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

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 K₂MnSn₂S₆ (KMS-1) overcomes these limitations and is exceptionally capable in selectively and rapidly sequestering high (ppm) as well as trace (ppb) quantities of UO₂²⁺ 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 K₂MnSn₂S₆ (KMS-1) overcomes these limitations and is exceptionally capable in selectively and rapidly sequestering high (ppm) as well as trace (ppb) quantities of UO₂²⁺ 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: RE₅Mn₄Al_{23–<i>x</i>}Si_<i>x</i> (RE = Ho, Er, Yb) and Er₄₄Mn₅₅(AlSi)₂₃₇

Four novel intermetallic silicides, RE₅Mn₄Al_{23–<i>x</i>}Si_<i>x</i> (<i>x</i> = 7.9(9), RE = Ho, Er, Yb) and Er₄₄Mn₅₅(AlSi)₂₃₇, have been prepared by reaction in aluminum flux. Three RE₅Mn₄Al_{23–<i>x</i>}Si_<i>x</i> compounds crystallize in the tetragonal space group <i>P</i>4/<i>mmm</i> with the relatively rare Gd₅Mg₅Fe₄Al_{18–<i>x</i>}Si_<i>x</i> structure type. Refinement of single-crystal X-ray diffraction data yielded unit cell parameters of <i>a</i> = 11.3834(9)–11.4171(10) Å and <i>c</i> = 4.0297(2)–4.0575(4) Å with volumes ranging from 522.41(5) to 528.90(8) ų. Structure refinements on single-crystal diffraction data show that Er₄₄Mn₅₅(AlSi)₂₃₇ adopts a new cubic structure type in the space group <i>Pm</i>3̅<i>n</i> with a very large unit cell edge of <i>a</i> = 21.815(3) Å. 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. Ho₅Mn₄Al_{23–<i>x</i>}Si_<i>x</i> 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-, β-, and Glassy K₂ZnSn₃S₈

We show that K₂ZnSn₃S₈ 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 K₂CO₃/S/Sn/Zn leads to different kinetically stable products (crystalline layered α-K₂ZnSn₃S₈, <b>1</b>, and glassy K₂ZnSn₃S₈, <b>A</b>, respectively). These forms convert to a thermodynamically stable compound (crystalline cubic β-K₂ZnSn₃S₈, <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 C₂H₆ versus CH₄

Mesoporous polymeric organic frameworks (mesoPOF)­s 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 m² g^–1 and have pores of several size ranges from micropores to large mesopores depending on the amount of surfactant used. The adsorption uptakes of CO₂, C₂H₆, and CH₄ measured at 273 K at 1 bar are linearly correlated to the micropore volume. The mesoPOFs display high adsorption selectivity of C₂H₆ over CH₄ by a factor of 40, and this property is dictated by their pore diameter

Scandium Selenophosphates: Structure and Properties of K₄Sc₂(PSe₄)₂(P₂Se₆)

The new compound K₄Sc₂P₄Se₁₄ 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 ¹/_∞[Sc₂P₄Se₁₄^4–] chains that are separated by K⁺ cations. The structural motif features two [PSe₄]^3– units and one [P₂Se₆]^4– unit bridging the Sc centers and has not been reported for any other compound. The ¹/_∞[Sc₂P₄Se₁₄^4–] chains pack in a crosshatched pattern perpendicular to the <i>c</i> axis of the crystal, forming channels for half of the K⁺ atoms while the other half occupy empty space between the chains. The orange-yellow crystals of K₄Sc₂P₄Se₁₄ 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^–1 for the [PSe₄]^3– and [P₂Se₆]^4– units, respectively. Solid-state ³¹P MAS NMR of K₄Sc₂P₄Se₁₄ shows two prominent peaks at 11.31 and −23.07 ppm and one minor peak at −106.36 ppm, most likely due to degradation of the product or an unknown second phase

Nb–Nb Interactions Define the Charge Density Wave Structure of 2H-NbSe₂

2H-NbSe₂ is a canonical Charge-Density-Wave (CDW) layered material the structural details of which remained elusive. We report the detailed structure of 2H-NbSe₂ 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 Nb–Nb distances were found to distort between 3.4102(8) and 3.4928(8) Å in the CDW phase from the average undistorted distance of 3.4583(4) Å

Phase-Change Materials Exhibiting Tristability: Interconverting Forms of Crystalline α-, β-, and Glassy K₂ZnSn₃S₈

We show that K₂ZnSn₃S₈ 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 K₂CO₃/S/Sn/Zn leads to different kinetically stable products (crystalline layered α-K₂ZnSn₃S₈, <b>1</b>, and glassy K₂ZnSn₃S₈, <b>A</b>, respectively). These forms convert to a thermodynamically stable compound (crystalline cubic β-K₂ZnSn₃S₈, <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

Na₂EuAs₂S₅, NaEuAsS₄, and Na₄Eu(AsS₄)₂: Controlling the Valency of Arsenic in Polysulfide Fluxes

The reactivity of europium with As species in Lewis basic alkali-metal polysulfide fluxes was investigated along with compound formation and the As³⁺/As⁵⁺ interplay vis-à-vis changes in the flux basicity. The compound Na₂EuAs₂S₅ containing trivalent As³⁺ is stabilized from an arsenic-rich polysulfide flux. It crystallizes in the monoclinic centrosymmetric space group <i>P</i>2₁/<i>c</i>. Na₂EuAs₂S₅ has [As₂S₅]^4– units, built of corner sharing AsS₃ pyramids, which are coordinated to Eu²⁺ ions to give a two-dimensional (2D) layered structure. A sodium polysulfide flux with comparatively less arsenic led to the As⁵⁺ containing compounds NaEuAsS₄ (orthorhombic, <i>Ama</i>2) and Na₄Eu­(AsS₄)₂ (triclinic, <i>P</i>1) depending on Na₂S/S ratio. The NaEuAsS₄ and Na₄Eu­(AsS₄)₂ have a three-dimensional (3D) structure built of [AsS₄]^3– tetrahedra coordinated to Eu²⁺ ions. All compounds are semiconductors with optical energy gaps of ∼2 eV