Silver Indium Telluride Semiconductors and Their Solid Solutions with Cadmium Indium Telluride: Structure and Physical Properties

Ag0.8In2.4Te4 (= AgIn3Te5) and Ag0.5In2.5Te4 (= AgIn5Te8) form solid solutions with CdIn2Te4, which are interesting as materials for photovoltaics or with respect
to their thermoelectric properties. The corresponding crystal structures
are related to the chalcopyrite type. Rietveld refinements of high-resolution
synchrotron powder diffraction data measured at K-absorption edges
of Cd, Ag, In, and Te and electron diffraction reveal the symmetry
as well as the element and vacancy distribution in Ag0.8In2.4Te4 (= AgIn3Te5)/Ag0.5In2.5Te4 (= AgIn5Te8) mixed crystals such as Ag0.25Cd0.5In2.25Te4 and Ag0.2Cd0.75In2.1Te4. All compounds of the solid solution
series (CdIn2Te4)x(Ag0.5In2.5Te4)1–x exhibit the HgCu2I4 structure
type (space group I4̅2m) with
completely ordered vacancies but disordered cations. The uniform cation
distribution and thus the local charge balance are comparable to that
of CdIn2Te4. In contrast, Ag0.8In2.4Te4 (= AgIn3Te5) crystallizes
in the space group P4̅2c with
disordered cations and partially ordered vacancies. This is corroborated
by bond-valence sum calculations and the fact that there is a Vegard-like
behavior for compounds with 0.5 < x in the pseudobinary
system (CdIn2Te4)x(Ag0.8In2.4Te4)1–x. Owing to the different structures, there is no
complete solid solution series between CdIn2Te4 and AgIn3Te5. All compounds in this work are
n-type semiconductors with a low electrical conductivity (∼1
S/m) and rather high absolute Seebeck coefficients (up to −750
μV/mK; 225 °C). Electrical band gaps (Eg) determined from the Seebeck coefficients as well as
(more reliably) from the electrical conductivity range between 0.19
and 1.13 eV.