Molybdenum Oxides MoO_x: Spark-Plasma Synthesis and Thermoelectric Properties at Elevated Temperature

Molybdenum oxides MoO_x in the composition range 2 ≤ x ≤ 3 were synthesized and compacted by the solid-state reaction of powdered α-MoO₃ with Mo in the spark-plasma synthesis (SPS) process at temperatures up to 973 K. From six known compounds in the Mo–O system, Mo₁₈O₅₂ (x = 2.889), Mo₁₇O₄₇ (x = 2.760), and γ-Mo₄O₁₁ (x = 2.750) were synthesized as practically single-phase products despite their narrow composition ranges (≈0.1 at %) with a yield (0.5–1 g per synthesis) sufficient for thermoelectric property measurements on the polycrystalline bulk material from room temperature to 763 K. For Mo₁₈O₅₂, which appears to be an intrinsic narrow-gap semiconductor (activation energy ≈ 0.3 eV), exceptionally low thermal conductivity over the full temperature range (0.5–0.9 W m^–1 K^–1), high p-type Seebeck coefficient at room temperature (+140 μV K^–1), and a p–n transition at 440 K were found. The phases MoO₂, γ-Mo₄O₁₁, and Mo₁₇O₄₇ show poor metallic conductivity. The highest power factor is achieved by Mo₁₇O₄₇ (30 Wm^–1 K^–2 for 440 K ≤ T ≤ 610 K). In γ-Mo₄O₁₁, a hitherto unknown reversible transition is found at 450 K from specific heat measurements.