Comparison of the defective pyrochlore and ilmenite polymorphs of AgSb$O_3$ using GGA and hybrid DFT

Silver antimonate, AgSb$O_3$, in both its defective pyrochlore and ilmenite structural polymorphs, has been suggested as a possible candidate mixed metal oxide for use in the photocatalytic splitting of water in visible light. In this study, we report electronic-structure calculations, using both standard and hybrid density-functional-theory approaches, on both structural forms of AgSb$O_3$ to fully characterize the band structure and composition of the valence and conduction bands. Analysis of conduction properties and optical absorption is also used to compare the predicted properties of the two materials. Results show that the valence band is dominated by O 2$p$ and Ag 4$d$ states, whereas the conduction band is composed mainly of Ag and Sb 5$s$ states. Band-edge effective-mass calculations indicate the materials operate via an $n$-type mechanism, with conduction properties being comparable for the two materials. The fundamental and optical band gaps are also predicted to be compatible with visible light adsorption.

Figure 1

Schematic showing the optimized AgSbO${}_3$ defective pyrochlore structure. Silver, antimony, and oxygen atoms are colored blue (light gray), purple (medium gray), and red (dark gray), respectively. The antimony atoms are also shown as polyhedra in the main image. Coordination environments of the AgO${}_6$ and SbO${}_6$ octahedra are also shown.

Figure 2

Electronic density of states for AgSbO${}_3$ in a defective pyrochlore structure, split into (i) the total EDOS, (ii) Ag PEDOS, (iii) Sb PEDOS, and (c) O PEDOS, as calculated using the (a) PBE and (b) HSE06 method. The $s$, $p$, and $d$ states are colored blue (dot-dash), red (solid), and green (dash), respectively. Vertical dashed gray lines represent divisions between different regions in the DOS. The Fermi energy has also been set to $0\hspace{0.5em}\mathrm{eV}$.

Figure 3

Band structure of the defective pyrochlore form of AgSbO${}_3$ calculated using (a) the PBE approach and (b)–(d) the HSE06 method. The HSE06-determined band structures are shown in fatband format with the contributions to the bands from the (b) Ag 4$d$ (green), (c) Sb 5$s$ (blue), and (d) O 2$p$ (red) states indicated. The Fermi level is set to $0\hspace{0.5em}\mathrm{eV}$, as indicated by the horizontal dashed gray line.

Figure 4

Schematic showing the optimized AgSbO${}_3$ ilmenite structure. Silver, antimony, and oxygen atoms are colored blue (light gray), purple (medium gray), and red (dark gray), respectively. The antimony atoms are also shown as polyhedra in the main image. Coordination environments of the AgO${}_6$ and SbO${}_6$ octahedra are also shown for reference.

Figure 5

Electronic density of states for AgSbO${}_3$ in an ilmenite structure, split into (i) the total EDOS, (ii) Ag PEDOS, (iii) Sb PEDOS, and (c) O PEDOS, as calculated using the (a) PBE and (b) HSE06 method. The $s$, $p$, and $d$ states are colored blue (dot-dash), red (solid), and green (dash), respectively. Vertical dashed gray lines represent divisions between different regions in the DOS. The Fermi energy has also been set to $0\hspace{0.5em}\mathrm{eV}$.

Figure 6

Band structure of the ilmenite form of AgSbO${}_3$ calculated using (a) the PBE approach and (b)–(d) the HSE06 method. The HSE06-determined band structures are shown in a fatband format with the contributions to the bands from the (b) Ag 4$d$ (green), (c) Sb 5$s$ (blue), and (d) O 2$p$ (red) states indicated. The Fermi level is set to $0\hspace{0.5em}\mathrm{eV}$, as indicated by the horizontal dashed gray line.

Figure 7

Calculated optical absorption ($\alpha$${}^2$) of the two forms of AgSbO${}_3$ summed over all possible direct VB to CB transitions. Solid lines represent calculated absorption using the HSE06 method with the extrapolation used to determine the calculated optical band gap shown by the dotted lines. The experimental optical band gaps are given by the dashed lines.[21, 22]