Optical properties and visible light-induced photocatalytic activity of bismuth sillenites (Bi12XO20, X = Si,Ge, Ti)

Three bismuth sillenite compounds with the general composition, Bi₁₂XO₂₀ (X = Si, Ge, Ti), were prepared by a conventional solid state synthesis route and their local crystal structure, light absorption and visible-light induced photocatalytic activities were systematically investigated. The prepared powder samples show comparable particle size distribution (~ 0.5–5 µm) and BET surface area (~ 0.8 m²/g). The optical band gap energies were estimated from diffuse reflectance spectra to be in the range 2.6–2.9 eV. The local crystal structure variation and its influence on the electronic band structure was elucidated in the three sillenite compounds by Raman analysis and density functional theory (DFT). The visible-light induced photocatalytic activity was evaluated by monitoring the degradation of rhodamine B dye solution under visible-light irradiation (> 420 nm). We found that the Bi₁₂TiO₂₀ powder shows the highest photocatalytic activity, which is attributed to the higher visible light absorption and the smaller band gap resulting from its distinct local crystal structure (longer Bi-O distance).     

压力作用下Cr2AC相(A=Si、Ge)的第一性原理研究

采用基于密度泛函理论的第一性原理方法,研究了高压下Cr2 AC相(A=Si、Ge)的力学性能和电子结构特点.结果表明:0～80 GPa范围内,Cr2 SiC和Cr2 GeC的晶格常数a、c和体积V均随压力增加而下降,a轴方向上比c轴方向上更容易被压缩;弹性性质计算结构表明在0～80 GPa范围内两个化合物均是力学稳定的,弹性模量(体积模量B、剪切模量G)随着压力的不断增加而增大,说明增加压力可以使两个化合物的硬度提高;此外,还从电子态密度的角度考察了Cr2 SiC和Cr2 GeC的电子性质,在0～80 GPa范围内,压力对整体态密度影响较小,费米能级处的电子态密度值随着压力的不断增加而降低,说明增加压力能够提高两个化合物的稳定性.     

Diffusion Across M/Pb(Zr,Ti)O3 Interfaces (M=Pt3Pb or Pt) Under Different System Conditions

Interfaces between functional ceramics, such as Pb(Zr_0.5Ti_0.5)O₃ or PZT, and metal electrodes, such as Pt, are important for many devices. Maintaining an interface that is free of secondary phases is necessary for the efficient transfer of electrons and device function. However, there are instances where unstable transient phases form at the interface due to atomic diffusion, such as Pt₃Pb. Here, we investigate the migration barriers for the diffusion of Pb across the PZT/Pt and PZT/Pt₃Pb interfaces using density functional theory (DFT) and the climbing image nudge elastic band (c‐NEB) method. Our calculation models take into account the influence of atmospheric conditions on Pb diffusion through the preferential stabilization of defects near the interface as a result of changes to the Pb and O chemical potentials. In addition, the PZT structures that are stable above and below the Curie temperature are considered. The migration barriers are predicted to be strongly dependent on atmospheric conditions and the phase of the PZT, tetragonal or cubic. In particular, an inversion of the Pb diffusion direction at the PZT/Pt interface is predicted to take place as the oxygen partial pressure increases. This prediction is confirmed by experimental in situ X‐ray diffraction measurements of a PZT/Pt interface.     

Sintering of mixtures of the MgO-MgCr2O4-MeC (Me=Si,Ti) system

A thermodynamic calculation of the probable reactions of interaction in mixtures of the MgO-MgCr₂O₄-MeC (Me=Si, Ti) system is presented. Sintering of MgO+MgCr₂O₄ mixture with additions of titanium and silicon carbides is investigated. It is established that the maximum density (96% of the theoretical value) of sintered specimens of MgO+MgCr₂O₄ mixture is attained at a 0.5% mass fraction of titanium or silicon carbide and a firing temperature of 1850°C.Translated from Ogneupory, No. 11, pp. 11–13, November, 1995.     

Keggin型多阴离子[XW12O40]n-(X=Si,Ge,P,As)电子性质的密度泛函理论研究

采用密度泛函理论方法研究了Keggin型多阴离子[XW12O40]n-(X=Si,Ge,P,As)的电子性质,在几何优化基础上,分析杂原子对体系的几何结构,电子性质,氧化还原性质的影响。结果表明:杂原子对多阴离子的键长有影响,变化规律与杂原子半径递变一致;杂原子的电子性质影响多阴离子的氧化性,多阴离子的氧化能力随杂原子的电负性增大而提高;杂原子的核电荷数越大,该多阴离子的氧化性越强;多阴离子的酸强度与杂原子的电负性次序一致。     

M2(Si,Al)4(N,C)7 (M=La,Y,Ca) carbonitrides: I. Synthesis and structural characterisation by XRD and NMR

A series of new nitrides and carbonitrides has been identified with crystal structures similar to those of the hexagonal quaternary nitrides of the type (Ba,Sr,Eu)YbSi₄N₇. The large divalent cations in these structures can be replaced by trivalent cations such as Ln and/or Y, if valency balance is preserved by the simultaneous substitution of carbon for nitrogen in the unique [4]-coordinated anion site. This has been demonstrated by carbon-13 magic-angle spinning NMR spectra which for the yttrium member of this series shows a peak at 36.7 ppm corresponding to carbon atoms occupying the central non-metal atom site in the characteristic [C(NSi₃)₄] structural unit. The resulting compounds have compositions of the types La₂Si₄N₆C, Y₂Si₄N₆C or (La,Y)₂Si₄N₆C; the crystal structure of a related mixed (Ca,Y) derivative of composition (Ca,Y)₂Si₄(N,C)₇ is reported in Part II of this series. When the two large cations are different, the hexagonal symmetry characteristic of the (Ba,Sr,Eu)YbSi₄N₇ compounds is maintained; when both cations are the same, lower symmetries are observed. The powder diffraction pattern of La₂Si₄N₆C indexes on an orthorhombic unit cell with a=6.0360(7), b=10.1246(9), c=10.5664(11) Å and the crystal structure has been determined. An alternative way of achieving valency balance without incorporation of carbon is to replace some of the silicon by aluminium; related derivatives of the type M₂Si₃AlN₇, where M=La, Y or mixed La,Y have been prepared and their unit cell dimensions are reported.     

A study of nanoporous carbon obtained from ZC powders (Z=Si, Ti, and B)

Nanoporous carbon (NPC) material prepared from polycrystalline powders of SiC, TiC, or B₄C carbides, using different techniques, was studied by means of ESR, Hall and conductance measurements for three groups of each type of sample—control group, and those annealed at T=960 and 1180 °C. It is found, according to Hall measurements, that holes are the majority charge carriers in NPC. The concentration of free carriers is found to be rather high (about 10¹⁹–10²¹ cm⁻³). The ESR spectrum has the so-called ‘Dyson line shape’ with a large signal asymmetry. Two groups of carriers with strongly different electronic and magnetic properties are found. The values of the g-factor and their variability are related to the existence of two different kinds of particles: small carbon nanoclusters (1–3 nm) and large-scale carbon skeleton structural elements (up to 1000 nm) of complex shape, where the holes move along graphite-like sp² bonds. The temperature dependence of the integral intensities shows the existence of two non-separable spin subsystems—charge carriers and localised spins. A role of oxygen atoms in the electronic system of NPC is studied.     

Reaction Pathway of Combustion Synthesis of Ti5Si3 in Cu–Ti–Si System

The reaction pathway of combustion synthesis (CS) of Ti₅Si₃ in Cu–Ti–Si system was explored through a delicate microstructure and phase analysis on the resultant products during differential thermal analysis (DTA). The formation of Cu–Si eutectic liquids plays a key role in the reaction pathway, which provides easy route for reactant transfer and accelerates the occurrence of complete reaction. Cu initially reacted with Si to form Cu₃Si by a solid‐state diffusion reaction, which further reacted with Cu to form Cu–Si liquids at the eutectic point of ~802°C; then Ti was dissolved into the surrounding Cu–Si liquids and led to the formation of Cu–Ti–Si ternary liquids; finally, Ti₅Si₃ was precipitated out of the saturated liquids by a solution–reaction–precipitation mechanism. The reaction pathway in CS of titanium silicide (Ti₅Si₃) could be described briefly as: Cu_(s) + Ti_(s) + Si_(s)→Cu₃Si_(s) + Ti_(s) + Si_(s)→(Cu–Si)_(l) + Ti_(s)→(Cu–Ti–Si)_(l)→Cu_(l) + Ti₅Si_3(s).     

Microwave dielectric properties of silico-carnotite Ca3M2Si3O12 (M= Yb,Y) ceramics synthesized via high energy ball milling

The Ca₃M₂Si₃O₁₂ (M = Yb, Y) ceramics with orthorhombic silico-carnotite structure were fabricated via high-energy ball milling and solid-state reaction route. Dense Ca₃Yb₂Si₃O₁₂ and Ca₃Y₂Si₃O₁₂ ceramics sintered at 1260 °C and 1240 °C revealed promising microwave dielectric properties with ε_r = 9.2 and 8.7, Q×f = 56,400 GHz and 29,094 GHz, τ_f = −77.5 ppm/°C and −76.8 ppm/°C, respectively. The connection between crystal structure and Q×f values of Ca₃M₂Si₃O₁₂ (M = Yb, Y) ceramics was discussed with respect to the packing fraction, and their intrinsic microwave dielectric properties were examined using the infrared reflectivity spectra analysis. The thermal stability of Ca₃Yb₂Si₃O₁₂ was improved successfully by forming 0.91Ca₃Yb₂Si₃O₁₂‐0.09CaTiO₃ composite ceramics with τ_f = +2.9 ppm/°C, ε_r = 12.93 and Q×f = 26,729 GHz.     

Processing and Dielectric Properties of Sillenite Compounds Bi12MO20−δ (M = Si, Ge, Ti, Pb, Mn, B1/2P1/2)

Six sillenite compounds Bi₁₂MO_20-δ (M = Si, Ge, Ti, Pb, Mn, B_1/2P_1/2) were synthesized, and the resulting single-phase powders were then sintered to obtain ∼97% dense ceramics. An analysis of their microwave dielectric properties, performed at ∼5.5 GHz, revealed a permittivity of ∼40 for all six compounds. The temperature coefficient of resonant frequency was the lowest for the Pb analogue (−84 ppm/K) and was found to increase with increasing ionic radius of the B-site ion to a value of −20 ppm/K for the Bi₁₂SiO₂₀ and Bi₁₂(B_1/2P_1/2)O₂₀ compounds. The Q × f value is a maximum for Bi₁₂SiO₂₀ and Bi₁₂GeO₂₀ with 8100 and 7800 GHz, respectively. The dielectric properties of the sillenites have been correlated with the structure of the oxygen network of the sillenite crystal lattice. As a result of its low sintering temperature (850°C), chemical compatibility with silver, low dielectric losses, and temperature-stable permittivity, the Bi₁₂SiO₂₀ compound is a suitable material for applications in low-temperature cofiring ceramic (LTCC) technology.     

Synthesis,microstructure, physical and mechanical properties of phase-pure entropy-enhanced (Nb0.8Ti0.05Ta0.05V0.05M0.05)4AlC3 (M = Hf,Zr) ceramics

The first 413-phase entropy-enhanced (Nb_0.8Ti_0.05Ta_0.05V_0.05M_0.05)₄AlC₃ (M = Hf, Zr) (EEMAX_Hf and EEMAX_Zr) ceramics were successfully consolidated by spark plasma sintering (SPS) using Nb, Ti, Ta, V, Zr, Hf, Al and graphite as initial materials. The formation of solid solution with five transition metals at the M sites of hexagonal M₄AlC₃ unit cell was confirmed by elemental analyses. Compared with pure Nb₄AlC₃, both the electrical and thermal conductivities of the entropy-enhanced ceramics showed a slight decrease, which is attributed to the lattice distortion and the increasing lattice defects that prevents the transfer of electrons and phonons. On the other hand, the mechanical properties of entropy-enhanced ceramics were greatly enhanced compared to pure Nb₄AlC₃. The measured fracture toughness of EEMAX_Hf and EEMAX_Zr ceramics were 8.2 MPa·m^1/2 and 10.0 MPa·m^1/2, respectively, which were increased by 18.8% and 44.9% compared to Nb₄AlC₃. The compressive strength of EEMAX_Hf and EEMAX_Zr ceramics were 987 MPa and 1187 MPa, respectively, being 92.0% and 130.9% higher than that of Nb₄AlC₃, respectively. EEMAX_Hf and EEMAX_Zr ceramics also possessed the higher Vickers hardness of 6.8 GPa and 7.4 GPa, respectively.     

Reaction of Cp2ZrR2 with M(CO)6 (M=Mo,W): Cp transfer from dialkylzirconocenes to molybdenum or tungsten carbonyl complexes

Dialkylzirconocenes react with molybdenum or tungsten hexacarbonyl and iodine or NBS to form cyclopentadiene molybdenum or tungsten tricarbonyl iodine or bromide.     

Ti3AC2(A=Si,Al)结构、弹性和电子性质的第一性原理研究

采用第一性原理方法,系统地研究了MAX相材料Ti3AC2(A=Si,Al)的结构、弹性和电子性质.对比LDA和GGA计算结果可知,采用GGA近似得到的结果更接近实验值.计算分析了Ti3AC2(A=Si,Al)的弹性性质,并根据弹性常数证明了其力学稳定性.此外,还从电子态密度和Mulliken布居分析的角度考察了Ti3AC2(A=Si,Al)的电子性质和价键特性,认为其具有共价键、离子键和金属键的综合性质.本文计算结果与文献报道吻合较好.     

Local orders,lattice distortions,and electronic structure dominated mechanical properties of (ZrHfTaM1M2)C (M = Nb,Ti, V)

High-entropy carbides (HECs) are regarded as potential candidate structural materials with attractive mechanical properties due to their ultra-high hardness. It is essential to reveal the atomic and electronic basis for strengthening mechanism in order to develop the advanced HECs. In the present work, C (M = Nb, Ti, V) are selected as case studies. The effects of transition metals (M) on the lattice parameters, bulk modulus, enthalpy of formation, electron work function (EWF), and bonding morphology/strength of HECs are comprehensively studied by first-principles calculations. It is found that the lattice parameters, equilibrium volumes, and bulk modulus of HECs are improved with the increase of M atomic volumes. The atomic-size differences among various groups of elements not only result in the lattice mismatch/distortion but also contribute to the formation of weak spots. In the view of bonding charge density, the electron redistributions caused by the coupling effect of the lattice distortion and valance electron differences can be revealed obviously, which identify the different bonding strength. Moreover, in terms of EWF, the proposed power-law-scaled hardness of HECs is validated and matches well with those reported theoretical and experimental results, providing a strategy to design advanced HECs with excellent mechanical properties.     

Microwave dielectric characteristics of Li2(Mg0.94M0.06)Ti3O8 (M=Zn,Co, and Mn) ceramics

Li₂(Mg_0.94M_0.06)Ti₃O₈ (M=Zn, Co, and Mn) ceramics were synthesized by the conventional solid-state reaction route. The effect of M (Zn, Co, and Mn) substitution on the structure, microstructure and microwave dielectric properties of Li₂(Mg_0.94M_0.06)Ti₃O₈ has been investigated. The XRD patterns of sintered samples revealed the single-phase formation with spinel structure. With the increase in ionic radius of M, the Qf value decrease is attributed to the decrease of packing fraction and grain size. The Li₂(Mg_0.94Zn_0.06)Ti₃O₈ ceramic sintered at 1075 °C for 4 h showed the best microwave dielectric properties with a dielectric constant of 27.1, a Qf value of 44 800 GHz, and a temperature coefficient of resonant frequency of (+)1.9 ppm/°C.     

Tuning of microwave dielectric properties of garnets Ca3Mg2CV2O12 (C = Si,Ti) through compression and expansion effects

Two novel low-ε_r Ca₃Mg₂CV₂O₁₂ (C = Si, Ti) ceramics with the garnet structure were synthesized by a traditional solid-state reaction method. Rietveld refinements based on XRD patterns show both the compounds crystallized into a cubic structure with the Ia-3d space group. Outstanding microwave dielectric properties (ε_r = 9.70, Q × f = 35,680 GHz, and τ_f = ?60.1 ppm/°C for Ca₃Mg₂SiV₂O₁₂ ceramic; ε_r = 11.70, Q × f = 48,530 GHz, and τ_f = ?43.7 ppm/°C for Ca₃Mg₂TiV₂O₁₂ ceramic) were obtained at 1240 °C and 1260 °C, respectively. The bond valence calculations reveal that Ca²⁺ at the A-site and Mg²⁺ at the B-site are slightly compressed, in combination with “rattling” Si⁴⁺ and “compressed” V⁵⁺ in the C-site of Ca₃Mg₂SiV₂O₁₂ compared to “rattling” V⁵⁺and “compressed” Ti⁴⁺ in Ca₃Mg₂TiV₂O₁₂, resulting in a negative deviation of ?7.16% for Ca₃Mg₂SiV₂O₁₂ and a positive value of 5.66% for Ca₃Mg₂TiV₂O₁₂ between the porosity corrected ε_r(Corr) (10.11 and 11.95) and theoretical ε_th (10.89 and 11.31) calculated by the C-M equation. The overall “rattling” effect in Ca₃Mg₂TiV₂O₁₂ results in a higher ε_r and a nearer to zero τ_f compared to Ca₃Mg₂SiV₂O₁₂. Besides, the Q × f values of Ca₃Mg₂CV₂O₁₂ (C = Si, Ti) ceramics were correlated with relative density and Raman mode (A_1g).     

Green Synthesis of M0 Nanoparticles (M=Pd,Pt, and Ru) for Electrocatalytic Hydrogen Evolution

In recent years the demand for green synthesis and green energy has increased immensely. Herein M⁰ nanoparticles were synthesized using a metal precursor and NaHCO₃ under hydrothermal conditions without using any reducing agent. The nanoparticles were characterized using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX). Surprisingly, it found that the Pt and Pd nanoparticles contain a high concentration of carbon. The presence of carbon is also evident from the attenuated transmission resonance infra-red (ATRIR) spectroscopy. Moreover, acetic acid was detected as a byproduct of the hydrothermal reaction. NaHCO₃ can be considered as the dissolution of CO₂ in a mildly alkaline solution. Thus, during the reaction, CO₂ was converted to carbonaceous material, which can be considered as a fixation of CO₂. The synthesized nanoparticles are also efficient catalysts for the electrocatalytic hydrogen evolution reaction (HER). It was found that Ru nanoparticles exhibit the highest activity among the three catalysts studied.     

M2(Si,Al)4(N,C)7 (M = La,Y,Ca) carbonitrides: II. The crystal structure of Ca0.8Y1.2Si4N6.8C0.2

A new (Ca,Y)Si₄(N,C)₇ phase has been characterised lying between the two end-members Y₂Si₄N₆C and CaYSi₄N₇. This phase is similar to BaYbSi₄N₇, which is made up of a network of [N(SiN₃)₄] structural units linked together in a three-dimensional network, with the large cations located in the interstices, but (Ca,Y)Si₄(N,C)₇ is a disordered variant, with nitrogen atoms partially occupying two sets of equivalent sites related by the combined operations of rotation and tilt. The crystal of (Ca,Y)Si₄(N,C)₇ used for structure determination contained Ca and Y in the atomic ratio 2:3, the excess positive charge in the cation sites being balanced by the partial replacement of nitrogen by carbon in the central non-metal site of the [N(SiN₃)₄] unit. Powder diffraction data are listed for Ca_0.8Y_1.2Si₄N_6.8C_0.2, which is hexagonal with a=5.9874(4), c=9.7849(8) Å at ambient temperature. The crystal structure has been determined from single crystal data; Z=2; S.G. P6₃mc (no. 186); R_int=0.0274, R1=0.0384, wR2=0.0993 for all data.     

Synthesis and electrochemical characteristics of isostructural LiMTiO4 (M = Mn,Fe, Co)

Isostructural LiMTiO₄ (M = Mn, Fe, Co) have been successfully synthesized by a facile sol-gel route and evaluated their possibility as an anode materials for lithium ion batteries (LIBs). All the samples exhibit high operating potential plateaus (>1.7 V vs. Li⁺/Li), which can prevent the formation of the solid electrolyte interphase (SEI) film on the electrode surface effectively. In addition, electrochemical investigations show that LiCoTiO₄ has a superior cycling performance, better rate capability, lower charge transfer resistance and higher lithium-ion diffusion coefficient than those of LiMnTiO₄ and LiFeTiO₄. These electrochemical results indicate that LiCoTiO₄ is the most promising lithium storage material among all the examined spinel-type LiMTiO₄ samples.