Electronic structure of single-crystalline thermoelectric ${\mathrm{Bi}}_{2-x}{\mathrm{Pb}}_x{\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{O}}_y$ $(x=0,0.6)$ from photoemission and x-ray absorption

The electronic structures of ${\mathrm{Bi}}_{2-x}{\mathrm{Pb}}_x{\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{O}}_y$ $(x=0,0.6)$ thermoelectric single crystals have been investigated by employing photoemission spectroscopy (PES) and polarization-dependent soft x-ray absorption spectroscopy. Co ions are found to be in the ${\mathrm{Co}}^{3+}\text{−}{\mathrm{Co}}^{4+}$ mixed-valent states with the low-spin configuration. The valence-band PES study reveals that the Co $3d$ states are located at the top of the valence bands, corresponding to the occupied electron configuration of $e_g^{\pi 4}{a}_{1g}^{1+x}$ $(x<1)$. Both the topmost occupied states and the lowest unoccupied states have the primarily out-of-plane Co $a_{1g}$ character, suggesting that carriers have the Co $a_{1g}$ hole character. Angle resolved photoemission spectroscopy measurements show that the lowest binding-energy states are shifted toward $E_F$ with increasing $x$ in ${\mathrm{Bi}}_{2-x}{\mathrm{Pb}}_x{\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{O}}_y$, indicating that the carrier states are shifted toward $E_F$. This study suggests that the thermoelectric property of ${\mathrm{Bi}}_{2-x}{\mathrm{Pb}}_x{\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{O}}_y$ is improved for $x>0$ mainly by the reduced resistivity.

Figure 1

(a) $\mathrm{Co}{\mathrm{O}}_6$ octahedron. A small circle at the center denotes the cobalt (Co) ion and the large open circles indicate oxygen (O) ions. $x^{\prime },y^{\prime },z^{\prime }$ are along the cubic crystal axes. (b) A schematic picture of the $\mathrm{Co}{\mathrm{O}}_2$ triangular lattice. The shaded circles indicate the Co ions centered at the $\mathrm{Co}{\mathrm{O}}_6$ octahedra. The $z$ axis is the trigonal threefold rotational axis. (c) The crystal-field splitting of the distorted $\mathrm{Co}{\mathrm{O}}_6$ octahedron. $e_g^{\pi }$ is used to be distinguished from the $e_g$ ($x^2\text{−}{y}^2$ and $3z^2\text{−}{r}^2$) states. (d) The first Brillouin zone of the hexagonal $\mathrm{Co}{\mathrm{O}}_2$ layer. The arrow along $\Gamma M$ indicates the direction along which the ARPES data were taken (see Fig.8 later).

Figure 2

(Color online) $\mathrm{Co}2p$ XAS spectra of ${\mathrm{Bi}}_{2-x}{\mathrm{Pb}}_x{\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{O}}_y$ with $x=0$ (BSCO) and $x=0.6$ (BPSCO), in comparison to those of $\mathrm{Ag}\mathrm{Co}{\mathrm{O}}_2$ (Ref. 24), $\mathrm{Sr}\mathrm{Co}{\mathrm{O}}_3$ (Ref. 25), and the fitting results (Ref. 21). See the text for details.

Figure 3

(Color online) (Top) Polarization-dependent $\mathrm{O}1s$ XAS spectra of BSCO with varying the incidence angle $\left(\theta \right)$ of the incoming photons. (Bottom) Polarization-dependent $\mathrm{Co}2p$ XAS spectra of BSCO.

Figure 4

Left: Valence-band PES spectra of BPSCO near the $\mathrm{Co}2p\to 3d$ absorption edge. Inset: The $\mathrm{O}2p_{3∕2}$ XAS spectrum of BPSCO. Arrows denote $h\nu$’s where the valence-band PES spectra were obtained. Right: Constant-initial-state (CIS) spectra of BPSCO for several initial-state energies, taken across the $\mathrm{Co}3p\to 3d$ absorption threshold.

Figure 5

(Color online) Top: (Left) The $\mathrm{Co}3d$ PSW and the $\mathrm{O}2p$ PSW of BPSCO. (Right) The $\mathrm{O}1s$ XAS spectrum of BPSCO.

Figure 6

(a) The paramagnetic electronic configurations for a ${\mathrm{Co}}^{3+}$ ion in the undistorted $\mathrm{Co}{\mathrm{O}}_6$ octahedron under the $O_h$ symmetry. (b) The low-spin electronic configurations for ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions in the compressed $\mathrm{Co}{\mathrm{O}}_6$ octahedron under the $D_{3d}$ symmetry. (c) Schematic PDOS for ${\mathrm{Bi}}_{2-x}{\mathrm{Pb}}_x{\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{O}}_y$.

Figure 7

(Color online) Comparison of the valence-band PES spectra of ${\mathrm{Bi}}_{2-x}{\mathrm{Pb}}_x{\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{O}}_y$ for $x=0$ and $x=0.6$ for $18\mathrm{eV}⩽h\nu ⩽811\mathrm{eV}$.

Figure 8

(Color online) Left: ARPES spectra of BSCO (top) and BPSCO (middle), respectively, obtained with $h\nu =20\mathrm{eV}$ for ${\vec{k}}_{\Vert }$ along $\Gamma M$. Right: The experimental band structures determined from the measured ARPES spectra. Dark parts correspond to the energy bands. Bottom: Comparison of the experimental band structures between BSCO and BPSCO.