Scandium Selenophosphates: Structure and Properties of K4Sc2(PSe4)2(P2Se6)

Article abstract of DOI:10.1021/acs.inorgchem.6b00535

The new compound K₄Sc₂P₄Se₁₄ was synthesized via the polychalcogenide flux method. It crystallizes in the space group C2/c, and the structure is composed of ¹/_∞[Sc₂P₄Se₁₄^4-] chains that are separated by K⁺ cations. The structural motif features two [PSe₄]^3- units and one [P₂Se₆]^4- unit bridging the Sc centers and has not been reported for any other compound. The ¹/_∞[Sc₂P₄Se₁₄^4-] chains pack in a crosshatched pattern perpendicular to the c axis of the crystal, forming channels for half of the K⁺ atoms while the other half occupy empty space between the chains. The orange-yellow crystals of K₄Sc₂P₄Se₁₄ are air-sensitive and gradually turn red over the course of a couple hours. The band gap of the phase is 2.25(2) eV, and Raman spectroscopy shows the symmetric stretches of the selenophosphate groups to be at 231 and 216 cm^-1 for the [PSe₄]^3- and [P₂Se₆]^4- units, respectively. Solid-state ³¹P MAS NMR of K₄Sc₂P₄Se₁₄ shows two prominent peaks at 11.31 and -23.07 ppm and one minor peak at -106.36 ppm, most likely due to degradation of the product or an unknown second phase.

Full text of DOI:10.1021/acs.inorgchem.6b00535

Article
pubs.acs.org/IC
Scandium Selenophosphates: Structure and Properties of
K4Sc2(PSe4)2(P2Se6)
Jonathan C. Syrigos and Mercouri G. Kanatzidis*
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
S
* Supporting Information
ABSTRACT: The new compound K4Sc2P4Se14 was synthesized via the
polychalcogenide flux method. It crystallizes in the space group C2/c, and
the structure is composed of 1/∞[Sc2P4Se144−] chains that are separated by
K+ cations. The structural motif features two [PSe4]3− units and one
[P2Se6]4− unit bridging the Sc centers and has not been reported for any
1
other compound. The /∞[Sc2P4Se144−] chains pack in a crosshatched
pattern perpendicular to the c axis of the crystal, forming channels for half
of the K+ atoms while the other half occupy empty space between the
chains. The orange-yellow crystals of K4Sc2P4Se14 are air-sensitive and
gradually turn red over the course of a couple hours. The band gap of the
phase is 2.25(2) eV, and Raman spectroscopy shows the symmetric
stretches of the selenophosphate groups to be at 231 and 216 cm−1 for the
[PSe4]3− and [P2Se6]4− units, respectively. Solid-state 31P MAS NMR of
K4Sc2P4Se14 shows two prominent peaks at 11.31 and −23.07 ppm and one minor peak at −106.36 ppm, most likely due to
degradation of the product or an unknown second phase.
both compounds, Ag+ ions incorporate into the sheets
INTRODUCTION
■
themselves instead of positioning themselves between the
Alkali- and rare-earth-metal-containing chalcophosphates have
extremely varied crystal structures. These are a result of the
diversity of selenophosphate ligands available1,2 and, to a
greater extent, the identity of the rare-earth metal that is
coordinated in the solid.1e,h,3 In the case of the lanthanides, the
larger coordination sphere of the metal centers further
contributes to the variety of structures that are possible in
this class of compounds. One of note is KSmP2S7, which has
layers.7
Here we report the new compound K4Sc2P4Se14 (or
K2ScP2Se7 empirically), composed of chains of
1/∞[Sc2P4Se144−], which was synthesized from a flux-containing
reaction. Each chain has two distinct selenophosphate ligands
bonded to the metal: [PSe4]3− units that form Sc2P2Se8 clusters
and [P2Se6]4− units that bridge the Sc2P2Se8 clusters along the
chain direction. Thus, a more precise formula is
K4Sc2(PSe4)2(P2Se6). Other examples of compounds contain-
ing two types of selenophosphate ligands exist, such as
K4Ti2(P2Se9)2(P2Se7) and K3Ru(P3Se4)(P2Se6), but they are
quite rare.1c,i
To the best of our knowledge, this is the first report of an
alkali-metal and scandium-containing chalcophosphate to date,
and we could find no reported compounds with a similar chain
structure. Other quaternary rare-earth-metal selenophosphates
have similar compositions, but they consist of either 2D sheet
substructures or a three-dimensional network.1k,2d,5a,c
−
intriguing layers of [SmP2S7 ] that snake up and down in the
[001] direction.4 Another example is LiEuPSe4, which has
channels of Li+ cations.2 It is important to note that very little
research has been done on such compounds,1a,b,h,k,2a,b,d,3−5 and
physical property analysis has only been done on about half of
the few compounds that have been discovered.1a,b,h,k,2b,d,4,5
Scandium and yttrium are not lanthanides but are,
nonetheless, similar to them. Of the rare-earth elements, Y is
most similar to Sc in its chemistry. There are only a few
quaternary yttrium selenophosphates.2b,6 KYP2Se6, for example,
contains sheets composed of Y3+ centers that are bridged by
[P2Se6]4− units. These infinite sheets are then separated by the
K+ ions. The compound has a band gap of around 2.75 eV and
exhibits no magnetic activity, which makes sense because of the
d0 metal centers.
EXPERIMENTAL SECTION
■
Reagents. All reagents were used as obtained from the specified
supplier: diethyl ether (ACS grade, BDH Chemical, Leicestershire,
U.K.); N,N-dimethylformamide (DMF; ACS grade, BDH Chemical,
Leicestershire, U.K.); red phosphorus powder (99%, Sigma-Aldrich, St.
Louis, MO); potassium metal (98%, Sigma-Aldrich, St. Louis, MO);
scandium metal powder (99.9%, Alfa Aesar, Ward Hill, MA); selenium
It is unusual how little to no research has been done on
scandium in this category of compounds. To date, only two
known scandium-containing quaternary chalcophosphates exist:
7
AgScP2S6 and its selenium analogue, AgScP2Se6.2e The
compounds have two-dimensional (2D) sheets wherein
Received: March 2, 2016
octahedrally coordinated Sc is bridged by [P2Se6]4− units. In
© XXXX American Chemical Society
A
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pellets (99.99%, Sigma-Aldrich, St Louis, MO). K2Se was prepared by
the reaction of stoichiometric amounts of potassium and selenium in
liquid ammonia as described elsewhere.8 P2Se5 was prepared by the
stoichiometric reaction of the elements in a 13 mm evacuated fused-
silica tube at 460 °C for 48 h as described elsewhere.1e
Differential Thermal Analysis (DTA). Experiments were
performed using a Shimadzu DTA-50 thermal analyzer. Ground
crystalline samples (∼20 mg) were sealed under vacuum (∼10−4
mbar) in a fused-silica ampule. A similar amount of Al2O3 was sealed
in a separate ampule under vacuum and used as a reference. The
samples were heated and cooled at a rate of 5 °C/min to a maximum
temperature of 600 °C.
Synthesis. K4Sc2P4Se14. A mixture of 0.157 g (1.0 mmol) of K2Se,
0.011 g (0.25 mmol) of scandium, 0.228 g (0.5 mmol) of P2Se5, and
0.079 g (1.0 mmol) of selenium was added to a carbon-coated fused-
silica tube with an outer diameter of 9 mm, an inner diameter of 7 mm,
and a length of 20 cm inside a nitrogen-filled glovebox. The tube was
sealed under vacuum and subjected to the following heating profile:
the reaction mixture was heated to 500 °C in 7 h and remained at that
temperature for 24 h. The reaction was then cooled to 350 °C in 24 h
and then cooled to room temperature in 1 h. The remaining flux in the
ingot was dissolved in dinitrogen-degassed DMF, replacing the DMF
every 10 min until the solvent remained clear, yielding orange-yellow,
needlelike crystals of K4Sc2P4Se14, which became a bright-yellow
powder when crushed. Energy-dispersive X-ray spectroscopy (EDS)
analysis on several crystals gave an average stoichiometry of
K4.0Sc2.0P4.4Se8.8 (normalized for scandium). The crystals were air-
sensitive and were stored in a glovebox to prevent oxidation.
Single-Crystal X-ray Crystallography. Data collection was
performed on a STOE IPDS II diffractometer using Mo Kα radiation
(λ = 0.71073 Å) operating at 50 kV and 40 mA at 293 K. The crystal
was coated in superglue on the mounting needle to prevent it from
oxidizing in air. The crystal was then collected under a flow of nitrogen
gas. Integration and numerical absorption corrections were performed
on the structure using X-AREA, X-RED, and X-SHAPE.9 The structure
was solved using direct methods and refined by full-matrix least
squares on F2 using the SHELXTL program package.10
Powder X-ray Diffraction (PXRD). A silicon-calibrated CPS 120
INEL powder X-ray diffractometer (Cu Kα graphite-monochromat-
ized radiation) equipped with a position-sensitive detector operating at
40 kV and 20 mA was used to collect PXRD data on ground crystalline
samples of the product with a flat sample geometry. Samples were
covered in paratone oil to prevent oxidation during measurement. The
Visualizer program within FindIt was used to generate simulated
patterns using the crystallographic information file (CIF) of the
refined structure.
Scanning Electron Microscopy (SEM). A Hitachi S-3400
scanning electron microscope equipped with a PGT energy-dispersive
X-ray analyzer was used to acquire images and semiquantitative EDS
analyses. Spectra were collected using an accelerating voltage of 25 kV
and an accumulation time of 60 s.
RESULTS AND DISCUSSION
■
Synthesis. The synthesis of K4Sc2P4Se14 was relatively easy
via the flux synthesis method because it appears to form as a
single phase and the flux dissolved readily in DMF. This is true
as long as a ratio of 4:1:2:4 K2Se/Sc/P2Se5/Se is used. The
compound would not form when a direct combination reaction
was attempted, forming only KPSe6 and K2P2Se6.
K4Sc2P4Se14 is air- or water-sensitive and will quickly oxidize
in around 30 min, turning redder as it is exposed to ambient air
and eventually turning to a black powder. This is most likely
due to the hard nature of the hard Lewis acid Sc3+ center, which
would much rather bond to the hard O of water than the soft
Se in the compound. K4Sc2P4Se14 is also very weakly diffracting,
so PXRD of the sample under paratone oil yielded only an
amorphous background (see the Supporting Information).
EDS was done on the oxidized samples, yielding seemingly
random compositions depending on the point observed. On
average, the composition of the oxidized samples was rich in O,
Se, and K and deficient in Sc. PXRD of the fully oxidized
samples was also done, and elemental Se peaks along with
heavily shifted K4Sc2P4Se14 peaks were observed (see the
Supporting Information).
Crystal Structure Description. K4Sc2P4Se14 crystallizes in
the C2/c (No. 15) space group, in the 2/m Laue class, and is
composed of /∞[Sc2P4Se144−] chains separated by K+ cations.
1
See Table 1 for crystallographic data. The chains are composed
of Sc2P2Se8 clusters bridged by [P2Se6]4− units (Figures 1 and
2). This chain structure has not been previously reported to the
best of our knowledge, but it is most similar to that of
Ag3Y(PSe4)2.6 Unlike K4Sc2P4Se14, Ag3Y(PSe4)2 lacks the
[P2Se6]4− unit and instead bridges every metal center with
two [PSe4]3− units. In K4Sc2P4Se14, the two Sc centers must be
3+ in order to balance the 4− charge of the [P2Se6]4− unit and
the 6− charge of the two [PSe4]3− units. The final 4− charge of
the chain is balanced by the four K+ cations present.
It is good to note here that the ratio of the maximum/
minimum residual density of the solved structure is a bit large at
2.67. However, this can be accounted for by some slight
disorder in the position on the terminal Se atoms on the
[P2Se6]4− (Se6) and [PSe4]3− (Se3) units as well as K1, which
is normal for such compounds. This disorder is expressed by
their larger than usual thermal ellipsoids.
Solid-State 31P Magic Angle Spinning (MAS) NMR. 31P MAS
NMR spectra were collected using a Varian VNMRS 400 MHz
spectrometer. NH4H2PO4 (δ = 0.8 ppm) was used as a reference for
chemical shifts. Pure samples of K4Sc2P4Se14 were loaded into a 5 mm
airtight zirconia rotor in a glovebox, and experiments were performed
using a pulse width (pwx90) of 3.5 μs and a relaxation delay of 5 min.
A total of 1024 scans were collected for K4Sc2P4Se14, with a spin rate of
10000 rpm.
Solid-State UV−Vis Spectroscopy. A Shimadzu UV-3101 PC
double-beam, double-monochromator spectrophotometer was used to
collect diffuse-reflectance spectra in the range of 200−2500 nm. The
instrument was equipped with an integrating sphere and controlled by
a personal computer. BaSO4 was used as a standard and set to 100%
reflectance. Samples were prepared by quickly placing the ground
crystalline products on a bed of BaSO4 in air. Collected reflectance
data were converted to absorbance according to the Kubelka−Munk
equation α/S = (1 − R)2/2R, where α is the absorption coefficient, S is
the scattering coefficient, and R is the reflectance.1e,8
Raman Spectroscopy. Raman spectra of ground crystalline
samples were collected on a DeltaNu Advantage NIR spectrometer
equipped with a CCD detector. The beam source was emitted from a
diode laser with 785 nm radiation. The samples were loaded into
borosilicate glass capillaries for measurement in a glovebox and sealed
with clay. A maximum power of 60 mW and a beam diameter of 35
μm were used. The spectrum was collected using an integration time
of 10 s.
The d0 Sc centers are in a distorted octahedral coordination
environment, which is most likely due to second-order Jahn−
Teller distortion. Despite this, the compound as a whole is
centrosymmetric because any distortion of one Sc in the
Sc2P2Se8 cluster is perfectly mirrored by the other Sc in the
cluster, effectively canceling out any polar moments. The chains
themselves pack in a crosshatched pattern when viewed along
the [001] direction. This pattern forms channels that are filled
by atom K2, while K1 is positioned elsewhere between the
chains (Figure 2c).
In the compound, there are five distinct bonding environ-
ments for the Se atoms: two of which are part of the [P2Se6]4−
unit and three of which are in the [PSe4]3− units. Of the six Se
B
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Table 1. Crystallographic Refinement Details of K4Sc2P4Se14
compound
1
empirical formula
K4Sc2P4Se14.
737.82
fw, g/mol
wavelength, Å
0.71073
habit
needle
color
orange-yellow
monoclinic
C2/c
cryst syst
space group
a, Å
22.668(5)
6.815(2)
19.828(4)
113.91(3)
2800(1)
8
b, Å
c, Å
β, deg
V, Å3
Z
ρ, g/cm3
3.500
μ, mm−1
19.499
F(000)
2616
θmax, deg
34.87
reflns collected
21264
Rint
0.0991
no. of param
109
refinement method
GOF
full-matrix least squares on F2
Figure 2. (a) [110], (b) [1−10], and (c) [001] directions. The view
along the [110] and [1−10] directions reveals the crosshatched
orientation of the chains within the unit cell, while the view along the
[001] direction reveals the channels in which atom K2 resides.
1.242
a
final R indices [>2σ(I)], R1/wR2
0.0672/0.1363
0.0842/0.1432
a
R indices (all data), R1/wR2
a
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑[w(|Fo|2 − |Fc|2)2]/
2
2
∑[w(Fo )2]1/2] and calc w = 1/[σ2(Fo ) + (0.0559P)2 + 1.2737P],
where P = (Fo + 2Fc2)/3.
2
nonbonding 3.942(2) Å. Between chains, the nearest interchain
separation is between Se3 and Se6 with a distance of 3.748(2)
Å. This is just below double the ionic radius of Se2− (1.84−1.9
Å),11 meaning there should be slight interchain interactions
contributing to the electronic structure.
Thermal Properties. DTA of K4Sc2P4Se14 showed no
peaks, either endothermic or exothermic, up to 600 °C. The
sample solidified from a powder into a solid agglomerate and
changed color from yellow to brown after heating, so it is
puzzling that no thermal transitions were observed in the DTA
signal. The PXRD pattern of the sample after DTA was
unchanged, yielding only an amorphous background under
paratone oil.
Optical Absorption Spectroscopy. The band gap of
K4Sc2P4Se14 was measured to be 2.25(2) eV, which matches its
orange-yellow color (Figure 3). Because of the large band gap
Figure 1. (a) Chain structures of [Sc2P4Se14]4− and (b) K4Sc2P4Sc14
along the [010] direction (with labeled atoms).
atoms in [P2Se6]4−, there are four that are bonded to both Sc
and P (Se1 and Se2) and two that are bonded only to the P
atom (Se3). Of the Se atoms in the [PSe4]3− group, there are
two that are monodentate to the Sc atom (Se4 and Se7), one
that is bridging two Sc centers (Se5), and one that does not
bond to the Sc atom (Se6).
Concerning [P2Se6]4−, the Se−Sc distance is 2.668(2)−
2.677(2) Å. For the Se atoms in the [PSe4]3− group, the Sc−Se
bond distance is 2.730(2)−2.756(2) Å for Se5 and 2.679(2)−
2.699(2) Å for Se4 and Se7. The Sc−Sc distance is a
Figure 3. Absorption spectrum of K4Sc2P4Se14. The band gap was
found to be around 2.25(2) eV. The sample was ground in the
glovebox and quickly analyzed rapidly in air to prevent oxidation.
C
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compared to those of other selenophosphates, this compound
could be useful for study in third harmonic generation because
a larger band gap is needed in order to prevent absorption
processes. Measurement of the band gap was done in air, so
there is a possibility that the compound began to oxidize during
the measurement. If so, the expected band gap would be higher
than that of the measured one as the compound turns more red
in color with increased air exposure. As a result, the actual band
gap of K4Sc2P4Se14 could actually be slightly greater than 2.25
eV.
ppm (see Figure 5), which correspond to the P in the [P2Se6]4−
and [PSe4]3− units, respectively. Both peaks have an integration
Solid-state Raman spectroscopy was performed on
K4Sc2P4Se14, yielding the spectra seen in Figure 4. The
Figure 5. Solid-state 31P MAS NMR of K4Sc2P4Se14. Two peaks at
11.31 and −23.07 ppm correspond to the [PSe4]3− and [P2Se6]4−
units, respectively. A minor impurity peak can be found around −106.4
ppm, and slight decomposition of the compound results in broadness
and extra peaks between 5 and −15 ppm.
of approximately 1, which should be the case. It is also
important to note a small peak at −106.36 ppm, which is either
from a minor unknown second phase or partial oxidation of the
compound. Seeing as no second unknown phases could be
found using SEM/EDS, partial oxidation of the compound is
more likely to be the source of this peak. The broadness
observed between the peaks at 11.31 and −23.07 ppm is
probably due to partial decomposition of the compound over
the course of the measurement or in storage.
Figure 4. Raman spectrum of K4Sc2P4Se14.
symmetric stretch of the [PSe4]3− unit can be found at 231
cm−1, while the asymmetric stretch is located at 438 cm−1.2d
Comparing the symmetric and asymmetric stretches of
K4Sc2P4Se14 to those of other [PSe4]3− containing rare-earth
metals (237 and 443−454 cm−1, respectively),2d this compound
has lower-energy stretching vibrations, suggesting slightly
weaker P−Se bonds. This is perhaps due to the relatively
hard and electronegative nature of the Sc3+ centers, which
would draw electron density from the [PSe4]3− units into the
Se−Sc bonds. Other similar compounds also compare well with
the measured Raman spectra.12 The bending modes of [PSe4]3−
can also be determined by analogy to be at 143 and 178 cm−1.2d
The P−P stretch of the [P2Se6]4− group can be found at 216
cm−1, which matches well to the P−P stretch at 217 cm−1 of
Sn2P2Se6, which also contains the [P2Se6]4− unit.2f The bending
modes of the [P2Se6]4− unit are most likely at 114 and 156
cm−1, while the other stretching modes can be found at 266,
372, 462, and 471 cm−1.1h,2f,13 All peaks except for the signal at
402 cm−1 could be assigned to the bending and stretching
frequencies of the selenophosphate groups by analogy.13 This
peak at 402 cm−1 is most likely another asymmetric stretching
group of either the [P2Se6]4− or [PSe4]3− group. See Table 2
for a list of the categorized Raman stretches and bends.
The peaks of the P atom match well with known 31P NMR
peak positions. Compared to that of the similar one-
dimensional compound KNb2PSe6 (−8.32 ppm),14 the peak
location in K4Sc2P4Se14 (−23.07 ppm) is more negative and
therefore more shielded. This makes sense because Nb4+ would
draw electron density from the [PSe4]3− unit, shielding the P
atom less compared to the Sc3+ center. The peak from the
[P2Se6]4− unit is close to that of the peak location in Pb2P2Se6
(29.1 ppm)15 although it is more shielded comparatively. Most
other [P2Se6]4−-containing compounds have even more
positive resonance peaks. Rb2CdP2Se6, for example, has a
peak at around 62.0 ppm.15 The reason that the P atoms in
K4Sc2P4Se14 are so shielded may be related to the bonding
configuration of the [P2Se6]4− groups to the metals. In the Pb
and Cd cases, all six Se atoms are bonded to the metal centers,
whereas in the scandium compound, only four Se atoms are
bonded. As a result, less electron density can be drawn away
from the ligands to the metals, which makes the 31P NMR peak
in the scandium compound have a smaller value.
CONCLUSIONS
The K4Sc2P4Se14 compound forms via flux synthesis and adopts
■
a
Solid-State 31P MAS NMR. Solid-state 31P MAS NMR of
K4Sc2P4Se14 shows two prominent peaks at 11.31 and −23.07
n e w s t r u c t u r e w i t h u n i q u e c h a i n s o f
Table 2. Raman Shifts of K4Sc2P4Se14 Sorted According to Which Bending and Stretching Modes of Each Ligand They Can Be
Attributed
[PSe4]3−
[PSe4]3−
[PSe4]3−
bend
[P2Se6]4−
symmetric stretch
[P2Se6]4−asymmetric
stretches
[P2Se6]4−
bend
[PSe4]3− or
symmetric stretch asymmetric stretch
[P2Se6]4− bend
Raman shift
(cm−1
231 (s) 438 (s)
143 (w), 178
(w)
216 (s)
266 (s), 372 (w), 462 (w), 114 (w), 156
471 (w) (s)
402 (w)
)
D
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Inorganic Chemistry
Article
1/∞[Sc2(PSe4)2(P2Se6)4−], which themselves organize in a
crosshatched pattern. The compound showed no thermal
transitions up to 600 °C, and the band gap was measured to be
∼2.3 eV. Raman spectroscopy shows the symmetric and
asymmetric stretches of the selenophosphate groups clearly,
and they were assigned by analogy to known compounds. This
system shows the versatility of the rare-earth-metal seleno-
phosphate family because it forms another unique chain
structure in an already varied library of interesting and complex
compounds.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00535.
■
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CIF of K4Sc2P4Se14 (CIF)
PXRD, SEM images, EDS analysis, atom crystallographic
coordinates, and selected bond lengths (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: m-kanatzidis@northwestern.edu.
Author Contributions
The manuscript was written through contributions of all
authors.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Science Foundation
(Grant DMR- 1410169) and from the Nicholson Fellowship
is gratefully acknowledged. The SEM/EDS work was
performed in the EPIC facility of the NUANCE Center at
Northwestern University. The NUANCE Center is supported
by the NSF-DMR, Keck Foundation, State of Illinois, and
Northwestern University.
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DOI: 10.1021/acs.inorgchem.6b00535
Inorg. Chem. XXXX, XXX, XXX−XXX