177 research outputs found
Cova de Can Sadurní, la transformació d’un jaciment. L’episodi sepulcral del neolític postcardial
The
present study deals with the structural characterization and classification
of the novel compounds <b>1</b>–<b>8</b> into perovskite
subclasses and proceeds in extracting the structure–band gap
relationships between them. The compounds were obtained from the employment
of small, 3–5-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 “perovskitoid”
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 “hexagonal perovskite polytypes”
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 “RbSnF<sub>2</sub>I” (<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> = 1.90–2.40
eV, with the band gap progressively decreasing with increased corner-sharing
functionality and increased torsion angle in the octahedral connectivity
Flux Crystal Growth of the RE2Ru3Ge5 (RE = La, Ce, Nd, Gd, Tb) Series and Their Magnetic and Metamagnetic Transitions
Previously synthesized only as powders, single crystals of the RE2Ru3Ge5 (RE = La, Ce, Nd, Gd, Tb) series of compounds have now been obtained from molten In. These materials crystallize with the U2Co3Si5-type structure in orthorhombic space group Ibam with lattice parameters a ≈ 10.00–9.77 Å (La–Tb), b ≈ 12.51–12.35 Å, and c ≈ 5.92–5.72 Å. The structure is a three-dimensional framework consisting of RuGe5 and RuGe6 units, as well as Ge–Ge zigzag chains. This structure type and those of the other five (Sc2Fe3Si5, Lu2Co3Si5, Y2Rh3Sn5, Yb2Ir3Ge5, and Yb2Pt3Sn5) to compose the RE2T3X5 phase space are discussed in depth. For the three compounds with RE = Nd, Gd, Tb, multiple magnetic transitions and metamagnetic behavior are observed. Electronic band structure calculations performed on La2Ru3Ge5 indicate that these materials have a negative band gap and are semimetallic in nature
Lattice dynamics reveals a local symmetry breaking in the emergent dipole phase of PbTe
Local symmetry breaking in complex materials is emerging as an important
contributor to materials properties but is inherently difficult to study. Here
we follow up an earlier structural observation of such a local symmetry broken
phase in the technologically important compound PbTe with a study of the
lattice dynamics using inelastic neutron scattering (INS). We show that the
lattice dynamics are responsive to the local symmetry broken phase, giving key
insights in the behavior of PbTe, but also revealing INS as a powerful tool for
studying local structure. The new result is the observation of the unexpected
appearance on warming of a new zone center phonon branch in PbTe. In a harmonic
solid the number of phonon branches is strictly determined by the contents and
symmetry of the unit cell. The appearance of the new mode indicates a crossover
to a dynamic lower symmetry structure with increasing temperature. No
structural transition is seen crystallographically but the appearance of the
new mode in inelastic neutron scattering coincides with the observation of
local Pb off-centering dipoles observed in the local structure. The observation
resembles relaxor ferroelectricity but since there are no inhomogeneous dopants
in pure PbTe this anomalous behavior is an intrinsic response of the system. We
call such an appearance of dipoles out of a non-dipolar ground-state
"emphanisis" meaning the appearance out of nothing. It cannot be explained
within the framework of conventional phase transition theories such as
soft-mode theory and challenges our basic understanding of the physics of
materials
Alternative Organic Spacers for More Efficient Perovskite Solar Cells Containing Ruddlesden-Popper Phases
The halide perovskite Ruddlesden-Popper (RP) phases are a homologous layered subclass of solution-processable semiconductors that have aroused great attention, especially for developing long-term solar photovoltaics. They are defined as (A′)2(A)n-1PbnX3n+1 (A′ = spacer cation, A = cage cation, and X = halide anion). The orientation control of low-temperature self-assembled thin films is a fundamental issue associated with the ability to control the charge carrier transport perpendicular to the substrate. Here we report new chemical derivatives designed from a molecular perspective using a novel spacer cation 3-phenyl-2-propenammonium (PPA) with conjugated backbone as a low-temperature strategy to assemble more efficient solar cells. First, we solved and refined the crystal structures of single crystals with the general formula (PPA)2(FA0.5MA0.5)n-1PbnI3n+1 (n = 2 and 3, space group C2) using X-ray diffraction and then used the mixed halide (PPA)2(Cs0.05(FA0.88MA0.12)0.95)n-1Pbn(I0.88Br0.12)3n+1 analogues to achieve more efficient devices. While forming the RP phases, multiple hydrogen bonds between PPA and inorganic octahedra reinforce the layered structure. For films we observe that as the targeted layer thickness index increases from n = 2 to n = 4, a less horizontal preferred orientation of the inorganic layers is progressively realized along with an increased presence of high-n or 3D phases, with an improved flow of free charge carriers and vertical to substrate conductivity. Accordingly, we achieve an efficiency of 14.76% for planar p-i-n solar cells using PPA-RP perovskites, which retain 93.8 ± 0.25% efficiency with encapsulation after 600 h at 85 °C and 85% humidity (ISOS-D-3)
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