Interconnected effects of direct Gd doping and accompanying indirect Te-stoichiometry destroying on the thermoelectric properties of Te-rich Bi2-Gd Te3+ compounds

Solvothermal synthesis and spark plasma sintering were applied to prepare Te-rich Bi 2- x Gd x Te 3+ y compounds with 1; 2; 3 and 4 ​at. % Gd. To synthesize the Te-rich compositions, 3 ​at. % Te excess was introduced into all the starting powders with different x . Under sintering, high-temperature Te evaporation occurred. Owing to difference in electronegativity of Bi and Gd, resulting in increasing of ionicity degree and relevant strengthen of polar covalent Bi(Gd)-Te bond, a rate of the Te evaporation is remarkably decreasing with increasing x . As result, the x (Gd content) and y (Te content) coefficients happen to be interconnected. Specific point defects are formed via direct Gd doping (dopant Gd atoms substituting for Bi) and indirect destroying Te-stoichiometry (interstitial Te atoms and anti-site Te Bi defects). Effects of these defects on the thermoelectric properties of the Bi 2- x Gd x Te 3+ y compounds with interconnected x and y coefficients were explored in detail. Specific anomalies of all the thermoelectric properties (the specific electrical resistivity, the Seebeck coefficient and the total thermal conductivity), related to onset of intrinsic electrical conductivity, are shifted to higher temperatures with increasing Gd content, which is accompanying by increasing Te content. This x ( y )-dependent shifting can be originated from increasing energy gap, which is, in turn, due to strengthen of polar covalent Bi(Gd)-Te bonds in the Gd-doped samples. The best thermoelectric properties are observed in the composition with 1 ​at. % Gd and 59.94 ​at. % Te. This composition is very close to Te-stoichiometric one and is optimal, since it has minimal electron concentration and maximal electron mobility. As is shown in upper Figure, changing the Te-stoichiometry, which is due to the high-temperature Te evaporation, occurs during spark plasma sintering of Te-rich Bi 2- x Gd x Te 3+ y samples. The evaporation rate is governed by Gd content in starting powders. This effect is related to increasing the ionicity degree and strength of polar covalent Bi(Gd)-Te bonds. As is shown in bottom Figure, with increasing the ionicity degree, onset of intrinsic electrical conductivity in the samples is shifting to higher temperatures in accordance with the Manka expression. • Bi 2- x Gd x Te 3+ y samples prepared by solvothermal synthesis and spark plasma sintering. • 3 ​at. % Te excess introduced into starting powders to prepare the Te-rich samples. • Rate of high-temperature Te evaporation during sintering decreases with increasing x. • The x (Gd) and y (Te) coefficients in the samples after sintering are interconnected. • The Gd doping affects a strength of polar covalent Bi(Gd)-Te bonds in Bi 2- x Gd x Te 3+ y .