Pbcn Structure Research Articles

Elastic, electronic, and magnetic properties of MNb2O6 (M = Mn, Fe, Co, Ni) in the orthorhombic Pbcn structure, a computational study

The orthorhombic niobates containing magnetic transition ions such as Mn, Fe, Co, and Ni show excellent promise for materials in energy storage, electro-photoactivity, and low-dimensional magnetic character. We report on the computational studies focusing on the elastic, electronic, and magnetic properties of four compounds with chemical formulae MNb2O6 (M = Mn, Fe, Co, Ni) in orthorhombic Pbcn crystal structure using the plane wave density functional theory (DFT) computations. The fully optimized crystal structures were used to study the elastic stiffness, compliance, bulk, and young moduli, the Poisson ratios, and anisotropy constants in both single crystal and polycrystalline compounds. Moreover, the electronic structure including the bandgaps, density of states, and magnetic states was also determined. The bandgap values of 2.98 eV, 2.64 eV, 2.70 eV, and 1.86 eV were obtained for MNb2O6 (M = Mn, Fe, Co, Ni), respectively. The Young’s modulus in all samples showed anisotropic behavior. The sound velocity and Debye’s temperature showed a decrease across the series MNb2O6 (M = Mn, Fe, Co, Ni). All four compounds have an antiferromagnetic ground state and the spin-only moments of these ions were found to be 4.55 μB for Mn3+, 3.62 μB for Fe3+, 2.64 μB for Co3+, and 1.68 μB for Ni3+. Overall, this study provides a comprehensive theoretical investigation of the basic materials properties of MNb2O6 (M = Mn, Fe, Co, Ni) which will enable the researcher working on the application of these materials in devices to carefully tailor their properties according to the application requirements.

Raman spectra of vanadates [formula omitted] ([formula omitted] Mn, Co, Ni, Zn) crystallized in the non-usual columbite-type structure

We report the first Raman spectra for the vanadates with chemical formula MV2O6 (M=Mn, Co, Ni, Zn) crystallized in a columbite-type structure (Pbcn space group). The columbite-type structure, which is quite unusual for the compounds MV2O6, was obtained by synthesis at high temperature and pressure from precursor powder samples in either P1‾ or C2/m structures. The cell parameters, interatomic distances, and distortion indexes of the octahedra in the Pbcn structure, were obtained from the Rietveld refinement of powder X-ray diffraction patterns measured at room temperature for each sample. The Raman modes were obtained from high quality spectra recorded in the range of wavenumbers between 50 and 1170 cm-1; these were assigned by comparison with the known spectra of the isostructural niobates MNb2O6. Common sequences of peaks can be identified in the Raman spectra of all the studied samples, but the position of those peaks in wavenumber ω‾ does not follow the behavior that is expected from looking only at the interatomic distances between the atoms involved in the vibration. The influence of the electronegativity of the atoms M in the force constants of the V–O bonds is proven in the evolution of the Ag Raman mode observed at the highest wavenumber.

Phase Stability of Chloroform and Dichloromethane at High Pressure

Chloroform (CHCl3) and dichloromethane (CH2Cl2) are model systems for the study of intermolecular interactions, such as hydrogen bonds and halogen–halogen interactions. Here we report a joint computational (density-functional perturbation theory (DFPT) modelling) and experimental (Raman scattering) study on the behaviour of the crystals of these compounds up to a pressure of 32 GPa. Comparing the experimental information on the Raman band positions and intensities with the results of calculations enabled us to characterize the pressure-induced evolution of the crystal structure of both compounds. We find that the previously proposed P63 phase of CHCl3 is in fact a metastable structure, and that up to 32 GPa the ambient-pressure Pnma structure is the ground state polymorph of this compound. For CH2Cl2 we confirm the stability of the ambient-pressure Pbcn structure up to 32 GPa. We show that the high-pressure evolution of the crystal geometry of CHCl3 in the Pnma structure is a result of the subtle balance between dipole–dipole interactions, hydrogen bonds and Cl···Cl contacts. For CH2Cl2 (Pbcn structure) the dipole–dipole interactions and hydrogen bonds are the main factors influencing the pressure-induced changes in the geometry.

Lack of linear magnetoelectric effect in ferrimagnetic distorted honeycomb Ni4Nb2O9

M 4 A 2O9 transition metal oxides, with M a divalent cation and A Nb or Ta, that exhibit a structure derived from corundum constitute an interesting class of materials due to their possible magnetoelectric properties. The lack of a linear magnetoelectric effect in Ni4Nb2O9, unlike Mn4Nb2O9 or Co4Nb2O9, is explained by a comprehensive investigation, combining synchrotron x-ray and neutron diffraction with magnetization, dielectric, and polarization measurements. The m′m′m magnetic point group associated with the Pb′cn′ ferrimagnetic structure induced below 76 K by the orthorhombic Pbcn structure of Ni4Nb2O9 forbids indeed such an effect. The crystal structure and magnetic ground state of Ni4Nb2O9 are discussed and compared with those of magnetoelectric P3¯c1M4A2O9 compounds whose structure derives from corundum.

Ground states of Au2Pb and pressure-enhanced superconductivity

$\mathrm{A}{\mathrm{u}}_{2}\mathrm{Pb}$ is a candidate of natural topological superconductors that has attracted much attention in the last few years. Combining ab initio calculations with machine-learning accelerated crystal structure searches, we have found two ground states of $\mathrm{A}{\mathrm{u}}_{2}\mathrm{Pb}$: $Pca{2}_{1}$ phase at ambient pressure and $I\overline{4}2d$ phase at high pressure. The $Pca{2}_{1}$ phase is energetically more favorable than the known Pbcn structure and can be a candidate for x-ray diffraction refinement; our calculations suggest that high-pressure $I\overline{4}2d$ is a conventional superconductor. By the high-pressure electric resistance measurements, we observed evidence of a ${T}_{c}$ enhancement. ${T}_{c}$ reaches a maximum value of around 4 K at 5 GPa, then decreases with further compression. The superconductivity can remain unchanged after pressure releasing, in line with our theoretical predictions. These results show that $\mathrm{A}{\mathrm{u}}_{2}\mathrm{Pb}$ exhibits abundant behaviors under varied pressure and temperature, which can help to understand how to adjust its electronic properties by pressure.

High-Pressure Synthesis, Structural, and Spectroscopic Studies of the Ni-U-O System.

The first comprehensive structural study of the Ni-U-O system is reported. Single crystals of α-NiUO4, β-NiUO4, and NiU3O10 were synthesized, and their structures were refined-using synchrotron single-crystal X-ray diffraction data supported by X-ray absorption spectroscopic measurements. α-NiUO4 adopts an orthorhombic structure in space group Pbcn and is isostructural to CrUO4 containing corrugated two-dimensional (2D) layers of corner-sharing UO6 polyhedra and edge-sharing one-dimensional (1D) zigzag α-PbO2 rutile-like chains of NiO6 polyhedra in the [001] direction. β-NiUO4 is isostructural to MgUO4 and has an orthorhombic structure in space group Ibmm, which contains alternating 1D chains of edge-sharing UO6 and NiO6 polyhedra in the [001] direction as in regular TiO2 rutile. NiU3O10 forms a triclinic structure in space group P1̅ and is isostructural with CuU3O10, where it forms a three-dimensional (3D) framework structure built through a mixture of UO6 and UO7 polyhedra in which the NiO6 polyhedra sit isolated within the framework. X-ray absorption near-edge structure (XANES) measurements, conducted using XANES mapping of single crystals, support the presence of hexavalent uranium in the three structures. The polymorphs of NiUO4 were found to only form under high-pressure and high-temperature conditions (≥4 GPa and 700 °C). It is argued that this is a consequence of the relative size difference between the Ni2+ and U6+ cations, where the Ni2+ cation is effectively too small for the Ibmm structure and too large for the Pbcn structure to form under ambient pressure conditions. This does not appear to be an issue for NiU3O10, which forms under ambient pressure conditions, where NiO6 polyhedra sit isolated within the framework of 3D connected UO6/UO7 polyhedra. Synthesis conditions indicate that β-NiUO4 is the preferred higher-pressure phase and that the transformation to this occurs irreversibly at a temperature between 950 and 1000 °C, when P = 4 GPa. The routes toward the synthesis of the oxides and the associated structural and spectroscopic results are described with respect to the structural chemistry of the Ni-U-O system, the larger AUO4 family of oxides (A = divalent or trivalent cation), and also their relation to the rutile-related family of oxides.

High-pressure Raman study of solid hydrogen up to 300 GPa**Project supported by the National Basic Research Program of China (Grant No. 2011CB808200), the Program for Changjiang Scholars and Innovative Research Team in University, China (Grant No. IRT1132), the National Natural Science Foundation of China (Grant Nos. 51032001, 11074090, 10979001, 51025206, 11274137, 11474127, and 11504127), the National Found for Fostering Talents

The high-pressure behavior of solid hydrogen has been investigated by in situ Raman spectroscopy upon compression to 300 GPa at ambient temperature. The hydrogen vibron frequency begins to decrease after it initially increases with pressure up to 38 GPa. This softening behavior suggests the weakening of the intramolecular bond and the increased intermolecular interactions. Above 237 GPa, the vibron frequency softens very rapidly with pressure at a much higher rate than that of phase III, corresponding to transformation from phase III into phase IV. The phase transition sequence has been confirmed from phase I to phase III and then to phase IV at 208 and 237 GPa, respectively. Previous theoretical calculations lead to the proposal of an energetically favorable monoclinic C2/c structure for phase III and orthorhombic Pbcn structure for phase IV. Up to 304 GPa, solid hydrogen is not yet an alkali metal since the sample is still transparent.

Structural and electronic properties of the hydrogen storage compound Ca(BH 4) 2·2NH 3 from first-principles

Ca(BH 4) 2·2NH 3 is considered to be a promising candidate for hydrogen storage. First-principles calculations based on density functional theory (DFT) were performed to study the structural and electronic properties. The optimized crystal structure was determined to be an orthorhombic Pbcn structure, with all atomic positions fully relaxed. The corresponding densities of states and charge densities indicated that strong correlations occur between Ca–B, B–H and N–H, but not between B–N, which obviously excluded the combination of calcium hydride and ammonia borane as the alternative structure. The presence of partial N–H⋯H–B dihydrogen bonding was verified. The calculated band structures implied an indirect wide band gap of 5.85 eV. The Bader charge analysis and calculated hydrogen removal energies were further investigated to explain the improved dehydrogenation properties of Ca(BH 4) 2·2NH 3 compared to pristine Ca(BH 4) 2.

Electronic, Vibrational, and Superconducting Properties of High-Pressure Metallic SiH4: ab initio Calculations

We extensively explore the experimentally proposed metallic structure of hcp P63 for the hydrogen rich compound, SiH4. It is found that the lattice dynamic of this structure is severely unstable. By freezing the soften mode, an orthorhombic Pbcn structure is discovered to be dynamically stable up to 226 GPa. Within the conventional BCS theory, the calculated critical temperature Tc within the proposed Pbcn structure is 16.5K at 188GPa, in good agreement with the experimental result (17.5K). Thus, we propose that the current predicted orthorhombic phase is a better candidate for the metallic phase of SiH4.

New potential super-incompressible phase of ReN2

The structural, elastic, and electronic properties of ReN2 are investigated by first-principles calculations with density functional theory. The obtained orthorhombic Pbcn structure is energetically the most stable structure at ambient pressure. ReN2 is a metallic, super-incompressible solid and presents a rather elastic anisotropy. The estimated Debye temperature and hardness are 735 K and 17.1 GPa, respectively. Its estimated hardness is comparative to that of Si3N4.

Elastic and thermodynamic properties of OsSi, OsSi2 and Os2Si3

All components of the elasticity tensors for OsSi (P213), OsSi2 (Cmca) and Os2Si3 (Pbcn and P-4c2) were computed by means of the stress–strain method in the framework of the density functional theory. The total energies and stress were calculated using density functional theory within the local density and conjugate gradient approximations. From the knowledge of the elastic constants, the Born stability criteria were discussed and the Debye approximation was used to evaluate the enthalpies of formation (ΔHf) of the different compounds. The ΔHf calculated at 298K for OsSi is in correct agreement with the experimental data but for Os2Si3(Pbcn) ΔHf is 30kJ/mol of atoms more exothermic than the measured one. Moreover, we have found that from 1200K the P-4c2 phase of Os2Si3 is most stable from the energy point of view than the Pbcn structure.

Structural and theoretical studies of 4,4'-[1,4-phenylene-bis-(azanediyl)]dipent-3-en-2-one: evidence of a π-delocalized keto-enamine

In this work we present the synthesis of 4,4'-(1,4-phenylene-bis(azanediyl))dipent-3-en-2-one. Evidence is proposed for the presence of various tautomers of this molecule which are complemented with theoretical density functional theory (DFT)-B3LYP/6-31G* calculations to characterize the potential energy surface of these species. The experimentally observed 3b isomer crystallizes as an orthorhombic Pbcn structure; a = 10.8412(10) A, b = 8.9205(7)A, c = 14.9949(13)A, V = 1450.1(2) A 3 , Z = 4 with a final R value is 0.038. From our X-Ray crystallographic analysis an intramolecular hydrogen N-H···O interaction is observed.