Narrow Results

The ground state properties and the structural phase transformation of tin dioxide (SnO₂) have been investigated using first principle full potential-linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). We used local density approximation (LDA) and the generalized gradient approximation (GGA), which are based on exchange-correlation energy optimization, to optimize the internal parameters by relaxing the atomic positions in the force directions and to calculate the total energy. For band structure calculations, we utilized both the Engel-Vosko's generalized gradient approximation (EVGGA), which optimizes the exchange-correlation potential, and also GGA. From the obtained band structures, the electron (hole) valance and conduction effective masses are deduced. For compressed volumes SnO₂ is shown to undergo two structural phase transitions with increasing pressure from the rutile- to the CaCl₂-type phase at 12.4 GPa and to a cubic phase, space group at 22.1 GPa. The calculated total energy allowed us to investigate several structural properties, in particular, the equilibrium lattice constants, bulk modulus, cohesive energy, interatomic distances and the angles between different atomic bonds. In addition, we discuss the bonding parameter in term of charge density, which show the localization of charge around the anion side.

The electronic structure of the Iridium based L1₂ intermetallic compounds (A₃B) such as Ir₃Ti, Ir₃Zr, Ir₃Hf, Ir₃V, Ir₃Nb and Ir₃Ta, which have wide applications as high temperature structural materials are studied by means of Self-Consistent Tight Binding Linear Muffin Tin Orbital (TB-LMTO) method. These compounds are found to crystallize in the Cu₃Au type structure. The total energies are calculated as a function of volume and fitted to Birch equation of state to find the equilibrium lattice parameter and the bulk modulus. They are tabulated and compared with the available experimental and other theoretical data. The partial number of electrons at A and B sites of these compounds are calculated as a function of volume. We find that there is a continuous transfer of d-electrons from A-site to B-site. The band structure and density of states histograms are plotted. From the DOS histograms, we find that the plots are similar for all the compounds except for Ir₃V. In the case of Ir₃V, we find that there are hybridization between Ir-d like and V-d like states at the Fermi level. Hence, it is predicted that under compression there may be a Lifshitz type of transition in Ir₃V. The cohesive energy, heat of formation and the electronic specific heat coefficient of the compounds are also computed.

The non-monotonic relationship of T_c with the number of Cu-O planes (n) per unit cell for compounds of Tl₂Ba₂Ca_n-1Cu_nO_2n+3 (n=1, 2, 3, 4 and 5) is investigated from the reaction between different structural blocks. The unit cell of the Tl superconductors is treated as two blocks: the perovskite block where the Cu-O planes are located and the rock salt block, which is considered as a charge-reservoir. A model was used to calculate the combinative energy of the two blocks. It is found that the combinative energy between the two blocks is closely related to the value of T_c. The relation demonstrates an interesting way to understand the nonlinear change of T_c with the number of Cu-O planes in the layered superconductors. This means that the interaction between the two blocks plays an important role in superconductivity. The results are somewhat different from that of another Tl-system superconductor, Tl₂Ba₂Ca_n-1Cu_nO_2n+4, with 4 superconductive compounds.

We have investigated the elastic, cohesive and thermal properties of (Lu, Sc) VO₃ and Sc_1-xLu_xVO₃(0.6 ≤ x ≤ 0.9) perovskites by means of a modified rigid ion model (MRIM). The variation of specific heat is determined following the temperature driven structural phase transitions. Also, the effect of lattice distortions on the elastic and thermal properties of the present pure and doped vanadates has been studied by an atomistic approach. The calculated bulk modulus (B_T), reststrahlen frequency (ν₀), cohesive energy (ϕ), Debye temperature (θ_D) and Gruneisen parameter (γ) reproduce well with the corresponding experimental data. The specific heat results can further be improved by including the magnetic ordering contributions to the specific heat.

A simplified model that describes the size and shape dependence of melting thermodynamics of full free nanocrystals was established. Critical sizes of Fe, Co, Ni magnetic nanocrystals when the crystals keep their crystallinity were calculated and the corresponding minimum melting temperature was predicted. Theoretical predictions were consistent with experimental results.

A model is developed to account for the size- and coherence-dependent cohesive energy, melting temperature, melting enthalpy, vacancy formation energy and vacancy concentration of nanowires and nanofilms. It is found that the variation direction (increasing or decreasing) of the thermodynamic properties is determined by the coherent interface, and the quantity of variation depends on the crystal size. The calculated results on Pb nanowires and nanofilms are consistent with the available experimental values.

We present a Hamiltonian for the boson–boson interaction, based on elastic coupling through flux lines. This Hamiltonian may be used to study polaritons and plasmons and likewise the cohesive energy in crystals of noble gases. The presented results for crystals of noble gases are in a good agreement with experimental data.

Size-dependent melting temperature of metallic nanoparticles is studied theoretically based on cohesive energy. Three factors are introduced in the present model. The k factor, i.e. efficiency of space filling of crystal lattice is defined as the ratio between the volume of the atoms in a crystal cell and that of the crystal cell. The β factor is defined as the ratio between the cohesive energy of surface atom and interior atom of a crystal. The q_s factor represents the packing fraction on a surface crystalline plane. Considering the β, q_s and k factors, the relationship between melting temperature and nanoparticle size is discussed. The obtained model is compared with the reported experimental data and the other models.

To apply the high hardness of TiN film to soft and hard multilayer composite sheets, we constructed a new type of composite structural material with ultra-high strength. The strain of crystal and cohesive energy between the atoms in the eight structures of N atom, Ti atom, 2N2Ti island and TiN rock salt deposited on the Al(001) surface were calculated with the first-principle ultra-soft pseudopotential approach of the plane wave based on the density functional theory. The calculations of the cohesive energy showed that N atoms could be deposited in the face-centered-cubic vacancy position of the Al(001) surface and results in a cubic structure AlN surface. The TiN film could be deposited on the interface of $\beta$-AlN. The calculations of the strains showed that the strain in the TiN film deposited on the Al(001) surface was less than that in the 2N2Ti island deposited on the Al(001) surface. The diffusion behavior of interface atom N was investigated by a nudged elastic band method. Diffusion energy calculation showed that the N atom hardly diffused to the substrate Al layer.

Based on cohesive energy, the size and shape effect on Bandgap, Dielectric constant and Phonon frequency of low-dimension semiconductor nanomaterials are predicted with structural miniaturization down to the nanoscale. It is projected that nanomaterial’s optical and electrical properties no longer remain constant but become tunable. The model reports that the bandgap increases while the dielectric constant and phonon frequency drop on decreasing size to the nanoscale. The bandgap variation, dielectric constant and phonon frequency are reported for spherical, thin film, nanowire, regular tetrahedral and regular octahedral shapes of semiconductor nanosolids. The shape effect becomes prominent as the form changes from spherical to regular tetrahedral shape up to the size limit of 20 nm. A good agreement between our model predictions and the available experimental and simulation data justifies the theory’s validity.

We have systematically investigated the effect of phonons on elastic, thermal and cohesive properties for rare earth gallates RGaO₃(R = La, Ce, Nd, Pr, Sm, Gd) by means of a Rigid Ion Model after modifying its framework to incorporate the van der Waals interactions. Besides that, we have calculated the temperature dependence of the specific heat for the present orthogallates. The results on bulk modulus, Debye temperature and specific heat reproduce well with the available experimental data.

Ab initio computational studies of size-dependent Actinide-based High Entropy Alloys (HEAs) nanomaterials (AHEANMs) have emerged as a powerful tool to unravel the fundamental properties and behaviors of these complex systems. AHEAs have demonstrated remarkable mechanical, thermal, and radiation-resistant characteristics making them promising candidates for advanced technological applications. In this context, the size-dependent properties like cohesive energy, melting temperature, Debye temperature, Gibbs free energy of mixing, Heat of mixing, entropy of mixing, etc., have been studied.