Trends in stability, elastic and electronic properties of cubic Rh, Ir, Pd and Pt carbides depending on carbon content: MC versus M4C from first-principles calculations

First principles FLAPW-GGA calculations have been performed to understand the peculiarities of stability, elastic, electronic properties and chemical bonding for cubic carbides of four noble metals M=Rh, Pd, Ir and Pt depending on carbon stoichiometry: MC versus M₄C. Our main findings are as follows: (i) in contrast to mono-carbides MC with positive formation energies E_form>0, carbon-deficient sub-carbides M₄C are stable (E_form<0), thus carbon stoichiometry is one of the major factors determining successful synthesis of these materials, and (ii) as distinct from the majority of other 3d-5d metals (including Pd and Pt examined here), an unusual effect of Rh and Ir “metallization” and the increasing of ductility for these metals owing to the introduction of carbon has been established.     

Etudes electrique et Raman des verres des systemes B2O3M2OM3PO4 (M=Li,Na)

New glasses with alkali carriers have been prepared in the system B₂O₃M₂OM₃PO₄ (M = Li, Na). The variation of the ionic conductivity has been discussed. Raman spectroscopy allows to characterize the behavior of the orthophosphate M₃PO₄ with respect to the boron-oxygen glass matrix.     

Sequence of phase transformations in the formation of superstructures of the M6C5 type in nonstoichiometric carbides

A symmetry analysis has been made of monoclinic and trigonal superstructures of the M₆C₅ type, which are formed in nonstoichiometric cubic carbides MC _y with a B1 structure. Channels of disorder-order transitions MC _y → M₆C₅ have been revealed, and the distribution functions of carbon atoms in the M₆C₅ superstructures have been calculated. The atomic-vacancy ordering in nonstoichiometric cubic carbides of vanadium VC _y and niobium NbC _y has been investigated using neutron diffraction and X-ray diffraction analyses. It has been shown that, as the temperature decreases, the Group V transition metal carbides MC _y can undergo two physically admissible sequences of transformations associated with the formation of M₆C₅ phases.     

Correlative atomic scale characterisation of secondary carbides in M50 bearing steel

Correlative atom probe tomography (APT) and transmission electron microscopy (TEM) are used to characterise the microstructure and chemistry of carbide precipitation in M50 bearing steel. This is a prerequisite in establishing relationships between the microstructure and hydrogen embrittlement (HE) resistance. Secondary carbides are the focus of this study, as they play a major role in improving HE-resistance. Secondary carbides are observed in APT, with compositions close to M₄C_3, M₂C and M₃C. Correlative TEM measured orientation relationships between the martensite matrix and carbides, enabling the confirmation of M₃C cementite precipitates in the corresponding APT reconstruction. Additionally, other precipitates observed in TEM were correlated to the M₂C carbides in APT data. The M₄C₃ carbides are found to have a significantly lower volume fraction than the M₂C carbides.     

X-Ray M5 absorption limits of Au,Tl, Pb and Bi

The combination defects of the X-ray M₄ and M₅ absorption limits of Ta to Bi have been studied, and the energies of the absorption limits compared with the level energies obtained by XPS. The M₅ absorption spectra of Au, T1, Pb and Bi in the metallic state have been investigated carefully, and, as expected, a weak subsidiary absorption discontinuity found on the low energy side of the apparent M₅ absorption discontinuity in the respective spectra. Finally, it has been shown that the absorption limit on the subsidiary absorption discontinuity gives the true M₅ absorption limit for these elements.     

A nuclear magnetic resonance study of the phase transitions and electric quadrupole Raman processes of M5H3(SO4)4·H2O (M=Na, K, Rb, and Cs) single crystals

The spin–lattice relaxation times and spin–spin relaxation times for ¹H and M in M₅H₃(SO₄)₄·H₂O (M=Na, K, Rb, and Cs) single crystals grown using the slow-evaporation method were measured as functions of temperature. Two kinds of protons were identified in the M₅H₃(SO₄)₄·H₂O structure: acid protons and water protons. Our experimental results show that the acid and water protons in Cs₅H₃(SO₄)₄·H₂O are involved in phase transitions of this crystal, whereas neither type of proton is involved in the phase transitions of the other three crystal type (M₅H₃(SO₄)₄·H₂O; M=Na, K, and Rb). Moreover, the relaxation times for the M (=Na, K, and Rb) nuclei in these crystals were found to decrease with increasing temperature and can be described with (k=2). The T₁ results for M (=Na, K, and Rb) in M₅H₃(SO₄)₄·H₂O crystals can be explained in terms of a relaxation mechanism in which the lattice vibrations are coupled to the nuclear electric quadrupole moments.     

Elastic properties, thermal expansion coefficients and electronic structures of Ti0.75X0.25C carbides

Elastic properties, thermal expansion coefficients and electronic structures of Ti_0.75X_0.25C carbides (X=W, Mo, Ta, Nb, V, Hf, Zr, Cr and Al) were systematically investigated using ab initio density functional theory (DFT) calculations. The calculated elastic moduli, electronic structures and thermal expansion coefficients α(T) of pure TiC are in good agreement with experimental data and other DFT calculations. Based on a phenomenological formula, the trends of elastic properties and ductile/brittle behavior of Ti_0.75X_0.25C were analyzed. It was found that alloying elements W, Mo, Ta, Nb, V and Hf can increase elastic moduli, while Zr, Cr and Al reduce moduli. The nearly free electron model and Debye approximation were applied in the evaluation of α(T). The anharmonic effect was taken into account by including volume-dependent elastic moduli and Debye temperature. Results show that alloying additions of 3d V, 4d Zr and Mo slightly reduce α(T), while 3d Cr increases α(T), Al, 4d Nb, 5d Hf and W almost keep α(T) unchanged in Ti_0.75X_0.25C at high temperatures. The electronic structures of Ti_0.75X_0.25C were calculated and analyzed, and the electronic density of states was used to interpret variations of elastic properties and ductile/brittle behavior induced by alloying additions.     

Phonon-phonon interactions in (Lu, Y)Ni2B2C

The temperature dependence of the Δ₄ acoustic and optic mode, measured at q=(0.5,0,0) in LuNi₂B₂C, YNi₂B₂C were extended to 1000 K and new measurements on the mixed compound, Y_0.5Lu_0.5Ni₂B₂C, were undertaken to explore the mode coupling in these materials. The temperature behavior of each is different and cannot be explained by a simple mode coupling scheme of the acoustic and optic modes.     

A revisit of photoluminescence property for vanadate oxides AVO3 (A:K, Rb and Cs) and M3V2O8 (M:Mg and Zn)

We revisited the vanadium oxide phosphors, AVO₃ (A:K, Rb, and Cs) and M₃V₂O₈ (A:Mg and Zn) for a revaluation of possibility of these compounds for lighting applications, and the internal quantum efficiency (η) and luminescent colour properties for AVO₃ (A:K, Rb, and Cs) and M₃V₂O₈ (A:Mg, and Zn) have been presented. The AVO₃ showed the broadband emission from 380 to 800 nm, and the η for the KVO₃, RbVO₃ and CsVO₃ were 4%, 79% and 87%, respectively. The CIE colour coordinates are located at white region on the chromaticity diagram. The M₃V₂O₈ (A:Mg and Zn) also exhibited a quite broadband emission between 410 and 900 nm, indicating yellow luminescent colour. The Zn₃V₂O₈ showed high η value, 52%, compared to that of the Mg₃V₂O₈ (η=6%). This enhancement of η in the Zn₃V₂O₈ could be due to the increasing exciton diffusion assisted by the hybridizations of Zn 3d and O 2p orbitals for the valence band, and Zn 4s and Ti 3d orbitals for the conduction band.     

Influence of surface environment on Auger peaks : The Palladium M45N45N45 transition

The fine structure of the M₄₅N₄₅N₄₅ Palladium Auger transition was investigated. The energy of the peaks and the changes of their shapes due to surface modifications, either by alloying or by oxidation are studied. It is found that the fine structure is due to the overlapping of two “band-like” transitions of the type M₄VV and M₅VV.     

Investigation on densification of Al2O3/B4C ceramic by ab initio calculation and experiment

Compared to experiment, the adsorption energies, bonding properties, and electronic structure of two different Al₂O₃/B₄C bridge sites with seven different Al₂O₃ surfaces are investigated by ab initio periodic density functional theory. The Al₂O₃/B₄C ceramic sintered in Ar is synthesized and measured by XRD and TEM. The calculated results reveal that the densification of O_bridge site of Al₂O₃/B₄C surface is better than that of Al_bridge. The Al₂O₃ (1 1 3)/B₄C with O_bridge is the most favorable and stable. The electronic structure shows that the electron hybridization exists between Al, O atoms and C, B atoms. The results indicate that the calculated results are in good agreement with the experiment.     

Ab initio study of some fundamental properties of the M3X (M=Cr,V; X=Si,Ge) compounds

M₃X (M=Cr, V; X=Si, Ge) compounds are studied using first-principles calculations based on the Density Functional Theory (DFT). It is found that the bulk of Cr₃X (X=Si, Ge) compounds are comparable to those of Al₂O₃, the nearest-neighbor distance D_M−M and D_M−X in these compounds increase and the bulk modulus decrease, there is a strong interaction between M and M (M=Cr the interaction is stronger). Also the interaction between M (M=Cr, V) and X (X=Ge) is negative, an anti-bonding-type interaction is dominant between these atoms.     

Calculated elastic properties of M2AlC (M=Ti, V, Cr, Nb and Ta)

M₂AlC phases, where M is a transition metal, are layered ternary compounds that possess unusual properties. In this paper, we have calculated the elastic properties of M₂AlC, with M=Ti, V, Cr, Nb and Ta, by means of ab initio total energy calculations using the projector augmented-wave method. We have derived the bulk and shear moduli, Young's moduli and Poisson's ratio for ideal polycrystalline M₂AlC aggregates. We have estimated the elastic modulus of Cr₂AlC with 357.7 GPa while the values of all other phases are in the range 309±10 GPa. We suggest that this can be understood based on the calculated bond energies for the M-C bonds. Furthermore, our results indicate a profound elastic anisotropy of M₂AlC even compared to materials with a well-established anisotropic character such as α-alumina. Finally, we have estimated the Debye temperatures of M₂AlC from the average sound velocity.     

MO2,M2O5和MO3氧化物对锂硼酸盐玻璃离子导电性的影响

本工作测量了45Li₂O·50B₂O₃·5M_mO_n(M=Al,Ti,Zr,P,V,Nb,Ta,Cr,Mo,W)玻璃的总电导率和电子电导率,以Raman光谱研究了玻璃结构,依据作为锂离子定域体的阴离子团的性质讨论了MO₂,M₂O₃和MO₃氧化物对玻璃离子导电性的影响。 关键词：     

Proprietes electriques des phases de basse temperature M1, T et M2 dans VO2 dope au chrome et a l'aluminium

The electrical conductivity and thermopower have been measured on Cr and Al doped VO₂ single crystals. The insulating M₁, T and M₂ phases have a similar behaviour with an activation energy E_a ?0.40 eV. The conductivity halves abruptly at the T→M₂ transition but no discontinuity is observed for the thermopower. The ansiotropy of the thermopower may be due to the anisotropic mobility of the holes in the lower quasi one dimensional Hubbard band.     

Structural, elastic and electronic properties and formation energies for hexagonal (W0.5Al0.5)C in comparison with binary carbides WC and Al4C3 from first-principles calculations

First principle calculations have been performed with the purpose to understand the peculiarities of the structural, elastic parameters and electronic properties and interatomic bonding for novel hexagonal carbide (W_0.5Al_0.5)C in comparison with binary phases WC and Al₄C₃. The geometries of all phases were optimized and their structural, elastic parameters and theoretical density were established. Besides, we have evaluated the formation energies (E_form) of W_0.5Al_0.5C for different possible preparation routes (namely for the reactions with the participation of simple substances (metallic W, Al and graphite, binary W or Al carbides and metallic Al and W, or binary W and Al carbides). The results show that the synthesis of the ternary carbide from simple substances is more favorable in comparison with the reactions with participation of W and Al carbides. Moreover, band structures, total and partial densities of states were obtained and analyzed systematically for (W_0.5Al_0.5)C, WC and Al₄C₃ phases in comparison with available theoretical and experimental data. The bonding picture in W_0.5Al_0.5C was described as a mixture of metallic, ionic and covalent contributions with the high anisotropy for the covalent W-C and Al-C bonds, where p-p like Al-C bonds become weaker than p-d like W-C bonds.     

Emission color variation of M2SiO4:Eu (M=Ba, Sr, Ca) phosphors for light-emitting diode

The luminescent properties of alkaline earth orthosilicates M₂SiO₄ (M=Ba, Sr, Ca) doped with Eu²⁺ ions are investigated. Two emission bands are assigned to the f-d transitions of Eu²⁺ ions doped into two different cation sites in host lattices confirmed by electron paramagnetic resonance signal. Two emission bands show the different emission color variation with substituting M²⁺ cations with smaller cations. This behavior is discussed in terms of two competing factors of the crystal field strength and covalence. Also the decay times are in order of 600-1000 ns. These phosphors with maximum excitation of around 370 nm can be applied as a color-tunable phosphor for light-emitting diode based on ultraviolet chip/phosphor technology.     

Electronic structure of CdTe probed by Cd and Te M4,5 X-ray emission spectra

We have investigated the bulk electronic structure of CdTe focusing on the Cd 5p and Te 5p valence states by X-ray emission spectroscopy (XES). Despite the very low fluorescence yields the Cd and Te M_4,5 (5p → 3d_3/2,5/2) spectra have been recorded successfully. A good correspondence has been found between the valence band XES and X-ray photoelectron spectra (XPS) by comparison on a common binding energy scale. We also performed a density functional theory calculation of the CdTe valence band, obtaining the Cd 4d, 5s, 5p and Te 5s, 5p local partial densities of states. The experimental Cd 5p and Te 5p derived from the X-ray emission spectra are in good agreement with the calculation. The intensity ratio of the Cd M_4,5 to the Te M_4,5 spectrum is obtained to be 0.25, in agreement with the ratio of the calculated Cd 5p to the Te 5p density of states in the CdTe upper valence band (0.22).     

Dy3+ emission in M5(PO4)3F (M = Ca, Ba) phosphor

Ultrafine M₅(PO₄)₃F:Dy³⁺ (M = Ca, Ba) phosphors were prepared via combustion process using metal nitrates as precursors. The formation of crystalline phosphate was confirmed by X-ray diffraction pattern. The PL excitation spectra show the excitation peaks observed at 250 to 400 nm due to f → f transition of Dy³⁺ ion, which are useful for solid-state lighting purpose (mercury free excitation). The PL emission of Dy³⁺ ion by 348 nm excitation gave an emission at 489 nm (blue), 582 nm (yellow) and 675 nm (red). All the characteristics of BYR emissions like BGR indicate that Dy doped Ca₅(PO₄)₃F and Ba₅(PO₄)₃F phosphors are good candidates that can be applied in solid-state lighting phosphor (mercury free excited lamp phosphor) and white light LED.      

Ab initio electronic structure,bonding and optical properties of two relatively new ceramics Hf2Al3C4 and Hf3Al3C5: A comparative study

The structural, electronic and optical properties of the ternary carbides Hf₂Al₃C₄ and Hf₃Al₃C₅ are studied via first principles orthogonalized linear combination of atomic orbitals (OLCAO) method. Results on crystal structure, interatomic bonding, band structure, total and partial density of states (DOS), localization index (LI), effective charge (Q*), bond order (BO), dielectric function (ε), optical conductivity (σ) and electron energy loss function are presented and discussed in detail. The band structure plots show the conducting nature of Hf₂Al₃C₄ and Hf₃Al₃C₅ carbides. DOS results disclose that the total number of states at Fermi level N(E_F) are 1.89 and 2.38 states/(eV unit cell) for Hf₂Al₃C₄ and Hf₃Al₃C₅ respectively. The Q* calculations show an average charge transfer of 0.723 and 0.711 electrons from Hf and 0.809 and 0.807 electrons from Al to C sites in Hf₂Al₃C₄ and Hf₃Al₃C₅ respectively. The BO results provide the dominating role of Al–C bonds with BO value of 6.62 (BO%?=?59%) and 6.66 (BO%?=?49%) for Hf₂Al₃C₄ and Hf₃Al₃C₅ respectively and are considered responsible for the crystals cohesion. The LI results reflect the presence of highly delocalized states in the vicinity of the Fermi level. The dielectric function plots of the real (?₁(?ω)) and imaginary (?₂(?ω)) parts show the anisotropic behavior of Hf₂Al₃C₄ and Hf₃Al₃C₅. The results on optical conductivity (σ) support the trends observed in dielectric functions. The electron energy loss functions reveal the presence of sharp peaks both in ab-plane and along c-axis around 20?eV in Hf₂Al₃C₄ and Hf₃Al₃C₅ ternary carbides.