Phase Transitions of Thermoelectric TAGS-85

Abstract

The alloys (GeTe)x(AgSbTe2)100–x, commonly known as TAGS-x, are among the best performing p-type thermoelectric materials for the composition range 80 ≤ x ≤ 90 and in the temperature range 200–500 °C. They adopt a rhombohedrally distorted rocksalt structure at room temperature and are reported to undergo a reversible phase transition to a cubic structure at ∼250 °C. However, we show that, for the optimal x = 85 composition (TAGS-85), both the structural and thermoelectric properties are highly sensitive to the initial synthesis method employed. Single-phase rhombohedral samples exhibit the best thermoelectric properties but can only be obtained after an annealing step at 600 °C during initial cooling from the melt. Under faster cooling conditions, the samples obtained are inhomogeneous, containing multiple rhombohedral phases with a range of lattice parameters and exhibiting inferior thermoelectric properties. We also find that when the room-temperature rhombohedral phase is heated, an intermediate trigonal structure containing ordered cation vacancy layers is formed at ∼200 °C, driven by the spontaneous precipitation of argyrodite-type Ag8GeTe6 which alters the stoichiometry of the TAGS-85 matrix. The rhombohedral and trigonal phases of TAGS-85 coexist up to 380 °C, above which a single cubic phase is obtained and the Ag8GeTe6 precipitates redissolve into the matrix. On subsequent cooling a mixture of rhombohedral, trigonal, and Ag8GeTe6 phases is again obtained. Initially single-phase samples exhibit thermoelectric power factors of up to 0.0035 W m–1 K–2 at 500 °C, a value that is maintained on subsequent thermal cycling and which represents the highest power factor yet reported for undoped TAGS-85. Therefore, control over the structural homogeneity of TAGS-85 as demonstrated here is essential in order to optimize the thermoelectric performance.

Chalcogenides by Design: Functionality through Metavalent Bonding and Confinement

“…GeTe is a well‐known PCM of its own and even more widely applied when combined with Sb 2 Te 3 . In addition, it is used in thermoelectric materials like TAGS, [ 69,70 ] a solid solution of GeTe and AgSbTe 2 . Recently, GeTe has also been shown to be the basis of the novel class of ferroelectric Rashba semiconductors.…”

A cubic room temperature polymorph of thermoelectric TAGS-85

“…In some cases ingots are obtained by direct quenching from the melt 14,15 or held rst at an intermediate temperature, 11,12 and in other reports additional processing steps are carried out on as-prepared ingots such as subsequent annealing 10 or grinding and hot-pressing. 9,16,17 Both from our own work 18 and from other previous reports, 9,[13][14][15] there are suggestions that TAGS-85 samples are oen inhomogeneous or multi-phase in nature as evidenced by irregular or asymmetric peak shapes in X-ray diffraction measurements. This has motivated us to look more closely at whether the crystal structure and homogeneity can be inuenced by careful control of the chemical synthesis conditions.…”

“…The high electrical conductivity is reected in a high thermal conductivity, which rises slightly from $2.5 W m À1 K À1 at 50 C to $2.9 W m À1 K À1 at 400 C. Consequently, the maximum ZT is just over 0.4 at 400 C. The signicantly worse thermoelectric properties of the C RT /21P phase might be partially due to the higher volume fraction of the Ag 8 GeTe 6 precipitate, which is $2% compared to $1% for our previous rhombohedral samples. 18 This issue might also be linked to the different density of cation vacancy layers in the 21P structure of the current study and the 39-layer (39P) structure previously obtained on heating rhombohedral samples. The 21P structure has roughly double the density of vacancy layers, which might lead to a self-doping effect and a hole concentration that is higher than optimal with respect to thermoelectric performance.…”

Argyrodite‐Type Cu8GeSe6–xTex (0 ≤ x ≤ 2): Temperature‐Dependent Crystal Structure and Thermoelectric Properties

“…The introduction of Ag in GST materials leads to the well‐known TAGS (Te/Sb/Ge/Ag) thermoelectric materials , . In TAGS materials, argyrodite‐type Ag 8 GeTe 6 has been found as a side phase (depending on the synthesis conditions) and may influence mechanical properties like creep behavior , . In quinary phases of the system Cu/Ge/Sb/Te/Se with the nominal compositions Cu 2 Ge 11 Sb 2 Te 12 Se 3 and Cu 2 Ge 11 Sb 2 Te 7.5 Se 7.5 , an argyrodite‐type second phase with composition Cu 8 GeSe 4 Te 2 is formed in a matrix of Cu‐ and Se‐doped GST phases (cf.…”

“…This study aims at exploring the maximum substitution range for Te and at extending structural investigations into the temperature range of good thermoelectric properties. Since Ag 8 GeTe 6 has also been detected in Te/Sb/Ge/Ag (= TAGS) thermoelectric materials,, and since GeTe‐Ag 8 GeTe 6 eutectic composites have been discussed, possibilities for heterostructuring in the quinary system Cu/Ge/Sb/Se/Te also seemed intriguing.…”