Crystal Structure and Atomic Vacancy Optimized Thermoelectric Properties in Gadolinium Selenides

Thermoelectric materials enable the energy conversion of waste heat into electricity, helpful to relieve global energy crisis. Here, we report a systematic investigation on high-temperature thermoelectric gadolinium selenides, cubic Gd_3-xSe₄ (x = 0.16, 0.21, and 0.25) and orthorhombic Gd₂Se_3-y (y = 0.02, 0.06, and 0.08). High energy synchrotron X-ray diffraction and total scattering have been used to investigate the crystallographic and local structures. Atomic-scale clusters of Gd vacancy in the cubic phase are observed by employing the reverse Monte Carlo simulation. For cubic Gd_3-xSe₄, adjusting Gd vacancy triggers the effect of multiple conduction bands, confirmed by the increase in effective masses. A reasonable peak zT of 0.27 is achieved at 850 K for Gd_3-xSe₄ (x = 0.16). On the other hand, tuning Se vacancy enables the optimization of electron concentration for the orthorhombic Gd₂Se_3-y. More significantly, its low deformation potential (Ξ = 12 eV) gives rise to enhanced electron mobility and a higher peak zT of 0.54 at 850 K for Gd₂Se_3-y (y = 0.02). Intriguingly, a higher zT of 1.2 at 1200 K is reasonably predicted by quality factor analysis. This work extends the scope of high-temperature thermoelectric materials and facilitates the exploration of novel high-temperature thermoelectric materials.