posted on 2018-09-12, 00:00authored byJames
M. Hodges, Yi Xia, Christos D. Malliakas, Grant C. B. Alexander, Maria K. Y. Chan, Mercouri G. Kanatzidis
Metal
chalcogenides underpin a wide variety of energy-related applications
and are ideal systems for probing lattice dynamics and fundamental
transport phenomena. Here we describe the synthesis and transport
properties of CsAg5TeS2 and its solid solution
CsAg5Te3–xSx (x = 1–2), new semiconductors
with tunable band gaps ranging from 0.17 to 0.30 eV. CsAg5TeS2 has a fully ordered two-dimensional structure that
includes a group of Ag atoms in a heteroleptic tetrahedral coordination
geometry (AgTe2S2). Single-crystal X-ray diffraction
indicates that the compounds crystallize in the tetragonal space group P4/mmm, while pair distribution function
(PDF) analysis reveals off-centering at the heteroleptic Ag sites,
signifying the lower-symmetry I4/mcm space group. The underlying disorder acts as a phonon-blocking mechanism
that helps facilitate an ultralow lattice thermal conductivity below
0.40 W·m–1·K–1 at ∼300
K , highlighting the importance of local disorder in thermal transport.
Density functional theory provides additional insight into the electronic
and thermal properties of the materials, which are good candidates
for p-type thermoelectrics.