Effect Of Anion Substitution Research Articles

Mixed anion control of enhanced negative thermal expansion in the oxysulfide of PbTiO3.

The rare physical property of negative thermal expansion (NTE) is intriguing because materials with a large NTE over a wide temperature range can serve as high-performance thermal expansion compensators. However, the applications of NTE are hindered by the fact that most of the available NTE materials show small magnitudes of NTE, and/or NTE occurs only in a narrow temperature range. Herein, for the first time, we investigated the effect of anion substitution instead of general Pb/Ti-site substitutions on the thermal expansion properties of a typical ferroelectric NTE material, PbTiO3. Intriguingly, the substitution of S for O in PbTiO3 further increases the tetragonality of PbTiO3. Consequently, an unusually enhanced NTE with an average volumetric coefficient of thermal expansion of V = -2.50 × 10-5 K-1 was achieved over a wide temperature range (300-790 K), which is in contrast to that of pristine PbTiO3 (V = -1.99 × 10-5 K-1, RT-763 K). The intensified NTE is attributed to the enhanced hybridization between Pb/Ti and O/S atoms by the substitution of S, as evidenced by our theoretical investigations. We therefore demonstrate a new technique for introducing mixed anions to achieve a large NTE over a wide temperature range in PbTiO3-based ferroelectrics.

Electrocatalytic Activities of Sr2BO3Cl (B = Mn, Fe, Co, Ni) for Oxygen Reactions in Alkaline Solutions

The practical application of zinc-air rechargeable batteries, which are expected to be the next-generation rechargeable batteries, is challenged by the kinetically very slow oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Therefore, an air electrode catalyst with high activity for OER and ORR is required, and perovskite oxides have attracted attention as a candidate. To improve the catalytic activity of perovskite oxides, optimization of metal cations has been studied. On the other hand, anion substitution has also been reported to improve activity, and the oxychloride Sr2CoO3Cl with the Ruddlesden-Popper (RP) structure (composition: An+1BnO3n+1), which is a type of layered perovskite structure showed superior catalytic activities. However, there are still few studies on oxychlorides and the effect of anion substitution on the catalytic activity is still unknown. In this study, we compared the air electrode catalytic activity of RP-type acid chlorides Sr2 BO3Cl (B = Mn, Fe, Co, Ni) and LaSrBO4, an oxide with the same structure and valence, and investigated the B-site element dependence of the chloride ion substitution effect.The ORR activity measurement results show that the oxychlorides with Co and Mn at the B site exhibited lower ORR overpotentials than the oxides, and that the ORR reaction electron number calculated from the Koutecký-Levich plots increases. Comparing the results for the same B-site elements, the oxychlorides had higher OER activity for all B-site elements. This suggests that chloride ion substitution may be effective in improving OER activity for all B-site elements, not only Co.

Combined Effect of Halogenation and SiO2 Addition on the Li-Ion Conductivity of LiBH4

In this work, the combined effects of anion substitution (with Br− and I−) and SiO2 addition on the Li-ion conductivity in LiBH4 have been investigated. Hexagonal solid solutions with different compositions, h-Li(BH4)1−α(X)α (X = Br, I), were prepared by ball milling and fully characterized. The most conductive composition for each system was then mixed with different amounts of SiO2 nanoparticles. If the amount of added complex hydride fully fills the original pore volume of the added silica, in both LiBH4-LiBr/SiO2 and LiBH4-LiI/SiO2 systems, the Li-ion conductivity was further increased compared to the h-Li(BH4)1−α(X)α solid solutions alone. The use of LiBH4-LiX instead of LiBH4 in composites with SiO2 enabled the development of an optimal conductive pathway for the Li ions, since the h-Li(BH4)1−α(X)α possesses a higher conductivity than LiBH4. In fact, the Li conductivity of the silica containing h-Li(BH4)1−α(X)α is higher than the maximum reached in LiBH4-SiO2 alone. Therefore, a synergetic effect of combining halogenation and interface engineering is demonstrated in this work.

Effect of Extra-Framework Anion Substitution on the Properties of a Chiral Crystalline Sponge.

Chiral metal-organic materials, CMOMs, are of interest as they can offer selective binding sites for chiral guests. Such binding sites can enable CMOMs to serve as chiral crystalline sponges (CCSs) to determine molecular structure and/or purify enantiomers. We recently reported on the chiral recognition properties of a homochiral cationic diamondoid, dia, network {[Ni(S-IDEC)(bipy)(H2O)][NO3]}n (S-IDEC = S-indoline-2-carboxylicate, bipy = 4,4'-bipyridine), CMOM-5[NO3]. The modularity of CMOM-5[NO3] means there are five feasible approaches to fine-tune structures and properties via substitution of one or more of the following components: metal cation (Ni2+); bridging ligand (S-IDEC); linker (bipy); extra-framework anion (NO3-); and terminal ligand (H2O). Herein, we report the effect of anion substitution on the CCS properties of CMOM-5[NO3] by preparing and characterizing {[Ni(S-IDEC)(bipy)(H2O)][BF4]}n, CMOM-5[BF4]. The chiral channels in CMOM-5[BF4] enabled it to function as a CCS for determination of the absolute crystal structures of both enantiomers of three chiral compounds: 1-phenyl-1-butanol (1P1B); methyl mandelate (MM); ethyl mandelate (EM). Chiral resolution experiments revealed CMOM-5[BF4] to be highly selective toward the S-isomers of MM and EM with enantiomeric excess, ee, values of 82.6 and 78.4%, respectively. The ee measured for S-EM surpasses the 64.3% exhibited by [DyNaL(H2O)4] 6H2O and far exceeds that of CMOM-5[NO3] (6.0%). Structural studies of the binding sites in CMOM-5[BF4] provide insight into their high enantioselectivity.

Carbon-Encapsulated Cobalt Phosphide Nanowires as Trifunctional Catalysts for Water Splitting Devices

Cobalt-based catalysts have been widely proved to be promising electrocatalysts for oxygen reduction (ORR), hydrogen evolution (HER), and oxygen evolution (OER). In this paper, to maximize the trifunctional reactivity of the catalyst, a series of one-dimensional carbon-coated cobalt-based compound nanowires with changed anions were reported, and their trifunctional activities were systematically investigated. Among the cobalt-based compounds with different anions, carbon-encapsulated cobalt phosphide nanowires (CoP/C NWs) show impressive trifunctional catalytic activity toward ORR, OER, and HER. Using CoP/C NWs as a trifunctional electrocatalyst, an integrated device of the two-electrode water splitting device powered by the zinc–air battery system was realized. The assembled zinc–air battery exhibits high power density (115 mW cm–2) and long-term cycle stability (over 350 h, 1050 cycles). This work provides an insight into the effects of anion substitution on electrocatalytic activity and provides a highly effective feasibility for the design of energy-related devices with high-activity and durable catalysts.

Enhanced free-exciton luminescence in Cs2SnBr6−xIx: A first-principles study

A2BX6-type tin halide perovskites have received extensive attention as environment-friendly lead-free luminescent materials. However, the limited knowledge of the partial anion substitution effects on their luminescent properties hinders their further development in this field. Therefore, we investigated such effects using Cs2SnBr6 (CSB) double perovskite as an example in this work. We first found that CSB remains stable upon partial anion substitution of bromine by iodine. Next, we explored the electronic origin of the tunable band gaps upon the presence of both Jahn-Teller-like or non-Jahn-Teller-like distortions in [SnBr6−xIx]2− (x = 1–3) octahedrons. As a further stage, we performed hybrid HSE06 functional calculations considering spin–orbit coupling (SOC) to obtain more accurate band gaps. Importantly, all substituted materials except Cs2SnBr4I2 (structure 2a) show direct band gap characteristics and enhanced light emissions. Finally, we identified the free exciton (FE) emission mechanism is more favorable than self-trapped exciton (STE) emission in these materials owning to their high electronic dimensionality, small effective mass, non-Jahn-Teller-like distortion, small exciton binding energy, small energy separation, small self-trapping depth, low detrapping barrier, and non-local deformation in the presence of heavy halogen. We believe our findings of Cs2SnBr6−xIx would promote the design of tin halide perovskites with better luminescent properties.

Concentration dependences of the structural, magnetic and kinetic properties of manganese chalcogenides doped with tellurium

The effect of anionic substitution on the structural, magnetic, and kinetic properties of chalcogenides of the MnSe1-XTeX system in the temperature range of 77-400K in magnetic fields up to 12 kOe was studied. With an increase in the substitution concentration, changes in the structural and magnetic characteristics, which correlate with changes in the electronic structure, were found. The type and mobility of the main current carriers were determined from the Hall effect measurements.

Divalent Ion Specific Outcomes on Stern Layer Structure and Total Surface Potential at the Silica:Water Interface.

The second-order nonlinear susceptibility, χ(2), in the Stern layer and the total interfacial potential drop, Φ(0)tot, across the oxide:water interface are estimated from SHG amplitude and phase measurements for divalent cations (Mg2+, Ca2+, Sr2+, and Ba2+) at the silica:water interface at pH 5.8 and various ionic strengths. We find that interfacial structure and total potential depend strongly on ion valency. We observe statistically significant differences between the experimentally determined χ(2) value for NaCl and that of the alkali earth series but smaller differences between ions of the same valency in that series. These differences are particularly pronounced at intermediate salt concentrations, which we attribute to the influence of hydration structure in the Stern layer. Furthermore, we corroborate the differences by examining the effects of anion substitution (SO42- for Cl-). Finally, we identify that hysteresis in measuring the reversibility of ion adsorption and desorption at fused silica in forward and reverse titrations manifests itself both in Stern layer structure and in total interfacial potential for some of the salts, most notably for CaCl2 and MgSO4 but less so for BaCl2 and NaCl.

Li-Ion Conductivity in Metal Hydride-Based Nanocomposite Electrolytes: The Effect of Nanoscaffold Porosity and Surface Chemistry

The development of energy storage technologies, such as rechargeable batteries, is crucial for the transition to a sustainable energy supply. Lithium-ion batteries are an effective means of energy storage, which is demonstrated by their wide application ranging from mobile phones to laptops and electric vehicles. Unfortunately, Li-ion batteries suffer from safety issues arising from their combustible organic electrolytes. All-solid-state batteries (ASSBs), in which the common liquid organic electrolyte is replaced by a solid-state electrolyte (SSE), could potentially lead to safer batteries with increased energy density.[1] Recently, metal hydrides (e.g. LiBH4 and LiCB11H12) have gained attention as promising solid electrolytes due to their electrochemical and thermal stability, low density and high ionic conductivity albeit at elevated temperatures. However, for successful incorporation of metal hydride SSEs in ASSBs, sufficient ionic conductivity at room temperature is a prerequisite. Therefore, the development of strategies that enhance the room temperature conductivity in complex hydrides (10-8 S cm-1 for LiBH4) is of major importance.In this contribution, we combined two promising strategies to enhance ion mobility in metal hydrides, namely, partial ionic substitution[2] and nanoconfinement[3], which led to highly conductive metal hydride-based nanocomposites (Figure 1). Specifically, via partial ion substitution with LiNH2, followed by nanoconfinement in a mesoporous oxide scaffold, LiBH4-LiNH2/metal oxide nanocomposites with conductivities reaching 5⸱10-4 S cm-1 at 30 °C were obtained[4], compared to 2⸱10-8 S cm-1 for pure LiBH4.Interestingly, the conductivity of the LiBH4-LiNH2/metal oxide nanocomposites is strongly influenced by the chemical and physical nature of the mesoporous metal oxide. We systematically studied the influence of the scaffold properties on the conductivity of nanoconfined LiNH2-substituted LiBH4 using mesoporous silica scaffolds (SBA-15) with varying surface chemistry and pore structure. The conductivity varied over three orders of magnitude when tuning both the porosity and surface chemistry of the metal oxide scaffold.[5] Our study reveals that the LiBH4-LiNH2/metal oxide conductivity is affected by the chemical nature of the scaffold, similar to LiBH4/metal oxide nanocomposites. A conductivity improvement of a factor of two is achieved by changing the SiO2 (SBA-15) surface chemistry through alumination (Figure 2a). On the other hand, different from nanoconfined LiBH4, the conductivity of LiBH4-LiNH2/metal oxide nanocomposites is largely dictated by the scaffold pore volume (Figure 2b). Notably, the conductivity can be varied from 4⸱10-7 S cm-1 to 5⸱10-4 S cm-1 by increasing the scaffold pore volume from 0.51 to 1.00 cm3 g-1.Our work demonstrates that the origin of the conductivity enhancement in anion-substituted complex hydride-based solid electrolytes is different from other nanoconfined complex hydrides, e.g. LiBH4. In particular, for nanoconfined LiBH4-LiNH2, the conductivity improvement is attributed to stabilization of a highly conductive phase inside the scaffold pores, rather than the formation of a conductive interfacial layer at the hydride/oxide interface as observed for nanoconfined LiBH4. Thus, it is clear that the conductivity of metal hydride-based nanocomposite ion conductors is closely linked to the properties of scaffold material. These fundamental insights on the influence of scaffold properties on ion mobility in nanocomposite materials could be applicable to other cation- and anion substituted ion conductors as well. Thereby, this work provides useful insights for the design novel solid-state electrolytes with excellent ionic conductivity, which are crucial for the development of next generation batteries. References Janek, Jürgen, and Zeier, Wolfgang G. "A solid future for battery development." Nature Energy 1.9 (2016): 1-4.Maekawa, Hideki, et al. "Halide-stabilized LiBH4, a room-temperature lithium fast-ion conductor." Journal of the American Chemical Society 131.3 (2009): 894-895.Blanchard, Didier, et al. "Nanoconfined LiBH4 as a fast lithium ion conductor." Advanced Functional Materials 25.2 (2015): 184-192Zettl, Roman, et al. "Combined Effects of Anion Substitution and Nanoconfinement on the Ionic Conductivity of Li-Based Complex Hydrides." The Journal of Physical Chemistry C 124.5 (2020): 2806-2816.de Kort, Laura M., et al. "The effect of nanoscaffold porosity and surface chemistry on the Li-ion conductivity of LiBH4–LiNH2/metal oxide nanocomposites." Journal of Materials Chemistry A 8.39 (2020): 20687-20697. Figure 1

Effect of anionic substitution on the high temperature stability of polymer‐derived SiOC glasses

AbstractIn this study the high temperature stability (crystallization and decomposition) of two silicon oxycarbide glasses with a similar amount of free carbon (8.3 vs 9.6 wt%) but different content of Si‐C bonds (SiC0.22O1.57 vs SiC0.07O1.86) is presented. The two SiOC glasses are obtained from the same precursor (2 µm methyl‐silsesquioxane spheres) via pyrolysis at 1100°C in inert (Ar) or reactive (CO2) atmospheres. Further annealing in Ar flow at temperatures above 1100°C and up to 1500°C is performed and the samples are characterized by Fourier Transformed Infrared Spectroscopy (FT‐IR) and X‐ray diffraction (XRD). For comparison purposes the same precursor was annealed in air flow to obtain SiO2 and its high temperature evolution is also studied. Results suggest that the onset for the carbothermal reduction is not dependent on the amount of Si‐C bonds. Moreover, contrary to what is usually reported in the scientific literature, silica phase present in the SiOC glasses does not show, in the same experimental conditions, superior crystallization resistance compared to pure silica glass.

Influence of anion substitution on 3D-architectured Ni-Co-A (A=H, O, P) as efficient cathode materials towards rechargeable Zn-based battery

Among various Zn-based batteries, transition metal compounds have been widely studied as cathode materials in alkaline electrolyte. However, the correlation between anion species and the electrochemical performance of corresponding compounds is still not well understood. Here, we construct a 3D-architecrured F doping Ni/Co hydroxides/oxides/phosphides (FNCA, A = H, O, P) as the model materials and deeply explore the effect of anion substitution on cathode material for Zn batteries. Electrochemical measurements are combined with theoretical calculations to reveal the mechanisms of FNCA delivering considerable dissimilarity in electrochemical properties. In-depth analysis suggests that the surface-dominated redox behaviors are greatly affected by the anion substitution, though the reversible capacity is just produced by the valence change of metal cations. The FNCP exhibits the highest capacity among three samples but an inferior cycling stability. As contrast, the FNCO shows a balanced characteristic of good capacity and superior cycling performance. Based on the understanding, we construct FNCA//Zn batteries. As expected, the FNCP//Zn batteries delivers the highest specific capacity of 318 mAh g−1 and a superior energy density of 532.7 Wh kg−1 at a power density of 1.673 kW kg−1, but an inferior cycling stability reserved from the individual FNCP electrode. This work provides a rational insight for deep understanding the behaviors that anions affect electrochemical energy storage but not participate in redox reactions, and offer more effective feasibility to design high capacity and durable cathode materials for aqueous Zn-based batteries.

Combined Effects of Anion Substitution and Nanoconfinement on the Ionic Conductivity of Li-Based Complex Hydrides.

Solid-state electrolytes are crucial for the realization of safe and high capacity all-solid-state batteries. Lithium-containing complex hydrides represent a promising class of solid-state electrolytes, but they exhibit low ionic conductivities at room temperature. Ion substitution and nanoconfinement are the main strategies to overcome this challenge. Here, we report on the synthesis of nanoconfined anion-substituted complex hydrides in which the two strategies are effectively combined to achieve a profound increase in the ionic conductivities at ambient temperature. We show that the nanoconfinement of anion substituted LiBH4 (LiBH4–LiI and LiBH4–LiNH2) leads to an enhancement of the room temperature conductivity by a factor of 4 to 10 compared to nanoconfined LiBH4 and nonconfined LiBH4–LiI and LiBH4-LiNH2, concomitant with a lowered activation energy of 0.44 eV for Li-ion transport. Our work demonstrates that a combination of partial ion substitution and nanoconfinement is an effective strategy to boost the ionic conductivity of complex hydrides. The strategy could be applicable to other classes of solid-state electrolytes.

Effect of Anionic Substitution of S by Se in CZTSSe Films Prepared from Electron Beam Evaporation

Abstract Cu2ZnSnSxSe4-x (CZTSSe) is a prominent candidate as a photon absorber layer in solar cell owing to its earth abundant constituents and high absorption coefficient with tunable bandgap. In this study, the effect of metallic stacking order and anionic substitution of S by Se on the properties of CZTSSe is investigated by varying the S/Se ratio. CZT metallic precursor films with stacking sequences SLG/Sn/Zn/Cu and SLG/Cu/Sn/Zn/Cu are fabricated on soda lime glass substrate by using electron beam evaporation of elemental metallic precursors. In order to obtain CZTSSe films with various S/Se ratio, the films are annealed in the presence of elemental S and Se environment with various ratio of elemental S and Se powder at 550 °C for 1 hour at a ramping rate of 10 °C/min with N2 as a carrier gas. The effect of variation of S/Se powder on crystallinity, morphology and composition of the films are confirmed through X-Ray diffraction, Raman spectroscopy, FEGSEM and EDS analysis. The optical bandgap of the films is estimated to decrease with increasing Se content in the films. The Se incorporation is noticed the highest in case of SLG/Sn/Zn/Cu stack and its bandgap is found to be decreasing from 1.53 to 1.2 eV with increasing Se content.

Effect of anion substitution on the structural and transport properties of argyrodites Cu7PSe6-xSx.

Inspired by the good performance of argyrodites as ion conducting thermoelectrics and as solid electrolytes we investigated the effect of isovalent S2- substitution for Se2- in Cu7PSe6. At room temperature Cu7PSe6 crystallizes in the primitive cubic β-polymorph of the argyrodite structure and transforms to the face-centered high-temperature (HT) γ-modification above 320 K. The transition for the homologous Cu7PS6 occurs at 510 K. Promising thermoelectric and ion conducting properties are observed only in the HT modification, where the cations are mobile. Using Rietveld refinements against X-ray diffraction data the effect of isovalent S2- substitution for Se2- on the structural and transport properties in Cu7PSe6-xSx was analyzed. While a step-wise incorporation of S2- showed typical behavior for a homogeneous solid solution series, the analysis of the diffraction data gave clear evidence of anion ordering due to site preference of the sulfide ions, which can be rationalized using Pearson's HSAB concept. This leads to a stabilization of the HT structure even at lower temperatures. This selective control enables new strategies for the design of argyrodite materials, as isovalent substitution not only allows an engineering of properties, but also permits the stabilization of the polymorph with the most promising properties.

Mixed Lithium Indium Halides As Solid-Electrolytes: Computational Experiments on Drivers of Li+ Diffusion

Introduction The search for solid electrolytes with fast Li+ diffusion is essential to the development of safer and higher energy-density all-solid state batteries. The presented simulations aim to understand the effect of the dynamic electronic structure on Li+ diffusion by not only identifying the atomic scale Li+ diffusion process, but also the effect of anion substitution on the interaction of the Li+ and the surrounding lattice. In this report, the diffusion processes of Li+ in Li3InCl­6 and Li3InBr6-xClx are probed using molecular dynamics simulation. Methodology Ab-initio molecular dynamics using Quantum ESPRESSO is used to simulate Li+ diffusion in Li3InCl­6 electrolytes. A 2x2x1 supercell structure containing 80 atoms is sufficient to capture the local and mid-range environment around the diffusing Li+. Computational experiments examined the effect of different changing temperature, volume, and the ratio of Br to Cl. The activation energies of Li+ diffusion in the electrolytes are determined by plotting the mean squared displacement versus time and temperatures. Maximally localized Wannier functions (MLWFs) are used for further electronic structure analysis of bond breaking and forming and polarization, thus allowing for an innovative analysis of the correlation of the electronic and ionic dynamics1. Results and Discussion Mixed lithium indium halides have unusual non-monotonic conductivity2 that cannot be explained by a simple picture of ionic interaction between Li+ and the anions sub-lattice. Li3InBr6 has the highest conductivity; if the decrease in conductivity is due to the smaller radius of Cl and thus a smaller volume, then the conductivity should decrease monotonically as Cl is introduced. Instead the Li3InBr3Cl3 alloy has a similar conductivity to the pure Br electrolyte, while other ratios have significantly lower conductivity. This trend can only be understood with first principles simulations of the Li+ diffusion pathways in the lithium indium halides. Molecular dynamics simulations of Li+ (at different temperatures) allows for the estimation of the activation energy and comparison to experiment. The simulations that vary temperature and volume also test the hypothesis posed in previous work1 that the driving force for Li+ mobility in the electrolyte arises from an internal frustration between the electronic (bonding) and structural (geometric) lattice. Mobility of Li+, a “jumping” ion, reveals a conduction pathway that involves transitions between octahedral sites through a tetrahedral interstitial site. The presence of a Cl­- anion can aid or disrupt the frustration that affects Li+ diffusion through correlation of Li+ diffusion events. The Cl- anions change the nature of the Li+ - halide polar-covalent bond, which can be uniquely analyzed with our novel dynamical methods. Therefore, in comparison with Br-, Cl- tips the scale of the bonding character of Li3InCl­6 just enough to disrupt the electronic-geometrical frustration.

Effects of Anion Substitution in (Mo,V)5O14 on Catalytic Performance in Selective Propene Oxidation to Acrolein

AbstractMolybdenum‐based mixed oxides represent well‐known model catalysts for the selective oxidation of light alkenes. Here, the anion lattice of (Mo,V)5O14 was for the first time modified by substituting oxygen ions with nitrogen ions. Investigations by XRD analysis, X‐ray absorption spectroscopy, and FTIR spectroscopy revealed that the incorporation of nitrogen in the structure of Mo5O14 proceeded without changing the average valence of metal centers. Additionally, impedance spectroscopy confirmed the formation of oxygen vacancies. Significant changes in conductivities remained after the removal of nitrogen. Temperature‐programmed reduction measurements were performed to investigate oxygen mobility. The enhanced reducibility of oxide nitrides correlated with the increased conductivity. Catalytic performance in selective propene oxidation was determined by online mass spectrometry und gas chromatography at different temperatures. Selectivity towards acrolein increased with increasing conductivity whereas the formation of total oxidation products COx decreased.

Effects of anion substitution on hydration, ionic conductivity and mechanical properties of anion-exchange membranes

Ionic conductivity and the mechanical properties of ionomers with various anions.

Notable Effects of Aliovalent Anion Substitution on the Electronic Structure and Properties of Metal Oxides and Sulfides

Although it is customary to substitute cations in metal oxides, sulfides, and other materials to modify their structure and properties, effects of anion substitution have not been investigated sufficiently. This is particularly true of materials cosubstituted by two anions (such as N3– and F– in place of O2– or P3– and Cl– in place of S2–). Substitution of a trivalent anion along with a monovalent anion helps to eliminate defects, the three anions being isoelectronic and of nearly the same size. Furthermore, such aliovalent anion substitution gives rise to marked changes in the electronic structure and properties. Isovalent anion substitution (e.g., S2– in place of O2– or Se2– in place of S2–) does not bring about such changes. In this Perspective, we examine the electronic structures and properties of several oxides involving cosubstitution of N and F for oxygen. The oxides discussed are TiO2, ZnO, Cr2O3, and BaTiO3. Aliovalent anion substitution decreases the band gaps of the oxides and affect the magne...

Control of the Valence Instability of Tetravalent Chromium in 1T-CrSe2 by Anion Substitution and Pressure

We report on the anion substitution and pressure effects on the layered compound 1T-CrSe2 with unusual-valent chromium. Owing to the competition between the orbital degrees of freedom and the valence instability of Cr4+, the systems of Cr(Se1−xSx)2 and Cr(Se1−yTey)2 exhibit several types of phase transition. As well as the parent compound CrSe2, Te-substituted samples and slightly S-substituted samples of x ≤ 0.025 show two phase transitions, which are likely to be related to both the valence change and orbital ordering of Cr4+. The highly S-substituted samples of x > 0.04 show one phase transition, which can be attributed to orbital ordering without a valence change. The intermediate S-substituted samples of \(0.025 < x \lesssim 0.04\) exhibit the above three types of phase transition and, in addition, another type of transition. In contrast, the pressure effect on CrSe2 is qualitatively different from these anion substitution effects, which indicates that these anion substitution effects are not simply explained by a chemical pressure effect but also by the change of the stability in the Cr4+ state.

Effect of anionic substitution on the structural and magneto-electrical properties of La–Ca–Mn–O perovskite manganites

The role of anion substitution of oxygen by fluorine in La0.7Ca0.3MnO3−δFx (x=0.0 and 0.6) system has been investigated by means of X-ray powder diffraction, electrical resistivity and magnetization measurements. The oxygen content of the samples, estimated by a redox back titration method, decreases with doping while the lattice parameters and unit cell volume increase slightly. The Curie temperature and the metal-insulator transition temperatures of the samples are also found to increase with doping. The deduced magnetoresistance is slightly higher for the doped sample.