Ab initio study of Nb2SC and Nb2S2C: Differences in coupling between the S and Nb-C layers

Using ab initio calculations, we have studied the structurally related compounds Nb₂SC and Nb₂S₂C. In Nb₂S₂C (space group , prototype Bi₂Te₃), S atoms are nearest neighbours, while in Nb₂SC (space group P6₃/mmc, prototype Cr₂AlC) this is not the case. The calculated equilibrium volume for these two phases deviates by 1.6-3.7% to previously-published experimental data and the bulk modulus-to-c₄₄ ratios obtained are 1.5 and 5.9, respectively. These results indicate a resemblance of Nb₂S₂C to hexagonal BN and graphite. Furthermore, we have demonstrated that the uniform compression method is adequate for estimating the elastic properties of Nb₂SC, a so-called MAX phase. It is our ambition that these calculations will stimulate further experimental research on these compounds.     

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.     

Crystal Structure and High Hardness Mechanism of Orthorhomic Si2N2O

A quasi-single-phase orthorombic Si2N20 compound is obtained by hot-pressing sintering using homogeneous precursors as raw materials under nitrogen atmosphere. The bulk hardness of orthorombie Si2N20 （o-Si2N2 O） is investigated by a nanoindenter experiment; the results show that o-Si2N20 with maximal value about 19 GPa has a high hardness covalent crystal besides β-Si3N4. It is discovered that the high hardness is mainly attributed to the unique crystal structure. The bridging O atoms in the o-Si2N20 are responsible for decreasing hardness. It is found that the Si-O bonds in the open tetrahedral crystal structure are more easily broken and tilted than other bonds.     

Structural phase transition (zincblende-rocksalt) and elastic properties in AlY (Y=N, P and As) compounds: Pressure-induced effects

The present paper addresses the pressure-induced structural aspects of ZnS-type (B3) to NaCl-type (B1) structure in AlY (Y=N, P, As). An effective-interionic interaction potential (EIoIP) with long-range Coulomb and three-body interactions and the Hafemeister-and-Flygare-type short-range overlap repulsion extended up to the second-neighbour ions and the van der Waals (vdW) interaction is developed. Emphasis has been given on evaluating the vdW coefficients by the Slater-Kirkwood variational method, as both the ions are polarizable. The lattice model calculations have revealed reasonably good agreement with the available experimental data on the phase-transition pressures (P_t=16, 14, 7.5 GPa) and the elastic properties of AlY (Y=N, P, As). The equation of state curves (plotted between V(P)/V(0) and pressure) for both the B3 and B1 structures obtained are in fairly good agreement with the experimental results. The calculated values of the volume collapses [ΔV(P)/V(0)] are also close to their observed data. Further, the variations of the second-order elastic constants with pressure follow a systematic trend that is almost identical to that exhibited by the observed data measured for other semiconducting compounds with B3→B1 structural phase transitions.     

Crystal structure of the quaternary compound CuTa2InTe4 from X-ray powder diffraction

The crystal structure of the new quaternary compound CuTa₂InTe₄ was studied using X-ray powder diffraction data. The powder pattern refined by the Rietveld method indicates that this material crystallizes in the tetragonal system with space group I-4¯2m (No. 121), Z=2, and unit cell parameters a=6.1963(2) Å, c=12.4164(4) Å, c/a=2.00 and V=476.72(3) Å³. The structural and instrumental refinement of 28 parameters led to R_p=10.4%, R_wp=11.1%, R_exp=6.8% and χ²=2.7 for 96 independent reflections.     

On the symmetry and structure of the double perovskites Ba2LnRuO6 (Ln=La, Pr and Nd)

The structures of the double perovskites Ba₂LnRuO₆ (Ln=La, Pr and Nd) at room temperature were re-investigated by profile analysis of X-ray diffraction data. It has been shown that neither the triclinic nor the monoclinic P2₁/n space group correctly describes their structures. Ba₂LaRuO₆ is actually rhombohedral, space group , cell parameters a=6.03196(10) Å and α=60.298(2)°. On the other hand, both Ba₂PrRuO₆ and Ba₂NdRuO₆ are found to be cubic, space group , with the cell parameters a=8.48416(6) and 8.47061(5) Å, respectively.     

Extended X-ray absorption fine-structure (EXAFS) of a complex oxide structure: a full multiple scattering analysis of the Au L3-edge EXAFS of Au2O3

Crystalline Au₂O₃ was obtained by hydrothermal synthesis at 3000 atm and its extended X-ray absorption fine-structure (EXAFS) at the Au L₃-edge was measured at room temperature. A detailed full multiple scattering (MS) analysis using FEFF8 theory shows that only a small number of scattering paths contribute significantly to the EXAFS of Au₂O₃. Because of the complex unit cell (low local symmetry) of the Au₂O₃ structure, contributions of MS paths are almost negligible. The results indicate that FEFF8 theory provides a good reference for the analysis of Au-O phases.     

Electronic structure and site occupation for the intermediate phase of LaNi4.5Al0.5Hy

The crystal structure, electronic structure and hydrogen site occupancy of LaNi_4.5Al_0.5H_y intermediate phase (y=2.0, 3.0, 4.0) have been investigated using the full-potential linearized augmented plane wave (FLAPW) method. For the first time we analyzed the interstitial site occupation of hydrogen atoms. The H atoms were found to prefer the 6m, 3f and 12n sites, while no 4h sites were occupied. A narrowed Ni-d band is found due to the lattice expansion: the total DOS at E_F increases with y, which indicates that the compounds become less stable. The interaction between Al and Ni, with H plays a dominant role in the stability of LaNi_4.5Al_0.5H_y intermediate phase. The smaller the shift of E_F towards the higher energy region, the more stable the compounds will be. Our results are compared with experimental data and discussed in the light of previous works.     

A facile method to the cube-like MnSe2 microcrystallines via a hydrothermal process

A convenient hydrothermal process was applied to prepare the cube-like MnSe₂ microcrystallines through the reaction of MnSO₄·H₂O with Se and NaH₂PO₂·H₂O in aqueous solution at 160 °C for 12 h. Powder X-ray diffraction (XRD) analysis confirmed that the product was the cubic phase of MnSe₂ with cell parameter a=6.440 Å. The chemical composition of the MnSe₂ was determined by XPS. The Raman spectrum of MnSe₂ presented the peaks of the Se-Se stretching mode at 232.44 and 266.58 cm⁻¹. The images of scanning electron microscope (SEM) and transmission electron microscope (TEM) showed the cube-like morphology of the product with the edge length ranging from 20 to 30 μm. The formation mechanism of the MnSe₂ microcrystallines was discussed as well.     

Synthesis, crystal structure and thermoelectric properties of IrSn1.5Te1.5 -based skutterudites

The mixed anion skutterudite IrSn_1.5Te_1.5 has been synthesized and characterized by X-ray powder diffraction, thermopower and electrical resistivity measurements. Structural analysis reveals that Sn and Te order in layers perpendicular to the [111] direction of the skutterudite unit cell, and a distortion of the anion sublattice is evident. The thermopower (S) is 160 μV/K at room temperature, while the electrical resistivity (ρ) is . The effects of chemical substitutions on the Ir site (Ru, Pd) and Sn site (In, Sb) have been investigated. The power factor (S²/ρ) was found to improve with In substitution but, at 0.9 μW/K² cm, is too small for this material to be useful for thermoelectric applications.     

Structure and electrical conductivity of a novel inorganic solid electrolyte: Na14.5[Al(PO4)2F2]2.5[Ti(PO4)2F2]0.5 (NATP)

A novel inorganic solid electrolyte with a layered framework structure stable up to 1043 K, Na_14.5[Al(PO₄)₂F₂]_2.5[Ti(PO₄)₂F₂]_0.5 (NATP), has been hydrothermally prepared and characterized by single-crystal and powder X-ray diffraction techniques, X-ray fluorescence (XRF) analysis, IR spectroscopic measurement, thermogravimetric and differential thermal analysis (TGA and DTA). NATP crystallizes in the acentric hexagonal space group P3 with a=10.448(2), b=10.448(2), , Z=1, containing a large number of Na⁺ cations in the interlamellar space and the cavities of its framework. There are six different crystallographic Na⁺ cationic sites, in which 8% Na(5) and 12% Na(6) sites are vacant. Electrical conductivity measurements show that Na⁺ cations exhibit a high mobility with two domains for the electrical conductivity versus temperature.     

A study of the mechanical and structural properties of polonium

We have performed an ab initio study of the structure of polonium. By calculating total energies in a number of tetragonal lattice configurations, we have shown that the simple cubic structure is preferred by the system. The other two zero-stress structures, bcc and fcc, correspond to inflection points along this path. These calculations agree with experimental evidence that polonium is the only known element to assume the simple cubic structure at room temperature. We have found an LDA lattice constant of 3.28 Å, and we have obtained two of the elastic constants: and      

Structural, electrical and magnetic characterization of the double perovskites Sr2CrMO6 (M=Mo, W): B′ 4d-5d system

We report on the preparation and characterization of the variation of B′-site transition metal in Sr₂CrMO₆ (M=Mo, W) with double perovskites structure. The magnetic susceptibility shows that Sr₂CrMoO₆ and Sr₂CrWO₆ are antiferromagnets with T_N=40 and 30 K at H=1 T, respectively. In addition, a large magnetoresistance ratio (MR) of ∼38% (H=3 T) at 5 K was observed in the Sr₂CrWO₆ compound. However, the Sr₂CrMoO₆ compound does not show any significant MR even at high fields (MR∼4%; H=3 T and 5 K). The measured O K-edge X-ray absorption is in agreement with the calculated O p-density of states for both compounds.     

A Simple Theoretical Method to Predict the Hardness of Pure Metal Crystals

Design of superhard bulk materials requires predicting their hardness, challenging current theories for material design. By introducing a concept of condensing force （CF）, it is shown via ab initio calculations for fcc （Ni, Cu, Al, Ir, Rh, Au, Ag, Pd） and hcp Re crystals that materials with larger CF can have greater hardness. Since the calculation of CF is easy, this method might prove a convenient way to evaluate the hardness of newly designed materials.     

Prediction study of structural and elastic properties under pressure effect of M2SnC (M=Ti, Zr, Nb, Hf)

Using first-principles calculations, we have studied the structural and elastic properties of M₂SnC, with M=Ti, Zr, Nb and Hf. Geometrical optimization of the unit cell is in good agreement with the available experimental data. The effect of high pressures, up to 20 GPa, on the lattice constants shows that the contractions along the a-axis were higher than those along the c-axis. We have observed a quadratic dependence of the lattice parameters versus the applied pressure. The elastic constants and their pressure dependence are calculated using the static finite strain technique. A linear dependence of the elastic stiffnesses on the pressure is found. We derived the bulk and shear moduli, Young's moduli and Poisson's ratio for ideal polycrystalline M₂SnC aggregates. We estimated the Debye temperature of M₂SnC from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of Ti₂SnC, Zr₂SnC, Nb₂SnC, and Hf₂SnC compounds.     

Rational synthesis of α-MnO2 and γ-Mn2O3 nanowires with the electrochemical characterization of α-MnO2 nanowires for supercapacitor

An acidification-hydrothermal method was developed to synthesize α-MnO₂ nanowires, which was subsequently treated with ethanol, resulting in γ-Mn₂O₃ nanowire bundles on a large scale. The electrochemical characterization was carried out by cyclic voltammetry, which indicated that the α-MnO₂ nanowires in 0.5 mol L⁻¹ Na₂SO₄ aqueous electrolyte was of an excellent electrode material for supercapacitor at the scan rate of 10 mV S⁻¹ in the range of 0.0-0.85 V.     

Theoretical Study of Compressibility and Thermoelasticity of LaNi5-xAlx （x= 0.3, 0.5 and 1.0）

The compressibility, the temperature dependence of bulk modulus, the pressure dependence of normalized volume V/V0, thermal expansion coefficient and Debye temperature of LaNi5-xAlx compounds are successfully obtained using the first-principles plane-wave pseudopotential （PW-PP） method, the EOSFIT6.0 software and the quasiharmonic Debye model. The rapid decrease of relative lattice constant a/a0 shows that the deformation is easier in directions normal to the c-axis than that along it. The relationships between bulk modulus B and pressure at different temperatures are also analysed. It is found that the bulk modulus B increases monotonically with increasing pressure. Moreover, the pressure dependences of thermal expansion and Debye temperature are also successfully obtained. The calculated results are in agreement with the experimental data.     

First-principles electronic-structure calculation on the spin distribution and the half-metallic properties of the mixed valence iron compound Ba2F2Fe1.5S3

The first principles within the full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA) approach were applied to study the new mixed valence compound Ba₂F₂Fe_1.5S₃. The density of states, the electronic band structure and the spin magnetic moment are calculated. The calculations reveal that the compound has an antiferromagnetic interaction between the Fe^III and Fe^II ions arising from the bridging S atoms, which validate the experimental assumptions that there is a low-dimensional antiferromagnetic interaction in Ba₂F₂Fe_1.5S₃. The spin magnetic moment mainly comes from the Fe^III and Fe^II ions with smaller contribution from S anion. By analysis of the band structure, we find that the compound has half-metallic property.     

Structure and dielectric properties of (Na0.50Bi0.50)1−xBaxTiO3: 0≤x≤0.10

Solid solution of (1−x)Na_0.5Bi_0.5TiO₃-xBaTiO₃ is investigated in the composition range 0.00≤x≤0.10. It is shown that the system exhibits rhombohedral structure up to x=0.055 and then becomes ‘nearly cubic’ for x≥0.06. Temperature dependent dielectric measurements reveals three peaks in the imaginary part of the dielectric constant for compositions exhibiting rhombohedral as well as ‘nearly cubic’ structures. The first of these three peaks exhibits Vogel-Fulcher type relaxation behaviour.     

Synthesis, structure and luminescence of LaSi3N5:Ce phosphor

In this work, new LaSi₃N₅:Ce³⁺ phosphors have been synthesized by solid-state reaction. Rietveld refinement of the crystal structure of La_1−xCe_xSi₃N₅ reveals that Ce atoms substituted for La atoms occupy 4a crystallographic positions. Broad emission and excitation bands observed were attributed to the transitions between the doublet ground state of the 4f¹ configuration and the crystal field components of the 5d¹ excited state. At 77 K, the centroid and crystal field splitting ε_cfs of the 5d levels of Ce³⁺ in LaSi₃N₅:Ce³⁺ compounds were valuated at 33.4×10³ and 11.3×10³ cm⁻¹, respectively. The zero-phonon line and the Stokes shift were measured to be 26.0×10³ and 5.0×10³ cm⁻¹, respectively.