Discovery of hexagonal ternary phase Ti2InB2 and its evolution to layered boride TiB

Mn+1AXn phases are a large family of compounds that have been limited, so far, to carbides and nitrides. 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\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar 6$$\end{document}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.


Synthesis of Ti2InB2:
Since indium has a much lower boiling point (2072 o C) than that of Ti (2836 o C) and B (3927 o C), the compound cannot be grown from the melt. The synthesis using arc melting (under 600mbar extra pure Ar, purity >99.99995%) was failed, because In evaporate seriously before reaction. Therefore Ti2InB2 was prepared by using a solid-state reaction route in the present study. Still, the yield was very small, because Ti2InB2 competed with other impurity phases (mainly TiB2 and Ti-In phases). In order to improve the yield, many experiment conditions: temperature effect, crucible effect, atmosphere effect, annealing effect and the initial composition ratio effect, were considered.
( Supplementary Fig. 12-17 and Table 3) The growth of Ti2InB2 is very sensitive to temperature, only intermediate temperatures (1100 o C to 1200 o C) work well for synthesis ( Supplementary Fig. 12). Above 1200 o C, the target phase becomes thermodynamically less stable than other phases, while below 1000 o C, the growth was rather slow.
Sample container also has a big influence on the synthesis (Supplementary Fig. 13 and Table 3). In a sealed stainless steel (SUS) tube, indium did not tend to react with other elements, and a lot of TiB2 was produced. A sealed quartz tube with Mo foil covering the sample worked much better. The reaction atmosphere was also taken into account (Supplementary Fig. 14 and Table 3). When the tube was sealed with Ar gas inside, indium tended to react with the other elements and the formation of TiB2 was suppressed; however, if the tube was sealed under vacuum, indium evaporated and covered the inner wall of the tube, leading to an indium loss and a high yield of the impurity phase TiB2. The influence of the reaction atmosphere is mainly ascribed to the low triple point of indium (157 o C and 1 kPa), therefore, in order to suppress the vapor loss of indium, a sealed environment was chosen rather than an open environment with Ar gas flow. Annealing was not effective to improve the growth of Ti2InB2, but on the contrary, the yield of Ti2InB2 was gradually decreasing under annealing at different conditions (Supplementary Table 3). The influence of initial composition ratio of Ti, In and B was considered at the end. By increasing the amount of only Ti by 15%, the yield of Ti2InB2 was much improved and the yield of TiB2 was largely decreased. However a further increase of Ti to 30% dramatically suppressed the growth of Ti2InB2. A similar behavior was observed when the initial amount of In was adjusted, and the results were summarized in the Supplementary Table 3.
Surprisingly, when excess amount of both In and Ti were added at the initial step, the yield of TiB2 monotonously decreased as the excess amount increased, and yield of Ti2InB2 reached a maximum value (Supplementary Fig. 15(a)). Finally, the obtained samples were washed by a diluted hydrochloric acid, and the impurity phases except TiB2 were removed.
The open circuit voltages (OCVs) for an intercalation reaction involving Li + and Na + ions can then be estimated from the energy difference of charge/discharge process as following:    [001] [110] [001] [010] [001] [001] [010] P6 ത m2 Cmcm The reaction process can be described as the equation as below:

Supplementary
Ti 2 InB 2 → 2TiB + In (vap. ) The obtained product TiB was puce color, different from dark grey of starting materials Ti2InB2.

EDS (upper) and XRD (lower) spectra in blue rectangle indicates that the formed metal species in the
inner wall of the quartz tube during the evacuation process was In metal. were observed in the EDS pattern, it is reasonable to conclude that all of the In was extracted during the dealloying process. The carbon signals in EDS results mainly originated from the carbon tape that we used to attach the powder sample onto the sample holder (copper stage). And the oxygen species should be the adsorbed oxygen molecule from the air because the TiB sample was exposed and stored in the air. can not be obtained due to the harsh reaction condition. Instead, another stable layered TiB phase with orthorhombic group (Cmcm) was generated. At the same time, tiny TiB phase with another orthorhombic group (Pnma) was also generated (Fig. S25).

Supplementary
Our calculations (Table S1) show that the stability of layered TiB phase with orthorhombic symmetry (Cmcm) and another orthorhombic TiB structure (Pnma) are comparable and is slightly more stable than the structure with hexagonal group (P6 ത 2). It was intuitive to expect that the removal of In atoms from Ti2InB2 could directly produce hexagonal TiB (Fig. S8). However, local displacements of Ti and B atoms motivated by the high temperature led to a generation of orthorhombic (Cmcm) phase with layered structure as shown in Fig. S10. Therefore, we consider the obtained orthorhombic phase was originated from a phase change of hexagonal (P6 ത 2) with the similar layered structure. The layered structure of TiB is kept after the hexagonal-to-orthorhombic phase change. Moreover, the difference of the calculated energies of the two TiB structures with orthorhombic symmetry Cmcm and Pnma are quite small (Fig. S25 and Table S1). This means that these two orthorhombic structures possess similar thermodynamic stabilities. However, a significant energy barrier can be expected for the phase change from layered structures (P6 ത m2 and Cmcm) to 3D Pnma structure. Therefore, the formation of TiB with Pnma symmetry is not dynamically favored. Consequently, the enlarged XRD pattern ( Figure S21  Annealing effect, e: the annealing was made based on 2.1; f: the annealing was made based on 1.1; g,h,i: the annealing was made based on 5, 5.1, and 5.2, respectively. Before annealing, the products were crushed and mixed again for homogeneity; j: The specified condition is, "QM, 1100 o C, 36h, Ar".