Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub>: A Lillianite
Homologue with Promising Thermoelectric Properties
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Abstract
Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12) with <i>a</i> = 13.991(3)
Å, <i>b</i> = 4.262(2) Å, <i>c</i> =
23.432(5) Å, and β = 98.3(3)° at 300 K. In its three-dimensional
structure, two NaCl-type layers A and B with respective thicknesses <i>N</i><sub>1</sub> = 5 and <i>N</i><sub>2</sub> = 4
[<i>N</i> = number of edge-sharing (Pb/Bi)Se<sub>6</sub> octahedra along the central diagonal] are arranged along the <i>c</i> axis in such a way that the bridging monocapped trigonal
prisms, PbSe<sub>7</sub>, are located on a pseudomirror plane parallel
to (001). This complex atomic-scale structure results in a remarkably
low thermal conductivity (∼0.33 W m<sup>–1</sup> K<sup>–1</sup> at 300 K). Electronic structure calculations and
diffuse-reflectance measurements indicate that Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> is a narrow-gap semiconductor with an indirect
band gap of 0.23 eV. Multiple peaks and valleys were observed near
the band edges, suggesting that Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> is a promising compound for both n- and p-type doping. Electronic-transport
data on the as-grown material reveal an n-type degenerate semiconducting
behavior with a large thermopower (∼−160 μV K<sup>–1</sup> at 300 K) and a relatively low electrical resistivity.
The inherently low thermal conductivity of Pb<sub>7</sub>Bi<sub>4</sub>Se<sub>13</sub> and its tunable electronic properties point to a
high thermoelectric figure of merit for properly optimized samples