Zhimei Sun

Structure and bulk modulus of M2AlC (M=Ti, V, and Cr)

We have performed theoretical studies of the bulk modulus of M2AlC, where M=Ti, V, Cr by means of ab initio total energy calculations using the projector augmented wave methods. Our estimated equilibrium volume and the lattice parameters (c/a) agree well (within ±2% and ±0.06%, respectively) with experimental data. The bulk modulus of M2AlC increases as Ti is substituted with V and Cr by 19% and 36%, respectively. This can be understood since the substitution of Ti by V and Cr is associated with an extensive increase in the M–Al and M–C bond energy.

Electronic origin of shearing in M2AC (M = Ti,V,Cr,A = Al,Ga)

We have studied shearing in M2AC (space group P63/mmc, prototype Cr2AlC), where M is Ti, V and Cr, and A is Al and Ga, using ab initio calculations. These compounds can be described as interleaved layers of MC and A. As Ti in Ti2AlC is substituted by V, c44 increases by 24.3%. Increasing the transition metal valence electron concentration further, through a substitution of V by Cr, results in 2.2% decrease of c44. The electronic origin of this c44 versus valence electron concentration dependence may be understood by analysing the decomposed band structure: in the vicinity of the Fermi level, we find two types of dd bonding. One contributes to shearing (t2g+eg symmetry or MC–MC coupling) and the other does not (eg symmetry). We provide evidence that filling of the transition metal dd bonding states with the t2g+eg symmetry may be responsible for the behaviour of c44. The results presented enable tailoring of the shear properties of M2AC phases.

Ab initio study of M2AlN (M = Ti,V,Cr)

We have studied M2AlN phases, where M = Ti, V, and Cr ,b y means of ab initio tota le nergy calculations. The bulk modulus of M2AlN increases as Ti is replaced with V and Cr by 19.0% and 26.5%, respectively, which can be understood on the basis of the increased number of valence electrons filling the p–d hybridized bonding states. The bulk modulus of M2AlN is generally higher than that of the corresponding M2AlC phase, which may be explained by an extr ae lectron in the former phases contributing to stronger chemical bonding. This work is important for fundamental understanding of elastic properties of these ternary nitrides and may inspire future experimental research. M2AX phases (space group P63/mmc ,p roto type Cr2AlC), where M is an early transition metal, A is a group IIIA or IVA element, and X is either C or N, have attracted great attention due to their unusual properties associated with metals and ceramics (for details see [1] and the references cited therein). While there are approximately 50 experimentally known M2AX phases, only a very limited number of M2AN phases have been synthesized. In our previous paper [2], we have investigated the effect of valence electron population on bonding and elastic properties of M2AlC phases, where M = Ti, V, and Cr ,b y means of ab initio total energy calculations. The bulk modulus of M2AlC increases as Ti is replaced with V and Cr by 19% and 36%, respectively, which is associated with an extensive increase in the M–Al and M–C bond energy [2]. In this letter, usingabinitio total energy calculations, we systematically study M2AlN phases (M = Ti, V, Cr). Ti2AlN has already been synthesized [1], while the other two phases have not been experimentally investigated. The main aim of this letter is to correlate the chemical bonding and elastic properties of M2AlN phases as a function of valence electron population in order to gain insight into this fascinating class of materials.

Effect of the valence electron concentration on the bulk modulus and chemical bonding in Ta2AC and Zr2AC (A=Al, Si, and P)

We have studied the effect of the valence electron concentration, on the bulk modulus and the chemical bonding in Ta2AC and Zr2AC (A=Al, Si, and P) by means of ab initio calculations. Our equilibrium volume and the hexagonal ratio (c∕a) agree well (within 2.7% and 1.2%, respectively) with previously published experimental data for Ta2AlC. The bulk moduli of both Ta2AC and Zr2AC increase as Al is substituted with Si and P by 13.1% and 20.1%, respectively. This can be understood since the substitution is associated with an increased valence electron concentration, resulting in band filling and an extensive increase in cohesion.

Ab initio study of the Cr2AlC(0001) surface

Using an ab initio total energy method, we have calculated the surface energy and surface stress of Cr2AlC (0001) with the configuration of the top layer as Al [(0001)(Al)], Cr [(0001)(Cr)], and C ...

Ab initio study of basal slip in Nb2AlC

Using ab initio calculations, we have studied shearing in Nb(2)AlC, where NbC and Al layers are interleaved. The stress-strain analysis of this deformation mode reveals Nb-Al bond breaking, while the Nb-C bond length decreases by 4.1%. Furthermore, there is no evidence for phase transformation during deformation. This is consistent with basal slip and may be understood on the basis of the electronic structure: bands below the Fermi level are responsible for the dd bonding between NbC basal planes and only a single band with a weak dd interaction is not resistant to shearing, while all other bands are unaffected. The Al-Nb bonding character can be described as mainly metallic with weak covalent-ionic contributions. Our study demonstrates that Al layers move with relative ease under shear strain. Phase conservation upon shearing is unusual for carbides and may be due to the layered nature of the phase studied. Here, we describe the electronic origin of basal slip in Nb(2)AlC, the atomic mechanism which enables reversible plasticity in this class of materials.

Electronic structure of M2AlC(0001) surfaces (M = Ti,V,Cr)

We have studied the correlation between the valence electron configuration and the electronic structure of M2AlC(0001) surfaces (M = Ti, V, Cr) using ab initio calculations. Based on our surface energy data, we find that the Al termination is the most stable configuration. As the M valence electron population is increased, the surface energy increases. This can be understood by analysing the valence electron concentration induced changes of the electronic structure. Antibonding states are present as Ti is substituted by Cr in M2AlC(0001). These results are of relevance for vapour phase condensation as well as oxidation.

Theoretical investigation of the bonding and solubility in Nb2−xWxAlC

We have performed theoretical studies of the solubility within Nb2−xWxAlC by means of ab initio total energy calculations. If x is increased from 0 to 2 the bulk modulus can be increased by as much as 31%. The bulk modulus deviates from Vegards rule, which may be understood based on substitution-induced changes in the CNb2−xWx bond angle resulting in flattening of the Nb2−x WxC layers upon solid solution formation. This rather extensive increase in the bulk modulus can be understood by considering the changes caused by the substitution of Nb through W for the equilibrium volume and chemical bonding. Based on the energy of formation analysis we suggest that the investigated system shows complete solubility. The bond length calculations suggest that both the Nb–C bond length as well as the W–C bond length are not significantly affected by variations in x. Based on a comparison to other solid solutions we suggest that this anomaly may be specific to the family of nanolaminates investigated here.

Ab initio investigation on the phase stability of Ti3SiC2, Ti3Si0.5Ge0.5C2 and

Phase stability of Ti 3SiC 2, Ti 3Si 0.5Ge 0.5C 2 and Ti 3GeC 2 under high hydrostatic pressures has been studied by ab initio calculations. The present results show that every phase undergoes a phase transformation from α- to β-phase at high hydrostatic pressures. The transition pressures for the three phases were calculated to be 380, 384 and 369 GPa by generalized gradient approximation, respectively; whereas according to the local density approximation results, they were 397, 412 and 400 GPa, respectively. The α- to β-phase transformation is generally accompanied by an increase of ∼0.8% in volume and an increased c/a ratio. Furthermore, the α- to β-phase transition also results in a reduction in the bulk modulus by<4%.

Ab initio study of the chemical bonding and mechanical properties of Li2SiZn

Using ab initio calculations, we have investigated the chemical bonding and elastic properties of Li2SiZn (space groups P63∕mmc and P3¯m1). Both structures have been observed experimentally. These compounds exhibit a layered structure where Li–Zn layers are interleaved with Si layers. The most dominant chemical bonding is ionic, and the bulk moduli calculated are 42 and 22 GPa, respectively. While the layered nature thereof is similar to the so-called MAX phases (where M=transition metal, A=group element, and X=C or N) [M. W. Barsoum, Prog. Solid State Chem. 28, 201 (2000)], the elastic properties are rather different. This can be understood by analyzing the differences in chemical bonding between Li2SiZn and MAX phases. It is our ambition that these calculations may inspire future research on the Li2SiZn phases.