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Register a new userFirst Principles NMR Study of Fluorapatite under Pressure
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NMR is the technique of election to probe the local properties of materials. Herein we present the results of density functional theory (DFT) textit{ab initio} calculations of the NMR parameters for fluorapatite (FAp), a calcium orthophosphate mineral belonging to the apatite family, by using the GIPAW method [Pickard and Mauri, 2001]. Understanding the local effects of pressure on apatites is particularly relevant because of their important role in many solid state and biomedical applications. Apatites are open structures, which can undergo complex anisotropic deformations, and the response of NMR can elucidate the microscopic changes induced by an applied pressure. The computed NMR parameters proved to be in good agreement with the available experimental data. The structural evaluation of the material behavior under hydrostatic pressure (from --5 to +100 kbar) indicated a shrinkage of the diameter of the apatitic channel, and a strong correlation between NMR shielding and pressure, proving the sensitivity of this technique to even small changes in the chemical environment around the nuclei. This theoretical approach allows the exploration of all the different nuclei composing the material, thus providing a very useful guidance in the interpretation of experimental results, particularly valuable for the more challenging nuclei such as $^{43}$Ca and $^{17}$O.
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This article reports the study of SnO by using the first-principles pseudopotential plane-wave method within the generalized gradient approximation (GGA). We have calculated the structural, elastic, electronic and optical of SnO under high pressure. The elastic properties such as the elastic constants Cij bulk modulus, shear modulus, Young modulus, anisotropic factor, Pugh ratio, Poisson ratio are calculated and analyzed. Mechanical stability of SnO at all pressure are confirmed by using Born stability criteria in terms of elastic constants and are associated with ductile behaviour based on G/B ratios. It is also found that SnO exhibits very high anisotropy. The energy band structure and density of states are also calculated and analyzed. The results show the semiconducting and metallic properties at 0 (zero) and high pressure, respectively. Furthermore, the optical properties such as dielectric function, refractive index, photoconductivity, absorption coefficients, loss function and reflectivity are also calculated. All the results are compared with those of the SnO where available but most of the results at high pressure are not compared due to unavailability of the results.
Electronic and magnetic properties of Ga$_{1-x}$Mn$_{x}$As, obtained from first-principles calculations employing the hybrid HSE06 functional, are presented for $x=6.25%$ and $12.5%$ under pressures ranging from 0 to 15 GPa. In agreement with photoemission experiments at ambient pressure, we find for $x=6.25%$ that non-hybridized Mn-3$d$ levels and Mn-induced states reside about 5 and 0.4 eV below the Fermi energy, respectively. For elevated pressures, the Mn-3$d$ levels, Mn-induced states, and the Fermi level shift towards higher energies, however, the position of the Mn-induced states relative to the Fermi energy remains constant due to hybridization of the Mn-3$d$ levels with the valence As-4$p$ orbitals. We also evaluate, employing Monte Carlo simulations, the Curie temperature ($T_{{rm C}}$). At zero pressure, we obtain $T_{{rm C}}=181$K, whereas the pressure-induced changes in $T_{{rm C}}$ are d$T_{{rm C}}$/d$p=+4.3$K/GPa for $x=12.5%$ and an estimated value of d$T_{{rm C}}$/d$papprox+2.2$K/GPa for $x=6.25%$ under pressures up to 6 GPa. The determined values of d$T_{{rm C}}$/d$p$ compare favorably with d$T_{{rm C}}$/d$p=+$(2-3) K/GPa at $pleq1.2$GPa found experimentally and estimated within the $p$-$d$ Zener model for Ga$_{0.93}$Mn$_{0.07}$As in the regime where hole localization effects are of minor importance [M. Gryglas-Borysiewicz $et$ $al$., Phys. Rev. B ${bf 82}$, 153204 (2010)].
Interactions of two-dimensional MXene sheets and electron beam of (scanning) transmission electron microscope are studied via first-principles calculations. We simulated the knock-on displacement threshold for Ti$_3$C$_2$ MXene sheet via ab initio molecular dynamics simulations and for five other MXenes (Ti$_2$C, Ti$_2$N, Nb$_2$C, Mo$_2$TiC$_2$, and Ti$_3$CN) approximately from defect formation energies. We evaluated sputtering cross section and sputtering rates, and based on those the evolution of the surface composition. We find that at the exit surface and for low TEM energies H and F sputter at equal rates, but at high TEM energies the F is sputtered most strongly. In the enter surface, H sputtering dominates. The results were found to be largely similar for all studied MXenes, and although the displacement thresholds varied between the different metal atoms the thresholds were always too high to lead to significant sputtering of the metal atoms. We simulated electron microscope images at the successive stages of sputtering, and found that while it is likely difficult to identify surface groups based on the spot intensities, the local contraction of lattice around O groups should be observable. We also studied MXenes encapsulated with graphene and found them to provide efficient protection from the knock-on damage for all surface group atoms except H.
We investigate the temperature-pressure phase diagram of BaTiO_3 using a first-principles effective-Hamiltonian approach. We find that the zero-point motion of the ions affects the form of the phase diagram dramatically. Specifically, when the zero-point fluctuations are included in the calculations, all the polar (tetragonal, orthorhombic, and rhombohedral) phases of BaTiO_3 survive down to 0 K, while only the rhombohedral phase does otherwise. We provide a simple explanation for this behavior. Our results confirm the essential correctness of the phase diagram proposed by Ishidate et al. (Phys. Rev. Lett. 78, 2397 (1997)).
In this article, we report emergence of topological phase in XMR material TmSb under hydrostatic pressure using first principles calculations. We find that TmSb, a topologically trivial semimetal, undergoes a topological phase transition with band inversion at X point without breaking any symmetry under a hydrostatic pressure of 12 GPa. At 15 GPa, it again becomes topologically trivial with band inversion at $Gamma$ as well as X point. We find that the pressures corresponding to the topological phase transitions are far below the pressure corresponding to structural phase transition at 25.5 GPa. The reentrant behaviour of topological quantum phase with hydrostatic pressure would help in finding a correlation between topology and XMR effect through experiments.
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