CuAu Structure Research Articles

Lattice inversion for interionic pair potentials

The ab initio total energies of alkali chloride crystals in B1 (rock salt), B2 (CsCl-type), B3 (zinc blende) and P4/mmm structures are calculated by ultrasoft pseudopotential method in this paper. Based on the different combinations of these total energies, the effective charges on ions are determined for long-range Coulomb interaction, and the short-range potential curves are obtained by the lattice inversion method. As the test for the quality of potentials, the static properties of ACl in the rock salt phase are calculated. The results are in good agreement with the experimental data. And the calculated lattice energies for ACl in wurtzite, CuAu, NiAs, and litharge structures are consistent with the predictions of pseudopotential calculations. In terms of the molecular dynamics simulations, the temperature dependences of volume, bulk modulus, and elastic constants are also investigated from room temperature to 800 K, the results are in good agreement with the experimental values. This indicates that the new interionic potentials are valid over a wide range of interionic separations and coordination numbers.

Interdiffusion, Crystallinity and Exchange Bias in MnxPt1-x/Co Bilayers

AbstractThe structure of the antiferromagnet (AF) in the Ta/ MnxPt1-x/Co /Ta system must have the face-centered tetragonal CuAu structure in order to obtain a measurable exchange bias (HE). The formation of this structure has an onset temperature of approximately 240 °C. Increasing the annealing temperature increases the amount of CuAu ordering, which increases HE, and also increases the MnPt/Co interdiffusion, which decreases HE. These two competing processes result in an optimum annealing temperature that maximizes HE.

Order and disorder in epitaxially grown CuInS 2

This study investigates the ordering effects which occur on the cation sublattice of the CuInS 2 compound. CuInS 2 is grown on single-crystalline silicon substrates of (001) orientation. The ordered, sulfur-terminated surface Si(001)(1×1)-S constitutes well-defined starting conditions for the epitaxial growth process using molecular beams. The silicon surfaces and epitaxial CuInS 2 films are characterized in situ by means of Auger electron spectroscopy and low-energy electron diffraction. X-Ray diffraction, and transmission electron microscopy are employed for an ex situ structural characterization. We demonstrate the coexistence of CuAu-type ordering and disorder and a complete absence of the equilibrium chalcopyrite order. CuInS 2 with the tetragonal CuAu structure is shown to grow exclusively in c-axis direction.

The non-monotonic temperature dependence of the yield stress in TiAl and CuAu alloys

An anomalous temperature dependence of the yield stress in the ordered CuAu alloy was detected. Blocked <101] superdislocations were observed in the region of the anomalous trend from room temperature to 300°C. Blocking of single dislocations was not detected at these temperatures. The comparison with the known data on the TiAl intermetallic was carried on. The causes of the nonmonotonous temperature dependence of the yield stress with two extremum points were analyzed. The set of possible states of dislocations in TiAl was discussed. A unified scheme with account of mutual dislocation transitions was proposed for the description of the deformation behavior over the whole temperature interval. Specific features of the dislocation structure of CuAu and TiAl were explained. The connection between the brittleness of TiAl and dislocation transformations was discussed.

Anisotropy of Interfaces in an Ordered HCP Binary Alloy

A multiple-order-parameter mean-field theory of ordering on a binary hexagonal- close-packed (HCP) crystal structure is developed, and adapted to provide a continuum formulation that incorporates the underlying symmetries of the HCP crystal in both the bulk and gradient energy terms of the free energy. The work is an extension of the previous treatment by Braun et al. [Phil. Trans. Roy. Soc. Lond. A355:1787 (1997)] of order–disorder transitions on a face-centered-cubic crystal (FCC) lattice. The theory is used to compute the orientation dependence of the structure and energy of interphase and antiphase boundaries in ordering to the Cd3Mg and CdMg structures, which are the HCP analogs of Cu3Au and CuAu structures in FCC. As in the corresponding FCC case, the multiple order parameters do not form a vector. Anisotropy is a natural consequence of the underlying crystal symmetries and the multiple-order-parameter continuum formation presented here. The isotropy transverse to the sixfold axis expected for a scalar order parameter is not found.

Ordering and recrystallization of CuAu alloy after cold deformation

Abstract The evolution of the microstructure of cold-deformed CuAu alloy is analyzed as a function of temperature and annealing time. Different temperature ranges were determined where the behavior of the microstructure formation during ordering of the alloy differs. The influence of the typical lamellar structure of CuAu on recrystallization was studied. Transmission electron micrographs suggest that a cooperative ‘tandem-mechanism’ involving ordering and dislocation resaturation operates and contributes to the formation of new grains. A comparison with other ordered alloys having the L1 0 , L1 2 or B2 superstructure is conducted in the paper. The role played by the immobilization of dislocation networks in retardation of recrystallization is also considered.

Band offsets and optical bowings of chalcopyrites and Zn-based II-VI alloys

Using first-principles band-structure theory we have systematically calculated the (i) alloy bowing coefficients, (ii) alloy mixing enthalpies, and (iii) interfacial valence- and conduction-band offsets for three mixed-anion (CuInX2, X=S, Se, Te) and three mixed-cation (CuMSe2, M=Al, Ga, In) chalcopyrite systems. The random chalcopyrite alloys are represented by special quasirandom structures (SQS). The calculated bowing coefficients are in good agreement with the most recent experimental data for stoichiometric alloys. Results for the mixing enthalpies and the band offsets are provided as predictions to be tested experimentally. Comparing our calculated bowing and band offsets for the mixed-anion chalcopyrite alloys with those of the corresponding Zn chalcogenide alloys (ZnX, X=S, Se, Te), we find that the larger p−d coupling in chalcopyrite alloys reduces their band offsets and optical bowing. Bowing parameters for ordered, Zn-based II-VI alloys in the CuAu, CuPt, and chalcopyrite structures are presented: we find that ordered Zn2SeTe has bowing coefficients of 1.44 and 3.15 eV in the CuAu and CuPt structures, while the random ZnSexTe1−x alloy has a bowing of 1.14 eV. The band alignment between CuInSe2 and CuInSe2-derived ordered vacancy compounds are also presented.

Atomically Layered Structures for Perpendicular Magnetic Information Storage

ABSTRACTLayered structures can possess a high volumetric density of interfaces, which can result in novel magnetic properties. We report magnetic characteristics of two types of atomically layered structures, (001) oriented intermetallics with the CuAu(I) crystal structure and (111) oriented artificial multilayers. C-axis oriented superlattices of PtFe, PtCo, PdFe and PdCo with the layered CuAu(I) structure, possess large magnetic anisotropies and novel magneto-optic properties relative to the corresponding random alloys. These films are produced by annealing, with tetragonality and magnetic anisotropy developing in a fashion which depends on the initial bilayer period and annealing parameters. (111) oriented artificially grown Pd/CoCr multilayers are reported for perpendicular magnetic recording applications. Using a perpendicular contact probe transducer, the multilayers exhibit two times the readback signal of CoCr media. The multilayers can be made with low roughness suitable for contact recording, and can generate narrow readback pulses with large amplitude. Many aspects of Pd/Co performance, such as overwrite, can be predictably optimized for a given set of transducer properties. The reported results show that compositionally modulated structures can be made which have a wide variety of useful properties for perpendicular hard disk and magneto-optic recording.

Structural effects on the magnetic properties of FePd and FeRh.

The magnetic structures of the ordered transition-metal compounds FePd and FeRh are found in both the CsCl and CuAu atomic structures by first-principles total-energy calculations. The ground state of FePd is shown to be the CuAu structure and ferromagnetic, in agreement with experiment. However, at equilibrium in the CsCl structure, where FePd is metastable, FePd behaves just like FeRh and has an antiferromagnetic ground state. A ferromagnetic phase of FePd lies 0.5 mRy higher in energy at the equilibrium volume and, like FeRh, but more readily, undergoes a first-order phase transition from the antiferromagnetic phase to the ferromagnetic phase at an expanded lattice constant or in a magnetic field.

Electronic Structure and Spin Fluctuations in the fcc Fe?Pd System

The electronic structure of the fcc Fe~Pd alloy system is investigated by band structure calculations employing the augmented spherical wave (ASW) method. The three compositions Fe3Pd, FePd and FePd3 are studied assuming the ordered CU3Au and the CuAu structure respectively. The total energy is computed as a function of volume and magnetic moment E(V, lvJ) by performing fixed spin moment (FSM) calculations for all three systems. The energy surface of Fe3Pd shows two minima, a ground state with an average magnetic moment of 2� Olj1.B/atom and a metastable minimum with a very small moment. The intermetallic compound FePd shows the usual behaviour of a strong itinerant ferromagnet with an average magnetic moment of 1� 56j1.B, where iron carries about 2� 8j1.B as for the high-spin state of pure fcc Fe. In FePd3 the iron states form rather localised bands and the magnetic moment rises to 3�2j1.B. Inclusion of spin fluctuations allows treatment of the finite-temperature behaviour of such systems. The magnetic contribution to the thermal expansion coefficient at room temperature for Fe3Pd is Qm::::; -18xlO~6 K~l, which largely compensates the positive phonon (Griineisen) part of Q.

An effective-medium theory approach to ordering in Cu-Au alloys

The authors present a new approach, based on the effective-medium theory of metallic cohesion, to the study of alloy phase stability. The efficiency and applicability of the method is demonstrated by studying ordering in Cu-Au alloys. The ground-state properties of the three stoichiometric Cu-Au structures are found to be in excellent agreement with experiment. Monte Carlo simulations have been carried out in an effort to understand the finite-temperature phase transitions. These simulations give a very good description of the complete temperature-concentration phase diagram, and illustrate the importance of going beyond a fixed-lattice Ising model in describing phase transitions in these alloys.

Ordered structures and metastable alloys grown by OMVPE

Our understanding of the thermodynamics of mixing in semiconductor solids has evolved from the purely empirical regular solution model to models based on the electron energy states in the solid, including the delta-lattice parameter (DLP) model and, more recently, first principles calculations. These models are in agreement that the enthalpy of mixing is invariably ⩾ 0 for III/V and II/VI alloys, and increases with increasing difference in lattice constant for the constituent binary compounds. In terms of the simple thermochemical mixing models, this suggests the occurrence of miscibility gaps. Solid phase immiscibility has indeed been observed in a number of systems. Nevertheless, such alloys can be grown by OMVPE, including the highly metastable alloys GaPSb and InPSb. Initially surprising was the occurrence of ordered structures in these same alloys. The regular solution model apparently specifically excludes immiscibility and ordering in the same system. However, when the positive enthalpy of mixing is due to strain energy effects, as in III/V and II/VI alloys, Hume-Rothery recognized very early that such phenomena should be anticipated. This was later confirmed by the detailed first principles calculations. In fact, the tendency for ordering is anticipated to increase as the difference in tetrahedral radii of the elements sharing a common sublattice increases. Ordered structures have now been observed in several III/V alloy systems including the ternary systems GaAsSb, GaInP, AllnAs, AlGaAs, and GaInAs, and the quarternaries GaInAsP, GaInAsSb, and AlGaInP. In this paper, ordering in other alloy systems such as GaPSb, InPSb, InAsSb, and GaAsP will be described. An unexpected observation is that the preferred ordered structure for the ternaries GaInP, GaPSb, InPSb, InAsSb, and GaAsP involves ordering along the <111 directions, forming the Cu-Pt (L1 1) structure. This is also true of GaAsSb grown by MBE. Both the first principles calculations and simple strain energy calculations indicate that such ordered structures are more stable than the disordered solid solution, but less stable than other ordered structures. The chalcopyrite structure (E1 0), with ordering along <210 directions, and the Cu-Au structure (L1 0), where ordering occurs along the <100 directions, are considerably more stable. The occurrence of these structures must be explained in terms of the relative strain energies of the various ordered structures at the surface and/or kinetic processes occurring during epitaxial growth. The importance of kinetic effects is indicated by the importance of growth parameters such as growth rate, temperature, and substrate orientation in determining both the degree of ordering and the specific ordered structures observed.

A tight-binding calculation of the magnetic properties of TPt (T identical to 3d transition element) ordered alloys with CuAu structure

A tight-binding type self-consistent band calculation is performed to study the magnetic properties of the ordered TPt (T identical to V, Cr, Mn, Fe, Co and Ni) alloys with CuAu structure within the Hartree-Fock approximation of the Hubbard model for their spin polarisation. Global features of their ground-state magnetic properties such as phase stability and local moments are explained successfully. The characteristic tetragonality of the lattice is argued from the calculated uniaxial pressures to be a consequence of the spin polarisation.

Growth of copper-gold and copper-aluminum particles by gas-evaporation technique

The composition, crystal structure and habit of Cu-Au and Cu-Al particles prepared by evaporation of mother alloys in argon gas were investigated by X-ray diffraction and electron microscopy. With the progress of evaporation, the Al concentration of the Cu-Al particle changed so as to fall at about 35 at%, while the Au concentration of the Cu-Au particles always increased up to 100 at%, irrespective of the composition of the starting materials. Both the Cu-Au particles over the whole range of Au concentration and the Cu-Al particles containing up to about 35 at% Al showed the same well-defined crystal habits as pure metal particles, corresponding to their crystal structures.

Modulated structure of CuAu II (one-dimensional modulation)

Modulated structure of CuAu II (one-dimensional modulation)

Concentration Modulation and Stability of Long Period Superlattice in the Off-Stoichiometric Cu–Au Alloys

The equilibrium atomic distributions in the off-stoichiometric Cu–Au alloys whose high and low temperature phases are of the long period superlattice (LPS) and the Ll 0 (CuAu) structures respectively are investigated on the basis of the mean field theory. The obtained results are as follows. LPS with uniformly distributed excess atoms are generally unstable against the modulation of the concentration of excess atoms along the direction perpendicular to antiphase boundaries. The period of the concentration modulation (CM) is a half of that of LPS and its phase shows that excess atoms enter into sites at antiphase boundaries preferentially. CM produced by LPS increases the stability of LPS and sometimes LPS becomes stable only by accompanying CM.

Optical properties and electronic structure of dilute Cu-Au, Cu-Zn, Cu-Al, Cu-Ga, Cu-Si, Cu-Ge, Cu-Sn, and Cu-As alloys

A systematic study of the optical properties of a large range of $\ensuremath{\alpha}$-phase copper-based alloys was undertaken. The solutes were selected to include elements with one, two, three, four, and five valence electrons. The differential reflectometer, which is capable of determining within a hundredth of an electron volt the energy for interband transitions of electrons, was used for the investigations. Using a line-shape analysis, three different transitions could be identified: (i) A threshold energy for interband transitions around 2.2 eV which involves the upper $d$ bands and the Fermi energy. This transitional energy does not change for solute concentrations up to approximately 1 at.%, confirming the theory by Friedel concerning screening of solute charges at low solute concentrations. Experimental evidence is given which suggests that both the $d$ bands and the Fermi energy are raised by alloying. (ii) Lower $d$-band to Fermi energy transitions around 5 eV behave in many respects similar to the upper $d$-band to Fermi energy transitions, except that the lower $d$ bands appear to be raised much less due to alloying. (iii) A conduction-band to conduction-band transition around 4 eV was observed to decrease in energy with alloying. Copper-gold alloys do not show any changes in the threshold energy due to solute additions. Additional broad structure around 3 eV indicates that transitions originating from gold $d$ bands may be involved.

Distribution of interstitial atoms over octahedral pores of ordered alloys with CuAu structure

The authors consider the kinetics of processes of redistributions of interstitial atoms over the octahedral interstices in ordered alloys of the CuAu type as a result of a sudden change in temperature. The relaxation time of the process is determined and the equilibrium concentrations of interstitial atoms are found. These quantities are considered as functions of the degree of long-range order, the alloy composition, and the temperature.

Changes in energy structure of Cu 3Au and CuAu 3 alloys studied by the method of X-ray photoelectron spectroscopy

It is shown that the structures of X-ray photoelectron spectra of Cu 3Au and CuAu 3 in the disordered state differ from those in the ordered state. It enables us to conclude that the energy spectra of valence electrons in ordered and disordered states are slightly different.

The band structure of long-period superlattice alloys

Jones' method for calculating the band structure of a long-period superlattice structure is extended to a three-dimensional crystal. In this method the antiphase boundaries themselves appear as perturbations rather than the whole crystal potential. The band structure of CuAu II is calculated, and as the length of the antiphase domains increases the long-period superlattice band structure tends to the band structure of CuAu I. The stability of CuAu II is discussed in terms of its band structure.