By the fine manipulation of the exceptional long-range germanium-telluride (Ge─Te) bonding through charge transfer engineering, we have achieved exceptional thermoelectric (TE) and mechanical properties in lead-free GeTe. This chemical bonding mechanism along with a semiordered zigzag nanostructure generates a notable increase of the average zT to a record value of ~1.73 in the temperature range of 323 to 773 K with ultrahigh maximum zT ~ 2.7. In addition, we significantly enhanced the Vickers microhardness numbers ( H _v ) to an extraordinarily high value of 247 H _v and effectively eliminated the thermal expansion fluctuation at the phase transition, which was problematic for application, by the present charge transfer engineering process and concomitant formation of microstructures. We further fabricated a single-leg TE generator and obtained a conversion efficiency of ~13.4% at the temperature difference of 463 K on a commercial instrument, which is located at the pinnacle of TE conversion.

This study uses two in situ techniques to investigate the geometry and phase change behaviour of bundled ultrathin W ₁₈ O ₄₉ nanowires and WO ₃ nanoparticles. The in situ X-ray diffraction (XRD) results have shown that the phase transition of WO ₃ nanoparticles occurs in sequence from monoclinic (room temperature) → orthorhombic (350°C) → tetragonal (800°C), akin to bulk WO ₃ ; however, W ₁₈ O ₄₉ nanowires remain stable as the monoclinic phase up to 500°C, after which a complete oxidation to WO ₃ and transformation to the orthorhombic β-phase at 550°C is observed. The in situ Raman spectroscopy investigations have revealed the Raman peak downshifts as the temperature increases, and have identified the 187.6 cm ⁻¹ as the fingerprint band for the phase transition from γ- to β-phase of the WO ₃ nanoparticle. Furthermore, WO ₃ nanoparticles exhibit the γ- to β-phase conversion at 275°C, which is about 75°C lower than the relaxation temperature of 350°C for the monoclinic γ-W ₁₈ O ₄₉ nanowires. These new fundamental understandings on the phase transition behaviour offer important guidance for the design and development of tungsten oxide-based nanodevices by defining their allowed operating conditions.