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Electrides contain interstitial electrons with the states that are spatially separated from the crystal framework states and form a detached electronic subsystem. In mayenite [Ca

We studied structural and magnetic phase transitions under applied pressure for the doped cobaltite ${\mathrm{La}}_{0.75}{\mathrm{Ba}}_{0.25}{\mathrm{CoO}}_{2.9}$. Neutron and x-ray diffraction experiments established the coexistence of rhombohedral and cubic phases in the sample. The magnetic state at 2 K is best described as a long-range ordered antiferromagnet (AFM) with small ferromagnetic (FM) clusters. With application of pressure, the rhombohedral phase gradually transforms into a cubic one. At room temperature and the highest applied pressure of 16 GPa, the cubic phase accounts for 70% of the sample volume. Quantum mechanical modeling confirmed the experimental findings and provided more insights into the structural and magnetic phase transitions at pressures exceeding 16 GPa. While the cubic crystal structure was preserved above 10 GPa, the AFM to FM phase transition was found at around 16 GPa. Further increase of the pressure resulted in suppression of magnetic order above 45 GPa. Using density functional theory (DFT)$+\mathrm{U}$ calculations, we were able to relate macroscopic magnetic properties induced by pressure with corresponding spin-state transitions in Co ions.

The electronic and magnetic structure, including the Heisenberg model exchange interaction parameters, was explored for the recently proposed novel cuprate ${\mathrm{Cu}}_{2}{\mathrm{F}}_{5}$. Using the $\mathrm{DFT}+\mathrm{U}$ calculation, it is shown that the compound is formed by two types of copper ions with ${d}^{9}$ and ${d}^{8}$ electronic configurations. We have found a very stable antiferromagnetic ordering with strong anisotropy of exchange interaction that results in the appearance of an unusual 2D-magnetism: within the (100)-plane the exchange between the S = 1 and S = 1/2 Cu ions has almost the same strength as between the two S = 1 ions. The interplane magnetic interaction is five times weaker than the in-plane one.

On the basis of the first-principles evolutionary crystal structure prediction of stable compounds in the Cu-F system, we predict two experimentally unknown stable phases -- Cu2F5 and CuF3. Cu2F5 comprises two interacting magnetic subsystems with the Cu atoms in the oxidation states +2 and +3. CuF3 contains magnetic Cu 3+ ions forming a lattice with the antiferromagnetic coupling. We showed that some or all of Cu 3+ ions can be reduced to Cu 2+ by electron doping, as in the well known KCuF3. Significant similarities between the electronic structures calculated in the framework of DFT+U suggest that doped CuF3 and Cu2F5 may exhibit high-temperature superconductivity with the same mechanism as in cuprates.

Increasing external pressure gives rise to s–d electron transfer in calcium that results in the localization of the charge density in the interstices of the crystal structure, i.e., the formation of an electride. The corresponding electronic states are partially filled and localized and, hence, electronic correlations could arise. We have carried out theoretical calculations for the high-pressure phases of Ca taking into account the Coulomb interactions between the electronic states centered on the interstitial site. The results of our calculations and proposed microscopic model showed that the structural phase transition under high pressure is due to an interplay of hybridization and correlation effects. Furthermore, it was found that the Coulomb repulsion can explain the experimentally observed anomalous increase in resistivity of the simple cubic phase of calcium under pressure.

The electronic structures of native oxides formed on Fe-15Cr, Fe-5Al and Fe-15Cr-5Al (all in wt.%) alloys, and the corresponding effects on diamond nucleation and growth in plasma enhanced CH4-H2 atmosphere were investigated. The ambient environment exposure produces a mixed surface oxide layer containing the oxides of both Fe and Cr or Al on Fe-15Cr or Fe-5Al alloys, respectively. A selective oxidation of Al occurs on Fe-15Cr-5Al and it protects the other two substrate elements from oxidation. The diamond deposition behavior indicates that prior to diamond growth, a graphite intermediate layer preferentially forms on Fe-15Cr and Fe-5Al alloys, along with an internal carburization of the substrates. For Fe-15Cr-5Al, only a single diamond coating with enhanced interfacial adhesion is produced, which is attributed to the effective barrier property of the aluminum oxide passive layer.

We investigate the changes in the electronic structure and magnetic properties of BiMnO 3 + δ accompanied the phase transition C 2 ∕ c → P 2 1 ∕ c which occurs either with an increase in the oxygen content or with external pressure applied. We have established that although both factors lead to similar structural changes, their mechanisms have reverse causal relationships. The application of pressure leads to the change in the volume of MnO 6 octahedra. The latter, due to strong p - d hybridization, leads to a decrease in the localization degree of manganese electrons, which causes the reduction in Mn magnetic moments. Meanwhile, when there is excess oxygen in the system, the reverse process occurs wherein the charge density redistribution leads to a decrease in the occupation number of the Mn- e g states causing the drop in the value of the local magnetic moments and, as a consequence, to the contraction in the lengths of the Mn-O bonds.

A correlated metallic state can arise as a result of the presence either strong charge or strong spin fluctuations. In the first case, as was shown first in (2004 Phys. Today 57 53) for the Hubbard model on the Bethe lattice, the system is a correlated metallic state close to the Mott-insulator state if the ratio of the value of the Coulomb interaction parameter U and the band width W is [Formula: see text]. The later case exist if [Formula: see text] and Hund's exchange parameter [Formula: see text]. In both cases narrowing of the bands near the Fermi level and renormalization of the effective electron mass is observed, although the mechanism for realizing this state will be fundamentally different. We performed the electronic structure calculations of the paramagnetic phase [Formula: see text]-iron which is a typical Hund's metal. We showed that the statistical distribution of charge between possible electronic d-configurations has a very weak dependence on the exchange interaction and is specific for metals. At the same time, the distribution of statistical weights between different spin configurations fundamentally changes with the inclusion of J. If we neglect Hund's interaction by setting J = 0, the contributions from the low-spin configurations for all possible charge states dominate. The exchange interaction causes a redistribution of probability in favor of high-spin multiplets, leading to the formation of a larger local moment. We also performed calculations for the two-bands half-filled model. By varying the values of the Coulomb and Hund's exchange interaction parameters, we reproduced the region of the phase diagram of the model in which the system undergoes a transition from the Mott-insulator state to the Hund's metal. This transition is accompanied by a change in the statistical probability distribution of possible multiple configurations. In the region corresponding to the Hund's metal state, a change of J leads to the effect of weights redistribution similar that we observe in [Formula: see text]-iron.

The first-principles density functional theory (DFT) calculations of layered RMn2Si2 (R = Y, La) intermetallic compounds with Mn-adjacent layers separated by two Si non-magnetic layers and between them – R-layer (the ThCr2Si2-type structure), are presented. The analysis of the partial and total densities of states (DOS) of both compounds (YMn2Si2 and LaMn2Si2) demonstrates their metallic and magnetic character as well. The noncollinear magnetic ordering with canted moments is found in LaMn2Si2 with moments of 2.99 μB and 0.02 μB for the Mn and the La ions, respectively. In YMn2Si2 the Mn moments form the collinear antiferromagnetic order, and the Y ions are nonmagnetic. Calculated charge density distribution within the cell of RMn2Si2 indicates a covalent bonding between the Mn- and Si-layers of the compound and ionic type of bonding for R ions. It is found that hybridization of the Mn 3d-states with Si 3p-states induces a slight magnetization of Si ions for both compounds. In LaMn2Si2, as compared with YMn2Si2, the Mn Si and Mn-R hybridization is weakened as a result of crystal lattice expansion, orbital momentum appearance and spin-orbit interaction influence. Therefore, the Mn d3z2-r2 and dx2-y2 states can participate in the Mn Mn intra- and interplane interactions, which is likely the reason of formation of the canted magnetic structure. The results of DFT calculations are in agreement with specially performed XPS measurements of YMn2Si2 and LaMn2Si2.

Theoretical studies using the state-of-the-art density functional theory and dynamicalmean-field theory (DFT + DMFT) method show that weak electronic correlation effects are crucial for reproducing the experimentally observed pressure-induced phase transitions of calcium from β-tin to Cmmm and then to the simple cubic structure. The formation of an electride state in calcium leads to the emergence of partially filled and localized electronic states under compression. The electride state was described using a basis containing molecular orbitals centered on the interstitial site and Ca-d states. We investigate the influence of Coulomb correlations on the structural properties of elemental Ca, noting that approaches based on the Hartree-Fock method (DFT + U or hybrid functional schemes) are poorly suited for describing correlated metals. We find that only the DFT + DMFT method reproduces the correct sequence of high-pressure phase transitions of Ca at low temperatures.

We suggest a possible scenario for magnetic transition under pressure in dimerised systems where electrons are localised on molecular orbitals. The mechanism of transition is not related with competition between kinetic energy and on-site Coulomb repulsion as in Mott-Hubbard systems, or between crystal-field splitting and intra-atomic exchange as in classical atomic spin-state transitions. Instead, it is driven by the change of bonding-antibonding splitting on part of the molecular orbitals. In the magnetic systems with few half-filled molecular orbitals external pressure may result in increase of the bonding-antibonding splitting and localise all electrons on low-lying molecular orbitals suppressing net magnetic moment of the system. We give examples of the systems, where this or inverse transition may occur and by means of ab initio band structure calculations predict that it can be observed in α−MoCl4 at pressure P ~ 11 GPa.

XPS measurements of coarse-grained and nanostructured nitinol (Ni 50.2Ti49.8) before and after chemical treatment in hydrofluoric acid (40% HF, 1 min) are presented. The nanostructured state, providing the excellent mechanical properties of nitinol, is achieved by severe plastic deformation. The near-surface layers of nitinol were studied by XPS depth profiling. According to the obtained results, a chemical treatment in hydrofluoric acid reduces the thickness of the protective TiO2 oxide layer and induces a nickel release from the nitinol surface and an arsenic contamination, and can therefore not be recommended as conditioning to increase the roughness of NiTi-implants. A detailed evaluation of the resulting toxicological risks is given. © 2012 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2012. Copyright © 2012 Wiley Periodicals, Inc.

The results of XPS measurements of nanostructured Ti (ns-Ti) prepared with a help of severe plastic deformation (SPD) have been presented. We have measured XPS spectra of core levels (Ti 2p, O 1s, C 1s, F 1s and valence bands before and after treatment of ns-Ti-implants in HF. The obtained data have been compared with XPS measurements of untreated and acid treated coarsegrained Ti (cg-Ti). According to these measurements the surface composition has not practically been changed by reduction of grains size of Ti-implants. It has been found that the surface of both types of implants is covered with thick TiO 2 layer. The acid treatment reduces the surface contamination of ns-Ti and cg-Ti by hydrocarbons and induces better passivation and formation of more thick TiO2 layer. It has been shown that severe plastic deformation not only improves mechanical properties but also preserves corrosion stability of Ti-implants. Copyright © 2012 American Scientific Publishers.