First-principles simulations of structural, electronic, and magnetic properties of vacancy-bearing Fe silicates

We study the structural, electronic, and magnetic properties of vacancy-bearing silicate mineral, Fe<SUB>2</SUB>SiO<SUB>4</SUB> using first-principles density-functional theory (DFT). Our DFT-simulated structure, which is compositionally close to naturally occurring laihunite compound shows good agreement in the general trend in the change in Fe<SUB>2</SUB>SiO<SUB>4</SUB> crystal structure upon vacancy introduction. Our study shows that the introduction of vacancy creates charge disproportionation of Fe ions into Fe<SUP>2+</SUP>-like and Fe<SUP>3+</SUP>-like ions with a charge difference larger than 0.5, keeping the valences of other ions unaltered. Fe<SUP>2+</SUP>-like ions are found to occupy octahedral sites of specific symmetry while Fe<SUP>3+</SUP>-like occupy the other leading to charge ordering at zero temperature. We also study the magnetic ordering of Fe ions

Ni Doping: A Viable Route to Make Body-Centered-Cubic Fe Stable at Earth’s Inner Core

With the goal of answering the highly debated question of whether the presence of Ni at the Earth’s inner core can make body-centered cubic (bcc) Fe stable, we performed a computational study based on first-principles calculations on bcc, hexagonal closed packed (hcp), and face-centered cubic (fcc) structures of the Fe1−xNix alloys (x = 0, 0.0312, 0.042, 0.0625, 0.084, 0.125, 0.14, 0.175) at 200–364 GPa and investigated their relative stability. Our thorough study reveals that the stability of Ni-doped bcc Fe is crucially dependent on the nature of the distribution of Ni in the Fe matrix. We confirm this observation by considering several possible configurations for a given concentration of Ni doping. Our theoretical evidence suggests that Ni-doped bcc Fe could be a stable phase at the Earth’s inner core condition as compared to its hcp and fcc counterparts

Site preference of Fe atoms in FeMgSiO<SUB>4</SUB> and FeMg(SiO<SUB>3</SUB>)<SUB>2</SUB> studied by density functional calculations

Using first-principles density functional calculation we investigate site preference of Fe in FeMgSiO4 olivine and FeMg(SiO3)2 pyroxene, which are the major constituents of the earth's upper mantle. A combination of state-of-the-art methods has been used for this purpose. The strong correlation effect at Fe site has been taken care of by means of local-density approximation+U calculations, and the crystal structures have been optimized by means of total-energy calculations. Our T=0 K study in the total-energy-minimized structures indicate a strong preference for Fe to occupy M2 site in case of pyroxene and a preference for Fe to occupy M1 site in case of olivine. We provide the microscopic understanding of our finding in terms of density of states and charge densities

Crossover of cation partitioning in olivines: a combination of ab initio and Monte Carlo study

We report studies based on a combination of ab initio electronic structure and Monte Carlo (MC) technique on the problem of cation partitioning among inequivalent octahedral sites, M1 and M2 in mixed olivines containing Mg<SUP>2+</SUP> and Fe<SUP>2+</SUP> ions. Our MC scheme uses interactions derived out of ab initio, density functional calculations carried out on measured crystal structure data. Our results show that there is no reversal of the preference of Fe for M1 over M2 as a function of temperature. Our findings do not agree with the experimental findings of Redfern et al. (Phys Chem Miner 27:630-637, 2000), but are in agreement with those of Heinemann et al. (Eur J Mineral 18:673-689, 2006) and Morozov et al. (Eur J Mineral 17:495-500, 2005)

Photosensitivity and charge injection dynamics of pentacene based thin-film transistors: influence of substrate temperature

In this work, we have performed an in-depth analysis to investigate the effect of substrate temperature on the molecular packing arrangement and energy levels of pentacene films. We have also explored their influence on the charge injection mechanism and photosensing behaviour of pentacene-based organic field-effect transistors (OFETs). In this study, we find the contact resistance and photosensitivity of the devices to be severely influenced by the active layer processing condition owing to the aforementioned structural and energy level modifications. Contact resistance of the devices at metal-semiconductor interfaces was observed to be reduced significantly upon increase in the substrate temperature; however, above a certain temperature, formation of pentacene thin-films was severely affected and no transistor characteristics were obtained afterwards. Detailed experimental analysis and theoretical investigations revealed that the processing temperature could strongly influence the grain structure and unit cell volume of the pentacene molecules, which consequently enhanced the carrier injection across the interface through a control over carrier mobility and the distribution of electronic states in the proximity of Fermi energy. Furthermore, our study demonstrates the role of substrate temperature in effectively enhancing the photosensitivity of the transistors. This report thus represents a step forward towards understanding a correlation between the processing temperature and the dynamics of charge injection in pentacene based organic transistors. The results also illustrate the viability of using proper substrate temperature to achieve an efficient photosensitivity from OFET devices