Comparative NMR study of hybridization effect and structural stability in $D{0}_{22}$-type $\mathrm{Nb}{\mathrm{Al}}_3$ and $\mathrm{Nb}{\mathrm{Ga}}_3$

With the aim of providing experimental information for the correlation between $p\text{−}d$ hybridization and phase stability in the $D{0}_{22}$ structure, we performed a comparative investigation on $\mathrm{Nb}{\mathrm{Al}}_3$ and $\mathrm{Nb}{\mathrm{Ga}}_3$ using ${}^{93}\mathrm{Nb}$ NMR spectroscopy. The quadrupole splittings, Knight shifts, and spin-lattice relaxation times ($T_1$’s) for each individual compound have been identified. The larger quadrupole interaction and higher anisotropic Knight shift have been observed in $\mathrm{Nb}{\mathrm{Al}}_3$, indicative of the stronger hybridization effect for this material, as compared with its isostructural compound $\mathrm{Nb}{\mathrm{Ga}}_3$. Results of experimental $T_1$ together with theoretical band structure calculations provide a measure of $d$-character Fermi-level density of states $N_d\left(E_F\right)$ and an indication of orbital weights. In addition, we found evidence that $N_d\left(E_F\right)$ correlates with the structural stability of the studied materials. Our NMR measurements confirm that $\mathrm{Nb}{\mathrm{Al}}_3$ is more stable than $\mathrm{Nb}{\mathrm{Ga}}_3$ with respect to the $D{0}_{22}$ structure, attributed to the stronger $p\text{−}d$ hybridization in the former material.

Figure 1

(Color online) Crystal structure for the $D{0}_{22}$-type ${\mathrm{MX}}_3$.

Figure 2

Resolved satellite line shapes for the Nb site in $\mathrm{Nb}{\mathrm{Al}}_3$ and $\mathrm{Nb}{\mathrm{Ga}}_3$.

Figure 3

${}^{93}\mathrm{Nb}$ central transition NMR spectra for $\mathrm{Nb}{\mathrm{Al}}_3$ and $\mathrm{Nb}{\mathrm{Ga}}_3$ measured at room temperature. The dotted curves are simulated according to Eq. (1).

Figure 4

Total density of states (the thin solid lines) for $\mathrm{Nb}{\mathrm{Al}}_3$ (a) and $\mathrm{Nb}{\mathrm{Ga}}_3$ (b) and the corresponding orbital projected partial density of states for Nb atoms in the systems. The zero of the energy denotes the Fermi level of the system.