Family Of Halides Research Articles

Titanium‐Based Vacancy‐Ordered Double Halide Family in Perovskite Solar Cells

The toxicity and stability risk of perovskite structured materials have raised concerns in respective to utilization as a solar energy conversion material. The most common perovskite structured material is lead (Pb)‐based, which is an element that is knowingly toxic to humans and the environment. Although the stability issue has been well allayed with several optimizations, the ruinous Pb remains a future challenge for perovskite solar cells. Compositional and structural derivatives of the perovskite family, specifically vacancy‐ordered double halide perovskites (DHPs), have attracted the attention of researchers in terms of efficiency and toxicity issues subjugation. Although tin (Sn)‐based vacancy‐ordered DHPs have been widely explored, the intrinsic property conduces low performance output. Titanium (Ti) is a potential substituting candidate of Sn in a vacancy‐ordered DHPs structure. It is an environment‐friendly element ideal for sustainable perovskite structured compositions. Rudimentary studies of Ti‐based vacancy‐ordered DHPs emphasized its potential development as an eco‐friendly and stable solar cell. In promoting the development of Ti‐based vacancy‐ordered DHPs as potential absorbers, we summarized herein the recent progress of experimental and theoretical studies of this perovskite material.

Li-rich channels as the material gene for facile lithium diffusion in halide solid electrolytes

Halide solid electrolytes have attracted intense research interest recently for application in all-solid-state lithium-ion batteries. Herein, we present a systematic first-principles study of the Li3MX6 (M: multivalent cation; X: halogen anion) halide family that unveils the link between Li-rich channels and ionic conductivity, highlighting the former as a material gene in these compounds. By screening a total of 180 halides for those with high thermodynamic stability, wide electrochemical window, low chemical reactivity, and decent Li-ion conductivity, we identify seven unexplored candidates for solid electrolytes. From these halides and another four prototype compounds, we discover that the facile Li diffusion is rooted in the availability of diffusion pathways which can avoid direct connection with M cations—that is, where the local environment is Li-rich. These findings shed light on strategies for regulating cation and anion frameworks to establish Li-rich channels in the design of high-performance inorganic solid electrolytes.

KCu(SeO4)Cl(H2O)2, a first copper chloride selenate

Abstract A first copper chloride selenate was obtained upon attempted preparation of a selenate analog of chlorothionite. The new compound is monoclinic, P21/c, a = 7.1833(5) Å, b = 11.7784(8) Å, c = 8.2419(6) Å, β = 91.083(2)°, V = 697.20(8) Å3, R 1 = 0.033. KCu(SeO4)Cl(H2O)2 has no structural analogs and adds to the small family of transition metal selenate halides. The CuO3(H2O)2Cl strongly distorted octahedra share common O–O edges thus forming dimeric units with a Cu–Cu distance of 3.49 Å. Dimeric units and SeO4 tetrahedra in KCu(SeO4)Cl(H2O)2 share common O atoms to produce unique [Cu(SeO4)Cl(H2O)2]− chains. We discuss further perspectives of the selenate halide family and expected differences in crystal chemistry of sulfate and selenate halides.

Quasi-collinear IR AOTF based on mercurous halide single crystals for spatio-spectral hyperspectral imaging.

The paper aims to show the advantages of the infrared-optimised quasi-collinear AOTF (acousto-optic tunable filter) for the spatio-spectral hyperspectral imaging system. The optimisation process is presented based on the selected tetragonal anisotropic materials with exceptional optical and acousto-optical properties in IR (infrared) spectral region. These materials are further compared in terms of their features and suitability for AOTF design. The spectral resolution is considered as the main optimising parameter. Resulting from the analysis, the mercurous chloride (Hg2Cl2) single crystal is selected as a representative of the mercurous halide family for the presentation of the quasi-collinear AOTF model operating in LWIR (long-wave infrared) spectral band. The overall parameters of the AOTF model such as spectral resolution, chromatic field of view, acoustic frequency, and operational power requirements are estimated and discussed in results.

A generic Slater-Koster description of the electronic structure of centrosymmetric halide perovskites.

The halide perovskites have truly emerged as efficient optoelectronic materials and show the promise of exhibiting nontrivial topological phases. Since the bandgap is the deterministic factor for these quantum phases, here, we present a comprehensive electronic structure study using first-principle methods by considering nine inorganic halide perovskites CsBX3 (B = Ge, Sn, Pb; X = Cl, Br, I) in their three structural polymorphs (cubic, tetragonal, and orthorhombic). A series of exchange-correlation (XC) functionals are examined toward accurate estimation of the bandgap. Furthermore, while 13 orbitals are active in constructing the valence and conduction band spectra, here, we establish that a 4 orbital based minimal basis set is sufficient to build the Slater-Koster tight-binding (SK-TB) model, which is capable of reproducing the bulk and surface electronic structures in the vicinity of the Fermi level. Therefore, like the Wannier based TB model, the presented SK-TB model can also be considered an efficient tool to examine the bulk and surface electronic structures of the halide family of compounds. As estimated by comparing the model study and DFT band structure, the dominant electron coupling strengths are found to be nearly independent of XC functionals, which further establishes the utility of the SK-TB model.

Sensitizer-free photon up conversion in (HQ)2ZnCl4 and HQCl crystals: systems involving resonant energy transfer and triplet-triplet annihilation.

This work deals with normal luminescence and up-conversion luminescence involving charge transfer and triplet-triplet annihilation in the lead free hybrid materials (HQ)2[ZnCl4] and HQCl salt; HQ is the hydroxyquinolate cation (HQ+ = C9H8NO+). The crystal structures were determined by X-ray diffraction and the optical properties were investigated by optical absorption and photoluminescence measurements and electronic band structure calculations. Under UV excitation, the normal luminescence is associated with π-π* transitions within the organic cation and involves energy and charge transfer between the inorganic ion and organic cation. Moreover, photoluminescence measurements under various excitation wavelengths performed on the hybrid (HQ)2[ZnCl4] and the salt HQCl have shown efficient up conversion of light from the near infrared (855 nm) to the visible region at 471 nm and 490 nm respectively. This behavior is described as sensitizer-free up-conversion luminescence based on the triplet-triplet annihilation process (TTA-UCL). These compounds are believed to be the first sensitizer-free TTA up-converting materials found in the organic metal halide family. Compared with the conventional TTA-UC solid systems based on precious and heavy metal organic complexes, the title compounds exhibit efficient photon up-conversion. Furthermore, they have extended the NIR conversion photons to a wide spectral interval of about 300 nm centred around 850 nm.

Synthesis of Easily Transferred 2D Layered BiI3 Nanoplates for Flexible Visible-Light Photodetectors.

Bismuth triiodide, BiI3, is one of the promising 2D layered materials from the family of metal halides. The unique electronic structure and properties make it an attractive material for the room-temperature gamma/X-ray detectors, high-efficiency photovoltaic absorbers, and Bi-based organic-inorganic hybrid perovskites. Other possibilities including optoelectronic devices and optical circuits are envisioned but rarely experimentally confirmed yet. Here, we report the synthesis of vertical 2D BiI3 nanoplates using the physical vapor deposition mechanism. The obtained products were found easy to be separated and transferred to other substrates. Photodetectors employing such 2D nanoplates on polyethylene terephthalate substrate are demonstrated to be quite sensitive to red light (635 nm) with good responsivity (2.8 A W-1), fast stable photoresponse (3/9 ms for raise/decay times), and remarkable specific detectivity (1.2 × 1012 jones), which attest to high comparability of the assembled components with many latest 2D nanostructured light sensors. In addition, such photodetectors exhibit outstanding mechanical stability and durability under different bending strains within the theoretically affordable levels, suggesting a variety of potential applications of 2D BiI3 for flexible devices.

Cesium Titanium(IV) Bromide Thin Films Based Stable Lead-free Perovskite Solar Cells

Summary Significant effort is being devoted to the search for all-inorganic Pb-free halide perovskites (HPs) for photovoltaic applications. However, candidate HPs that combine all the desirable attributes — ease of synthesis, favorable bandgaps, outstanding optoelectronic properties, high stability, no toxicity — are extremely rare. Here, we demonstrate experimentally the promise of cesium titanium(IV) bromide (Cs 2 TiBr 6 ), a part of the Ti-based vacancy-ordered double-perovskite halides family, in perovskite solar cells (PSCs). We show, for the first time, that high-quality Cs 2 TiBr 6 thin films can be prepared through a facile low-temperature vapor-based method. These films exhibit a favorable bandgap of ∼1.8 eV, long and balanced carrier-diffusion lengths exceeding 100 nm, suitable energy levels, and superior intrinsic and environmental stability. The first demonstration of Cs 2 TiBr 6 thin films-based PSCs shows stable efficiency of up to 3.3%. Insights into the Cs 2 TiBr 6 film-formation mechanisms and the PSC device operation are provided, pointing to directions for improving Ti-based PSCs.

Comparative study of perovskite-type scintillator materials CsCaI3 and KCaI3 via first-principles calculations

Several members of a large family of perovskite-like halides with a common chemical formula, ABX3 (A = monovalent, B = divalent, and X = halogen ion), are being investigated for their interesting properties and potential technological applications. CsCaI3 and KCaI3 are two such ionic compounds who are of interest in the quest for superior and cost-effective alternatives to NaI or CsI based scintillators. They are the subject of this first-principles based computational study. Both are wide-gap materials having primarily I 5p and Ca 3d characters near the valence and conduction band edges, respectively. Although built from [CaI6] octahedral motifs, structural differences between the two compounds is reflected in anisotropic electron effective mass and distinctive formation and migration of self-trapped holes. We discuss these properties as they relate to scintillation decay and proportional light yield.

Structural Evolution in BaSn2F5X (X = Cl, Br, I): A Family of Alkaline Earth Metal Tin Mixed Halides.

As the first family of Sn-based alkaline earth metal mixed halides, three new compounds, BaSn2F5X (X = Cl, Br, and I), are synthesized by hydrothermal method. These compounds are crystallized in the centrosymmetric space groups of P21/c, P4/nmm, and Pmma for BaSn2F5Cl, BaSn2F5Br, and BaSn2F5I, respectively, and their microscopic frameworks are all composed of the fundamental structural unit [SnF4]2- and its derivatives ([SnF4Cl]3- and [SnF5]3- groups). Interestingly, the structures in BaSn2F5X are significantly changed from one-dimensional (1D) to two-dimensional (2D) and then to 1D motifs as X varies from Cl, Br, to I. Structural analysis combined with theoretical calculations reveals that the structural diversities are caused by the difference of ionic radius and electronegativity of X- anions as well as the orientation of the lone-pair electrons on Sn2+ cations. Moreover, the optical, electronic, and thermal properties for these three compounds are determined. This work provides a representative example to show how microscopic ions influence the structures, thus in favor of the design for new mixed halides, a type of important functional materials with many optoelectronic applications.

Characterization of highly crystalline lead iodide nanosheets prepared by room-temperature solution processing

Two-dimensional (2D) semiconducting materials are particularly appealing for many applications. Although theory predicts a large number of 2D materials, experimentally only a few of these materials have been identified and characterized comprehensively in the ultrathin limit. Lead iodide, which belongs to the transition metal halides family and has a direct bandgap in the visible spectrum, has been known for a long time and has been well characterized in its bulk form. Nevertheless, studies of this material in the nanometer thickness regime are rather scarce. In this article we demonstrate an easy way to synthesize ultrathin, highly crystalline flakes of PbI2 by precipitation from a solution in water. We thoroughly characterize the produced thin flakes with different techniques ranging from optical and Raman spectroscopy to temperature-dependent photoluminescence and electron microscopy. We compare the results to ab initio calculations of the band structure of the material. Finally, we fabricate photodetectors based on PbI2 and study their optoelectronic properties.

Photovoltaic Rudorffites: Lead-Free Silver Bismuth Halides Alternative to Hybrid Lead Halide Perovskites.

Hybrid CPbX3 (C: Cs, CH3 NH3 ; X: Br, I) perovskites possess excellent photovoltaic properties but are highly toxic, which hinders their practical application. Unfortunately, all Pb-free alternatives based on Sn and Ge are extremely unstable. Although stable and non-toxic C2 ABX6 double perovskites based on alternating corner-shared AX6 and BX6 octahedra (A=Ag, Cu; B=Bi, Sb) are possible, they have indirect and wide band gaps of over 2 eV. However, is it necessary to keep the corner-shared perovskite structure to retain good photovoltaic properties? Here, we demonstrate another family of photovoltaic halides based on edge-shared AX6 and BX6 octahedra with the general formula Aa Bb Xx (x=a+3 b) such as Ag3 BiI6 , Ag2 BiI5 , AgBiI4 , AgBi2 I7 . As perovskites were named after their prototype oxide CaTiO3 discovered by Lev Perovski, we propose to name these new ABX halides as rudorffites after Walter Rüdorff, who discovered their prototype oxide NaVO2 . We studied structural and optoelectronic properties of several highly stable and promising Ag-Bi-I photovoltaic rudorffites that feature direct band gaps in the range of 1.79-1.83 eV and demonstrated a proof-of-concept FTO/c-m-TiO2 /Ag3 BiI6 /PTAA/Au (FTO: fluorine-doped tin oxide, PTAA: poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], c: compact, m: mesoporous) solar cell with photoconversion efficiency of 4.3 %.

DFT investigation of electronic structures and magnetic properties of halides family MeHal3 (Me=Ti, Mo,Zr,Nb, Ru, Hal=Cl,Br,I) one dimensional structures

Using DFT GGA calculations, electronic structure and magnetic properties of wide family of transition metal trihalides (TMHal3) (Zr, Ti and Nb iodides, Mo, Ru, Ti and Zr bromides and Ti or Zr chlorides) are investigated. These structures consist of transition metal atoms chains surrounded by halides atoms. Chains are connected to each other by weak interactions. All TMHal3 compounds were found to be conductive along chain axis except of MoBr3 which is indirect gap semiconductor. It was shown that NbI3 and MoBr3 have large magnetic moments on metal atoms (1.17 and 1.81µB, respectively) but other TMHal3 materials have small or zero magnetic moments. For all structures ferromagnetic and anti-ferromagnetic phases have almost the same energies. The causes of these properties are debated.

Analysis of self-trapped hole mobility in alkali halides and metal halides

The small radius hole polarons (self-trapped holes (STH) known also as the Vk centers) are very common color centers observed in numerous alkali halides and alkaline-earth halides. Their mobility controls the rate of secondary reactions between electron and hole defects and thus radiation stability/sensitivity of materials. We have analysed here the correlation between the temperatures at which hole polarons start migration in a series of alkali halides (fluorites, chlorides, bromides, iodides) and the lattice displacement around quasi-molecule. These results are especially important for identification of the self-trapped holes, for example, in novel scintillating materials such as SrI2, as well as in a large family of perovskite halides and more complex halide materials.

Preferential Eu Site Occupation and Its Consequences in the Ternary Luminescent HalidesAB2I5∶Eu2+(A=Li–Cs;B=Sr, Ba)

Several rare-earth-doped, heavy-metal halides have recently been identified as potential next-generation luminescent materials with high efficiency at low cost. AB2I5:Eu2+ (A=Li–Cs; B=Sr, Ba) is one such family of halides. Its members, such as CsBa2I5:Eu2+ and KSr2I5:Eu2+, are currently being investigated as high-performance scintillators with improved sensitivity, light yield, and energy resolution less than 3% at 662 keV. Within the AB2I5 family, our first-principles-based calculations reveal two remarkably different trends in Eu site occupation. The substitutional Eu ions occupy both eightfold-coordinated B1(VIII) and the sevenfold-coordinated B2(VII) sites in the Sr-containing compounds. However, in the Ba-containing crystals, Eu ions strongly prefer the B2(VII)sites. This random versus preferential distribution of Eu affects their electronic properties. The calculations also suggest that in the Ba-containing compounds one can expect the formation of Eu-rich domains. These results provide atomistic insight into recent experimental observations about the concentration and temperature effects in Eu-doped CsBa2I5. We discuss the implications of our results with respect to luminescent properties and applications. We also hypothesize Sr, Ba-mixed quaternary iodides ABaVIIISrVIII5:Eu as scintillators having enhanced homogeneity and electronic properties.

Crystal Growth and Scintillation Properties of <formula formulatype="inline"><tex Notation="TeX">${\rm Cs}_{2}{\rm NaGdBr}_{6}{:}{\rm Ce}^{3+}$</tex></formula>

Single crystals of Cs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> NaGdBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> with different Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+3</sup> activator concentrations were grown by a two-zone Bridgman method. This new compound belongs to a large elpasolite halide (A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> BLnX <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> ) family. Many of these elpasolite compounds have shown high luminosity, good energy resolution and excellent proportionality in comparison to traditional scintillators such as CsI and NaI; therefore, they are particularly attractive for gamma-ray spectroscopy applications. This study investigated the scintillator properties of Cs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> NaGdBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> :Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+3</sup> crystals as a new material for radiation detection. Special focus has been placed on the effects of activator concentration (0 to 50 mol.%) on the photoluminescence responses. Results of structural refinement, photoluminescence, radioluminescence, lifetime and proportionality measurements for this new compound are reported.

A review of thermochemical cycles for hydrogen production: analysis of potential of membrane technology in I-S, UT-3 and CuCl cycles

Global warming and climate change necessitate a serious move away from fossil-based systems toward hydrogen economy. A study has been carried out on different thermochemical cycles for hydrogen production to bring about their important aspects. The study involves process description of different routes followed by thermochemical cycles. These include metal oxide processes, sulphur family processes like iodine sulphur cycle and Westinghouse cycle, halide family processes like UT-3 and Ispra Mark cycle, copper chlorine cycle and others. This paper gives an insight into the advantages and disadvantages associated with the processes. The review is done keeping in view the relevant and useful aspects in research to present information in terms of species, operating parameters, reactors, costs involved, safety, etc. This will provide a platform for further research to save effort by referring the basic information on thermochemical cycles, so that an appropriate and satisfactory cycle may be chosen.

Measurement and analysis of the pure rotational spectra of tin monochloride, SnCl, using laser ablation equipped chirped pulse and cavity Fourier transform microwave spectroscopy

The pure rotational spectrum of tin monochloride, SnCl, has been measured and analyzed using chirped pulse and cavity Fourier transform microwave spectrometers equipped with a laser ablation source. SnCl, in its X2Π12 electronic state, has been measured in the 8–17GHz region. Rotational constants, centrifugal distortion constants, Λ-doubling constants, magnetic hyperfine constants, and nuclear electric quadrupole coupling constants for multiple isotopologues have been determined and are reported. The bond length, nuclear magnetic and quadrupole constants have been analyzed and compared against the family of tetral halides. Analysis of the bond length and hyperfine interactions point to a Sn–Cl single bond which is largely ionic in nature.

Energy transport and scintillation of cerium-doped elpasolite Cs2LiYCl6: Hybrid density functional calculations

Elpasolites are a large family of halides which have recently attracted considerable interest for their potential applications in room-temperature radiation detection. Cs2LiYCl6 is one of the most widely studied elpasolite scintillators. In this paper, we will show hybrid density functional calculations on electronic structure, energetics of small electron and hole polarons and self-trapped excitons, and the excitation of luminescence centers (Ce impurities) in Cs2LiYCl6. The results provide important understanding in energy transport and scintillation mechanisms in Cs2LiYCl6 and rare-earth elpasolites in general.

On Bonding in Ionic Crystals

This article details the description of ionic bonding derived from the electron localization function (ELF) topology in the alkali halide family (i.e., ionic sizes, shapes, critical points, atomic coordinations, etc.). This approach establishes a relationship between ELF topological characteristics and basic ionic and crystal properties. We show how many deeply rooted concepts about ionic bonding can be derived from a (counterintuitive) analysis of electron pairing. In other words, we show how principles of covalent bonding are also applicable to ionic bonds. For example, ionic compounds are found to have a definite valence shell, which is responsible for chemical activity and crystalline strains under pressure. The valence may be the outermost shell of the softest ion, or the outermost shells of both ions, cation and anion, if their hardnesses are similar. The valence shows partial electron pairs (shell electron pairs, SEPs) whose spatial distribution around the valence shell can be related to the classical definition of polarization. Core SEPs (which might include the outermost shell of the hardest ion) are arranged in the crystal maximizing the distance between them. Hence, analysis of electron pairing in the whole crystal reveals that covalent rules for geometry, the valence shell electron pair repulsion theory, is also applicable to ionic crystals. Furthermore, it is found to describe all electron pairs in the crystal, so that it is indeed an electron pair repulsion theory. Finally, we use global properties from the ELF topology (i.e., ionic volumes) to understand Pauling’s electronegativity scale.