5d Transition Elements Research Articles

A first-principles assessment of the thermoelectric properties in half-heusler compound NbIrSn

Converting waste heat into electric power using thermoelectric materials could significantly address global energy needs. Half-Heusler compounds exhibit significant promise as thermoelectric materials suitable for high temperatures, thereby offering a potential solution to address the energy crisis. By employing density functional theory (DFT), semi-classical Boltzmann transport theory (BTE), and density functional perturbation theory (DFPT), this study thoroughly examines the structural, electronic, magnetic, phonon, mechanical, and thermoelectric properties of 18 valence electron half Heusler compound NbIrSn. Considering the presence of heavy 5d transition element Ir in our compound, all calculations are carried out with and without spin–orbit coupling (SOC). This material display both dynamic and mechanical stability, and also possess the property of ductility as indicated by Pugh’s ratio and Poisson’s ratio. NbIrSn is identified as non-magnetic semiconductors with indirect band gaps of 0.65 eV and it reduces to 0.63 eV when SOC is included. The different transport parameters are analyzed in relation to the chemical potential and doping concentrations for different temperatures. The lattice thermal conductivity of the material at room temperature is measured to be 13.40 Wm−1K−1 and 14.81 Wm−1K−1without and with SOC respectively. The optimal zT values for NbIrSn at 1200 K are 0.98 with p-type doping and 0.31 with n-type doping. Incorporating SOC leads to a substantial improvement, raising the optimal zT values to 1.33 for p-type doping and 0.47 for n-type doping. In conclusion, incorporating SOC is essential when analyzing the characteristics of the proposed compound. The present study highlights NbIrSn as a potentially a favorable candidate for p-type doping on high-temperature power generation.

A general method for locating stationary points on the mixed-spin surface of spin-forbidden reaction with multiple spin states.

Some chemical reactions proceed on multiple potential energy surfaces and are often accompanied by a change in spin multiplicity, being called spin-forbidden reactions, where the spin-orbit coupling (SOC) effects play a crucial role. In order to efficiently investigate spin-forbidden reactions with two spin states, Yang et al. [Phys. Chem. Chem. Phys. 20, 4129-4136 (2018)] proposed a two-state spin-mixing (TSSM) model, where the SOC effects between the two spin states are simulated by a geometry-independent constant. Inspired by the TSSM model, we suggest a multiple-state spin-mixing (MSSM) model in this paper for the general case with any number of spin states, and its analytic first and second derivatives have been developed for locating stationary points on the mixed-spin potential energy surface and estimating thermochemical energies. To demonstrate the performance of the MSSM model, some spin-forbidden reactions involving 5d transition elements are calculated using the density functional theory (DFT), and the results are compared with the two-component relativistic ones. It is found that MSSM DFT and two-component DFT calculations may provide very similar stationary-point information on the lowest mixed-spin/spinor energy surface, including structures, vibrational frequencies, and zero-point energies. For the reactions containing saturated 5d elements, the reaction energies by MSSM DFT and two-component DFT agree very well within 3 kcal/mol. As for the two reactions OsO+ + CH4 → OOs(CH2)+ + H2 and W + CH4 → WCH2 + H2 involving unsaturated 5d elements, MSSM DFT may also yield good reaction energies of similar accuracy but with some counterexamples. Nevertheless, the energies may be remarkably improved by a posteriori single point energy calculations using two-component DFT at the MSSM DFT optimized geometries, and the maximum error of about 1 kcal/mol is almost independent of the SOC constant used. The MSSM method as well as the developed computer program provides an effective utility for studying spin-forbidden reactions.

Alloying effects on site preference, mechanical properties, and deformation behavior of L12 Co–Ti-based alloys

Alloying plays an important role in controlling the phase stability, mechanical properties, and deformation behavior of ordered intermetallic compounds. In this study, the effects of 3d, 4d, and 5d transition elements on site preference, elastic properties, ideal shear strength, and planar fault energies of L12 Co–Ti-based alloys were systematically investigated by using first-principles calculations. The calculated transfer energy and formation enthalpy indicate that Sc, V, Cr, Y, Zr, Nb, Mo, W, Hf, Ta, and W tend to occupy the Ti site, which reduce the structural stability of L12-Co3Ti. The elastic moduli and ideal shear strength of L12-Co3(Ti,M) increase with average electron density. The electron localization function (ELF) analysis reveals that the Co–M bonds have a stronger covalent character than the Co–Ti bond, which plays an important role in the strengthening of the alloys. The ratio of superlattice intrinsic stacking fault (SISF) to anti-phase boundary (APB) energy of L12-Co3(Ti,M) decreases with increasing atomic number of alloying elements in each period in the range of studied elements, which tends to change the deformation mode from the APB-favored to SISF-favored ones. The APB anisotropy ratio of L12-Co3(Ti,M) decreases with increasing atomic number of alloying elements in each period, which enhances the yield strength anomaly. This study not only sheds insight into the fundamental understanding of phase stability and mechanical behavior of multicomponent L12 compounds, but also provides useful guidelines for designing novel L12-stregnthened Co-based superalloys with superior mechanical properties.

Magnetism and magnetocaloric effect of rare earth <italic>RET</italic><sub>2</sub><italic>X</italic><sub>2</sub> compounds

Magnetic refrigeration (MR) technology is considered to be an energy-saving alternative to the commonly used gas compression/expressing refrigeration technology. Currently, searching or designing magnetic solids with an excellent magnetocaloric effect (MCE) that works at different temperature regions is one of the most important targets. Therefore, the investigation of magnetism and MCE of rare-earth-based compounds is a prominent topic in the field of refrigeration. Among them, the ternary rare-earth compounds of RET 2 X 2 ( T = 3d, 4d, or 5d transition elements; X = B, P, Si, and Ge) with a ThCr2Si2-type crystal structure have attracted considerable attention owing to their unique physical and chemical properties. In this study, the crystal structure, magnetism, and MCE of RE Mn2(Si, Ge)2, RE Ru2(Si, Ge)2, RE Co2B2 and related compounds are briefly reviewed. Some of these compounds show excellent magnetocaloric properties with potential application in the field of active cryogenic MR.

Balance between strength and ductility of dilute Fe2B by high-throughput first-principles calculations

In boride ceramics, Fe2B typically functions as the primary wear-resistant phase owing to its high hardness, but the elastic and ductile-brittle properties of Fe2B greatly influence its wear resistance and industrial application. First-principles calculations using density functional theory (DFT) were utilized to systematically investigate the effects of Al, Si, and transition elements (including 3d, 4d, and 5d transition elements) on the mechanical properties and electronic structure of Fe31MB16 with a dilute solution model. The bulk modulus (B), shear modulus (G), and Young's modulus (E) were calculated through the stress-strain method. The Vickers hardness (HV) and fracture toughness (KIC) were calculated by semi-empirical models. The electronic structures and chemical bonding were analyzed using the charge density difference, electron localization function, and Mulliken bond population. Overall, Fe31YB16 had the largest moduli (B, G, and E) among all the Fe31MB16 compounds that were investigated, while Fe31CrB16 had the highest hardness. Compared with pure Fe2B, the ductility of Fe31MB16 improved to varying degrees according to the B/G and Poisson's ratio (σ) criteria, respectively. However, adding alloying elements in this concentration did not change the intrinsic brittleness of Fe2B. Moreover, the trends in the calculated KIC of Fe31MB16 (M = Cr, Mn, Mo, and W respectively) were consistent with those of experiments. The electronic structure and Mulliken population analysis showed that the differences in the mechanical properties of the Fe31MB16 compounds were primarily determined by the M-B and B–B bonds because the alloying element M replaced Fe. These results provide guidance for improving the ductility of Fe2B and its industrial application.

On Jeff =0 Ground State Iridates(V): Tracking Residual Paramagnetism in New Bi2 NaIrO6.

Open-shell transition metal oxides are capable of developing a rich diversity of electronic phases. The specific features evolving crucially depend on an intricate interplay of various local and long-range electronic interactions. Recently, the 5d transition elements have come into sharp focus because for these elements spin-orbit coupling (SOC) and onsite Coulomb repulsion (U) are on a comparable energy scale. For Ir4+ the t2g level associated to an octahedral crystal field (CF) is split by SOC, giving rise to a Jeff = spin state and rendering respective oxides like Sr2 IrO4 as Mott insulators. Transferring this scenario to iridium(V) oxides would lead to a diamagnetic ground state, Jeff =0. However, reported experimental results do not lend unambiguous support for such an electronic state. Theoretical explanations for the breakdown of the J=0 magnetic state suffer from conspicuous discrepancies. In an attempt to empirically contribute to resolving the puzzle, Bi2 NaIrO6 was synthesized in high purity by precipitation from homogeneous solution; it represents an iridium(V) oxide where long range band structure effects and magnetic superexchange are minimized, and the t2g degeneracy is lifted geometrically. We managed to reduce the strength of paramagnetic response, lending support to a Jeff =0 ground state of Bi2 NaIrO6 , exhibiting van Vleck type behavior.

Accurate electronic free energies of the 3d,4d , and 5d transition metals at high temperatures

Free energies of bulk materials are nowadays routinely computed by density functional theory. In particular for metals, electronic excitations can significantly contribute to the free energy. For an ideal static lattice, this contribution can be obtained at low computational cost, e.g., from the electronic density of states derived at T=0 K or by utilizing the Sommerfeld approximation. The error introduced by these approximations at elevated temperatures is rarely known. The error arising from the ideal lattice approximation is likewise unexplored but computationally much more challenging to overcome. In order to shed light on these issues we have computed the electronic free energies for all 3d,4d, and 5d transition elements on the ideal lattices of the bcc, fcc, and hcp structures using finite-temperature density-functional theory. For a subset of elements we have explored the impact of explicit thermal vibrations on the electronic free energies by using ab initio molecular dynamics simulations. We provide an analysis of the observed chemical trends in terms of the electronic density of states and the canonical d band model and quantify the errors in the approximate methods. The electronic contribution to the heat capacities and the corresponding errors due to the different approximations are studied as well.

Electronic structure and local magnetism of 3d–5d impurity substituted CeFe2

We present here a systematic first-principles study of electronic structure and local magnetic properties of Ce[Fe0.75M0.25]2 compounds, where M is a 3d, 4d or 5d transition or post-transition element, using the generalized gradient approximation of the density functional theory. The d–f band hybridizations existing in CeFe2 get modified by the impurity M in an orderly manner across a period for each impurity series: the hybridization is strongest for the Mn group impurity in the period and gets diminished on either side of it. The weakening of the d–f hybridization strength is also associated with a relative localization of the Ce 4f states with respect to the delocalized 4f states in CeFe2. The above effects are most prominent for 3d impurity series, while for 4d and 5d impurities, the hybridizations and relocalizations are relatively weak due primarily to the relatively extended nature of 4d and 5d wavefunctions. The Ce local moment is found to decrease from the CeFe2 value in proportion to the strength of relocalization, thus following almost the same orderly trend as obeyed by the d–f hybridization. Further, depending on the way the spin-up and spin-down densities of states of an impurity shift relative to the Fermi energy, the impurity local moments are highest for Mn or Fe group, reduce on either side, become zero for Ni to Ga, and are small but negative for V and Ti. The Ce hyperfine field is found to follow the M local moment in a linear fashion, and vice–versa.

Diverse structures of mixed-metal oxides containing rare earths and their magnetic properties

The solid state chemistry of mixed-metal oxides containing rare earths (4f elements) and 4d or 5d transition elements has attracted a great deal of interest, because these materials adopt a diverse range of structures and show a wide range of electronic properties due to 4f and 4d (or 5d) electrons. We have focused our attention on the structural chemistry and magnetic properties of perovskite-type oxides with the general formula AnLnMn11O3n (A = Ca, Sr, Ba; Ln = rare earths; M = Ru, Ir; n = 1­4). Their structures are controlled by changing the ratio of the Ln and M ions, and peculiar magnetic properties and magnetic structures have been investigated by magnetic susceptibility, specific heat and neutron diffraction measurements.

M X-ray cross sections from 75 to 300 keV proton bombardment of selected 5d transition elements

M X-ray production cross sections for selected thick targets of Hf, W, Re, Pt, Au and Pb impacted by 75–300keV protons are presented and compared with ECPSSR and RPWBA theories using different atomic data bases, including comparisons with other investigators. The results for the major M X-ray transitions exhibit a universal character with respect to the reduced proton velocity in agreement with expectations from theory.

E/a determination for 4d- and 5d-transition metal elements and their intermetallic compounds with Mg, Al, Zn, Cd and In

The present work is devoted to the determination of the effective electrons per atom ratio e/a by means of first-principles full-potential linearized augmented plane wave-Fourier method for elements from Rb to Ag in Period 5 and from Cs to Au in Period 6 of the periodic table and is regarded as a continuation of the preceding work done for elements from K to Cu in Period 4. The value of e/a was determined by reading off the square of the Fermi diameter, from the dispersion relation for electrons outside the Muffin-Tin spheres. A straightforward reading of the ordinate at the Fermi level, i.e. local reading method was validated for Rb and Cs in Group 1, Sr in Group 2, Y in Group 3, Pd and Pt in Group 10 and Ag and Au in Group 11. Instead, the nearly free electron (NFE) method was found to be indispensable for TM elements from Zr to Rh in Period 5 and those from Ba to Ir in Period 6. The composition dependence of e/a values for intermetallic compounds in X–TM (X = Mg, Al, Zn, Cd and In) alloy systems was also studied. The new Hume–Rothery electron concentration rule was established by constructing e/uc, the number of electrons per unit cell, vs. square of critical reciprocal lattice vector, , diagram for structurally complex metallic alloys having a pseudogap at the Fermi level. A proper use of either the local reading- or the NFE-e/a for the elements as indicated above is found to be essential.

Energetics of interstitial oxygen in β-TiX (X = transition elements) alloys using first principles methods

The energetics of interstitial oxygen (O) in β-Ti and β-Ti–X alloys (X=3d-, 4d-, or 5d-transition elements) was studied using first principles methods based on density functional theory (DFT). For small concentrations of X (=1.8at.%), the insertion energy EinsX for interstitial oxygen along the three transition series follows a similar trend: it first decreases reaching a minimum between Groups 4 and 5 elements, then increases to reach a maximum for Group 8 elements (Fe, Ru, Os) before decreasing further. Except for the early transition elements, the interaction between X and O is repulsive. The lattice strain is found to be the dominant contribution to the insertion energy and can explain the trends along the three series. Using the calculated energetics and the lattice strain due to interstitial oxygen, a model is constructed to determine trends in maximum oxygen solubility CO,max in β-Ti–X alloys. The CO,maxX decreases in the middle of each transition series.

Investigation of Possible Half-Metallic Antiferromagnets on Double Perovskites A2BB′O6 (A=Ca, Sr Ba; B,B′=Transition Metal Elements)

AbstractA search was made for possible half-metallic (HM) antiferromagnet (AFM) in all the double perovskites structures of Sr2BB'O6 where BB' pairs are any combination of 3d, 4d or 5d transition elements with the exception of La. Sr can also be replaced by Ca or Ba whenever HM-AFM was found and similar calculations were then performed in order to probe further possibilities. It was found that A2MoOsO6, A2TcReO6, A2CrRuO6, where A=Ca, Sr, Ba, are all potential candidates for HM-AFM. The AFM of A2BB'O6 comes from both the superexchange mechanism and the generalized double exchange mechanism via the B(t2g)-O2pπ-B'(t2g) coupling, With the latter also being the origin of their HM. Also considered were the effects of spin-orbit coupling (SOC) and correlation (+U) by introducing +SOC and +U corrections. It is found that the SOC effect has much less influence than the correlation effect on the HM property of the compounds. For A2TcReO6 and A2CrRuO6, after +U, they become nearly Mott-Insulators. In the future, it is hoped that there will be further experimental confirmation for these possible HM-AFM candidates.

Investigation of possible half-metallic antiferromagnets on double perovskites LaABB′O6 (A=Ca,Sr,Ba; B,B′=transition elements)

We thoroughly investigated all the possible (C229=406) candidates of half-metallic (HM) antiferromagnet (AFM) with the structure LaSrBB′O6, where BB′ pairs are any combination of two 3d, 4d, and 5d transition elements. Sr was also replaced by Ca or Ba whenever HM-AFM was found and similar calculation was performed in order to probe more possibilities. We have found that LaAWXO6, where A=Ca,Sr,Ba and X=Tc,Re are potential candidates of HM-AFM. Among those HM-AFM candidates, LaAWReO6 (A=Sr,Ba) are rather robust than others for the structural optimization. Including previous Wang and Guo’s work in 2006 and this work, we believe that we have found all possible HM-AFM candidates for double perovskites structure LaABB′O6 (A=Ca,Sr,Ba and B,B′=transition elements).

Crystal chemical simulation of superconductors on the basis of oxide and intermetalliclayers

Simulation of ‘hybrid’ superconductors of 3d, 4d and 5d transition elements consisting oftwo different superconducting fragments located between positively charged ionsplanes—B′O2 oxideplanes and B2C2 intermetallic layers—has been performed on the basis of the structure ofSr2Mn3As2O2 (A2(B2C2)(B′O2)). The oxide planesare similar to those of CuO2 in high-temperature superconducting cuprates while the intermetallic layers are similar to those ofNi2B2 in low-temperature superconducting borocarbidesRNi2B2C and Fe2As2 layers in high-temperature superconducting oxypnictidesRFeAsO1−xFx.

Chemical Bonding in Metallic Rutile-type Oxides TO2 (T = Ru, Rh, Pd, Pt)

Abstract Synthesis routes to rutile-type oxides with 4d and 5d transition elements are summarized. Trends in electronic structure have been established through an analysis in the framework of density functional theory presenting the band structure, the density of states and the properties of chemical bonding. The metal-oxygen bond is found to play the major role in bonding of the system in the valence band. Throughout the series 4d → 5d (RuO2, RhO2, PdO2 and PtO2) the crystal field analysis of the band structure shows a lowering of eg towards t2g manifolds and a broadening of the overall density of states. In the vicinity of the Fermi level the role of the antibonding metal-oxygen character is investigated in the context of instability towards possible magnetic polarization, especially for RuO2.

Donor Excitons in LiH and LiD Single Crystals

The formation of bound excitons of the donor type was observed in LiH and LiD single crystals activated by 4d or 5d transition elements (Rh, Ru, Ir). The bound excitons were found to form under direct optical excitation of impurity ions.

HYBRIDIZATION EFFECTS IN SOME UTSi (2:1:3; 1:1:1; 2:3:1) COMPOUNDS

The presence of magnetic moments in uranium atoms in some of U 2 TSi 3, UTSi and U 2 T 3 Si ( T is a 3d, 4d or 5d transition element) compounds is discussed in terms of the hybridization of s, p, d and f atomic orbitals. The hybridization strength in these ternary series of isostructural compounds was calculated using the semiquantitative band structure approach of Straub and Harrison. The prediction criterion for the presence of magnetic ordering, comparing the band width with a critical energy, seems to hold. The dominant influence of fd hybridization effects in the change of magnetic properties inside an isostructural group is in evidence.

Optical properties of Bi12SiO20 single crystals doped with 4d and 5d transition elements

Bi 12 SiO 20 single crystals doped with Ru, Rh, Pd, Re, Os, and Ir transition elements were obtained. Optical absorption and optical activity were measured in the visible spectral range. Strong changes were observed in transmission spectra when doping with various elements. Doping with Os and Pd causes strong bleaching to the crystals, changes the transmission curve shoulder, and decreases the absorption coefficient. Transmission spectra of Ru, Re, and Ir doped crystals are shifted to the red region and the absorption coefficient is considerably increased. Optical rotatory power remains unchanged by doping with Rh, Pd, Re, and Ir. It is increased negligibly by Pd doping, considerably increased by Os doping, and strongly decreased by Ru doping. The influence of doping elements on optical rotatory power is discussed on the basis of the two elementary structural unit cells (BiOn polyhedra and SiO4 tetrahedra) and of the transition energy of electrons located in their vicinity.

Photoelectrochemical properties of TiO 2 rutile microalloyed with 4d and 5d transition elements

The microalloying effects of 4d and 5d transition metals on the photoelectrochemical properties of rutile titanium oxide are investigated by a series of experiments and DV-Xα molecular orbital calculation. Rutile TiO 2 and (Ti, M)O 2 oxides (M: Nb, Mo, Ta, W) can be made by the oxidation of pure Ti and Ti–5 mol% M alloys, respectively. The saturated photocurrent density of rutile (Ti, M)O 2 is dependent on the additional element, M, and it lowers in the order, Nb>Ta>Mo>W. The visible light response at 620 nm in rutile (Ti, Ta)O 2 and at 620 and 730 nm in rutile (Ti,W)O 2 is found by measuring the spectral dependence of anodic photocurrent density at 1000 mV vs. SCE. The experimental results can be explained by the calculated level structures of the microalloyed rutile oxides.