Preparation and characterization of nonstoichiometric Te-deficient and Te-rich thermoelectric Bi2-Gd Te3± compounds

• Nonstoichiometric Te-deficient and Te-rich Bi 2- x Gd x Te 3± y samples were prepared. • Solvothermal synthesis and spark plasma sintering were applied to prepare the samples. • Highest electron content and texturing degree found for Te-rich Bi 2- x Gd x Te 3+ y samples. • Best thermoelectric properties found for Te-deficient sample with x = 0.01 and y = 0.011. The Gd-doped Bi 2 Te 3 compounds were prepared by solvothermal synthesis and spark plasma sintering. It was found that for the Bi 2- x Gd x Te 3± y compounds with x = 0.01; 0.05; 0.1; 0.15 and 0.2, the Gd doping results in relevant changing the Te content, y , i.e. x and y are inter-dependent. Governing by the Gd content, the Te-deficient Bi 2- x Gd x Te 3- y compositions with x = 0; 0.01; 0.05 and y = 0.018; 0.011; 0.003, and the Te-rich Bi 2- x Gd x Te 3+ y compositions with x = 0.1; 0.15; 0.2 and y = 0.013; 0.023; 0.04, were successfully prepared. The Te-vacancies and the anti-site Bi Te defects are specific for the Te-deficient compositions, whereas the interstitial Te atoms and the anti-site Te Bi defects are characteristic for the Te-rich compositions. Deviation from Te-stoichiometry is originated from high-temperature Te evaporation under sintering. The Te-enriched starting Bi 2- x Gd x Te 3 powders were applied to tune the Te-stoichiometry in desired manner. A rate of the Te evaporation decreases with increasing x that is related to difference in electronegativites of the Bi and Gd atoms, which, in turn, results in increasing the strength of polar covalent Gd-Te bond as compared to that for polar covalent Bi-Te bond. Crystal lattice parameters, concentration and mobility of electrons, thermoelectric properties, and grain ordering degree are different for the nonstoichiometric Te-deficient and Te-rich compositions. Highest thermoelectric figure-of-merit is found for the Te-deficient Bi 1.99 Gd 0.01 Te 2.989 composition ( ZT ≈0.7 at ~400 K).