Figure 4. Electron localization function (ELF) in (110) plane for (a) Ti3AlB2, (b) Ti2ZrAlB2-1 and (c) Ti2ZrAlB2-2.
In Figure 4, the electron localization function in (110) plane for Ti3AlB2, Ti2ZrAlB2-1 and Ti2ZrAlB2-2 are given. It can be seen the electron localization function on Ti and B atoms are almost zero, which mean there is almost no charge. But on Zr atoms, the electron localization function is about 0.8, which means there is strong charge localization. This charge distribution shows that the ionic bonds between Ti and B are similar to covalent bonds to some extent. There is almost no local charge on the atoms, and the charges are distributed between two atoms, and the interaction between atoms by sharing electron pairs. While the chemical bonds between Zr and B are typical ionic bonds which formed by one party loses electrons and the other gets electrons, the charge distribution of the two electrons is obviously different. This difference further results in that the different binding strength of the chemical bond formed between Ti and B and between Zr and B. Another obvious difference is, compared with Ti3AlB2 and Ti2ZrAlB2-2, there is a small uncharged or almost no charge region between closest B-B atoms in Ti2ZrAlB2-1, which indicates that this structure is not so closely as the other two structures. This also leads to the fact that Ti2ZrAlB2-1 is easier to be compressed and deformed than the other two structures, so its bulk modulus and shear modulus are also lower than them, which is in good agreement with the results given in Table 3. This will further affect its thermal performance, which we will discuss in the Section 3.4.