Panoramic Synthesis as an Effective Materials Discovery Tool: The System Cs/Sn/P/Se as a Test Case

The common approach to
the synthesis of a new material
involves reactions held at high temperatures under certain conditions
such as heating in a robust vessel in the dark for a period until
it is judged to have concluded. Analysis of the vessel contents afterward
provides knowledge of the final products only. Intermediates that
may form during the reaction process remain unknown. This lack of
awareness of transient intermediates represents lost opportunities
for discovering materials or understanding how the final products
form. Here we present new results using an emerging in situ monitoring
approach that shows high potential in discovering new compounds. In
situ synchrotron X-ray diffraction studies were conducted in the Cs/Sn/P/Se
system. Powder mixtures of Cs2Se2, Sn, and PSe2 were heated to 650 °C and then cooled to room temperature
while acquiring consecutive in situ synchrotron diffraction patterns
from the beginning to the end of the reaction process. The diffraction
data was translated into the relationship of phases present versus
temperature. Seven known crystalline phases were observed to form
on warming in the experiment: Sn, Cs2Se3, Cs4Se16, Cs2Se5, Cs2Sn2Se6, Cs4P2Se9, and Cs2P2Se8. Six unknown phases
were also detected; using the in situ synchrotron data as a guide
three of them were isolated and characterized ex situ. These are Cs4Sn­(P2Se6)2, α-Cs2SnP2Se6, and Cs4(Sn3Se8)­[Sn­(P2Se6)]2. Cs4(Sn3Se8)­[Sn­(P2Se6)]2 is a two-dimensional compound that behaves as an n-type
doped semiconductor below 50 K and acts more like a semimetal at higher
temperatures. Because all crystalline phases are revealed during the
reaction, we call this approach “panoramic synthesis”.