Thermoelectric properties and impedance spectroscopy of polycrystalline samples of the beta-gallia rutile intergrowth, (Ga,In)4(Sn,Ti)5O16

Polycrystalline samples of Ga3In(Sn1−yTiy)5O16, y≤0.3 were prepared by solid-state synthesis and characterized to determine their optical properties, thermoelectric properties and electrical impedance. Using diffuse reflectance data and assuming a direct band gap, the band gap of the material ranges from 3.58 eV for y=0 to 3.74 eV for y=0.2. The dc conductivity decreased with increasing Ti content and was thermally activated, ranging from ≤10−5 S/cm at 300 °C to 0.03 S/cm at 1000 °C for Ga3InSn5O16. The Seebeck coefficient was negative indicating n-type conduction. The activation energies for dc conduction and thermopower were similar, ranging from 1.4±0.1 eV for y=0 to 1.8±0.2 eV for y=0.2, suggesting band conduction. The thermal conductivity of Ga3InSn5O16 ranged from 1.1 W/m K at 500 oC to 0.75 W/m K at 800 °C. The highest figure of merit measured was 2×10−4 at ∼900 °C, which is much lower than desired for thermoelectric materials. A comparison of dc conductivity and impedance data indicated a substantial ionic contribution for samples containing titanium. - Graphical abstract: The thermoelectric properties of polycrystalline Ga3In(Sn1−yTiy)5O16, y≤0.3 were measured in air at 300–900 °C. The materials are broad-band n-type semiconductors with non-negligible ionic conduction for samples prepared with y>0. The thermoelectric figure of merit is much lower than desired for practical thermoelectric devices. Highlights: ► Ga3In(Sn1−yTiy)5O16, y≤0.2 is a broad-band n-type semiconductor. ► The conductivity of Ti-containing samples show evidence of an ionic as well as an electronic contribution. ► The thermal conductivity of polycrystalline Ga3In(Sn1−yTiy)5O16 decreases with increasing temperature. ► The thermoelectric figure of merit for Ga3In(Sn1−yTiy)5O16 is lower than desired for practical thermoelectric devices.