Structure-property investigation of ZnSb, ZnAs, and SiB 3: binary semiconductors with electron poor framework structures

In today’s society, where energy conservation and green energy are buzz words, new scientific discoveries in green energy harvesting is key. This work focuses on materials capable of recycling low value thermal energy. Low value thermal energy, waste heat, is for free, and can be transformed into valuable electricity via thermoelectric technology. A thermoelectric device cleanly converts heat into electricity through the Seebeck effect. Thermoelectric devices can play an important role in satisfying the future global need for efficient energy management, however, the primary barrier of improving thermoelectric devices is the materials themselves. The aim of this thesis is to identify new compositions and structures for thermoelectric materials. In particular, the concept of “electron poor framework semiconductors” is explored. Electron Poor Framework Semiconductors (EPFS) are materials at the border between metals and non-metals, which often show intricate and unique structures with complex bonding schemes. Generally, constituting elements should be from group 12(II) (Zn, Cd), 13(III) (B, Al, Ga, In), 14(IV) (Si, Ge, Sn, Pb), 15(V) (Sb, Bi), and 16(VI) (Te), i.e. elements which have a similar electronegativity (between 1.5-2.0). All EPFS materials have in common highly complex crystal structures, which are thought to be a consequence of their electron-poor bonding patterns. EPFS materials have an intrinsically very low – glass like - lattice thermal conductivity. The focus of this thesis is on combinations of group 12(II) (Zn) with 16 (V) (As, Sb), and 13(III) (B) with 14(IV) (Si). ZnSb possesses a simple structure with 8 formula units in an orthorhombic unit cell, it is considered a stoichiometric compound without noticeable structural disorder. In this thesis ZnSb is used as a model system to establish more broadly structure–property correlations in Sb based EPFS materials. ZnSb was established to possess an impurity band that determines electrical transport properties up to 300–400 K. Doping of ZnSb with Ag seems to enhance the impurity band by increasing the number of acceptor states and improving charge carrier density by two orders of magnitude. ZT values of Ag doped ZnSb are found to exceed 1 at 350 K. The origin of the low thermal conductivity of ZnSb was traced back to a multitude of localized low energy optic modes, acting as Einstein-like rattling modes. ZnAs was accessed through high pressure synthesis. The compound is isostructural to ZnSb and