Renewable energy in focus: In5Se5Br, a solid material with promising thermoelectric properties for industrial applications

• In5Se5Br contains indium simultaneously in three different oxidation states.
• Bulk sample of In5Se5Br shows n-type conductivity.
• The Seebeck voltage increases linearly with the temperature difference increase.
• In bulk In5Se5Br the resistivity oscillates between 2.6 MΩ and 23 MΩ.
• DTA and HT-powder XRD data show incongruent melting of the compound.

We obtained via solid state synthesis needle-shaped crystals of In5Se5Br crystallizing in the space group Pmn21 and containing indium simultaneously in three different oxidation states: In+, formal In2+ and In3+. Bulk sample of In5Se5Br shows n-type conductivity and linear increase of Seebeck voltage with the temperature difference increase. Seebeck voltage of approx. 720 mV is recorded at a temperature difference of 80 K, corresponding to a Seebeck coefficient −8900 μV/K. A voltage increase up to 250 mV is recorded within 10 min upon application of a 27 K temperature difference between the contacts. On-off switching of the heating source unveils repeatable results. Linear I–U behavior with a resistivity of 2.32 × 1011 Ω is observable for individual needles of In5Se5Br. In bulk In5Se5Br the resistivity oscillates between 2.6 MΩ and 23 MΩ. DTA and HT-powder XRD data show incongruent melting to InBr, InSe and In2Se3 at 805 K. The ternary compound expands 1.02% along [0 1 0] showing a coefficient of thermal expansion αb = 2.3(4) × 10−5 K−1. Lower expansions of 0.6% and 0.16% along a and c   axes corresponding to mean coefficients of thermal expansion of αa¯ = 1.3(1) × 10−5 K−1, αc¯ = 4.4(5) × 10−6 K−1 are observed. Thin layer growing of In5Se5Br on glass substrate with targeted doping/substitutions can improve the sample conductivity, increase the Seebeck coefficient and lower the thermal conductivity making In5Se5Br a good alternative material for industrial thermoelectric applications.

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