241 research outputs found
Cova de Can SadurnĂ, la transformaciĂł dâun jaciment. Lâepisodi sepulcral del neolĂtic postcardial
The
present study deals with the structural characterization and classification
of the novel compounds <b>1</b>â<b>8</b> into perovskite
subclasses and proceeds in extracting the structureâband gap
relationships between them. The compounds were obtained from the employment
of small, 3â5-atom-wide organic ammonium ions seeking to discover
new perovskite-like compounds. The compounds reported here adopt unique
or rare structure types akin to the prototype structure perovskite.
When trimethylammonium (TMA) was employed, we obtained TMASnI<sub>3</sub> (<b>1</b>), which is our reference compound for a âperovskitoidâ
structure of face-sharing octahedra. The compounds EASnI<sub>3</sub> (<b>2b</b>), GASnI<sub>3</sub> (<b>3a</b>), ACASnI<sub>3</sub> (<b>4</b>), and IMSnI<sub>3</sub> (<b>5</b>)
obtained from the use of ethylammonium (EA), guanidinium (GA), acetamidinium
(ACA), and imidazolium (IM) cations, respectively, represent the first
entries of the so-called âhexagonal perovskite polytypesâ
in the hybrid halide perovskite library. The hexagonal perovskites
define a new family of hybrid halide perovskites with a crystal structure
that emerges from a blend of corner- and face-sharing octahedral connections
in various proportions. The small organic cations can also stabilize
a second structural type characterized by a crystal lattice with reduced
dimensionality. These compounds include the two-dimensional (2D) perovskites
GA<sub>2</sub>SnI<sub>4</sub> (<b>3b</b>) and IPA<sub>3</sub>Sn<sub>2</sub>I<sub>7</sub> (<b>6b</b>) and the one-dimensional
(1D) perovskite IPA<sub>3</sub>SnI<sub>5</sub> (<b>6a</b>).
The known 2D perovskite BA<sub>2</sub>MASn<sub>2</sub>I<sub>7</sub> (<b>7</b>) and the related all-inorganic 1D perovskite âRbSnF<sub>2</sub>Iâ (<b>8</b>) have also been synthesized. All
compounds have been identified as medium-to-wide-band-gap semiconductors
in the range of <i>E</i><sub>g</sub> = 1.90â2.40
eV, with the band gap progressively decreasing with increased corner-sharing
functionality and increased torsion angle in the octahedral connectivity
Layered Metal Sulfides Capture Uranium from Seawater
Uranium is the main source for nuclear energy but also
one of the
most toxic heavy metals. The current methods for uranium removal from
water present limitations, such as narrow pH operating range, limited
tolerance to high salt concentrations, or/and high cost. We show here
that a layered sulfide ion exchanger K<sub>2</sub>MnSn<sub>2</sub>S<sub>6</sub> (KMS-1) overcomes these limitations and is exceptionally
capable in selectively and rapidly sequestering high (ppm) as well
as trace (ppb) quantities of UO<sub>2</sub><sup>2+</sup> under a variety
of conditions, including seawater. KMS-1 can efficiently absorb the
naturally occurring U traces in seawater samples. The results presented
here reveal the exceptional potential of sulfide-based ion-exchangers
for remediating of uranium-containing wastes and groundwater and for
extracting uranium from the sea
Layered Metal Sulfides Capture Uranium from Seawater
Uranium is the main source for nuclear energy but also
one of the
most toxic heavy metals. The current methods for uranium removal from
water present limitations, such as narrow pH operating range, limited
tolerance to high salt concentrations, or/and high cost. We show here
that a layered sulfide ion exchanger K<sub>2</sub>MnSn<sub>2</sub>S<sub>6</sub> (KMS-1) overcomes these limitations and is exceptionally
capable in selectively and rapidly sequestering high (ppm) as well
as trace (ppb) quantities of UO<sub>2</sub><sup>2+</sup> under a variety
of conditions, including seawater. KMS-1 can efficiently absorb the
naturally occurring U traces in seawater samples. The results presented
here reveal the exceptional potential of sulfide-based ion-exchangers
for remediating of uranium-containing wastes and groundwater and for
extracting uranium from the sea
Quaternary Aluminum Silicides Grown in Al Flux: RE<sub>5</sub>Mn<sub>4</sub>Al<sub>23â<i>x</i></sub>Si<sub><i>x</i></sub> (RE = Ho, Er, Yb) and Er<sub>44</sub>Mn<sub>55</sub>(AlSi)<sub>237</sub>
Four novel intermetallic silicides,
RE<sub>5</sub>Mn<sub>4</sub>Al<sub>23â<i>x</i></sub>Si<sub><i>x</i></sub> (<i>x</i> = 7.9(9), RE
= Ho, Er, Yb) and Er<sub>44</sub>Mn<sub>55</sub>(AlSi)<sub>237</sub>, have been prepared by
reaction in aluminum flux. Three RE<sub>5</sub>Mn<sub>4</sub>Al<sub>23â<i>x</i></sub>Si<sub><i>x</i></sub> compounds crystallize in the tetragonal space group <i>P</i>4/<i>mmm</i> with the relatively rare Gd<sub>5</sub>Mg<sub>5</sub>Fe<sub>4</sub>Al<sub>18â<i>x</i></sub>Si<sub><i>x</i></sub> structure type. Refinement of single-crystal
X-ray diffraction data yielded unit cell parameters of <i>a</i> = 11.3834(9)â11.4171(10) Ă
and <i>c</i> =
4.0297(2)â4.0575(4) Ă
with volumes ranging from 522.41(5)
to 528.90(8) Ă
<sup>3</sup>. Structure refinements on single-crystal
diffraction data show that Er<sub>44</sub>Mn<sub>55</sub>(AlSi)<sub>237</sub> adopts a new cubic structure type in the space group <i>Pm</i>3Ì
<i>n</i> with a very large unit cell
edge of <i>a</i> = 21.815(3) Ă
. This new structure
is best understood when viewed as two sets of nested polyhedra centered
on a main group atom and a manganese atom. These polyhedral clusters
describe the majority of the atomic positions in the structure and
form a perovskite-type network. We also report the electrical and
magnetic properties of the title compounds. All compounds except the
Ho analogue behave as normal paramagnetic metals without any observed
magnetic transitions above 5 K and exhibit antiferromagnetic correlations
deduced from the value of their Curie constants. Ho<sub>5</sub>Mn<sub>4</sub>Al<sub>23â<i>x</i></sub>Si<sub><i>x</i></sub> exhibits a ferromagnetic transition at 20 K and an additional
metamagnetic transition at 10 K, suggesting independent ordering temperatures
for two distinct magnetic sublattices
Phase-Change Materials Exhibiting Tristability: Interconverting Forms of Crystalline α-, ÎČ-, and Glassy K<sub>2</sub>ZnSn<sub>3</sub>S<sub>8</sub>
We show that K<sub>2</sub>ZnSn<sub>3</sub>S<sub>8</sub> is a phase-change
system that exhibits tristability. Kinetic and thermodynamic forms
of different compounds in the K/Zn/Sn/S system have been synthesized
and thoroughly characterized. We report an example where slow and
rapid cooling of a melt of K<sub>2</sub>CO<sub>3</sub>/S/Sn/Zn leads
to different kinetically stable products (crystalline layered α-K<sub>2</sub>ZnSn<sub>3</sub>S<sub>8</sub>, <b>1</b>, and glassy
K<sub>2</sub>ZnSn<sub>3</sub>S<sub>8</sub>, <b>A</b>, respectively).
These forms convert to a thermodynamically stable compound (crystalline
cubic ÎČ-K<sub>2</sub>ZnSn<sub>3</sub>S<sub>8</sub>, <b>2</b>) upon annealing below their melting points. The band gaps of compounds <b>1</b>, <b>A</b>, and <b>2</b> are 2.30, 2.15, and
2.55 eV, respectively
Mesoporous Hydrophobic Polymeric Organic Frameworks with Bound Surfactants. Selective Adsorption of C<sub>2</sub>H<sub>6</sub> versus CH<sub>4</sub>
Mesoporous polymeric organic frameworks (mesoPOF)Âs have
been synthesized through surfactant mediated polymerization of phlorglucinol
(1,3,5-trihydroxybenzene) and terephthalaldehyde under solvothermal
conditions. The materials contain bound surfactant and exhibit hydrophobic
properties. The mesoPOFs present high surface areas up to 1000 m<sup>2</sup> g<sup>â1</sup> and have pores of several size ranges
from micropores to large mesopores depending on the amount of surfactant
used. The adsorption uptakes of CO<sub>2</sub>, C<sub>2</sub>H<sub>6</sub>, and CH<sub>4</sub> measured at 273 K at 1 bar are linearly
correlated to the micropore volume. The mesoPOFs display high adsorption
selectivity of C<sub>2</sub>H<sub>6</sub> over CH<sub>4</sub> by a
factor of 40, and this property is dictated by their pore diameter
Phase-Change Materials Exhibiting Tristability: Interconverting Forms of Crystalline α-, ÎČ-, and Glassy K<sub>2</sub>ZnSn<sub>3</sub>S<sub>8</sub>
We show that K<sub>2</sub>ZnSn<sub>3</sub>S<sub>8</sub> is a phase-change
system that exhibits tristability. Kinetic and thermodynamic forms
of different compounds in the K/Zn/Sn/S system have been synthesized
and thoroughly characterized. We report an example where slow and
rapid cooling of a melt of K<sub>2</sub>CO<sub>3</sub>/S/Sn/Zn leads
to different kinetically stable products (crystalline layered α-K<sub>2</sub>ZnSn<sub>3</sub>S<sub>8</sub>, <b>1</b>, and glassy
K<sub>2</sub>ZnSn<sub>3</sub>S<sub>8</sub>, <b>A</b>, respectively).
These forms convert to a thermodynamically stable compound (crystalline
cubic ÎČ-K<sub>2</sub>ZnSn<sub>3</sub>S<sub>8</sub>, <b>2</b>) upon annealing below their melting points. The band gaps of compounds <b>1</b>, <b>A</b>, and <b>2</b> are 2.30, 2.15, and
2.55 eV, respectively
Scandium Selenophosphates: Structure and Properties of K<sub>4</sub>Sc<sub>2</sub>(PSe<sub>4</sub>)<sub>2</sub>(P<sub>2</sub>Se<sub>6</sub>)
The new compound K<sub>4</sub>Sc<sub>2</sub>P<sub>4</sub>Se<sub>14</sub> was synthesized via the polychalcogenide
flux method. It crystallizes in the space group <i>C</i>2/<i>c</i>, and the structure is composed of <sup>1</sup>/<sub>â</sub>[Sc<sub>2</sub>P<sub>4</sub>Se<sub>14</sub><sup>4â</sup>] chains that are separated by K<sup>+</sup> cations.
The structural motif features two [PSe<sub>4</sub>]<sup>3â</sup> units and one [P<sub>2</sub>Se<sub>6</sub>]<sup>4â</sup> unit
bridging the Sc centers and has not been reported for any other compound.
The <sup>1</sup>/<sub>â</sub>[Sc<sub>2</sub>P<sub>4</sub>Se<sub>14</sub><sup>4â</sup>] chains pack in a crosshatched pattern
perpendicular to the <i>c</i> axis of the crystal, forming
channels for half of the K<sup>+</sup> atoms while the other half
occupy empty space between the chains. The orange-yellow crystals
of K<sub>4</sub>Sc<sub>2</sub>P<sub>4</sub>Se<sub>14</sub> 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<sup>â1</sup> for the [PSe<sub>4</sub>]<sup>3â</sup> and
[P<sub>2</sub>Se<sub>6</sub>]<sup>4â</sup> units, respectively.
Solid-state <sup>31</sup>P MAS NMR of K<sub>4</sub>Sc<sub>2</sub>P<sub>4</sub>Se<sub>14</sub> shows two prominent peaks at 11.31 and â23.07
ppm and one minor peak at â106.36 ppm, most likely due to degradation
of the product or an unknown second phase
NbâNb Interactions Define the Charge Density Wave Structure of 2H-NbSe<sub>2</sub>
2H-NbSe<sub>2</sub> is a canonical Charge-Density-Wave
(CDW) layered
material the structural details of which remained elusive. We report
the detailed structure of 2H-NbSe<sub>2</sub> below the CDW transition
using a (3 + 2)-dimensional crystallographic approach on single crystal
X-ray diffraction data collected at 15 K. Intensities of main reflections
as well as CDW satellites of first order were measured. Quantitative
information about the magnitude of the structural distortions and
clustering of Nb atoms were extracted from the refined model. The
NbâNb distances were found to distort between 3.4102(8) and
3.4928(8) Ă
in the CDW phase from the average undistorted distance
of 3.4583(4) Ă
NbâNb Interactions Define the Charge Density Wave Structure of 2H-NbSe<sub>2</sub>
2H-NbSe<sub>2</sub> is a canonical Charge-Density-Wave
(CDW) layered
material the structural details of which remained elusive. We report
the detailed structure of 2H-NbSe<sub>2</sub> below the CDW transition
using a (3 + 2)-dimensional crystallographic approach on single crystal
X-ray diffraction data collected at 15 K. Intensities of main reflections
as well as CDW satellites of first order were measured. Quantitative
information about the magnitude of the structural distortions and
clustering of Nb atoms were extracted from the refined model. The
NbâNb distances were found to distort between 3.4102(8) and
3.4928(8) Ă
in the CDW phase from the average undistorted distance
of 3.4583(4) Ă
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