J. S. de Almeida

TUNING THE STRUCTURAL, ELECTRONIC, AND OPTICAL PROPERTIES OF BEXZN1?XTE ALLOYS

A series of first principles calculations have been carried out to investigate structural, electronic, and optical properties of BexZn1−xTe alloys for five beryllium compositions. Our results show that the lattice constant scales linearly with beryllium composition and there is a direct-to-indirect band gap crossover nearly at the composition of 20%. It is also found that no bowing effect in the absorption edge is observed unlike other II-VI semiconductor alloys. Our results are in good qualitative agreements with experimental observations.

Dynamical stability of the hardest known oxide and the cubic solar material: TiO2

The authors have studied dynamical stability of different polymorphs of TiO2 using ab initio phonon calculations based on density functional theory in conjunction with force-constant method. Rutile TiO2 was found stable at ambient pressure, but unstable at high pressure. The calculated Raman frequency and phonon density of states (PDOS) of rutile TiO2 are in a good agreement with experiment. Concerning two cubic phases (solar materials), fluorite stabilized under pressure, whereas pyrite showed instability throughout the whole pressure range. Furthermore, the PDOS of cotunnite (the hardest known oxide) phase confirmed that it exists at high pressure and can be quenched down to a low pressure limit.

Electronic properties of III-nitride semiconductors: A first-principles investigation using the Tran-Blaha modified Becke-Johnson potential

In this work, we use density functional theory to investigate the influence of semilocal exchange and correlation effects on the electronic properties of III-nitride semiconductors considering zinc-blende and wurtzite crystal structures. We find that the inclusion of such effects through the use of the Tran-Blaha modified Becke-Johnson potential yields an excellent description of the electronic structures of these materials giving energy band gaps which are systematically larger than the ones obtained with standard functionals such as the generalized gradient approximation. The discrepancy between the experimental and theoretical band gaps is then significantly reduced with semilocal exchange and correlation effects. However, the effective masses are overestimated in the zinc-blende nitrides, but no systematic trend is found in the wurtzite compounds. New results for energy band gaps and effective masses of zinc-blende and wurtzite indium nitrides are presented.

Mechanical stability of TiO2 polymorphs under pressure : ab initio calculations

First-principles calculations using plane-wave basis sets and ultrasoft pseudopotentials have been performed to study the mechanical stabilities of the rutile, pyrite, fluorite and cotunnite phases of titanium dioxide (TiO2). For these polymorphs, we have calculated the equilibrium volumes, equations of state, bulk moduli and selected elastic constants. Compared to the three phases rutile, pyrite and fluorite, the recently discovered cotunnite phase shows the highest c44 for all pressures considered. Cotunnite also shows the highest bulk modulus amongst the four studied phases at an ambient pressure of B0 = 272 GPa.

Novel semiconducting materials for optoelectronic applications: Al1−xTlxN alloys

We proprose the ternary semiconducting Al1-xTlxN alloys as new material for optoelectronic applications. Ab initio calculations have been performed to study structural, electronic, and optical prop ...

High-pressure synthesis and physical properties of an orthorhombic phase of chromium dioxide

High-pressure synthesis and physical properties of an orthorhombic phase of chromium dioxide

Energy-level and optical properties of nitrogen doped TiO2: An experimental and theoretical study

Photoacoustic spectroscopy was used to study nitrogen-doped titanium dioxide film. The energy positions of defect and impurity centre levels are reported. The energy levels were obtained using the excitation method and the mechanisms of the photoacoustic signal generation are discussed. The visible light absorption of the yellow film was explained considering electronic transitions between localized states within the band gap and the transitions from these states into the conduction band. Moreover, first principles calculations revealed that nitrogen doping and oxygen vacancies in titanium dioxide induce defect levels within the gap which account for the absorption in the visible light.

On the dynamical stability and metallic behavior of YH3 under pressure

We report on the behavior of structural and electronic properties of yttrium trihydride under pressure using first principles calculations. We show that YH3 undergoes a structural transformation and its high pressure phase is dynamically stable under pressure since the peak at the imaginary frequencies of the phonon density of states, which account for the structural instability disappears at high pressure. Additionally, our GW calculations indicate a metallization of the high pressure cubic phase of YH3.

Insights in the electronic structure and redox reaction energy in LiFePO4 battery material from an accurate Tran-Blaha modified Becke Johnson potential

The main goals of this paper are to investigate the accuracy of the Tran-Blaha modified Becke Johnson (TB-mBJ) potential to predict the electronic structure of lithium iron phosphate and the related redox reaction energy with the lithium deintercalation process. The computed electronic structures show that the TB-mBJ method is able to partially localize Fe-3d electrons in LiFePO4 and FePO4 which usually is a problem for the generalized gradient approximation (GGA) due to the self interaction error. The energy band gap is also improved by the TB-mBJ calculations in comparison with the GGA results. It turned out, however, that the redox reaction energy evaluated by the TB-mBJ technique is not in good agreement with the measured one. It is speculated that this disagreement in the computed redox energy and the experimental value is due to the lack of a formal expression to evaluate the exchange and correlation energy. Therefore, the TB-mBJ is an efficient method to improve the prediction of the electronic structures coming form the standard GGA functional in LiFePO4 and FePO4. However, it does not appear to have the same efficiency for evaluating the redox reaction energies for the investigated system.

Influence of the external pressure on the quantum correlations of molecular magnets

The study of quantum correlations in solid state systems is a large avenue for research and their detection and manipulation are an actual challenge to overcome. In this context, we show by using first-principles calculations on the prototype material KNaCuSi