Thermoelectric materials utilize the Seebeck effect to convert thermal to electrical energy. The Seebeck coefficient (thermopower), S, depends on the free (mobile) carrier concentration, n, and effective mass, m*, as In some materials the influence of m* on the Seebeck coefficient can be significant and its determination is important for better understanding of thermoelectric materials. 125 Te nuclear magnetic resonance (NMR) [4][5][6][7][8][9] can be used to study complex tellurides and derive the carrier concentration from spin-lattice relaxation time, T 1 . In a theoretical model by Selbach et al. [10], the coefficient of proportionality between T 1 and n, "Bloembergen constant", C B , depends on m*. Hence, if the effective mass in a series of materials is not constant, it can affect C B , and, therefore, the carrier concentration deriving from 125 Te NMR T 1 measurements should be different compared to that obtained if m* is constant.Here it is shown that the combination of the (Table 1) were synthesized by a direct reaction of the constituent elements in 10 mm diameter fused silica ampoules in argon backfilled up to ~17 kPa pressure atmosphere. For synthesis we used Ge (Materion G-1038, 99.999% purity), and Te (Alfa Aesar, 99.9999% purity, product #12607, lot 004). Ampoules with the constituent elements were heated up to 1323 K to melt the constituents, and after ~4 hours the melt was cooled down with the furnace at the rate of 100 K/hour. Each ingot had a mass of 20 g, diameter of 10 mm, length of ~40 mm. Analysis of each material was performed by X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy, and showed that all materials are nearly single phase; only ~1 at.% Ge is present in the samples matrix due to naturally occurring process of Ge rejection from the melt [8,14]. 125 Te NMR experiments were performed at 126 MHz using a Bruker 400WB spectrometer with TOPSPIN software in a magnetic field of 9.4 T without sample spinning 4 (static regime). Powder samples were prepared from ingots and placed in 4-mm diameter insert. 125 Te NMR spin-lattice relaxation measurements were used to obtain the spin-lattice relaxation time, T 1 , using GeTe as a reference material [5,8]; the uncertainty of T 1 measurements is less than ±0.2 ms [5].More experimental details including pulse sequence for 125 Te NMR can be found in Refs. 5,8,9,14. Samples for the Seebeck coefficient measurements were prepared by cutting the ingot with a diamond saw [9]. The Seebeck coefficient was measured by a LSR-3 measuring system (Linseis Inc.) in a helium atmosphere relative to the Pt legs of a Pt-(Pt+Rh) thermocouple and then the absolute Seebeck coefficient was calculated; the uncertainty of measurements is about ±3% [9]. Table 1). 125 Te NMR spectra for these two materials are shown on inset in Fig. 1(a), demonstrating that signals can be easily detected; spectra for the rest of materials also are located between those for GeTe and TAGS-85.Relaxation curves shown on Fig. 1(a) as well as for ...