1-19. Discovery of Boron-based MAX Phase by High-throughput Calculations

Junjie Wang1, 2, *, Nanxi Miao1, 2, Artem R. Oganov1, 2, 3

1. State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, People's Republic of China.
2. International Center for Materials Discovery, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, People's Republic of China.
3. Skolkovo Institute of Science and Technology, Skolkovo Innovation Center, 3 Nobel St., Moscow 143026, Russia.

Abstract:Mn+1AXn (MAX) phases are a class of unique materials that exhibit a combination of ceramic and metallic properties, and a mixture of covalent and metallic bonding. Therefore, MAX phases possess the features of elastically stiff, strong, and heat-tolerant ceramics, although their electrical and heat conductivities drop linearly with increasing temperature, as with a metal. For the reported MAX phases, M represents an early transition metal, A is generally a metal element in group 13 or 14, while X is limited to C or N. Here we report the prediction of a compound, Ti2InB2, a stable boron-based ternary phase in the Ti-In-B system, using a computational structure search strategy. This predicted Ti2InB2 compound is successfully synthesized using a solid-state reaction route and its space group is confirmed as P 6 m2 (No. 187), which is in fact a hexagonal subgroup of P63/mmc (No. 194), the symmetry group of conventional Mn+1AXn phases. Moreover, a strategy for the synthesis of MXenes from Mn+1AXn phases is applied, and a layered boride, TiB, is obtained by the removal of the indium layer through dealloying of the parent Ti2InB2 at high temperature under a high vacuum. We theoretically demonstrate that the TiB single layer exhibits superior potential as an anode material for Li/Na ion batteries than conventional carbide MXenes such as Ti3C2. (Nature Communications, 2019, 10, 2284).

基于高通量计算的硼基 MAX 相发现

王俊杰*, 苗楠茜，Artem R. Oganov 

西北工业大学凝固技术国家重点实验室，材料基因组国际合作研究中心 

摘要:本文综合运用高通量结构搜索、密度泛函理论计算与实验验证相结合的方法。Ti2InB2 由 Ti，In 和 B 粉末使用固态反应途径合成，层状 TiB 由 Ti2InB2 脱合金化制备而得。右图(a)展示了 搜索得到热动力学稳定 TixAyBz 化合物的过程。作者在前期的准备工作中，首先用二元变分法确 定了 TixBz 的稳定性。在三元初始化结构搜索中，作者首先使用二元变成分高通量结构搜索，将 TixBz 和块体 A 代替 Ti, B, A 作为起止搜索产物。当一个三元硼化物在初始化结构搜索中被发现 是热力学稳定的，紧接着对它展开三元变成分搜索，以确保该三元硼化物的全局稳定性。对于 搜索得到的全局稳定的三元硼化物，将展开高精度的理论计算和进一步的实验验证探究。上图 (b)展示了高精度结构优化以后 Ti2InB2 的晶体结构。研究发现 Ti2InB2 化合物具有 MAX 相的典 型特征:空间群属于 P m2 的层状六方结构，两个 M (Ti)层和一个 A (In)层以 A-B-A 的次序构 成六方密排堆积。Ti2InB2 中 B/Ti(1.0)比例高于常规 MAX 相中 X/M(1/2,2/3 或 3/4)的比例, 硼原 子占据在两个 M 层之间的中心位置形成一层类石墨烯的 B 层，而非 MAB 相中常见的面内等边三角形。

      在计算预测指导下，通过调整实验参数实现了 Ti2InB2 的固相反应合成(上图(c)所示)。在 HCl 酸蚀刻后，仅剩余 Ti2InB2(93.7wt%)和少量 TiB2(6.3wt%)，结晶所得的 Ti2InB2 晶体结 构符合预期，属于 P6m̅ 2 空间群。图(c)扫描电镜和透射电微镜分析进一步证实了 Ti2InB2 是具有 层状晶体结构的材料，这与理论预测结论一致。 HAADF 图像的快速傅里叶变换(FFT)也表 明 Ti2InB2 以六方结构结晶。为了得到层状 TiB，采用高温真空脱合金方法来剥离 In 层，得到如 上图(d)所示具正交结构(Cmcm)的 TiB 化合物。如上图(d)扫描电镜显示，提取 In 之后所获得了 摩尔比约为 1:1 的层状结构 TiB。透射电子显微镜图像观察到堆叠的 Ti 原子以及不同表面的晶 面间距和理论预测结果一致。此外，计算的自由能显示，在 0K 至 2000K 的整个温度范围内， Pnma TiB 总是比 Cmcm 相更稳定，尽管能量差非常小(约 0.005~0.01kJ / mol)。这意味着由于 热力学驱动力较弱，从 Cmcm 向 Pnma 过渡的可能性很小，这解释了为什么亚稳态 Cmcm 相是 1050°C热处理下的主要产物。
关键词:高通量计算;硼基 MAX 相;结构预测;密度泛函计算