Tl2Ag12Se7: A New pnp Conduction Switching Material with Extraordinarily Low Thermal Conductivity

While continuing our investigations of thallium chalcogenides because of their outstanding thermoelectric properties, we discovered a new selenide with an interesting pnp switching behavior around 400 K. Tl2Ag12Se7 was prepared via high temperature reaction from the elements in the stoichiometric ratio. This selenide crystallizes in a new structure type, namely a √3 × √3 × 1 super cell of the Zr2Fe12P7 type, adopting space group P3̅, a = b = 18.9153(18) Å, c = 4.3783(4) Å, and V = 1356.6(2) Å3 (Z = 3). The structure consists of a complex network of three-dimensionally connected AgSe4 tetrahedra that include linear channels filled with thallium atoms. This material is a semiconductor with an experimentally derived activation gap of 0.8 eV and extraordinarily low thermal conductivity of –1 K–1. A reversible phase transition causes a pnp switch from p-type to n-type and back to p-type conduction beginning around 390 K, with the thermopower changing fast from +230 μV K–1 at 390 K to −230 μV K–1 at 410 K, and then back up to +75 μV K–1 at 420 K.