Solvothermal and mechanochemical intercalation of Cu into La2O2S2 enabled by the redox reactivity of (S2)2- pairs.

Intercalation of Cu into layered polychalcogenide La2O2S2 was demonstrated to be viable both under solvothermal conditions at 200 °C and mechanical ball milling at ambient temperature. This result evidences the soft-chemical nature of metal intercalation into layered polychalcogenides driven by the redox reactivity of anion-anion bonds.

Design of metastable oxychalcogenide phases by topochemical (de)intercalation of sulfur in La2O2S2.

Designing and synthesising new metastable compounds is a major challenge of today's material science. While exploration of metastable oxides has seen decades-long advancement thanks to the topochemical deintercalation of oxygen as recently spotlighted with the discovery of nickelate superconductor, such unique synthetic pathway has not yet been found for chalcogenide compounds. Here we combine an original soft chemistry approach, structure prediction calculations and advanced electron microscopy techniques to demonstrate the topochemical deintercalation/reintercalation of sulfur in a layered oxychalcogenide leading to the design of novel metastable phases. We demonstrate that La2O2S2 may react with monovalent metals to produce sulfur-deintercalated metastable phases La2O2S1.5 and oA-La2O2S whose lamellar structures were predicted thanks to an evolutionary structure-prediction algorithm. This study paves the way to unexplored topochemistry of mobile chalcogen anions.

Unravelling the Origin of the Yellow-Orange Luminescence in Natural and Synthetic Scapolites.

After decades of speculation without material proof, the yellow-orange luminescence of scapolite is definitely assigned to (S2)- activators trapped in [Na4] square cages. Synthetic sulfur-doped scapolites confirm the implication of sulfur species in luminescence. Formally, the emission and excitation spectra of various polysulfide species were calculated. The excellent match between theory and experiments for (S2)- dimers provides definitive proof that it is the cause of the yellow-orange luminescence in scapolite.

Layered Quaternary Compounds in the Cu2S-In2S3-Ga2S3 system.

Several new materials with four structure-types (e.g., Cu0.32In1.74Ga0.84S4 (CIGS4), Cu0.65In1.75Ga1.4S5 (CIGS5), Cu1.44In2.77Ga0.76S6 (CIGS6), and Cu1.1In2.49Ga1.8S7 (CIGS7)) have been evidenced in the Cu2S-In2S3-Ga2S3 pseudo-ternary system. All of them present a 2D structure built upon infinite 2/∞[InS2] layers ((InS6) octahedra sharing edges) on which condense on both sides mono-, bi-, or tri-2/∞[MS] layers ((MS4) tetrahedra (M = Cu, In, Ga) sharing corners). (M(Td))n-2(In(Oh))Sn slabs are separated from each other by a van der Waals gap, and subscript n refers to the number of sulfur layers within the building block. These compounds have the propensity to display stacking faults but also polymorphic forms. Their optical gap (ca. 1.7 eV) is quite similar to the one of the Cu(In0.7Ga0.3)S2 chalcopyrite absorbers used in tandem solar cells, and the major charge carriers are holes. This suggests that they might be very attractive for photovoltaic applications in thin film devices but also for photocatalysis.

Unexplored reactivity of (Sn)2- oligomers with transition metals in low-temperature solid-state reactions.

We demonstrate here the low temperature topochemical insertion of transition elements (Fe, Ni, and Cu) in precursors containing pre-formed (Sn)2- (n = 2 and 3) oligomers. Indeed, this soft chemistry route opens the door to the easy, orientated synthesis of low dimensional transition metal compounds provided that the elemental metal can retrocede electron(s) to empty antibonding sulfur σ* levels.

Crystal Chemistry, Optical-Electronic Properties, and Electronic Structure of Cd1- xIn2+2 x/3S4 Compounds (0 ≤ x ≤ 1), Potential Buffer in CIGS-Based Thin-Film Solar Cells.

CdIn2S4 and In2S3 compounds were both previously studied as buffer layers in CIGS-based thin-film solar cells, each of them exhibiting advantages and disadvantages. Thus, we naturally embarked on the study of the CdIn2S4-In2S3 system, and a series of Cd1- xIn2+2 x/3S4 (0 ≤ x ≤ 1) materials were prepared and characterized. Our results show that two solid solutions exist. The aliovalent substitution of cadmium(II) by indium(III) induces a structural transition at x ≈ 0.7 from cubic spinel Fd3̅ m to tetragonal spinel I41/ amd that is related to an ordering of cadmium vacancies. Despite this transition, the variation of optical gap is continuous and decreases from 2.34 to 2.11 eV going from CdIn2S4 to In2S3 while all compounds retain an n-type behavior. In contrast with the Al xIn2-xS3 solid solution, no saturation of the gap is observed. Moreover, XPS analyses indicate a difference between surface and volume composition of the grains for Cd-poor compounds. The use of Cd1- xIn2+2 x/3S4 compounds could be a good alternative to CdIn2S4 and In2S3 to improve CIGS/buffer interfaces with a compromise between photovoltaic conversion efficiency and cadmium content.

A Topochemical Approach to Synthesize Layered Materials Based on the Redox Reactivity of Anionic Chalcogen Dimers.

Layered transition metal compounds represent a major playground to explore unconventional electric or magnetic properties. In that framework, topochemical approaches that mostly preserve the topology of layered reactants have been intensively investigated to tune properties and/or design new materials. Topochemical reactions often involve the insertion or deinsertion of a chemical element accompanied by a change of oxidation state of the cations only. Conversely, cases where anions play the role of redox centers are very scarce. Here we show that the insertion of copper into two dimensional precursors containing chalcogen dimers (Q2 )2- (Q=S, Se) can produce layered materials with extended (CuQ) sheets. The reality of this topochemical reaction is demonstrated here for different pristine materials, namely La2 O2 S2 , Ba2 F2 S2 , and LaSe2 . Therefore, this work opens up a new synthetic strategy to design layered transition metal compounds from precursors containing polyanionic redox centers.

Cationic and Anionic Disorder in CZTSSe Kesterite Compounds: A Chemical Crystallography Study.

The cationic and anionic disorder in the Cu2ZnSnSe4-Cu2ZnSnS4 (CZTSe-CZTS) system has been investigated through a chemical crystallography approach including X-ray diffraction (in conventional and resonant setup), 119Sn and 77Se NMR spectroscopy, and high-resolution transmission electron microscopy (HRTEM) techniques. Single-crystal XRD analysis demonstrates that the studied compounds behave as a solid solution with the kesterite crystal structure in the whole S/(S + Se) composition range. As previously reported for pure sulfide and pure selenide compounds, the 119Sn NMR spectroscopy study gives clear evidence that the level of Cu/Zn disorder in mixed S/Se compounds depends on the thermal history of the samples (slow cooled or quenched). This conclusion is also supported by the investigation of the 77Se NMR spectra. The resonant single-crystal XRD technique shows that regardless of the duration of annealing step below the order-disorder critical temperature the ordering is not a long-range phenomenon. Finally, for the very first time, HREM images of pure selenide and mixed S/Se crystals clearly show that these compounds have different microstructures. Indeed, only the mixed S/Se compound exhibits a mosaic-type contrast which could be the sign of short-range anionic order. Calculated images corroborate that HRTEM contrast is highly dependent on the nature of the anion as well as on the local anionic order.

Numerical and experimental study of suspensions containing carbon blacks used as conductive additives in composite electrodes for lithium batteries.

Suspensions of carbon blacks and spherical carbon particles are studied experimentally and numerically to understand the role of the particle shape on the tendency to percolation. Two commercial carbon blacks and one lab-synthesized spherical carbon are used. The percolation thresholds in suspensions are experimentally determined by two complementary methods: impedance spectroscopy and rheology. Brownian dynamics simulations are performed to explain the experimental results taking into account the fractal shape of the aggregates in the carbon blacks. The results of Brownian dynamics simulations are in good agreement with the experimental results and allow one to explain the experimental behavior of suspensions.

Effects of water uptake on the inherently oxygen-deficient compounds Ln26O27 square(BO3)8 (Ln=La, Nd).

The inherently oxygen-deficient compounds Ln26O27 square(BO3)8 (Ln=La, Nd) react with water vapor leading to Ln26O26(OH)2(BO3)8 phases, and this reaction is reversible. The crystal structure of Nd26O27 square(BO3)8 has been determined from single-crystal data (space group P with a=6.7643(10) A, b=12.663(2) A, c=14.271(2) A, alpha=90.553(8) degrees, beta=99.778(10) degrees, and gamma=90.511(9) degrees). It is a triclinic distorted version of the monoclinic structure of La26O27 square(BO3)8. The Ln26O26(OH)2(BO3)8 phases both crystallize in the monoclinic system (space group P21/c with a=6.7445(4) A, b=12.6177(9) A, c=14.4947(10) A, and beta=100.168(7) degrees for Nd26O26(OH)2(BO3)8 and a=6.9130(15) A, b=12.896(3) A, c=14.792(4) A, beta=99.698(16) degrees for La26O26(OH)2(BO3)8), and their crystal structure has been determined from single-crystal data, showing that the hydroxyl groups are localized mainly on one of the oxygen sites at room temperature (RT). For the Nd phases, the change in crystal system can result from two different phenomena depending on the atmosphere, either a phase transformation corresponding to a water uptake under wet conditions (triclinic Nd26O27 square(BO3)8 at RT-->monoclinic Nd26O26(OH)2(BO3)8) or a phase transition at approximately 300 degrees C for the anhydrous phase under dry conditions (triclinic Nd26O27 square(BO3)8 at RT-->monoclinic Nd26O27 square(BO3)8 at T>300 degrees C). For Nd26O26(OH)2(BO3)8, the conductivity measured under wet conditions at 300 degrees C is sigma300 degrees C approximately 0.5x10(-5) S cm(-1). Due to the dehydration process, the proton contribution to the total conductivity of the Nd phase is no longer observed above 500 degrees C whereas it was still clearly visible at 600 degrees C for the La phase.