The multiple hydrogen location near a α-Fe vacancy

The interaction between 10 hydrogen atoms and a α-Fe structure having a vacancy (V) has been studied using a cluster model and a semi-empirical theoretical method. The energy of the system was calculated by the atom superposition and electron delocalization molecular orbital method. The electronic structure was studied using the concept of density of states and crystal orbital overlap population curves.For the study of a sequential absorption, the hydrogen atoms were positioned in their energy minima configurations, near to the tetrahedral sites neighboring the vacancy, except the last H atom that was located far from the vacancy. The energy difference for H agglomeration was also computed. The vacancy-H_n complexes become less stable than VH species for more than three hydrogen's.The changes in the electronic structure of Fe atoms near the vacancy were also analyzed. The interactions mainly involve Fe 3d and 4s atomic orbitals. The contribution of Fe p orbitals is much less important. The Fe-Fe bond weakened as new Fe-H and H-H pairs were formed. The effect of H atoms is limited to its first Fe neighbors. The detrimental effect of H atoms on the Fe-Fe bonds can be related to the mechanism for embrittlement in α-Fe.     

Light-induced structural changes in sodiumnitroprusside (Na2(Fe(CN)5NO)·2D2O) at 80 K

Groundstate and electronic excited state (MS_I) of deuterated sodiumnitroprusside (Na₂(Fe(CN)₅NO)·2D₂O) have been investigated by neutron diffraction as well as by optical and Mössbauer techniques. Significant structural changes occur predominantly in the O–N–Fe–C-bond. It has been shown, that the N–O bond-length is not the order parameter, as expected from other studies. We found an increase in the bond lengths Fe–N4 of 0.019(2) Å and N–O of 0.004(4) Å respectively, which is in qualitative agreement with changes determined by Raman spectroscopy and predictions based on diatomic correlations (Badger/Herschbach/Laurie). Additionally we observed a change in the Fe–C1 bond length of 0.012(3) Å in agreement with Raman meaurements.     

On the Phase Transformations in Cr–FeB and Fe–CrB Systems at High Temperature

The solid state interactions occurring at high temperature in the Cr–FeB and Fe–CrB systems were studied by transmission Mössbauer and X-ray diffraction techniques on samples prepared by powders carefully mixed, cold-compacted and then treated at 1000°C for times up to 16 h. In the Cr–FeB system, iron atoms liberated by the substitutional diffusion of Cr into FeB lattice preferentially destabilize the iron monoboride with formation of Cr-containing Fe₂B. In the Fe–CrB system, chromium atoms liberated by the substitutional diffusion of Fe into CrB lattice interacts with iron forming an Fe–Cr metal alloy. Moreover, zones of Cr-containing FeB and Fe₂B form at the contact between metal iron and chromium monoboride, and tend to disappear as iron is consumed by the alloying process.     

Exhibition of High- and Low-spin States of the High-temperature Fcc Phase in Nanoparticles of Fe,Fe-rich and Co-rich Alloys

The results of combined X-ray and Mössbauer studies of structure and local magnetic ordering in massive substances Fe, Fe–Ni, Fe–Mn, Fe–Ni–Mn, Fe–Pt, Fe–Co and aerosol nanoparticles produced by their evaporation in rare Ar atmosphere are discussed. This technique provides a stochiometric composition of alloys in nanoparticles. The smallest (5–8 nm) particles for all alloys containing Fe 60–65% are shown to have a bcc structure whereas with doubling a size the particles acquire a fcc structure. This is explained by the fact that by cooling the particles in the course of preparation they quickly reach a state close to the equilibrium and, according to the constitution diagram, must decompose into two phases. Such decomposition in massive alloys was never observed at temperatures below 300°C because of diffusive difficulties. It is found that as-fresh aerosol particles are covered with an X-ray amorphous oxide shell, which is displayed in the room temperature Mössbauer spectra as a superparamagnetic doublet and is transformed into sextet at lower temperatures. An availability of the oxide shell has no practical influence on the particles structure. The obtained Mössbauer spectra are considered with the model suggested by R.J. Weiss in 1963, on existence of two-spin states in the high-temperature fcc modification of Fe and its alloys. Both states coexist, moreover, in the Mössbauer spectra the ferromagnetic state dominates at high temperature and anti-ferromagnetic one at low temperature. The ferromagnetic state manifests itself as a remnant of the frozen magnetic ordering of the high-temperature fcc modification in the resulting bcc structure, whereas the anti-ferromagnetic state is related to some fcc fraction retained under the particles quenching.     

Antisite defects and Mg doping in LiFePO<Subscript>4</Subscript>: a first-principles investigation

Atomic and electronic structures of LiFePO₄ with the antisite defect and Mg doping at Li and Fe sites have been investigated using first-principles density-functional theory with the on-site Coulomb interaction taken into account. It is demonstrated that the most favorable antisite defect type is the exchange defect, in which Li and Fe ions exchange positions. The resultant longer Fe–O bond and narrower band gap drop a hint that the electronic and ionic transport properties may be improved. For the case of Mg doping, Mg is preferentially doped at the Fe site instead of the Li site to form a new LiFe_1−y Mg _y PO₄ solid solution, leading to a higher ionic conductivity. Moreover, the dependence of the electrochemical properties on the concentration of Mg dopant has also been discussed.     

金属元素替代对Li4BN3H10储氢材料释氢影响机理的第一性原理研究

应用基于密度泛函理论的赝势平面波第一性原理方法研究了金属元素替代对Li₄BN₃H₁₀ 释氢的影响机理.计算给出了结合能、电子态密度、密集数, 分析了结构的稳定性和原子间的成键情况.结果表明: 晶体的结合能与(LiM)₄BN₃H₁₀(M=Ni,Ti,Al,Mg)释氢性能没有直接的关联.带隙的宽窄和带隙中是否存在杂质能级是决定(LiM)< 关键词： LiBNH系储氢材料 第一性原理 元素替代 释氢机理     

H,O,N,He原子掺杂Al原子的第一性原理研究

采用密度泛函理论框架下的第一性原理平面波赝势方法,对Al中分别加入H,O,N和He原子后的晶体状态进行了研究.通过晶体结构和形成能分析比较了杂质原子占据不同位置的难易程度及对晶体稳定性的影响,并从态密度、电荷密度和电荷布居的角度,分析了其电子结构.结果表明:H、O和N原子占据金属Al的四面体间隙最稳定,而He原子主要占据金属Al的八面体间隙. O和N原子与Al原子具有强烈的共价作用,H原子与Al原子存在共价作用但相对较弱,而He原子与Al原子的相互作用以范德华力为主.进一步揭示了四种原子在金属Al中不同行为的电子机制.     

Stability of the (H2O)2 dimer in the ground and lowest-lying excited electronic states

A theoretical study of the (H₂O)₂ dimer has been carried out in which the lowest S₁ and T₁ excited electronic states of the dimer complex, the influence of hydrogen bond formation on the shift in the maximum of the absorption band, and the stability of the dimer complex in the ground and excited states have been examined. It was found that there is only a single global maximum for the system — a nonplanar dimer complex formed by a linear hydrogen bond. Cyclic and bifurcated structures are transition states which do not form stable configurations when electronically excited. For the structure having a minimum in the ground electronic state, two nondissociating S₁ and T₁ states were found with bond energies of 2.0 and 4.4 kcal/mole, respectively. Formation of hydrogen bonds leads to a shift in the absorption maximum to the blue region with respect to the monomer. The hydrogen bond was found to weaken in the excited electronic states of the dimer.V. D. Kuznetsov Siberian Physicotechnical Scientific-Research Institute at the State University, Tomsk, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 11–15, March, 1993.     

Correlation of57Fe hyperfine interactions with Fe-Fe distance in Laves phase compounds RFe2 (R=rare earth)

The⁵⁷Fe Mössbauer isomer shifts and magnetic hyperfine fields in Laves phase compounds RFe₂ (R=Pr, Nd and Sm) are studied with particular reference to the effect of the Fe–Fe interatomic distance on the hyperfine interactions. It is shown that the charge density at the Fe nuclei scales linearly with fractional volume change up to 20%. The⁵⁷Fe hyperfine field corrected for the influence of rare-earth moment shows a systematic variation with the distance, which can be understood in terms of the Bethe-Slater curve arguments. The similarity of the atomic volume dependence of the⁵⁷Fe hyperfine interactions in Lves phase compounds to those in iron with close-packed structure is emphasized.This paper is based on a paper presented at the 5th Int. Conf. on hyperfine interactions, Berlin, July 21–25, 1980.     

Formation and ordering of local magnetic moments in Fe–Al alloys

The density functional theory is used to study the local magnetic moments in Fe–Al alloys depending on concentration (from 29 to 44 at% Al) and the Fe nearest environment. We have found three different solutions for the system: a spin-spiral wave (SSW) which has a minimum energy and two collinear states, a ferromagnetic one and a state with both positive and negative Fe magnetic moments (the Fe atoms with many neighboring Al atoms around them have negative magnetic moments, while the other Fe atoms—positive). Both the SSW and the negative Fe moments agree with the experiments. Magnetization curves taken from the literature are analyzed. The assumption of percolation character of the size distribution of magnetic clusters describes well the experimental superparamagnetic behavior above 150 K.     

Single-Crystal Fe Q-Band ENDOR Study of the 4 Iron–4 Sulfur Cluster in its Reduced [4Fe–4S] State

⁵⁷Fe Q-band ENDOR has been used to study the [4Fe–4S]¹⁺ state created by γ irradiation of single crystals of the synthetic model compound [N(C₂H₅)₄]₂[Fe₄S₄(SCH₂C₆H₅)₄] enriched in ⁵⁷Fe. This compound is an excellent biomimetic model of the active sites of many 4 iron–4 sulfur proteins, enabling detailed and systematic studies of its oxidized [4Fe–4S]³⁺ and reduced [4Fe–4S]¹⁺ paramagnetic states. Taking advantage of the fact that Q-band ENDOR, in contrast with X-Band ENDOR, allows for a very good separation of the ⁵⁷Fe transitions from those of the protons, the complete hyperfine tensors of the four iron atoms for the [4Fe–4S]¹⁺ species has been measured with precision. For each iron atom, the electron orbital and electron spin isotropic contributions have been determined separately. Moreover, it is remarkable that two ⁵⁷Fe hyperfine tensors attributed to the ferrous pair of iron atoms are very different. In effect, one tensor presents a much larger anisotropic part and a much smaller isotropic part than those of the other. This difference has been interpreted in terms of a differential electron orbital hyperfine interaction among the two ferrous ions.     

The effect of interstitial hydrogen on the electronic structure of Fe-Pd alloys

The electronic structure and bonding in Fe-Pd alloys were computed using a tight binding method. Two phases have been identified for these alloys, a high temperature fcc and a low temperature fct structure. The hydrogen absorption turns out to be a favorable process in both structures. The hydrogen at tetrahedral interstitial site for the fct structure is 2.2 eV more stable than that impurity atom located at an octahedral interstitial site in the fcc structure.The density of states curves show a peak below the d metal band which is made up mostly of hydrogen based states (>50% H_1s) while the metal contribution includes mainly s and p orbitals.In the fcc structure, both Fe-H and Pd-H bonds are developed while the Fe-Pd interface shows antibonding filled states near the Fermi level. When the fct phase is considered, the Fe-H overlap population (OP) decreases, while the Pd-H remains similar to the previous case. The Fe-Fe OP decreases and the Pd-Pd bonds are almost unaltered. The interfacial Fe-Pd bonds are almost unaffected by hydrogen. The band structure of the hydrogenated alloys in the fcc and fct phases were also computed.     

元素替代对LiNH2储氢材料释氢能力影响的第一性原理研究

采用第一性原理赝势平面波方法,研究了元素替代对 LiNH₂释氢能力影响及作用机理.计算给出了结合能、电子态密度、电荷布居,分析了结构的稳定性和原子间的成键情况.结果表明：金属Ca,Na,Al替代LiNH₂部分Li时,可以使N—H键有所减弱.Mg,Al同时替代Li时,效果最好.在Li（Mg）NH₂中,非金属元素B,C,P替代N时,C的效果最好.预测Mg,Al,C共同替代时,会得到的一种较低释氢温度的储氢材料. 关键词： 2储氢材料')" href="#">LiNH₂储氢材料 密度泛函理论 元素替代行为 释氢能力     

Icosahedral-Cluster-Reinforced Structures in Irradiated Metallic Materials

A nonequilibrium state has been discovered which is induced by ion irradiation in metallic materials (solid solutions of Fe–Ni, Fe–Cr–Ni, Ni–Cr, Cu–Ni, Fe–Cr, and V–Ti–Cr systems and in pure metals Zr and Ti) at high levels of radiation damage, and the features of this state are considered. In the region of existence of this state, both the ion and the electron subsystems of the metal show highly anomalous properties. Moreover, the occurrence of this state is accompanied by substantial diffraction effects – X-ray line splitting – and, as indicated by electron microscopy, by the formation of a cluster structure. Simulation by the methods of molecular dynamics suggests that the clusters observed are atomic groups of icosahedral (quintuple) symmetry formed in the neighborhood of radiation vacancies. These clusters reinforce the matrix, and this should result in substantial changes in strength and electronic properties of the material. The results of the computer simulation agree with the observed diffraction effects.     

Thermodynamic Characteristics of Complexes with a Hydrogen Bond of Bromosubstituted Anilines with Proton Acceptors

In the range 290–330 K, the influence of temperature on the spectral characteristics of the absorption bands is studied for an amino group of bromosubstituted anilines in free molecules and in the hydrogenbond complexes with various proton acceptors. Using the van't Hoff formula, the thermodynamic characteristics of the complexes with the composition 1:1 are determined. It is shown that in these complexes the hydrogen bond turns out to be stronger than in those with the composition 1:2.     

La、Al在Fe晶界上协同作用的第一性原理研究

采用基于密度泛函理论的第一性原理,研究了稀土La和Al在bcc-Fe中Fe晶界处的协同作用影响.计算了La和Al在晶界、表面、晶内的形成能,建立La、Al共存于晶界模型,通过电荷密度、布居分布手段分析了La、Al原子对晶界的影响.结果表明:La、Al原子易偏聚在晶界处,且La-Al的原子间距与体系结合能成正比关系. La的掺杂改善了晶界处的电荷分布情况,促进了Al原子与周围Fe原子间的相互作用,态密度曲线的计算结果显示,La原子的加入能够使La、Fe及Al原子间的键的结合力更强,从而提高了界面结合强度.     

Ti, C, N在α-Fe基中的合金化效应及对键合性质的影响

基于第一性原理赝势平面波方法研究了合金元素Ti, C, N对α-Fe基电子结构及键合性质的影响, 计算了含Ti, C, N的Fe基固溶体的总能量、结合能, 分析了态密度、电荷布居数、交叠布居数和电荷密度, 从理论上解释了在Fe基中固溶Ti, C, N后其性能改善的原因. 结果表明, 随着Fe基固溶体中Ti(0-12.5 at%), C(0-11.11 at%), N(0-11.11 at%)含量增加, 结合能略有增加; Ti, C, N的固溶使各Fe基固溶体在费米能级处强烈成键, 结合能力增强, 并且在费米能级附近出现赝能隙, 表明固溶体中金属键与共价键共存; 随着Ti, C, N含量的增加, C, N分别与Ti, Fe之间的共价键结合强度加强, 部分C, N原子会与Ti原子结合形成TiC, TiN颗粒, 起到沉积相颗粒强韧化作用.     

Point defects in FeAl

Summary Point defects in annealed B2-phase FeAl samples in the range 47–53 at.% Fe were studied using⁵⁷Fe M?ssbauer spectroscopy. Spectra were analyzed using local environment models according to which point defects in atomic shells close to probe atoms induce shifts in the nuclear monopole interaction. For well-annealed samples, better results were obtained assuming only the presence of Fe_Al antisite and V_Fe vacancy defects, and not of Al_Fe antisite defects. Monopole interactions of⁵⁷Fe probes on the Fe and Al sublattices having no defects in the first two shells were about +0.27 and −0.03 mm s⁻¹, respectively, with respect to Fe in alpha-Fe metal. The shifts induced by Fe_Al and V_Fe defects in the first shells of Fe probes on the Fe and Al sublattices were −0.15 and −0.24 mm s⁻¹, respectively, and, in the second shells, +0.06 and +0.011 mm s⁻¹. In addition to structural defects needed to accommodate deviations from stoichiometry, annealed samples were found to contain several percents of Fe_Al and V_Fe defects due to lattice disorder, with greater disorder in Fe-deficient alloys. Paper presented at the ICAME-95, Rimini, 10–16 September 1995.     

Substitution effects on the hydrogen storage behavior of AB2 alloys by first principles

The hydrogen storage behavior of the TiCr₂ and ZrCr₂ alloys substituted with the third components (Zr, V, Fe, Ni) have been studied using first-principles calculations. The change of the hydrogen absorption energies caused by metal doping is arising from the charge transfer among the doped alloys interior. Zr and V atoms devoted abundant electrons, leading to a great enhancement of the H absorption energy, while Fe and Ni atoms always accepted electrons, yielding a remarkable decrease of the H absorption energy. The hydrogen diffusion energy barrier is closely correlated with the geometry effect rather than the electronic structure.     

Analysis of diffuse reflection and absorption spectra of ZnO in the near-IR region

It is established that the absorption of ZnO in the near-IR region of the spectrum is determined by free electrons and chemisorbed gases. The free-electron absorption spectrum is approximated by a power function with an exponent which varies within the range 2–3, depending on the defectiveness of the specimen. Chemisorbed gases yield absorption bands at 1.37; 1.18; 1.08–1.03; 0.95–0.93; 0.85; 0.75–0.73; and 0.65–0.63 eV. The bands at 1.18 and 0.85 eV are due to chemisorbed oxygen in the states O^– and O₂ ^–, while the other bands can be attributed mainly to atomic hydrogen and OH-groups.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 86–90, July, 1988.We thank E. V. Komarov for his help in conducting the experiments.