57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

Objective:

The goal of the ACS-PRF funded project was to prepare novel thermoelectric materials via chemical fusion of semimetallic RESb/REBi and insulating RE₂O₃ binaries (RE is a rare-earth element). The idea was to structurally combine the RESb/REBi and RE₂O₃ phases with different properties in order to obtain "electron-crystal phonon-glass" materials with improved thermoelectric performance (Figure 1).

Figure 1. Formation of (RESb)_n(RE₂O₃) from RESb, an "electron crystal", and RE₂O₃, a "phonon glass".

Results (2011-2012):

1) Decoupling the Electrical Conductivity and Seebeck Coefficient in the RE₂SbO₂ Compounds through Local Structural Perturbations

Compromise between electrical conductivity and Seebeck coefficient limits the efficiency of chemical doping in the thermoelectric research. An alternative strategy, involving the control of a local crystal structure, is demonstrated to improve the thermoelectric performance in the RE₂SbO₂ system. The RE₂SbO₂ phases, adopting a disordered anti-ThCr₂Si₂ type structure (I4/mmm), were prepared for RE = La, Nd, Sm, Gd, Ho and Er. By traversing the rare-earth series, the lattice parameters of the RE₂SbO₂ phases are gradually reduced, thus increasing chemical pressure on the Sb environment. As the Sb displacements are perturbed, different charge carrier activation mechanisms dominate the transport properties of these compounds. As a result, the electrical conductivity and Seebeck coefficient are improved simultaneously while the number of charge carriers in the series remains constant. This study illustrates possibilities of overcoming the classical challenges in the thermoelectric research.

Figure 2. DOS at the Fermi level in the middle of a pseudogap. The shaded region represents localized states. The mobility edge is marked at boundary between the localized and the non-localized states. Three charge carrier activation mechanisms are shown: (left) hopping between the localized states; (center) thermal activation into localized states (RE = Gd); (right) thermal activation into non-localized states (RE = Ho, Er).

Figure 3. Electrical conductivity and Seebeck coefficient for the RE₂SbO₂ phases.

2) Disorder-Controlled Electron Transport Properties in the Ho₂Sb_1-xBi_xO₂ Systems

The Ho₂Sb_1-xBi_xO₂ series was traversed by varying the Sb:Bi ratio. While the modification on the average Ho₂Sb_1-xBi_xO₂ structure is minimal, the lattice disorder caused by the pnictogen atomic displacement is reduced upon the Bi substitution. As a result, the energy range of Anderson-type localization within the Sb/Bi states diminishes. Since the Sb/Bi states dominated the electronic structure at the vicinity of the Fermi level, the electrical properties of the Ho₂Sb_1-xBi_xO₂ phases were altered systematically within a large range. With this study, we proved that the electrical properties of a disordered system can be tuned by controlling the degree of atomic localization. In a suitable system, such indirect physical property perturbation may be used in conjunctions with other methods such as chemical doping and superlattice construction to produce tailor-made materials for the future electronic and optical applications.

Figure 4. Electrical conductivities (a) and Seebeck coefficents (b) of the Ho₂Sb_1-xBi_xO₂ compounds in the rage of 0–400K.