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

Highlights: • High quality ceramic Ga₂O₃(ZnO)₉ has been synthesised. • Very low thermal conductivity for an oxide of 1.5–2.2 W/m K has been achieved. • HAADF-STEM showed the presence of nano-sized, wedge-shaped twin boundaries. • The nano-scale features, chemically investigated for the first time, revealed the Zn and Ga distribution. • Ga ions occupy the sites at the interfaces of twin boundaries and inversion boundaries. 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.