726 research outputs found
New Candidates for Topological Insulators : Pb-based chalcogenide series
Here, we theoretically predict that the series of Pb-based layered
chalcogenides, PbBiSe and PbSbTe, are possible
new candidates for topological insulators. As increases, the phase
transition from a topological insulator to a band insulator is found to occur
between and 3 for both series. Significantly, among the new topological
insulators, we found a bulk band gap of 0.40eV in PbBiSe which is one
of the largest gap topological insulators, and that PbSbTe is
located in the immediate vicinity of the topological phase boundary, making its
topological phase easily tunable by changing external parameters such as
lattice constants. Due to the three-dimensional Dirac cone at the phase
boundary, massless Dirac fermions also may be easily accessible in
PbSbTe
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
Tuning Ionic and Electronic Conductivities in the "Hollow" Perovskite { en}MAPbI<sub>3</sub>
The recently developed family of 3D halide perovskites with general formula (A)1-x(en)x(M)1-0.7x(I)3-0.4x (A = MA, FA; M = Pb2+, Sn2+ en = ethylenediammonium), often referred to as "hollow"perovskites, exhibits exceptional air stability and crystallizes in the high symmetry α phase at room temperature. These properties are counterintuitive, considering that these structures include the large divalent en cation charge-compensated by vacancies of Pb cations and I anions. Moreover, the understanding of their transport behavior is incomplete. To provide new insights into the ionic and electronic transport properties of these "hollow"perovskites, we performed DC polarization experiments and ab initio calculations on the {en}MAPbI3 material. We observe large variations of ionic and electronic conductivities with en concentration, which can be explained by charge and site arguments in conjunction with trapping effects. The latter is reflected by the increase of the activation energies for iodide ion transport with higher en content that we observe from both experimental and computational results. The connection between these transport phenomena and the stability of "hollow"perovskite materials and devices is discussed. </p
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