Crystallographic Site Occupancy Research Articles

Synthesis, densification, and cation inversion in high entropy (Co,Cu,Mg,Ni,Zn)Al2O4 spinel

ABSTRACT The synthesis, densification behavior, and crystallographic site occupancy were investigated for four different spinel-based ceramics, including a high-entropy spinel (Co0.2Cu0.2Mg0.2Ni0.2 Zn0.2)Al2O4. Each composition was reacted to form a single phase, but analysis of X-ray diffraction patterns revealed differences in cation site occupancy with the high-entropy spinel being nearly fully normal. Densification behavior was investigated and showed that fully dense ceramics could be produced by hot pressing at temperatures as low as 1375°C for all compositions. Vickers’ hardness values were at least 10 GPa for all compositions. The cations present in the high-entropy spinel appear to have a stabilizing effect that led to nearly normal site occupancy compared to full cation inversion behavior of nickel aluminate spinel. This is the first report that compares cation site occupancy of a high-entropy spinel to conventional spinel ceramics.

Mechanical Activation and Cation Site Disorder in MgAl2O4.

The synthesis and crystallographic site occupancy were investigated for MgAl2O4 with and without mechanical activation of the precursor powders. Heating to 1200 °C or higher resulted in the formation of a single spinel phase regardless of whether the powders were mechanically activated or not. Neutron diffraction analysis was used to determine cation site occupancy and revealed that mechanical activation resulted in a lower degree of cation site inversion compared to the nonactivated materials, which indicated that the powders were closer to thermodynamic equilibrium. This is the first study to characterize the effects of mechanical activation on crystallographic site occupancy in magnesium aluminate spinel using neutron diffraction.

Atom Probe Tomography of a Cu-Doped TiNiSn Thermoelectric Material: Nanoscale Structure and Optimization of Analysis Conditions.

Cu-doping and crystallographic site occupations within the half-Heusler (HH) TiNiSn, a promising thermoelectric material, have been examined by atom probe tomography. In particular, this investigation aims to better understand the influence of atom probe analysis conditions on the measured chemical composition. Under a voltage-pulsing mode, atomic planes are clearly resolved and suggest an arrangement of elements in-line with the expected HH (F-43m space group) crystal structure. The Cu dopant is also distributed uniformly throughout the bulk material. For operation under laser-pulsed modes, the returned composition is highly dependent on the selected laser energy, with high energies resulting in the measurement of excessively high absolute Ti counts at the expense of Sn and in particular Ni. High laser energies also appear to be correlated with the detection of a high fraction of partial hits, indicating nonideal evaporation behavior. The possible mechanisms for these trends are discussed, along with suggestions for optimal analysis conditions for these and similar thermoelectric materials.

Broad luminescence tuning in Mn2+-doped Rb2Zn3(P2O7)2via doping level control based on multiple synergies

A series of novel Mn2+-doped Rb2Zn3(P2O7)2 phosphors for tunable emission from green to orange-red due to Mn2+ preferential occupation of different crystallographic sites with an increasing Mn2+ doping level.

Tuning Transition Metal Oxides Towards Achieving Water-Stability and Electrochemical Stability As Electrode Materials for Sodium-Ion Batteries

Transition metal (TM) oxides are a fascinating class of materials, whose properties can be suitably tuned in a variety of ways; such as by selecting TM-ions/dopants having preferred electronic configurations, engineering the crystallographic site occupancy by dopants, controlling/modifying the degree of covalence of TM-O bonds, modifying lattice spacing(s), tuning phase assemblage etc. Such modifications done from the fundamental perspectives influence the performances of TM-oxides for a variety of applications, including their widespread usage as electrode-active materials in alkali metal-ion batteries.In the context of the upcoming Na-ion battery system, O3-type ‘layered’ Na-TM-oxides are promising as cathode-active materials due to their inherently high initial Na-content (as compared to the P2 counterparts). However, they suffer from instabilities caused due to multiple phase transformations during Na-removal/insertion and sensitivity to air/moisture. Against this backdrop, with the help of a dopant, having d 0 electronic configuration (viz., no octahedral site preference energy), we have been able to tune the composition and structural features to suppress the phase transitions upon Na-removal/insertion and also improve the air/water-stability in significant terms; so much so that long-term cyclic stability has been achieved with health/environment-friendly ‘aqueous processed’ electrodes (sans, usage of toxic/hazardous/expensive chemicals like NMP and PVDF) [1]. As will be explained in more elaborate terms during the talk, the changes in structural features, which have led to such outstanding water-stability, include differential contraction/dilation of the Na-‘inter-slab’/TM-‘slab’ spacing and partial occupancy of the dopant at tetrahedral sites of the structure.On the anode front, successful development of Na-ion battery system necessitates looking beyond hard carbon based anodes, which possess safety hazards due to the Na-insertion potential being a bit too close to the Na-plating potential. In this context, Na-titanates promise to be ‘safe’ anode materials; but suffer from cyclic instability [2]. Against this backdrop, we have developed carefully tuned bi-phase Na-titanate based electrodes, having Na2Ti3O7 and Na2Ti6O13 as the primary and secondary phases, respectively. The as-developed phase assemblage has been able to address the cyclic instability of single-phase Na-titanate, leading to long-term cyclic stability even at high current densities (up to 50C!) [3]. These are important steps towards the development of health/environment-friendly, cost-effective, safe and high-performance Na-ion batteries.Acknowledgement to RRCAT, Indore, India, for enabling the usage of Synchrotron facility.Parts of the concerned works have been published as (i.e., the associated publications); B. S. Kumar, A. Pradeep, A. Dutta, and A. Mukhopadhyay; J. Mater. Chem. A 8 (2020) 18064H. S. Bhardwaj, T. Ramireddy, A. Pradeep, M. K. Jangid, V. Srihari, H. K. Poswal, and A. Mukhopadhyay; ChemElectroChem 5[8] (2018) 1219A. Pradeep, B. S. Kumar, A. Kumar, V. Srihari, H. K. Poswal, and A. Mukhopadhyay; Electrochim. Acta 362 (2020) 137122

Addressing the High-Voltage Structural and Electrochemical Instability of Ni-Containing Layered Transition Metal (TM) Oxide Cathodes by "Blocking" the "TM-Migration" Pathway in the Lattice.

"Layered"/"cation-ordered"/O3-type Li-TM-oxides (TM: transition metal) suffer from structural instability due to "TM migration" from the TM layer to the Li layer upon Li removal (viz., "cation disordering"). This phenomenon gets exacerbated upon excessive Li removal, with Ni ions being particularly prone to migration. When used as cathode material in Li-ion batteries, the "TM migration" and associated structural changes cause rapid impedance buildup and capacity fade, thus limiting the cell voltages to ≤4.3 V for stable operation and lowering the usable Li-storage capacity (concomitantly, energy density). Looking closely at the "TM migration" pathway, one realizes that the tetrahedral site (t-site) of the Li layer forms an intermediate site. Accordingly, the present work explores a new idea concerning suppression of "Ni migration" by "blocking" the intermediate crystallographic site (viz., the t-site) with a dopant, which is the most stable at that site. In this regard, density functional theory (DFT)-based simulations indicate that the concerned t-site is energetically the most preferred and stable site for d10 Zn2+. Detailed analysis of crystallographic data (including bond valence sum) obtained with the as-prepared Zn-doped Li-NMC supports the same. Furthermore, the simulations also predict that Zn doping is likely to suppress "Ni migration" upon Li removal. Supporting these predictions, galvanostatic delithiation/lithiation studies with Zn-doped and undoped Li-NMCs demonstrate significantly improved cyclic stability, near-complete suppression of "cation mixing", and negligible buildup of impedance (as well as potential hysteresis) for the former, even upon being subjected to long-term cycling using a high upper cut-off potential of 4.7 V (vs Li/Li+). Accordingly, such subtle tuning of the composition and structure, in the light of electronic configuration of the dopant and specific crystallographic site occupancy, is likely to pave the way toward the development of Ni-containing stable high voltage O3-type Li-TM-oxide cathodes for the next-generation Li-ion batteries.

Synthesis, neutron diffraction and photoluminescence properties of a whitlockite structured Ca9MgLi(PO4)7:Pr3+ phosphor

The whitlockite structured compound Ca9MgLi(PO4)7 has been investigated as a potential host material for down-conversion phosphors. However, no detailed crystal structure and crystallographic site occupancy have been reported for this compound previously. In this work, Pr3+ activated Ca9MgLi(PO4)7 phosphors were successfully synthesized, and the structure and photoluminescence properties were characterized. Upon neutron and X-ray powder diffraction data, the crystallographic site occupancies of M1-M5 in the whitlockite-type structure were carefully refined by the Rietveld method. The M1, M2 and M3 sites are all found to be 8-fold coordinated and fully occupied with Ca and Pr. The results also revealed that the M4 and M5 sites are co-occupied by Li–Mg and Ca–Mg, respectively. Photoluminescence (PL) study showed that the phosphor has a characteristic excitation band at the visible light region of 430–500 nm. Three peaks observed at 442, 468, and 484 nm in the excitation spectra can be ascribed to the 3H4→3P2, 3H4→3P1, and 3H4→3P0 transition of Pr3+, respectively. The phosphor doped with 0.02 Pr3+ gives the strongest emission peaked at 612 nm, with the CIE chromaticity coordinate of (0.613, 0.366). This reddish-orange emitting phosphor has good thermal stability that it retains 83% of its emission intensity at 200 °C. Energy transfer mechanism of the activators was analyzed to be dipole-dipole multipolar interaction. The present work demonstrates that Ca9MgLi(PO4)7:Pr3+ is a promising phosphor for blue-pumped white light-emitting diodes (WLEDs).

Photoluminescent properties and Judd-Ofelt analysis of Eu3+-doped NaMg(PO3)3 red phosphor

A series of novel red-emitting Na1-xMg(PO3)3: xEu3+ phosphors were successfully synthesized using the high-temperature solid-state method. The crystal structure, photoluminescence spectra, decay lifetime and quantum efficiency of the phosphors were investigated as a function of Eu3+ concentration. A dominant red emission peak centered at 612 nm along with an intense 5D0→7F4 transition at 699 nm indicates that Eu3+ occupies low symmetry sites in NaMg(PO3)3 host lattice, which was confirmed by Judd-Ofelt analysis. The crystallographic site-occupations of Eu3+ were investigated by the site-selective excitation and emission spectra in the 5D0→7F0 region, which indicates that Eu3+ ion in NaMg(PO3)3 prefers to occupy the crystallographic site of C3 symmetry. The emission intensity is closely dependent on Eu3+ concentration with critical concentration of 8 mol%. NaMg(PO3)3: Eu3+ phosphor exhibits good thermal stability (67.2% at 473 K).

Doping of metastable Cu3N at different Ni concentrations: Growth, crystallographic sites and resistivity

Copper nitride, Cu3N, is a metastable material whose properties can be changed considerably by doping with metals which opens for a variety of applications in several areas (sensors, electrical connects, batteries, memories, etc.). The present work is a systematic study in the system Cu-Ni-N of preferences regarding occupation of interstitial and substitutional crystallographic sites in the Cu3N structure as the metal dopant level increases and how the occupation influences growth behavior, texture, microstructure and resistivity. Ni doped Cu3N films of different chemical composition were grown by a gas-pulsed Chemical Vapor Deposition technique. The occupation of the different crystallographic sites of the Cu3N by the Ni atoms was obtained from analysis of X-ray diffraction data. At low Ni content, less than about 21% in metal content, Ni replaced the Cu atoms in the structure. In the intermediate Ni metal content range from about 21 to 40% the vacant centre position became available. After filling the centre position, substitution of Cu for Ni occurred up to a Ni content of about 80% (Cu0.8Ni3.2N) which is the solid solubility limit of Ni in Cu3N. The film resistivity decreased rapidly by adding nickel to the Cu3N structure from about 109μΩ·cm without any Ni doping to about 100μΩ·cm with 80% Ni in the metal content. After filling the centre position the change in resistivity when Cu atoms were substituted for Ni was very small. Finally, the growth mechanism, texture and microstructure changed significantly with the uptake of Ni atoms in the structure.

New Eu2+-activated borophosphate phosphors Ba3(ZnB5O10)PO4 for near-ultraviolet-pumped white light-emitting diodes

We presented a series of Ba3(ZnB5O10)PO4: Eu2+ borophosphate phosphors and highlighted their photoluminescence characteristics of intrinsic Eu2+ emission. Under near-ultraviolet excitation, the composition-optimized Ba2.95(ZnB5O10)PO4: 0.05Eu2+ generated bright blue emission peaking at 415 nm with an internal quantum efficiency of 30.1%, corresponding to the electric-dipole allowed 4f-5d transition of the Eu2+ ions. The nonradiative transition between Eu2+ ions in Ba3(ZnB5O10)PO4: Eu2+ phosphors was attributable to be the dipole–dipole interaction, and the activation energy for thermal quenching was calculated to be 0.223 eV. Crystallographic site-occupations of Eu2+ ions in the Ba3(ZnB5O10)PO4 host were also assigned and only one Eu2+ luminescence center that substituted nine-coordinated Ba site was confirmed. The Ba2.95(ZnB5O10)PO4: 0.05Eu2+ phosphor exhibited a better light yield with chromaticity coordinates of (0.168, 0.049) and correlated color temperature of 2009 K than that of commercial phosphor BaMgAl10O17: Eu2+. These results indicate the Ba3(ZnB5O10)PO4: Eu2+ phosphors are competitive as the promising blue-emitting phosphor-converted materials for white light-emitting diodes.

Binary Alkali-Metal Silicon Clathrates by Spark Plasma Sintering: Preparation and Characterization.

The binary intermetallic clathrates K8-xSi46 (x = 0.4; 1.2), Rb6.2Si46, Rb11.5Si136 and Cs7.8Si136 were prepared from M4Si4 (M = K, Rb, Cs) precursors by spark-plasma route (SPS) and structurally characterized by Rietveld refinement of PXRD data. The clathrate-II phase Rb11.5Si136 was synthesized for the first time. Partial crystallographic site occupancy of the alkali metals, particularly for the smaller Si20 dodecahedra, was found in all compounds. SPS preparation of Na24Si136 with different SPS current polarities and tooling were performed in order to investigate the role of the electric field on clathrate formation. The electrical and thermal transport properties of K7.6Si46 and K6.8Si46 in the temperature range 4–700 K were investigated. Our findings demonstrate that SPS is a novel tool for the synthesis of intermetallic clathrate phases that are not easily accessible by conventional synthesis techniques.

New yellow-emitting Whitlockite-type structure Sr(1.75)Ca(1.25)(PO4)2:Eu(2+) phosphor for near-UV pumped white light-emitting devices.

New compound discovery is of interest in the field of inorganic solid-state chemistry. In this work, a whitlockite-type structure Sr1.75Ca1.25(PO4)2 newly found by composition design in the Sr3(PO4)2-Ca3(PO4)2 join was reported. Crystal structure and luminescence properties of Sr1.75Ca1.25(PO4)2:Eu(2+) were investigated, and the yellow-emitting phosphor was further employed in fabricating near-ultraviolet-pumped white light-emitting diodes (w-LEDs). The structure and crystallographic site occupancy of Eu(2+) in the host were identified via X-ray powder diffraction refinement using Rietveld method. The Sr1.75Ca1.25(PO4)2:Eu(2+) phosphors absorb in the UV-vis spectral region of 250-430 nm and exhibit an intense asymmetric broadband emission peaking at 518 nm under λex = 365 nm which is ascribed to the 5d-4f allowed transition of Eu(2+). The luminescence properties and mechanism are also investigated as a function of Eu(2+) concentration. A white LED device which is obtained by combining a 370 nm UV chip with commercial blue phosphor and the present yellow phosphor has been fabricated and exhibit good application properties.

Electrochemical Lithium Insertion Behavior of β-LixV2O5 (0 < x ≤ 3) as the Cathode Material for Secondary Lithium Batteries

Electrochemical lithium insertion/extraction reaction in the β-LixV2O5 phase (0 < x ≤ 3) as the cathode material for rechargeable lithium batteries was examined. Of interest is the rigid 3D host lattice of the monoclinic tunnel-like β lithium bronze that shows desirable structural reversibility upon high Li uptakes (∼3 mol per formula unit), and a remarkable discharge capacity exceeding 330 mA h g−1 during 50 cycles were obtained at 10 mA g−1 in the 4.0/1.8 V potential range. Owing to the complicated ordering processes of crystallographic site occupancy, the pure Li-inserted β phase exhibits rather complex electrochemical properties. New Li diffusivity data within the entire concentration range were obtained by electrochemical impedance spectroscopy and galvanostatic intermittent titration. The slow diffusion kinetics at the deeply discharged state (x > 2.0) is mainly responsible for the large polarization during the process as well as a notable capacity drop (around 24% of the initial low-rate capacity) at a higher rate (50 mA g−1). Further, an abrupt decline in the kinetics of charge-transfer during 0.33 < x ≤ 1.0 is discovered, which results from the re-arrangement of the inserted Li+ among the various interstatial sites in the tunnel framework as lithiation proceeds.

Site Preference and Ordering Induced by Au Substitution in the γ-Brass Related Complex Au–Cr–Zn Phases

The crystal chemistry of the ternary Au-Cr-Zn alloy was studied by means of synthesis, single crystal X-ray diffraction, and electron structure calculations. While the compound CrZn(∼17) represents the binary end-point of the homogeneity range, the inclusion of Au proves to be very site specific, and at the limiting composition Au10Cr4Zn89 the structure is completely ordered. The crystallographic site occupancy pattern calculated by the Local Density Approximation (LDA)-Density Functional Theory (DFT) parametrized extended Hückel (eH) Mulliken charge populations in Au10Cr4Zn89 agrees very well with the experimentally found site occupancy pattern.

Luminescent and thermal properties of a new green emitting Ca 6Sr 4(Si 2O 7) 3Cl 2:Eu 2+ phosphor for near-UV light-emitting diodes

A novel green emitting phosphor, Eu 2+-activated Ca 6Sr 4(Si 2O 7) 3Cl 2, was synthesized using the solid-state reaction and its temperature-dependent luminescence characteristic was reported for the first time. Crystallographic site-occupations of Eu 2+ ions in this host were assigned and two distinguishable Sr 2+ sites were confirmed. As the temperature increases, the emission lines of Ca 6Sr 3.99(Si 2O 7) 3Cl 2:0.01Eu 2+ show an anomalous blue-shift along with the broadening bandwidth and decreasing emission intensity, which is ascribed in terms of the phonon-assisted back tunneling from the excited state of low-energy emission band to the high-energy emission band in the configuration coordinate diagram. Further, the luminescence quenching temperature, the activation energy for thermal quenching (Δ E), and the chromaticity coordinates were also investigated. In view of its preferable excitation spectrum profile, intense green emission peaking at 511 nm, and high thermal luminescence stability, the as-prepared phosphor is expected to find applications as a new green emitting phosphor for near-UV light emitting diodes.

Crystallographic analysis of the structure of a series of lozenge-like sulfides with Bi, In, Pb

Crystallographic analysis is used to study the structure of (Cd,Pb)Bi2S4 (kudriavite), Bi3In5S12, Pb4In3Bi7S18, Pb1.6In8Bi4S19, Pb4In2Bi4S13, Bi2In4S9 lozenge-like sulfides (with the m plane perpendicular to the translation ∼4 A). It is shown that the structure bases contain regular cation and anion sublattices whose conjugation within the common translation cell is determined by the cation/anion ratio. The independent collective ordering of cations results in complex variants of their occupation of separate crystallographic sites. Sometimes independent anionic ordering leads to unusual coordination environments.

The luminescence spectroscopy and thermal stability of red-emitting phosphor Ca 9Eu(VO 4) 7

Eu-based vanadate Ca 9Eu(VO 4) 7 phosphor was synthesized by the solid state reaction method and was characterized by X-ray powder diffraction (XRD). The photoluminescence excitation and emission spectra, fluorescence decay curves and the dependence of luminescence intensity on temperature were investigated. The phosphor can be efficiently excited by near UV light to realize an intense red luminescence (614 nm) corresponding to the electric dipole transition 5D 0 → 7F 2 of Eu 3+ ions. The crystallographic site-occupations of the Eu 3+ ions in Ca 9Eu(VO 4) 7 were investigated by the site-selective excitation and emission spectra, and the fluorescence decay curves in the 5D 0 → 7F 0 region using a pulsed, tunable, narrowband dye laser. The red luminescence together with the thermal stability was discussed on the base of the Eu 3+ site-distribution in Ca 9Eu(VO 4) 7 host.

Luminescence Properties of Yb2+ Doped NaBaPO4 Phosphate Crystals

Yb2+-doped NaBaPO4 phosphors were synthesized by the conventional high-temperature solid-state reaction. The x-ray powder diffraction, photoluminescence excitation and emission spectra, and the decay curves, were measured. The crystallographic site-occupations of Yb2+ ions in NaBaPO4 host were assigned on the base of the luminescence spectra and the crystal structure. The luminescence natures of NaBaPO4:,Yb2+, e.g., the Stokes shift, the luminescence quenching temperature, and the activation energy of thermal quenching were reported. The thermal stability of luminescence was investigated on the base of the temperature-dependent emission intensities and the decay curves. Some anomalous luminescence properties, e.g., the larger Stokes shift, the short decay lifetimes, the lower quenching temperature and low activation energies for thermal quenching, were discussed. The luminescence was ascribed to impurity trapped exciton emission.

Luminescence and microstructures of Eu3+-doped Ca9LiGd2/3(PO4)7

A red-emitting phosphor, Eu(3+)-doped Ca(9)LiGd(2/3)(PO(4))(7), was synthesized by the conventional high-temperature solid-state reaction. X-ray powder diffraction (XRD) analyses confirmed the pure crystalline phase of Whitlockite-type structure. The excitation spectra of Eu(3+) doped Ca(9)LiGd(2/3)(PO(4))(7) were measured in the VUV and UV region indicating an efficient energy transfer process from the host and Gd(3+) to Eu(3+) ions. Upon excitation with VUV and UV radiation, the phosphor showed strong red emission around 611 nm corresponding to the forced electric dipole (5)D(0)→(7)F(2) transition of Eu(3+) ions. The VUV- and UV-excited luminescence spectra of Ca(9)LiGd(2/3)(PO(4))(7):Eu(3+) together with the dependence of the integrated emission intensities on the doping levels were investigated. The Eu(3+) ions were investigated by a tunable laser as an excitation source. The excitation spectra of (7)F(0)→(5)D(0) transitions suggest that there are two families of inequivalent sites for Eu(3+) in this host. The concentration quenching and crystallographic site-occupancy of Eu(3+) ions in Ca(9)LiGd(2/3)(PO(4))(7) host were discussed on the basis of the site selective excitation and emission spectra, the luminescence decay and its crystal structure.

Green-Emitting Ca6Sr4(Si2O7)3Cl2: Eu2+ Phosphors for White Light-Emitting Diodes

A series of new green-emitting Ca6Sr4-xEux(Si2O7)3Cl2 phosphors were synthesized using the high-temperature solid-state reaction. Thermogravimetry-differential thermal analysis (TG-DTA) and X-ray diffraction (XRD) were used in order to determine the crystallization process and crystal structure of the obtained product. Complete optical studies including the diffuse reflection, photoluminescence excitation and emission measurements were performed. Crystallographic site-occupations of Eu2+ ions in the Ca6Sr4(Si2O7)3Cl2 host were assigned and two distinguishable Eu2+ sites, viz., Eu(1)2+ and Eu(2)2+, were confirmed. The luminescence natures of the Eu2+ emission, e. g., the Stokes shift and the concentration-dependent luminescence behaviors were also investigated in detail. Based on the experimental results and theoretical calculation, the dipole-dipole interaction was dominantly involved in the energy transfer from Eu(1)2+ to Eu(2)2+ ions, and the critical distance was determined to be about 14.69 Å. The thermal-quenching behaviors were further presented. In view of its strong absorption in the near-UV region, the intense green light emission peaking at 511 nm, as well as the lower thermal quenching, the Eu2+-doped Ca6Sr4(Si2O7)3Cl2 phosphor can serve as a potential new material for phosphor-converted light-emitting diodes (LEDs).