CuAu Structure Research Articles

Theoretical study of structural, elastic and thermodynamic properties of Cu2MgSnX4 (X = S, Se and Te) quaternary compounds

A theoretical study of structural and elastic properties of Cu2MgSnSe4(CMTSe) and Cu2MgSnTe4 (CMTTe) compounds in stannite (ST), kesterite (KS), primitive-mixed CuAu (PMCA) and Wurtzite-Stannite (WS) structures is presented, by using the functional density theory in the framework of FP-LAPW + lo method within the generalized gradient approximation, based on Perdew 2008 functional (GGA-PBEsol). For each phase, the structural parameters, bulk modulus, and its pressure derivative are calculated. The relative stability of these phases is also discussed. In addition, the elastic constants have been calculated in order to examine the mechanical stability of all phases. Moreover, the anisotropy factor, shear modulus, Young's modulus, Lame's coefficient and Poisson's ratio have been estimated from the obtained single crystalline elastic constants. The thermodynamic properties such as the heat capacities, the coefficient of thermal expansion and the Debye temperature, in the pressure range 0–15 GPa and the temperature range from 0 to 1300 K, of the ground state structure are also investigated for the three materials Cu2MgSnS4 (CMTS), CMTSe, and CMTTe. The obtained results are compared with available experimental data and with other theoretical calculations.

Stability and electronic properties of planar defects in quaternary I2-II-IV-VI4 semiconductors

Extended defects such as stacking faults and anti-site domain boundaries can perturb the band edges in Cu2ZnSnS4 and Cu2ZnSnSe4, acting as a weak electron barrier or a source for electron capture, respectively. In order to find ways to prohibit the formation of planar defects, we investigated the effect of chemical substitution on the stability of the intrinsic stacking fault and metastable polytypes and analyzed their electrical properties. Substitution of Ag for Cu makes stacking faults less stable, whereas the other substitutions (Cd and Ge) promote their formation. Ge substitution has no effect on the electron barrier of the intrinsic stacking fault, but Cd substitution reduces the barrier energy, and Ag substitution enhances electron capture by the stacking fault. While Cd substitution stabilizes the stannite structure, chemical substitutions make the primitive-mixed CuAu structure less stable with respect to the ground-state kesterite structure.

Reaction between nickel or iron and xenon under high pressure

ABSTRACT Xe–Ni and Xe–Fe systems are studied in a pressure range relevant to the Earth's core (135–210 GPa) using laser-heated diamond anvil cells and synchrotron X-ray diffraction. The stability of several intermetallic compounds, including XeNi and XeFe, has been recently calculated using structural searches and density functional theory (DFT) above 155 and 190 GPa, respectively [Zhu L, Liu H, Pickard CJ, et al. Nat Chem. 2014;6:644–648]. We have synthesized XeNi around 150 GPa, confirming the prediction; however, it has a cubic -CuAu structure, different from the predicted one for XeNi but identical to the structure predicted for XeFe. -XeNi is calculated to be metastable with DFT. A disordered NiXe () alloy is observed to form prior to this compound. This alloy is interesting in the perspective of a possible storage of xenon in the Earth's core. We have not observed any reaction between Xe and Fe up to 210 GPa.

Orbital-frustration-induced ordering in semiconductor alloys

It is well known that ternary zinc-blende semiconductors are always more stable in the chalcopyrite (CH) structure than the Cu-Au (CA) structure because the CH structure has a large Coulomb interaction and a reduced strain energy. Surprisingly, an experimental study showed that the $\mathrm{ZnFeS}{\mathrm{e}}_{2}$ alloy takes the CA order as the ground-state structure, which is consistent with our density functional theory calculations showing that the CA order has lower energy than the CH order for $\mathrm{ZnFeS}{\mathrm{e}}_{2}$. We reveal that the orbital degree of freedom of a high-spin $\mathrm{F}{\mathrm{e}}^{2+}$ ion $({d}^{6})$ in the tetrahedral crystal field plays a key role in stabilizing the CA order. First, the spin-minority $d$ electron of the $\mathrm{F}{\mathrm{e}}^{2+}$ ion tends to occupy the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-like orbital instead of the ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$-like orbital because of its large negative Coulomb energy. Second, for a nearest-neighboring $\mathrm{F}{\mathrm{e}}^{2+}$ pair, two spin-minority $d$ electrons with occupied ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-like orbitals on the plane containing the Fe-Fe bond have lower electronic kinetic energies. Both conditions can be satisfied in the CA ordered $\mathrm{ZnFeS}{\mathrm{e}}_{2}$ alloy, whereas there is an orbital frustration in the CH structure. Our results suggest that the orbital degree of freedom provides a new way to manipulate the structure and properties of alloys.

Rapid Synthesis and Characterization of CuInS<sub>2</sub> Prepared Using Microwave-Assisted Heating

In this work, the CuInS2 nanoparticles are successfully synthesized by microwave-assisted heating technique and further calcined at 400 °C. The morphological, structural, and optical properties of the synthesized CuInS2 nanoparticles are investigated by scanning electron microscope (SEM), X-ray diffraction (XRD), Raman scattering, and transmittance measurement, respectively. The SEM image shows the clear particle shape of the calcined CuInS2 nanoparticles. After calcination treatment, the fundamental (112) peak of the XRD spectrum and a broad Raman peak mixed with chalcopyrite and CuAu structures support the improved crystallinity of the calcined CuInS2 nanoparticles.

Theoretical Study of Structural, Elastic Properties and Phase Transitions of Cu<sub>2</sub>ZnSnS<sub>4</sub>

The total energy, the electronic properties, phase transitions, and elastic properties of Cu2ZnSnS4(CZTS) in the three structures are investigated by first-principles calculations based on density functional theory. Results show that the total energies of stannite (ST) and primitive-mixed CuAu (PMCA) structures are higher than that of kesterite-type (KS), and the KS is the ground state structure. Relationships between enthalpy and pressure of the KS, ST and PMCA structure of CZTS are also investigated at 0 K, since the pressure can have profound impacts on the electronic structure, possible phase transitions and structure stability. And results also show that KS structure is always the most stable; ST is the second; and the PMCA structure is the most unstable; phase transitions of three structures could not occur in high pressure. The high ratios of shear modulus to bulk modulus (G/B) indicate that CZTS compounds in three types have ductile behaviors. The Poisson ratios for the three structures are from 0.27 to 0.31, which again proves that all structures of CZTS have better plasticity. The results can increase more hints about further research directions, and these effects can play an important role in future experimental preparation technology and theoretical work of CZTS materials.

Structural, elastic, thermodynamic and lattice dynamic properties of PrX (X = Sb, Bi)

AbstractFirst principles calculations for rare earth compounds PrSb and PrBi in NaCl(B1), CsCl(B2), ZB(B3), WC(Bh) and CuAu(L1o) structures have been performed using density functional theory within the generalized gradient approximation. For the total-energy calculation, the projected augmented plane-wave implementation of the Vienna ab-initio simulation package has been used. Our structural results are consistent with the available experimental and other theoretical data. The phonon dispersion curves and corresponding one-phonon density of states (DOS) for both compounds are also computed in NaCl(B1) structure which is energetically more stable than the other structures. In addition to these properties, the temperature/pressure variation of the volume, thermal expansion coefficient, heat capacity are calculated in over wide pressure and temperature ranges.

Crystalline ordered states of CuIn1−xGaxSe2 (x = 0, 0.3, and 1.0) thin-films on different substrates investigated by Raman scattering spectroscopy

Structural properties of Cu(In,Ga)Se2 absorber layers have been examined for photovoltaic applications. Thin-films with three different chemical compositions, CuInSe2 (CIS), Cu(In,Ga)Se2 (CIGS) and CuGaSe2 (CGS), were grown by co-evaporation on two kinds of substrates: Mo-coated soda-lime glass and bare soda-lime glass. Intriguing morphology and grain-growth behaviors were found in the surface of the films. X-ray diffraction of the films exhibited phase formation of the stoichiometric chalcopyrite phase of the materials while signs of secondary phases like Cu2Se and Cu–Se2 were also observed. The optical transmittance of the films was measured to obtain their optical bandgaps, which were well matched with the bulk values of CIS, CIGS, and CGS, which are 1.1, 1.4, and 1.7 eV, respectively. Using Raman scattering spectroscopy, the A1 mode was observed to shift from 177 cm−1 for CIS to 189 cm−1 for CGS as the Ga content increased. The films on Mo substrates are likely to have secondary phases, which is not the case for soda-lime glass. An indication of the formation of the CuAu structure is obtained from the CIS thin-films.

Calculation of the lattice dynamics and Raman spectra of copper zinc tin chalcogenides and comparison to experiments

The electronic structure, lattice dynamics, and Raman spectra of the kesterite, stannite, and pre-mixed Cu-Au (PMCA) structures of Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) were calculated using density functional theory (DFT). Differences in longitudinal and transverse optical (LO-TO) splitting in kesterite, stannite, and PMCA structures can be used to differentiate them. The Γ-point phonon frequencies, which give rise to Raman scattering, exhibit small but measurable shifts, for these three structures. Experimentally measured Raman scattering from CZTS and CZTSe thin films were examined in light of DFT calculations and deconvoluted to explain subtle shifts and asymmetric line shapes often observed in CZTS and CZTSe Raman spectra. Raman spectroscopy in conjunction with ab initio calculations can be used to differentiate between kesterite, stannite, and PMCA structures of CZTS and CZTSe.

A first-principle study of the structural, elastic, lattice dynamical and thermodynamic properties of PrX (X=P, As)

The structural, phase transition, elastic, lattice dynamic and thermodynamic properties of rare-earth compounds PrP and PrAs with NaCl (B1), CsCl (B2), ZB (B3), WC (B h) and CuAu (L1 0) structures are investigated using the first principles calculations within the generalized gradient approximation (GGA). For the total-energy calculation, we have used the projected augmented plane-wave (PAW) implementation of the Vienna Ab-initio Simulation Package (VASP). Specifically, some basic physical parameters, e.g. lattice constants, bulk modulus, elastic constants, shear modulus, Young's modulus and Poison's ratio, are predicted. The obtained equilibrium structure parameters are in excellent agreement with the experimental and theoretical data. The temperature and pressure variations of the volume, bulk modulus, thermal expansion coefficient, heat capacity and Debye temperature are calculated in wide pressure and temperature ranges. The phonon dispersion curves and corresponding one-phonon density of states (DOS) for both compounds are also computed in the NaCl (B1) structure.

Addition of Na into CuInS 2 thin film via co-evaporation

While most of studies focus on the addition of Na into CuInGaSe 2 as well as CuInGaS 2 thin films, this study examines the addition of Na into CuInS 2 (CIS) thin films. Moreover, an alternative approach has been used to incorporate Na into CIS thin films. Two source evaporation (Cu and In) was first performed to obtain Cu–In layers with desired thicknesses. Three source evaporation (Cu, In, and NaF) then followed subsequently to produce Na-doped Cu–In precursor films having different Na concentrations. The precursor films were immediately sulfurized in the same evaporation chamber to form CIS thin films. The addition of Na was found to enhance (112) preferred orientation and reduce the grain size. Raman spectra show that the addition of Na does not alter the needed phase transformation from CuInS 2–CuAu structure to CuInS 2–chalcopyrite structure during the sulfurization. Blue shift of the CIS Raman CH mode occurs as a result. The doping of Na was also found to decrease the film resistivity or increase the hole concentration in the films.

Atomic ordering in GaAsSb (0 0 1) grown by metalorganic vapor phase epitaxy

Epitaxial GaAsSb (0 0 1) semiconductor alloys grown by metalorganic vapor phase epitaxy exhibit several spontaneously ordered structures. A superlattice structure with three-fold ordering in the [1 1 0] direction has been previously observed by different groups. CuAu structures with (1 0 0) and (0 1 0) ordering planes have also been reported. The physical origin of CuAu ordering in III–V semiconductors has not yet been explained. In this work we report the effect of growth conditions on CuAu ordering in GaAsSb, including miscut from (0 0 1), growth rate, bismuth surfactant concentration, and growth temperature. These data point to a surface kinetic mechanism not based on dimer strain, but possibly due to one-dimensional ordering at step edges.

Deformation behavior and dislocation structure of CuAu ordered alloy

An ordered CuAu alloy, which had relatively coarse grains (5 to 10 μm), without the lamellar structure, is mechanically tested at temperatures from 77 to 658 K. The temperature dependence of the yield stress is shown to be anomalous with a maximum nearT p≈573 K. The TEM analysis reveals the presence of single dislocations, superdislocations and microtwins. Long rectilinear dislocations at 523 K are identified as blocked screw superdislocations with the Burgers vector type <101]. Numerous curved single dislocations are seen, but blocked single dislocations are not detected. Various models are discussed to account for the specific deformation behavior of the ordered CuAu alloy.

Structural stability of some CsCl structure HfTM (TM = Co, Rh, Ru, Fe) compounds

In order to investigate Hf-TM (TM = Fe, Co, Rh, Ru) phase diagrams in the region of 50:50% atomic ratio, we performed ab initio Full-Potential Linearized Augmented Plane Waves calculations of the most stable Hf and TM elemental phases and HfTM compounds of the CsCl and CuAu structure types. The obtained electronic structures, cohesive energies and enthalpies of formation are discussed and compared to some of the existing models and available experimental data. The non-existing compound HfFe is found to be at least metastable, and the reason for its absence from the phase diagram is discussed.

CuAu Structure in the Restricted Primitive Model and Oppositely Charged Colloids

We study the phase behavior of oppositely charged equal-size hard spheres both theoretically and experimentally, using Monte Carlo simulations and confocal microscopy. In the simulations, two systems are considered: the restricted primitive model (RPM) and a system of screened Coulomb particles. We construct the phase diagrams of both systems by computer simulations and predict a novel solid phase that has the CuAu structure. In addition, the CuAu structure is observed experimentally in a system of oppositely charged colloids. The qualitative agreement between the RPM, the screened Coulomb system, and the experiments shows that colloids form a suitable model system to study phase behavior in ionic systems.

Properties of a CuAu phase of AgGaSe2 grown on [100] GaAs substrate

Recently, it has been suggested theoretically [A. Janotti and S.-H. Wei, Appl. Phys. Lett. 81, 3957 (2002)] that, if AgGaSe2 with the CuAu structure can be grown epitaxially on ZnSe, it will be an excellent source for spin-polarized electrons. Here we report the growth and properties of epitaxial films of AgGaSe2 with the CuAu structure on (100) GaAs substrates and compared them with those of chalcopyrite AgGaSe2.

Morphology and structure of thin epitaxial Cu(In,Ga)S 2 films on Si substrates

Cu(In,Ga)S 2 (CIGS) thin films were grown epitaxially on Si substrates of various orientations. The sulphur-termination process, utilized to suppress the unfavourable native surface reconstructions of Si, was examined. Thereby, an anisotropic etching of Si by sulphur was observed at high substrate temperatures. Cu(In,Ga)S 2 was found to show a strong tendency for nucleation on tips and at step edges of the substrates. Furthermore, the material was observed to agglomerate to existing grains as well. These mechanisms lead to a three-dimensional growth mode. The resulting roughness showed a strong and monotonic dependence on the Ga content of the samples, whereas the influence of the lattice mismatch on the morphology of the samples was found to be less distinct. The coexistence of the highly ordered chalcopyrite structure with the metastable CuAu structure was found to be another prominent feature in epitaxial Cu(In,Ga)S 2 thin films on Si.

Structure and mechanical properties of CuAu and CuAuPd ordered alloys

The formation of a microstructure duringL10-type ordering in an equiatomic CuAu alloy and ternary CutĀu−Pd alloys having different initial states has been studied. Results of mechanical tests of the alloys have been given. Methods for improvement of strength properties of the ordered alloys have been discussed.

Simple model for localization in δ-Pu

First-principles methods are employed to study the effect of localization of the 5f electrons in δ-Pu. First, a full-potential linear muffin–tin orbitals (FPLMTO) method was applied to a model system, , where Puloc are Pu atoms with localized (nonbonding) 5f electrons and Puit atoms with itinerant (bonding) 5f electrons. Within the FPLMTO, this system was treated as an ordered compound, either in the Cu3Au or the CuAu structure to model δ-Pu which crystallize in a face-centred-cubic structure. A more realistic alloy treatment of our model system was provided by the Korringa–Kohn–Rostocker method within Green's function formalism in which compositional disorder is treated by means of the coherent potential approximation. With these two approaches best agreement with the experimental lattice constant for δ-Pu were achieved for a 67–68% fraction of itinerant (Puit) atoms. This corresponds to a little less than four itinerant 5f electrons/atom in δ-Pu which agrees well with some proposed theoretical models, but disagree with at least an other theoretical suggestion. We show that a good lattice constant (by construction), good bulk modulus, and full mechanical stability for δ-Pu follows from our model. The main problem with the present approach and some other presented models, trying to capture localization in δ-Pu, is that the contribution to the total energy from the localized 5f electrons cannot be calculated accurately and therefore one parameter (usually the lattice constant) needs to be fitted to experiment.

Interdiffusion and exchange bias in the MnxPt1−x/Co system

Mn x Pt 1−x (25 nm )/ Co (8.0 nm ) antiferromagnetic/ferromagnetic (AF/F) bilayers were grown via dc magnetron sputtering at room temperature on Si(111) substrates. Samples were annealed in a vacuum in a 1200 Oe magnetic field at various temperatures TA after growth. The Mn concentration x was determined from the MnPt lattice parameters and Rutherford backscattering spectroscopy. The maximum exchange bias (HE) was observed for x∼0.50, in agreement with previous work. Annealing caused the MnPt to form an ordered face-centered-tetragonal CuAu ordered structure which is necessary to observe HE. However, x-ray reflectivity indicates that an interdiffusion region exists at the MnPt/Co interface. The interdiffusion increases with TA, resulting in lower HE. The TA that maximizes HE, 318 °C, is a compromise between the volume fraction of MnPt forming the ordered CuAu structure, which increases HE, and the amount of interdiffusion, which decreases HE.