Crystal structures and thermoelectric properties of the series Tl_10−xLa_xTe₆with 0.2 ≤ x ≤ 1.15

Abstract

The tellurides Tl(10-x)La(x)Te(6) were synthesized from the elements in stoichiometric ratios at 873 K, followed by slow cooling. These materials are substitution variants of Tl(5)Te(3), crystallizing in space group I4/mcm, with lattice dimensions of a = 8.9220(4) Å, c = 13.156(1) Å, V = 1047.2(1) Å(3), for x = 1 (Z = 2). Increasing the La content occurs with an increase in the unit cell volume and the c axis, but a decrease of the a axis. Tl(5)Te(3) is a metallic compound, while Tl(9)LaTe(6) was calculated to be semiconducting. Correspondingly, the Seebeck coefficient increases with increasing x, while the electrical and thermal conductivity both decrease. The highest thermoelectric figure-of-merit determined thus far is 0.21 at 581 K for cold-pressed Tl(9)LaTe(6).

Phase range and physical properties of the thallium tin tellurides Tl10−xSnxTe6 (x≤2.2)

“…Following the aforementioned logic, x = 2.2 would yield (Tl + ) 7.8 (Sn 2+ ) 2.2 (Te 2− ) 6 (e − ) 0.2 leaving an overall electron excess of 0.2 in the proposed formula and hence n-type conduction, in contrast to the experimentally observed ptype conduction. It is noted that we also observed p-type carriers in the isostructural Tl 8.85 La 1.15 Te 6 that also formally contained excess electrons [20]. Some side products may be present below the detection limit of diffraction studies; for attempted x > 2.2, this entails one additional phase -SnTe, which is present between 3 and 6% for "Tl 7.6 Sn 2.4 Te 6 as found in its diffraction pattern.…”

“…Amongst those materials of interest, one will find a number of thallium tellurides including Tl 2 GeTe 3 [15], Tl 9 AgTe 5 [16], Tl-doped PbTe [17], and TlSbTe 2 [18]. Tl 9 BiTe 6 [19] and Tl 9 LaTe 6 [20], substitution variants of mixed-valent Tl 5 Te 3 [21], are narrow band gap semiconductors; strong potential benefits for thermoelectric performance originate from the structure's complex network of heavy atoms [22]. Similarly, studies of the compound Tl 4 SnTe 3 [23] ( Tl 8 Sn 2 Te 6 ) revealed a high thermoelectric figure-of-merit, ZT, culminating in 0.74 at 673 K [24].…”

Thermoelectric properties of hot-pressed Tl9LnTe6 (Ln=La, Ce, Pr, Nd, Sm, Gd, Tb) and Tl10−xLaxTe6 (0.90⩽x⩽1.05)

“…Its substantially higher thermoelectric figure-of-merit benefits significantly from the consolidation method, which yielded 98% of the theoretical density. By contrast, the coldpressed La sample had a density of only 80% [16].…”

“…4(b). Electronic structure calculations imply that as x increases in Tl 10Àx La x Te 6 , a transition from metallic (x = 0) to p-doped semiconducting (x < 1) then to n-doped semiconducting (x > 1) behavior is anticipated [16]. Actually, the above mentioned p-n transition has been demonstrated in Tl 9Àx Bi 1+x Te 6 [9].…”

“…We prepared a series of Tl 9 LnTe 6 by the cold-pressing and sintering method and determined their electrical and thermal properties [16,17]. At 500 K, thermal conductivity values as low as 0.40-1.25 W m À1 K À1 were observed in these materials.…”