Noncollinear Electric Dipoles in a Polar Chiral Phase of CsSnBr3 Perovskite.

Polar and chiral crystal symmetries confer a variety of potentially useful functionalities upon solids by coupling otherwise noninteracting mechanical, electronic, optical, and magnetic degrees of freedom. We describe two phases of the 3D perovskite, CsSnBr3, which emerge below 85 K due to the formation of Sn(II) lone pairs and their interaction with extant octahedral tilts. Phase II (77 K < T < 85 K, space group P21/m) exhibits ferroaxial order driven by a noncollinear pattern of lone pair-driven distortions within the plane normal to the unique octahedral tilt axis, preserving the inversion symmetry observed at higher temperatures. Phase I (T < 77 K, space group P21) additionally exhibits ferroelectric order due to distortions along the unique tilt axis, breaking both inversion and mirror symmetries. This polar and chiral phase exhibits second harmonic generation from the bulk and pronounced electrostriction and negative thermal expansion along the polar axis (Q22 ≈ 1.1 m4 C-2; αb = -7.8 × 10-5 K-1) through the onset of polarization. The structures of phases I and II were predicted by recursively following harmonic phonon instabilities to generate a tree of candidate structures and subsequently corroborated by synchrotron X-ray powder diffraction and polarized Raman and 81Br nuclear quadrupole resonance spectroscopies. Preliminary attempts to suppress unintentional hole doping to allow for ferroelectric switching are described. Together, the polar symmetry, small band gap, large spin-orbit splitting of Sn 5p orbitals, and predicted strain sensitivity of the symmetry-breaking distortions suggest bulk samples and epitaxial films of CsSnBr3 or its neighboring solid solutions as candidates for bulk Rashba effects.

Instant Macrocyclizations via Multicomponent Reactions.

Macrocycles fascinate chemists due to both their structure and their applications. However, we still lack efficient and sustainable synthetic methods, giving us straightforward access to them. Herein, a rapid macrocyclization utilizing a two-step, one-pot approach based on orthogonal multicomponent reaction (MCR) tactics is introduced. This synthetic protocol, which is based on Ugi and Groebke-Blackburn-Bienaymé reactions with isocyanides tethered to alkyl tosylates, yields medium sized macrocycles that are otherwise difficult to achieve. Single crystal structures reveal conformational reorganization via intramolecular hydrogen bonding, and modeling studies profile the synthesized libraries.

Diffuse Reflectance Spectroscopy with Dilution: A Powerful Method for Halide Perovskites Study.

Halide perovskites and their low-dimensional analogs are promising semiconductor materials for solar cells, LEDs, lasers, detectors and other applications in the area of photonics. The most informative optical property of semiconductor photonics materials is the absorption spectrum enabling observation of the fundamental absorption edge, exciton structure, defect-related bands, etc. Traditionally, in the study of halide perovskites, this spectrum is obtained by absorption spectroscopy of thin films or diffuse reflectance spectroscopy of powders. The first method is applicable only to compounds with the developed thin film deposition technology, and in the second case, a large absorption coefficient narrows the observations down to the sample transparency region. In this paper, we suggest the diffuse reflectance spectroscopy with dilution as a method for obtaining the full-range absorption spectrum from halide perovskite powders, and demonstrate its application to practically important cases.

Internal Vibrations of Pyridinium Cation in One-Dimensional Halide Perovskites and the Corresponding Halide Salts.

We investigate vibrations of the pyridinium cation PyH+ = C5H5NH+ in one-dimensional lead halide perovskites PyPbX3 and pyridinium halide salts PyHX (X- = I-, Br-), combining infrared absorption and Raman scattering methods at room temperature. Internal vibrations of the cation were assigned based on density functional theory modeling. Some of the vibrational bands are sensitive to perovskite or the salt environment in the solid state, while halide substitution has only a minor effect on them. These findings have been confirmed by 1H, 13C and 207Pb solid-state nuclear magnetic resonance (NMR) experiments. Narrower vibrational bands in perovskites indicate less disorder in these materials. The splitting of NH-group vibrational bands in perovskites can be rationalized the presence of nonequivalent crystal sites for cations or by more exotic phenomena such as quantum tunneling transition between two molecular orientations. We have shown how organic cations in hybrid organic-inorganic crystals could be used as spectators of the crystalline environment that affects their internal vibrations.

Tolerance Factor for Stabilizing 3D Hybrid Halide Perovskitoids Using Linear Diammonium Cations.

Three-dimensional (3D) halide perovskites have attracted enormous research interest, but the choice of the A-site cations is limited by the Goldschmidt tolerance factor. In order to accommodate cations that lie outside the acceptable range of the tolerance factor, low-dimensional structures usually form. To maintain the favorable 3D connection, the links among the metal-halide octahedra need to be rearranged to fit the large cations. This can result in a departure from the proper corner-sharing perovskite architectures and lead to distinctly different perovskitoid motifs with edge- and face-sharing. In this work, we report four new 3D bromide perovskitoids incorporating linear organic diammonium cations, A'Pb2Br6 (A' is a +2 cation). We propose a rule that can guide the further expansion of this class of compounds, analogous to the notion of Goldschmidt tolerance factor widely adopted for 3D AMX3 perovskites. The fundamental building blocks in A'Pb2Br6 consist of two edge-shared octahedra, which are then connected by corner-sharing to form a 3D network. Different compounds adopt different structural motifs, which can be transformed from one to another by symmetry operations. Electronic structure calculations suggest that they are direct bandgap semiconductors, with relatively large band dispersions created by octahedra connected by corner-sharing. They exhibit similar electronic band structures and dynamic lattice characteristics to the regular 3D AMX3 perovskites. Structures with smaller Pb-Br-Pb angles and larger octahedra distortion exhibit broad photoluminescence at room temperature. The emerging structure-property relationships in these 3D perovskitoids set the foundation for designing and investigating these compounds for a variety of optoelectronic applications.

Ultrafast Excitonic Response in Two-Dimensional Hybrid Perovskites Driven by Intense Midinfrared Pulses.

Two-dimensional organic-inorganic hybrid perovskites (2DHPs) are natural quantum-well-like materials, in which strong quantum and dielectric confinement effects due to the organic spacers give rise to tightly bound excitons with large binding energy. To examine the mutual interactions between the organic spacer cations and the inorganic charge-residing octahedral framework in 2DHPs, here we perform femtosecond pump-probe spectroscopy by direct vibrational pumping of the organic spacers, followed by a visible-to-ultraviolet probe covering their excitonic resonances. Measurements on prototypical lead-bromide based 2DHP compounds, (BA)_{2}PbBr_{4} and (BA)_{2}(FA)Pb_{2}Br_{7} (BA^{+}=butylammonium; FA^{+}=formamidinium), reveal two distinct regimes of the temporal response. The first regime is dominated by a pump-induced transient expansion of the organic spacer layers that reduces the exciton oscillator strength, whereas the second regime arises from pump-induced lattice heating effects primarily associated with a spectral shift of the exciton energy. In addition, vibrational excitation enhances the biexciton emission, which we attribute to a stronger intralayer exciton confinement as well as vibrationally induced exciton detrapping from defect states. Our study provides fundamental insights regarding the impact of organic spacers on excitons in 2DHPs, as well as the excited-state dynamics and vibrational energy dissipation in these structurally diverse materials.

High-efficiency two-dimensional Ruddlesden-Popper perovskite solar cells.

Three-dimensional organic-inorganic perovskites have emerged as one of the most promising thin-film solar cell materials owing to their remarkable photophysical properties, which have led to power conversion efficiencies exceeding 20 per cent, with the prospect of further improvements towards the Shockley-Queisser limit for a single­junction solar cell (33.5 per cent). Besides efficiency, another critical factor for photovoltaics and other optoelectronic applications is environmental stability and photostability under operating conditions. In contrast to their three-dimensional counterparts, Ruddlesden-Popper phases--layered two-dimensional perovskite films--have shown promising stability, but poor efficiency at only 4.73 per cent. This relatively poor efficiency is attributed to the inhibition of out-of-plane charge transport by the organic cations, which act like insulating spacing layers between the conducting inorganic slabs. Here we overcome this issue in layered perovskites by producing thin films of near-single-crystalline quality, in which the crystallographic planes of the inorganic perovskite component have a strongly preferential out-of-plane alignment with respect to the contacts in planar solar cells to facilitate efficient charge transport. We report a photovoltaic efficiency of 12.52 per cent with no hysteresis, and the devices exhibit greatly improved stability in comparison to their three-dimensional counterparts when subjected to light, humidity and heat stress tests. Unencapsulated two-dimensional perovskite devices retain over 60 per cent of their efficiency for over 2,250 hours under constant, standard (AM1.5G) illumination, and exhibit greater tolerance to 65 per cent relative humidity than do three-dimensional equivalents. When the devices are encapsulated, the layered devices do not show any degradation under constant AM1.5G illumination or humidity. We anticipate that these results will lead to the growth of single-crystalline, solution-processed, layered, hybrid, perovskite thin films, which are essential for high-performance opto-electronic devices with technologically relevant long-term stability.

Structural and thermodynamic limits of layer thickness in 2D halide perovskites.

In the fast-evolving field of halide perovskite semiconductors, the 2D perovskites (A')2(A) n-1M n X3n+1 [where A = Cs+, CH3NH3+, HC(NH2)2+; A' = ammonium cation acting as spacer; M = Ge2+, Sn2+, Pb2+; and X = Cl-, Br-, I-] have recently made a critical entry. The n value defines the thickness of the 2D layers, which controls the optical and electronic properties. The 2D perovskites have demonstrated preliminary optoelectronic device lifetime superior to their 3D counterparts. They have also attracted fundamental interest as solution-processed quantum wells with structural and physical properties tunable via chemical composition, notably by the n value defining the perovskite layer thickness. The higher members (n > 5) have not been documented, and there are important scientific questions underlying fundamental limits for n To develop and utilize these materials in technology, it is imperative to understand their thermodynamic stability, fundamental synthetic limitations, and the derived structure-function relationships. We report the effective synthesis of the highest iodide n-members yet, namely (CH3(CH2)2NH3)2(CH3NH3)5Pb6I19 (n = 6) and (CH3(CH2)2NH3)2(CH3NH3)6Pb7I22 (n = 7), and confirm the crystal structure with single-crystal X-ray diffraction, and provide indirect evidence for "(CH3(CH2)2NH3)2(CH3NH3)8Pb9I28" ("n = 9"). Direct HCl solution calorimetric measurements show the compounds with n > 7 have unfavorable enthalpies of formation (ΔHf), suggesting the formation of higher homologs to be challenging. Finally, we report preliminary n-dependent solar cell efficiency in the range of 9-12.6% in these higher n-members, highlighting the strong promise of these materials for high-performance devices.

High-Performance Thermoelectric α-Ag9 Ga1-x Te6 Compounds with Ultralow Lattice Thermal Conductivity Originating from Ag9 Te2 Motifs.

We have determined the complex atomic structure of high-temperature α-Ag9 GaTe6 phase with a hexagonal lattice (P63 mc space group, a=b=8.2766 Å, c=13.4349 Å). The structure has outer [GaTe4 ]5- tetrahedrons and inner [Ag9 Te2 ]5+ clusters. All of the Ag ions are disorderly distributed in the lattice. Seven types of the Ag atoms constitute the cage-like [Ag9 Te2 ]5+ clusters. The highly disordered Ag ions vibrate in-harmonically, producing strong coupling between low frequency optical phonons and acoustic phonons. This in conjunction with a low sound velocity of 1354 m s-1 leads to an ultralow thermal conductivity of 0.20 W m-1 K-1 at 673 K. Meanwhile, the deficiency of Ga in Ag9 Ga1-x Te6 compounds effectively optimizes the electronic transport properties. Ag9 Ga0.91 Te6 attains a highest power factor of 0.40 mW m-1 K-2 at 673 K. All these contribute to a much-improved ZT value of 1.13 at 623 K for Ag9 Ga0.95 Te6 , which is 41 % higher than that of the pristine Ag9 GaTe6 sample.

Hybrid Germanium Bromide Perovskites with Tunable Second Harmonic Generation.

Ge-based hybrid perovskite materials have demonstrated great potential for second harmonic generation (SHG) due to the geometry and lone-pair induced non-centrosymmetric structures. Here, we report a new family of hybrid 3D Ge-based bromide perovskites AGeBr3 , A=CH3 NH3 (MA), CH(NH2 )2 (FA), Cs and FAGe0.5 Sn0.5 Br3 , crystallizing in polar space groups. These compounds exhibit tunable SHG responses, where MAGeBr3 shows the strongest SHG intensity (5×potassium dihydrogen phosphate, KDP). Structural and theoretical analysis indicate the high SHG efficiency is attributed to the displacement of Ge2+ along [111] direction and the relatively strong interactions between lone pair electrons of Ge2+ and polar MA cations along the c-axis. This work provides new structural insights for designing and fine-tuning the SHG properties in hybrid metal halide materials.

Demonstration of Energy-Resolved γ-Ray Detection at Room Temperature by the CsPbCl3 Perovskite Semiconductor.

The detection of γ-rays at room temperature with high-energy resolution using semiconductors is one of the most challenging applications. The presence of even the smallest amount of defects is sufficient to kill the signal generated from γ-rays which makes the availability of semiconductors detectors a rarity. Lead halide perovskite semiconductors exhibit unusually high defect tolerance leading to outstanding and unique optoelectronic properties and are poised to strongly impact applications in photoelectric conversion/detection. Here we demonstrate for the first time that large size single crystals of the all-inorganic perovskite CsPbCl3 semiconductor can function as a high-performance detector for γ-ray nuclear radiation at room temperature. CsPbCl3 is a wide-gap semiconductor with a bandgap of 3.03 eV and possesses a high effective atomic number of 69.8. We identified the two distinct phase transitions in CsPbCl3, from cubic (Pm-3m) to tetragonal (P4/mbm) at 325 K and finally to orthorhombic (Pbnm) at 316 K. Despite crystal twinning induced by phase transitions, CsPbCl3 crystals in detector grade can be obtained with high electrical resistivity of ∼1.7 × 109 Ω·cm. The crystals were grown from the melt with volume over several cubic centimeters and have a low thermal conductivity of 0.6 W m-1 K-1. The mobilities for electron and hole carriers were determined to ∼30 cm2/(V s). Using photoemission yield spectroscopy in air (PYSA), we determined the valence band maximum at 5.66 ± 0.05 eV. Under γ-ray exposure, our Schottky-type planar CsPbCl3 detector achieved an excellent energy resolution (∼16% at 122 keV) accompanied by a high figure-of-merit hole mobility-lifetime product (3.2 × 10-4 cm2/V) and a long hole lifetime (16 µs). The results demonstrate considerable defect tolerance of CsPbCl3 and suggest its strong potential for γ-radiation and X-ray detection at room temperature and above.

Isothermal pressure-derived metastable states in 2D hybrid perovskites showing enduring bandgap narrowing.

Materials in metastable states, such as amorphous ice and supercooled condensed matter, often exhibit exotic phenomena. To date, achieving metastability is usually accomplished by rapid quenching through a thermodynamic path function, namely, heating-cooling cycles. However, heat can be detrimental to organic-containing materials because it can induce degradation. Alternatively, the application of pressure can be used to achieve metastable states that are inaccessible via heating-cooling cycles. Here we report metastable states of 2D organic-inorganic hybrid perovskites reached through structural amorphization under compression followed by recrystallization via decompression. Remarkably, such pressure-derived metastable states in 2D hybrid perovskites exhibit enduring bandgap narrowing by as much as 8.2% with stability under ambient conditions. The achieved metastable states in 2D hybrid perovskites via compression-decompression cycles offer an alternative pathway toward manipulating the properties of these "soft" materials.

25th Anniversary of Molecules-Recent Advances in Inorganic Chemistry.

Celebrating the "25th Anniversary of Molecules" with a Special Issue dedicated to "Recent Advances in Inorganic Chemistry" strengthens the renewed role that inorganic chemistry, one of the oldest chemistry divisions, has lately earned thanks to cutting-edge perspectives and interdisciplinary applications, eventually receiving the veneration and respect which its age might require [...].

Organic Cation Alloying on Intralayer A and Interlayer A' sites in 2D Hybrid Dion-Jacobson Lead Bromide Perovskites (A')(A)Pb2Br7.

Hybrid layered halide perovskites have achieved impressive performance in optoelectronics. New structural types in the two-dimensional (2D) halide system such as the Dion-Jacobson phases have attracted wide research attention due to the short interlayer distance and unique layer orientation that facilitate better charge-transport and higher stability in optoelectronic devices. Here, we report the first solid solution series incorporating both A and A' cations in the 2D Dion-Jacobson family, with the general formula (A')(A)Pb2Br7 ((A' = 3-(aminomethyl)piperidinium (3AMP) and 4-(aminomethyl)piperidinium) (4AMP); A = methylammonium (MA) and formamidinium (FA)). Mixing the spacing A' cations and perovskitizer A cations generates the new (3AMP)a(4AMP)1-a(FA)b(MA)1-bPb2Br7 perovskites. The crystallographically refined crystal structures using single-crystal X-ray diffraction data reveal that the distortion of the inorganic framework is heavily influenced by the degree of A' and A alloying. A rising fraction of 4AMP in the structure, decreases the Pb-Br-Pb angles, making the framework more distorted. On the contrary, higher FA fractions increase the Pb-Br-Pb angles. This structural evolution fine-tunes the optical properties where the larger the Pb-Br-Pb angle, the narrower the band gap. The photoluminescence emission energy mirrors this trend. Raman spectroscopy reveals a highly dynamical lattice similar to MAPbBr3 and consistent with the local distortion environment of the [Pb2Br7] framework. Density functional theory (DFT) calculations of the electronic structures reveal the same trend as the experimental results where (3AMP)(FA)Pb2Br7 has the smallest band gap while (4AMP)(MA)Pb2Br7 has the largest band gap. The structural effects from solely the organic cations in the 2D system highlight the importance of understanding the high sensitivity of the optoelectronic properties on the structural tuning in this broad class of materials.

Three-Dimensional Lead Iodide Perovskitoid Hybrids with High X-ray Photoresponse.

Large organic A cations cannot stabilize the 3D perovskite AMX3 structure because they cannot be accommodated in the cubo-octhedral cage (do not follow the Goldschmidt tolerance factor rule), and they generally template low-dimensional structures. Here we report that the large dication aminomethylpyridinium (AMPY) can template novel 3D structures which resemble conventional perovskites. They have the formula (xAMPY)M2I6 (x = 3 or 4, M = Sn2+ or Pb2+) which is double of the AMX3 formula. However, because of the steric requirement of the Goldschmidt tolerance factor rule, it is impossible for (xAMPY)M2I6 to form proper perovskite structures. Instead, a combination of corner-sharing and edge-sharing connectivity is adopted in these compounds leading to the new 3D structures. DFT calculations reveal that the compounds are indirect band gap semiconductors with direct band gaps presenting at slightly higher energies and dispersive electronic bands. The indirect band gaps of the Sn and Pb compounds are ∼1.7 and 2.0 eV, respectively, which is slightly higher than the corresponding AMI3 3D perovskites. The Raman spectra for the compounds are diffuse, with a broad rising central peak at very low frequencies around 0 cm-1, a feature that is characteristic of dynamical lattices, high anharmonicity, and dissipative vibrations very similar to the 3D AMX3 perovskites. Devices of (3AMPY)Pb2I6 crystals exhibit clear photoresponse under ambient light without applied bias, reflecting a high carrier mobility (µ) and long carrier lifetime (τ). The devices also exhibit sizable X-ray generated photocurrent with a high µτ product of ∼1.2 × 10-4 cm2 /V and an X-ray sensitivity of 207 µC·Gy-1·cm-2.

Negative Pressure Engineering with Large Cage Cations in 2D Halide Perovskites Causes Lattice Softening.

Organic-inorganic hybrid halide perovskites are promising semiconductors with tailorable optical and electronic properties. The choice of A-site cation to support a three-dimensional (3D) perovskite structure AMX3 (where M is a metal and X is a halide) is limited by the geometric Goldschmidt tolerance factor. However, this geometric constraint can be relaxed in two-dimensional (2D) perovskites, providing us an opportunity to understand how various A-site cations modulate the structural properties and thereby the optoelectronic properties. Here, we report the synthesis and structures of single-crystal (BA)2(A)Pb2I7 where BA = butylammonium and A = methylammonium (MA), formamidinium (FA), dimethylammonium (DMA), or guanidinium (GA), with a series of A-site cations varying in size. Single-crystal X-ray diffraction reveals that the MA, FA, and GA structures crystallize in the same Cmcm space group, while the DMA imposes the Ccmb space group. We observe that as the A-site cation becomes larger, the Pb-I bond continuously elongates, expanding the volume of the perovskite cage, equivalent to exerting "negative pressure" on the perovskite structures. Optical studies and DFT calculations show that the Pb-I bond length elongation reduces the overlap of the Pb s- and I p-orbitals and increases the optical bandgap, while Pb-I-Pb tilting angles play a secondary role. Raman spectra show lattice softening with increasing size of the A-site cation. These structural changes with enlarged A cations result in significant decreases in photoluminescence intensity and lifetime, consistent with a more pronounced nonradiative decay. Transient absorption microscopy results suggest that the PL drop may derive from a higher concentration of traps or phonon-assisted nonradiative recombination. The results highlight that extending the range of Goldschmidt tolerance factors for 2D perovskites is achievable, enabling further tuning of the structure-property relationships in 2D perovskites.

From Bowls to Capsules: Assembly of Hexanuclear NiII Rings Tailored by Alkali Cations.

An anionic hexanuclear NiII metallamacrocycle with endo and exo linking sites has been employed as a building block to generate a series of capsules and bowls of nanometric size. The supramolecular arrangement of the {Ni6 } rings was tailored by the size of the alkali cations, showing the transition from {Ni6 -M2 -Ni6 } capsules (M=LiI and NaI ) to {Ni6 -M} bowls (M=KI and CsI ). The alkyl co-cations are determinant to stabilize the assemblies by means of CH⋅⋅⋅π interactions on the exo side of the metallamacrocycles. The effect on the topology of the supramolecular assemblies of the cation size, cation charge, Et3 NH+ or Me4 N+ counter cations has been analyzed. Magnetic measurements reveal the presence of ferromagnetic and antiferromagnetic interactions inside the rings that allow a S=0 ground state.

Two-Dimensional Hybrid Halide Perovskites: Principles and Promises.

Hybrid halide perovskites have become the "next big thing" in emerging semiconductor materials, as the past decade witnessed their successful application in high-performance photovoltaics. This resurgence has encompassed enormous and widespread development of the three-dimensional (3D) perovskites, spearheaded by CH3NH3PbI3. The next generation of halide perovskites, however, is characterized by reduced dimensionality perovskites, emphasizing the two-dimensional (2D) perovskite derivatives which expand the field into a more diverse subgroup of semiconducting hybrids that possesses even higher tunability and excellent photophysical properties. In this Perspective, we begin with a historical flashback to early reports before the "perovskite fever", and we follow this original work to its fruition in the present day, where 2D halide perovskites are in the spotlight of current research, offering characteristics desirable in high-performance optoelectronics. We approach the evolution of 2D halide perovskites from a structural perspective, providing a way to classify the diverse structure types of the materials, which largely dictate the unusual physical properties observed. We sort the 2D hybrid halide perovskites on the basis of two key components: the inorganic layers and their modification, and the organic cation diversity. As these two heterogeneous components blend, either by synthetic manipulation (shuffling the organic cations or inorganic elements) or by application of external stimuli (temperature and pressure), the modular perovskite structure evolves to construct crystallographically defined quantum wells (QWs). The complex electronic structure that arises is sensitive to the structural features that could be in turn used as a knob to control the dielectric and optical properties the QWs. We conclude this Perspective with the most notable achievements in optoelectronic devices that have been demonstrated to date, with an eye toward future material discovery and potential technological developments.

Chemical and Structural Diversity of Hybrid Layered Double Perovskite Halides.

Hybrid halide double perovskites are a class of compounds attracting growing interest because of their richness of structure and property. Two-dimensional (2D) derivatives of hybrid double perovskites are formed by the incorporation of organic spacer cations into three-dimensional (3D) double perovskites. Here, we report a series of seven new layered double perovskite halides with propylammonium (PA), octylammonium (OCA), and 1,4-butyldiammonium (BDA) cations. The general formulas of the compounds are AmMIMIIIX8 (single-layered Ruddlesden-Popper type with m = 4 and A = PA or OCA, and single-layered Dion-Jacobson type with m = 2 and A = BDA, MI = Ag, MIII= In or Bi, X = Cl or Br) and PA2CsMIMIIIBr7 (bilayered, with MI = Ag, MIII = In or Bi). These families of compounds demonstrate great versatility, with tunable layer thickness, the ability to vary the interlayer spacing, and the ability to selectively tune the band gap by varying the MI and MIII cations along with the halide anions. The band gap of the single-layered materials varies from 2.41 eV for PA4AgBiBr8 to 3.96 eV for PA4AgInCl8. Photoluminescent emission spectra of the layered double perovskites at low-temperature (100 K) are reported, and density functional theory electronic structure calculations are presented to understand the nature of the band gap evolution. The development of new structural and compositions in layered double perovskite halides enhances the understanding of structure-property relations in this important family.

A Natural 2D Heterostructure [Pb3.1Sb0.9S4][Au xTe2- x] with Large Transverse Nonsaturating Negative Magnetoresistance and High Electron Mobility.

We report the two-dimensional (2D) natural heterostructure [Pb3.1Sb0.9S4][Au xTe2- x] ( x = 0.52-0.36) which shows anomalous, transverse nonsaturating negative magnetoresistance (MR). For x = 0.52, the material has a commensurately modulated structure with alternating [Pb3.1Sb0.9S4] rocksalt layers and atomically thin [Au xTe2- x] sheets, as determined by single-crystal X-ray diffraction using a (3 + 1)-dimensional space group; for other x compositions, the modulated structure is absent and the Au and Te atoms are disordered. The transport properties in this system at low temperature (<100 K) are dominated by an unusual 2D hopping mechanism, while at room temperature a high carrier mobility of ∼1352 cm2 V-1 s-1 is obtained ( x = 0.36). The confined electrons within the [Au xTe2- x] layers are also exposed to interlayer coupling with the insulating [Pb3.1Sb0.9S4] layers, and as a result, the properties of the heterostructures emerge not only from the constituent layers but also the interactions between them. Furthermore, the various Au and Te coordination patterns found in the [Au xTe2- x] sheets as a function of x further contribute to a unique electronic structure that leads to the anomalous nonsaturating negative MR with different field dependent behaviors. First-principles calculations indicate that the [Au xTe2- x] sheets are responsible for the unusual electrical transport properties in this 2D system.