A R T I C L E S
Chung et al.
2.3. Physical Measurements. 2.3.1. X-ray Powder Diffrac-
tion. X-ray powder diffraction analyses were performed using a
calibrated CPS 120 INEL X-ray powder diffractometer (Cu KR
graphite monochromatized radiation) operating at 40 kV/20 mA
and equipped with a position-sensitive detector with flat sample
geometry.
2.3.2. Scanning Electron Microscopy. Semiquantitative ana-
lyses of the compound was performed with a JEOL JSM-35C
scanning electron microscope (SEM) equipped with a Tracor
Northern energy dispersive spectroscopy (EDS) detector.
As a result, little is known about the behavior of inorganic
polymers in solution. In some cases, these solutions have shown
interesting mesogenic liquid crystal properties, for example,
[V0.837P2S6-],20 1∞[CrP2S6-],21 1∞[PdPS4-]22,23 and 1∞[Mo3Se3-].24-27
The possibility to exfoliate or even dissolve mineral compounds
is of major importance because it can allow for patterned
deposition and modification of the structure with solution
chemistry methods paving the way to new organic-inorganic
hybrid and nanocomposite materials. The solutions of K4P8Te4
exhibit slightly blue-tinted white photoluminescence at room
temperature when excited above the energy gap. Precipitation
with ethanol at room temperature gave nanospheres of K4P8Te4
exhibiting a blue-shifted energy band gap. With a wide battery of
spectroscopic techniques, we show that in solution K4P8Te4
exfoliates and with time the [P8Te44-] chains rearrange to small
molecular species. We also describe the results of ab initio density
functional theory calculations using the all-electron full-potential
linearized augmented plane wave (FLAPW)28 method, which show
that K4P8Te4 is an indirect band gap semiconductor and has a strong
covalent bonding character of P-Te.
2.3.3. Transmission Electron Microscopy (TEM) and High
Resolution (HR) TEM. TEM sample was diluted with ethanol.
TEM and HRTEM images were obtained with JEOL JEM 2200
FS Field emission TEM.
2.3.4. Solid-state UV-vis Spectroscopy. Optical diffuse re-
flectance measurements were performed at room temperature using
a Shimadzu UV-3101 PC double-beam, double-monochromator
spectrophotometer operating in the 200-2500 nm region. The
details of the energy gap measurements have been discussed
elsewhere.29-33
2.3.5. Infrared Spectroscopy. FT-IR spectrum was recorded
as a solid in a CsI matrix. The sample was ground with dry CsI
into a fine powder and pressed into a translucent pellet. The
spectrum was recorded in the far-IR region (600-100 cm-1, 4 cm-1
resolution) with the use of a Nicolet 740 FT-IR spectrometer
equipped with a TGS/PE detector and silicon beam splitter.
2.3.6. Thermogravimetric Analysis. Experiments were per-
formed on Shimadzu TGA-50 thermal analyzer by heating the
samples up to 500 °C at a rate of 10 °C min-1 under N2 flow of ca.
2. Experimental Section
2.1. Reagents. The reagents mentioned in this work were used
as obtained unless noted otherwise: K metal (analytical reagent,
Aldrich Chemical Co., Milwaukee, WI); red phosphorus powder
(99%, Sigma-Aldrich Inc., Saint Louis, MO); Te (99.999%,
Noranda Advanced Materials, Quebec, Canada); N,N-dimethylform-
amide (ACS reagent grade, Spectrum Chemicals,); diethyl ether
(anhydrous, ACS reagent grade, Columbus Chemical Industries,
Columbus, WI,); hydrazine (98%, anhydrous, Sigma-Aldrich Inc.,
Saint Louis, MO). K2Te starting material was prepared by reacting
stoichiometric amounts of the elements in liquid ammonia. Anhy-
drous hydrazine was distilled before use. CAUTION: Hydrazine
is highly toxic and should be handled using proper protective
equipments with special care to prevent contact with either the
vapors or liquid.
2.2. Synthesis. All sample preparation processes were carried
out under inert atmosphere. Pure K4P8Te4 was achieved by heating
a mixture of K2Te/P/Te ) 3:2:5 under vacuum in a silica tube at
450 °C for 6 days, followed by cooling to 250 at 2 °C h-1. The
excess flux was dissolved with degassed N,N-dimethylformamide
(DMF) under a N2 atmosphere to reveal deep red-tinted black needle
crystals. Energy-dispersive spectroscopy microprobe analysis on
five crystals showed an average composition of “K3.8P8Te3.7”. The
single crystals appear stable in DMF, N-methylformamide, deion-
ized H2O, and air. Attempts to synthesize other alkali metal
analogues with similar reaction conditions were unsuccessful.
20 mL min-1
.
2.3.7. Differential Thermal Analysis (DTA). Experiments were
performed on Shimadzu DTA-50 thermal analyzer. A sample (∼30
mg) of ground crystalline material was sealed in a silica ampule
under vacuum. A similar ampule of equal mass filled with Al2O3
was sealed and placed on the reference side of the detector. The
sample was heated to 550 at 5 °C min-1, and after 1 min it was
cooled at a rate of -5 °C min-1 to 50 °C. The residues of the
DTA experiments were examined by X-ray powder diffraction.
Reproducibility of the results was confirmed by running multiple
heating/cooling cycles. The melting and crystallization points were
measured at a minimum of endothermic peak and a maximum of
exothermic peak.
2.3.8. 31P Solid-State NMR Spectroscopy. Room temperature
31P NMR measurements were taken on a 9.4 T 400 MHz Varian
Infinity Plus NMR spectrometer using a double-resonance magic
angle spinning (MAS) probe using a 4 mm (outer) diameter zirconia
rotor. Bloch decay spectra were taken with the excitation/detection
channel tuned to 31P at 161.82 MHz with a 4.5 µs, 90° pulse
(calibrated to (0.5 µs), a relaxation delay of 20-13000 s, and
samples were spun at frequencies between 6 and 13 kHz. All spectra
were processed with up to 100 Hz of line broadening, up to a tenth-
order polynomial baseline correction, and the chemical shifts (CS)
were externally referenced to 85% H3PO4 at 0 ppm. The spin-lattice
relaxation time (T1) of each chemical shift was estimated from the
exponential buildup of the peak intensity area as a function of the
relaxation delay at 13 kHz between 20 and 13000 s. The chemical
shift anisotropy (CSA) principle value of each chemical shift was
derived from the average of the Herzfeld-Berger fitting34 of the
MAS isotropic and sideband peak intensity areas at spinning
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