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

Molybdenum oxides MoOx 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.