An ab initio study of 5d noble metal nitrides: OsN2, IrN2, PtN2 and AuN2

The crystal structure, phonon stability, elasticity and electronic properties of four noble metal nitrides (PtN₂, IrN₂, OsN₂ and AuN₂) with three structural types (pyrite, marcasite and CoSb₂ structure) were studied by first principles calculations. In agreement with experiments and previous theoretical predictions, it is found that the most stable structure for OsN₂ is marcasite, for PtN₂ is pyrite, and for IrN₂ is the CoSb₂ structure. It is found that these three compounds are thermodynamically metastable with respect to solid N₂ and the metal at zero pressure. The structures are mechanically and dynamically stable. The lowest energy structure of AuN₂ is the CoSb₂ structure. The formation energy of AuN₂ is found to be very high compared to the other three nitrides studied here. This underlies the experimental difficulty in the synthesis of this compound. OsN₂ is found to be metallic, while IrN₂ and PtN₂ are both semiconductors.     

Ohmic-rectification conversion that is tuned using H2O2 for enhanced rectification and optoelectronic performance in MoS2/ZnO nanorod devices

The characteristics of a p–n junction that consists of MoS₂ thin films and ZnO nanorods grown on heavily-doped n-type Si substrate are reported. The current–voltage characteristics for MoS₂/ZnO nanorod devices exhibit ohmic conduction. The measured current is limited by thermionic emission in MoS₂/ZnO nanorod devices that are treated with H₂O₂. H₂O₂ treatment results in the modification of the MoS₂–ZnO interface, so the rectification performance for MoS₂/ZnO nanorod devices is improved. H₂O₂ treatment also increases the responsivity of MoS₂/ZnO nanorod devices to solar irradiation. This phenomenon is caused by induced ohmic-rectification conversion due to H₂O₂ treatment.     

Phase transition and optoelectronic properties of MgH2

In this article, structural and electronic properties of MgH₂ have been studied. The aim behind this study was to find out the ground state crystal structure of MgH₂. For the purpose, density functional theory (DFT)-based full-potential linearized augmented plane wave (FP-LAPW) calculations have been performed in three different space groups: P4₂/mnm (α-MgH₂), Pa3 (β-MgH₂) and Pbcn (γ-MgH₂). It has been found that the ground state structure of MgH₂ is α-MgH₂. The present study shows that α-MgH₂ transforms into γ-MgH₂ at a pressure of 0.41 GPa. After further increase in pressure, γ-MgH₂ transforms into β-MgH₂ at a pressure of 3.67 GPa. The obtained results are in good agreement with previously reported experimental data. In all the studied phases, the behavior of MgH₂ is insulating and its optical conductivity is around 6.0 eV. The α-MgH₂ and γ-MgH₂ are anisotropic materials while β-MgH₂ is isotropic in nature.     

High Field ESR System and Its Application to the Study of Quantum Spin Systems

Developments of the high field ESR system in Kobe University is presented. Using Gunn oscillators and backward traveling oscillators (BWO), we can cover the frequency region from 30 to 1183.6 GHz with the use of InSb detector. Pulsed magnetic field up to 30 T is available and we are now trying to extend the field up to 40 T. Temperature range is from 1.8 to 300 K. Using this system, we studied S=1/2 ladder like system Cu₂(C₅H₁₂N₂)₂Cl₄, and found a new magnetic transition at 10.1 T at 1.8 K. The temperature dependence of ESR in Cu₂(C₅H₁₂N₂)₂CI₄ shows g-shift below 8 K which corresponds to the maximum of the magnetic susceptibility. The g-shift below 8 K suggests the increase of the quantum fluctuation in the system, and the role of the quantum fluctuation in Cu₂(C₅H₁₂N₂)₂CI₄ is discussed.     

Identification of the Li sites in the Li ion conductor, Li6SrLa2Nb2O12, through neutron powder diffraction studies

In this paper a neutron powder diffraction structural study of the Li ion conducting garnet-related system, Li₆SrLa₂Nb₂O₁₂, is reported. The results show that this phase is cubic, space group Ia-3d, and contains Li in two partially occupied crystallographic sites. The first site, Li1, corresponds to the ideal tetrahedral site in the garnet framework and possesses an occupancy of 0.59(1). The second site, Li2, is significantly more distorted and possesses an occupancy of 0.352(3). Compared to the related Li₅La₃Nb₂O₁₂ system, the Li2 site occupancy is greatly increased, while the Li1 site occupancy is reduced. Despite these large differences in site occupancies, the reported conductivities for Li₅La₃Nb₂O₁₂ and Li₆SrLa₂Nb₂O₁₂ are similar, showing the complexities of these new garnet Li ion conductors.     

Influence of cooperativity on the frequency shift of the Ar–H stretch vibration in HArF complexes

CH₃Li–FArH–X (X?=?H₂, OC, N₂, P₂, CO₂, CO, BeH₂) trimers have been investigated using quantum chemical calculations at the QCISD/6-311++G(2d,2p) level. The results show that the lithium bonding has a prominent effect on the strength and properties of the hydrogen bonding. The hydrogen-bonding interaction energy is increased by 160–340% due to the presence of lithium bonding. The Ar–H stretch vibration shows a blue shift in the FArH–X (X?=?H₂, OC, N₂, CO₂, CO) dimer, but a red shift in the FArH–X (X?=?P₂, BeH₂) dimer. The red shift is increased in the corresponding trimer, while the blue shift shows a different change. The blue shift is also increased in CH₃Li–FArH–X (X?=?H₂, OC, N₂, CO₂) trimers, but it changes to a red shift in the CH₃Li–FArH–CO trimer. The shift change is consistent with the explanation given by Joseph and Jemmis.     

5d过渡金属二硼化物的结构和热、力学性质的第一性原理计算

利用基于密度泛函理论的第一性原理系统地研究了5d过渡金属二硼化物TMB₂ (TM=Hf–Au) 的结构、热学、力学和电学性质. 我们考虑了三种结构, 分别为AlB₂, ReB₂和WB₂结构. 计算得到的晶格常数与先前的理论和实验研究符合得很好. 通过计算生成焓预测了化合物的热力学稳定性; 从HfB₂到AuB₂, 生成焓的整体趋势是逐渐增加的. 在所考虑的结构中, 对HfB₂和TaB₂, AlB₂结构是最稳定的; 对WB₂, ReB₂, OsB₂, IrB₂和AuB₂, ReB₂结构是最稳定的; 对PtB₂, WB₂结构是最稳定的. 在所考虑的化合物中, ReB₂结构的ReB₂具有最大的剪切模量(295 GPa), 是最硬的化合物, 与先前的理论和实验结果相符. 计算得到的总态密度显示所有结构都具有金属特性. 讨论了系列化合物的变化趋势. 关键词： 弹性性质 热力学性质 第一性原理 5d过渡金属二硼化物     

Carbon-13 enrichment by IR laser chemistry of CHF3-Cl2

IR laser chemistry of CHF₃ is investigated in both neat form and in the presence of Cl₂ for carbon-13 enrichment. Infrared multiple-photon dissociation of CHF₃ is an order of magnitude more efficient in the scavenged system compared to the neat case. The photolysis of CHF₃/Cl₂ mixture results in two products, viz., CF₂Cl₂ and C₂F₄Cl₂ but with different enrichment factors. The parametric studies show that C₂F₄Cl₂ arises due to MPD of CF₂Cl₂ in secondary photolysis.     

Au和3d过渡金属元素混合团簇结构、电子结构和磁性的研究

采用基于密度泛函理论的第一性原理方法系统研究了Au与3d过渡元素构成的混合小团簇的结构、稳定性、电子结构及磁性,得到了Au与3d过渡元素构成的混合小团簇的稳定结构.计算结果表明,Au与3d元素可形成大量的低能异构体,特别是有些异构体在结构上极相近,这不同于共价或离子键类型的团簇.与纯过渡金属团簇类似,这类团簇也表现出复杂的磁性.过渡金属元素的磁矩相比体材料而言既有增强的、也有减弱的,与轨道的交换劈裂密切相关.对于基态构型,AuCr₂,Au₂Cr_{2关键词： 密度泛函理论 第一性原理方法 团簇 电子结构}     

Magnetic properties and hyperfine interactions in RCo2B2 (R = Nd,Gd, Tb)

The magnetic properties of RCo₂B₂ compounds which crystallize in the ThCr₂Si₂ structure with R = Nd, Gd, Tb have been investigated. The magnetic structure is ferromagnetic for NdCo₂B₂ and GdCo₂B₂, T_c equals 32 and 26 K respectively and antiferromagnetic for TbCo₂B₂ (T_N = 19 K). Curie-Weiss behaviour is exhibited by all the compounds and the effective moments derived indicate that Co is diamagnetic. The difference in magnetic properties between RCo₂B₂ and other isomorphous RCo₂X₂ (X = Si, Ge) is discussed. Mössbauer studies of ¹⁵⁵Gd in GdCo₂B₂ yielded the hyperfine interaction parameters and determined the direction of the magnetization to be in the basal plane. The electric quadropole interaction at 4.1 K is 580 MHz sec^?1, this is the largest ever found in an intermetallic Gd containing compound.     

Investigation of half-metallic and magnetic phase transition in Co2TiZ (Z = Al,Ga, In) Heusler alloys

ABSTRACT

The half-metallic and ferromagnetic properties of Co₂TiZ (Z?=?Al, Ga, In) Heusler alloys are investigated. The structural stability is analyzed among the two possible structures, namely, L2₁ (Cu₂MnAl phase) and XA (Hg₂CuTi phase) structures. It is found that these alloys are stable in L2₁ (Cu₂MnAl phase) structure. The electronic structure of Co₂TiZ reveals that these alloys exhibit half-metallic ferromagnetism with small spin-flip gap at the minority spin state. As the pressure is increased, ferromagnetic to non-magnetic phase transition is observed at the pressures of 380.9, 363.0 and 317.8?GPa for Co₂TiAl, Co₂TiGa and Co₂TiIn, respectively. Half-metallic to metallic phase transition occurs in Co₂TiAl, Co₂TiGa and Co₂TiIn at the pressures of 76.5?GPa, 73.1?GPa and 63.9?GPa respectively.     

Half-metallicity in Fe-based Heusler alloys Fe2TiZ (Z = Ga,Ge, As,In, Sn and Sb)

The electronic structure and magnetic properties of new Fe-based Heusler alloys Fe₂TiZ (Z = Ga, Ge, As, In, Sn and Sb) have been studied by first-principles calculations. In these alloys, the 24-electron Fe₂TiGe, Fe₂TiSn are nonmagnetic semiconductors and other compounds are all ferrimagnetic metals. Fe₂TiAs and Fe₂TiSb are predicted to be half-metals with 100% spin polarization. The spin polarization ratio in Fe₂TiGa and Fe₂TiIn is also quite high. The calculated total moment for Fe₂TiAs and Fe₂TiSb is 1 μ_B, which is mainly determined by the Fe partial moment. The half-metallicity of Fe₂TiSb is stable under lattice distortion. The spin polarization of Fe₂TiSb is found to be 100% for the lattice variation in a range of 5.6–6.1 Å, which is attractive in practical applications.     

Luminescence of Ca-doped (Y1-x, Gdx)2O2S

Broad-band yellow luminescence peaking around 575–595 nm has been found in Ca-doped (Y_1-x, Gd_x)₂O₂S. The doping of Ca into Y₂O₂S with the concentration up to 1 mol% is possible. At the optimum concentration (about 0.2 mol%), the cathodoluminescence brightness of Ca-doped Y₂O₂S is 10% of that of yellow-emitting (Zn, Cd)S: Ag.The emission peak is 575 nm in Y₂O₂S: Ca and 595 nm in Gd₂O₂S: Ca. The phosphor exhibits strong thermoluminescence after UV excitation at 77 K. In (Y_1-x, Gd_x)₂O₂S, neither emission spectra nor the temperatures of thermal glow peak depend on x, indicating localized characters of the traps concerned. The photoluminescence is slow in buildup and persistent in decay. These results are explained by a model: the substitution of Y(Gd) by Ca creates acceptor levels in which holes are captured, giving rise to subsequent radiative recombination with excited electrons. Trapped electrons recombine with mobile holes nonradiatively, but exhibit thermoluminescence with high efficiency when they are thermally released.     

A hydrogen bonded complex C2H2…HBr isolated and characterized in the gas phase using pulsed-jet,Fourier transform microwave spectroscopy

A complex formed from ethyne and hydrogen bromide has been isolated and characterized by using a fast-mixing nozzle in conjunction with a pulsed-jet, Fourier transform microwave spectrometer. Any possible chemical reaction between the two components when mixed in the usual way was thereby precluded. The rotational constants A, B and C, the quartic centrifugal distortion constants δ_J, δ_Jk and δ_J and the Br nuclear hyperfine coupling constants X_aa and X_bb—X_cc (nuclear quadrupole) and M_bb (spin-rotation) were determined for each of the five isotopomers C₂H₂…H⁷⁹Br, C₂H₂…H⁸¹ Br, C₂H₂…D⁷⁹Br, C₂H₂…D⁸¹Br and C₂D₂…H⁷⁹Br. Interpretations of the spectroscopic constants show that the complex is planar and T shaped in the equilibrium conformation, with HCCH internuclear axis forming the cross of the T and the HBr internuclear axis lying along the C₂ axis of C₂H₂. The H of HBr is closer to the centre (?) of the π bond of ethyne, and therefore HBr is involved in a hydrogen bond to the π system, in which the distance of H from ? is r (?…H) = 2.469(1) A. The intermolecular stretching force constant is estimated as kσ = 5.38(2) Nm^?1 for the species involving a hydrogen bond (C₂H₂…H⁷⁹Br, C₂H₂…H⁸¹Br and C₂D₂…H⁷⁹Br) while this quantity increases to kσ = 5.68(2)Nm^?1 for those complexes bound through a deuterium bond (C₂H₂…D⁷⁹Br and C₂H₂…D⁷⁹Br). The opportunity is taken to consider similarities in the properties of complexes within the two series B…HBr and B…HC1 for a range of Lewis bases B, including B = C₂H₂. Some family relationships are identified in the two series.     

The specific heat of two cubic laves compounds: TiBe2 and ThMg2

The low temperature specific heats of two isomorphous Laves compounds, TiBe₂ and ThMg₂, have been measured. The shape of the transition for the itinerant antiferromagnet TiBe₂ is characteristic of magnetic ordering; the entropy of this transition is quite small, approximately 107 mJ/mole-K, a further indication of itinerancy. The γ, proportional to the density of states, is 42 mJ/mole-K² for TiBe₂, and the Debye temperature, θ_D, is 650 K. These numbers are compared to those for the itinerant ferromagnet, ZrZn₂. The results for ThMg₂ are γ=9.2 mJ/ mole-K² and θ_D = 217 K. Results for both compounds are discussed in regards to the theory of Enz and Matthias.     

Anomalous diamagnetic transitions in Ba2Cu3O4Cl2 and Sr2Cu3O4Cl2 antiferromagnets

The magnetic properties of Ba₂Cu₃O₄Cl₂ and Sr₂Cu₃O₄Cl₂ oxychlorides have been studied. A spontaneous manifestation of diamagnetism has been found at quasi-zero and low fields. The temperature of these transitions coincides with the temperature of the antiferromagnetic ordering in Ba₂Cu₃O₄Cl₂ (T _NI ≈ 337 K) and Sr₂Cu₃O₄Cl₂ (T _NI ≈ 386 K). The recorded diamagnetic signal is ?10^?4 emu. It is comparable with the percentage of the superconducting phase in the sample. It is supposed that the arising superconductivity is due to the formation of the superconducting clusters with the direct overlap of the copper d orbitals owing to the frustration processes.     

Quantum beats in two-photon resonance fluorescence

The two-photon resonance fluorescence spectrum of a three-level atom is shown to consist of the low frequency modes in addition to the high frequency ones in the limit of high photon densities. The spectral function for the low frequency modes consists of two lorentzian lines describing: the peak occuring at the renormalized beat frequency Δ₊ and that of the zero-photon excitation at the frequency Δ_-, where Δ_±=Δ-3Ω²/2ω_a±Ω²u/2ω_a, u²=1+(2Δω_a/Ω ²)². Here, 2Δ is the energy splitting between the two excited states, ω_a is the photon energy of the pump field and Ω is the Rabi frequency. The peak at the renormalized beat frequency Δ₊ occurs provided that the condition (2Δω_a/Ω²)² > 1 is satisfied. The two-photon laser spectroscopy is expected to be a useful tool for the observation of the low frequency modes in question.     

Formation pathways of HO2 and OH changing as a function of temperature in photolytically initiated oxidation of dimethyl ether

The time resolved product formation in oxidation of dimethyl ether (DME) has been studied between 298-625 K and 20-90 torr total pressure. Near-infrared frequency modulation spectroscopy (FMS) with Herriott type multi pass optics and UV absorption spectroscopy (UV) were conducted in the same cell. The reaction was initiated by pulsed photolysis in a mixture of Cl₂, O₂, and DME via CH₃OCH₂ radical formation. The reaction process was investigated through FMS measurement of HO₂ and OH, and UV measurement of CH₃OCH₂O₂. The yields of HO₂ and OH are obtained by comparison with reference mixtures, Cl₂, O₂, and CH₃OH for HO₂, and Cl₂, O₂, CH₃OH, and NO for OH, which convert 100% of initial Cl to HO₂ and OH. The CH₃OCH₂O₂ yield is also obtained. It was found that the HO₂ yield increases sharply over 500 K mainly with a longer time constant than that of R + O₂ reaction, while a prompt component exists throughout the temperature range at a few percent yield. OH was found to be produced promptly at a yield considerably larger than that known for the simplest alkanes. The CH₃OCH₂O₂ profile has a prompt rise followed by a gradual decay whose rate is consistent with the slow HO₂ formation. The species profiles were successfully predicted with a model constructed by modifying the existing one to suit the reduced pressure condition. After modification, it was inferred that the HO₂ formation over 500 K is secondary from HCHO + OH and HCO + O₂ and a part of HCO is formed directly from the O₂ adduct, whereas the HO₂ formation below 500 K is governed by CH₃OCH₂O₂ chemistry. The HCO forming pathway via isomerization-decomposition of the O₂ adduct, which was not included in the former models, was supported by our quantum-chemical calculations.     

The magnetic phases and structures of Ce(Ru1-xFex)2Ge2 studied by μ-SR

We report on the structures of the magnetic phases of CeRu₂Ge₂ and their evolution in the series Ce(Ru_1-xFe_x)₂Ge₂. The crossover behavior between the ferromagnetic ground state of CeRu₂Ge₂ to the paramagnetic ground state of CeFe₂Ge₂ is compared to the behavior in the series CeRu₂(Ge_1-xSi_x)₂. A profound difference is observed between these two series, which is in contradiction to the uniform behavior predicted by the Doniach-necklace model. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photocatalytic activity of Nb2O5/SrNb2O6 heterojunction on the degradation of methyl orange

The pure SrNb₂O₆ powders were prepared at 1400 °C by a conventional solid-state method and characterized by X-ray powder diffraction and UV-vis diffuse reflection spectrum. The powders of Nb₂O₅ and SrNb₂O₆ were ball-milled together and annealed to form the Nb₂O₅/SrNb₂O₆ composite. Photocatalytic activities of the composites were investigated on the degradation of methyl orange. The results show that the proportion of Nb₂O₅ to SrNb₂O₆ and the annealing temperature greatly influence the photocatalytic activities of the composites. The best photocatalytic activity occurs when the weight proportion of Nb₂O₅ to SrNb₂O₆ is 30% and the annealing temperature is 600 °C. The tremendously enhanced photocatalytic activity of the Nb₂O₅/SrNb₂O₆ composite compared to Nb₂O₅ or SrNb₂O₆ is ascribed to the heterojunction effect taking place at the interface between particles of Nb₂O₅ and SrNb₂O₆. The powders also show a higher photocatalytic activity than commercial anatase TiO₂.