The metastable phase diagram of the Gd2O3–SrO–CoO
x
system in air was established based on the powder X-ray diffraction results of the 1100 °C-synthesized and then furnace-cooled or slowly-cooled (1 K min−1) samples. It consists of two solid solutions, Gd1−x
Sr
x
CoO3−δ
(0.6 ≤ x ≤ 0.9) with a tetragonal I4/mmm superstructure and Gd
x
Sr2−x
CoO4−δ
(0.5 ≤ x ≤ 1.2) with a layered tetragonal I4/mmm K2NiF4-type structure, and one ternary compound Gd2SrCo2O7 with a tetragonal P42/mnm structure. The existence of six binary oxide compounds Gd2SrO4, GdCoO3, Sr2Co2O5 (R), Sr6Co5O15, Sr5Co4O12 and Sr14Co11O33 was confirmed. This metastable phase diagram is of technological interest in the controlled preparation of single-phase complex oxides. New phases Gd0.375Sr2.625Co2O7−δ
and τ4 with an orthorhombic Immm structure were found in the quenched samples. Differences between the present metastable phase diagram and the reported 1100 °C equilibrium one are discussed.
Double-layered manganites are natural superlattices with low thermal conductivity, which is of importance for potential thermoelectric applications. The Gd2−2xSr1+2xMn2O7 (x = 0.5; 0.625; 0.75) were prepared by the solid-state reaction method. All the samples crystallize in the tetragonal I4/mmm Sr3Ti2O7 type structure. The unit cell volume and the distortion in the MnO6 octahedra increase with increasing Gd content. Their thermoelectric properties were investigated between 300 and 1200 K. All exhibit an n-type semiconducting behavior. The electrical conductivity (σ) increases while the absolute value of the Seebeck coefficient (|S|) decreases with increasing Gd content. Simultaneous increases in σ and |S| with increasing temperature are observed at temperatures approximately higher than 600 K, and the power factor reaches a maximum value of 18.36 μW/(m K²) for x = 0.75 at 1200 K. The thermal conductivity (κ) is lower than 2 W/(m K) over the temperature range of 300–1000 K for all the samples and a maximum dimensionless figure of merit ZT of 0.01 is obtained for x = 0.75 at 1000 K.
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