Trivalent M3 Research Articles

Synthesis, Crystal Growth, Electronic Properties and Optical Properties of Y6IV2.5S14 (IV=Si, Ge)

AbstractTwo isostructural ternary acentric sulfides, Y6Si2.5S14 (1) and Y6Ge2.5S14 (2) were re‐investigated to understand the origin of the chemical flexibility of RE6BxCyCh14(RE=Y, La−Lu; B=Si, Ge, Sn, Al, Ga; C=monovalent M+ (Ag, Na, Li, etc.), divalent M2+ (Mg, Cr, Ni, Zn, etc.), trivalent M3+ (Al, In, Ga, etc.), tetravalent M4+ (Si, Ge, Sn) and pentavalent M5+ (Sb), Ch=S, Se), which consists of ∼444 isostructural compounds. Y6IV2.5S14 (IV=Si, Ge) were synthesized by a high‐temperature salt flux method. The crystal structures of Y6IV2.5S14 (IV=Si, Ge) are constructed by [YS8] polyhedra, [Si1S6] octahedra, and [Si2S4] tetrahedra. The Si1 atom displaces from the center of [Si1S6] octahedra with partial occupancy, which can be replaced by various metals, and mainly accounts for the chemical flexibility of the RE6BxCyCh14 family. The bonding pictures of Y6Si2.5S14 were studied by electron localization function (ELF) and crystal orbital Hamilton population (COHP) calculations. Y6Si2.5S14 is evaluated as an indirect semiconductor with a bandgap of 2.4(1) eV measured by UV‐Vis. The indirect bandgap of Y6Ge2.5S14 is 1.7(1) eV. Y6IV2.5S14 (IV=Si, Ge) are not type‐I phase‐matchable materials. For samples of 47 μm particle size, Y6Si2.5S14 and Y6Ge2.5S14 own good second harmonic generation (SHG) responses of ∼3.0×AGS and ∼2.8×AGS respectively. Y6Si2.5S14 and Y6Ge2.5S14 possess high laser damage threshold (LDT) of ∼5.5×AGS and ∼5.2×AGS respectively.

Experimental and Theoretical Investigations on the Structural, Electronic, and Vibrational Properties of Cs2AgSbCl6 Double Perovskite

Despite the rapid development and enormous success of organic–inorganic hybrid halide perovskites (AB′X3), such as CH3NH3PbI3 as absorbers for perovskite-based solar cells (PSCs), the commercial applications of photovoltaic techniques still face several challenges, such as decomposition when exposed to light and moisture, and lead toxicity. On the other hand, the double perovskites (A2B′B″X6) are derived from the AB′X3 when half of the octahedrally coordinated B′-cations are partially replaced by the suitable B″-cations. They are attracting attention due to a new design strategy to replace Pb2+ ions with the couple of a monovalent M+ ion and a trivalent M3+ ion, leading to a new family of quaternary double perovskites. In this way, we aim to synthesize and characterize Cs2AgSbCl6 powdered samples, designed for solar cell applications. The crystalline phase and morphological features are investigated by X-ray powder diffraction (XRPD), neutron powder diffraction (NPD), scanning electron microscopy (SEM) in complement with UV–vis spectroscopy, showing a suitable band gap of 2.7 eV. The solution synthesis method proved to be efficient in obtaining polycrystalline-Cs2AgSbCl6 samples in a cubic ordered phase. DFT calculations also provided insights on the vibrational properties of Cs2AgSbCl6, corroborating the experimental data and elucidating the optical activity of Raman and infrared modes.

Influence of Transition Metal Charge Compensation Species on Phase Assemblage in Zirconolite Ceramics for Pu Immobilisation

ABSTRACTImmobilisation of Pu in a zirconolite matrix (CaZrTi2O7) is a viable pathway to disposition. A-site substitution, in which Pu4+ is accommodated into the Ca2+ site in zirconolite, coupled with sufficient trivalent M3+/Ti4+ substitution (where M3+ = Fe, Al, Cr), has been systematically evaluated using Ce4+ as a structural analogue for Pu4+. A broadly similar phase assemblage of zirconolite-2M and minor perovskite was observed when targeting low levels of Ce incorporation. As the targeted Ce fraction was elevated, secondary phase formation was influenced by choice of M3+ species. Co-incorporation of Ce/Fe resulted in the stabilisation of a minor Ce-containing perovskite phase at high wasteloading, whereas considerable phase segregation was observed for Cr3+ incorporation. The most favourable substitution approach appeared to be achieved with the use of Al3+, as no perovskite or free CeO2 was observed. However, high temperature treatments of Al containing specimens resulted in the formation of a secondary Ce-containing hibonite phase.

From 0D Cs3Bi2I9 to 2D Cs3Bi2I6Cl3: Dimensional Expansion Induces a Direct Band Gap but Enhances Electron–Phonon Coupling

Alternative all-inorganic halide perovskites are sought to replace the hybrid lead halide perovskites because of their increased stability. Here, the (111)-oriented defect perovskite family A3M2X9 based on trivalent M3+ is expanded through the use of mixed halides, resulting in Cs3Bi2I6Cl3. This compound shares the (111)-oriented 2D bilayer structure of α-Cs3Sb2I9 (space group P3m1), with Cl occupying the bridging positions of the bilayers and I in the terminal sites, in contrast to the parent compound Cs3Bi2I9, which consists of 0D molecular [Bi2I9]3– dimers. The increased dimensionality induces a direct band gap as calculated by density functional theory but has an absorption edge of 2.07 eV, nearly identical to the indirect band gap compound Cs3Bi2I9. Intriguingly, there is a remarkable lack of Cl orbital contribution to the band edge states of Cs3Bi2I6Cl3, despite Bi–Cl bonds binding all octahedra together. This highlights the importance of interlayer interactions in the defect perovskite family, whi...

Targeting Vacancies in Nitridosilicates: Aliovalent Substitution of M2+ (M=Ca, Sr) by Sc3+ and U3+

Based on the known linking options of their fundamental building unit, that is the SiN4 tetrahedron, nitridosilicates belong to the inorganic compound classes with the greatest structural variability. Although facilitating the discovery of novel Si-N networks, this variability represents a challenge when targeting non-stoichometric compounds. Meeting this challenge, a strategy for targeted creation of vacancies in highly condensed nitridosilicates by exchanging divalent M2+ for trivalent M3+ using the ion exchange approach is reported. As proof of concept, the first Sc and U nitridosilicates were prepared from α-Ca2 Si5 N8 and Sr2 Si5 N8 . Powder X-ray diffraction (XRD) and synchrotron single-crystal XRD showed random vacancy distribution in Sc0.2 Ca1.7 Si5 N8 , and partial vacancy ordering in U0.5x Sr2-0.75x Si5 N8 with x≈1.05. The high chemical stability of U nitridosilicates makes them interesting candidates for immobilization of actinides.

Targeting Vacancies in Nitridosilicates: Aliovalent Substitution of M2+ (M=Ca, Sr) by Sc3+ and U3+

AbstractBased on the known linking options of their fundamental building unit, that is the SiN4 tetrahedron, nitridosilicates belong to the inorganic compound classes with the greatest structural variability. Although facilitating the discovery of novel Si–N networks, this variability represents a challenge when targeting non‐stoichometric compounds. Meeting this challenge, a strategy for targeted creation of vacancies in highly condensed nitridosilicates by exchanging divalent M2+ for trivalent M3+ using the ion exchange approach is reported. As proof of concept, the first Sc and U nitridosilicates were prepared from α‐Ca2Si5N8 and Sr2Si5N8. Powder X‐ray diffraction (XRD) and synchrotron single‐crystal XRD showed random vacancy distribution in Sc0.2Ca1.7Si5N8, and partial vacancy ordering in U0.5xSr2−0.75xSi5N8 with x≈1.05. The high chemical stability of U nitridosilicates makes them interesting candidates for immobilization of actinides.

Design of Lead-Free Inorganic Halide Perovskites for Solar Cells via Cation-Transmutation.

Hybrid organic-inorganic halide perovskites with the prototype material of CH3NH3PbI3 have recently attracted intense interest as low-cost and high-performance photovoltaic absorbers. Despite the high power conversion efficiency exceeding 20% achieved by their solar cells, two key issues-the poor device stabilities associated with their intrinsic material instability and the toxicity due to water-soluble Pb2+-need to be resolved before large-scale commercialization. Here, we address these issues by exploiting the strategy of cation-transmutation to design stable inorganic Pb-free halide perovskites for solar cells. The idea is to convert two divalent Pb2+ ions into one monovalent M+ and one trivalent M3+ ions, forming a rich class of quaternary halides in double-perovskite structure. We find through first-principles calculations this class of materials have good phase stability against decomposition and wide-range tunable optoelectronic properties. With photovoltaic-functionality-directed materials screening, we identify 11 optimal materials with intrinsic thermodynamic stability, suitable band gaps, small carrier effective masses, and low excitons binding energies as promising candidates to replace Pb-based photovoltaic absorbers in perovskite solar cells. The chemical trends of phase stabilities and electronic properties are also established for this class of materials, offering useful guidance for the development of perovskite solar cells fabricated with them.

Photoluminescence properties of M3+ (M3+ = Bi3+, Sm3+) activated Na5Eu(WO4)4 red-emitting phosphors for white LEDs

The red phosphor Na5Eu(WO4)4 activated with trivalent M3+ (M3+=Bi3+, Sm3+) ions were prepared through the solid state reaction technique. The properties of structure, morphology, photoluminescence were characterized systematically. The lattice parameters were refined by Rietveld method. The emission spectra, excitation spectra, and decay curves were employed to study the luminescent behaviors. Both of Bi3+ and Sm3+ doping could increase the red-emitting intensity and corresponding quantum efficiency (QE) of Na5Eu(WO4)4 host. Furthermore, the color rendering index of the commercial yellow phosphor YAG:Ce3+ was increased from 44.5 to 50.8 and 50.2 by mixed with Na5Eu0.8Bi0.2(WO4)4 and Na5Eu0.96Sm0.04(WO4)4 phosphors under 465nm excitation, respectively. The CIE chromaticity coordinates of these phosphors are close to the standard value for red. Herein, the present materials are promising candidates as red phosphors for white light emitting diodes with near-UV/blue GaN-based chips.

Relationship between the Electrochemical Behavior and Li Arrangement in LixMyMn2-yO4 (M = Co, Cr) with Spinel Structure

The relationship between the electrochemical behavior and the arrangement of lithium/vacancies has been investigated with electrochemical Li removal in Li(x)M(y)Mn(2-y)O4 (x < or = 1.0, 0.0 < or = y < or = 0.3, M = Co, Cr). It was shown that the electrochemical removal proceeds via two voltage regions: (1) approximately 3.9 V at x > or = approximately 0.5 and (2) approximately 4.2 V at x < or = approximately 0.5. To understand the stepwise behavior, entropy measurement of reaction, DeltaS(obs), was performed by using the electrochemical methods. The changes of the sign in deltaS(obs) from negative to positive at the composition x approximately 0.50 in Li(x)M(y)Mn(2-y)O4 indicated that the ordered arrangement of Li/vacancies was formed with electrochemical Li removal. Moreover, such an ordering was suppressed by the substitution of Co3+ and Cr3+ for Mn3+. To clarify the nature and origin of Li/vacancy ordering, the Monte Carlo simulation was performed in view of Coulombic interaction. The simulation reproduced the formation of a new phase arising from Li/vacancy ordering at x = 0.50 in Li(x)Mn2O4. In addition, the ordered arrangement of Li/vacancy at x = 0.5 was perturbed by the trivalent M3+ replacement in spinel structure due to the local clustering of Li+ around M3+. Consequently, the electrochemical behavior in spinel LiMn2O4 was deeply related to the Coulombic interactions, proved by the fact that experimentally observed changes in entropy agreed well with Monte Carlo simulation based on the Coulombic interaction.

A radiochemical study of the kinetics of exchange between manganese oxides and some cations in solution

The kinetics of the exchange between56Mn-labelled manganese dioxide and cations in aqueous solution was studied by measuring the β− activity acquired by the solution. The results of the exchange between a chemical γ MnO2 and a divalent M2+ ion (M=Mn, Co, Cu or Zn) or a trivalent M3+ ion (M=Ga, Fe, In, Rh or Al) indicate a fast initial process followed by a diffusion—controlled exchange. It is assumed that M2+ ions exchange with Mn2+ ions and M3+ ions exchange with Mn3+ ions in MnO2. The process depends on the radii of the host and substituent ions and on consideration of crystal field stabilisation energies. It seems that the γ MnO2 studied contains more Mn3+ than Mn2+ ions. The possibility of the exchange between Mn ions and cations of a different charge cannot be ruled out. The exchange between Co2+ ions and MnO2 was enhanced in presence of pyrophosphate, which stabilises Mn(III) as a complex. The fraction of Mn in different samples of MnO2 exchanged with a given cation depends on the type and not on the surface area of the sample.