Effect Of Cation Substitution Research Articles

The impact of Nd3+/La3+ substitution on the cation distribution and phase diagram in the La2SrAl2O7-Nd2SrAl2O7 system

The effect of isovalent cation substitution of lanthanum atoms in the structure of La2SrAl2O7 oxide, and phase equilibria (solidus-liquidus curves) in the binary system La2SrAl2O7-Nd2SrAl2O7 were studied. It was found that Nd3+ substitution for La3+ has effect on the structure of La2SrAl2O7 in respect of the character of cation distribution in the solid solution La2–xNdxSrAl2O7 from statistically disordered to the ordered one where strontium cations predominantly occupied the rock-salt layers, as reflected by the solidus-liquidus lines.

Compositional inhomogeneity and tuneable thermal expansion in mixed-metal ZIF-8 analogues.

We study the structural and thermomechanical effects of cation substitution in the compositional family of metal-organic frameworks Zn1-xCdx(mIm)2 (HmIm = 2-methylimidazole). We find complete miscibility for all compositions x, with evidence of inhomogeneous distributions of Cd and Zn that in turn affect framework aperture characteristics. Using variable-temperature X-ray powder diffraction measurements, we show that Cd substitution drives a threefold reduction in the magnitude of thermal expansion behaviour. We interpret this effect in terms of an increased density of negative thermal expansion modes in the more flexible Cd-rich frameworks.

Effect of cation substitution on luminescence properties of phosphor NaYPO4F:Ce3+: A comparison with LnPO4:Ce3+ (Ln = La and Y)

Phosphor NaYPO4F:Ce3+ was synthesized via a high-temperature solid-state reaction, and Na+ and Y3+ ion were partially replaced by Li+ and La3+ ion, respectively. X-ray diffraction patterns, excitation and emission spectra were recorded to investigate the phosphors. The phosphor shows a typical doublet emission of Ce3+ ion in the spectral range ~ 310–420nm. The emission peak of Ce3+ ion at short-wavelength side gradually enhances when Li+ ion gradually substitute for Na+ ion. The emission peak at short-wavelength side gradually decline when La3+ ion gradually substitute for Y3+ ion. The different luminescence behaviors among NaYPO4F:Ce3+, YPO4:Ce3+ and LaPO4:Ce3+ also are simply discussed according to the Ce3+ coordination environment. The results show that high concentration of Li+ or La3+ ion results in the occurrence of a second phase YPO4 or LaPO4, and that the optimal concentration of Li+ and La3+ ion are 0.4 and 0.2, respectively.

Effects of Fluorine and Chromium Doping on the Performance of Lithium-Rich Li1+xMO2(M = Ni, Mn, Co) Positive Electrodes

Lithium-rich metal oxides Li1+zMO2 (M = Ni, Co Mn, etc.) are promising positive electrode materials for high-energy lithium-ion batteries, with capacities of 250–300 mAh·g–1 that closely approach theoretical intercalation limits. Unfortunately, these materials suffer severe capacity fade on cycling, among other performance issues. While ion substitution can improve the performance of many of these materials, the underlying mechanisms of property modification are not completely understood. In this work we show enhanced performance of the Li1+zMO2 electrode, consisting of Li2MnO3 (with C2/m space group) and LiMO2 (with R3m space group) phases, and establish the effects of cationic and anionic substitution on the phase and structure evolution underpinning performance changes. While the undoped material has a high capacity of ∼270 mAh·g–1, only 79% of this remains after 200 cycles. Including ∼2% Cr in the material, likely at the R3m metal (3a) site, improved cycle performance by ∼13%, and including ∼5% F in...

Spectrally Selective Spectroscopy of Native Ca-Containing and Ba-Substituted LH1-RC Core Complexes from Thermochromatium tepidum.

The LH1-RC core complex from the thermophilic photosynthetic purple sulfur bacterium Thermochromatium tepidum has recently attracted interest of many researchers because of its several unique properties, such as increased robustness against environmental hardships and the much red-shifted near-infrared absorption spectrum of the LH1 antenna exciton polarons. The known near-atomic-resolution crystal structure of the complex well supported this attention. Yet several mechanistic aspects of the complex prominence remained to be understood. In this work, samples of the native, Ca2+-containing core complexes were investigated along with those destabilized by Ba2+ substitution, using various spectrally selective steady-state and picosecond time-resolved spectroscopic techniques at physiological and cryogenic temperatures. As a result, the current interpretation of exciton spectra of the complex was significantly clarified. Specifically, by evaluating the homogeneous and inhomogeneous compositions of the spectra, we showed that there is little to no effect of cation substitution on the dynamic or kinetic properties of antenna excitons. Reasons of the extra red shift of absorption/fluorescence spectra observed in the Ca-LH1-RC and not in the Ba-LH1-RC complex should thus be searched in subtle structural differences following the inclusion of different cations into the core complex scaffold.

Luminescence Property Upgrading via the Structure and Cation Changing in AgxEu(2–x)/3WO4 and AgxGd(2–x)/3–0.3Eu0.3WO4

The creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor for controlling the scheelite-type structure and luminescence properties. AgxEu3+(2–x)/3□(1–2x)/3WO4 and AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 (x = 0.5–0) scheelite-type phases were synthesized by a solid state method, and their structures were investigated using a combination of transmission electron microscopy techniques and powder synchrotron X-ray diffraction. Transmission electron microscopy also revealed the (3 + 1)D incommensurately modulated character of AgxEu3+(2–x)/3□(1–2x)/3WO4 (x = 0.286, 0.2) phases. The crystal structures of the scheelite-based AgxEu3+(2–x)/3□(1–2x)/3WO4 (x = 0.5, 0.286, 0.2) red phosphors have been refined from high resolution synchrotron powder X-ray diffraction data. The luminescence properties of all phases under near-ultraviolet (n-UV) light have been investigated. The excitation spectra of AgxEu3+(2–x)/3□(1–2x)/3WO4 (x = 0.5,...

Synthesis and crystal structure of C2/c Ca(Co,Mg)Si2O6 pyroxenes: effect of the cation substitution on cell volume

AbstractA series of clinopyroxenes along the CaMgSi2O6–CaCoSi2O6 join was synthesized by quenching from melts at 1500°C and subsequent annealing at 1250°C (at 0.0001 GPa). This protocol proved to be the most effective to obtain homogenous, impurity-free and stoichiometric pyroxenes. Electron microprobe analyses in energy dispersive mode were conducted and single-crystal X-ray diffraction data were collected on Ca (CoxMg1-x)Si2O6 pyroxenes with x = 0.2, 0.4, 0.5, 0.6. Effects of cation substitution at the M1 site are described. The experimental findings of this study allow us to extend the comparative analysis of the structural features of pyroxenes with divalent cations at the M1 and M2 sites.

Effect of cationic substitution on the double-well hydrogen-bond potential in [K1−x (NH4) x ]3H(SO4)2 proton conductors: a single-crystal neutron diffraction study

The structure of the mixed crystal [K1-x(NH4)x]3H(SO4)2 as obtained from single-crystal neutron diffraction is compared with the previously reported room-temperature neutron structure of crystalline K3H(SO4)2. The two structures are very similar, as indicated by the high value of their isostructurality index (94.8%). It was found that the replacement of even a small amount (3%) of K+ with NH4+ has a significant influence on the short strong hydrogen bond connecting the two SO42- ions. Earlier optical measurements had revealed that the kinetics of the superionic transition in the solid solution [K1-x(NH4)x]3H(SO4)2 are much faster than in K3H(SO4)2; this reported difference in the kinetics of the superionic phase transition in this class of crystal is explained on the basis of the difference in strength of the hydrogen-bond interactions in the two structures.

Thermodynamic Investigation of Selected Compositions in the LiNiO2-LiMnO2-LiCoO2 System As Lithium Ion Battery Cathode Materials

The choice of cathode material is critical for determining the performance and safety of lithium ion batteries. Therefore, the development and optimization of new chemistries for cathodes of lithium ion cells is of major importance. Layered structured lithium mixed transition metal oxides with the general formula of LiNixMnyCozO2 (x + y + z = 1) and an α-NaFeO2-type layered structure (space group ) are attractive intercalation type cathode materials which overcome the limitations of LiCoO2, the most commonly used cathode for lithium ion batteries in portable applications. These NMC materials not only are isostructural to LiCoO2, but also offer comparatively higher reversible capacity, lower cost, and improved thermal stability, which are important characteristics for large scale applications. Although the structural characteristics and electrochemical behavior of many promising compositions in the NMC system have been investigated to some extent, there are very few studies devoted to the thermodynamic properties of this class of materials. The enthalpies of formation of LiNi1−xCoxO2 compounds were determined by Wang and Navrotsky [1] using high temperature oxide solution calorimetry and the enthalpy of formation of LiNi1/3Mn1/3Co1/3O2 and its chemically delithiated phases were measured by Idemoto and Matsui [2] using acid solution calorimetry. In the present work, with the aim of investigating the effect of cation substitution on thermodynamic stability of NMC materials, the standard enthalpies of formation of two different series of sol-gel made NMC samples, namely LiNixMnxCo1-2xO2 (0 ≤ x ≤ 0.5) and LiNi0.8-yMnyCo0.2O2 (0 ≤ y ≤ 0.4), are determined by high-temperature oxide-melt drop solution calorimetry. The compositions are selected to include the technologically relevant NMC111, NMC442, NMC532 and NMC622 compounds. It is shown that LiNixMnyCozO2 with layered structure is a nearly ideal solid solution of LiCoO2, LiNiO2 and LiNi0.5Mn05O2. Our calorimetry results show that the standard enthalpies of formation of the compounds from the elements vary almost linearly with composition in the respective area. Furthermore, based on our experimental heat of formation data and available low temperature heat capacity data for LiCoO2, LiNiO2 and LiNi0.5Mn05O2, a model for the variation of the Gibb’s free energy of formation and equilibrium potentials of the NMCs with composition at room temperature is presented. In addition, key electrochemical tests were performed to establish a relationship between the thermodynamic properties and electrochemical performance of the cathode materials. This work should help clarify the structure–property relationships in these compounds. M.Wang, & A. Navrotsky, Solid State Ionics 166 (2004), 167-173.Y.Idemoto & T. Matsui, Solid State Ionics 179 (2008), 625-635.

(Invited) Enhancing Phase Stability and CO2 Tolerance of Ba0.5Sr0.5Co0.8Fe0.2O3-δ

The effect of B-site cation substitution on the phase stability of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) in O2- and CO2-containing atmospheres has been studied thoroughly by our group. In particular, a doping concentration of 10 mol-% yttrium (Y) was found to strongly reduce or suppress secondary phase formation extending the stability of the cubic phase regime. Characterization by electrochemical impedance spectroscopy (EIS) of both, pure and Y-doped BSCF electrodes, is performed in oxygen-nitrogen mixtures as well as in CO2-containing atmospheres using symmetrical SOFC cells with Ce0.9Gd0.1O2-δ (CGO) as electrolyte from 600 to 900 °C. Analysis of the EIS data in combination with the distribution of relaxation times (DRT) reveals an enhanced tolerance of the Y-doped BSCF cathodes towards the adsorption of CO2 molecules. The enhanced stability of the cubic phase and reduced poisoning effect on the oxygen reduction reaction in CO2- containing atmospheres by doping, is further confirmed by scanning electron microscopy (SEM) and analytical (scanning) transmission electron microscopy ((S)TEM) combined with energy dispersive X-ray spectroscopy (EDXS).

Studies on order – Disorder transition, lattice expansion and ionic conductivity in aliovalent cation substituted Sm2Zr2O7 System

The effect of simultaneous aliovalent cation substitution on a pyrochlore lattice in (Sm,Zr)2-x(Sc,Ce)xO7 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) was investigated by X-ray diffraction in ambient and high-temperature conditions, electron microscopy, Raman spectroscopy and impedance spectroscopy. The samples were prepared by a conventional high temperature ceramic route. Due to comparable sizes of the ions involved, both the substituent ions diffused randomly into both cation sites leading to an order-to-disorder transition. The lattice thermal expansion coefficient decreases continuously with increased aliovalent cations substitutions which is mainly attributed to the increased diatomic bond energy of the average cation-oxygen bond, unlike the usual trends observed in the systems having phase transition from ordered pyrochlore to disordered fluorite system. The trends in ionic conductivity suggest that it is more governed by the co-operative behaviour of the mobile charge carriers. in the higher substitutions leading to higher activation energies. Hence in the case of pyrochlore type composition, in addition to the lattice disordering, co-operative behaviour of the mobile charge carriers and ionization energy of the individual bond play an important part in determining the electrical behaviour of the system.

Effect of cation substitution in Cs1–2x Ba x H2PO4 on structural properties and proton conductivity

We synthesized compounds with partial substitution of Cs+ cations in CsH2PO4 by Ba2+ cations. The structural, electron transport and thermodynamic properties of Cs1–2x Ba x H2PO4 (x = 0–0.15) were studied for the first time with the help of a set of physicochemical methods: infrared and impedance spectroscopy, X-ray diffraction and synchronous thermal analysis. The proton conductivity of Cs1–2x Ba x H2PO4 at 50–230°C was investigated in detail by impedance measurements. The formation of solid substitution solutions isostructural with CsH2PO4 (P21/m) is observed in the range of substitution degrees of x = 0–0.1, with a slight decrease in the unit cell parameters and some salt amorphization. The conductivity of disordered Cs1–2x Ba x H2PO4 in the low-temperature region increases by two orders of magnitude at x = 0.02 and increases with an increasing fraction of barium cations by three or four orders of magnitude at x = 0.05–0.1; the superionic phase transition practically disappears. At x = 0.15, heterophase systems based on salts are formed, showing high conductivity and a further decrease in the activation energy of conductivity to 0.63 eV. The conductivity of the high-temperature phase of Cs1–2x Ba x H2PO4 does not change with increasing fraction of the substituent.

First principles study of 2D layered organohalide tin perovskites.

This article describes the structure and the electronic properties of a series of layered perovskites of a general formula (A+)2(SnX4)-2 where X = I, Br and A+ is an organic cation, either formamidinium, 1-methylimidazolium, or phenylethylammonium. For each system, two conformations are considered, with eclipsed or staggered stacking of the adjacent inorganic layers. Geometry optimizations are performed at the density functional theory level with generalized gradient approximation (GGA) functional and semiempirical correction for dispersion energies; band profiles and bandgaps are computed including both spin orbit coupling (SOC) and correlation (GW) effects through an additive scheme. The theoretical procedures are validated by reproducing the experimental data of a well known 3D tin iodide perovskite. The results, combined with the calculations previously reported on PbI4 analogues, allow us to discuss the effect of cation, metal, and halide substitution in these systems and in particular to explore the possibility of changing the electronic bandgap as required by different applications. The balance of SOC and GW effects depends on the chemical nature of the studied perovskites and strongly influences the value of the simulated bandgap.

Effect of anion and cation substitution in tungsten disulfide and tungsten diselenide on conductivity and thermoelectric power

The temperature dependences of the conductivity and thermoelectric power for a series of samples W1–x Nb x S2, W1–x Nb x Se2, WS2–y Se y , W1–x Nb x S2–y Se y are studied at low temperatures. It is found that the cation substitution of W atoms with Nb leads to an increase in the conductivity and a decrease in the thermoelectric power. The anion substitution of S with Se atoms results in a simultaneous increase in the conductivity and thermoelectric power. The highest power factor among the samples studied is inherent to W0.8Nb0.2Se2.

The design and synthesis of new double perovskite (Na,Li)YMg(W,Mo)O6:Eu3+ red phosphors for white light-emitting diodes

Doubly ordered double perovskites with AAʹBBʹO6 stoichiometry are promising new host materials for Eu3+ activator ions. In this paper, double perovskite (Na,Li)YMg(W,Mo)O6:Eu3+ phosphors were synthesized by a high-temperature solid-state method and the effect of cation substitutions in this series of monoclinic-type NaYMgWO6 compounds was investigated in order to control the photoluminescence properties. The composition modulation and structural evolution of (Na,Li)YMg(W,Mo)O6:Eu3+ were studied by powder X-ray diffraction and Rietveld refinement. The microstructure and composition of the phosphor materials were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. All compounds' photoluminescence properties were measured, and the luminescence properties were related to the structural properties of the materials. The (Na,Li)YMg(W,Mo)O6:Eu3+ phosphors emit intense red light dominated by the 5D0→7F2 transition at ∼618 nm, along with other characteristic transitions from Eu3+ 4f-4f transitions when excited at ∼395 nm and ∼466 nm. The red emission intensity of Eu3+ reaches a maximum for LiYMgW0.9Mo0.1O6:Eu3+ phosphor, which exhibits high quantum efficiency and excellent thermal stability. The obtained results suggest that these new double perovskites doped with Eu3+ are promising red phosphor candidates for white light-emitting diodes.

Cation Substitution Effect on a Molecular Analogue of Perovskite Manganites

A recently synthesized Mn-containing molecule Ce2IIICeIVMn8IIIO8(O2CPh)18(HO2CPh) (Ce3Mn8III) structurally resembles the repeating unit of a bulk perovskite manganite. This resemblance brings forth the intriguing possibility of studying physical properties and effects that appear in both the molecule and the far more complex bulk perovskites, for example, effects of cation substitution, which in bulk manganites may lead to phase transition from metallic-ferromagnetic to insulating-paramagnetic as well as the colossal magnetoresistance effect. We investigate divalent and trivalent cation substitution in Ce3Mn8III molecules and its effects on ground-state magnetic configuration using first-principles-based approaches. One MnIII ion changes its valence to MnIV after trivalent (including La, Gd) cation substitution, while four MnIII ions become MnIV upon divalent (including Ca, Sr, Ba, and Pb) cation substitutions, all accompanied by vanishing local Jahn–Teller distortion around MnIV. The valence state of MnIV induced by cation substitutions can hop among Mn sites in molecule, and the calculated energy barrier for such state hopping is 0.25 to 0.6 eV/molecule. In addition, the charging energies of the Ce3Mn8III molecule and its derivatives are found to be strongly dependent on the spin direction of added electron.

Solid-state NMR and short-range order in crystalline oxides and silicates: a new tool in paramagnetic resonances.

Most applications of high-resolution NMR to questions of short-range order/disorder in inorganic materials have been made in systems where ions with unpaired electron spins are of negligible concentration, with structural information extracted primarily from chemical shifts, quadrupolar coupling parameters, and nuclear dipolar couplings. In some cases, however, the often-large additional resonance shifts caused by interactions between unpaired electron and nuclear spins can provide unique new structural information in materials with contents of paramagnetic cations ranging from hundreds of ppm to several per cent and even higher. In this brief review we focus on recent work on silicate, phosphate, and oxide materials with relatively low concentrations of paramagnetic ions, where spectral resolution can remain high enough to distinguish interactions between NMR-observed nuclides and one or more magnetic neighbors in different bonding configurations in the first, second, and even farther cation shells. We illustrate the types of information available, some of the limitations of this approach, and the great prospects for future experimental and theoretical work in this field. We give examples for the effects of paramagnetic transition metal, lanthanide, and actinide cation substitutions in simple oxides, pyrochlore, zircon, monazite, olivine, garnet, pyrochlores, and olivine structures.

New insight into the structure evolution and site preferential occupancy of Na2Ba6(Si2O7)(SiO4)2:Eu2+ phosphor by cation substitution effect

A series of single-phased Na2Ba5.99-x(Si2O7)(SiO4)2:0.01Eu2+, xMg2+ and Na2Ba5.99-y(Si2O7)(SiO4)2:0.01Eu2+, yZn2+ phosphors have been successfully prepared via the solid-state reaction. Structural refinement, luminescence property, color tuning and thermally stability were systematically investigated. The as-prepared Na2Ba6(Si2O7)(SiO4)2:0.01Eu2+ phosphor showed broad excitation bands from 250 nm to 450 nm and presented wide emission bands under 365 nm excitation, which could be decomposed by Gaussian fitting into three peaks corresponding to different Eu luminous centers named Eu1, Eu2 and Eu3. By substituting Mg2+ or Zn2+ for Ba2+, the relative intensities of Eu1, Eu2 and Eu3 emission bands changed gradually. Combined with the variation tendencies of BaO bond lengths by Rietveld refinement, the possible mechanism of the luminescence transformation of Eu2+ ion was the preferential occupancy of Eu2+ at different crystallographic sites. In addition, the introduction of Mg2+ ion expressed more obvious improvement on the thermal quenching behaviors of Eu2+ than Zn2+ ion in this matrix. The emission intensities of Eu2 and Eu3 could be enhanced from 63.2% to 82.7% and 35.1%–46.8% at 373 K compared to the initial intensity when Mg2+ concentration was 0.10. Generally, this work sheds some new lights on the design and structure explanation of preferential occupancy at different crystallographic sites in phosphors.

Effect of cation substitution on bridgmanite elasticity: A key to interpret seismic anomalies in the lower mantle.

Seismological observations show that, in some regions of the lower mantle, an increase in bulk sound velocity, interestingly, occurs in the same volume where there is a decrease in shear velocity. We show that this anti-correlated behavior occurs on cation substitution in bridgmanite by making single crystal elasticity measurements of MgSiO3 and (Mg,Fe,Al)(Si,Al)O3 using inelastic x-ray scattering in the ambient conditions. Cation substitution of ferrous iron and aluminum may explain large low shear velocity provinces in the lower mantle.

Insights into the Redox Performance of Non–stoichiometric Lanthanum Manganite Perovskites for Solar Thermochemical CO2 Splitting

AbstractPerovskite structured mixed metal oxides are one category of redox materials that recently demonstrated high potential for solar fuel production from thermochemical H2O/CO2 splitting. Substituted lanthanum manganite perovskite was previously found to be one of the most suitable candidates among the perovskite family, owing to its unique redox properties. However, the effect of synthesis method and cationic substitutions in A and B‐sites of ABO3 perovskite need to be elucidated. Herein, a systematic study was performed by implementing various synthetic strategies such as solid‐state, pechini process, glycine combustion or glucose‐assisted methods, to obtain LaxSr1‐xMnO3 (LSM) as a single phase and to determine the effect of synthesis method on the redox efficiency and performance stability for CO2 splitting. The results indicated that the materials synthesized by the pechini method were the most reactive among the series, with a stable CO production of ∼260 μmol.g−1 for x=0.5. Also, various pure phase A and B‐site substituted perovskites were synthesized by the pechini method to study the effect of such substitutions on the fuel yield of these materials. Y/Ca/Ba A‐site and Al/Fe B‐site substitutions in (La,Sr)MnO3 did not enhance the redox cycling capability when compared with LSM. It was observed that Sr was the best A‐site substituent and the presence of single Mn cation alone in B‐site was the most suitable option for promoting CO2‐splitting activity. Further, the effect of the addition of promotional agents was explored and enhancement of CO evolution was observed when compared to pure LSM.