Crystal structure, magnetization, 125Te NMR, and Seebeck coefficient of Ge49Te50R1 (R = La, Pr, Gd, Dy, and Yb)

Abstract

GeTe, a self-doping semiconductor, is a well-known base compound for thermoelectric and phase-change materials. It is known, that replacement of Ge in Ag 6.5 Sb 6.5 Ge 37 Te 50 (TAGS-85) material by rare earth Dy significantly enhances both the power factor and thermoelectric figure of merit. Here we demonstrate how replacement of Ge in GeTe by rare earths with different atomic size and localized magnetic moments affect XRD patterns, magnetization, 125 Te NMR spectra and spin-lattice relaxation, and the Seebeck coefficient of the alloys with a nominal composition of Ge 49 Te 50 R 1 (R = La, Pr, Gd, Dy, and Yb). SEM, EDS and WDS data show that rare earth atoms in the matrix are present at smaller extent compared to a nominal composition, whereas rare earth also is present in the inclusions. Rare earths affect the Seebeck coefficient, which is a result of interplay between the reduction due to higher carrier concentration and enhancement due to magnetic contribution. The effect of replacement of Ge in GeTe by Dy on the Seebeck coefficient is smaller than that observed in Ag 6.5 Sb 6.5 Ge 36 Te 50 Dy 1. This can be explained by larger amount of rare earth, which can be embedded into the lattice of materials containing [Ag+Sb] atomic pairs and possible effect from these pairs.

125Te NMR for structural investigations in phase change materials: Optimization of experimental conditions coupled to NMR shift prediction

“…A similar method was also used by Sen et al [17] to study amorphous and fcc Ge1Sb2Te4 and Ge2Sb2Te5, by Levin et al and Cui et al [18][19][20][21][22][23] [24] for GeTe based materials and Bi2Te3 and by Njegic et al [25] for the PbTe-GeTe binary and PbTe-SnTe binary. It was also used by Garaga et al [26] on Te oxides but using a CPMG sequence which allows to obtain a better sensitivity.…”