Nan-Xian Chen

Theoretical study of the phase stability and site preference for R3(Fe,T)29 (R=Nd, Sm; T=V, Ti, Cr, Cu, Nb, Mo, Ag)

The phase stability of intermetallics R3(Fe,T)29 with Nd3(Fe,Ti)29 structure and site preference of some 3d or 4d transition elements T were investigated in molecular static and molecular dynamic methods with a series of ab initio pair potentials obtained though the lattice inversion method. Calculated results show that adding either Cr, Mo, Ti, V or Nb atoms makes the crystal cohesive energy of R3(Fe,T)29 decrease markedly, proving that these atoms can stabilize R3(Fe,T)29 with the structure of Nd3(Fe,Ti)29, even though the R3Fe29 crystal structure is itself metastable. The calculated lattice parameters are in good agreement with the experimental data. The degree of the decrease in cohesive energy corresponds with the species and occupation sites of the ternary atoms. The order of site preference of these stabilizing elements T is 4i2, 4i1 and 4g with the occupation of 4i2 corresponding to the greatest energy decrease. The calculated result further shows that the addition of Cu or Ag cannot play a role in stabilizing the structure. These calculated results correspond well to available experiments. Supported by the pair potentials, calculated structures are stable within a certain temperature range and the space group of the final structure remains unchanged with respect to a variety of initial deformations. So it was confirmed that there exist a series of R3(Fe,T)29 compounds with the stable structure of Nd3(Fe,Ti)29 in the R–Fe–T systems. The process of the evolution from the RFe5 structure to metastable R3Fe29 was well explained too with the pair potential in this paper. All these prove the effectiveness of ab initio pair potentials obtained through the lattice inversion method in the description of rare-earth materials.

Generation of nondiffracting beams by diffractive phase elements

We present a design of the diffractive phase elements (DPE’s) that produce nondiffracting beams according to the beam-shaping scheme, in which the incident Gaussian-profile beam is converted into a Bessel-function J0 beam. An optimization method is applied to solving this special beam-shaping problem. Numerical investigation of the generating J0 Bessel beam shows that the designed DPE can satisfactorily produce the J0 Bessel beam.

Selective field evaporation in field-ion microscopy for ordered alloys

Semiempirical pair potentials, obtained by applying the Chen-inversion technique to a cohesion equation of Rose et al. [Phys. Rev. B 29, 2963 (1984)], are employed to assess the bonding energies of surface atoms of intermetallic compounds. This provides a new calculational model of selective field evaporation in field-ion microscopy (FIM). Based on this model, a successful interpretation of FIM image contrasts for Fe3Al, PtCo, Pt3Co, Ni4Mo, Ni3Al, and Ni3Fe is given.

INTERATOMIC POTENTIALS BETWEEN DISTINCT ATOMS FROM FIRST-PRINCIPLES CALCULATION AND LATTICE-INVERSION METHOD

The effective interatomic potentials between distinct atoms in intermetallics, such as FeAl, Fe3Al, NiAl, Ni3Al, FeCr, Al3Cr, AlLi, Al3Li, and AlLi3, are obtained by inversion of first-principles cohesive energy curves based on the lattice inversion method of Chen. The obtained potentials are used to evaluate the phonon dispersions and linear thermal expansion of some intermetallic compounds, as well as the site preference of alloying element Cr in the D03-ordered Fe3Al.

Recursive algorithm for phase retrieval in the fractional Fourier transform domain

We first discuss the discrete fractional Fourier transform and present some essential properties. We then propose a recursive algorithm to implement phase retrieval from two intensities in the fractional Fourier transform domain. This approach can significantly simplify computational manipulations and does not need an initial phase estimate compared with conventional iterative algorithms. Simulation results show that this approach can successfully recover the phase from two intensities.

Insight into the high reactivity of commercial Fe–Si–B amorphous zero-valent iron in degrading azo dye solutions

Improving intrinsic reactivity is one of the key requirements in applying zero-valent iron in the field. As a new kind of zero-valent iron, iron based amorphous alloys were recently found to be capable of rapidly remediating wastewater. However, the mechanisms for the rapid degradation have not yet been fully understood. In this study, commercial Fe–Si–B amorphous alloy ribbons (Fe–Si–BAR) were used to degrade azo dyes (Direct Blue 6 and Orange II) to study the reaction kinetics, pathway and mechanism behind the high reactivity of these iron based amorphous alloys. The results show that, under the same conditions, the surface normalized reaction rate constants for the decomposition of Orange II and Direct Blue 6 by Fe–Si–BAR could be 1300 and 60 times larger respectively than those obtained by using 300 mesh iron powders. Through UV-vis spectrophotometry and mass spectrometry, it is found that the intermediate products of the azo dyes degraded by Fe–Si–BAR are similar to those produced in degradation by iron powders. However, the controlling step of the degradation reaction by Fe–Si–BAR turns out to be the diffusion process rather than the surface chemical reaction found in the reaction by iron powders. Further analysis indicates that the high degradation efficiency of Fe–Si–BAR results from its amorphous structure and the metalloid additions, which could enhance the catalytic effect and promote the formation of a non-compact and easily detached oxide layer on the surface. The experiments under different environmental conditions show that the factors that influence the degradation efficiency of crystalline iron powders affect that of Fe–Si–BAR in a similar way, but Fe–Si–BAR is capable of efficiently degrading wastewater under broader conditions than the crystalline iron powders. The results indicate that Fe–Si–BAR is a promising environmental catalyst for wastewater treatment.

Atomistic study on the structure and Curie temperature for Nd2Fe17−xCrx

The structural stability of ternary compounds Nd2Fe17� xCrx is evaluated by using a series of quasi-ab initio interatomic potentials. The results show that the substitution of Cr atoms for Fe almost does not change the crystal symmetry significantly and the calculated structural parameters of Nd2Fe17� xCrx correspond well to experimental data. The site preference of Cr atom is further evaluated and the order is given as 6c, 18f, 18h and 9d, which are close to experimental results. Moreover, the relaxed structure together with the site preference of Cr atoms could give some simple explanation on the variation behavior of Curie temperature. All results indicate that the present calculated potentials are effective for studying some structural properties of this kind of intermetallics. r 2002 Elsevier Science B.V. All rights reserved.

Pair potentials for a metal-ceramic interface by inversion of adhesive energy

A concise and general formula is introduced to obtain ab initio pair potentials between atoms across a metal–ceramic interface by inversion of the adhesive energies of the interface. Derivation of interfacial potentials ΦAg−Mg and ΦAg−O from ab initio adhesive energies is performed by applying the formula to the Ag/MgO(001) interface. Transferability of these potentials at Ag/MgO(100), Ag/MgO(110) and Ag/MgO(111) interfaces is discussed.

Theoretical study of phase forming of NaZn13-type rare-earth intermetallics

By using the interatomic pair potential obtained with the lattice inversion method, the stability of RT13-xMx (R = La, Ce, Pr and Nd; T = Co and Fe; M = Si, Al, Cr, V and Ti) of the NaZn13 type and its derivative structure are studied. The structural transition of LaT13-xSix (T = Co and Fe) between the cubic one with the space group Fm3c and the tetragonal one with 14/mcm is imitated from the viewpoint of energy. As for the function of the third elements, Al and Si are beneficial to the phase stability of RT13-xMx, whereas Cr, Ti and V are unfavourable to the stability. In the calculation, the range of x, with which RT13-xMx could crystallize in the cubic or tetragonal structures, agrees with the experiments very well. The calculated crystallographic parameters coincide with the experimental observation. In the cubic structure, Si and Al prefer the 96i site, and in the tetragonal structure Si first occupy the 16l(2) site, then the 16k site. In addition, all the site positions of the compounds with either the cubic or tetragonal structure are really congruent with the experimental one.

Phase stability and site preference of Nd2Fe17−xTx (TV, Ti, Nb) and Nd2−xZrxFe17

Abstract The phase stability of Nd 2 Fe 17− x T x (TV, Ti, Nb, x =0–1.2) is tested by many means including random atom shift, global deformation and high temperature disturbance under the control of the ab initio potentials. The T atoms substitute for Fe without changing the crystal symmetry and preferentially occupy the 6c site, which is in good agreement with experimental results. The Zr atoms prefer to substitute for Nd atoms in Nd 2 Fe 17 . Moreover, the introduction of ternary elements slightly enlarge the calculated Nd 2 Fe 17− x T x (TV, Ti, Nb) volume, which is also close to experiment. All the results indicate that the potentials are valid for studying some structural properties of the intermetallics.