Utilising unit-cell twinning operators to reduce lattice thermal conductivity in modular structures: Structure and thermoelectric properties of Ga₂O₃(ZnO)₉

The Ga2O3(ZnO)m family of homologous compounds have been identified as potential thermoelectric materials, but properties are often limited due to low densification. By use of B2O3 as an effective liquid phase sintering aid, high density, high quality ceramic samples of Ga2O3(ZnO)9 have been synthesised. The atomic structure and local chemical composition of Ga2O3(ZnO)9 have been determined by means of high resolution X-ray diffraction and atomic resolution STEM-HAADF, EDS and EELS measurements. X-ray analysis showed that the compound crystalizes in the Cmcm orthorhombic symmetry. Atomically resolved HAADF-STEM images unambiguously showed the presence of nano-sized, wedge-shaped twin boundaries, parallel to the b-axis. These nano-scale structural features were chemically investigated, for the first time, revealing the exact distributions of Zn and Ga; it was found that Ga ions occupy sites at the junction of twin boundaries and inversion boundaries. HAADF-EDS analysis showed that the calcination step has a significant impact on crystal structure homogeneity. By use of a sintering aid and optimization of processing parameters the ceramics achieved a low thermal conductivity of 1.5–2.2 W/m.K (for the temperature range 300–900 K), a power factor of 40–90 μW/K.m2, leading to a ZT of 0.06 at 900 K. The work shows a route to exploit nanoscale interface features to reduce the thermal conductivity and thereby enhance the thermoelectric figure of merit in complex thermoelectric materials.