Pb₇Bi₄Se₁₃: A Lillianite Homologue with Promising Thermoelectric Properties

Abstract

Pb₇Bi₄Se₁₃ 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>₁ = 5 and <i>N</i>₂ = 4 [<i>N</i> = number of edge-sharing (Pb/Bi)­Se₆ octahedra along the central diagonal] are arranged along the <i>c</i> axis in such a way that the bridging monocapped trigonal prisms, PbSe₇, 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^–1 K^–1 at 300 K). Electronic structure calculations and diffuse-reflectance measurements indicate that Pb₇Bi₄Se₁₃ 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 Pb₇Bi₄Se₁₃ 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^–1 at 300 K) and a relatively low electrical resistivity. The inherently low thermal conductivity of Pb₇Bi₄Se₁₃ and its tunable electronic properties point to a high thermoelectric figure of merit for properly optimized samples