Site-selective Substitution Research Articles

Promoting fast Na-storage and fabricating a stable layered cathode for sodium-ion batteries via site-selective substitution in triple crystallographic sites

An advanced Na0.66Ca0.05[Ni0.25Li0.11Mn0.64]O1.95F0.05 with fast Na storage and long-life was designed via a site-selective substitution of Ca2+, Li+ and F−, and investigated by a series of physical/electrochemical tests and theoretical calculation.

Synthesis of polyoxothiometalates through site-selective post-editing sulfurization of polyoxometalates.

Polyoxometalates (POMs) function as platforms for synthesizing structurally well-defined inorganic molecules with diverse structures, metals, compositions, and arrangements. Although post-editing of the oxygen sites of POMs has great potential for development of unprecedented structures, electronic states, properties, and applications, facile methods for site-selective substitution of the oxygen sites with other atoms remain limited. Herein, we report a direct site-selective oxygen-sulfur substitution method that enables transforming POMs [XW12O40]4- (X = Si, Ge) to Keggin-type polyoxothiometalates (POTMs) [XW12O28S12]4- using sulfurizing reagents in an organic solvent. The resulting POTMs retain the original Keggin-type structure, with all 12 surface W[double bond, length as m-dash]O groups selectively converted to W[double bond, length as m-dash]S without sulfurization of other oxygen sites. These POTMs show high stability against water and O2 in organic solvents and a drastic change in the electronic states and redox properties. The findings of this study represent a facile method for converting POMs to POTMs, leading to the development of their unique properties and applications in diverse fields, including (photo)catalysis, sensing, optics, electronics, energy conversion, and batteries.

Dual Site-Selective Substitution Strategy Achieving Anionic Redox Activity and Solid-Solution Reaction in P2-Type Cathode Materials

P2-type Na0.67Ni0.33Mn0.67O2 has been considered as the potential cathode for sodium-ion batteries. However, its practical application is plagued by Na+/vacancy ordering, harmful phase transition, and lattice oxygen loss. Herein, we develop a dual site-selective substitution strategy to fabricate a P2-type Na0.63Ca0.05(Ni0.26Li0.07Mn0.67)O2 cathode. The substitution of Li+ for Ni2+ introduces lone pair oxygen via forming a Li–O–Li configuration and make O 2p close to its Fermi level due to the weakened TM–O (TM: transition metal) bond, which triggers the anionic redox for charge compensation, while the introduction of Ca2+ in a Na layer enhances the electrostatic cohesion of neighboring TM layers by forming a strengthened O–Ca–O configuration, which suppresses the glide of adjacent TM layers and reduces the excessive lattice oxygen loss. Therefore, with a dual site-selective substitution strategy, the P2-type Na0.63Ca0.05(Ni0.26Li0.07Mn0.67)O2 cathode can suppress the Na+/vacancy ordering, P2–O2 phase transition, and lattice oxygen loss even at a potential of 4.35 V, achieving a reversible anionic redox and solid-solution reaction. The P2-type Na0.63Ca0.05(Ni0.26Li0.07Mn0.67)O2 cathode exhibits high discharge capacity (142.7 mA h g–1 at 20 mA g–1), excellent rate capability (57.1 mA h g–1 at 2 A g–1), and cyclic stability (a capacity retention of 83.2% after 700 cycles).

Role of site selective substitution, magnetic parameter tuning, and self heating in magnetic hyperthermia application: Eu-doped magnetite nanoparticles.

Various researchers have provided considerable insight into the fundamental mechanisms behind the power absorption of single-domain magnetic nanoparticles (MNPs) in magnetic hyperthermia applications. However, the role of all parameters pertinent to magnetic relaxation continues to be debated. Herein, to explore the role of magnetic anisotropy with the site selective substitution related to magnetic relaxation has generally been missing, which is critically essential in respective of hyperthermia treatment. Our study unravels contradictory results of rare earth (RE) interaction effects in ferrite to that of recently reported literature. Despite this, rare earth atoms have unique f-block properties, which significantly impact the magnetic anisotropy as well as the relaxation mechanism. Here, we use appropriate Eu doping concentration in magnetite and analyze its effect on the matrix. Furthermore, a positive SAR can effectively reduce the relative dose assigned to a patient to a minimal level. This study indicates that the introduction of Eu ion positively influenced the heating efficiency of the examined magnetite systems.

Utilization of sulfur function in directed catalytic C-H transformation: site-selective substitution on indole and naphthalene skeletons

Sulfur-containing functional groups, among various coordinating functions, have recently attracted attention as useful directing groups for the direct catalytic C-H transformation of aromatic and heteroaromatic compounds. Herein we briefly summarize our recent work on the site-selective substitution on indole and naphthalene skeletons using thioether directing groups under either Cp*Rh(III) or Cp*Ir(III) catalysis. The thioether groups can readily be removed and transformed to other functional groups. The reactions developed appear to be of importance in pharmaceutical and materials chemistry.

Tunable emission via dual-site occupancy in Ba2CaB2Si4O14: Bi3+, Sm3+ phosphors

Through site selective substitution engineering, the (Ba2Ca)1−2x/3BixB2Si4O14(BCBSO: xBi3+) phosphors were prepared by high-temperature solid-phase synthesis. BCBSO: xBi3+ phosphors show two obvious emission peaks at 330 and 500 nm, which are derived from the Bi3+ ions occupying Ba and Ca sites, respectively. The emission of the BCBSO: xBi3+ phosphors is tunable by varying the Bi3+ content. Through XRD and XPS, the crystal structure and element composition of the samples have been determined. The emission and decay curves confirm that the two emissions come from different luminescent centers. The two emission bands of the BCBSO: Bi3+ phosphor have distinctive thermal responses to temperatures. The Sm3+ ion activated BCBSO phosphor has a bright emission mainly at 601 nm. The temperature-dependent emission of BCBSO: 0.08Bi3+, 0.1Sm3+ indicates an optical thermometer which is designed with high relative sensitivity at 510 K (Sr = 2.3206% K–1).

Construction and Functionalization of Highly Strained N -Doped Zigzag Hydrocarbon Belts

Construction and Functionalization of Highly Strained <i>N</i> -Doped Zigzag Hydrocarbon Belts

Effects of local lattice deformation on magnetic anisotropy of W-type ferrites

We calculate the magnetic anisotropy (MA) energy of W-type ferrites, Ba(Sr)Me2Fe16O27, where Me is Mg, Co, and Ni, using a cluster model and LS coupling on magnetic ions, along with the structural data obtained via x-ray and neutron diffractions. We show that the local MA energy of Fe3＋ and Me2+ on each non-equivalent site depends strongly on the local lattice structure around the magnetic ions. The bulk MA energy calculated for W-type ferrite with Me=Mg is found to be weakly dependent on the details of local lattice structures, and the calculated result is consistent with that observed for SrMg 2Fe16O27. Results for W-type ferrite with Me=Co suggest the appearance of a cone-type magnetic structure; however, c− plane MA may appear when a small amount of Co2+ resides on 12k sites. The finding could resolve the experimental controversy for the magnetic structure of W-type ferrite with Me=Co. Based on the results, we discuss site-selective substitution of Me2+ (Me = Fe, Co, and Ni) to enhance the performance of permanent magnets.

Stabilization of layered perovskite structures via strontium substitution in Ca3Ti2O7 revealed via elemental mapping

Extensive studies have been performed on layered compounds, ranging from layered cuprates to van der Waals materials with critical issues of intergrowths and stacking faults. However, such structures have been studied less because of experimental difficulty. We present characteristic defect structures of intergrowths in the Ruddlesden–Popper Ca2.46Sr0.54Ti2O7, which is known to exhibit hybrid improper ferroelectricity. Transmission electron microscopy reveals that numerous intergrowths composed of 7 and 15 layers are introduced in the ferroelectric domains. Elemental maps demonstrate that Sr ions are selectively substituted into the perovskite layers of intergrowths. Density functional theory calculations support the site-selective substitution of Sr ions, favorably located in the intergrowths. The stabilization of the Ruddlesden–Popper phase and intergrowths via Sr substitution can be explained by the ionic-radius difference between Ca and Sr ions. The study reveals detailed defect structures originating from the layered perovskite structure of Ca2.46Sr0.54Ti2O7 and shows the usefulness of elemental mapping in probing the substitution effects in oxides.

Doping with W6+ ions enhances the performance of TiNb2O7 as an anode material for lithium-ion batteries

Monoclinic TiNb2O7 (TNO) has recently been reported as a promising anode candidate for next-generation high-energy–density lithium-ion batteries (LIBs). Unfortunately, poor electronic and ionic conductivities have limited its practical application. In this study, we prepared selectively W6+-doped TNO to overcome these drawbacks. We demonstrate herein the effect of site-selective substitution on the electrochemical performance of TNO after selectively doping its Ti4+ and Nb5+ sites with W6+ ions. The W6+ ions were incorporated into the TNO lattice without dramatically altering the lattice parameters. The ionic substitution of both Ti4+ and Nb5+ ions with W6+ ions in TNO improved the electronic conductivity because of partial reduction of the Ti4+ and Nb5+ ions as a result of charge compensation. Compared with the Nb-substituted TNO (i.e., the TNO derivative in which some of the Nb5+ ions had been replaced by W6+ ions), the Ti-substituted TNO displayed superior transfer properties, presumably because of its lower effective mass. As a result, the Ti-substituted TNO had the highest energy storage performance with its reversible capacity reaching 156.2 mA h/g, even at a rate of 20 C. After 500 repetitive cycles at 6 C, 85.5% of the capacity was retained without significant capacity fading.

Site selective substitution of Eu3+ and its effects on structural and luminescent properties of monoclinic YP5O14 pentaphosphate

Y1-xEuxP5O14 (0 < x ≤ 1) pentaphosphate polycrystallines were synthesized by solid state reaction and the effects of Eu3+ content on the structure and luminescent properties were investigated. X-ray diffraction and Infrared spectroscopy studies revealed a sequence of the composition-driven structural phase transitions C2/c → P21/c; occur at the lower Eu3+ concentration (below 40 mol% of Eu3+ doping). The spectroscopic properties in UV-excitable range of the doped compounds were investigated. Site-selective laser excitation in the 5D2 level of Eu3+ ion was performed and luminescence decay times were measured at 30 K for 5 and 10%Eu3+ -doped YP5O14 samples. The obtained results clearly show that the Eu3+ ions are distributed over two distinct crystallographic sites. Careful analysis of the main 5D2→7FJ (J = 0, 2) transitions revealed a good agreement between the observed and expected components numbers for the local symmetry predicated upon the crystal structure of the monoclinic YP5O14. The spectroscopic results are discussed and correlated with the structural data.

Magnetic anisotropy of doped Cr2O3 antiferromagnetic films evaluated by utilizing parasitic magnetization

In Cr2O3 thin films doped with Al or Ir, we have discovered a parasitic magnetization, accompanied by the antiferromagnetic order, with tunable direction and magnitude. In this study, by utilizing the parasitic magnetization, the antiferromagnetic anisotropy KAF of the doped Cr2O3 thin films was evaluated. A much greater improvement of KAF was obtained for Al-doped Cr2O3 films than that of bulk. The maximum KAF in this study was ∼9 × 104 J/m3, obtained for the Al 3.7%-doped Cr2O3 film sample. The enhancement of the magnetic dipole anisotropy KMD due to the site-selective substitution is speculated for the dominant origin of the enhancement. Furthermore, based on the obtained KAF, the influence of the parasitic magnetization on the exchange bias blocking temperature TB of the doped-Cr2O3/Co exchange coupled system was discussed. TB greatly increases when the parasitic magnetization is coupled antiparallel to ferromagnetic moment, such as Al-doped Cr2O3/Co systems.

Role of La3+ ion substitution sites on the photoluminescence properties of the SrTiO3:Eu3+ phosphors

Abstract The consequence of the site selective substitution of La3+ ions in modifying the local crystal structure in red emitting titanate perovskites and the resulting enhancement of the luminescence properties were investigated in two distinct series of red phosphors of Sr1-x-yLaxTiO3:yEu3+ and Sr1-x/2-yLaxTi1-x/2O3:yEu3+. Herein, the red emission line was found sharply magnified for the substitutions at the Sr2+ sites whereas broad peaks resembling the emission from the La2O3:Eu3+ red phosphor were detected for the substitution at both Sr2+ and Ti4+ sites, in the experiment of altering substitution sites of the La3+ ions in Sr1-xTiO3:xEu3+. The local asymmetry induced by the substitution of smaller aliovalent ions was found to enhance the luminescence properties in the phosphors with doping at the Sr2+ sites. The occupation of Eu3+ ions in an environment having La3+ ions as nearest neighbours relaxes the shielding of the 4f electrons of Eu3+ to produce a broad emission in the phosphors with double site doping. The substitution of rare earth ions at the Sr2+ sites in Sr1-x-yLaxTiO3:yEu3+ was verified by the enhancement in the intensity of the Raman modes at 184 cm−1 (TO2) and 476 cm−1 (LO2). In the case of the Sr1-x/2-yLaxTi1-x/2O3:yEu3+ phosphors, the presence of the substituent ions at the Ti4+ sites was confirmed by the increased intensity observed for the Raman mode at 114 cm 1 (TO1).

Co site preference and site-selective substitution in La–Co co-substituted magnetoplumbite-type strontium ferrites probed by 59Co nuclear magnetic resonance

In the La–Co co-substituted magnetoplumbite-type strontium (Sr–La–Co) ferrite, the base materials of the high-performance hard ferrite magnet, Co tends to occupy more than two crystallographically inequivalent sites. To reveal the Co site preference and the function of each site, 59Co nuclear magnetic resonance (NMR) spectrum was measured for the Sr–La–Co ferrites with various Co compositions. The intimate correlation between the anisotropy field and the relative intensity of the strongest NMR line clearly indicates that Co occupying the tetrahedrally coordinated 4f1 site is only responsible for the enhancement in the uniaxial anisotropy. In the samples with the Co concentrations much larger than commercial magnets, most Co selectively occupies the 4f1 site, indicating possible tuning of the magnetic performance with limited Co content in the future development. The close correlation between orbital moments and local strain is also demonstrated.

Compositional tuning and site selective excitations in SrTiO3:Y3+, Eu3+ red phosphors

The paper reports the effective control of charge transfer transitions in perovskite based titanate red emitting phosphors by site selective substitution and the resulting enhanced emission with increased colour purity. The influence of altering the substitution sites of Y3+ and Eu3+ ions at different cationic sites on local structure variations and photoluminescence properties is investigated by adopting three different protocols of composition, viz., Sr1-x-yYxTiO3: yEu3+, Sr1-x/2-yYxTi1-x/2O3: yEu3+ and Sr1-x/2-y/2YxTi1-x/2-y/2O3: yEu3+. It is found that optimum substitution of Y3+ ions at Sr2+ and Ti4+ sites in Sr0.95Eu0.05TiO3 enhances the intensity of luminescence by 14 times, accompanied by an increase in asymmetric ratio between 5D0→7F2 and 5D0→7F1 transitions from 1 to 5.6. Further, the double site substitution of Eu3+ and Y3+ ions in this system yields emission having close resemblance with that of the commercial red phosphor, Y2O3:Eu3+ with 99.9% colour purity, asymmetric ratio 6.8 and chromaticity co-ordinates (0.66, 0.34), close to that of standard red. The biexponential luminescence decay curves propose the occurrence of two nonequivalent Eu3+ crystallographic sites in Sr1-x-yYxTiO3: yEu3+ phosphors and the triple exponential behavior observed in the case of Sr1-x/2-yYxTi1-x/2O3: yEu3+ and Sr1-x/2-y/2YxTi1-x/2-y/2O3: yEu3+ shows the occurrence of three nonequivalent Eu3+ crystallographic sites. The increase in local asymmetry for Sr1-x/2-y/2YxTi1-x/2-y/2O3: yEu3+, which prompted the colour pure emission, compared to that of Sr1-x/2-yYxTi1-x/2O3: yEu3+, is evident in the increase of J-O intensity parameter (Ω2) from 6.07 × 10−20 to 8.98 × 10−20 cm2.

Site-selective substitution of gold atoms in the Au24(SR)20 nanocluster by silver

The synthesis and structure determination of atomically precise alloy nanoclusters have attracted much attention in recent research. Herein, we report a new alloy nanocluster Au24-xAgx(TBBM)20 (x∼1) synthesized via a ligand-exchange-induced size/structure transformation method. Its X-ray structure is obtained successfully and the dopant Ag is found to occupy three special positions in the kernel, rather than equivalently over all the kernel sites. This selective occupancy is interesting and DFT calculation results suggest that the relative oxidation state (rationalized as difference in the charge) of the Ag when doped into the cluster is likely determining the most favorable doping positions. This work provides a new strategy for controlled synthesis of new Au-Ag nanoclusters and also reveals a new scenario for the doping position of Ag atoms in Au-Ag bimetal nanoclusters.

Mixed-Metal Hybrid Polyoxometalates with Amino Acid Ligands: Electronic Versatility and Solution Properties.

Eight new members of a family of mixed-metal (Mo,W) polyoxometalates (POMs) with amino acid ligands have been synthesized and investigated in the solid state and solution using multiple physical techniques. While the peripheral POM structural framework is conserved, the different analogues vary in nuclearity of the central metal-oxo core, overall redox state, metal composition, and identity of the zwitterionic α-amino acid ligands. Structural investigations reveal site-selective substitution of Mo for W, with a strong preference for Mo to occupy the central metal-oxo core. This core structural unit is a closed tetrametallic loop in the blue reduced species and an open trimetallic loop in the colorless oxidized analogues. Density functional theory calculations suggest the core as the favored site of reduction and reveal that the corresponding molecular orbital is much lower in energy for a tetra- versus trimetallic core. The reduced species are diamagnetic, each with a pair of strongly antiferromagnetically coupled MoV centers in the tetrametallic core, while in the oxidized complexes all Mo is hexavalent. Solution small-angle X-ray scattering and circular dichroism (CD) studies indicate that the hybrid POM is stable in aqueous solution on a time scale of days within defined concentration and pH ranges, with the stability enhanced by the presence of excess amino acid. The CD experiments also reveal that the amino acid ligands readily exchange with other α-amino acids, and it is possible to isolate the products of amino acid exchange, confirming retention of the POM framework. Cyclic voltammograms of the reduced species exhibit an irreversible oxidation process at relatively low potential, but an equivalent reductive process is not evident for the oxidized analogues. Despite their overall structural similarity, the oxidized and 2e-reduced hybrid POMs are not interconvertible because of the respective open- versus closed-loop arrangement in the central metal-oxo cores.

(C6 F5 )3 B Catalyzed Chemoselective and ortho-Selective Substitution of Phenols with α-Aryl α-Diazoesters.

The development of an efficient method for the site-selective substitution of unprotected phenols has long been considered as an attractive but challenging task. Herein, we describe a highly chemo- and ortho-selective substitution reaction of phenols with α-aryl α-diazoacetates with commercially available (C6 F5 )3 B as the catalyst. This reaction proceeds under simple and mild conditions with high efficiency, it features a wide substrate scope and can be easily scaled up.

On the Question of Site-Selective Ligand Exchange in Carboxylate-Substituted Metal Oxo Clusters.

Reaction of [Ti4Zr4O6(OBu)4(OMc)16] (OMc = methacrylate) with acetylacetone (acacH) resulted in dissection of the cluster and formation of [Ti(OBu)2(acac)2] and the smaller cluster [Ti2Zr4O4(OMc)16]. In contrast, the same reaction with [Zr6O4(OH)4(OOCR)12]2·6RCOOH (R = Et, CH2CH=CH2) led to site-selective substitution of two carboxylate ligands and formation of isostructural [Zr6O4(OH)4(OOCR)12–x(acac)x]2·6RCOOH (x ≤ 1).

Gallium hole traps in irradiated KTiOPO4:Ga crystals

Nominally pure and gallium doped single crystals of potassium titanyl phosphate (KTiOPO4) have been studied by Electron Paramagnetic Resonance at low temperatures before and after irradiation. Irradiation with 20 MeV electrons performed at room temperature and liquid nitrogen temperature caused an appearance of electrons and holes. Gallium impurities act as hole traps in KTiOPO4 creating Ga4+ centers. Two different Ga4+ centers were observed, Ga1 and Ga2. The Ga1 centers are dominant in Ga-doped samples. For the Ga1 center, a superhyperfine structure with one nucleus with nuclear spin ½ was registered and attributed to the interaction of gallium electrons with a phosphorus nucleus or proton in its surrounding. In both Ga1 and Ga2 centers, Ga4+ ions substitute for Ti4+ ions, but with a preference to one of two electrically distinct crystallographic positions (site selective substitution). The Ga doping eliminates one of the shortcomings of KTP crystals—ionic conductivity of bulk crystals. However, this does not improve significantly the resistance of the crystals to electron and γ-radiation.