Nanostructures in Te/Sb/Ge/Ag (TAGS) Thermoelectric Materials Induced by Phase Transitions Associated with Vacancy Ordering

Te/Sb/Ge/Ag
(TAGS) materials with rather high concentrations of
cation vacancies exhibit improved thermoelectric properties as compared
to corresponding conventional TAGS (with constant Ag/Sb ratio of 1)
due to a significant reduction of the lattice thermal conductivity.
There are different vacancy ordering possibilities depending on the
vacancy concentration and the history of heat treatment of the samples.
In contrast to the average α-GeTe-type structure of TAGS materials
with cation vacancy concentrations <∼3%, quenched compounds
like Ge0.53Ag0.13­Sb0.27□0.07Te1 and Ge0.61Ag0.11­Sb0.22□0.06Te1 exhibit “parquet-like”
multidomain
nanostructures with finite intersecting vacancy layers. These are
perpendicular to the pseudocubic ⟨111⟩ directions but
not equidistantly spaced, comparable to the nanostructures of compounds
(GeTe)n­Sb2Te3. Upon heating, the nanostructures transform into long-periodically
ordered trigonal phases with parallel van der Waals gaps. These phases
are slightly affected by stacking disorder but distinctly different
from the α-GeTe-type structure reported for conventional TAGS
materials. Deviations from this structure type are evident only from
HRTEM images along certain directions or very weak intensities in
diffraction patterns. At temperatures above ∼400 °C, a
rock-salt-type high-temperature phase with statistically disordered
cation vacancies is formed. Upon cooling, the long-periodically trigonal
phases are reformed at the same temperature. Quenched nanostructured
Ge0.53Ag0.13­Sb0.27□0.07Te1 and Ge0.61Ag0.11­Sb0.22□0.06Te1 exhibit ZT values
as high as 1.3 and 0.8, respectively, at 160 °C, which is far
below the phase transition temperatures. After heat treatment, i.e.,
without pronounced nanostructure and when only reversible phase transitions
occur, the ZT values of Ge0.53Ag0.13­Sb0.27□0.07Te1 and Ge0.61Ag0.11­Sb0.22□0.06Te1 with extended van der Waals gaps amount to 1.6 at 360 °C
and 1.4 at 410 °C, respectively, which is at the top end of the
range of high-performance TAGS materials.