Stability of FeVO4-II under Pressure: A First-Principles Study

In this work, we report first-principles calculations to study FeVO4 in the CrVO4 -type (phase II) structure under pressure. Total-energy calculations were performed in order to analyze the structural parameters, the electronic, elastic, mechanical, and vibrational properties of FeVO4 -II up to 9.6 GPa for the first time. We found a good agreement in the structural parameters with the experimental results available in the literature. The electronic structure analysis was complemented with results obtained from the Laplacian of the charge density at the bond critical points within the Quantum Theory of Atoms in Molecules methodology. Our findings from the elastic, mechanic, and vibrational properties were correlated to determine the elastic and dynamic stability of FeVO4 -II under pressure. Calculations suggest that beyond the maximum pressure covered by our study, this phase could undergo a phase transition to a wolframite-type structure, such as in CrVO4 and InVO4. Keywords: FeVO4 under pressure; CrVO4-type structure; first-principles; mechanical properties; vibrational properties; electronic propertie

Energetics and the magnetic state of Mn2 adsorbed on Au(111): Dimer bond distance dependence

"In this work we present a theoretical study of the adsorption Mn2 dimer on the Au(111) surface. Here we use the density functional theory to construct a map of adsorption energies, EA, of Mn2 on a Au(111) surface as a function of interatomic bond distance, , among Mn atoms. We employed a 4×4 supercell of Au(111) surface which lead us to reach values in the range from 2.6 to 6.8 Å. To make a full study of the adsorption energies we considered the antiferromagnetic (AFM) and ferromagnetic (FM) states of the Mn2 on the surface. The energy landscape contains local minima when the Mn atoms are adsorbed above triangular sites and barriers that the Mn adatoms have to overcome when they move across the Au(111) surface along various paths. Our results show that the lowest energy state corresponds to the state in which the Mn atoms are next-nearest neighbors and are antiferromagnetically coupled. Furthermore, all the local minima with higher bonding energy are also those in the antiferromagnetic state. Nevertheless we find a short interval in which the FM state has lower energy than the AFM one. Finally, scanning tunneling microscope simulations for various dimer configurations on surface are reported.

Phase transition systematics in BiVO4 by means of high-pressure-high-temperature Raman experiments

"We report here high-pressure-high-temperature Raman experiments performed on BiVO4. We characterized the fergusonite and scheelite phases (powder and single crystal samples) and the zircon polymorph (nanopowder). The experimental results are supported by ab initio calculations, which, in addition, provide the vibrational patterns. The temperature and pressure behavior of the fergusonite lattice modes reflects the distortions associated with the ferroelastic instability. The linear coefficients of the zircon phase are in sharp contrast to the behavior observed in the fergusonite phase. The boundary of the fergusonite-to-scheelite second-order phase transition is given by TF-Sch (K) = -166(8)P(GPa) + 528(5). The zircon-to-scheelite, irreversible, first-order phase transition takes place at T-Z-(Sch )(K) = -107(8)P(GPa) + 690(10). We found evidence of additional structural changes around 15.7 GPa, which in the downstroke were found to be not reversible. We analyzed the anharmonic contribution to the wave-number shift in fergusonite using an order parameter. The introduction of a critical temperature depending both on temperature and pressure allows for a description of the results of all the experiments in a unified way.

Structural, electronic, vibrational, and elastic properties of SWCNTs doped with B and N: an

A systematic density functional study of structural, electronic, vibrational, and elastic properties of single walled carbon nanotubes (SWCNTs) pristine and doped with B or N, is presented. The properties of zig-zag ((n, 0), n = 4, 6, 7, 8), and armchair ((n,n), n = 4, 5) SWCNTs are reported. To study the impurity effects on the electronic structure, we used the electron localization function, charge density, electronic partial density of states, and band structures. The total energy was calculated and the geometrical structure determined. We analyzed the effect of impurities in the vibrational frequencies, by calculating the radial breathing mode (RBM). We also determined the strain energy, the Poisson ratio, and the Young modulus, and compared to the properties of pristine systems

Anharmonic contribution to the stabilization of Mg(OH)2 from first principles

"Geometrical and vibrational characterization of magnesium hydroxide was performed using density functional theory. Four possible crystal symmetries were explored: P[3 with combining macron] (No. 147, point group −3), C2/m (No. 12, point group 2), P3m1 (No. 156, point group 3m) and P[3 with combining macron]m1 (No. 164, point group −3m) which are the currently accepted geometries found in the literature. While a lot of work has been performed on Mg(OH)2, in particular for the P[3 with combining macron]m1 phase, there is still a debate on the observed ground state crystal structure and the anharmonic effects of the OH vibrations on the stabilization of the crystal structure. In particular, the stable positions of hydrogen are not yet defined precisely, which have implications in the crystal symmetry, the vibrational excitations, and the thermal stability. Previous work has assigned the P[3 with combining macron]m1 polymorph as the low energy phase, but it has also proposed that hydrogens are disordered and they could move from their symmetric position in the P[3 with combining macron]m1 structure towards P[3 with combining macron]. In this paper, we examine the stability of the proposed phases by using different descriptors. We compare the XRD patterns with reported experimental results, and a fair agreement is found. While harmonic vibrational analysis shows that most phases have imaginary modes at 0 K, anharmonic vibrational analysis indicates that at room temperature only the C2/m phase is stabilized, whereas at higher temperatures, other phases become thermally competitive.