Zero-Dimensional Hybrid Organic-Inorganic Indium Bromide with Blue Emission.

Low-dimensional hybrid organic-inorganic metal halides have received increased attention because of their outstanding optical and electronic properties. However, the most studied hybrid compounds contain lead and have long-term stability issues, which must be addressed for their use in practical applications. Here, we report a new zero-dimensional hybrid organic-inorganic halide, RInBr4, featuring photoemissive trimethyl(4-stilbenyl)methylammonium (R+) cations and nonemissive InBr4- tetrahedral anions. The crystal structure of RInBr4 is composed of alternating layers of inorganic anions and organic cations along the crystallographic a axis. The resultant hybrid demonstrates bright-blue emission with Commission Internationale de l'Eclairage color coordinates of (0.19, 0.20) and a high photoluminescence quantum yield (PLQY) of 16.36% at room temperature, a 2-fold increase compared to the PLQY of 8.15% measured for the precursor organic salt RBr. On the basis of our optical spectroscopy and computational work, the organic component is responsible for the observed blue emission of the hybrid material. In addition to the enhanced light emission efficiency, the novel hybrid indium bromide demonstrates significantly improved environmental stability. These findings may pave the way for the consideration of hybrid organic In(III) halides for light emission applications.

(CH3 NH3 )AuX4 ⋠H2 O (X=Cl, Br) and (CH3 NH3 )AuCl4 : Low-Band Gap Lead-Free Layered Gold Halide Perovskite Materials.

Perovskite solar cells have recently enabled power conversion efficiency comparable to established technologies such as silicon and cadmium telluride. Ongoing efforts to improve the stability of halide perovskites in ambient air has yielded promising results. However, the presence of toxic heavy element lead (Pb) remains a major concern requiring further attention. Herein, we report three new Pb-free hybrid organic-inorganic perovskite-type halides based on gold (Au), (CH3 NH3 )AuBr4 ⋅H2 O (1), (CH3 NH3 )AuCl4 ⋅H2 O (2), and (CH3 NH3 )AuCl4 (3). Hydrated compounds 1 and 2 crystallize in a brand-new structure type featuring perovskite-derived 2D layers and 1D chains based on pseudo-octahedral AuX6 building blocks. In contrast, the novel crystal structure of the solvent-free compound 3 shows an exotic non-perovskite quasi-2D layered structure containing edge- and corner-shared AuCl6 octahedra. The use of Au metal instead of Pb results in unprecedented low band gaps below 2.5 eV for single-layered metal chlorides and bromides. Moreover, at room temperature the three compounds show a weak blue emission due to the electronic transition between Au-6s and Au-5d, in agreement with the density function theory (DFT) calculation results. These findings are discussed in the context of viability of Au-based halides as alternatives for Pb-based halides for optoelectronic applications.

Rb4Ag2BiBr9: A Lead-Free Visible Light Absorbing Halide Semiconductor with Improved Stability.

Replacement of the toxic heavy element lead in metal halide perovskites has been attracting a great interest because the high toxicity and poor air stability are two of the major barriers for their widespread utilization. Recently, mixed-cation double perovskite halides, also known as elpasolites, were proposed as an alternative lead-free candidate for the design of nontoxic perovskite solar cells. Herein, we report a new nontoxic and air stable lead-free all-inorganic semiconductor Rb4Ag2BiBr9 prepared using the mixed-cation approach; however, Rb4Ag2BiBr9 adopts a new structure type (Pearson's code oP32) featuring BiBr6 octahedra and AgBr5 square pyramids that share common edges and corners to form a unique 2D layered non-perovskite structure. Rb4Ag2BiBr9 is also demonstrated to be thermally stable with the measured onset decomposition temperature of To = 520 °C. Optical absorption measurements and density functional theory calculations suggest a nearly direct band gap for Rb4Ag2BiBr9. Room temperature photoluminescence (PL) measurements show a broadband weak emission. Further, temperature-dependent and power-dependent PL measurements show a strong competition between multiple emission centers and suggest the coexistence of defect-bound excitons and self-trapped excitons in Rb4Ag2BiBr9.

Exploratory Work in the Quaternary System of Ca⁻Eu⁻Cd⁻Sb: Synthesis, Crystal, and Electronic Structures of New Zintl Solid Solutions.

Investigation of the quaternary system, Ca⁻Eu⁻Cd⁻Sb, led to a discovery of the new solid solutions, Ca1-xEuxCd2Sb2, with the CaAl2Si2 structure type (x ≈ 0.3⁻0.9, hP5, P 3 ¯ m1, a = 4.6632(5)⁻4.6934(3) Å, c = 7.630(1)⁻7.7062(7) Å), Ca2-xEuxCdSb2 with the Yb2CdSb2 type (x ≈ 0.6, oS20, Cmc21, a = 4.646(2) Å, b = 17.733(7) Å, c = 7.283(3) Å), and Eu11-xCaxCd6Sb12 with the Sr11Cd6Sb12 type (x ≈ 1, mS58, C2/m, a = 32.407(4) Å, b = 4.7248(5) Å, c = 12.377(1) Å, ß = 109.96(1)°). Systematic crystallographic studies of the Ca1-xEuxCd2Sb2 series indicated expansion of the unit cell upon an increase in the Eu content, in accordance with a larger ionic radius of Eu2+ vs. Ca2+. The Ca2-xEuxCdSb2 composition with x ≈ 0.6 adopts the non-centrosymmetric space group, Cmc21, although the parent ternary phase, Ca2CdSb2, crystallizes in the centrosymmetric space group, Pnma. Two non-equivalent Ca sites in the layered crystal structure of Ca2-xEuxCdSb2 get unevenly occupied by Eu, with a preference for the interlayer position, which offers a larger available volume. Similar size-driven preferred occupation is observed in the Eu11-xCaxCd6Sb12 solid solution with x ≈ 1.

Broadband Emission in Hybrid Organic-Inorganic Halides of Group 12 Metals.

We report syntheses, crystal and electronic structures, and characterization of three new hybrid organic-inorganic halides (R)ZnBr3(DMSO), (R)2CdBr4·DMSO, and (R)CdI3(DMSO) (where (R) = C6(CH3)5CH2N(CH3)3, and DMSO = dimethyl sulfoxide). The compounds can be conveniently prepared as single crystals and bulk polycrystalline powders using a DMSO-methanol solvent system. On the basis of the single-crystal X-ray diffraction results carried out at room temperature and 100 K, all compounds have zero-dimensional (0D) crystal structures featuring alternating layers of bulky organic cations and molecular inorganic anions based on a tetrahedral coordination around group 12 metal cations. The presence of discrete molecular building blocks in the 0D structures results in localized charges and tunable room-temperature light emission, including white light for (R)ZnBr3(DMSO), bluish-white light for (R)2CdBr4·DMSO, and green for (R)CdI3(DMSO). The highest photoluminescence quantum yield (PLQY) value of 3.07% was measured for (R)ZnBr3(DMSO), which emits cold white light based on the calculated correlated color temperature (CCT) of 11,044 K. All compounds exhibit fast photoluminescence lifetimes on the timescale of tens of nanoseconds, consistent with the fast luminescence decay observed in π-conjugated organic molecules. Temperature dependence photoluminescence study showed the appearance of additional peaks around 550 nm, resulting from the organic salt emission. Density functional theory calculations show that the incorporation of both the low-gap aromatic molecule R and the relatively electropositive Zn and Cd metals can lead to exciton localization at the aromatic molecular cations, which act as luminescence centers.

Synthesis, Crystal and Electronic Structures, and Optical Properties of (CH3NH3)2CdX4 (X = Cl, Br, I).

We report the synthesis, crystal and electronic structures, as well as optical properties of the hybrid organic-inorganic compounds MA2CdX4 (MA = CH3NH3; X = Cl, Br, I). MA2CdI4 is a new compound, whereas, for MA2CdCl4 and MA2CdBr4, structural investigations have already been conducted but electronic structures and optical properties are reported here for the first time. Single crystals were grown through slow evaporation of MA2CdX4 solutions with optimized conditions yielding mm-sized colorless (X = Cl, Br) and pale yellow (X = I) crystals. Single crystal and variable temperature powder X-ray diffraction measurements suggest that MA2CdCl4 forms a 2D layered perovskite structure and has two structural transitions at 283 and 173 K. In contrast, MA2CdBr4 and MA2CdI4 adopt 0D K2SO4-derived crystal structures based on isolated CdX4 tetrahedra and show no phase transitions down to 20 K. The contrasting crystal structures and chemical compositions in the MA2CdX4 family impact their air stabilities, investigated for the first time in this work; MA2CdCl4 is air-stable, whereas MA2CdBr4 and MA2CdI4 partially decompose when left in air. Optical absorption measurements suggest that MA2CdX4 have large optical band gaps above 3.9 eV. Room temperature photoluminescence spectra of MA2CdX4 yield broad peaks in the 375-955 nm range with full width at half-maximum values up to 208 nm. These PL peaks are tentatively assigned to self-trapped excitons in MA2CdX4 following the crystal and electronic structure considerations. The bands around the Fermi level have small dispersions, which is indicative of high charge localization with significant exciton binding energies in MA2CdX4. On the basis of our combined experimental and computational results, MA2CdX4 and related compounds may be of interest for white-light-emitting phosphors and scintillator applications.

The Ternary Alkaline-Earth Metal Manganese Bismuthides Sr2MnBi2 and Ba2Mn1-xBi2 (x ≈ 0.15).

Two new ternary manganese bismuthides have been synthesized and their structures established based on single-crystal X-ray diffraction methods. Sr2MnBi2 crystallizes in the orthorhombic space group Pnma (a = 16.200(9) Å, b = 14.767(8) Å, c = 8.438(5) Å, V = 2018(2) Å3; Z = 12; Pearson index oP60) and is isostructural to the antimonide Sr2MnSb2. The crystal structure contains corrugated layers of corner- and edge-shared [MnBi4] tetrahedra and Sr atoms enclosed between these layers. Electronic structure calculations suggest that Sr2MnBi2 is a magnetic semiconductor possessing Mn2+ (high-spin d5) ions, and its structure can be rationalized within the Zintl concept as [Sr2+]2[Mn2+][Bi3-]2. The temperature dependence of the resistivity shows behavior consistent with a degenerate semiconductor/poor metal, and magnetic susceptibility measurements reveal a high degree of frustration resulting from the two-dimensional nature of the structure. The compositionally similar Ba2Mn1-xBi2 (x ≈ 0.15) crystallizes in a very different structure (space group Imma, a = 25.597(8) Å, b = 25.667(4) Å, c = 17.128(3) Å, V = 11253(4) Å3; Z = 64; Pearson index oI316) with its own structure type. The complex structure boasts Mn atoms in a variety of coordination environments and can be viewed as consisting of two interpenetrating 3D frameworks, linked by Bi-Bi bonds. Ba2Mn1-xBi2 can be regarded as a highly reduced compound with anticipated metallic behavior.

Efficient Generation of Long-Lived Triplet Excitons in 2D Hybrid Perovskite.

Triplet excitons form in quasi-2D hybrid inorganic-organic perovskites and diffuse over 100 nm before radiating with >11% photoluminescence quantum efficiency (PLQE) at low temperatures.

Employing Lead Thiocyanate Additive to Reduce the Hysteresis and Boost the Fill Factor of Planar Perovskite Solar Cells.

Lead thiocyanate in the perovskite precursor can increase the grain size of a perovskite thin film and reduce the conductivity of the grain boundaries, leading to perovskite solar cells with reduced hysteresis and enhanced fill factor. A planar perovskite solar cell with grain boundary and interface passivation achieves a steady-state efficiency of 18.42%.

Organic-Inorganic Perovskites: Structural Versatility for Functional Materials Design.

Although known since the late 19th century, organic-inorganic perovskites have recently received extraordinary research community attention because of their unique physical properties, which make them promising candidates for application in photovoltaic (PV) and related optoelectronic devices. This review will explore beyond the current focus on three-dimensional (3-D) lead(II) halide perovskites, to highlight the great chemical flexibility and outstanding potential of the broader class of 3-D and lower dimensional organic-based perovskite family for electronic, optical, and energy-based applications as well as fundamental research. The concept of a multifunctional organic-inorganic hybrid, in which the organic and inorganic structural components provide intentional, unique, and hopefully synergistic features to the compound, represents an important contemporary target.

Photovoltaic Properties of Two-Dimensional (CH3NH3)2Pb(SCN)2I2 Perovskite: A Combined Experimental and Density Functional Theory Study.

We explore the photovoltaic-relevant properties of the 2D MA2Pb(SCN)2I2 (where MA = CH3NH3(+)) perovskite using a combination of materials synthesis, characterization and density functional theory calculation, and determine electronic properties of MA2Pb(SCN)2I2 that are significantly different from those previously reported in literature. The layered perovskite with mixed-anions exhibits an indirect bandgap of ∼2.04 eV, with a slightly larger direct bandgap of ∼2.11 eV. The carriers (both electrons and holes) are also found to be confined within the 2D layers. Our results suggest that the 2D MA2Pb(SCN)2I2 perovskite may not be among the most promising absorbers for efficient single-junction solar cell applications; however, use as an absorber for the top cell of a tandem solar cell may still be a possibility if films are grown with the 2D layers aligned perpendicular to the substrates.

Room-Temperature Ba(Fe1-x Cox)2 As2 is not Tetragonal: Direct Observation of Magnetoelastic Interactions in Pnictide Superconductors.

Lattice distortions corresponding to Ba displacements with respect to the FeAs sublattice are revealed to break the room-temperature tetragonal symmetry in Ba(Fe1-x Cox)2 As2. The displacements yield twin domains of the size of ≈10 nm. The domain size correlates with the magnitude of the local Fe magnetic moment and its non-monotonic dependence on Co concentration.

Robust antiferromagnetism preventing superconductivity in pressurized (Ba 0.61 K 0.39)Mn2Bi2.

BaMn2Bi2 possesses an iso-structure of iron pnictide superconductors and similar antiferromagnetic (AFM) ground state to that of cuprates, therefore, it receives much more attention on its properties and is expected to be the parent compound of a new family of superconductors. When doped with potassium (K), BaMn2Bi2 undergoes a transition from an AFM insulator to an AFM metal. Consequently, it is of great interest to suppress the AFM order in the K-doped BaMn2Bi2 with the aim of exploring the potential superconductivity. Here, we report that external pressure up to 35.6 GPa cannot suppress the AFM order in the K-doped BaMn2Bi2 to develop superconductivity in the temperature range of 300 K-1.5 K, but induces a tetragonal (T) to an orthorhombic (OR) phase transition at ~20 GPa. Theoretical calculations for the T and OR phases, on basis of our high-pressure XRD data, indicate that the AFM order is robust in the pressurized Ba0.61K0.39Mn2Bi2. Both of our experimental and theoretical results suggest that the robust AFM order essentially prevents the emergence of superconductivity.

Ferromagnetism of Fe3Sn and alloys.

Hexagonal Fe(3)Sn has many of the desirable properties for a new permanent magnet phase with a Curie temperature of 725 K, a saturation moment of 1.18 MA/m. and anisotropy energy, K1 of 1.8 MJ/m(3). However, contrary to earlier experimental reports, we found both experimentally and theoretically that the easy magnetic axis lies in the hexagonal plane, which is undesirable for a permanent magnet material. One possibility for changing the easy axis direction is through alloying. We used first principles calculations to investigate the effect of elemental substitutions. The calculations showed that substitution on the Sn site has the potential to switch the easy axis direction. However, transition metal substitutions with Co or Mn do not have this effect. We attempted synthesis of a number of these alloys and found results in accord with the theoretical predictions for those that were formed. However, the alloys that could be readily made all showed an in-plane easy axis. The electronic structure of Fe(3)Sn is reported, as are some are magnetic and structural properties for the Fe(3)Sn(2), and Fe(5)Sn(3) compounds, which could be prepared as mm-sized single crystals.

Cu substitution effects on the local magnetic properties of Ba(Fe(1-x)Cu(x))(2)As(2): a site-selective (75)As and (63)Cu NMR study.

We take advantage of the site-selective nature of the ^{75}As and ^{63}Cu NMR techniques to probe the Cu substitution effects on the local magnetic properties of the FeAs planes in Ba(Fe_{1-x}Cu_{x})_{2}As_{2}. We show that the suppression of antiferromagnetic Fe spin fluctuations induced by Cu substitution is weaker than a naive expectation based on a simple rigid band picture, in which each Cu atom would donate three electrons to the FeAs planes. Comparison between ^{63}Cu and ^{75}As NMR data indicates that spin fluctuations are suppressed at the Cu and their neighboring Fe sites in the tetragonal phase, suggesting the strongly local nature of the Cu substitution effects. We attribute the absence of a large superconducting dome in the phase diagram of Ba(Fe_{1-x}Cu_{x})_{2}As_{2} to the emergence of a nearly magnetically ordered FeAs plane under the presence of orthorhombic distortion.

Annealing effects on the properties of BFe2As2 (B = Ca, Sr, Ba) superconducting parents.

The effects of thermal-annealing on the antiferromagnetic (TN) and structural (Ts) transition temperatures of ThCr2Si2-type BaFe2As2 and SrFe2As2 ('122') crystals are reported and compared to that of CaFe2As2. Although the shift in transition temperature for CaFe2As2 can be as high as 75 K, we find modest changes of ∼6 K for BaFe2As2 and SrFe2As2. Such findings are based on the measurements of temperature dependence of electrical resistivity, magnetization, and heat capacity. Residual resistivity ratios show an improvement of crystal quality upon annealing for both BaFe2As2 and SrFe2As2. We confirm the pressure-like influence of annealing on the 122 crystals.

Complex structures of different CaFe2As2 samples.

The interplay between magnetism and crystal structures in three CaFe2As2 samples is studied. For the nonmagnetic quenched crystals, different crystalline domains with varying lattice parameters are found, and three phases (orthorhombic, tetragonal, and collapsed tetragonal) coexist between TS=95 K and 45 K. Annealing of the quenched crystals at 350°C leads to a strain relief through a large (~1.3%) expansion of the c-parameter and a small (~0.2%) contraction of the a-parameter, and to local ~0.2 Šdisplacements at the atomic-level. This annealing procedure results in the most homogeneous crystals for which the antiferromagnetic and orthorhombic phase transitions occur at TN/TS=168(1) K. In the 700°C-annealed crystal, an intermediate strain regime takes place, with tetragonal and orthorhombic structural phases coexisting between 80 to 120 K. The origin of such strong shifts in the transition temperatures are tied to structural parameters. Importantly, with annealing, an increase in the Fe-As length leads to more localized Fe electrons and higher local magnetic moments on Fe ions. Synergistic contribution of other structural parameters, including a decrease in the Fe-Fe distance, and a dramatic increase of the c-parameter, which enhances the Fermi surface nesting in CaFe2As2, are also discussed.

Local inhomogeneity and filamentary superconductivity in Pr-doped CaFe2As2.

We use multiscale techniques to determine the extent of local inhomogeneity and superconductivity in Ca0.86Pr0.14Fe2As2 single crystal. The inhomogeneity is manifested as a spatial variation of the praseodymium concentration, local density of states, and superconducting order parameter. We show that the high-Tc superconductivity emerges from cloverlike defects associated with Pr dopants. The highest Tc is observed in both the tetragonal and collapsed tetragonal phases, and its filamentary nature is a consequence of nonuniform Pr distribution that develops localized, isolated superconducting regions within the crystals.

Crystal, magnetic, and electronic structures, and properties of new BaMnPnF (Pn = As, Sb, Bi).

New BaMnPnF (Pn = As, Sb, Bi) are synthesized by stoichiometric reaction of elements with BaF2. They crystallize in the P4/nmm space group, with the ZrCuSiAs-type structure, as indicated by X-ray crystallography. Electrical resistivity results indicate that Pn = As, Sb, and Bi are semiconductors with band gaps of 0.73 eV, 0.48 eV and 0.003 eV (extrinsic value), respectively. Powder neutron diffraction reveals a G-type antiferromagnetic order below T(N) = 338(1) K for Pn = As, and below T(N) = 272(1) K for Pn = Sb. Magnetic susceptibility increases with temperature above 100 K for all the materials. Density functional calculations find semiconducting antiferromagnetic compounds with strong in-plane and weaker out-of-plane exchange coupling that may result in non-Curie Weiss behavior above TN. The ordered magnetic moments are 3.65(5) µ(B)/Mn for Pn = As, and 3.66(3) µ(B)/Mn for Pn = Sb at 4 K, as refined from neutron diffraction experiments.