Bonding nature of metal/oxide incoherent interfaces by first-principles calculations

A bonding mechanism of large-mismatched metal/oxide heterointerfaces, classified as incoherent interfaces, is investigated by first-principles calculations. As a model system, incoherent $\mathrm{Ni}∕\mathrm{Zr}{\mathrm{O}}_2\left(111\right)$ interfaces are selected, and the interfacial bonding characters and their relevance to the interface strength are analyzed. It is found that the chemical bonds of the interfacial atomic pairs are strongly dependent on the atomic configurations in the interface structures, and show a site-dependent character from ionic through covalent/metallic bonding. Thus, even in the presence of a large misfit, stable interfaces can be formed by an effective chemical bonding transition along the interfaces. First-principles tensile tests show that such a bonding multiplicity strongly affects the atomic-scale fracture behavior and ideal mechanical strength of the interfaces.

Figure 1

Atomic arrangement of the interface supercells, projected on the interface plane. (a) the translation state of $T_0$ and (b) of $T_1$. In this figure, one Ni layer and two $\mathrm{Zr}{\mathrm{O}}_2$ layers adjacent to the interface are only shown. The arrow in (a) indicates a minimum translation vector of $\mathbf{t}=a_{\mathrm{Zr}\mathrm{O}2}∕3\left[\overline{2}11\right]$. In each model, Ni atoms located at equivalent sites on the $\mathrm{Zr}{\mathrm{O}}_2$ surface are assigned by using alphabetical characters from “$a$” to “$g$.”

Figure 2

Excess energies of (a) $\mathrm{Zr}{\mathrm{O}}_2$ (111) surfaces and (b) $\mathrm{Ni}∕\mathrm{Zr}{\mathrm{O}}_2$ (111) interfaces as a function of oxygen chemical potential $\left({\mu }_{\mathrm{O}}\right)$. Vertical thin lines indicate ${\mu }_{\mathrm{O}}$ values at the oxidation and reduction limits. In the plot of (b), solid lines denote energies for the $T_0$-type Zr or O terminated interfaces, while broken lines indicate results for the $T_1$-type interfaces (also see Fig. 1).

Figure 3

Relaxed interface structures for Zr terminated and O terminated interfaces, viewed along the $\left[\overline{1}01\right]$ direction. (a) The Zr terminated interface in the $T_0$ translation state, and (b) the one in $T_1$. In the similar way, the results for the $T_0$ and $T_1$ translation states for the O terminated case are drawn in (c) and (d), respectively. Works of separation $W_{\mathrm{sep}}$ (unit: $\mathrm{J}∕{\mathrm{m}}^2$) are shown in the parentheses below the respective structures.

Figure 4

Atom-projected DOS profiles for (a) the Zr terminated and (b) O terminated interface having the $T_0$-type structure. In these plots, DOSs for atoms around the centers of Ni and $\mathrm{Zr}{\mathrm{O}}_2$ slabs (denoted as “bulk”), and those for interfacial Ni, Zr, and O in the on-top configuration are displayed. The Fermi level of each supercell is set at $0\mathrm{eV}$.

Figure 5

DOS profiles for Ni atoms at different interfacial sites in (a) the Zr terminated and (b) the O terminated interface having the $T_0$-type structure. Dotted curves overlaid on each figure correspond to DOS profiles of Ni at the center of the Ni slab. The Fermi level of each supercell is set at $0\mathrm{eV}$.

Figure 6

ED difference maps for the $T_0$ structures of the Zr and O terminated interfaces. (a) and (c) are the plots on the $\left(01\overline{1}\right)$ plane, while (b) and (d) are those on the $\left(\overline{2}11\right)$ plane. The contour lines are drawn from $-0.12$ to 0.12 with an interval of 0.01 in $\text{electrons}∕{\mathrm{\AA }}^2$, except for the zero contour line. Solid lines indicate positive values, while broken ones negative. On these cross sections, the on-top Ni $\left({\mathrm{Ni}}^a\right)$, the two kinds of hollow-site Ni (${\mathrm{Ni}}^c$ and ${\mathrm{Ni}}^d$) and the asymmetric-site Ni $\left({\mathrm{Ni}}^b\right)$ are present (also see Fig. 1).

Figure 7

ED difference maps on the $\left(01\overline{1}\right)$ planes for the $T_1$ structures of the Zr terminated (a) and O terminated (b) interfaces. The contour maps are drawn in the similar manner with Fig. 3.

Figure 8

The stress-strain curves for the Zr terminated and O terminated interfaces. The $T_0$ structure of the Zr terminated interface showed cleavage fracture inside $\mathrm{Zr}{\mathrm{O}}_2$. In (a), the profile of $T_0$ is only displayed, but it is expected that the $T_1$-type structure shows the similar curve, since that translation state also have a rather large $W_{\mathrm{sep}}$ value, resulting in cleavage inside $\mathrm{Zr}{\mathrm{O}}_2$. In contrast, the O terminated interfaces exhibited fracture at the interface planes. However, it is interesting to point out the different stress-strain behaviors between the two translation states.

Figure 9

The bond-length changes as a function of tensile strain. It is noted that the $T_0$-type interfaces contain four kinds of bonding-pair configurations, whereas in the case of the $T_1$-type O terminated interface, three kinds of Ni-O configurations with different bond lengths are present (also see Fig. 1).