To overcome this contradiction we take into account the interaction between an individual nanotube and its environment. As a result, the nanotube loses its Goldstone degrees of freedom and essential reconstruction of the lowest-energy part of its phonon spectrum occurs. The theory proposed explains experimental data on thermal capacity in the ultra-low temperature range and simultaneously demonstrates better agreement with experiments in the region 2.5 K< T <40 K than the preceding models. [ABSTRACT FROM AUTHOR]

calculation of heat capacity of diamond using reliable data on the diamond spectroscopic and elastic-dynamic characteristics is performed together with an experimental investigation of a low-temperature (50-300 K) heat capacity of synthetic diamond produced by General Electric. Based on the established temperature dependence of heat capacity, the values of the main thermodynamic functions of diamond under standard conditions have been calculated. The results obtained have been tabulated and recommended for practical use. [ABSTRACT FROM AUTHOR]

The characteristics of the electronic and phonon subsystems of lanthanum tetraboride (LaB 4) studied using first-principles calculations. The calculated lattice parameters, as well as the positions of atomic, are satisfactorily consistent with the experimental data. The partial densities of states, band structure, Fermi surface, phonon dispersion curve of LaB 4 are calculated and analysed. The reliability of the calculation results is confirmed by a satisfactory agreement between the calculated thermodynamic parameters of LaB 4 (temperature changes in heat capacity, entropy, Grüneisen parameter and volume modulus) with experimental data. • The electronic and phonon properties of LaB 4 studied by first-principles band theory. • The experimental data of specific heat and lattice parameters at 2–300 K are analysed. • The calculated thermodynamic parameters of LaB 4 correspond to experimental data. [ABSTRACT FROM AUTHOR]

A deficiency of Ga in wide band‐gap AgGa1‐xTe2 semiconductors (1.2 eV) can be used to optimize the electrical transport properties and reduce the thermal conductivity to achieve ZT > 1 at 873 K. First‐principles density functional theory calculations and a Boson peak observed in the low temperature heat capacity data indicate the presence of strong coupling between optical phonons with low frequency and heat carrying acoustical phonons, resulting in a depressed maximum of Debye frequency in the first Brillouin zone and low phonon velocities. Moreover, the AgTe bond lengths and TeAgTe bond angles increase with rising temperature, leading to a significant distortion of the [AgTe4]7− tetrahedra, but an almost unmodified [GaTe4]5− tetrahedra. This behavior results in lattice expansion in the ab‐plane and contraction along the c‐axis, corresponding to the positive and negative Gruneisen parameters in the phonon spectral calculations. This effect gives rise to the large anharmonic behavior of the lattice. These factors together with the low frequency vibrations of Ag and Te atoms in the structure lead to an ultralow thermal conductivity of 0.18 W m−1 K−1 at 873 K. A significant distortion of the [AgTe4]7− tetrahedra but an almost unchanged [GaTe4]5− tetrahedra in the structure of AgGaTe2 upon heating leads to lattice expansion in the ab‐plane and contraction along the c‐axis, producing a large anharmonicity in the lattice and intrinsic low thermal conductivity. Maximum ZT above 1 is achieved through Ga deficiency in wide band‐gap AgGa1‐xTe2 semiconductors. [ABSTRACT FROM AUTHOR]