A nitrogen-rich compound, ReN(8)xN(2), was synthesized by a direct reaction between rhenium and nitrogen at high pressure and high temperature in a laser-heated diamond anvil cell. Single-crystal X-ray diffraction revealed that the crystal structure, which is based on the ReN8 framework, has rectangular-shaped channels that accommodate nitrogen molecules. Thus, despite a very high synthesis pressure, exceeding 100GPa, ReN(8)xN(2) is an inclusion compound. The amount of trapped nitrogen (x) depends on the synthesis conditions. The polydiazenediyl chains [-N=N-] that constitute the framework have not been previously observed in any compound. Abinitio calculations on ReN(8)xN(2) provide strong support for the experimental results and conclusions.
The electronic properties of monolayer graphene grown epitaxially on SiC(0001) are known to be highly sensitive to the presence of NO2 molecules. The presence of small areas of bilayer graphene, on the other hand, considerably reduces the overall sensitivity of the surface. We investigate how NO2 molecules interact with monolayer and bilayer graphene, both free-standing and on a SiC(0001) substrate. We show that it is necessary to explicitly include the effect of the substrate in order to reproduce the experimental results. When monolayer graphene is present on SiC, there is a large charge transfer from the interface between the buffer layer and the SiC substrate to the molecule. As a result, the surface work function increases by 0.9 eV after molecular adsorption. A graphene bilayer is more effective at screening this interfacial charge, and so the charge transfer and change in work function after NO2 adsorption is much smaller.
The phase stability of Ti_{2}AlC upon oxygen incorporation has been studied by means of first-principles calculations. Recent experimental observations of this so-called MAX phase (M = early transition metal, A = A-group element, and X = C or N) show that the characteristic nanolaminated structure is retained upon oxygen incorporation, with strong indications of O substituting for C. Therefore, a solid solution of C and O on the carbon sublattice has been simulated by the so-called special quasirandom structure method. Through a developed systematic approach, the enthalpy of formation of Ti_{2}Al(C_{1−}_{x},O_{x}) has been compared to all experimentally known competing phases, and has been found favorable for all C to O ratios at the composition of the MAX phase. A negative isostructural formation enthalpy has also been predicted for Ti_{2}Al(C_{1−}_{x},O_{x}). Altogether, the results indicate that a large amount of oxygen, at least up to x=0.75, might be present in the Ti_{2}AlC MAX-phase structure without decomposition of the material into its competing phases. Furthermore, an effect of an increased oxygen content is a corresponding increase in the bulk modulus and a change in electronic properties. These results are of importance for further understanding and identification of possible composition range of the MAX-phase oxycarbide, and hence for the prospect of tuning the material properties by a varying incorporation of oxygen.
The M(n+1)AX(n) (MAX) phases are nanolaminated compounds with a unique combination of metallic and ceramic properties, not yet including magnetism. We carry out a systematic theoretical study of potential magnetic MAX phases and predict the existence of stable magnetic (Cr(1-x)Mn(x))(2)AlC alloys. We show that in this system ferromagnetically ordered Mn layers are exchange coupled via nearly nonmagnetic Cr layers, forming an inherent structure of atomic-thin magnetic multilayers, and that the degree of disorder between Cr and Mn in the alloy can be used to tune the sign and magnitude of the coupling.
Inherently layered magnetic materials, such as magnetic M(n+1)AX(n) (MAX) phases, offer an intriguing perspective for use in spintronics applications and as ideal model systems for fundamental studies of complex magnetic phenomena. The MAX phase composition M(n+1)AX(n) consists of M(n+1)AX(n) blocks separated by atomically thin A-layers where M is a transition metal, A an A-group element, X refers to carbon and/or nitrogen, and n is typically 1, 2, or 3. Here, we show that the recently discovered magnetic Mn2GaC MAX phase displays structural changes linked to the magnetic anisotropy, and a rich magnetic phase diagram which can be manipulated through temperature and magnetic field. Using first-principles calculations and Monte Carlo simulations, an essentially one-dimensional (1D) interlayer plethora of two-dimensioanl (2D) Mn-C-Mn trilayers with robust intralayer ferromagnetic spin coupling was revealed. The complex transitions between them were observed to induce magnetically driven anisotropic structural changes. The magnetic behavior as well as structural changes dependent on the temperature and applied magnetic field are explained by the large number of low energy, i.e., close to degenerate, collinear and noncollinear spin configurations that become accessible to the system with a change in volume. These results indicate that the magnetic state can be directly controlled by an applied pressure or through the introduction of stress and show promise for the use of Mn2GaC MAX phases in future magnetoelectric and magnetocaloric applications.
The lattice parameters of InxAl1-xN in the whole compositional range are studied using first-principle calculations. Deviations from Vegards rule are obtained via the bowing parameters, delta(a)=0.0412 +/- 0.0039 A and delta(c)=-0.060 +/- 0.010 A, which largely differ from previously reported values. Implications of the observed deviations from Vegards rule on the In content extracted from x-ray diffraction are discussed. We also combine these results with x-ray diffraction and Raman scattering studies on InxAl1-xN nanocolumns with 0.627 <= x <= 1 and determine the E-2 phonon frequencies versus In composition in the scarcely studied In-rich compositional range.
Study and design of magneto-optically active single point defects in semiconductors are rapidly growing fields due to their potential in quantum bit (qubit) and single photon emitter applications. Detailed understanding of the properties of candidate defects is essential for these applications, and requires the identification of the defects microscopic configuration and electronic structure. In multicomponent semiconductors point defects often exhibit several non-equivalent configurations of similar but different characteristics. The most relevant example of such point defect is the divacancy in silicon carbide, where some of the non-equivalent configurations implement room temperature qubits. Here, we identify four different configurations of the divacancy in 4H-SiC via the comparison of experimental measurements and results of first-principle calculations. In order to accomplish this challenging task, we carry out an exhaustive numerical accuracy investigation of zero-phonon line and hyperfine coupling parameter calculations. Based on these results, we discuss the possibility of systematic quantum bit search.
Point defects in semiconductors are relevant for use in quantum technologies as room temperature qubits and single photon emitters. Among suggested defects for these applications are the negatively charged silicon vacancy and the neutral divacancy in SiC. The possible nonequivalent configurations of these defects have been identified in 4H-SiC, but for 6H-SiC, the work is still in progress. In this paper, we identify the different configurations of the silicon vacancy and the divacancy defects to each of the V1-V3 and the QL1-QL6 color centers in 6H-SiC, respectively. We accomplish this by comparing the results from ab initio calculations with experimental measurements for the zero-phonon line, hyperfine tensor, and zero-field splitting. Published under license by AIP Publishing.
Metallic osmium (Os) is one of the most exceptional elemental materials, having, at ambient pressure, the highest known density and one of the highest cohesive energies and melting temperatures(1). It is also very incompressible(2-4), but its high-pressure behaviour is not well understood because it has been studied(2-6) so far only at pressures below 75 gigapascals. Here we report powder X-ray diffraction measurements on Os at multi-megabar pressures using both conventional and double-stage diamond anvil cells(7), with accurate pressure determination ensured by first obtaining self-consistent equations of state of gold, platinum, and tungsten in static experiments up to 500 gigapascals. These measurements allow us to show that Os retains its hexagonal close-packed structure upon compression to over 770 gigapascals. But although its molar volume monotonically decreases with pressure, the unit cell parameter ratio of Os exhibits anomalies at approximately 150 gigapascals and 440 gigapascals. Dynamical mean-field theory calculations suggest that the former anomaly is a signature of the topological change of the Fermi surface for valence electrons. However, the anomaly at 440 gigapascals might be related to an electronic transition associated with pressure-induced interactions between core electrons. The ability to affect the core electrons under static high-pressure experimental conditions, even for incompressible metals such as Os, opens up opportunities to search for new states of matter under extreme compression.
The magnetic properties of fcc-FeMn alloys, especially at the Fe(0.5)Mn(0.5) composition, have been the subject of intense experimental and theoretical investigations for several decades. We carry out an ab initio theoretical study of this system, including simultaneous optimization of structural and magnetic properties, and find that the ground state is the locally relaxed noncollinear 3Q antiferromagnetic structure. We also show that the two most frequently used parameterizations of the generalized gradient approximation not only fail to reproduce the equilibrium lattice constant of FeMn alloys, and consequently the magnetic properties, but also internally yield qualitatively different results. For practical studies of these alloys, which currently attract great attention, we propose a set of approximations, which is internally consistent, and brings the equilibrium lattice constant and magnetic properties in good agreement with the experiment in the whole range of alloy compositions.
Density functional theory is a standard model for condensed-matter theory and computational material science. The accuracy of density functional theory is limited by the accuracy of the employed approximation to the exchange-correlation functional. Recently, the so-called strongly constrained appropriately normed (SCAN) [Sun, Ruzsinszky, and Perdew, Phys. Rev. Lett. 115, 036402 (2015)] functional has received a lot of attention due to promising results for covalent, metallic, ionic, as well as hydrogen- and van der Waals-bonded systems alike. In this work, we focus on assessing the performance of the SCAN functional for itinerant magnets by calculating basic structural and magnetic properties of the transition metals Fe, Co, and Ni. We find that although structural properties of bcc-Fe seem to be in good agreement with experiment, SCAN performs worse than standard local and semilocal functionals for fcc-Ni and hcp-Co. In all three cases, the magnetic moment is significantly overestimated by SCAN, and the 3d states are shifted to lower energies, as compared to experiments.
We perform a combined theoretical and experimental study of the phase stability and magnetism of the off-stoichiometric Ni1-xMn1+xSb in the half-Heusler crystal phase. Our work is motivated by the need for strategies to engineer the magnetism of potentially half-metallic materials, such as NiMnSb, for improved performance at elevated temperatures. By means of ab initio calculations we investigate Ni1-xMn1+xSb over the whole composition range 0 andlt;= x andlt;= 1 of Ni replacing Mn and show that at relevant temperatures, the half-Heusler phase should be thermodynamically stable up to at least x=0.20 with respect to the competing C38 structure of Mn2Sb. Furthermore we find that half-Heusler Ni1-xMn1+xSb retains half-metallic band structure over the whole concentration range and that the magnetic moments of substitutional Mn-Ni atoms display magnetic exchange interactions an order of magnitude larger than the Ni-Mn interaction in NiMnSb. We also demonstrate experimentally that the alloys indeed can be created by synthesizing off-stoichiometric Ni1-xMn1+xSb films on MgO substrates by means of magnetron sputtering.
By means of ab-initio calculations, we perform an analysis of the configurational thermodynamics, effects of disorder, and structural energy differences in Fe-Ni alloys at the pressure and temperature conditions of the Earths core. We show from ab-initio calculations that the ordering energies of fcc and hcp-structured Fe-Ni solid solutions at these conditions depend sensitively on the alloy configuration, i.e., on the degree of chemical disorder, and are on a scale comparable with the structural energy differences. From configurational thermodynamic simulations we find that a distribution of Fe and Ni atoms in the solutions should be very close to completely disordered at these conditions. Using this model of the Fe-Ni system, we have calculated the fcc-hcp structural free energy difference in a wide pressure-temperature range of 120-360 GPa and 1000-6600K. Our calculations show that alloying of Fe with Ni below 3000 K favours stabilisation of the fcc phase over the hcp, in agreement with experiments. However, above 3000 K the effect is reversed, and at conditions corresponding to those of the Earths inner core, Ni acts as an agent to stabilise the hcp phase.
We have performed calculations for the hyperfine field in the disordered Fe0.5Ni0.5 alloy using supercells with up to 864 atoms. The computational scheme is based on exact muffin-tin orbitals and the locally self-consistent Green’s function formalism, which scales linearly with the number of atoms in the supercell. This scheme allows local environment effects, such as chemical and magnetic environment, and short-range order, to be explicitly included. Supercell calculations for Fe-Ni show that while the average fields coincides with that obtained using the coherent potential approximation, there is a significant distribution of the hyperfine fields depending on the local environment. The fields of Fe and Ni show qualitatively different behaviour as a function of the on-site magnetic moment, but scale linearly with the average magnetic moment in the first coordination shell.
In magnetic alloys, the effect of finite temperature magnetic excitations on phase stability below the Curie temperature is poorly investigated, although many systems undergo phase transitions in this temperature range. We consider random Ni-rich Fe-Ni alloys, which undergo chemical order-disorder transition approximately 100 K below their Curie temperature, to demonstrate from ab initio calculations that deviations of the global magnetic state from ideal ferromagnetic order due to temperature induced magnetization reduction have a crucial effect on the chemical transition temperature. We propose a scheme where the magnetic state is described by partially disordered local magnetic moments, which in combination with Heisenberg Monte Carlo simulations of the magnetization allows us to reproduce the transition temperature in good agreement with experimental data.
The chelating gadolinium-complex is routinely used as magnetic resonance imaging (MRI) -contrast enhancer. However, several safety issues have recently been reported by FDA and PRAC. There is an urgent need for the next generation of safer MRI-contrast enhancers, with improved local contrast and targeting capabilities. Cerium oxide nanoparticles (CeNPs) are designed with fractions of up to 50% gadolinium to utilize the superior MRI-contrast properties of gadolinium. CeNPs are well-tolerated in vivo and have redox properties making them suitable for biomedical applications, for example scavenging purposes on the tissue-and cellular level and during tumor treatment to reduce in vivo inflammatory processes. Our near edge X-ray absorption fine structure (NEXAFS) studies show that implementation of gadolinium changes the initial co-existence of oxidation states Ce3+ and Ce4+ of cerium, thereby affecting the scavenging properties of the nanoparticles. Based on ab initio electronic structure calculations, we describe the most prominent spectral features for the respective oxidation states. The as-prepared gadolinium-implemented CeNPs are 3-5 nm in size, have r(1)-relaxivities between 7-13 mM(-1) s(-1) and show clear antioxidative properties, all of which means they are promising theranostic agents for use in future biomedical applications.
The electron spins of semiconductor defects can have complex interactions with their host, particularly in polar materials like SiC where electrical and mechanical variables are intertwined. By combining pulsed spin resonance with ab initio simulations, we show that spin-spin interactions in 4H-SiC neutral divacancies give rise to spin states with a strong Stark effect, sub-10(-6) strain sensitivity, and highly spin-dependent photoluminescence with intensity contrasts of 15%-36%. These results establish SiC color centers as compelling systems for sensing nanoscale electric and strain fields.
The TiN/SiNx nanocomposite and nanolaminate systems are the archetype for super if not ultrahard materials. Yet, the nature of the SiNx tissue phase is debated. Here, we show by atomically resolved electron microscopy methods that SiNx is epitaxially stabilized in a NaCl structure on the adjacent TiN(001) surfaces. Additionally, electron energy loss spectroscopy, supported by first-principles density functional theory calculations infer that SiNx hosts Si vacancies.
The large class of layered ceramics encompasses both van der Waals (vdW) and non-vdW solids. While intercalation of noble metals in vdW solids is known, formation of compounds by incorporation of noble-metal layers in non-vdW layered solids is largely unexplored. Here, we show formation of Ti3AuC2 and Ti3Au2C2 phases with up to 31% lattice swelling by a substitutional solid-state reaction of Au into Ti3SiC2 single-crystal thin films with simultaneous out-diffusion of Si. Ti3IrC2 is subsequently produced by a substitution reaction of Ir for Au in Ti3Au2C2. These phases form Ohmic electrical contacts to SiC and remain stable after 1,000 h of ageing at 600 degrees C in air. The present results, by combined analytical electron microscopy and ab initio calculations, open avenues for processing of noble-metal-containing layered ceramics that have not been synthesized from elemental sources, along with tunable properties such as stable electrical contacts for high-temperature power electronics or gas sensors.
We investigated the structural and electrical properties of 2D MXene sheets by means of firstprinciples density functional theory (DFT) calculations. To describe the Kohn-Sham states, plane wave basis set and projector augmented wave method (PAW) were used as implemented in the Vienna ab initio Simulation Package (VASP). We applied PBE parameterization of the generalized gradient approximation of the exchange and correlation energy functional to account for many-body effects of the interacting electron system. Convergent sampling of the Brillouin-zone was achieved by a Γ-centered 15×15×1 grid. In order to model a single sheet of MXene we ensured at least 30 Å vacuum between the periodically repeated sheets. For the structural optimization 1×10−3 eV/Å force criteria was used. The relativistic spin-orbit coupling effects were also included in our simulations regarding band structure and density of states.
The influence of pressure on the electronic structure of Os has attracted substantial attention recently due to reports on isostructural electronic transitions in this metal. Here, we theoretically investigate the Fermi surface of Os from ambient to high pressure, using density functional theory combined with dynamical mean field theory. Weprovide a detailed discussion of the calculated Fermi surface and its dependence on the level of theory used for the treatment of the electron-electron interactions. Although we confirm that Os can be classified as weakly correlated metal, the inclusion of local quantum fluctuations between 5d electrons beyond the local density approximation explains the most recent experimental reports regarding the occurrence of electronic topological transitions in Os.
Based on the Kohn-Sham Pauli potential and the Kohn-Sham electron density, the upper bound of the Pauli kinetic energy is tested as a suitable replacement for the exact Pauli kinetic energy for application in orbital-free density functional calculations. It is found that bond lengths for strong and moderately bound systems can be qualitatively predicted, but with a systematic shift toward larger bond distances with a relative error of 6% up to 30%. Angular dependence of the energy-surface cannot be modeled with the proposed functional. Therefore, the upper bound model is the first parameter-free functional expression for the kinetic energy that is able to qualitatively reproduce binding curves with respect to bond distortions. (C) 2016 Wiley Periodicals, Inc.
We use the color diffusion (CD) algorithm in nonequilibrium (accelerated) ab initio molecular dynamics simulations to determine Ti monovacancy jump frequencies in NaCl-structure titanium nitride (TiN), at temperatures ranging from 2200 to 3000 K. Our results showthat theCDmethod extended beyond the linear-fitting rate-versus-force regime [Sangiovanni et al., Phys. Rev. B 93, 094305 (2016)] can efficiently determine metal vacancy migration rates in TiN, despite the low mobilities of lattice defects in this type of ceramic compound. We propose a computational method based on gamma-distribution statistics, which provides unambiguous definition of nonequilibrium and equilibrium (extrapolated) vacancy jump rates with corresponding statistical uncertainties. The acceleration-factor achieved in our implementation of nonequilibrium molecular dynamics increases dramatically for decreasing temperatures from 500 for T close to the melting point T-m, up to 33 000 for T approximate to 0.7 T-m
We have studied the influence of additions of Al and Si on the lattice stability of face-centred-cubic (fcc) versus hexagonal-closed-packed (hcp) Fe-Mn random alloys, considering the influence of magnetism below and above the fcc Neel temperature. Employing two different ab initio approaches with respect to basis sets and treatment of magnetic and chemical disorder, we are able to quantify the predictive power of the ab initio methods. We find that the addition of Al strongly stabilizes the fcc lattice independent of the regarded magnetic states. For Si a much stronger dependence on magnetism is observed. Compared to Al, almost no volume change is observed as Si is added to Fe-Mn, indicating that the electronic contributions are responsible for stabilization/destabilization of the fcc phase.
We have studied the lattice stability of face centred cubic (fcc) versus hexagonal close packed (hcp) Fe-Mn random alloys using ab initio calculations. In the calculations we considered the antiferromagnetic order of local moments, which for fcc alloys models the magnetic configuration of this phase at room temperature (below its Neel temperature) as well as their complete disorder, corresponding to paramagnetic fcc and hcp alloys. For both cases, the results are consistent with our thermodynamic calculations, obtained within the Calphad approach. For the room temperature magnetic configuration, the cross-over of the total energies of the hcp phase and the fcc phase of Fe-Mn alloys is at the expected Mn content, whereas for the magnetic configuration above the fcc Neel temperature, the hcp lattice is more stable within the whole composition range studied. The increase of the total energy difference between hcp and antiferromagnetic fcc due to additions of Mn as well as the stabilizing effect of antiferromagnetic ordering on the fcc phase are well displayed. These results are of relevance for understanding the deformation mechanisms of these random alloys.
The influence of the Mn content on the elastic properties of face centered cubic Fe-Mn alloys was studied using the combinatorial approach. Fe-Mn thin films with a graded chemical composition were synthesized. Nanoindentation experiments were carried out to investigate the elastic properties as a function of the Mn content. As the Mn content increases from similar to 23 to similar to 39 at.%, the average bulk modulus varies from 143 to 105 GPa. Ab initio calculations served to probe the impact of magnetic effects on the elastic properties. The magnetic state description with disordered local moments yields the best agreement with the experimental results, whereas with non-magnetic and antiferromagnetic configurations the bulk modulus is overestimated. The strong impact of the magnetic configuration may be understood based on the differences in the chemical bonding and the magnetovolume effect. It is suggested that, owing to minute energy differences of competing antiferromagnetic configurations, a mixture of these with a "notional magnetic disorder" is present, which is in fact well described by the disordered local moments model. These results show that the combinatorial thin film synthesis with subsequent nanoindentation is an appropriate tool for investigating the elastic properties of Fe-Mn alloys systematically as a function of the chemical composition, to validate theoretical models.
We have studied the influence of Al and Si additions on the elastic properties of face-centered cubic (fcc) Fe-Mn random alloys with Fe/Mn ratios of 4.00 and 2.33 using ab initio calculations. When Al is added up to 8 at.% the shearing elastic constants (C-11-C-12)/2 and C-44 decrease, resulting in a drop of similar to 20% in shear and similar to 19% in Youngs modulus. In fcc Fe-Mn-Si alloys, the trends in the elastic constants are similar, but less drastic, with a similar to 7% shear and similar to 6% Youngs modulus decrease when Si is added up to 8 at.%. The Fe/Mn ratio exhibits a minor influence on the shear and Youngs modulus values at constant Al and Si contents. To assess the quality of the ab initio data Fe-Mn-Al and Fe-Mn-Si thin films with an fcc structure were combinatorially synthesized and the elastic properties measured using nanoindentation. For both systems the measured and calculated lattice parameters are in good agreement. Although the measured Youngs modulus data showed significant scatter due to the high surface roughness, they are in good agreement with the predicted values. For the Fe-Mn-Al system the calculations generally underestimate the experimental data by similar to 15%. For the Fe-Mn-Si system the calculated data are in general lower by similar to 10% than the experimentally determined values. The presented results are of relevance for multicomponent alloy design, since the effect of Si and Al addition on the elastic properties of Fe-Mn alloys can be predicted based on ab initio data.
Transition metal multilayers are prime candidates for high reflectivity soft x-ray multilayer mirrors. In particular, Cr/Sc multilayers in the amorphous state have proven to give the highest reflectivity in the water window. We have investigated the influence of impurities N and O as residual gas elements on the growth, structure, and optical performance of Cr/Sc multilayers deposited in high vacuum conditions by a dual cathode direct current magnetron sputter deposition. Multilayer structures with the modulation periods in the range of 0.9–4.5 nm and Cr layer to bilayer thickness ratios in the range of 0.17–0.83 were deposited with an intentionally raised base pressure (p_{B}), ranging from 2 × 10-7 to 2 × 10-5 Torr. Compositional depth profiles were obtained by elastic recoil detection analysis and Rutherford backscattering spectroscopy, while the structural investigations of the multilayers were carried out using hard x-ray reflectivity and transmission electron microscopy. By investigating stacked multilayers, i.e., several multilayers with different designs of the modulation periods, stacked on top of each other in the samples, we have been able to conclude that both N and O are incorporated preferentially in the interior of the Sc layers. At p_{B} = 2 × 10-6 Torr, typically <3 at.% of N and <1.5 at.% of O was found, which did not influence the amorphous nanostructure of the layers. Multilayers deposited with a high p_{B} ~2 × 10-5 Torr, a N content as high as ~37 at.% was measured by elastic recoil detection analysis. These multilayers mainly consist of understoichiometric face-centered cubic CrN x /ScN y nanocrystalline layers, which could be grown as thin at 0.3 nm and is explained by a stabilizing effect on the ScN y layers during growth. It is also shown that by adding a background pressure of as little as 5 × 10-6 Torr of pure N_{2} the soft x-ray reflectivity (? = 3.11 nm) can be enhanced by more than 100% by N incorporation into the multilayer structures, whereas pure O_{2} at the same background pressure had no effect.
Isostructural stability of B1-NaCl type SiN on (001) and (111) oriented ZrN surfaces is studied theoretically and experimentally. The ZrN/SiNx/ZrN superlattices with modulation wavelength of 3.76 nm (dSiNx similar to 0.4 nm) were grown by dc-magnetron sputtering on MgO(001) and MgO(111). The results indicate that 0.4 nm thin SiNx layers utterly influence the preferred orientation of epitaxial growth: on MgO(001) cube-on-cube epitaxy of ZrN/SiNx superlattices were realized whereas multilayers on MgO(111) surface exhibited an unexpected 002 texture with a complex fourfold 90 degrees-rotated in-plane preferred orientation. Density functional theory calculations confirm stability of a (001) interface with respect to a (111) which explains the anomaly.
We discover that hcp phases of Fe and Fe0.9Ni0.1 undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio, and Mossbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe. DOI: 10.1103/PhysRevLett.110.117206
The structural and magnetic properties of siderite FeCO3 have been studied by means of neutron powder diffraction at pressures up to 7.5 GPa and first-principles theoretical calculations. The lattice compression in the rhombohedral calcite-type structure is dominated by the reduction of the Fe-O bonds, while the changes of the C-O bonds are much less pronounced. The Neel temperature of the antiferromagnetic ( AFM) ground state increases substantially under pressure with a coefficient dT(N)/dP = 1.8K/GPa, which is about 1.5 times larger in comparison with those predicted by the empirical Bloch rule. The ab initio calculations were performed in the framework of the density functional theory including Hubbard-U correction. The calculated structural parameters and Neel temperature as functions of pressure provide a reasonable agreement with the experimental results. The analysis of the density of electronic states points toward increased covalent bonding between the Fe and O atoms upon pressure, giving rise to unexpectedly large pressure coefficient of the Neel temperature and reduced ordered magnetic moments of Fe atoms.
We provide experimental and theoretical evidence for a pressure-induced Mott insulator-metal transition in Fe2O3 characterized by site-selective delocalization of the electrons. Density functional plus dynamical mean field theory (DFT + DMFT) calculations, along with Mossbauer spectroscopy, x-ray diffraction, and electrical transport measurements on Fe2O3 up to 100 GPa, reveal this site-selective Mott transition between 50 and 68 GPa, such that the metallization can be described by ((FE3+HS)-F-VI)(2)O-3 [R (3) over barc structure]-amp;gt;(50) (GPa) (Fe-VIII(3+HS) Fe-VI(M))O-3 [P2(1)/n structure]-amp;gt;(68 Gpa)(Fe-VI(M))(2)O-3[Aba2/PPv structure]. Within the P2(1)/n crystal structure, characterized by two distinct coordination sites (VI and VIII), we observe equal abundances of ferric ions (Fe3+) and ions having delocalized electrons (Fe-M), and only at higher pressures is a fully metallic high-pressure structure obtained, all at room temperature. Thereby, the transition is characterized by delocalization/metallization of the 3d electrons on half the Fe sites, with a site-dependent collapse of local moments. Above approximately 50 GPa, Fe2O3 is a strongly correlated metal with reduced electron mobility (large band renormalizations) of m*/m similar to 4 and 6 near the Fermi level. Importantly, upon decompression, we observe a site-selective (metallic) to conventional Mott insulator phase transition (Fe-VIII(3+HS) Fe-VI(M))O-3 -amp;gt;(50) (GPa)(Fe-VIII(3+HS) Fe-VI(3+HS))O-3 within the same P2(1)/n structure, indicating a decoupling of the electronic and lattice degrees of freedom. Our results offer a model for understanding insulator-metal transitions in correlated electron materials, showing that the interplay of electronic correlations and crystal structure may result in rather complex behavior of the electronic and magnetic states of such compounds.
The thermodynamic properties of the Mo-Ru binary σ-phase are investigated using a combination of ab initio calculations and CALPHAD modeling. Total energy calculations have been performed for the complete set of 32 end-member compounds of a 5-sublattice compound energy model. The internal crystallographic parameters for each end-member compound have been determined by minimising the total energy. A simpler, 3-sublattice model of the Mo-Ru σ-phase is formulated on the basis of calculated total energies. The site occupancy is acquired by minimising the free energy given by the compound energy model. A strong preference of Mo and Ru towards high-coordination sites and icosahedral sites in the Mo-Ru σ-phase is found and analysed in terms of the electronic structure.
The optical signature of niobium in the low-temperature photoluminescence spectra of three common polytypes of SiC (4H, 6H, and 15R) is observed and confirms the previously suggested concept that Nb occupies preferably the Si-C divacancy with both Si and C at hexagonal sites. Using this concept we propose a model considering a Nb-bound exciton, the recombination of which is responsible for the observed luminescence. The exciton energy is estimated using first-principles calculation and the result is in very good agreement with the experimentally observed photon energy in 4H SiC at low temperature. The appearance of six Nb-related lines in the spectra of the hexagonal 4H and 6H polytypes at higher temperatures is tentatively explained on the grounds of the proposed model and the concept that the Nb center can exist in both C1h and C3v symmetries. The Zeeman splitting of the photoluminescence lines is also recorded in two different experimental geometries and the results are compared with theory based on phenomenological Hamiltonians. Our results show that Nb occupying the divacancy at the hexagonal site in the studied SiC polytypes behaves like a deep acceptor.
According to the so-called Janak’s theorem, the eigenstates of the Kohn-Sham Hamiltonian are given by the derivative of the total energy with respect to the occupation numbers of the corresponding one-electron states. The linear dependence of the Kohn-Sham eigenvalues on the occupation numbers is often assumed in order to use the Janak’s theorem in applications, for instance, in calculations of the core-level shifts in materials by means of the Slater-Janak transition state model. In this work first-principles density-functional theory calculations using noninteger occupation numbers for different core states in 24 different random alloy systems were carried out in order to verify the assumptions of linearity. It is found that, to a first approximation, the Kohn-Sham eigenvalues show a linear behavior as a function of the occupation numbers. However, it is also found that deviations from linearity have observable effects on the core-level shifts for some systems. A way to reduce the error with minimal increase of computational efforts is suggested.
Most applications of thermodynamic databases to materials design are limited to ambient pressure. The consideration of elastic contributions to thermodynamic stability is highly desirable but not straight-forward to realise. We present examples of existing physical models for pressure-dependent thermodynamic functions and discuss the requirements for future implementations given the existing results of experiments and first-principles calculations. We briefly summarize the calculation of elastic constants and point out examples of nonlinear variation with pressure, temperature and chemical composition that would need to be accounted for in thermodynamic databases. This is particularly the case if a system melts from different phases at different pressures. Similar relations exist between pressure and magnetism and hence set the need to also include magnetic effects in thermodynamic databases for finite pressure. We present examples to illustrate that the effect of magnetism on stability is strongly coupled to pressure, temperature, and external fields. As a further complication we discuss dynamical instabilities that may appear at finite pressure. While imaginary phonon frequencies may render a structure unstable and destroy a crystal lattice, the anharmonic effects may stabilize it again at finite temperature. Finally, we also outline a possible implementation scheme for strain effects in thermodynamic databases.
The temperature-dependent effective potential (TDEP) method is generalized beyond pair interactions. The second- and third-order force constants are determined consistently from ab initio molecular dynamics simulations at finite temperature. The reliability of the approach is demonstrated by calculations of the mode Grüneisen parameters for Si. We show that the extension of TDEP to a higher order allows for an efficient calculation of the phonon life time, in Si as well as in ε-FeSi; a system that exhibits anomalous softening with temperature.
An accurate and easily extendable method to deal with lattice dynamics of solids is offered. It is based on first-principles molecular dynamics simulations and provides a consistent way to extract the best possible harmonic-or higher order-potential energy surface at finite temperatures. It is designed to work even for strongly anharmonic systems where the traditional quasiharmonic approximation fails. The accuracy and convergence of the method are controlled in a straightforward way. Excellent agreement of the calculated phonon dispersion relations at finite temperature with experimental results for bcc Li and bcc Zr is demonstrated.
We have developed a thorough and accurate method of determining anharmonic free energies. The technique is based in ab initio molecular dynamics and map a model Hamiltonian to the fully anharmonic system. We can accurately deal with low-symmetry systems, such as random alloys. The formalism and the numerics are described in great detail. A number of practical examples are given, and favourable results are presented, both with respect to experiment and established techniques.
We have developed a thorough and accurate method of determining anharmonic free energies, the temperature dependent effective potential technique (TDEP). It is based on ab initio molecular dynamics followed by a mapping onto a model Hamiltonian that describes the lattice dynamics. The formalism and the numerical aspects of the technique are described in detail. A number of practical examples are given, and results are presented, which confirm the usefulness of TDEP within ab initio and classical molecular dynamics frameworks. In particular, we examine from first principles the behavior of force constants upon the dynamical stabilization of the body centered phase of Zr, and show that they become more localized. We also calculate the phase diagram for ^{4}He modeled with the Aziz et al. potential and obtain results which are in favorable agreement both with respect to experiment and established techniques.
The oscillating magnetic interlayer coupling of Fe over spacer layers consisting of CuxPdt-x alloys is investigated by first principles density functional theory. The amplitude, period, and phase of the coupling, as well as the disorder-induced decay, are analyzed in detail and the consistency to the Ruderman-Kittel-Kasuya-Yoshida theory is discussed. An effect of the Fermi surface nesting strength on the amplitude is established from first principles calculations. An unexpected variation of the phase and disorder-induced decay is obtained and the results are discussed in terms of asymptotics.
We propose a nondestructive technique based on atomic core-level shifts to characterize the interface quality of thin film nanomaterials. Our method uses the inherent sensitivity of the atomic core-level binding energies to their local surroundings in order to probe the layer-resolved binary alloy composition profiles at deeply embedded interfaces. From an analysis based upon high energy x-ray photoemission spectroscopy and density functional theory of a Ni/Cu fcc (100) model system, we demonstrate that this technique is a sensitive tool to characterize the sharpness of a buried interface. We performed controlled interface tuning by gradually approaching the diffusion temperature of the multilayer, which lead to intermixing. We show that core-level spectroscopy directly reflects the changes in the electronic structure of the buried interfaces, which ultimately determines the functionality of the nanosized material. © 2006 The American Physical Society.
The segregation energies of 3d (Sc-Cu), 4d (Y-Ag) and 5d (La-Au) transition metal impurities on the (1 0 0) surface of TiC have been obtained using first-principles electronic structure calculations. The results are in agreement with available experimental data and show that the difference in atomic size between the impurity and host species, as well as the difference in surface energies determines if the impurity will segregate towards the surface or not. The results indicate that the difference in size is the dominant factor for the trends in segregation of transition metal impurities towards the (1 0 0) surface of TiC. © 2005 Elsevier B.V. All rights reserved.
We have performed an ab initio study of the surface energies, surface electronic structures and work functions for the (1 0 0) surface of the, existent and hypothetical, cubic 3d (Sc-Cu), 4d (Zr-Ag) and 5d (La-Au) transition metal carbides. The calculated surface energies have been compared to predictions using a so-called bond-cutting model and a model based on the so-called bonding energies. The absolute values and rough trends of the surface energies are fairly well predicted within the simple bond-cutting model, as compared to fully self-consistent calculations, while both trends and absolute values are well reproduced within the bonding energy model. The electronic structure (densities of states) of the transition metal carbides at the surface and in the bulk have been calculated. The trends are discussed in relation to the behavior of the surface energy and the work function across the series. © 2004 Elsevier B.V. All rights reserved.
Layered magnetic materials are fascinating from the point of view of fundamental science as well as applications. Discoveries such as giant magnetoresistance (GMR) in magnetic multilayers have revolutionized data storage and magnetic recording, and concurrently initiated the search for new layered magnetic materials. One group of inherently nanolaminated compounds are the so called M_{n+1}AX_{n} (MAX) phases. Due to the large number of isostructural compositions, researchers are exploring the wide range of interesting properties, and not primarily functionalization through optimization of structural quality. Magnetic MAX phases have been discussed in the literature, though this is hitherto an unreported phenomenon. However, such materials would be highly interesting, based on the attractive and useful properties attained with layered magnetic materials to date. Here we present a new MAX phase, (Cr_{1–x}Mn_{x})_{2}GeC, synthesized as thin film in heteroepitaxial form, showing single crystal material with unprecedented structural MAX phase quality. The material was identified using first-principles calculations to study stability of hypothetical MAX phases, in an eort to identify a potentially magnetic material. The theory predicts a variety of magnetic behavior depending on the Mn concentration and Cr/Mn atomic conguration within the sublattice. The analyzed thin films display a magnetic signal well above room temperature and with partly ferromagnetic ordering. These very promising results open up a field of new layered magnetic materials, with high potential for electronics and spintronics applications.
he first experimental realization of a magnetic M_{n+1}AX_{n} (MAX) phase, (Cr_{0.75}Mn_{0.25})_{2}GeC, is presented, synthesized as a heteroepitaxial single crystal thin film, exhibiting excellent structural quality. This self-organized atomic laminate is based on the well-known Cr_{2}GeC, with Mn, a new element in MAX phase research, substituting Cr. The compound was predicted using first-principles calculations, from which a variety of magnetic behavior is envisaged, depending on the Mn concentration and Cr/Mn atomic configuration within the sublattice. The analyzed thin films display a magnetic signal at room temperature.