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

Article abstract of DOI:10.1021/jacs.7b05423

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 .

Full text of DOI:10.1021/jacs.7b05423

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Article
Panoramic synthesis as an effective materials
discovery tool: The system Cs/Sn/P/Se as a test case
Alyssa S Haynes, Constantinos C. Stoumpos, Haijie Chen, Daniel Chica, and Mercouri G. Kanatzidis
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05423 • Publication Date (Web): 30 Jun 2017
Downloaded from http://pubs.acs.org on July 2, 2017
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Panoramic synthesis as an effective materials discovery tool: The
system Cs/Sn/P/Se as a test case
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Alyssa S. Haynes,1 Constantinos Stoumpos,1 Haijie Chen, 1,2 Daniel Chica,1 and Mercouri G.
Kanatzidis1,2
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1Department of Chemistry, Northwestern University, Evanston, Illinois, 60208
2Materials Science Division, Argonne National Laboratory, Lemont, Illinois, 60439
Abstract
The common approach to a new material synthesis 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 Xꢀray diffraction studies were conducted in the Cs/Sn/P/Se
system. Powder mixtures of Cs2Se2, Sn, and PSe2 were heated to 650 ºC 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, αꢀCs2SnP2Se6,
and Cs4(Sn3Se8)[Sn(P2Se6)]2. Cs4(Sn3Se8)[Sn(P2Se6)]2 is a twoꢀdimensional compound that
behaves as an nꢀtype 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”.
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Introduction
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Solid state synthesis methods for materials discovery such as the molten salt flux and
solvothermal techniques have been developed to overcome problems of low diffusion and
energyꢀminimum thermodynamic products observed with traditional “heat and beat” synthesis.1ꢀ9
However, with current solid state synthetic techniques, we can only observe the final products.
We generally do not know if these products form on heating, soaking, or cooling or how long
they survive in the reaction. In this regard, our syntheses are “blind” as we are unaware of any
interesting intermediates and mechanistic insight they would provide.
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As a way to overcome these barriers, we have proposed a materials discovery method,
which allows us to examine the entire synthetic reaction process using in situ synchrotron
powder Xꢀray diffraction (PXRD).10 By using temperature as a variable, the relationships among
phases can be probed as the reaction proceeds. The goal is to utilize in situ synchrotron PXRD to
uncover and characterize phases not accessible through traditional “blind” synthesis.
In situ synchrotron Xꢀray diffraction (XRD) studies have previously been employed to
monitor the formation and growth of crystalline inorganic solids, and most studies have used
hydrothermal,11ꢀ15 solvothermal,16ꢀ18 solꢀgel,14,19,20 molten salt flux,10,21 ionꢀthermal,22,23 or
surfactantꢀthermal methods.23,24 The majority of these investigations aimed to understand the
reaction mechanism, and some studies compared syntheses using different parameters (e.g.
temperature, pH, source materials, stoichiometry) to determine how the reaction pathway was
affected. Many of these experiments revealed previously unknown intermediates, however, only
meltꢀquench reactions were employed to isolate the intermediates ex situ, and none were
successful.11,13ꢀ16,20 The one exception is our previous in situ synchrotron PXRD study with the
goal of discovering metastable inorganic materials.10 We showed that even wellꢀinvestigated
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systems of K/Cu/S and K/Sn/S still had new materials to yield that were recovered ex situ.
Through these experiments, four known and four new phases were observed whose structures
were solved and refined using the in situ PXRD as a guide.
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Additionally, we have conducted in situ synchrotron PXRD measurements on amorphous
CsPSe6, which uncovered the crystallization of the new phase, βꢀCsPSe6, and its phase transition
to the known αꢀCsPSe6.25 The energy of the phase transition had been too low to be detected via
differential thermal analysis (DTA) so the in situ experiment was needed to accurately observe
the kinetics of the system. Using the information obtained from these experiments, we
successfully isolated and characterized βꢀCsPSe6 ex situ and discovered it possessed unique
perpendicular chain packing within the structure. Therefore, these early examples point to the
high utility of in situ synchrotron XRD studies as a direct means to understand the details of a
synthetic reaction. This approach allows us to become aware not only of the final products but of
all the crystalline intermediates as well as when they form, how long they exist, etc. This
information is crucial to design a successful synthesis for them as we have done in this paper.
Herein we have applied this in situ PXRD approach to the Cs/Sn/P/Se system as a new
model to further explore its power to help discover new phases. We chose this system because
we have extensive expertise in the synthesis and study of chalcophosphate compounds.26
Chalcophosphates have useful properties including reversible phase transitions,27ꢀ31
ferroelectricity,32 photoluminescence,33,34 γꢀray detection,35 and nonlinear optics.25,36ꢀ39 Powder
mixtures of Cs2Se2, Sn, and PSe2 were heated to 650 ºC then cooled while continuously
monitoring the crystalline phases present with in situ synchrotron PXRD. From this data, we
created a phase versus temperature map containing seven known compounds (Sn,40 Cs2Se3,41
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Cs4Se16,42 Cs2Se5,43 Cs2Sn2Se6,44 Cs4P2Se9,45 and Cs2P2Se8 ) and six unknowns. Our
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experiments reveal that the phase space is very rich on heating, but only one unknown crystalline
phase ultimately forms on cooling.
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Using the in situ synchrotron PXRD data as a guide, we successfully synthesized and
determined the structures of three new tin selenophosphates. The first one, Cs4Sn(P2Se6)2,
contains molecular ions of [Sn(P2Se6)2]4−, with octahedrally coordinated Sn4+ sandwiched
between two [P2Se6]4− units. The second, αꢀCs2SnP2Se6, features 1/∞[SnP2Se6]2− chains with Sn2+
ions adopting a seesaw coordination geometry. The third compound was Cs4(Sn3Se8)[Sn(P2Se6)]2,
a complex phase comprising of defective SnSe2 layers, formulated as 2/∞[Sn3Se8]4−, decorated on
both sides of the layers with terminal [Sn(P2Se6)] groups. These groups bind to the 2/∞[Sn3Se8]4−
layers through three selenium ions that complete the octahedral coordination of the Sn4+ ions.
Cs4(Sn3Se8)[Sn(P2Se6)]2 displays semimetallic behavior above 50 K and nꢀtype doped
semiconductor properties at lower temperatures.
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Experimental Section
Reagents. All chemicals were used as obtained: cesium metal (99.9+%, Strem Chemicals,
Inc., Newburyport, MA); tin powder (99.5% Sigma Aldrich, St. Louis, MO); red phosphorus
powder (99%, Sigma Aldrich, St. Louis, MO); selenium pellets (99.99%, Sigma Aldrich, St.
Louis, MO). Cs2Se and Cs2Se2 were synthesized by reacting stoichiometric amounts of the
elements in liquid ammonia as described elsewhere.46,47 Glassy PSe2 was formed by reacting P
(0.082 g, 2.65 mmol) and Se (0.418 g, 5.29 mmol) under an evacuation of ~10−4 mbar at 600 ºC
for 12 h. P2Se5 was made by reacting a stoichiometric ratio of elements using traditional solid
state synthesis as described previously.47
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Preparation of Starting Material for in situ Synchrotron PXRD. Amounts listed
below of Cs2Se2, Sn, and PSe2 were combined in a mortar in a dry, N2 atmosphere glovebox. The
reactants in a mortar were ground with a pestle and mixed with a spatula for ~10 minutes after
the mixture looked homogeneous. The powder mixture was loaded into 0.3 mm outer diameter
fused silica capillaries and packed tightly to ~1 cm in length of the capillary. A layer of fused
silica granules (63ꢀ90 µm) were added and served as the point of sealing of the capillary while
keeping the tight packing of the powder. The capillaries were flame sealed under an evacuation
of ~10−4 mbar.
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Sn1: Cs2Se2 (0.085 g, 0.20 mmol), Sn (0.012 g, 0.10 mmol), PSe2 (0.076 g, 0.40 mmol)
Sn2: Cs2Se2 (0.085 g, 0.20 mmol), Sn (0.024 g, 0.20 mmol), PSe2 (0.076 g, 0.40 mmol)
Sn3: Cs2Se2 (0.085 g, 0.20 mmol), Sn (0.047 g, 0.40 mmol), PSe2 (0.076 g, 0.40 mmol)
Ex Situ Synthesis of Cs4Sn(P2Se6)2 (Newꢀ6). Amounts of Cs2Se (0.169 g, 0.40 mmol),
Sn (0.024 g, 0.20 mmol), P (0.025 g, 0.80 mmol), and Se (0.126 g, 1.6 mmol) were combined in
a 9 mm carbonꢀcoated fused silica tube in a dry, nitrogenꢀfilled environment then flame sealed
after evacuating the tube to ~10−4 mbar. The tube was placed in a furnace and heated to 650 ºC,
soaked for 10 h, then cooled to 250 ºC in 10 h where the furnace was turned off to allow the tube
to cool to room temperature. The product contained black hexagonal plate crystals of
Cs4Sn(P2Se6)2 (100%), which are air and water stable. Energy dispersive Xꢀray spectroscopy
(EDS) gave an average composition of multiple crystals of Cs3.8SnP4.2Se11.7
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Ex Situ Synthesis of αꢀCs2SnP2Se6 (Newꢀ4). Cs2Se (0.169 g, 0.40 mmol), Sn (0.048 g,
0.40 mmol), P (0.050 g, 1.6 mmol), and Se (0.568 g, 7.2 mmol) were combined in a carbonꢀ
coated fused silica 9 mm tube in a dry, N2 atmosphere glovebox. The tube was evacuated to
~10−4 mbar, flame sealed, then subjected to a heating profile of heating to 650 ºC, soaking for 10
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h, then cooling to room temperature in 15 h. The product contained translucent orange rod
crystals of αꢀCs2SnP2Se6 (85%) and SnSe (15%).48 EDS gave an average composition of
multiple crystals of Cs1.9SnP2.2Se5.5. αꢀCs2SnP2Se6 was stored in an evacuated desiccator because
it is air and water sensitive.
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Ex Situ Synthesis of Cs4(Sn3Se8)[Sn(P2Se6)]2 (Newꢀ5). A mixture of Cs2Se (0.276 g,
0.80 mmol), Sn (0.237 g, 2.0 mmol), P (0.050 g, 1.6 mmol), and Se (0.568 g, 7.2 mmol) were
loaded into a 9 mm carbonꢀcoated fused silica tube in a dry, nitrogenꢀfilled glovebox. The tube
was removed, evacuated to ~10−4 mbar then flame sealed. It was then placed into a furnace
where it heated to 850 ºC, soaked for 24 h, then cooled to room temperature in 24 h. The product
was irregularꢀshaped shiny black crystals of Cs4(Sn3Se8)[Sn(P2Se6)]2 (90%) and a small amount
of SnSe2 (10%).49 EDS gave an average composition of multiple crystals of Cs4Sn4.9P4.3Se19.9
This material is air and water stable.
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Physical Measurements
HighꢀResolution in situ PXRD with Synchrotron Radiation. In situ synchrotron
PXRD were taken at beamline 17ꢀBMꢀB in the Advanced Photon Source at Argonne National
Laboratory running at 18 keV (λ = 0.72768 Å) with a Perkin Elmer aꢀSi Cꢀwindow detector at a
distance of 500 mm. A dark frame was taken before each PXRD frame, and 35 1 s exposures
were averaged together for each PXRD pattern. The capillary furnace setup translated
horizontally across the entire capillary twice with respect to the synchrotron beam for every
averaged PXRD pattern to obtain data of the entire capillary. An electrical resistance furnace
detailed by Chupas, et al. was used to heat the capillaries.50 The samples were heated to 200 ºC at
a rate of 10 ºC/min then further heated to 650 ºC at 2 ºC/min. With no soaking time at 650 ºC, the
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capillaries cooled to 200 ºC at a rate of 2 ºC/min, then cooled further to room temperature at a
rate of 10 ºC/min. LaB6 was used as the standard to refine the sampleꢀtoꢀdetector distance and
imaging plate tilt relative to the beam. The raw images were processed in GSAS II.51 Peak
matching of the data was conducted by comparing calculated PXRD patterns obtained from the
ICSD database to the experimental data.
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Ex situ PXRD. PXRD patterns were obtained from a Rigaku Miniflex600 diffractometer
with Cu Kα radiation operating at 40 kV and 15 mA with a highꢀspeed silicon strip detector. A
zeroꢀbackground Si sample holder with 0.2 mm indent was used. The experimental PXRD
patterns were compared to simulated ones from CIFs using the FINDIT software for the ICSD.
Single Crystal XRD. Single crystals were fixed to glass fibers using super glue then
analyzed in a STOE IPDS II single crystal diffractometer with Mo Kα radiation (λ = 0.71073 Å)
operating at 50 kV and 40 mA. The STOE programs XꢀArea, XꢀRED, and XꢀSHAPE were used
to conduct the data collection, integration, and numerical absorption, respectively. The atomic
sites without disorder from the analogous Rb4Sn5P4Se2052 were used to solve the crystal structure
of Cs4(Sn3Se8)[Sn(P2Se6)]2 and SHELXT53 was used to solve the crystal structures of
Cs4Sn(P2Se6)2 and αꢀCs2SnP2Se6. The structures were all refined in the SHELXTL program
package.54 The twinning in αꢀCs2SnP2Se6 was subsequently refined using Jana2006.55 Details on
refining disorder and twinning are in the Supporting Information (SI). The CIFs were finalized
using EnCIFer.56
Scanning Electron Microscopy (SEM). Semiquantitative microprobe analyses and EDS
were performed on a Hitachi Sꢀ3400 scanning electron microscope equipped with a PGT energyꢀ
dispersive Xꢀray analyzer. For EDS, parameters of 25 kV accelerating voltage, 60 mA probe
current, and 60 s acquisition time were used.
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SolidꢀState UV/Vis/NearꢀIR Spectroscopy. Powdered samples were placed on a bed of
BaSO4, which was set to 100% reflectance. Diffuse reflection measurements from 200 – 2000
nm were taken using a Shimadzu UVꢀ3600 PC doubleꢀbeam, doubleꢀmonochromator
spectrophotometer. The KubelkaꢀMunk equation: α/S = (1–R)2/2R, where R is reflectance, α is
the absorption coefficient, and S is the scattering coefficient, was employed to convert the
reflectance data into absorption.46 The band gap was estimated by linearly fitting the absorption.
Differential Thermal Analysis (DTA). Granules of the sample (~80 mg) were flame
sealed in an ampoule evacuated to ~10−4 mbar. An ampoule of the standard, αꢀAl2O3, with a
similar mass was also prepared. A Shimadzu DTAꢀ50 thermal analyzer carried out the DTA
experiments with parameters of ± 5 ºC/min rate and a maximum temperature of 650 ºC. The
temperature at the center of the peaks and valleys are reported as the crystallization and melting
temperatures, respectively.
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Transport Measurements. Transport measurements were conducted on
Cs4(Sn3Se8)[Sn(P2Se6)]2 single crystals. Resistivity and Hall effect were measured on a Quantum
Design PPMSꢀ9T. Contacts were made with gold wires attached to the sample surface using
carbon paste, and sample dimensions were obtained from SEM images.
Results and Discussion
In Situ Synchrotron PXRD. We chose to explore the Cs/Sn/P/Se system because we
have studied it previously and found the quaternaries Cs2SnP2Se657 and Cs6Sn2P2Se14.58 Being a
known system to us presented the possibility to compare the effectiveness of the inꢀsitu approach
we describe here to the conventional “blind” approach. We were also interested in answering the
question of how these phases form and whether any intermediate phases existed, that had not
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been observed previously. Cs2SnP2Se6 was picked as the starting point, and we decided to
explore how the phase composition changes by varying the amount of tin in the Cs/P/Se flux.
Figure 1a depicts the three reactions set up for in situ synchrotron PXRD. The three reactions are
labeled as Sn1, Sn2, and Sn3, according to the equivalence of tin in the reaction.
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A schematic of the glass capillary furnace used for the in situ synchrotron PXRD is
shown in Figure 1b. PXRD patterns were continuously generated as the sample heated to 650 ºC
then cooled to room temperature. Each in situ experiment lasted ~8 h, and produced 486 PXRD
patterns (Figure 2). All three experiments produced data that showed a complex phase space on
heating, a full melt before 650 ºC, and fewer phases on cooling. The number of phases present in
the Sn2 and Sn3 experiments becomes so high around ~250 ºC on heating that the peaks are no
longer sharp and separated, and the identity of crystalline phases present can no longer be
determined in confidence. However, Sn2 and Sn3 showed similar characteristics to Sn1 overall,
and some qualitative comparisons can still be made.
For the remainder of this paper we focus on the Sn1 experiment because we were able to
extensively analyze this in situ synchrotron PXRD data. The PXRD patterns of Sn1 were
compared to simulated patterns of the starting materials and all known combinations of Cs, Sn, P,
and/or Se from the elements to quaternaries. Seven known crystalline phases were identified in
this process: Sn,40 Cs2Se3,41 Cs4Se16,42 Cs2Se5,43 Cs2Sn2Se6,44 Cs4P2Se9,45 and Cs2P2Se8.36 There
were numerous unaccounted diffraction peaks remaining, and these were designated as 6 unique
unknown phases based on the temperature of appearance and disappearance of each group of
peaks. The amount of each phase present was estimated by comparing peak intensity between all
PXRD frames as done previously.10 The relationship between temperature and phases of the Sn1
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reaction are visualized in Figure 3 and summarized in Table 1. Below we describe what happens
during the Sn1 reaction.
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At the onset, all starting materials (Sn, “Cs2Se2”, and PSe2) are present. Sn is crystalline
and easily identified in the in situ synchrotron PXRD.40 “Cs2Se2” exists nominally; the synthesis
procedure used to form Cs2Se2 yielded a mixture of cesium polyselenides with 3 crystalline
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stoichiometries (Cs2Se3,41 Cs4Se16,42 and Cs2Se5 ) that average together to nominal “Cs2Se2”
when including mole percent. The final starting material, PSe2, is amorphous, so its presence is
unseen by PXRD. We presume that PSe2 is present in the system until phosphorusꢀcontaining
phases crystallize.
On heating, nominal “Cs2Se2” anneals to preferentially give Cs2Se341 as the only cesium
polyselenide present after 114 ºC. Soon after at 125 ºC, the first unknown phase, Newꢀ1, appears.
This phase is shortꢀlived, disappearing by 204 ºC. The intensity of the Sn peaks remain strong
around the temperature range of Newꢀ1, hence we do not expect Newꢀ1 to contain Sn.
Experiments to identify Newꢀ1 ex situ indicated that it is very air sensitive.
Around 200 ºC on heating, many changes occur: Sn,40 Cs2Se3,41 Newꢀ1, and PSe2 react
and ternaries begin to form. First is the transient Cs2Sn2Se644 from 204ꢀ224 ºC. Cs4P2Se945 and
two unknowns (Newꢀ2 and Newꢀ3) appear at 206 ºC. Both unknowns are present in the system
for a range of over 150 ºC. Cs2P2Se836 emerges at 211 ºC and is the most prominent phase until it
dissipates at 322 ºC. Three additional unknowns are seen on heating: Newꢀ4 from 220ꢀ327 ºC,
Newꢀ5 from 261ꢀ389 ºC, and Newꢀ6 from 323ꢀ483 ºC. The final phase to melt is Newꢀ3 at 560
ºC. We will show below that Newꢀ4, ꢀ5, and ꢀ6 are all quaternaries, and the known phases
present in this temperature region are ternaries. From 560 ºC up to 650 ºC on heating, the entire
sample is a full melt and all crystalline phases disappear. On cooling from 650 ºC, the reaction
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mixture remains a melt until 444 ºC, when Newꢀ6 crystallizes. This is the only phase formed on
cooling, and it remains until the end of the experiment at room temperature.
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Once the relationship between temperature and phases has been established, the unknown
phases can be identified. There are three main routes to identifying unknowns from in situ PXRD.
First, the in situ PXRD can be used as a full guide to synthesize single crystals ex situ for
characterization. Second, the structure can be deduced by chemical analogy to known
compounds by comparing calculated PXRD patterns to the experimental in situ PXRD patterns.
Third, computational software (e.g. FOX59) can be employed to solve structures from the in situ
PXRD.
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Additionally, examining how the intensity of a phase is affected by the presence of
unknown phases gives additional information to help identify the unknowns. For example,
Cs4P2Se945 and Newꢀ4 both decrease in intensity when Newꢀ5 forms on heating. It is therefore
likely that Newꢀ5, Cs4P2Se9,45 and Newꢀ4 all have similar structural building blocks, which can
help us identify Newꢀ4 and Newꢀ5. The anionic units (i.e. potential building blocks) of the
known compounds seen in the in situ synchrotron PXRD of Sn1 are depicted in Figure 4.
Identifying Newꢀ6 as Cs4Sn(P2Se6)2. The unknown, Newꢀ6, was identified using method
1 (i.e. synthesize ex situ). The major product of the Sn1 reaction was Newꢀ6 so this
stoichiometric ratio was used along with a similar albeit longer heating profile. The product was
hexagonal plate crystals of Cs4Sn(P2Se6)2. This material crystallizes in the trigonal space group
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ꢀ
R3 with crystal parameters of a = 7.808(1) Å, c = 37.905(8) Å, V = 2001.4(6) Å , and Z = 3
(Table 2). It is comprised of molecular ions of [Sn(P2Se6)2]4− charge balanced by Cs+ cations
(Figure 5). Sn has an oxidation state of +4 and coordinates octahedrally to six total selenium
atoms, three from each [P2Se6]4− ligand. The [Sn(P2Se6)2]4− molecular ions sit in the middle of a
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3 improper rotation, however, the molecules themselves do not possess this symmetry (D3d).
Therefore, the [P2Se6]4− units disorder into three positions to accommodate the D3d symmetry
(Figure S10). Additional details of the disorder are described in the SI. The structure of the
molecular ion [Sn(P2Se6)2]4− is analogous to [M(P2Se6)2]5− (M = As, Bi, In) in Cs5As(P2Se6)2,60
Cs5Bi(P2Se6)2,61and Cs5In(P2Se6)2,62 which crystallize in the space groups P42/m, Pmc21, and
P42/m, respectively.
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Cs4Sn(P2Se6)2 can be synthesized as a pure phase by direct combination, and it is stable
in air and water. It does not form a glass by water quenching the melt. Differential thermal
analysis reveals congruent melting at 542 ºC and recrystallization on cooling at 526 ºC (Figure
S11). This material has a band gap of 1.7 eV (Figure S12).
Identifying Newꢀ4 as αꢀCs2SnP2Se6. The Newꢀ4 phase was identified using a
combination of methods 1 and 2 (i.e. synthesize ex situ and chemical analogy, respectively).
Newꢀ4 has a similar PXRD pattern to the known oneꢀdimensional Cs2SnP2Se657 (Figure S13),
hence their structures are possibly similar. Newꢀ4 is also seen in the Sn2 and Sn3 in situ
synchrotron PXRD, and with higher intensity in both as compared to Sn1. This gave evidence
that Newꢀ4 is a Snꢀrich phase. Based on this information, we targeted synthesizing Newꢀ4 ex situ
with Sn ratios greater than that used in the Sn1 in situ synchrotron PXRD experiment. The best
sample with matching PXRD to Newꢀ4 was formed using a Cs:Sn:P:Se stoichiometry of 2:1:2:6.
EDS analysis confirmed the composition of the Newꢀ4 orange rod crystals to be Cs2SnP2Se6. The
structure was determined at 293 K while the previously determined literature structure was
solved at 100 K, thus we call these phases αꢀ and βꢀCs2SnP2Se6, respectively. Both αꢀ and βꢀ
Cs2SnP2Se6 crystallize in the monoclinic space group P21/c and have similar unit cell parameters
(Table 2). The key structural difference between αꢀ and βꢀCs2SnP2Se6 is that there is disorder in
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the [P2Se6]4− units in the βꢀphase, but there is no disorder in the αꢀphase. This alters the PXRD
pattern slightly between the two similar phases (Figure S13). The structures of both phases are
oneꢀdimensional chains of 1/∞[SnP2Se6]2−, which are composed of alternating Sn2+ and the
ethaneꢀlike [P2Se6]4− units (Figure 6). The chains are chargeꢀbalanced by Cs+ cations. Each Sn2+
ion has distorted seesaw geometry (Table S7) and coordinates to two [P2Se6]4− units, with two
regular Sn–Se bonds and one nonbonding Sn–Se interaction per [P2Se6]4− unit. The lone pair on
the Sn2+ center is therefore stereochemically expressed and alternates pointing up or down on in
the chain. There are numerous examples of selenophosphate structures comprised of oneꢀ
dimensional chains of 1/∞[MP2Se6]2− (M = Cd, Fe, Hg, Mn, Pd, Zn),47,63ꢀ65 but αꢀ and βꢀ
Cs2SnP2Se6 are the first examples with seesaw metal coordination. Further in situ XRD studies
of these two phases would reveal if there is a relationship between them, i.e. if one phase
transitions into the other.
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Identifying Newꢀ5 as Cs4(Sn3Se8)[Sn(P2Se6)]2. The structure of Newꢀ5 was first
hypothesized by method 2 (i.e. chemical analogy). The (001) peak of Newꢀ5 has high intensity
and an uncommonly low 2θ value (2θ(001) = 2.24º with λ = 0.72768 Å and below we determine c
= 18.797(4) Å). This easily identifiable peak along with the other peaks assigned to Newꢀ5 were
recognized as analogous to Rb4(Sn3Se8)[Sn(P2Se6)]2.52 We deduced Newꢀ5 to be
Cs4(Sn3Se8)[Sn(P2Se6)]2 and synthesized it ex situ by following a modified literature procedure
for Rb4(Sn3Se8)[Sn(P2Se6)]2.
ꢀ
Cs4(Sn3Se8)[Sn(P2Se6)]2 crystallizes in the trigonal space group P3m1 with a = 7.695(1)
Å, c = 18.797(4) Å, V = 964.0(3) Å3, and Z = 1 (Table 2). The structure is analogous to
Rb4(Sn3Se8)[Sn(P2Se6)]252 and is comprised of twoꢀdimensional 2/∞[Sn3Se8]4− infinite slabs
derived from the structure of SnSe2 missing ¼ of the Sn atoms;49 these layers are capped by
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coordinating to one [Sn(P2Se6)] fragment per side (i.e. two total) of each unit of the 2/∞[Sn3Se8]4−
layer (Figure 7). The Sn4+ in [Sn(P2Se6)] has octahedral coordination with three Se from the
2/∞[Sn3Se8]4− layer and three Se from [P2Se6]4−. The resulting −4 charge is balanced by the four
Cs+ cations. Similar to Cs4Sn(P2Se6)2, the [P2Se6]4− units distort into three positions because they
sit on a symmetry element they do not possess, namely C3v (Figure S14). The Cs ions are
disordered in the interlayer space. This material is air and water stable.
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The targeted ex situ synthesis of Cs4(Sn3Se8)[Sn(P2Se6)]2 gave SnSe2 as a minor second
phase, but we were able to pick out single crystals large enough for electronic property
measurements.
Temperatureꢀdependent
resistivity
and
carrier
density
for
Cs4(Sn3Se8)[Sn(P2Se6)]2 are shown in Figure 8. The resistivity decreases from 300 K to 50 K and
then increases with lower temperature. Compared to the resistivity of 0.0287 ꢀ⋅cm at 300 K, the
one at 2 K (0.0824 ꢀ⋅cm) is comparable. The compound behaves as a heavily doped
semiconductor. Hall resistivity has linear field dependent behavior and shows the material is nꢀ
type (Figure S15). As listed in Table S8, the calculated carrier concentration (n) at 300 K is
~1.79 × 1019 1/cm3. At 2 K, however, n decreased to be ~7.78 × 1017 1/cm3. This may be due to
slight bandgap widening and subsequent carrier relaxation to the donor levels. The obtained
mobilities (ꢁ = 1/nq⋅σ
) at 300 K and 5 K are 12.2 cm2/V⋅s and 97.5 cm2/V⋅s, respectively.
Conclusion
In this work, we presented a powerful tool to discover new materials. The in situ
synchrotron PXRD technique we employed yields a fingerprint of each crystalline phase present,
including metastable intermediates, over a wide range of temperatures. Even previously
investigated systems using the conventional “blind” approaches can yield new materials that
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have been missed. With just one stoichiometry in the Cs/Sn/P/Se system, stunningly we
uncovered six new phases; three have been isolated and characterized, and three need further
work to identify their structures and exact compositions. Because the in situ synchrotron PXRD
monitoring of the reaction provides information on all occurring crystalline phases at once, it
gives us total phase awareness under the specific reaction conditions. In this sense, it gives us a
“panoramic” view of the reaction and provides the necessary evidence for where and when the
compounds form and thus enabling the design of the synthesis. By conducting “panoramic
synthesis”, we are opening our eyes where we had been previously “blind” to what occurs during
the “reaction arrow” of solid state synthesis. Our in situ synchrotron PXRD data described above
provides clues to identifying the remaining unknowns for future studies. “Panoramic synthesis”
is a powerful approach to accelerating materials discovery and can expand to a wide variety of
new systems. We believe there is a multitude of new structures that “panoramic synthesis” can
reveal where other synthetic techniques fall short. By developing this approach and combining
with other in situ local structural techniques and molecular dynamics simulations, we can move
toward better structure predictability and expedited discovery of inorganic materials.
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Acknowledgements
We gratefully acknowledge support from the National Science Foundation Grant DMRꢀ
1410169 and a Graduate Research Fellowship for A. S. H. under Grant No DGEꢀ1324585. This
work made use of the EPIC facility of the NUANCE Center at Northwestern University, funded
by the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCIꢀ
1542205); the MRSEC program (NSF DMRꢀ1121262) at the Materials Research Center; the
International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois,
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through the IIN. A. S. H. acknowledges Prof. Daniel Shoemaker for his guidance on analyzing
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the in situ PXRD and Ms. Charlotte Stern for her crystallography mentorship.
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Supporting Information Available: Details of disorder and twinning, atomic coordinates,
displacement parameters, in situ synchrotron PXRD patterns, figures of disorder in crystal
structures, absorption spectra, DTA, and transport measurements.
Xꢀray crystallographic data of Cs4Sn(P2Se6)2, αꢀCs2SnP2Se6, and Cs4(Sn3Se8)[Sn(P2Se6)]2 (CIF).
Corresponding Author
*Email: mꢀkanatzidis@northwestern.edu
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(62) Chondroudis, K.; Chakrabarty, D.; Axtell, E. A.; Kanatzidis, M. G. Z. Anorg. Allg. Chem.
1998, 624, 975ꢀ979.
(63) Chondroudis, K.; Kanatzidis, M. G. J. Solid State Chem. 1998, 138, 321ꢀ328.
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(64) Chondroudis, K.; Kanatzidis, M. G.; Sayettat, J.; Jobic, S.; Brec, R. Inorg. Chem. 1997, 36,
5859ꢀ5868.
(65) Haynes, A. S.; Lee, K.; Kanatzidis, M. G. Z. Anorg. Allg. Chem. 2016, 642, 1120ꢀ1125.
(66) Chung, I.; Kanatzidis, M. G. Inorg. Chem. 2011, 50, 412ꢀ414.
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Table 1. Phases observed from the in situ synchrotron PXRD of Sn1 reaction.
Heating or Cooling
Phase
Sn40
Start (º C)
RT
End (º C)
201
Heating
Heating
Heating
“Cs2Se2”a
Glassy PSe2
RT
RT
114
204
b
41
10
11
12
13
14
15
16
17
18
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20
21
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30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Heating
Cs2Se3
114
201
Heating
Heating
Heating
Heating
Heating
Heating
125
204
206
206
206
211
204
224
363
450
560
322
Newꢀ1
Cs2Sn2Se6
Cs4P2Se9
Newꢀ2
Newꢀ3
Cs2P2Se8
Newꢀ4
αꢀCs2SnP2Se6
44
45
36
Heating
Heating
Heating
220
261
323
444
327
389
483
RT
Newꢀ5
Cs4(Sn3Se8)[Sn(P2Se6)]2
Newꢀ6
Cs4Sn(P2Se6)2
Newꢀ6
Cs4Sn(P2Se6)2
Cooling
aNominally Cs2Se2; mixture of crystalline Cs2Se3,41 Cs2Se5,43 and Cs4Se16
bAmorphous so not observed in PXRD, but it is a known starting material; end temperature
presumed when crystalline Pꢀcontaining phases appear
42
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Table 2. Crystal Refinements of Cs4Sn(P2Se6)2, αꢀCs2SnP2Se6, βꢀCs2SnP2Se6 (for comparison to
the αꢀphase),66 and Cs4(Sn3Se8)[Sn(P2Se6)]2.a
Cs4(Sn3Se8)
Cs4Sn(P2Se6)2
Newꢀ6
αꢀCs2SnP2Se6
Newꢀ4
βꢀCs2SnP2Se6
[Sn(P2Se6)]2
Newꢀ5
2828.17
ref66
Structural Formula
Formula weight
Temperature
1721.73
920.21
920.21
10
11
12
13
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16
17
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21
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41
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293(2) K
0.71073 Å
Trigonal
ꢀ
293(2) K
0.71073 Å
Monoclinic
P21/c
100 K
0.71073 Å
Monoclinic
P21/c
293(2) K
0.71073 Å
Trigonal
ꢀ
Wavelength
Crystal system
Space group
R3
P3m1
a = 7.808(1) Å
a = 10.230(1) Å
a = 10.116 Å
a = 7.695(1) Å
b = 7.808(1) Å
b = 12.990(1) Å b = 12.7867 Å
b = 7.695(1) Å
Unit cell dimensions
c = 37.905(8) Å c = 10.950(2) Å
β = 94.53(1)°
c = 11.0828 Å c = 18.797(4) Å
β = 94.463°
V (Å3)
Z
Calculated ρ (g/cm3)
µ (mm−1)
2001.4(6)
3
1450.5(3)
4
1429.2(1)
4
964.0(3)
1
4.285
22.932
2214
4.213
4.277
22.264
1576
4.872
21.937
1576
25.966
1210
F(000)
Reflections collected
Independent reflections
Rint
5141
11524
2625
10599
2955
9435
942
1058
0.0769
100%
942 / 2 / 63
1.054
0.1206
100%
0.0670
N/A
0.0412
99.60%
1058 / 0 / 57
1.238
Completeness to θ = 25°
Data/restraints/parameters
Goodnessꢀofꢀfit
2625 / 0 / 101
3.17
N/A
1.123
Robs = 0.0566
wRobs = 0.1085
Robs = 0.0691
wRobs = 0.1456 wRobs = 0.0764 wRobs = 0.0837
Robs = 0.0438
Robs = 0.0397
Final R indices [>2σ(I)]
Rall = 0.0930
wRall = 0.1241
Rall = 0.0973
wRall = 0.1488
Robs = 0.0650
wRobs = 0.0812 wRall = 0.0865
Rall = 0.0473
R indices [all data]
Extinction coefficient
N/A
N/A
N/A
0.00111(16)
Largest diff. peak and
hole (e·Å−3)
1.505 and
−1.172
1.395 and
−2.682
1.161 and
−1.153
1.942 and
−1.592
aR = Σ||Fo|ꢀ|Fc|| / Σ|Fo|, wR = {Σ[w(|Fo|2 ꢀ |Fc|2)2] / Σ[w(|Fo|4)]}1/2 and
calc w=1/[σ2(Fo2)+(0.0234P)2+0.0000P] where P=(Fo2+2Fc2)/3
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Figure 1. Experimental setup for in situ synchrotron PXRD: a) Reaction procedure with the
three reactions labeled as Sn1, Sn2, and Sn3, according to the equivalence of tin in the reaction;
b) Model of capillary furnace used.
Figure 2. Select in situ synchrotron PXRD patterns every 10 ºC exhibiting the entire reaction
progression of Sn1 to show a sample of the raw data collected. Color scheme: starting materials
(purple); formation of Newꢀ1 (red); formation of Cs4Sn2Se6, Cs4P2Se9, Newꢀ2, Newꢀ3, Cs2P2Se8,
and Newꢀ4 (yellow); formation of Newꢀ5 (green); formation of Newꢀ6 (blue); and full melt
(black).
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b
Figure 3. Reaction map of Sn1: a) Full cycle; b) On heating only. * = Nominal composition (the
mixed crystalline phases that make up “Cs2Se2” are Cs2Se3, Cs4Se16, and Cs2Se5). Dashed lines
represent nonꢀcrystalline phase present.
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Figure 4. Structure of anionic units in known crystalline phases detected in Sn1 in situ
synchrotron PXRD experiment: a) Polyselenide units in cesium polyselenides; b) Part of
2/∞[Sn2Se6]4− layer with octahedrally coordinated Sn in Cs2Sn2Se6;44 c) Molecular ions [P2Se8]2−
and [P2Se9]4− with tetrahedrally coordinated P from Cs2P2Se836 and Cs4P2Se9,45 respectively.
Color scheme: selenium (red), tin (gray), phosphorus (black).
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Figure 5. a) Molecular ion of [Sn(P2Se6)2]4−; b) Unit cell of Cs4Sn(P2Se6)2 viewed down the
[540] direction. Disorder in the structure can be seen in Figure S10.
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Figure 6. a) Chain of 1/∞[SnP2Se6]2−; b) Unit cell of αꢀCs2SnP2Se6 viewed along the aꢀaxis.
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Figure 7. a) Two unit cells of Cs4(Sn3Se8)[Sn(P2Se6)]2 viewed in the [010] direction; b)
2/∞[Sn3Se8]4− layer viewed down the cꢀaxis. Disordered P, Se, and Cs atoms removed for clarity.
Disordered unit cell can be seen in Figure S14.
Figure 8. a) Temperatureꢀdependent resistivity; b) Carrier density of Cs4(Sn3Se8)[Sn(P2Se6)]2.
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Figure TOC.
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