cKey Laboratory of State Ethnic Affairs Commission for Electronic and Information Engineering, Southwest Minzu University, Chengdu 610041, China
Abstract: We have designed Ti3AlB2 and two new layered ordered double-transition metals MAX compound Ti2ZrAlB2 based on the structure of Ti3AlC2. By first-principles calculations with density functional theory, their structure, phase stability, elastic properties, electronic properties and thermal properties have been further investigated. Results show that they are all energetic, thermodynamically and mechanically stable. The bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio and Debye temperature were computed to discuss their elastic and thermal properties. Results show that they are all good ductile materials with high melting points. Density of states and electron localization function of these three phases were presented to research the chemical bonds and explore the reason limiting their melting points.
Keywords: first-principles calculations, ordered double-transition metals MAX, phase stability, thermal properties
Introduction
Since Nowotny et al synthesized the first member of Mn+1AXn (MAX) compounds–Ti3SiC2 in 1965 [1]. Hundreds of new carbides of this class were synthesized in the past several decades [2-7]. As a new class of ceramic compounds, Mn+1AXn crystallizes in the form of with layered structures, where n varies from 1 to 3, M is an early transition metal, A is primarily an element in group IIIA or IVA, X is the element of C, B or N. This new class of ceramic materials usually have the advantages of both metal and traditional ceramic materials, such as good conductivity, thermal conductivity and easy processing similar to metal, high temperature resistance, light weight, creep resistance and fatigue resistance similar to traditional ceramic materials. These excellent characteristics make them as potential candidates in a wide range of technological applications [8-10]. In addition, based on the structure of MAX compounds, a new class of 2D materials–MXenes, can be gotten by selectively etching the A-layers and leaving the Mn+1Xn layers intact [11-13]. This will greatly increase the variety of 2D materials. What’s more, theoretical and experimental studies show that the performance and stability of double transition metal MXenes are better than that of single transition metal MXenes. However, the double transition metal MXenes should be obtained by chemical etching the corresponding double transition metal MAX. For example, Rose et al [14] obtained a set of novel MXenes–Mo1.33C from their independently synthesized (Mo2/3Sc1/3)2AlC structure (Mo and Sc are chemically ordered in the same plane) by selective etching of the Al and Sc elements. Anasori et al[15] have obtained 2D MXenes structures Mo2TiC2Tx, Mo2Ti2C3Txand Cr2TiC2Tx based on the parent ordered double transition metal MAX phases.
In view of its application prospect in the industrial field and the design requirements of two-dimensional double transition metal MXenes, the design of double transition metal MAX structures has an important scientific significance. Now many double transition metal MAX [16-17], such as (V0.5Cr0.5)n+1AlCn(n=2, 3) and (Ti0.5Nb0.5)5AlC4, have been successfully synthesized by introducing another solid solution metal into the layered ceramic compounds in the laboratory. Liu et al [18-19] also reported the crystal structure of two newly ordered double transition metal MAX phases, Cr2TiAlC2 and Cr5/2Ti3/2AlC3, by various techniques. By mixing and heating different elemental powder mixtures method, Anasori et al [20] has successfully obtained two newly double ordered transition metal MAX phases Mo2TiAlC2 and Mo2Ti2AlC3. And more and more chemical ordered double transition metal MAX structures have been synthesized in the laboratory [21]. However, both theoretical and experimental studies focus on MAX (X=C, N), but there are few studies on MAX (X=B) [22], and no report on the double transition metal MAB. Ti3AlC2 is the most typical representative of MAX phase, and the research on this phase is also the most. Therefore, we design the aimed structures on the basis of Ti3AlC2 structure.
As an effective way to predict condensed material structures and properties, first-principles calculations can reproduce experimental data at a high accuracy. So this method has been widely used in the study field of material science [23-26]. In this study, based on the structure of Ti3AlC2, we predicted Ti3AlB2 and two Ti2ZrAlB2 double transition metal MAX phases and further investigate the phase stability and potential properties of them by first-principle calculations.