Metal halide perovskite crystal structures have emerged as a class of optoelectronic materials, which combine the ease of solution processability with excellent optical absorption and emission qualities. Restricting the physical dimensions of the perovskite crystallites to a few nanometers can also unlock spatial confinement effects, which allow large spectral tunability and high luminescence quantum yields at low excitation densities. However, the most promising perovskite structures rely on lead as a cationic species, thereby hindering commercial application. The replacement of lead with nontoxic alternatives such as tin has been demonstrated in bulk films, but not in spatially confined nanocrystals. Here, we synthesize CsSnX3 (X = Cl, Cl0.5Br0.5, Br, Br0.5I0.5, I) perovskite nanocrystals and provide evidence of their spectral tunability through both quantum confinement effects and control of the anionic composition. We show that luminescence from Sn-based perovskite nanocrystals occurs on pico- to nanosecond time scales via two spectrally distinct radiative decay processes, which we assign to band-to-band emission and radiative recombination at shallow intrinsic defect sites.

Solution-processed organo-lead halide perovskites are produced with sharp, color-pure electroluminescence that can be tuned from blue to green region of visible spectrum (425–570 nm). This was accomplished by controlling the halide composition of CH3NH3Pb(BrxCl1–x)3 [0 ≤ x ≤ 1] perovskites. The bandgap and lattice parameters change monotonically with composition. The films possess remarkably sharp band edges and a clean bandgap, with a single optically active phase. These chloride–bromide perovskites can potentially be used in optoelectronic devices like solar cells and light emitting diodes (LEDs). Here we demonstrate high color-purity, tunable LEDs with narrow emission full width at half maxima (FWHM) and low turn on voltages using thin-films of these perovskite materials, including a blue CH3NH3PbCl3 perovskite LED with a narrow emission FWHM of 5 nm.

Organometallic lead-halide perovskite-based solar cells now approach 18% efficiency. Introducing a mixture of bromide and iodide in the halide composition allows tuning of the optical bandgap. We prepare mixed bromide-iodide lead perovskite films CH3NH3Pb(I1-xBrx)3 (0 ≤ x ≤ 1) by spin-coating from solution and obtain films with monotonically varying bandgaps across the full composition range. Photothermal deflection spectroscopy, photoluminescence, and X-ray diffraction show that following suitable fabrication protocols these mixed lead-halide perovskite films form a single phase. The optical absorption edge of the pure triiodide and tribromide perovskites is sharp with Urbach energies of 15 and 23 meV, respectively, and reaches a maximum of 90 meV for CH3NH3PbI1.2Br1.8. We demonstrate a bromide-iodide lead perovskite film (CH3NH3PbI1.2Br1.8) with an optical bandgap of 1.94 eV, which is optimal for tandem cells of these materials with crystalline silicon devices.

A quantum magnet, LiCuSbO 4 , with chains of edge-sharing spin-½ CuO 6 octahedra is reported. While short-range order is observed for T < 10 K, no zero-field phase transition or spin freezing occurs down to 100 mK. Specific heat indicates a distinct high field phase near the 12 T saturation field. Neutron scattering shows incommensurate spin correlations with q = (0.47±0.01)π/a and places an upper limit of 70 µeV on any spin gap. Exact diagonalization of 16-spin easy-plane spin-½ chains with competing ferro-and antiferromagnetic interactions (J 1 = -75 K, J 2 = 34 K) accounts for the T > 2 K data. PACS: 75.10.Jm, 75.30.Kz, 75.40.Cx, 75.30.Et 2 The Heisenberg spin-½ chain is one of very few quantum critical systems to be realized in a crystalline solid. An element of frustration is added by next-nearest-neighbor (NNN) interactions (J 2 ). [1][2][3] In such systems, theoretical work [4][5][6][7][8][9] indicates that qualitatively different quantum phases are possible as a function of α = J 2 /J 1 , axial exchange anisotropy, ∆, and the applied field h = gµ B H/|J 1 |. Finite values of α are observed in copper oxide spin-chains formed by corner-or edge-sharing Jahn-Teller distorted CuO 6 polyhedra. [10][11][12][13][14][15][16][17] While the sign and magnitude of J 1 is dependent on the α c ) from a short-range ordered (SRO) phase with a small yet finite gap (α < α c ). 20Dimer, spiral, vector-chiral and multipolar phases have been predicted as a function of α, ∆ and h. 4,6,8 Interest has been further amplified by the observation that ferroelectricity accompanies magnetic ordering in some of these systems. 14,21,22 Known spin-½ FM-AFM chains span a wide range of α and include Li 2 CuO 2 , 17,21,22 Here we present the structure and magnetic properties of the one-dimensional (1D) spin chain compound LiCuSbO 4 . Due to weak inter-chain interactions it has no 3D ordering down to 100 mK and can be driven to a saturated-FM phase above the experimentally accessible field <µ o H s > = 12 T. The magnetic susceptibility, isothermal magnetization and specific heat can be simultaneously modelled for T > 2 K using the approximate spin Hamiltonian:Here θ is the angle between the applied field, h, and the unique molecular axis, z. The parameters J 1 = -75K, α = -0.45, and ∆ = 0.83 account for the main features in the thermomagnetic data and are also consistent with the magnetic excitations measured by inelastic neutron scattering.Powder samples of LiCuSbO 4 were prepared using a ceramic synthesis. 25LiCuSbO 4 has an orthorhombic structure 26, 27 related to that of LiFeSnO 4 . 28Rietveld analysis 29 of powder neutron diffraction data acquired on the BT1 diffractometer at NIST 30 was carried out using FULLPROF. 31The final fit, with the corr...

Layered hybrid metal-halide perovskites with non-centrosymmetric crystal structure are predicted to show spin-selective band splitting from Rashba effects. Thus, fabrication of metal-halide perovskites with defined crystal symmetry is desired to control the spin-splitting in their electronic states. Here, we report the influence of halogen parasubstituents on the crystal structure of benzylammonium lead iodide perovskites (4-XC 6 H 4 CH 2 NH 3 ) 2 PbI 4 (X = H, F, Cl, Br). Using X-ray diffraction and second-harmonic generation, we study structure and symmetry of single crystal and thin film samples. We report that introduction of a halogen atom lowers the crystal symmetry such that the chlorine-and bromine-substituted structures are non-centrosymmetric. The differences can be attributed to the nature of the intermolecular interactions between the organic molecules. We calculate electronic band structures and find good control of Rashba splittings. Our results present a facile approach to tailor hybrid layered metal halide perovskites with potential for spintronic and non-linear optical applications.

We demonstrate short-range ordering in Li-ion battery material Li1.25Nb0.25Mn0.5O2, and identify its local structure and correlation length—which is sensitive to synthesis conditions and has important consequences for the material's electrochemistry.

Rechargeable battery systems based on Mg-ion chemistries are generating significant interest as potential alternatives to Li-ion batteries. Despite the wealth of local structural information that could potentially be gained from Nuclear Magnetic Resonance (NMR) experiments of Mg-ion battery materials, systematic Mg solid-state NMR studies have been scarce due to the low natural abundance, low gyromagnetic ratio, and significant quadrupole moment ofMg (I = 5/2). This work reports a combined experimental Mg NMR and first principles density functional theory (DFT) study of paramagnetic Mg transition metal oxide systems MgMnO and MgCrO that serve as model systems for Mg-ion battery cathode materials. Magnetic parameters, hyperfine shifts and quadrupolar parameters were calculated ab initio using hybrid DFT and compared to the experimental values obtained from NMR and magnetic measurements. We show that the rotor assisted population transfer (RAPT) pulse sequence can be used to enhance the signal-to-noise ratio in paramagnetic Mg spectra without distortions in the spinning sideband manifold. In addition, the value of the predicted quadrupolar coupling constant of MgMnO was confirmed using the RAPT pulse sequence. We further apply the same methodology to study the NMR spectra of spinel compounds MgVO and MgMnO, candidate cathode materials for Mg-ion batteries.

The structural misfit compound ͑PbSe͒ 1.16 ͑TiSe 2 ͒ 2 is reported. It is a superconductor with a T c of 2.3 K. ͑PbSe͒ 1.16 ͑TiSe 2 ͒ 2 derives from a parent compound, TiSe 2 , which shows a charge-density wave transition and no superconductivity. The crystal structure, characterized by high-resolution electron microscopy and powder x-ray diffraction, consists of two layers of 1T-TiSe 2 alternating with a double layer of ͑100͒ PbSe. Transport measurements suggest that the superconductivity is induced by charge transfer from the PbSe layers to the TiSe 2 layers.