New Candidates for Topological Insulators : Pb-based chalcogenide series

Here, we theoretically predict that the series of Pb-based layered chalcogenides, Pb$_n$Bi$_2$Se$_{n+3}$ and Pb$_n$Sb$_2$Te$_{n+3}$, are possible new candidates for topological insulators. As $n$ increases, the phase transition from a topological insulator to a band insulator is found to occur between $n=2$ and 3 for both series. Significantly, among the new topological insulators, we found a bulk band gap of 0.40eV in PbBi$_2$Se$_4$ which is one of the largest gap topological insulators, and that Pb$_2$Sb$_2$Te$_5$ 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 Pb$_2$Sb$_2$Te$_5$

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 TMASnI₃ (<b>1</b>), which is our reference compound for a “perovskitoid” structure of face-sharing octahedra. The compounds EASnI₃ (<b>2b</b>), GASnI₃ (<b>3a</b>), ACASnI₃ (<b>4</b>), and IMSnI₃ (<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 GA₂SnI₄ (<b>3b</b>) and IPA₃Sn₂I₇ (<b>6b</b>) and the one-dimensional (1D) perovskite IPA₃SnI₅ (<b>6a</b>). The known 2D perovskite BA₂MASn₂I₇ (<b>7</b>) and the related all-inorganic 1D perovskite “RbSnF₂I” (<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>_g = 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}MAPbI₃

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