Enhancement of Electrical Conduction and Phonon Scattering in Ga2O3(ZnO)9-In2O3(ZnO)9 Compounds by Modification of Interfaces at the Nanoscale

Abstract

The Ga2O3(ZnO)9 and In2O3(ZnO)9 homologous phases have attracted attention as thermoelectric (TE) oxides due to their layered structures. Ga2O3(ZnO)9 exhibits low thermal conductivity, while In2O3(ZnO)9 possesses higher electrical conductivity. The TE properties of the solid solution of Ga2O3(ZnO)9-In2O3(ZnO)9 were explored and correlated with changes in the crystal structure. High-quality (1−x)Ga2O3(ZnO)9-(ZnO)9 (x = 0.0 to 1.0) ceramics were prepared by the solid-state route using B2O3 and Nd2O3 as additives. The crystal structures were analysed by x-ray diffraction, high-resolution transmission electron microscopy and atomic resolution scanning transmission electron microscopy–high-angle annular dark field imaging–energy dispersive x-ray spectroscopy (STEM–HAADF–EDS) techniques. A layered superstructure with compositional modulations was observed in all samples in the (1−x)Ga2O3(ZnO)9-xIn2O3(ZnO)9 system. All the ceramics exhibited nanoscale structural features identified as Ga- and In-rich inversion boundaries (IBs). Substitution of 20 mol.% In (x = 0.2) in the Ga2O3(ZnO)9 compounds generated basal and pyramidal indium IBs typically found in the In2O3(ZnO)m system. The (Ga0.8In0.2)2O3(ZnO)9 compound does not exhibit the structural features of the $$ Cmcm $$CmcmGa2O3(ZnO)9 compound, which is formed by a stacking of Ga-rich IBs along the pyramidal plane of the wurtzite ZnO, but features that resemble the crystal structure exhibited by the $$ R\overline{3} m $$R3¯mIn2O3(ZnO)m with basal and pyramidal indium IBs. The structural changes led to improved TE performance. For example, (Ga0.8In0.2)2O3(ZnO)9 showed a low thermal conductivity of 2 W/m K and a high power factor of 150 μW/m K2 giving a figure of merit (ZT) of 0.07 at 900 K. This is the highest ZT for Ga2O3(ZnO)9-based homologous compounds and is comparable with the highest ZT reported for In2O3(ZnO)9 homologous compounds.