Isostructural Alloys Research Articles

Structural stability, mechanical, and thermodynamic properties under pressure of B2-type CuM (M = Be, Al, and Zn) alloys: a DFT investigation

In this work, we systematically examined the structural stability, mechanical properties, and thermodynamic behavior of B2-type CuBe alloy, and compared the results with isostructural Cu-based alloys (CuAl and CuZn) by employing first-principles calculations in the pressure range of −22 to 100 GPa. This study revealed the stable existence of CuBe alloy at low-density expansion states (e.g. ∼ −20 GPa), indicating its superior structural stability compared to CuAl and CuZn. The pressure dependence of properties such as cell parameter a (a/a 0) and density ρ (ρ/ρ 0), elastic parameters (elastic constants C ij , bulk modulus B, shear modulus G, and Young's modulus E), deduced parameters (B/G ratio, Poisson's ratio ν, Vickers hardness, sound velocity, and Debye termperature ΘD), and thermodynamic parameters (free energy F, entropy S, and heat capacity C v) were investigated. All CuM (M = Be, Al, and Zn) alloys had more difficulty undergoing uniaxial stress than shear stress. External pressure reduced the ductility of the CuBe alloy, while excess pressure (P > 50 GPa) resulted in increased ductility, which was similar to CuAl but different from CuZn. The hardness and ΘD values demonstrated consistent variation corresponding to the ductility changes. Thermodynamic parameters were minimally affected by pressure, and the stronger interactions led to greater F in the CuBe alloy. These findings offer confidence for the future design of ordered Cu-Be alloys with exceptional properties.

Structural and electronic properties of Tl films on Ag(111): From (3×3) surface alloy to moiré superstructure

Surface alloys between the heavy metals Bi, Pb, or Sb and noble metals, such as Ag(111), are known for their giant Rashba splitting. Although thallium (Tl) should result in isostructural surface alloys, its structural and electronic properties remained elusive. The authors present here a detailed work on the structural and electronic properties of Tl films epitaxially grown on Ag(111) surfaces. By combining experimental LEED, AES, STM/STS, and IPE with $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ band structure and charge distribution calculations, the $s$,${p}_{z}$-derived surface state of the TlAg${}_{2}$ surface alloy is identified, with a weak, but finite, Rashba splitting.

Lanthanide-Based Coordination Polymers Molecular Alloys Stability: A Thermochemical Approach

In this study, we investigate the thermodynamics of lanthanide-based coordination polymer molecular alloys. We demonstrate that if lanthanide ions have many chemical similarities, the solubility of homo-lanthanide-based coordination polymers can vary significantly from one lanthanide ion to another. Indeed, we experimentally determine the solubility constants of a series of isostructural homo-lanthanide coordination polymers, with general chemical formula [Ln2(bdc)3(H2O)4]∞ with Ln = La-Er plus Y and where bdc2- symbolizes 1,4-benzene-di-carboxylate. Then, we extend the study to two series of isostructural molecular alloys with general chemical formula [Ln2xLn'2-2x(bdc)3(H2O)4]∞ with 0 ≤ x ≤ 1 based either on heavy ([Eu2xTb2-2x(bdc)3(H2O)4]∞) or light ( [Nd2xSm2-2x(bdc)3(H2O)4]∞) lanthanide ions. We found that whatever the solubility difference of the homo-nuclear compounds is, the configurational entropy is the main driving force of the stabilization of molecular alloys.

Van der Waals imprinting of black-phosphorus-like binary alloyed monolayers with tunable band gaps and moiré superstructures

Fabrication of isostructural isovalent alloys is a powerful approach to discovering novel materials with emergent properties. This approach has been well established in traditional bulk materials, yet remains to be fully exploited in two-dimensional systems. Here we use black- phosphorus (BP)-like group-VA binary alloyed monolayers on BP-like group IV-VI substrates to demonstrate the enabling power of isomorphism and isovalency in fabricating two-dimensional alloyed monolayers with tunable properties. We achieve the formation of nearly freestanding BP-like ${\mathrm{Sb}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x} (0\ensuremath{\le}x\ensuremath{\le}1)$ monolayers on the GeSe substrate, with a semiconductor-to-metal transition and tunable moir\'e superstructures as $x$ increases. In contrast, BP-like ${\mathrm{Pb}}_{x}{\mathrm{Bi}}_{1\ensuremath{-}x}$ monolayers can be stabilized only for low Pb concentrations, and the contrasting behaviors are attributable to preservation or breakdown of isovalency in the two cases. This study establishes a generic approach to growing alloyed monolayers via van der Waals imprinting, opening a new avenue for materials discoveries with desirable functionalities.

Phase transition behavior of heterostructural alloys: Effects of size mismatch and site preference

Unlike isostructural alloys, phase transitions of heterostructural alloys between two different lattice types involve more complexity and are less explored. In this work by analyzing the local environment around each atom, we study the distinct features of phase transitions in three prototypical alloy systems (Cd,Pb)Te, (Cd,Ge)Te and (Zn,Mg)O with different size and chemical mismatches. We show that for the (Zn,Mg)O alloy with small size mismatch and low site preference energy between the Zn and Mg compounds, the phase transition occurs abruptly from the zinc-blende to rock-salt structures, showing the feature of a first-order transition. On the contrary, many distorted local configurations unlike either tetrahedron or octahedron ones in the zinc-blende or rock-salt structures, respectively, exist near the phase transition in (Cd,Ge)Te and (Cd,Pb)Te alloys, which have low formation energies. This is attributed to the large size mismatch between CdTe and PbTe, and stronger preference of Cd occupying the tetrahedral sites, whereas Pb and Ge occupying the octahedral sites in (Cd,Pb)Te and (Cd,Ge)Te alloys. Consequently, the phase transitions in (Cd,Pb)Te and (Cd,Ge)Te alloys happen gradually in a wide composition range, showing the feature of a second-order transition. Our study, therefore, shows that phase transition behaviors of heterostructural alloys can be rather complex, either abrupt or gradual, largely depending on the size mismatch and site preference of the constituents. This provides deep understanding on the phase transition behavior of these types of heterostructural alloys.

Equilibrium phase diagrams of isostructural and heterostructural two-dimensional alloys from first principles

SummaryAlloying is a successful strategy for tuning the phases and properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs). To accelerate the synthesis of TMDC alloys, we present a method for generating temperature-composition equilibrium phase diagrams by combining first-principles total-energy calculations with thermodynamic solution models. This method is applied to three representative 2D TMDC alloys: an isostructural alloy, MoS2(1-x)Te2x, and two heterostructural alloys, Mo1-xWxTe2 and WS2(1-x)Te2x. Using density-functional theory and special quasi-random structures, we show that the mixing enthalpy of these binary alloys can be reliably represented using a sub-regular solution model fitted to the total energies of relatively few compositions. The cubic sub-regular solution model captures 3-body effects that are important in TMDC alloys. By comparing phase diagrams generated with this method to those calculated with previous methods, we demonstrate that this method can be used to rapidly design phase diagrams of TMDC alloys and related 2D materials.

Heterostructural phase diagram of Ga2O3–based solid solution with Al2O3

Ga2O3, which is emerging as semiconductor material due to the ultra-wide bandgap, has tunability in bandgap and lattice constant by alloying Al. However, successful control of alloying phase is still challenging due to its heterostructural nature and rich polymorphs. Here, we identified the thermodynamic phase diagram of heterostructural (AlxGa1-x)2O3 alloy. Using density-functional theory (DFT) calculations and regular solution model, we calculated the Gibbs-free energy of mixing of heterostructural polymorphs. Based on the calculation, we show the phase diagram of (AlxGa1-x)2O3 alloy system with a markedly increased metastability than the isostructural alloy, which can make a vast phase space for homogeneous single-phase alloys. We also investigated the correlation between the bandgap and lattice constant within these systems using hybrid DFT calculations, which can guide the device design of Ga2O3 power electronics.

Giant low field magnetocaloric effect near room temperature in isostructurally alloyed MnNiGe-FeCoGe systems

According to the strategy of isostructural alloying, the magnetostructural coupling was realized by doping appropriate stable hexagonal Ni2In-type FeCoGe alloy into MnNiGe systems, which results in the achievement of large magnetization difference and giant low field magnetic entropy change (ΔSm) in the isostructurally alloyed (MnNiGe)-(FeCoGe) systems. Giant ΔSm as much as −51.3 J∙kg-1K−1 at 296 K and −56.2 J∙kg-1K−1 at 275 K under 5 T magnetic field have been observed in (MnNiGe)0.91-(FeCoGe)0.09 and (MnNiGe)0.89-(FeCoGe)0.11 alloys, respectively. Moreover, the ΔSm of (MnNiGe)0.89-(FeCoGe)0.11 alloy are reached −25.6 J∙kg-1K−1 (2 T) and −43.6 J∙kg-1K−1(3 T), respectively. The observed values of -ΔSM under the magnetic field of 2 T and 3 T are larger than most of the reported MnMX(M = Co, Ni and X = Ge, Si) based systems.

Electronic and vibrational spectroscopy of miscible MgO–ZnO ternary alloys

The ordered structure of MgO-ZnO alloy system is a versatile tunable optical material promising for diverse optoelectronic applications. However, isovalent and isostructural alloy compositions of MgO-ZnO are generally unstable at ambient conditions. Using state-of-the-art \textit{ab initio} evolutionary simulations, we predict and study the properties of stable phases of MgO-ZnO. We establish the dynamical stability of the predicted crystal structures through the phonon and Raman spectroscopy. Detailed analyses of two of the most stable structures reveal highly tunable properties that could be explored for photonic and optical applications.

Windows open for highly tunable magnetostructural phase transitions

An attempt was made to tailor the magnetostructural transitions over a wide temperature range under the principle of isostructural alloying. A series of wide Curie-temperature windows (CTWs) with a maximal width of 377 K between 69 and 446 K were established in the Mn1−yCoyNiGe1−xSix system. Throughout the CTWs, the magnetic-field-induced metamagnetic behavior and giant magnetocaloric effects are obtained. The (Mn,Co)Ni(Ge,Si) system shows great potential as multifunctional phase-transition materials that work in a wide range covering liquid-nitrogen and above water-boiling temperatures. Moreover, general understanding of isostructural alloying and CTWs constructed in (Mn,Co)Ni(Ge,Si) as well as (Mn,Fe)Ni(Ge,Si) is provided.

Wide temperature window of magnetostructural transition achieved in Mn0.4Fe0.6NiSi1−xGax by a two-step isostructural alloying process

A new approach has been proposed in this work, which provides an effective way for tuning the structural transition in MM’X systems with relatively high transition temperature. With this method, a temperature window as wide as 275 K for the magnetostructural transition has been achieved in the MnNiSi alloy system. The maximum magnetic entropy change of the system is as high as 13.3 J/kgK, which, together with the large temperature window, enables the Mn0.4Fe0.6NiSi1−xGax system to be a promising candidate for magnetic refrigerant applications.

FeSn2 and CoSn2 Electrode Materials for Na-Ion Batteries

Alloys have the potential to fill the void for a reliable, high-performing anode material for sodium ion batteries. Here we investigate for the first time CoSn2 and FeSn2 and their reaction with sodium. These tin alloys can achieve capacities of up to 680 mAh/g if fully exploited, almost double of the standard anode material used today. We elucidate the reaction mechanism of these two isostructural alloys by analogy to the well understood Sn-Na system, and demonstrate that the transition metal believed to be inactive by the community in fact strongly influences the reaction mechanism.

Effects of hydrostatic pressure on magnetostructural transitions and magnetocaloric properties in (MnNiSi)1−x(FeCoGe)x

The isostructural alloying of two compounds with different magnetic and thermo-structural properties has resulted in a new system, (MnNiSi)1−x(FeCoGe)x, that exhibits large magnetocaloric effects with acute sensitivity to both compositional variation and applied hydrostatic pressure. The maximum isothermal entropy change reaches a value of −ΔSmax = 143.7 J/kg K for a field change of ΔB = 5 T at atmospheric pressure. The first-order magnetostructural transition responsible for the entropy change shifts to lower temperature with applied hydrostatic pressure (∼−10 K/kbar) but maintains a large value of −ΔSmax.

Giant magnetocaloric effect in isostructural MnNiGe-CoNiGe system by establishing a Curie-temperature window

An effective scheme of isostructural alloying was applied to establish a Curie-temperature window in isostructural MnNiGe-CoNiGe system. With the simultaneous accomplishment of decreasing structural-transition temperature and converting antiferromagnetic martensite to ferromagnetic state, a 200 K Curie-temperature window was established between Curie temperatures of austenite and martensite phases. In the window, a first-order magnetostructural transition between paramagnetic austenite and ferromagnetic martensite occurs with a sharp jump in magnetization, showing a magnetic entropy change as large as −40 J kg−1 K−1 in a 50 kOe field change. This giant magnetocaloric effect enables Mn1−xCoxNiGe to become a potential magnetic refrigerant.

Determination of the crystal structures of In70-Ni30 and In70-Pd30 using perturbed angular correlation

According to phase diagrams based on x-ray measurements, In70-Pt30 has the cubic Sn7Ir3 crystal structure (D8f, cI40) but the alloys In70-Ni30 and In70-Pd30 have been variously reported to have either a cubic gamma-brass (D81-3, cI52) or the Sn7Ir3 structures. In this study, hyperfine interaction measurements are applied as an alternate method to identify phases. Perturbed angular correlation (PAC) measurements were made of characteristic nuclear quadrupole interactions of 111In/Cd probe atoms, and demonstrated a common, characteristic "signature" of the Sn7Ir3 structure in all three alloys. The Sn7Ir3 structure has two inequivalent Sn-sites with a 3:4 ratio of atoms and point symmetries indicate that the electric-field gradients at both sites should be axially symmetric. Measured perturbation functions for all three alloys exhibited two axially symmetric quadrupole interaction signals having the expected 3:4 ratio of amplitudes, as expected for the structure. Furthermore, ratios of the two quadrupole interaction frequencies in each alloy were characteristically large, with frequencies for probe atoms on In(3) sites roughly five times greater than on In(4) sites. Taken together, these observations confirm that all three phases have the Sn7Ir3 structure. Quadrupole interaction frequencies are also reported for isostructural alloys of gallium with Pt, Pd and Ni. Negligible inhomogeneous broadening was observed in measurements near room temperature in all six phases, indicating excellent atomic ordering at the stoichiometric 70:30 compositions.

Search for quantum criticality in a ferromagnetic system UNi1−xCoxSi2

Polycrystalline samples of the isostructural alloys UNi${}_{1\ensuremath{-}x}$Co${}_{x}$Si${}_{2}$ ($0\ensuremath{\leqslant}x\ensuremath{\leqslant}1$) were studied by means of x-ray powder diffraction, magnetization, electrical resistivity, and specific-heat measurements, at temperatures down to 2 K and in magnetic fields up to 5 T. The experimental data revealed an evolution from strongly anisotropic ferromagnetism with pronounced Kondo effect, observed for the alloys with $x<0.98$ and being gradually suppressed with rising Co content, to spin-glass-like states with dominant spin fluctuations, seen for the sample with $x=0.98$. Extrapolation of the value of ${T}_{\mathrm{C}}(x)$ yields a critical concentration ${x}_{\mathrm{c}}=1$, at which the magnetic ordering entirely disappears. This finding is in line with preliminary data collected for stoichiometric UCoSi${}_{2}$.

Variational study of the magnetically induced metal-insulator transition in FeSi1−xGex

Recently, a minimal microscopic model for the magnetically induced metal-insulator transition in isostructural and isoelectronic alloys of FeSi${}_{1\ensuremath{-}x}$Ge${}_{x}$ has been introduced and solved within the mean-field approximation. Reasonable qualitative agreement with the experimental data has been found except for the type of transition between the paramagnetic insulator and the ferromagnetic metal. In this paper we argue that when correlation effects are taken into account, the model predicts a first-order metal-insulator transition in agreement with experiments.

Modular Inorganic Polyoxometalate Frameworks Showing Emergent Properties: Redox Alloys

Alloy, alloy: A pure cobalt-based Keggin network linked by W-O-Co interactions is “alloyed” with a pure manganese-based Keggin network, linked by W-O-Mn interactions. These isostructural alloys are solid solutions of the Co and Mn nets. The alloys are prepared by the stoichiometric mixing of the components of each discrete network, leading to a series of single-crystalline mixed transition-metal frameworks with novel redox properties.

On the electronic origin of the inverse magnetocaloric effect in Ni–Co–Mn–In Heusler alloys

In order to understand the electronic origin of the inverse magnetocaloric effect observed in a Ni–Co–Mn–In system we used a combination of indirect experimental probes as magnetization, resistivity and specific heat. The findings are compared with the band structure of isostructural Heusler alloys such as Ni–Mn–Ga. We suggest that the inverse magnetocaloric effect in Ni–Co–Mn–In originates from the high density of states close to Fermi energy. Within the austenite state this causes ferromagnetic band splitting. The structural change to the martensite allows an alternative way to reduce the high density of states at lower temperatures, which does not require band splitting and thus does not support ferromagnetic order.

Highly coercive L10-FePd thin films grown on MgO single crystal substrates

L10-ordered systems are of great interest from both the technological and research point of view since their high magnetocrystalline anisotropy makes them suitable candidates for future high density recording media. FePd and FePt are isostructural alloys that in their ordered phase show very different magnetic behaviour. In this work ultra-thin films of FePd and FePt films have been grown showing very hard magnetic properties. The coercivity of FePd films grown at 700°C reaches values well above 10 kOe much greater than that obtained in FePt films grown in the same conditions.