Trigonal Prismatic Sites Research Articles

Toward an understanding of the role of structural organization and point defects in the formation of functional pure and Tm3+-doped (Ca3-xSrx)(VO4)2 for diode-pumped and SRS lasers

Whitlockite-type polycrystalline and single-crystal (Ca3−xSrx)(VO4)2 solid solutions were successfully obtained by solid-state synthesis and Czochralski method, respectively. Synchrotron powder X-ray diffraction allowed to reveal actual compositions of polycrystalline (Ca3−xSrx)(VO4)2 solid solutions with х = 0.3, 0.54, 0.9, 1.2, 1.35 refined by Rietveld method, which are slightly different from the initial ones. Synchrotron X-ray single-crystal diffraction analysis of Czochralski-grown undoped (Ca1.8Sr1.2)(VO4)2 (CSVO) revealed its actual composition (Ca2.982(11)Sr0.031)(VО4)2. Difference between initial and refined compositions is analyzed and discussed. In the CSVO structure, Sr2+ ions partially occupy monocapped Ca2 and two-capped Ca3 trigonal prismatic sites and Ca2+ ions are in the Ca5 and Ca5A sites, forming two independent strongly distorted tetrahedral Ca5O4 and octahedral Ca5AO6 oxygen environments. Synchrotron X-ray single-crystal diffraction analysis together with X-ray absorption spectroscopic study of (Сa2.4Sr0.6)(VO4)2 doped with 0.5 and 1.0 wt% Tm2O3 (СSVO:0.5Tm and CSVO:1.0Tm) allowed to establish the absence of cations in the Ca5A site, the localization of Sr2+ ions in the Ca2 and Ca3 sites and additionally in octahedral Ca4 site together with Tm3+ ions, forming an individual eight-coordinated environment, in the refined compositions (Ca2.776(7)Sr0.210(4)Tm3+0.008(1))(VO4)2 and (Ca2.855(7)Sr0.140(7)Tm3+0.009(1)□0.022(7))(VO4)2, respectively, with vacancies (marked by a square,□) in the two-capped Ca1 trigonal prismatic site in the CSVO:1.0Tm structure. Additional interstitial oxygen, participating in the formation of the Tm3+ environment, found in the residual electron density, together with vacancies in the Ca1 site provides electroneutrality to CSVO:0.5Tm and CSVO:1.0Tm crystals. Several structural models of the coordination environment of Tm3+ ions were analyzed and the most correct one was proposed.

Structure and titanium distribution of feiite characterized using synchrotron single-crystal X-ray diffraction techniques

Abstract A solid solution of the mineral feiite (Fe3TiO5) was recently discovered in a shock-induced melt pocket of the Shergotty martian shergottite. It is particularly interesting for its potential as an indicator of pressure-temperature (P-T) and oxygen fugacity in martian crustal and mantle material. To date, complete crystallographic analysis of feiite has not been conducted, as the mineral was previously analyzed by electron backscatter diffraction on micrometer-size grains (Ma et al. 2021). Here we report a convergent crystal-structure model for feiite based on synchrotron single-crystal X-ray diffraction data collected on three grains of feiite synthesized at 12 GPa and 1200 °C. Feiite adopts the CaFe3O5 structure type (Cmcm, Z = 4), which is composed of two octahedral M1 and M2 sites and one trigonal prismatic M3 site (M = metal) in a ratio of 1:2:1. The three feiite grains with composition Ti0.46–0.60Fe3.54–3.40O5 were best modeled by substituting Ti4+ into only the octahedral M2 site, accounting for 30% of this site. Comparisons of the measured average bond lengths in the coordination polyhedra with the optimized Ti4+–O, Fe2+–O, and Fe3+–O bond lengths suggest that ferrous iron occupies the trigonal M3 site, while iron is mixed valence in the octahedral M1 and M2 sites. The Ti4+ and Fe3+ content constrained by our crystal-chemical analyses suggests that at least ~30% of the available iron must be ferric (i.e., Fe3+/Fetotal = 0.3) for the sample synthesized at 12 GPa and 1200 °C and higher P-T conditions may be needed to form the end-member feiite (Fe32+TiO5).

Insights into P-Type Iron- and Manganese-Based Layered Sodium Cathodes

Sodium-ion batteries are a low-cost alternative to lithium-ion batteries in large-scale electric energy storage applications due to the natural-abundant sodium resources and similar working principle to LIBs during cycling [1]. Until now, many different cathodes for SIBs have been studied, including layered transition metal oxides (NaxTMO2, x<1), Prussian blue-type compounds, polyanionic and organic compounds [2]. Among them, NaxTMO2 has been extensively reported, due to its layered structure, easy synthesis, promising electrochemical properties, and feasibility for commercial production [3]. According to the occupation of Na ions and oxygen stacking, NaxTMO2 can be classified into P2 and O3 types, where P2 structure contasins trigonal prismatic sites of sodium ions and ABBA oxygen stacking sequence and O3 structure represents octahedral sites of sodium ions and ABCABC oxygen stacking sequence [4]. At low potentials (<2.0V), there is a phase transition P2-P2’ connected with the manganese redox activity and the Jahn-Teller distortion [5]. Compared with O3-type structure, P2-type NaxTMO2 provides higher specific capacity and better structural stability due to the lower diffusion barriers through Na-ion prismatic sites [6].Iron- and manganese-based layered oxides for sodium-ion batteries have attracted renewed interest due to their low cost and environmental friendliness. However, the phase change at high voltage and the Jahn-Teller effect lead to shortening cycle life and poor rate capability. We design a structure optimization of the Na2/3Mn1/2Fe1/2O2 cathode through a partial substitution of Fe3+ to explore the mechanism of redox activity of P2-type Mn/Fe-based layered cathodes. We present the investigation of the effect of simultaneous cationic substitution on the electrochemical performance and structural stability of P2-Na2/3Mn1/2Fe1/2O2. The obtained material exhibits a solid solution mechanism during desodiation/resodiation process and delivers an initial discharge capacity of 168 mA h g-1 at 0.1C and a capacity retention of 80% after 50 cycles. The modified P2 composition has stable cycle performance in the potential window from 1.5V to 4.5V. The main focus here is to understand the fundamental electrochemical mechanism of this layered cathode material by exploring the redox processes of transition metal cations and oxygen anions upon cycling. In situ X-ray synchrotron radiation diffraction is used to explore the structural changes during cycling. In situ X-ray Absorption Near Edge Spectroscopy is used to elucidate the local electronic structure of Fe and Mn atoms as it evolves throughout this electrochemical process. 23Na solid-state nuclear magnetic resonance spectroscopy is used to investigate Na coordination environments during desodiation/resodiation. Ex situ soft X-ray absorption spectroscopy results reveal the participation of oxygen anions in the electrochemical reactions. Thus, we explore the redox reactions of transition metals and oxygen to explain the mechanism and the electrochemical performance for the P2-type Mn/Fe-based layered cathodes.

Symmetry-Mismatched SBU Transformation in MOFs: Postsynthetic Metal Exchange from Zn to Fe and Its Effects on Gas Adsorption and Dye Selectivity.

This research explores the alteration of metal-organic frameworks (MOFs) using a method called postsynthetic metal exchange. We focus on the shift from a Zn-based MOF containing a [Zn4O(COO)6] secondary building unit (SBU) of octahedral site symmetry (ANT-1(Zn)) to a Fe-based one with a [Fe3IIIO(COO)6]+ SBU of trigonal prismatic site symmetry (ANT-1(Fe)). The symmetry-mismatched SBU transformation cleverly maintains the MOF's overall structure by adjusting the conformation of the flexible 1,3,5-benzenetribenzoate linker to alleviate the framework strain. The process triggers a decrease in the framework volume and pore size alongside a change in the framework's charge. These alterations influence the MOF's ability to adsorb gas and dye. During the transformation, core-shell MOFs (ANT-1(Zn@Fe)) are formed as intermediate products, demonstrating unique gas sorption traits and adjusted dye adsorption preferences due to the structural modifications at the core-shell interface. Heteronuclear clusters, located at the framework interfaces, enhance the heat of CO2 adsorption. Furthermore, they also influence the selectivity of the dye size. This research provides valuable insights into fabricating novel MOFs with unique properties by modifying the SBU of a MOF with flexible organic linkers from one site symmetry to another.

First-principles study of oxygen segregation and its effect on the embrittlement of molybdenum symmetrical tilt grain boundaries

Elemental segregation can pronouncedly affect the cohesion of grain boundary (GB), which shows tremendous potential in altering the thermal stability and mechanical properties of metals. In this work, we systematically investigate the segregation behavior of oxygen (O) atoms at a series of GBs in molybdenum (Mo), using first principles calculations, to reveal the effect of O segregation on the embrittlement of Mo GBs. It is found that O atoms energetically prefer to segregate at sites in both the GB core and the vicinity of the GB plane. The strengthening energy is strongly associated with the GB structure, and it linearly decreases with the continuous deformation of the GB. O segregation can lead to an increase in the volume of polyhedron sites and a decrease in the charge density among adjacent Mo atoms across the GB plane. The weakened Mo-Mo bonds are believed to disrupt the GB cohesion, thereby increasing the intergranular cleavage tendency of Mo. Interestingly, O segregation does not always trigger GB embrittlement. The Σ5(310)[001] symmetric tilt grain boundary (STGB) with an O atom doped at the cap trigonal prism (CTP) site possesses a strength as strong as the clean GB. O segregation promotes the generation of Mo-O bonds across the GB plane, which counteracts the adverse effect of weakened Mo-Mo bonds. This work deepens the understanding of the possible effect of O segregation on GB cohesion of Mo from the atomic and electronic scales, providing guidance for enhancing the mechanical properties of Mo via GB structure optimization.

(Digital Presentation) Cobalt-Free Spinel-Layered Composite As a Positive Electrode for Sodium-Ion Batteries

Sodium-ion battery (NIB) system is an emerging technology and can be considered as a suitable alternative for lithium-ion batteries (LIBs) due to the large abundance and distribution of sodium on earth and similar working principles to LIB. Among those cathodes for NIBs, layered transition metal oxides (NaxMO2) receive more attention because of their higher capacity, appropriate operating potentials, higher ionic conductivity, and ease of synthesis [1]. According to the stacking sequence of oxygen layers and Na occupation sites, layered transition metal oxides are mainly classified as P2, O3, P3, and O2 structures. The letters P and O imply that the sodium occupies trigonal prismatic sites and octahedral sites, respectively. The numbers indicate the no. of oxygen stacking layers [2]. Among these, the P2 type layered transition metal oxides gained more recognition as cathode materials for NIBs due to their superior rate capability from the migration of sodium ions through the face-sharing trigonal prismatic sites [3]. However, the intercalation/de-intercalation of large sodium ions creates some structural deterioration and irreversibility. Designing multiphase materials is an effective strategy to improve the electrochemical performance of the material to avail the synergistic effects from each phase [3,4]. In this work, a cobalt-free layered-spinel composite was synthesized by sol-gel method as positive electrode material for NIBs. It is highly attractive, as it is cobalt-free and hence, cost-effective and environmentally benign. The layered phase provides a smoother diffusion pathway and the spinel phase could enhance the electronic conductivity [3,4]. The presence of layered and spinel phases was confirmed by the X-ray diffraction technique. Scanning electron microscopic investigations reveal particles of layered morphology with well-defined edges.The electrochemical investigations were done in Na-half cells in the voltage range of 1.5- 4.0 V vs. Na+/Na. The cyclic voltammogram of the layered-spinel composite in Na half-cell shows two sets of peaks corresponding to the redox activity of Mn and Ni. When the upper cut-off voltage was increased above 4 V, contributions from the Fe electrochemical activity were also observed. To investigate the sodium storage performance, galvanostatic charge-discharge studies were done. The material displayed an initial discharge capacity of 171 mAh g-1 and promising high-rate behavior. To investigate the electrochemical mechanism, in operando X-ray absorption spectroscopic studies were done and the results will be discussed in detail. Acknowledgement A.Bhaskar gratefully acknowledges financial support from “DST-IISc Energy Storage Platform on Supercapacitors and Power Dense Devices” through the MECSP-2K17 program under grant no. DST/TMD/MECSP/2K17/20”. D. Dixon acknowledges the financial support from SERB, New Delhi, India through Ramanujan Fellowship, under the grant number SB/S2/ RJN-162/2017. A. Thottungal is grateful to CSIR New Delhi for the CSIR SRF grant and AcSIR, Ghaziabad- 201002, India. DESY, Hamburg, Germany is acknowledged for the beamtime allocation at the P65 beamline at PETRA III and the beamline scientist Dr. Edmund Welter is acknowledged for his support. This work contributes to the research performed at CELEST, and was partially funded by the DFG under Project ID 390874152 (POLiS Cluster of Excellence). Reference D. Slater , D. Kim , E. Lee and C. S. Johnson , Adv. Funct. Mater., 2013, 23 , 947 —958 Delmas, C. Fouassier and P. Hagenmuller, Phys. B, 1980, 99, 81 Zheng, P. Yan, W. H. Kan, C. Wang and A. Manthiram, J. Electrochem. Soc., 2016, 163(3), A584 Hou , J. Yin , X. Lu , J. Li , Y. Zhao and X. Xu , Nanoscale, 2018, 10 , 6671 —6677

Density Functional Theory Approach for Muon Sites Estimation in Mn&lt;sub&gt;3&lt;/sub&gt;Sn

We report the estimation of muon sites inside Mn3Sn using density functional theory based on the full-potential linearized augmented plane wave (FLAPW) calculation. Our calculation shows that the Perdew–Burke–Ernzerhof (PBE) Generalized-Gradient Approximation (GGA) functional is closer to the experimental structure compared to the von Barth-Hedin Local Density Approximation (LDA)-optimized geometry. The PBE GGA is therefore subsequently used in FLAPW post-calculation for the electrostatic potential calculation to find the local minima position as a guiding strategy for estimating the muon site. Our result reveals at least two muon sites of which one is placed at the center between two Mn-Sn triangular layers (A site) and the other at the trigonal prismatic site of Sn atom (B site). The total energy of Mn3Sn system in the presence of muon at A site or B site are compared and we find that A site is a more favorable site for muon to stop.

High-pressure and high-temperature phase relations in the systems KAlSiO4-MgAl2O4 and CaAl2O4-MgAl2O4: Stability fields of NAL phases

Phase relations in the system KAlSiO4-MgAl2O4 were determined up to 28 GPa and 1500 °C. A hexagonal aluminous (NAL) phase is stable above 16 GPa with a narrow compositional range of (1-x)KAlSiO4·xMgAl2O4, x ≈ 0.61–0.65. The stability field of NAL phase in the CaAl2O4-MgAl2O4 system was also determined up to 29 GPa and 1200 °C, resulting in a compositional range of NAL phase of (1-x)CaAl2O4·xMgAl2O4, x ≈ 0.67–0.74. Compared with the above compositional widths of NAL phases, the stability field of NAL phase in the NaAlSiO4-MgAl2O4 system is much wider, (1-x)NaAlSiO4·xMgAl2O4, x ≈ 0.47–0.70 (Ono et al., 2009). The difference may be caused by the fact that both of Na+ and Mg2+ with similar cation sizes enter 6-fold trigonal prism sites in the NAL structure. In the system KAlSiO4-MgAl2O4, KAlSiO4 kalsilite and MgAl2O4 spinel coexist below ~8 GPa, above which an assemblage of MgAl2O4 spinel, corundum, K2Mg2Si2O7-rich phase X and pyrope becomes stable. The assemblage further changes into NAL phase at 16 GPa. These results and compositional similarity suggest that composite inclusions consisting of MgAl2O4-rich spinel and kalsilite-nepheline solid solution found in diamonds in the Juina-5 kimberlite, Brazil, were formed below ~8 GPa presumably by decomposition of K-rich NAL phase trapped in diamonds in the lower mantle conditions.

Quaternary Arsenides REHfCu2+δAs3 (RE = La-Nd; δ ≈ 0.17): Superstructures of the Zr2Ni3P3-Type Structure.

The quaternary rare-earth-metal hafnium copper arsenides REHfCu2+δAs3 (where RE = La-Nd; δ ≈ 0.17) were obtained by direct reactions of the elements at 1073 K. They adopt a noncentrosymmetric orthorhombic crystal structure (space group Pmn21; a = 397.73(3)-392.37(2) pm, b = 1062.68(8)-1055.23(6) pm, c = 1307.25(9)-1291.40(7) pm for RE = La-Nd), which can be considered to be a Cu-deficient superstructure of the ternary Zr2Ni3P3-type structure found among metal-rich pnictides with a metal-to-nonmetal ratio close or equal to 2:1. The RE atoms occupy trigonal prismatic (CN6) and monocapped trigonal prismatic (CN7) sites, the latter resulting from the occurrence of Cu vacancies, the Hf atoms occupy octahedral sites (CN6), and the Cu atoms occupy tetrahedral sites (CN4). Band structure calculations on idealized models LaHfCu2As3 and LaHfCu2.5As3 suggest that the bonding situation improves by fractional occupation of the Cu3 site via depopulation of antibonding Cu-As states.

On symmetry of rare-earth scandium borate RESc3(BO3)4 (RE = Ce, Nd) laser crystals

Symmetry, structural features, and point defects in the Czochralski-grown single crystals with the initial compositions CeSc3(BO3)4 (CSB-1.0) and Nd1.25Sc2.75(BO3)4 (NSB-1.25) are studied by the full-profile Rietveld method on synchrotron X-ray powder data. The space group P321 is first established and confirmed for CSB-1.0 and NSB-1.25 crystals, respectively. The features of synchrotron diffraction patterns that distinguish the space group Р321 from the R32 one are given. The site occupancies in both structures are refined and resulted in the stoichiometric composition for CeSc3(BO3)4, which coincides with the nominal composition, and the composition [Nd0.455(1)Sc0.045(39)(1)]Nd0.500(2)Sc3(BO3)4 ((Nd0.955Sc0.045(39))Sc3(BO3)4), which differs from the initial Nd1.25Sc2.75(BO3)4 one, with partial occurrence of Sc ions in one of the trigonal-prismatic sites. An analysis of literature data on single crystals with the general formula RESc3(BO3)4 (RE ​= ​Ce, Pr, Nd) together with the results of present study of powdered single crystals gives reason to exclude a crystallization of these compounds and solid solutions based on them in the space group R32. The correlation between the refined compositions of RESc3(BO3)4 crystals (RE ​= ​La, Ce, Pr, Nd), having a specific space group, and compositions of initial charge is given.

Synthesis by solution combustion of inorganic blue pigments MFe2(P2O7)2: Influence of the cation M (Zn2+, Co2+, Cu2+, and Mg2+) on their optical properties

The aim of this work is to find the effect of the cation M (Zn2+, Co2+, Cu2+, and Mg2+) on the optical properties of MFe2(P2O7)2 synthesized by solution combustion. The crystal structure of the powders was determined by X-ray diffraction (XRD), Infrared spectroscopy (IR), and Raman spectroscopy. The presence of Fe2+ and Fe3+ ions in the pigments was studied using Mössbauer spectroscopy (MS), these spectra showed at 298 K three well-defined Mössbauer doublets belonging to octahedral S1 site and S3 site of Fe3+ and trigonal prismatic S2 site of Fe2+, confirming the formation of ZnFe2(P2O7)2, CoFe2(P2O7)2 and MgFe2(P2O7)2 structures, while in CuFe2(P2O7)2 there is no presence of trigonal prismatic S2 site of Fe2+ ion. Therefore, the mechanism of color on the ZnFe2(P2O7)2, CoFe2(P2O7)2, and MgFe2(P2O7)2 structures are due to intervalence charge transfer between Fe2+ and Fe3+ ions. In contrast, on the CuFe2(P2O7)2 structure is due to d-d electronic transitions of the Cu2+ ion, these evidences can also be observed in their UV–Vis diffuse reflectance spectra. Moreover, the inorganic blue pigment MgFe2(P2O7)2, which is free of cobalt, copper, and zinc could be used as a pigment in applications of low temperatures.

Photoluminescence properties of Eu2+ activator ions in the SrS–Ga2S3 system

Various phosphors with activator Eu2+ ions in the host compositions of the SrS–Ga2S3 system were investigated on the basis of the temperature–composition diagram for the SrS–Ga2S3 system. The phosphors were prepared by solid-state reaction using stoichiometric amounts of SrS, EuS, and Ga2S3. The SrS:Eu2+, Sr2Ga2S5:Eu2+, and SrGa2S4:Eu2+ emitted red (610 nm), yellow-green (550 nm), and green (532 nm) light, respectively. For the SrS:Eu2+ phosphor, the host SrS had a cubic crystal structure and the Eu2+ activator ions were substituted at the Sr2+ sites of the host, surrounded by six sulfur anions (S2−) in Oh group symmetry. In the Sr2Ga2S5:Eu2+ phosphor, the Eu2+ cations occupied monocapped trigonal prism sites surrounded by seven S2– anions. In the SrGa2S4:Eu2+ phosphor, the Eu2+ cations occupied square anti-prismatic sites surrounded by eight S2– anions in D4d symmetry. The photoluminescence properties of the SrS:Eu2+, Sr2Ga2S5:Eu2+, and SrGa2S4:Eu2+ phosphors prepared at temperatures from 700 to 1000 °C were investigated.

Short O–O separation in layered oxide Na0.67CoO2 enables an ultrafast oxygen evolution reaction

The layered oxide Na0.67CoO2 with Na+ occupying trigonal prismatic sites between CoO2 layers exhibits a remarkably high room temperature oxygen evolution reaction (OER) activity in alkaline solution. The high activity is attributed to an unusually short O-O separation that favors formation of peroxide ions by O--O- interactions followed by O2 evolution in preference to the conventional route through surface O-OH- species. The dependence of the onset potential on the pH of the alkaline solution was found to be consistent with the loss of H+ ions from the surface oxygen to provide surface O- that may either be attacked by solution OH- or react with another O-; a short O-O separation favors the latter route. The role of a strong hybridization of the O-2p and low-spin CoIII/CoIV π-bonding d states is also important; the OER on other CoIII/CoIV oxides is compared with that on Na0.67CoO2 as well as that on IrO2.

(Invited) The P2 Sodium Layered Oxides in Na Ion-Batteries

The development of very large scale renewable energy systems requires optimizing the lifetime, the price and the material availability. From these points of view, sodium based batteries have to be investigated. Our research group studied layered oxides as positive electrode for 30 years. Now a special focus is devoted on P2-type layered phases.One of the main interests of this structure is the existence of an ion conduction plane made of face sharing trigonal prims which exhibits a high ionic diffusivity thanks to the existence of a large bottleneck (oxygen rectangle) for sodium diffusion. This structure is able to accommodate a lot of transition metal cations, allowing the optimization of the properties by cationic substitution.Studies was performed on the P2- Nax(Mn,Co)O2, Nax(Mn,Fe)O2, Nax(Mn,Fe,Co)O2 and Nax(Mn,Fe,Ni)O2 systems with various amounts of transition metal ions. All materials crystallize in the hexagonal system (P63/mmc space group). The structure is made of MO2 slabs built of corner sharing MO6 octahedra. The sodium ions are in trigonal prismatic sites. Half of them share edges (Nae) with the MO6 octahedra while the other one share faces (Naf). As the two types of prisms share a common face, they cannot be occupied simultaneously at the atomic scale. The sodium distribution depends of the cationic charge distribution and of the sodium amount is order to minimize the Na+-Na+ and Na+-Mn+ electrostatic repulsions. This lead to a large number of ordered Na+ distribution as it was shown in the case of the NaxCoO2 systems. In materials with several transition metal cations, their statically distribution in the slabs prevent generally from Na+ ordering.In all systems, the P2 type packing is preserved in the 0.3 < x < 1 range. The fully intercalated phase is difficult to be obtained due to the very low ionic conductivity when the amount of vacancies is very small. For these compositions, only the Naf prisms are occupied. The specific capacity is in the 130-170 mAh.g-1 depending on the voltage range.For large deintercalation amounts (x < 0.3) a large polarization occurs in discharge. The GITT experiment shows that this “polarization” is not kinetics. The in-situ XRD during battery cycling from the Na0.7(Mn0.5Fe0.3Ni0.2)O2 phase shows the existence at high voltage of two phases with different interslab distances. The change in the shape of the diffraction line profiles is characteristic of the continuous formation of stacking faults. For manganese based materials, structural distortions (Jahn-Teller effect) occurs at the end of discharge. Even if these distortions are fully reversible, they have a very bad effect on the cycling behavior.In the intermediate composition range the reversibility is very good. Therefore, these materials can be interesting for practical applications in stationary batteries due to their low price and the availability of the raw materials.

Effect of thermal treatment upon the structure incommensurability and magnetism of the spin chain oxide Sr3CaMn2CoO9+δ

Spin chain oxides with different incommensurate hexagonal composite structures have been obtained for a same cationic composition according to the formula Sr3CaMn2CoO9+δ by varying the thermal treatments in air. The cell parameters of these oxides, a∼9.5 Å, c1∼2.5 Å and c2∼3.8 Å, decrease as the deviation from commensurability increases, i.e. as γ = c1/c2 decreases and falls away from 2/3. All the investigated samples exhibit a signature typical of single-ion magnet (SIM), as well as a competition between single-chain magnet (SCM) and short-range ordering (SRO) responses. The balance between the latter two features turns out to be clearly affected by the thermal treatment. The sample quenched at 1450 °C, which structure is almost commensurate (γ = 0.6659, close to 2/3), exhibits a SCM behavior without sizeable indication of SRO, whereas, for the samples annealed at 1300 °C or furnace cooled from 1400 °C with an incommensurate structure (γ = 0.6622–0.6631), the SRO character is strongly reinforced. These changes in the magnetic response are assumed to originate from incorporation of extra oxygen into the commensurate stoichiometric Sr3CaMn2CoO9 oxide, leading to the replacement of some trimeric units built up of two octahedra and one trigonal prism (Oh2Tp) by tetrameric units (Oh3Tp) containing one additional octahedron, with a concomitant appearance of cationic vacancies in the trigonal prismatic sites.

High-Pressure Phase Relations and Crystal Structures of Postspinel Phases in MgV2O4, FeV2O4, and MnCr2O4: Crystal Chemistry of AB2O4 Postspinel Compounds.

We have investigated high-pressure, high-temperature phase transitions of spinel (Sp)-type MgV2O4, FeV2O4, and MnCr2O4. At 1200-1800 °C, MgV2O4 Sp decomposes at 4-7 GPa into a phase assemblage of MgO periclase + corundum (Cor)-type V2O3, and they react at 10-15 GPa to form a phase with a calcium titanite (CT)-type structure. FeV2O4 Sp transforms to CT-type FeV2O4 at 12 GPa via decomposition phases of FeO wüstite + Cor-type V2O3. MnCr2O4 Sp directly transforms to the calcium ferrite (CF)-structured phase at 10 GPa and 1000-1400 °C. Rietveld refinements of CT-type MgV2O4 and FeV2O4 and CF-type MnCr2O4 confirm that both the CT- and CF-type structures have frameworks formed by double chains of edge-shared B3+O6 octahedra (B3+ = V3+ and Cr3+) running parallel to one of orthorhombic cell axes. A relatively large A2+ cation (A2+ = Mg2+, Fe2+, and Mn2+) occupies a tunnel-shaped space formed by corner-sharing of four double chains. Effective coordination numbers calculated from eight neighboring oxygen-A2+ cation distances of CT-type MgV2O4 and FeV2O4 and CF-type MnCr2O4 are 5.50, 5.16, and 7.52, respectively. This implies that the CT- and CF-type structures practically have trigonal prism (six-coordinated) and bicapped trigonal prism (eight-coordinated) sites for the A2+ cations, respectively. A relationship between cation sizes of VIIIA2+ and VIB3+ and crystal structures (CF- and CT-types) of A2+B23+O4 is discussed using the above new data and available previous data of the postspinel phases. We found that CF-type A2+B23+O4 crystallize in wide ionic radius ranges of 0.9-1.4 Å for VIIIA2+ and 0.55-1.1 Å for VIB3+, whereas CT-type phases crystallize in very narrow ionic radius ranges of ∼0.9 Å for VIIIA2+ and 0.6-0.65 Å for VIB3+. This would be attributed to the fact that the tunnel space of CT-type structure is geometrically less flexible due to the smaller coordination number for A2+ cation than that of CF-type.

Orthotellurate Cathode Frameworks for Potassium-Ion Battery

As a low-cost alternative to lithium, potassium-ion battery (PIB) has recently attracted great attention. PIB can be a high-voltage contender considering the significantly negative potential of the K+/K redox couple, which is close to or even lower than Li depending on the solvent [1,2]. Nonetheless, the large ionic radius of potassium coupled with significant strain accompanying potassium (de)insertion, the number of potassium-based compounds (particularly, cathode materials) that can practically be utilized is limited. Exploration of the broad class of orthotellurate frameworks based on K6TeO6 has led us to identify new cathode frameworks that can reversibly (de) insert potassium ions, not only at high voltage but also at decent capacities. Figure 1 shows a layered crystal structure of K2Ni2TeO6, a new cathode material synthesized via the conventional solid state method. The structure entails a honeycomb-like network of NiO6 octahedra (in purple) surrounding each TeO6 octahedon (in blue). Potassium atoms (in brown) occupy trigonal prismatic sites sharing faces with NiO6 or TeO6 octahedra. During the meeting, we will highlight the synthesis, crystal structure and electrochemical performance of K2Ni2TeO6 as a new cathode material for PIB [3]. Reference s : [1] Y. Marcus, Pure Appl. Chem., 57 (1985) 1129-1132. [2] S. Komaba, T. Hasegawa, M. Dahbi and K. Kubota, J . Power Sources, 60 (2015) 172-175. [3] T. Masese, K. Yoshii, T. Okumura, H. Senoh, M. Shikano, manuscript submitted. Figure 1. Schematic presentation of K2Ni2TeO6 viewed along [110] direction. NiO6 tetrahedra (blue), TeO6 octahedra (purple), and K atoms (brown) are illustrated. Figure 1

Structural Instability in Single-Crystal Rare-Earth Scandium Borates RESc3(BO3)4

An influence of the type of rare-earth (RE) cation and composition of initial charge on the symmetry, structural features, and real composition of the single-crystal huntite family rare-earth scandium borates RESc3(BO3)4 (RE = Ce, Pr, Nd), grown by the Czochralski method, has been studied by single crystal and powder X-ray diffraction. A crystallization of scandium borates in the space groups C2/c (RE = Ce) and P321 (RE = Pr, Nd) has been found. Disordering in the structures with the space group P321, which has been first determined for the huntite family compounds, is due to the RE and Sc redistribution over two trigonal-prismatic sites to maintain the stability of crystal structure. The crystals grown from the initial charges NdSc3(BO3)4 and Nd1.25Sc2.75(BO3)4 are characterized by the greatest disordering, and they are isotypic, rather than isostructural, to the crystals obtained from the charges Pr1.1Sc2.9(BO3)4 and Pr1.25Sc2.75(BO3)4. A change in the unit cell parameters and interatomic distances depe...

Synthesis and crystal structure of Mg-bearing Fe9O11: New insight in the complexity of Fe-Mg oxides at conditions of the deep upper mantle

A novel Mg-bearing iron oxide Mg0.87(1)Fe4.13(1)2+Fe43+O11 was synthesized at 12 GPa and 1300 °C using a large volume press. Rietveld structural analysis was conducted with a laboratory X‑ray diffraction pattern obtained at ambient conditions. The crystal structure, which has one oxygen trigonal prism site and four octahedral sites for the cations, was found to be isostructural with Ca2Fe7O11. The unit-cell lattice parameters are a = 9.8441(5) Å, b = 2.8920(1) Å, c = 14.1760(6) Å, β = 99.956(4)°, V = 397.50(3) Å3, and Z = 2 (monoclinic, C2/m). Mg and Fe cations are disordered on the trigonal prism site and on two of the four octahedral sites, and the remaining Fe is accommodated at the other two octahedral sites. The present structure is closely related to the other recently discovered Fe oxide structures, e.g., Fe4O5 and Fe5O6, by distortion derived either from incorporation (Fe4O5) or removal (Fe5O6) of an edge-shared FeO6 single octahedral chain in their structures. The present synthesis at deep upper mantle conditions and the structural relationships observed between various novel Mg-Fe oxides indicate that a series of different phases become stable above 10 GPa and that their relative stabilities (Fe2+/Fe3+) must be controlled by oxygen fugacity.

TiS2 as a high performance potassium ion battery cathode in ether-based electrolyte

Potassium ion batteries are potential energy storage devices owing to their low cost and good K+ diffusion kinetics due to the small Stoke's radius. Here, we report a layered TiS2 cathode material, demonstrating outstanding potassium storage cycling and rate performances in ether-based electrolyte, with a capacity of 80mAhg−1 at 20C and 63mAhg−1 after 600 cycles (1.5–3.0V, corresponding to 4.8Ag−1). The phase transitions during K ion intercalation at the atomic level are explored via High Resolution Transmission Electron Microscopy (HRTEM) and ex-situ X-ray diffraction (XRD) combined with Rietveld refinements. It undergoes a second-stage structure to be K0.11TiS2 and then first-stage K0.56TiS2. The K cations intercalate into trigonal prismatic sites with a sliding of Ti-S plane by 120 degree-rotation, rendering a 3×3 commensurate superstructure along the [001] zone. The K stage intercalation is in favor of a Daumas-Hérold model rather than Rüdorff model. This study demonstrates that the layer structure materials are promising candidates for high performances K-ion batteries and highlights the importance of improving electrochemical performances with electrolyte modification.