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In the present study, we describe a new approach to the synthesis of a Bi-containing apatite, which allows for a reduced reaction time and temperature. Using in-situ variable temperature X-ray diffraction measurements, we have refined the crystal structure of the material, which shows that its composition may be described as [Ca3.88±0.01Bi0.12±0.01]F[Ca4.42±0.01Bi1.58±0.01]T(PO4)6O1.85±0.01. The compound displays isotropic thermal expansion in the temperature range (173–373) K. A thermodynamic investigation of the sample was undertaken, which revealed the low-temperature heat capacity, C p ,m o , which was determined using adiabatic calorimetry from T = 6.4 K to 305.0 K. Smoothed C p ,m o ( T ) values between 6.5 K and 305.0 K are presented, along with the functions [ S m o ( T ) - S m o ( 6.5 ) ] , [ H m o ( T ) - H m o ( 6.5 ) ] , and [ Φ m o ( T ) - Φ m o ( 6.5 ) ] . Possible causes of the abnormal increase on the heat capacity curve in the low-temperature region are discussed.

A novel high-voltage solid solution of Li2CoxMn1–x(SO4)2 (x = 0–1) was synthesized using a simple solid-state reaction. Single-phase monoclinic crystal structure was identified, and there was no evidence for the presence of an orthorhombic polymorph at a sample preparation temperature of 450 °C. Interestingly, the bimetallic sulfates were found to be thermally stable up to 550 °C, and it was validated through various characterization techniques. Beyond 550 °C, the materials start to decompose and form Li2SO4 and α-CoSO4 in the case of Li2Co(SO4)2, while Li2Mn(SO4)2 decomposes to form Li2Mn2(SO4)3. In addition, an important property called sensitivity toward moisture was also characterized, which plays a critical role in the handling of the sulfate-based cathode during the cell fabrication. Thus, the reaction mechanism involved during the absorption of the atmospheric moisture was also investigated carefully. Finally, for the first time electrochemical activity of both Co and Mn systems was discovered wher...

To control the process of cation exchange (CE) in a multielemental system, a detailed understanding of structural changes at the microscopic level is imperative. However, the synthesis of a multielemental system has so far relied on the CE phenomenon of a binary system, which does not necessarily extend to the higher-order systems. Here, direct experimental evidence supported by theoretical calculations reveals a growth model of binary Cu–S to ternary Cu–Sn–S to quaternary Cu–Zn–Sn–S, which shows that cations preferentially diffuse along a specific lattice plane with the preservation of sulfuric anionic framework. In addition, we also discover that, unlike the commonly accepted structure (P63mc), the metastable crystal structure of Cu–Zn–Sn–S phase possesses fixed Sn occupancy sites. By revealing the preferential nature of cations diffusion and growth mechanism, our work provides insight into controlling the stoichiometry and phase purity of novel multielemental materials. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version We acknowledge financial support from National Research Foundation (NRF), Singapore, through the Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE) and Nanomaterials for Energy and Water Management (SHARE NEW) CREATE program. L.H.W. thanks the funding support from Singapore Ministry of Education, Tier 2 (2016-T2-1-030). S.L. acknowledges the funding support from Singapore Ministry of Education Tier 1 (107/15). H.Z. thanks the funding support from U.S. DOE BES Materials Sciences and Engineering Division Under Contract No. KC22ZH. X.Y.L. thanks the funding support from Singapore Ministry of Education, Tier 1 (RG21/16) and Tier 2 (MOE2016-T2-1- 043) grants.. The work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We thank Fiona Doyle for lending us her synthetic laboratory in University of California Berkeley (UCB), Song Chengyu and Karen Bustilo for their help and assistance on TEM, and Matthew P. Sherburne for nanoparticle growth discussion.

Altering cation and anion ratios in perovskites has been an excellent avenue in tuning the perovskite properties and enhancing the performance. Recently, MA/FA/Cs triple cation mixed halide perovskites have demonstrated efficiencies reaching up to 22 %. Similar to the widely explored MAPbI3, excess PbI2 is added in these perovskite films to enhance the performance. Previous reports demonstrate that the excess PbI2 is beneficial for the performance. However, not much work has been conducted about its impact on stability. Triple cation perovskites (TCP) deploy excess PbI2 up to 8 %. Thus, it is imperative to analyze the role of excess PbI2 in the degradation kinetics. In this paper, we have varied the amount of PbI2 in the triple cation perovskite films and monitored the degradation kinetics by X-ray diffraction (XRD) and optical absorption spectroscopy. We found that the inclusion of excess PbI2 adversely affects the stability of the material. Faster degradation kinetics is observed for higher PbI2 samples. However, excess PbI2 samples showed superior properties such as enhanced grain sizes and better optical absorption. Thus, careful management of the PbI2 quantity is required to obtain better stability and alternative pathways should be explored to achieve better device performance rather than adding excess PbI2. NRF (Natl Research Foundation, S’pore) Accepted version

With the objective of moving towards the commercialisation of Fe-based high voltage cathode Li2Fe(SO4)2, the effect of fluorinated binder (polyvinylidene fluoride – PVDF) and non-fluorinated binder (polyacrylonitrile – PAN) was evaluated. First, the redox mechanism involved in the cycling of Li2Fe(SO4)2 cathode in the presence of these two binders was investigated through ex situ X-ray Absorption Spectroscopy (XAS) studies which can directly determine oxidation state changes of the Fe ion during the charge–discharge process. Subsequently, the effect of the binders on the cycling and rate performance was evaluated. In the present work, consistent cycling performance with discharge capacity of 60 mA h g−1 was recorded at low current rate of C/15. However, it was noted that at high charging rate the cell with binder retained only 29% of the discharge capacity in comparison to the cell without binder. This was attributed to the high charge transfer resistance of binders through cyclic voltammogram and electrochemical impedance spectroscopy studies. This in turn suggests the requirement of high conductivity binders for such sulphate based electrodes.

In this work, we present the first report on thin film solar cells that employ Cu 2 MnSnS 4 (CMTS) and Cu 2 MnSn(S, Se) 4 (CMTSSe) as the absorber. CMTS and CMTSSe thin films are fabricated using a low cost spray pyrolysis technique in ambient atmosphere using water as a solvent. The crystal structure of the materials are similar to the established kesterite type photovoltaic materials such as Cu 2 ZnSn(S, Se) 4 (CZTSSe). The bandgap of these materials is between 1.4−1.7 eV, which is ideal for optimum solar absorption and can be tuned by varying the ratio of the elemental constituents. The photovoltaic device with a structure of Mo/CMTS/CdS/TCO/top electrode is fabricated as a proof-of-concept and yields a power conversion efficiency of ~0.73% for the best optimized CMTS device with Na doping. Through analysis of the device characteristics, we identify a key problem of very high carrier density in the CMTS/CMTSSe absorber that leads to short collection length, low V oc , low quantum efficiency at long wavelength and high shunt conductance that quench the fill factor. We also discuss several routes to improve the device performance.

Copper-doped hydroxyapatite (HA) of nominal composition Ca10(PO4)6[Cux(OH)2-2xOx] (0.0 &le, x &le, 0.8) was prepared by solid-state and wet chemical processing to explore the impact of the synthesis route and mode of crystal chemical incorporation of copper on the antibacterial efficacy against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) strains. Apatites prepared by solid-state reaction showed unit cell volume dilation from 527.17 &Aring, 3 for copper-free HA to 533.31 &Aring, 3 for material of the putative composition Ca10(PO4)6[Cu0.8(OH)0.4O0.8] consistent with Cu+ insertion into the [001] hydroxyapatite channel. This was less pronounced (528.30 &Aring, 3 to 529.3 &Aring, 3) in the corresponding wet chemical synthesised products, suggesting less complete Cu tunnel incorporation and partial tenancy of Cu in place of calcium. X-ray absorption spectroscopy suggests fast quenching is necessary to prevent oxidation of Cu+ to Cu2+. Raman spectroscopy revealed an absorption band at 630 cm&minus, 1 characteristic of symmetric O-Cu+-O units tenanted in the apatite channel while solid-state 31P magic-angle-spinning nuclear magnetic resonance (MAS NMR) supported a vacancy-Cu+ substitution model within the apatite channel. The copper doping strategy increases antibacterial efficiency by 25% to 55% compared to undoped HA, with the finer particle sizes and greater specific surface areas of the wet chemical material demonstrating superior efficacy.

CeNbO4.25 is reported to exhibit fast oxygen ion diffusion at moderate temperatures, making this the prototype of a new class of ion conductor with applications in a range of energy generation and storage devices. To date, the mechanism by which this ion transport is achieved has remained obscure, in part due to the long-range commensurately modulated structural motif. Here we show that CeNbO4.25 forms with a unit cell ∼12 times larger than the stoichiometric tetragonal parent phase of CeNbO4 as a result of the helical ordering of Ce(3+) and Ce(4+) ions along z. Interstitial oxygen ion incorporation leads to a cooperative displacement of the surrounding oxygen species, creating interlayer "NbO6" connectivity by extending the oxygen coordination number to 7 and 8. Molecular dynamic simulations suggest that fast ion migration occurs predominantly within the xz plane. It is concluded that the oxide ion diffuses anisotropically, with the major migration mechanism being intralayer; however, when obstructed, oxygen can readily move to an adjacent layer along y via alternate lower energy barrier pathways.

Despite their extremely good performance in solar cells with efficiencies approaching 20% and the emerging application for light-emitting devices, organic-inorganic lead halide perovskites suffer from high content of toxic, polluting, and bioaccumulative Pb, which may eventually hamper their commercialization. Here, we present the synthesis of two-dimensional (2D) Cu-based hybrid perovskites and study their optoelectronic properties to investigate their potential application in solar cells and light-emitting devices, providing a new environmental-friendly alternative to Pb. The series (CH3NH3)2CuCl(x)Br(4-x) was studied in detail, with the role of Cl found to be essential for stabilization. By exploiting the additional Cu d-d transitions and appropriately tuning the Br/Cl ratio, which affects ligand-to-metal charge transfer transitions, the optical absorption in this series of compounds can be extended to the near-infrared for optimal spectral overlap with the solar irradiance. In situ formation of Cu(+) ions was found to be responsible for the green photoluminescence of this material set. Processing conditions for integrating Cu-based perovskites into photovoltaic device architectures, as well as the factors currently limiting photovoltaic performance, are discussed: among them, we identified the combination of low absorption coefficient and heavy mass of the holes as main limitations for the solar cell efficiency. To the best of our knowledge, this is the first demonstration of the potential of 2D copper perovskite as light harvesters and lays the foundation for further development of perovskite based on transition metals as alternative lead-free materials. Appropriate molecular design will be necessary to improve the material's properties and solar cell performance filling the gap with the state-of-the-art Pb-based perovskite devices.

Pressure-induced crystallographic transitions and optical behavior of MAPbI3 (MA=methylammonium) were investigated using in-situ synchrotron X-ray diffraction and laser-excited photoluminescence spectroscopy. We observed that the tetragonal phase that presents under ambient pressure transformed to a ReO3-type cubic phase at 0.3 GPa, which further converted into a putative orthorhombic structure at 2.7 GPa. The sample was finally separated into crystalline and amorphous fractions beyond 4.7 GPa. During the decompression, the phase-mixed material restored the original structure in two distinct pathways depending on the peak pressures. Being monitored using a laser-excited photoluminescence technique under each applied pressure, it was determined that the bandgap reduced with an increase of the pressure till 0.3 GPa and then enlarged with an increase of the pressure up to 2.7 GPa. This work lays the foundation for understanding pressure-induced phase transitions and bandgap tuning of MAPbI3, enriching potentially the toolkit for engineering perovskites related photovoltaic devices. National Research Foundation (NRF) Accepted version This work was partially supported by NRF-CRP14-2014-03 and Custom Electronics, Inc. CHESS was supported by the NSF award DMR-1332208. Bandgap calculations were contributed by Hai Xiao, Jason Crowley and William A. Goddard III from Materials and Process Simulation Center (MSC) and Joint Center for Artificial Photosynthesis (JCAP), California Institute of Technology. S.J. acknowledges the support from Binghamton University.

Bismuth-based ternary halides have recently gained a lot of attention as lead-free perovskite materials. However, photovoltaic performances of these devices remain poor, mostly due to their low-dimensional crystal structure and large bandgap. Here, a dynamic hot casting technique to fabricate silver bismuth iodide-based perovskite solar cells under an ambient atmosphere with power conversion efficiencies above 2.5% is demonstrated. Silver bismuth iodides are 3D analogs of complex ternary bismuth halides with a suitable bandgap for a single junction solar cell. As far as it is known, these results represent the highest efficiency for solution processed air-stable lead-free perovskite solar cells. The enhanced solar cell performance via this dynamic hot casting technique can be attributed to long carrier lifetimes, micrometer-sized crystalline grains, and pinhole free thin-film formation with uniform morphology. This work provides a new direction for fabrication of solution-processed lead-free perovskite solar cells with a rapid fabrication strategy irrespective of the processing environment. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

Catalyst deactivation is a persistent problem not only for the scientific community but also in industry. Isolated single-site heterogeneous catalysts have shown great promise to overcome these problems. Here, a versatile anchoring strategy for molecular complex immobilization on a broad range of semiconducting or insulating metal oxide (e.g. titanium dioxide, mesoporous silica, cerium oxide, and tungsten oxide) nanoparticles to synthesize isolated single-site catalysts has been studied systematically. An oxidatively stable anchoring group, maleimide, is shown to form covalent linkages with surface hydroxyl functionalities of metal oxide nanoparticles by photoclick chemistry. The nanocomposites have been thoroughly characterized by techniques including UV-visible diffuse reflectance spectroscopy (UV-DRS), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), and X-ray absorption spectroscopy (XAS). The IR spectroscopic studies confirm the covalent linkages between the maleimide group and surface hydroxyl functionalities of the oxide nanoparticles. The hybrid nanomaterials function as highly efficient catalysts for essentially quantitative oxidations of terminal and internal alkenes, and show molecular catalyst product selectivities even in more eco-friendly solvents. XAS studies verify the robustness of the catalysts after several catalytic cycles. We have applied the photoclick anchoring methodology to precisely control the deposition of a luminescent variant of our catalyst on the metal oxide nanoparticles. Overall, we demonstrate a general approach to use irradiation to anchor molecular complexes on oxide nanoparticles to create recyclable, hybrid, single-site catalysts that function with high selectivity in a broad range of solvents. We have achieved a facile, spatially and temporally controllable photoclick method that can potentially be extended to other ligands, catalysts, functional molecules, and surfaces. MOE (Min. of Education, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

In this work we have studied the feasibility of highly symmetric guanidinium (GA; CH6N3+) cation in perovskite based solar cells. It is an alternative to the methyl ammonium (MA; CH3NH3+) or formamidinium (FA; CH(NH2)2+) ions commonly utilized in the perovskite solar cells, may bring additional advantages due to its triad rotational symmetry and zero dipole moment (μ). We noticed that due to steric factors, GA preferably forms two-dimensional (2D) layer, where GA2PbI4 is favored monoclinic structure (E g = 2.5 eV) and the obtained device efficiency is η = 0.45%. This study provides a guideline for designing guanidinium based perovskite absorbers for solar cell devices. In addition, through further compositional engineering with mixed organic cations using GA may enhance the device performance. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

Carbon based perovskite solar cells (PSCs) are fabricated through easily scalable screen printing techniques, using abundant and cheap carbon to replace the hole transport material (HTM) and the gold electrode further reduces costs, and carbon acts as a moisture repellent that helps in maintaining the stability of the underlying perovskite active layer. An inorganic interlayer of spinel cobaltite oxides (Co3O4) can greatly enhance the carbon based PSC performance by suppressing charge recombination and extracting holes efficiently. The main focus of this research work is to investigate the effectiveness of Co3O4 spinel oxide as the hole transporting interlayer for carbon based perovskite solar cells (PSCs). In these types of PSCs, the power conversion efficiency (PCE) is restricted by the charge carrier transport and recombination processes at the carbon–perovskite interface. The spinel Co3O4 nanoparticles are synthesized using the chemical precipitation method, and characterized by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and UV-Vis spectroscopy. A screen printed thin layer of p-type inorganic spinel Co3O4 in carbon PSCs provides a better-energy level matching, superior efficiency, and stability. Compared to standard carbon PSCs (PCE of 11.25%) an improved PCE of 13.27% with long-term stability, up to 2500 hours under ambient conditions, is achieved. Finally, the fabrication of a monolithic perovskite module is demonstrated, having an active area of 70 cm2 and showing a power conversion efficiency of >11% with virtually no hysteresis. This indicates that Co3O4 is a promising interlayer for efficient and stable large area carbon PSCs. NRF (Natl Research Foundation, S’pore) Accepted version

The hybrid organic–inorganic perovskite (CH3NH3)PbI3 may find application in next generation solid-state sensitised solar cells. Although this material and related perovskites were discovered many decades ago, questions remain concerning their diverse structural chemistry and unusual properties. The article presents a review of previous work and provides a detailed description of the preparation, structural characterisation and physical characteristics of (CH3NH3)PbI3. The phase changes exhibited by (CH3NH3)PbI3 have been probed using variable temperature powder and single crystal X-ray diffraction, combined with differential scanning calorimetry, thermogravimetric analysis and phase contrast transmission electron microscopy. The optical band gap for (CH3NH3)PbI3 determined by UV-Visible spectroscopy was compared to values obtained from density-of-state simulation of the electronic band structure. Accepted version

We report the pressure-induced crystallographic transitions and optical behavior of MAPbI3 (MA=methylammonium) using in situ synchrotron X-ray diffraction and laser-excited photoluminescence spectroscopy, supported by density functional theory (DFT) calculations using the hybrid functional B3PW91 with spin-orbit coupling. The tetragonal polymorph determined at ambient pressure transforms to a ReO3-type cubic phase at 0.3 GPa. Upon continuous compression to 2.7 GPa this cubic polymorph converts into a putative orthorhombic structure. Beyond 4.7 GPa it separates into crystalline and amorphous fractions. During decompression, this phase-mixed material undergoes distinct restoration pathways depending on the peak pressure. In situ pressure photoluminescence investigation suggests a reduction in band gap with increasing pressure up to ≈0.3 GPa and then an increase in band gap up to a pressure of 2.7 GPa, in excellent agreement with our DFT calculation prediction. This work lays the foundation for understanding the pressure-dependent phase transition of MAPbI3 and potentially enriches the toolkit for engineering perovskite polymorphs with exceptional optical properties. NRF (Natl Research Foundation, S’pore) Accepted version

Although being considered as one of the most promising cathode materials for Lithium-ion batteries (LIBs), LiNi1/3Co1/3Mn1/3O2 (NCM) is currently limited by its poor rate performance and cycle stability resulting from the thermodynamically favorable Li+/Ni2+ cation mixing which depresses the Li+ mobility. In this study, we developed a two-step method using fluffy MnO2 as template to prepare hierarchical porous nano-/microsphere NCM (PNM-NCM). Specifically, PNM-NCM microspheres achieves a high reversible specific capacity of 207.7 mAh g−1 at 0.1 C with excellent rate capability (163.6 and 148.9 mAh g−1 at 1 C and 2 C), and the reversible capacity retention can be well-maintained as high as 90.3% after 50 cycles. This excellent electrochemical performance is attributed to unique hierarchical porous nano-/microsphere structure which can increase the contact area with electrolyte, shorten Li+ diffusion path and thus improve the Li+ mobility. Moreover, as revealed by XRD Rietveld refinement analysis, a negligible cation mixing (1.9%) and high crystallinity with a well-formed layered structure also contribute to the enhanced C-rates performance and cycle stability. On the basis of our study, an effective strategy can be established to reveal the fundamental relationship between the structure/chemistry of these materials and their properties.

et al., Rare earth silicate apatites are one-dimensional channel structures that show potential as electrolytes for solid oxide fuel cells (SOFC) due to their high ionic conductivity at intermediate temperatures (500-700 °C). This advantageous property can be attributed to the presence of both interstitial oxygen and cation vacancies, that create diffusion paths which computational studies suggest are less tortuous and have lower activation energies for migration than in stoichiometric compounds. In this work, neutron diffraction of NdAlSiO (0 ≤ x ≤ 1.5) single crystals identified the locations of oxygen interstitials, and allowed the deduction of a dual-path conduction mechanism that is a natural extension of the single-path sinusoidal channel trajectory arrived at through computation. This discovery provides the most thorough understanding of the O transport mechanism along the channels to date, clarifies the mode of interchannel motion, and presents a complete picture of O percolation through apatite. Previously reported crystallographic and conductivity measurements are re-examined in the light of these new findings., We are pleased to acknowledge the Agency for Science, Technology and Research (A*STAR) PSF grant 082 101 0021 “Optimization of Oxygen Sublattices in Solid Oxide Fuel Cell Apatite Electrolytes” for funding the work and the Ministry of Education (MOE) Tier 2 grant T208B1212 for enabling the purchase of a single crystal X-ray diffractometer.

Improved photoelectrochemical activity for tetragonal BiV 0.8 Mo 0.2 O 4 fabricated by hydrothermal synthesis is reported in the present study. The enhanced water oxidation efficiency is attributed to the formation of cation vacancies (Bi 5+ )/oxygen interstitials due to high amount of Mo doping. Efficient charge transport and electron hole separation for water oxidation using BiV 0.8 Mo 0.2 O 4 photoanode was supported by electrochemical impedance investigations and open circuit photovoltage measurements. The present study gives a significant insight into the role of nonstoichiometry related efficient water oxidation using tetragonal BiVO 4 .

et al., High oxygen content apatite germanates, La10Ge 6-xWxO27+x, have been prepared by doping on the Ge site with W. In addition to increasing the oxygen content, this doping strategy is shown to result in stabilisation of the hexagonal lattice, and yield high conductivities. Structural studies of La10Ge 5.5W0.5O27.5 show that the interstitial oxygen sites are associated to a different degree with the Ge/WO4 tetrahedra, leading to five coordinate Ge/W and significant disorder for the oxygen sites associated with these units. Raman spectroscopy studies suggest that in the case of the WO5 units, the interstitial oxygen is more tightly bonded and therefore not as mobile as in the case of the GeO5 units, thus not contributing significantly to the conduction process. © 2011 The Royal Society of Chemistry., Financial support from Spanish project MAT2007-64486-C07-02 is acknowledged.

Solid-state reaction between SrCO{sub 3}, Cr{sub 2}O{sub 3} and SrF{sub 2} has produced the apatite phase Sr{sub 10}(CrO{sub 4}){sub 6}F{sub 2} and Sr{sub 2}CrO{sub 4} which adopts the K{sub 2}NiF{sub 4}-type structure. The reaction outcome was very sensitive to the heating rate with rapid rise times favouring the formation of Sr{sub 2}CrO{sub 4}, which has been synthesised at ambient pressure for the first time. Powder X-ray diffraction and electron diffraction confirmed that Sr{sub 2}CrO{sub 4} adopts a body centred tetragonal cell (space group I4/mmm) with lattice parameters a=3.8357(1) A and c=12.7169(1) A, while a combination of neutron and X-ray diffraction verified Sr{sub 10}(CrO{sub 4}){sub 6}F{sub 2} is hexagonal (space group P6{sub 3}/m) with lattice parameters a=9.9570(1) A and c=7.4292(1) A. X-ray photoelectron spectroscopy and magnetic measurements were used to characterise the oxidation states of chromium contained within these phases. - Graphical abstract: A solid-state reaction between SrCO{sub 3}, Cr{sub 2}O{sub 3} and SrF{sub 2} produced Sr{sub 10}(CrO{sub 4}){sub 6}F{sub 2} apatite and Sr{sub 2}CrO{sub 4} which adopts the K{sub 2}NiF{sub 4}-type structure. Powder X-ray and electron diffraction confirmed that Sr{sub 2}CrO{sub 4} is body-centred tetragonal, while a combination of neutron and X-ray diffraction verified Sr{sub 10}(CrO{sub 4}){sub 6}F{sub 2} ismore » hexagonal. X-ray photoelectron spectroscopy and magnetic measurements identified the oxidation states of chromium in these phases.« less

International audience; In this paper, we report on the magnetic properties of Ba2CoS3, a spin–chain compound recently found to be the first Co2+-containing one-dimensional sulfide to show metallic-like conductivity and negative magnetoresistance. We carried out an in-depth experimental investigation of the local structure of the cobalt atoms, and ab initio calculations of the resulting electronic configuration of Co2+. From theoretical considerations, the intra-chain coupling was predicted to be antiferromagnetic. Experimentally, several estimates of this magnetic coupling were derived by analysing the temperature dependence of the magnetic susceptibility. Magnetic and heat capacity measurements also provided evidence of a three-dimensional antiferromagnetic ordering, a feature indicative of a noticeable inter-chain coupling in this quasi-1D system.

The pressure-induced structural evolution of formamidinium-based perovskite FAPbI3 was investigated using in situ synchrotron X-ray diffraction and laser-excited photoluminescence methods. Cubic α-FAPbI3 ( Pm3̅ m) partially and irreversibly transformed to hexagonal δ-FAPbI3 ( P63 mc) at a pressure less than 0.1 GPa. Structural transitions of α-FAPbI3 followed the sequence of Pm3̅ m → P4/ mbm → Im3̅ → partial amorphous during compression to 6.59 GPa, whereas the δ-phase converted to an orthorhombic Cmc21 structure between 1.26 and 1.73 GPa. During decompression, FAPbI3 recovered the P63 mc structure of the δ-phase as a minor component (∼18 wt %) from 2.41-1.40 GPa and the Pm3̅ m structure of the α-phase becomes dominant (∼82 wt %) at 0.10 GPa but with an increased fraction of δ-FAPbI3. The photoluminescence behaviors from both the α- and δ-forms were likely controlled by radiative recombination at the defect levels rather than band-edge emission during pressure cycling. FAPbI3 polymorphism is exquisitely sensitive to pressure. While modest pressures can engineer FAPbI3-based photovoltaic devices, irreversible δ-phase crystallization may be a limiting factor and should be taken into account.