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.