Effect Of Valence Electron Concentration Research Articles

Crystal structure, electronic structure and phase stability of the Cu2-xMxCd (M=Zn, Ga, Ge, Sn) pseudo-binary Laves phases: effect of valence electron concentration

Starting from Cu2Cd, a series of compounds with the approximate formula Cu2-xMxCd (x ​∼ ​0.1–1) (M ​= ​Zn, Ga, Ge, Sn) were prepared by high temperature solid state synthesis. X-ray diffraction and energy dispersive X-ray spectroscopy were used for structure determination, phase analysis and compositional study. A correlation between the valence electron concentration (VEC) and phase stability was noticed in Cu2–xMxCd (M ​= ​Zn, Ga, Ge, Sn). A slight incorporation of M (M ​= ​Zn, Ga, Ge, Sn) for Cu into Cu2Cd (C14, MgZn2 type) leads to the formation of a solid solution of cubic γ-brass type Cu5Cd8 as a major phase. Further incorporation of M leads to the appearance of MgCu2 (C15) type cubic Laves phase as the major phase. The cubic Laves phases are stable within a narrow homogeneity range. At higher atomic percentage of M, the stable pseudo-binary cubic phase ceases to form and binary compounds start to appear. One ordered configuration from each of the four systems was constructed to calculate the density of states (DOS) and crystal orbital Hamilton population (COHP) in order to explain the stability and bonding characteristics of the pseudo-binary Laves phases applying first principles density functional theory. These phases are electronically stable and poor metallic in nature; where partially covalent hetero-atomic bonds have significant contribution towards the overall stability and bonding.

Effects of Cr/Fe Ratio on Structural Evolution and Tensile Properties of the Fe75.7−xCrxNi15.1Al4.6Ti4.6 High‐Entropy Alloys

According to the effect of valence electron concentration (VEC) on the phase evolution of high‐entropy alloys (HEAs), Fe75.7−xCrxNi15.1Al4.6Ti4.6 (at%, x = 17.1, 18.9, 20.6, 22.3, and 25.3) HEAs are designed. Effects of the Cr addition on the microstructure and tensile properties are investigated systemically. As the Cr/Fe ratio increases (i.e., decreasing VEC), there is a phase transition from face‐centered‐cubic (FCC) to body‐centered‐cubic (BCC) and Heusler‐type β′ ‐Ni2TiAl (L21). When x is between 17.1 and 22.3, the alloys, with the mixed structure of the FCC and BCC/L21 phases, behave like composites with a sharp increase in strength but reduced ductility. When x = 25.3, the alloy, with only BCC/L21 phases, becomes extremely brittle. Specifically, Fe56.8Cr18.9Ni15.1Al4.6Ti4.6 exhibits a good combination of a high strength of 1142 MPa and an elongation strain of 9.5%. The excellent tensile properties of this alloy are attributed to the synergetic effects of a soft FCC phase, a hard BCC phase, and the uniform distribution of high‐density L21 nanoparticles in BCC matrix. The phase‐evolution principle is delineated by parameters of the VEC and the atomic size difference (δ), and the strengthening mechanism is well discussed. These findings offer a guidance for the development of BCC/L21‐containing HEAs for practical applications.

Effects of Bi-doping on electronic structure and martensitic transformation in Ni2MnGa and Mn2NiGa MSMAs: A theoretical approach

The effects of Bi-doping on the electronic structure, martensitic transformation and magnetic properties of Heusler alloys Ni2MnGa1-xBix and Mn2NiGa1-xBix (x = 0, 0.25, 0.5, 0.75, 1) were investigated theoretically. The substitution of Bi for Ga has a negative effect on the martensitic transformation. The total energy difference ΔEM between the martensite and austenite decreases gradually with the doping of Bi. When x = 0.5, 0.75 and 1.0 in Ni2MnGa1-xBix and x = 1.0 in Mn2NiGa1-xBix, the martensite phase becomes higher in energy comparing with the austenite. Thus the doping of Bi can lower the martensitic transformation temperature TM and finally suppresses it. The possible reasons for this trend are related to the increasing valence electron concentration e/a and cell volume V with Bi-doping. The competition between them may be ascribed further by the electron density, which combines the effect of valence electron concentration and size effect. The electron densities of Ni2MnGa1-xBix and Mn2NiGa1-xBix decrease monotonously with increasing Bi content. This agrees well with the variation of ΔEM or TM. The doping of Bi does not change the magnetic structure of Ni2MnGa1-xBix and Mn2NiGa1-xBix. The former is ferromagnetic and the latter is ferrimagnetic in both martensite and austenite state. These results can help to introduce new component like Bi to Heusler alloys and discover new functional materials in them.

Effect of Valence Electron Concentration in High Entropy Shape Memory Alloys/ Review

The shape memory effect is the ability to recover the shape after deformation on heating or stress. High entropy alloys were found by mixing an equiatomic number of complex components; nowadays, a new generation of un equiatomic components are produced. Properties like high strength and slow diffusion are properties of HEAs, and these properties are perfect circumstances for the appearance of the martensitic diffusionless transformation. Combining these two properties of alloy in a new class called high entropy shape memory alloys (HESMAs) opens a door for many new applications in terms of shape recovery and deformation resistance. This paper reviews the combination of (HE and SMA) properties in an alloy, VEC was found for all the different compositions of HESMAs, and the focus is to see how VEC affects this new family of alloys. Configurational entropy of the compositions was all calculated, and the values are mostly greater than 1.5R.

ChemInform Abstract: Influence of Valence Electron Concentration on Laves Phases: Structures and Phase Stability of Pseudo‐Binary MgZn2‐xPdx.

AbstractThe series of title compounds (0.15 ≤ x ≤ 1.0) is synthesized from the elements (Ta tubes, Ar, 750 °C, 18 h, followed by annealing at 400 °C for 5 d, 90—95% yields) and characterized by single crystal XRD and TB‐LMTO‐ASA electronic structure calculations to analyze the effect of valence electron concentration on MgZn2.

Effect of Valence Electron Concentration on Elastic Properties of 4d Transition Metal Carbides MC (M = Y, Zr, Nb, and Rh)

The electronic structure and elastic properties of the 4d transition metal carbides MC (M = Y, Zr, Nb, Rh) were studied by means of extended Huckel tight-binding band electronic structure calculations. As the valence electron population of M increases, the bulk modulus of the MC compounds in the rocksalt structure does not increase monotonically. The dominant covalent bonding in these compounds is found to be M-C bonding, which mainly arises from the interaction between M 4d and C 2p orbitals. The bonding characteristics between M and C atoms affecting the variation of the bulk modulus can be understood on the basis of their electronic structure. The increasing bulk modulus from YC to NbC is associated with stronger interactions between M 4d and C 2p orbitals and the successive filling of M 4d-C 2p bonding states. The decreased bulk modulus for RhC is related to the partial occupation of Rh-C antibonding states. clarified in these studies are useful in interpreting their physical properties. The common viewpoint is that covalent bonds can be formed between both metal-carbon and metal-metal pairs of atoms. At the same time, a transfer of electrons from M to C takes place, implying a certain degree of ionic bonding. We stress the importance of covalent metal-carbon bonds in explaining the stability and observed mechanical properties of these compounds. These pro- perties are understood to arise in large part from strong covalent bonding between transition metal d and carbon p orbitals. In this paper, we present the results of the electronic structures calculated for the carbides of 4d transition metals Y, Zr, Nb, and Rh. The presence of covalent M-C bonds and metallic M-M bonds is confirmed and analyzed in detail. In addition, the number of valence d electrons in the metals induces changes in bonding and elasticity in these compounds. It is, therefore, our goal to understand the relationship between the M-C bond strength and elastic properties of 4d transition metal carbides (MC) in the rocksalt structure as a function of the valence electron concentration (VEC) per atom, which is the total number of valence electrons in the structure divided by the number of atoms in a unit cell. As the VEC increases, the bulk modulus (B) increases from YC to NbC, and then decreases for RhC. This variation of B with the number of metal d electrons can be explained as arising from the orbital mixing between metal and carbon and the successive filling of the consequent bonding and antibonding M 4d-C 2p states. The focus is on the trends in the electronic structure and the nature of bonding, which are essential for the understanding of the elastic properties of the transition metal carbides. Our approach is based on a thorough complementary analysis of the density of states (DOS) and crystal orbital overlap population (COOP) to gain more profound insight into the bonding of this class of materials and get more detailed information on the bonding character than previous- ly achieved. The COOP curves are indicative of the nature and strength of the bonding between a given pair of atoms, with positive and negative regions indicating bonding and antibonding states, respectively. The chemical bonding of M-C and M-M is quantitatively discussed by use of the bond overlap population based on supercell band structure calcu- lations.

Crystal Structures and Stabilities of γ‐ and γ′‐Brass Phases in Pd2–xAuxZn11 (x = 0.2–0.8): Vacancies vs. Valence Electron Concentration

AbstractTo probe the effect of valence electron concentration (vec or e–/atom) on γ‐Pd2+xZn11–x phases a series of compoundsPd2–xAuxZn11 (x = 0.3–0.8; e–/atom = 1.70–1.75) was synthesized and structurally characterized. The gold‐substituted γ‐brass type phase was observed for x ≤ 0.3, having a refined composition of Pd1.93Au0.27(1)Zn10.80(1) (space group I$\bar{4}$3m; a = 9.0953 (2) Å). Further addition of gold (0.4 ≤ x ≤ 0.8) leads to a 2 × 2 × 2 superstructure of theγ‐brass type phase (denoted as the γ′‐phase), with noticeable phase width (F$\bar{4}$3m; 18.1827(4)–18.1799(4) Å). The γ′‐phase consists of four independent 26‐atoms clusters, which are arranged around four distinct high symmetry points. The structural stability and phase width ofγ′‐phase are primarily controlled by two of these clusters, both of which show mixed occupancies and occurrence of non‐stoichiometric vacancies. Amongst all the observed γ′‐phases in the Pd‐Au‐Zn system, the vacancy concentration increases with increasing values of gold substitution. The vec of all observed γ′‐phases fall between 1.636 and 1.654 e–/atom values, which are greatly influenced by the nonstoichiometric vacancies present in the structure.

Effect of valence electron concentration on stability of fcc or bcc phase in high entropy alloys

Phase stability is an important topic for high entropy alloys (HEAs), but the understanding to it is very limited. The capability to predict phase stability from fundamental properties of constituent elements would benefit the alloy design greatly. The relationship between phase stability and physicochemical/thermodynamic properties of alloying components in HEAs was studied systematically. The mixing enthalpy is found to be the key factor controlling the formation of solid solutions or compounds. The stability of fcc and bcc solid solutions is well delineated by the valance electron concentration (VEC). The revealing of the effect of the VEC on the phase stability is vitally important for alloy design and for controlling the mechanical behavior of HEAs.

Phase stability and elastic properties ofXMgB14studied byab initiocalculations(X=Al,Ge,Si,C,Mg,Sc,Ti,V,Zr,Nb,Ta,Hf)

The effect of valence electron concentration (VEC) and size of the $X$ element in $X{\text{MgB}}_{14}$ (space group $Imma$ $X=\text{Al}$, C, Si, Ge, Mg, Sc, Ti, V, Zr, Hf, Nb, Ta) on stability and elastic properties was studied using ab initio calculations. Generally, icosahedral bonds, present in these compounds, are electron deficient. Based on the Bader charge analysis [Bader, Atoms in Molecules: A Quantum Theory (Oxford University Press, New York, 1990)] and density of states, $X$ elements and Mg are shown here to transfer electrons to the boron network. Hence, the stability of the compounds studied increases as more electrons are transferred. As the VEC of the $X$ element increases, fewer electrons are transferred to the boron network, and therefore the phase stability decreases. The bulk moduli of all compounds are in the range from 205 to 220 GPa. This can be understood analyzing the cohesive energy thereof. As the bulk modulus increases, the cohesive energy decreases.

A covalent view of chemical bonding in Laves phases CaLi xAl 2 − x

The series of compounds CaLi xAl 2 − x was synthesized and structurally characterized in order to probe the effect of valence electron concentration on both global and local structural features in main group intermetallics. CaLi x Al 2 − x compounds adopt Laves phase structures and show the following trend: (i) x = 0, MgCu 2 type; (ii) 0.25 < x < 0.75, MgNi 2 type; (iii) 0.80 < x < 1.0, MgZn 2 type; (iv) 1.0 < x < 2.0, two-phase region; (v) x = 2, MgZn 2 type. Extended Hückel calculations are used to examine local chemical bonding effects and to rationalize the observed bond distance trends and arrangement of atomic constituents in this system.