Unprecedented robust antiferromagnetism in fluorinated hexagonal perovskites

The diversification of antiferromagnetic (AFM) oxides with high Néel temperature is of fundamental as well as technical interest if one considers the need for robust AFM in the field of spin-tronics (exchange bias, multiferroics, etc.). Within the broad series of so-called hexagonal perovskites (HP), the existence of face-sharing octahedral units drastically lowers the strength of magnetic exchanges as compared to corner-sharing octahedral edifices. Here, we show that the partial introduction of F(-) in several Fe-based HP types leads to a drastic increase of the AFM ordering close to the highest values reported in iron oxides (T(N) ≈ 700 K). Our experimental results are supported by ab initio calculations. The T(N) increase is explained by the structural effect of the aliovalent F(-) for O(2-) substitution occurring in preferred anionic positions: it leads to local changes of the Fe-O-Fe connectivity and to chemical reduction into predominant Fe(3+), both responsible for drastic magnetic changes.     

Sub-ODE method and soliton solutions for the variable-coefficient mKdV equation

In this work, an auxiliary equation is used for an analytic study on the time-variable coefficient modified Korteweg-de Vries (mKdV) equation. Five sets of new exact soliton-like solutions are obtained. The results show that the pulse parameters are time-dependent variable coefficients. Moreover, the basic conditions for the formation of derived solutions are presented.     

Dark solitons for a combined potential KdV and Schwarzian KdV equations with t-dependent coefficients and forcing term

In this work we formally derive the dark soliton solutions for the combined potential KdV and Schwarzian KdV equations. The combined KdV and Schwarzian KdV equations with time-dependent coefficients and forcing term are then investigated to obtain dark soliton solutions. The solitary wave ansatz is used to carry out the analysis for both models.     

Nonstoichiometric oxides with a layer structure: The compounds A1−x(Ti1−xM1+x)O5

New nonstoichiometric oxides A_1?x(Ti_1?xNb_1+x)O₅ and tantalates ATiTaO₅ with a layer structure of the KTiNbO₅ type have been isolated, with A = K, Rb, Tl, Cs. These oxides, which are orthorhombic, space group Pnma, are characterized by a preferential occupation of one type of site 4c by the titanium atoms. The structural evolution as a function of composition and the stability of these compounds are discussed.     

Electronic states of MgO: Spectroscopy, predissociation, and cold atomic Mg and O production

We used multiconfigurational methods and a large basis set to compute the potential energy curves of the valence and valence-Rydberg electronic states of MgO molecule. New bound electronic states are found. Using these highly correlated wave functions, we evaluated their mutual spin-orbit couplings and transition moment integrals. For the bound electronic states of MgO, we deduced an accurate set of spectroscopic constants that agree remarkably well with experimental results. Moreover, our potentials, transition moments, and spin-orbit coupling evolutions are incorporated into Fermi golden rule calculations to deduce the radiative lifetimes of MgO(B?(1)Σ(+)) rovibrational levels and the natural lifetimes of MgO(A?(1)Π) vibrational levels, where a good agreement is found with experimental values. Finally, we suggest new routes for the production of cold Mg and O atoms and cold MgO molecules.     

Two‐Orbital Three‐Electron Stabilizing Interaction for Direct Co2+?As3+ Bonds involving Square‐Planar CoO4 in BaCoAs2O5

The quest for new oxides with cations containing active lone‐pair electrons (E) covers a broad field of targeted specificities owing to asymmetric electronic distribution and their particular band structure. Herein, we show that the novel compound BaCoAs₂O₅, with lone‐pair As³⁺ ions, is built from rare square‐planar Co²⁺O₄ involved in direct bonding between As³⁺E and Co²⁺ d_z2 orbitals (Co As=2.51 Å). By means of DFT and Hückel calculations, we show that this σ‐type overlapping is stabilized by a two‐orbital three‐electron interaction allowed by the high‐spin character of the Co²⁺ ions. The negligible experimental spin‐orbit coupling is expected from the resulting molecular orbital scheme in O₃AsE–CoO₄ clusters.     

Effects of temperature and Nd composition on non-linear transport properties in substituted Ce1−xNdxO2−δ cerium dioxides

Cerium dioxides doped or substituted by neodymium have been prepared using low- (320°C) and high-temperature (1600°C) processes. The Nd substituted ceria phase obtained at high temperature is a solid solution Ce_1−xNd_xO_2−δ 0?x?0.30. Electrical impedance spectroscopy analyses have been performed in the temperature range 40-700°C. At temperatures above 400°C, Nyquist representations allow to separate three signals corresponding to bulk, grain boundary and electrode responses. Non-linear variations of the resistance and the capacitance as functions of temperature and composition x are observed. In the case of grain boundary and electrode interface signals, constant phase elements with non-integer exponent n have been used to represent the equivalent circuits. For each contribution, the conductance strongly increases then reaches a limit value, above x=0.10. When composition x increases, the condensation of Nd-vacancy defect clusters might be at the origin of the non-linear evolution of the conductance. Bulk and grain boundary conductions present different activation energies (0.7 and 1.3 eV).     

Electronic structure of the [MgO3]+ cation

Accurate ab initio calculations are performed to investigate the stable isomers of [MgO(3)](+) and its lowest electronic states at both molecular and asymptotic regions. The calculations are done using large basis sets and configuration interaction methods including the complete active space self-consistent field, the internally contracted multi-reference configuration interaction, the standard coupled cluster (RCCSD(T)) approaches and the newly implemented explicitly correlated coupled cluster method (RCCSD(T)-F12). The presence of three stable forms is predicted: a cyclic global minimum c-MgO(3)(+), which is followed by a quasi-linear isomer, l2-MgO(3)(+). A third isomer of C(s) symmetry (l1-MgO(3)(+)) is also found. Moreover, we computed the one-dimensional cuts of the six-dimensional potential energy surfaces of the lowest doublet and quartet electronic states of [MgO(3)](+) along the R(MgO) and R(OO) stretching coordinates covering both the molecular and the asymptotic regions. These curves are used later for discussing the metastability of this cation and to propose plausible mechanisms for the Mg(+) + O(3) atmospherically important ion-molecule reaction and related reactive channels.     

1-Soliton solution of the D(m, n) equation with generalized evolution

This paper obtains the 1-soliton solution of the nonlinear dispersive Drinfel’d-Sokolov equation with power law nonlinearity. In the first case the soliton solution is without the generalized evolution. The solitary wave ansatz method is used to carry out the integration. Subsequently, the He’s semi-inverse variational principle is used to integrate the equation with power law nonlinearity. Parametric conditions for the existence of envelope solitons are given.     

Isotopes to assess sustainability of overexploited groundwater in the Souss–Massa system (Morocco)

Groundwater depletion and changes in isotopic and chemical contents constitute the main indicators of overexploitation, recharge, and flow paths in the Souss–Massa aquifer. These indicators highlight processes concerning sustainability of water resources in the aquifer (e.g. surface/groundwater interaction, recharge processes, and marine intrusion). The spatial variation of stable and radioactive isotopic contents indicates a mixing of modern and old water within the system. Recent recharge was observed mainly along the Souss River (the major surface-water drainage in the study area) and in the irrigated areas. Mapping of chemical and isotopic variation shows that the area is affected by abstraction, irrigation water return, and the evolution of modern recharge in time and space. The processes, distribution, and timing of groundwater flow are influenced by short- and long-term effects; long-term recharge is dependent on climatic conditions. This study can be used to make informed decisions about water-resource allocation and alternative management practices.