High-temperature thermoelectric properties of Sr2RuYO6 and Sr2RuErO6 double perovskites influenced by structure and microstructure

The high-temperature thermoelectric properties of Sr2RuYO6 and Sr2RuErO6 double perovskites were evaluated and reported for the first time. These compounds show high Seebeck coefficients not only at room temperature, but also at high temperature (for Sr2RuYO6, SRT ≈ −475 μV K−1 and S1200K ≈ −250 μV K−1; Sr2RuErO6, SRT ≈ −400 μV K−1 and S1200K ≈ −250 μV K−1). The n-type semiconducting behaviour dominates the resistivity values. Both compounds crystallize in a monoclinic unit cell (space group P21/n). The lattice parameters are a = 5.7761(2), b = 5.7804(1), c = 8.1689(1), α = γ = 90° and β = 90.2087(8)° for the Sr2RuYO6, and a = 5.7760(1), b = 5.7722(0), c = 8.1544(4), α = γ = 90° and β = 90.2099(7)° for Sr2RuErO6. The unit cell can be described approximately as √2ap × √2ap × 2ap, where ap is the unit cell parameter of the ideal cubic perovskite structure. High-resolution transmission electron microscopy shows an interesting three-dimensional micro-twin-domain texture where the c axis is placed in the three space directions. Structural transitions at high temperatures (Tt(Sr2RuYO6) ≈920 K and Tt(Sr2RuErO6) ≈890 K) are observed by specific heat measurement in both compounds, which are found to have a strong influence on the Seebeck coefficient and electrical conductivity.