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The quest for broadening the materials base of topological insulators (TIs) beyond the handful of presently known examples has recently led to exploratory calculations of the topological Z2 metric from the band structure of various candidate compounds in assumed crystal structures. However, structural transformations such as volume compression, lattice straining, or atom swaps that are used to instigate in a trivial insulator the band inversion underlying TI-ness, might also destabilize the system to the point that it either distorts into a more stable structure or does not form at all. Whether the more stable form of the candidate material is a TI, it is to be determined. Yet, often TI discovery calculations do not assess whether the postulated structural forms predicted to be TIs are also the stable forms of these compounds. Here, we show that in the broad family of III-Bi-O3 oxides (III = Al, Ga, In, Sc, Y, and La), the cubic AIIIBiO3 perovskite structure, which has been recently predicted to be TI for YBiO3, is unstable, whereas the stable AIIIBiO3 structural forms are trivial band insulators. [ABSTRACT FROM AUTHOR]

Rashba spin splitting is a two-dimensional(2D) relativistic effectclosely related to spintronics. However, so far there is no pristine2D material to exhibit enough Rashba splitting for the fabricationof ultrathin spintronic devices, such as spin field effect transistors(SFET). On the basis of first-principles calculations, we predictthat the stable 2D LaOBiS2with only 1 nm of thicknesscan produce remarkable Rashba spin splitting with a magnitude of 100meV. Because the medium La2O2layer producesa strong polar field and acts as a blocking barrier, two counter-helicalRashba spin polarizations are localized at different BiS2layers. The Rashba parameter can be effectively tuned by the intrinsicstrain, while the bandgap and the helical direction of spin statessensitively depends on the external electric field. We propose anadvanced Datta-Das SFET model that consists of dual gates and 2D LaOBiS2channels by selecting different Rashba states to achievethe on–off switch via electric fields. [ABSTRACT FROM AUTHOR]

New quaternary thioiodides Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5 have been synthesized by isothermal heating as well as chemical vapor transport. Pb2BiS2I3 and Sn2BiS2I3 crystallize in the space group, Cmcm, with unit cell parameters a = 4.3214 (9), b = 14.258 (3), and c = 16.488 (3) Å; a = 4.2890 (6), b = 14.121(2), and c = 16.414 (3) Å, respectively. Sn2BiSI5 adopts a unique crystal structure that crystallizes in C2/m with cell parameters a = 14.175 (3), b = 4.3985 (9), c = 21.625 (4) Å, and β = 98.90(3)°. The crystal structures of Pb2BiS2I3 and Sn2BiS2I3 are strongly anisotropic and can be described as three-dimensional networks that are composed of parallel infinite ribbons of [M4S2I4] (M = Pb, Sn, Bi) running along the crystallographic c-axis. The crystal structure of Sn2BiSI5 is a homologue of the M2BiS2I3 (M = Pb, Sn) which has two successive ribbons of [M4S2I4] separated by two interstitial (Sn1-xBixI6) octahedral units. These compounds were characterized by scanning electron microscopy, differential thermal analysis, and X-ray photoelectron spectroscopy. Pb2SbS2I3, Pb2BiS2I3, "Pb2Sb1-xBixS2I3" (x ∼ 0.4), Sn2BiS2I3 and Sn2BiSI5 are highly resistive and exhibit electrical resistivities of 3.0 GΩ cm, 100 MΩ cm, 65 MΩ cm, 1.2 MΩ cm, and 34 MΩ cm, respectively, at room temperature. Pb2BiS2I3, Sn2BiS2I3, Pb2SbS2I3, "Pb2Sb1-xBixS2I3" (x ∼ 0.4), and Sn2BiSI5 are semiconductors with bandgaps of 1.60, 1.22, 1.92, 1.66, and 1.32 eV, respectively. The electronic band structures of Pb2BiS2I3, Sn2BiS2I3, and Sn2BiSI5, calculated using density functional theory, show that all compounds are direct bandgap semiconductors. [ABSTRACT FROM AUTHOR]

The synthesis and properties of the hybrid organic/inorganic germanium perovskite compounds, AGeI3, are reported (A = Cs, organic cation). The systematic study of this reaction system led to the isolation of 6 new hybrid semiconductors. Using CsGeI3 (1) as the prototype compound, we have prepared methylammonium, CH3NH3GeI3 (2), formamidinium, HC(NH2)2GeI3 (3), acetamidinium, CH3C(NH2)2GeI3 (4), guanidinium, C(NH2)3GeI3 (5), trimethylammonium, (CH3)3NHGeI3 (6), and isopropylammonium, (CH3)2C(H)NH3GeI3 (7) analogues. The crystal structures of the compounds are classified based on their dimensionality with 1-4 forming 3D perovskite frameworks and 5-7 1D infinite chains. Compounds 1-7, with the exception of compounds 5 (centrosymmetric) and 7 (nonpolar acentric), crystallize in polar space groups. The 3D compounds have direct band gaps of 1.6 eV (1), 1.9 eV (2), 2.2 eV (3), and 2.5 eV (4), while the 1D compounds have indirect band gaps of 2.7 eV (5), 2.5 eV (6), and 2.8 eV (7). Herein, we report on the second harmonic generation (SHG) properties of the compounds, which display remarkably strong, type I phase-matchable SHG response with high laser-induced damage thresholds (up to ∼3 GW/cm2). The second-order nonlinear susceptibility, XS(2), was determined to be 125.3 ± 10.5 pm/V (1), (161.0 ± 14.5) pm/V (2), 143.0 ± 13.5 pm/V (3), and 57.2 ± 5.5 pm/V (4). First-principles density functional theory electronic structure calculations indicate that the large SHG response is attributed to the high density of states in the valence band due to sp-hybridization of the Ge and I orbitals, a consequence of the lone pair activation. [ABSTRACT FROM AUTHOR]

Cs2Hg3S4was synthesized by slowlycooling a melted stoichiometric mixture of Hg and Cs2S4. Cs2Hg3S4crystallizes inthe Ibamspacegroup with a= 6.278(1)Å, b= 11.601(2) Å, and c= 14.431(3)Å; dcalc= 6.29 g/cm3. Its crystal structure consists of straight chains of [Hg3S4]n2n–that engage in side-by-side weak bonding interactionsforming layers and are charge balanced by Cs+cations.The thermal stability of this compound was investigated with differentialthermal analysis and temperature dependent in situ synchrotron powderdiffraction. The thermal expansion coefficients of the a, b, and caxes were assessed at1.56 × 10–5, 2.79 × 10–5, and 3.04 × 10–5K–1, respectively.Large single-crystals up to ∼5 cm in length and ∼1 cmin diameter were grown using a vertical Bridgman method. Electricalconductivity and photoconductivity measurements on naturally cleavedcrystals of Cs2Hg3S4gave resistivityρ of ≥108Ω·cm and carrier mobility-lifetime(μτ) products of 4.2 × 10–4and5.82 × 10–5cm2V–1for electrons and holes, respectively. Cs2Hg3S4is a semiconductor with a bandgap Eg∼ 2.8 eV and exhibits photoluminescence (PL)at low temperature. Electronic band structure calculations withinthe density functional theory (DFT) framework employing the nonlocalhybrid functional within Heyd–Scuseria–Ernzerhof (HSE)formalism indicate a direct bandgap of 2.81 eV at Γ. The theoreticalcalculations show that the conduction band minimum has a highly dispersiveand relatively isotropic mercury-based s-orbital-like character whilethe valence band maximum features a much less dispersive and moreanisotropic sulfur orbital-based band. [ABSTRACT FROM AUTHOR]

We report the synthesis, physicalcharacterization, and crystalgrowth of Tl4CdI6. We show that this materialhas good photoconductivity and is a promising semiconductor for roomtemperature X-ray and γ-ray detection. Large single crystalswere grown by the vertical Bridgman method and cut to dimensions appropriatefor detector testing. Single crystal X-ray diffraction refinementsconfirm that Tl4CdI6crystallizes in the tetragonalcrystal system with a centrosymmetric space group of P4/mnc, with a calculated density of 6.87 g/cm3. Thermal analysis and high-temperature synchrotron powderdiffraction studies were used to determine phase relationships andcrystallization behavior during crystal growth. We have elucidatedthe reason for different colors encountered when synthesizing or growingsingle crystals of Tl4CdI6(yellow, red, andblack), and it is the presence of a small amount of TlI impurity.We report proper crystal growth conditions to obtain essentially pureyellow Tl4CdI6crystals. The material havingthe yellow color has a band gap of 2.8 eV. First-principles densityfunctional theory calculations indicate a direct band gap at the Γpoint of the Brillouin zone. The Tl4CdI6crystalshave a resistivity of 1010Ω·cm. Photoconductivitymeasurements on the as-grown crystals show mobility-lifetime producton the order of 10–4cm2/V for both electronsand holes. The promising detector properties of this material areconfirmed by preliminary measurements showing a clear spectral responseto an Ag X-ray source, which classifies Tl4CdI6as an emerging material for radiation detection. [ABSTRACT FROM AUTHOR]

The chemical concept of lattice hybridizationwas applied to identify new chalcohalide compounds as candidates forX-ray and γ-ray detection. Per this approach, compound semiconductormaterials with high density and wide band gaps can be produced thatcan absorb and detect hard radiation. Here, we show that the mixedchalcogenide–halide compound Tl6SI4isa congruently melting, mechanically robust chalcohalide material withstrong photoconductivity response and an impressive room-temperaturefigure of merit. Tl6SI4crystallizes in thetetragonal P4/mncspace group, with a= 9.1758(13) Å, c= 9.5879(19) Å, V= 807.3(2) Å3, and a calculated density of 7.265 g·cm–3. The new material requires a more simplified crystal growth comparedto the leading system Cd0.9Zn0.1Te, which isthe benchmark room-temperature hard radiation detector material. We successfully synthesized Tl6SI4crystals toproduce detector-grade wafers with high resistivity values (∼1010Ω·cm) and high-resolution detection of X-rayspectra from an Ag (22 keV) source. [ABSTRACT FROM AUTHOR]

AGeI3 (A: Cs+ (I), MeNH3+ (II), HC(NH2)2+ (III), CH3C(NH2)2+ (IV), C(NH2)3+ (V), Me3NH+ (VI), iPrNH3+ (VII)) polar hybrid inorganic/organic semiconductors are prepared from a solution of GeI4 in aqueous HI and H3PO2 by precipitation and crystallization through the addition of stoichiometric amounts of AI (stirring at 120 °C until half of the solution is evaporated, 25 °C, 24 h, 80-90% yield). [ABSTRACT FROM AUTHOR]

The title compounds (MII: Mg, Zn, Cd, Hg; MIV: Ge, Sn), prepared by flame-melting rapid cooling reactions or by a traditional tube furnace synthesis with <90% yields, are characterized by single crystal XRD, SEM, solid state UV/VIS spectroscopy, Raman spectroscopy, electrical resistivity and photoconductivity measurements, and DFT calculations. [ABSTRACT FROM AUTHOR]