L. T. Kong

Correlation of lattice constant versus tungsten concentration of the Ni-based solid solution examined by molecular dynamics simulation

Abstract In order to investigate the Ni-enriched Ni–W solid solution at an atomic level, an n-body Ni–W potential was firstly derived based on the cohesion energies, lattice constants and bulk moduli of four Ni–W non-equilibrium solid phases obtained by ion mixing experiment or ab initio calculation. Based on the derived potential, molecular dynamics simulation was then performed to examine the correlation of the lattice constant of the Ni–W solid solution vs. the W concentration and the results were in good agreement with the experimental data.

Metastable phase formation in an immiscible Cu-Ta system studied by ion-beam mixing, ab initio calculation, and molecular dynamics simulation

Ion-beam mixing experiments show that amorphous phases are formed in the Cu70Ta30, Cu65Ta35, Cu60Ta40, and Cu50Ta50 multilayered samples, while a crystalline structure is retained in the Cu75Ta25 sample. These results suggest that the composition limit for forming an amorphous phase is around 30 at.% Ta, which confirms the same value of 30 at.% Ta predicted by recent atomistic modeling. For the metastable crystalline Cu75Ta25 phase, ab initio calculations identify its relatively stable structure to be L12 (fcc type) and determine some associated physical properties, which agree well with those derived from a proven realistic Cu-Ta potential through molecular dynamics simulations. Interestingly, an fcc Cu75Ta25 phase is indeed observed in ion-beam mixing experiments and its lattice constant determined by diffraction analysis matches well with those from theoretical calculations.

Ab initio Calculation to Predict Possible Non-Equilibrium Solid Phases in an Immiscible Y–Nb System

In the equilibrium immiscible Y–Nb system, the total energies of the possible structures for Y 3 Nb and YNb 3 non-equilibrium phases were calculated as a function of their lattice constant(s), under the frame work of the Vienna ab initio simulation package (VASP) and the calculated results predicted the relative stability of the Y 3 Nb and YNb 3 phases crystallizing in four possible simple structures, i.e. A15, D0 19 , L1 2 and L6 0 structures, respectively. Experimentally, a fcc Y 3 Nb non-equilibrium phase was indeed obtained by ion beam mixing and its lattice constant determined by diffraction analysis was in agreement with the calculated value.

Comparative study of metastable phase formation in the immiscible Cu–W system by ab initio calculation and n-body potential

The lattice constants and cohesive energies of some possible metastable Cu–W compounds are obtained by ab initio calculation and the formation of a metastable phase at Cu75W25 is predicted for the equilibrium immiscible Cu–W system. The prediction is in agreement with the fact that a metastable hcp phase was indeed observed in the Cu75W25 multilayer films upon ion beam mixing. Furthermore, some of the ab initio calculated properties are used in deriving an n-body Cu–W potential under the embedded atom method. The constructed Cu–W potential is then used to predict the phase stability of the metastable Cu–W phases over the entire composition and the prediction is also supported by some experimental observations.

Evidence of a high-spin ferromagnetic state in fcc-structured Fe-Ag (Au) alloys synthesized by ion beam manipulation

Based on the design of interfacial free energy in Fe–Ag (Au) multilayered films, metastable Fe40Ag60 and Fe40Au60 alloys were synthesized by ion beam manipulation in the Fe–Ag and Fe–Au systems. The synthesized alloys were identified to crystallize in a large fcc structure with a lattice constant of 0.38–0.39 nm and the associated magnetic moments per Fe atom were measured to be around 2.10–2.30 μB. The above experimental observations, together with some theoretical analysis, confirm the existence of a high-spin ferromagnetic state in the fcc-structured Fe-containing alloys.

Interfacial Reaction of W/Cu Examined by an n-body Potential through Molecular Dynamics Simulations.

Concerning the possible barrier function of W in the case of Cu metallization technology, the interfacial reaction between the equilibrium immiscible Cu and W was studied by molecular dynamics simulations, based on a proven realistic Cu–W potential derived under the embedded atom method. The simulation results revealed that the Cu (W) atoms did not diffuse into the W (Cu) lattice within a temperature range of 300–900 K. Moreover, even a disordered interlayer was preset at the Cu–W interface, mutual diffusion was not detectable and the disordered interlayer did not expand either, while it was reported to occur in the Ta/Cu bilayer at around 450–600°C.

Atomistic Modeling of Metastable Phase Selection of a Highly Immiscible Ag-W System

Ab initio calculation is performed for predicting the cohesive energies and lattice constants of some possible metastable compounds in the Ag-W system with the largest positive formation enthalpy among the binary transition metal alloys. The calculated results together with the experimental data are then used in deriving an n-body Ag-W potential under the framework of the embedded atom method. Based on the proven realistic Ag-W potential, molecular dynamics simulations are performed to reveal the metastable phase selection over the entire composition of the system and the results predict that the metastable Ag 100-x W x alloy in an fcc structure is more stable than in the bcc structure when 0 ≤ x < 57, whereas the bcc structure becomes energetically favored when 57 < x≤100. Interestingly, the prediction is in good agreement with the experimental results.

Construction of an N-Body Cu–Ta Potential and Study of Interfacial Behavior between Immiscible Cu and Ta through Molecular Dynamics Simulation

We proposed a method of constructing a realistic n-body potential based on the embedded atom method (EAM) for the equilibrium immiscible Cu–Ta system, in which no equilibrium compound exists. The Cu–Ta cross potential was derived by fitting some physical properties of the nonequilibrium B2 CuTa and L1 2 Cu 3 Ta phases determined by ab initio calculations. The constructed potential was then applied to study the interfacial behavior between Cu and Ta through molecular dynamics simulations performed at 300 and 800 K, respectively. Two fcc-Cu/bcc-Ta bilayer models were employed and one model included a preset disordered interlayer. It was found that a few Ta atoms could move into the Cu lattice at the imperfect Cu/Ta interface at high temperature.

First-principles calculations of the structural stability and magnetic property of the metastable phases in the equilibrium immiscible Co–Au system

To reveal the energetic sequence of the alloy phases in the Co-Au system, the lattice constants, cohesive energies, and bulk modulus of the fcc Au, hcp Co, the B1, B2, and L1(0) structured CoAu phases, and the D0(3), L1(2), and D0(19) structured Co(3)Au and CoAu(3) phases, respectively, are acquired by first-principles calculations within the generalized-gradient approximation (GGA) as well as within the local density approximation (LDA). In addition, the magnetic moment of the Co atom in the studied phases are also calculated. To further examine the structural stability, the elastic constants of the studied phases are calculated and the results suggest that the fcc-type structures could be elastically stable at Co/Au = 1:3, 1:1, and 3:1, whereas the hcp-type structures could be stable at Co/Au = 1:3 and 3:1. Moreover, the spatial valence charge density (SVCD) and spin density of the studied phases are also calculated to clarify the physical origin of the structural stability. It turns out that, in the relatively stable phases, the high SVCDs mostly distribute between the similar atoms, thus forming the attractive covalent bonding to stabilize the respective structures, and that the spin density may also play an important role in influencing the stability of the ferromagnetic metastable phases.

Structural Stability and the Correlation of Lattice Constant versus Tantalum Concentration of the Ag-Based Fcc Solid Solutions Studied by Molecular Dynamics Simulation

Based on the cohesion energies and lattice constants of some Ag–Ta non-equilibrium solid phases obtained by ab initio calculations, an embedded atom method (EAM) potential of the equilibrium immiscible Ag–Ta system was derived. Applied the derived potential, molecular dynamics simulation was carried out to predict, at an atomic scale, the quantitative correlation of the lattice constant of the Ag-based solid solutions versus the Ta concentration. The simulation results predict that the fcc crystalline structure can be preserved in the Ag-based solid solutions until the Ta concentration reaches 10 at.% and that the correlation of the lattice constant of the Ag-based solid solutions versus the Ta concentration is in good agreement with that deduced by Vegards Law only when the Ta concentration is less than 6 at.%.