Site Symmetry Research Articles

Multi-strategy ratiometric luminescent thermometry based on Dy doped CaWO4 phosphors independent of excitation wavelength

This paper explores the detailed photoluminescence (PL) behaviors of Dy3+ doped CaWO4 (Dy:CaWO4) phosphors under multi-ultraviolet wavelength excitation. Moreover, it proposes multi-strategy ratiometric thermometry based on luminescence intensity ratio (LIR) from the transitions of thermally coupled energy levels (TCLs) of Dy3+. Under excitation of multiple ultraviolet wavelengths, the Dy:CaWO4 phosphors exhibited multi-wavelength responsive PL characteristics of Dy3+ with strong blue and yellow emissions from Dy3+ TCLs transitions. The temperature-dependent PL properties of Dy:CaWO4 phosphors were investigated over a temperature range of 300∼650 K. Multi-strategy ratiometric luminescent thermometry was examined under various excitation wavelengths by the thermal equilibrium population of Dy3+ TCLs. The proposed six LIR schemes based on the PL emissions of Dy3+ exhibited a good quantitative relationship with temperature. They showed well-luminescent thermometric behaviors with high absolute and relative sensitivities due to the largest energy gap of TCLs and a relatively larger LIR value. The thermometry properties of the multiple LIR schemes were independent of excitation wavelength, attributed to the stable site symmetry of Dy3+ and the thermal equilibrium population of TCLs under different excitation wavelengths. The multi-strategy ratiometric thermometry of Dy3+ demonstrated the merit of high sensitivity, excellent stability, and repeatability, indicating the potential luminescent thermometric application of Dy:CaWO4 phosphors.

Europium-doped strontium gadolinium oxide phosphor: Investigating structural and photoluminescence characteristics via sol-gel combustion synthesis

SrGd2O4 phosphors doped with Eu3+ were successfully synthesized through a sol-gel combustion method, covering a range of dopant concentrations from 0.25 mol% to 3 mol%. The structural analysis of these phosphor materials was comprehensively conducted utilizing various techniques, including X-ray powder diffraction analysis (XRD), Energy Dispersive X-ray (EDX), and Fourier-transform infrared spectroscopy (FTIR). In addition to unveiling the structural characteristics, these analyses provide valuable insights into the compositional aspects, enhancing our understanding of the synthesized SrGd2O4:Eu3+ phosphors across different doping levels. XRD analysis findings validate the successful generation of the intended SrGd2O4 host, demonstrating orthorhombic system structures consistent with JPCD card number 98-019-3592. FTIR analyses conducted on the phosphor samples not only identify bending modes but also reveal intricate details about small vibration bonds within the material. When excited by the 349 nm laser, SrGd2O4:xEu3+ phosphors exhibit distinct photoluminescence (PL) properties like red emission at 614 nm from Eu3+. From the emission spectra, one can clearly observe that Eu3+ with an ionic radius close to the Gd3+ ion preferentially occupies the symmetry sites of the host lattice. The optimal doping concentration was determined to be 0.5 mol%, as revealed by the data in our study. Additionally, a deeper understanding of the luminescence quenching mechanism was attained, pinpointing the involvement of dipole-dipole (d-d) energy transfer in this intriguing phenomenon. This optimal concentration not only enhances the material's properties but also underscores the pivotal role of d-d interactions in governing the luminescence behavior within the doped system.

(Invited) Unravelling Structural Implications on Charge Transport Mechanism in Nanostructured Electrode Materials for Energy Storage Devices

The development of new and novel electrode materials for energy storage devices has become the intensive research by the materials science community because of its importance in the portable electronic devices, hybrid electric vehicles and many other applications. Since the discovery by Goodenough and his co-workers on the electrochemical behavior of olivine LiFePO4 (LFP), different theoretical and experimental investigations have been undertaken to optimize it as cathode material in lithium-ion batteries and trying to understand the charge/discharge mechanism. Despite having many fascinating electrochemical properties, the main drawback of LFP lies with its low gravimetric density and poor electrical conductivity (both electronic and ionic) which limits the lithium intercalation/deintercalation rates, and hence the practical specific capacity. Therefore, it becomes necessary to gain the fundamental understanding of electronic structure of the LFP system by adopting appropriate experimental technique. We exploit the combined Mössbauer and X-ray absorption spectroscopy to unravel the electronic structure and local site symmetry of Fe in olivine structured LFP with different crystallite sizes (CS). The lattice parameters are found to contract with decrease in CS monotonously, whereas the electronic structural parameters exhibit two different regions with threshold anomaly around ≈30 nm CS. The 57Fe Mössbauer studies reveal the coexistence of Fe2+ and Fe3+sites and their relative concentration are mainly determined by the CS, which provides the comprehensive insight into the electronic structure of LFP at mesoscopic level. The soft X-ray absorption unravels unequivocally the valance states of Fe 3d electrons in the proximity of Fermi level, which are prone to the local lattice distortion. An obtained spectra fingerprint the effect of CS supplying rich information on valence state of iron, lithium-ion vacancy concentration, covalency and crystal field. The unique structural and electronic properties of the LFP are closely interlinked with changes in the bonding character, which shows the strong dependency on the CS. The evolution of the 3d states is in overall agreement with the local lattice distortion and provides the origin of the size effects on the electronic structure olivine phosphate and other transition metal ion containing materials. We observe polaronic conductivity enhancement of approximately two orders of magnitude at the nanoscale level as compared with its bulk counterpart. The volumetric changes with respect to crystallite size are related to the compressive strain resulting into the improvement in the electronic diffusivity. The nano-crystalline LFP with better kinetics will open the new avenue for its usage as cathode material in rechargeable batteries.

AgNPs incorporated in Eu(III)-doped hydroxyapatite biomaterial: Spectroscopic, cytotoxicity, and antimicrobial investigation

This study aimed to synthesize and to characterize hydroxyapatite with antimicrobial action. For the synthesis, the precursors calcium nitrate and phosphoric acid were employed; europium(III), 1 % w/w in relation to calcium(II), was used as spectroscopic probe, to investigate structural alterations. Silver nanoparticles, as antimicrobial agents, were added to the resulting material. Formation of the hydroxyapatite phase was confirmed by X-ray diffraction and vibrational spectroscopy in the infrared region. Incorporation of silver nanoparticles in the material was confirmed by electronic absorption spectroscopy in the ultraviolet–visible region, through the absorption band at approximately 400 nm. Scanning electron microscopy revealed that the material had adequate morphology to induce bone formation. The excitation, emission, and lifetime measurements of the structural probe, europium(III), showed that the hydroxyapatite unit cell had two symmetry sites. Cytotoxicity evaluation in human cells showed that the material was toxic at concentrations above 1250 μg/mL. At 2000 ppm, the material displayed antimicrobial activity against E. coli, but the maximum evaluated concentration was insufficient to verify activity against S. aureus. Therefore, hydroxyapatite bearing silver nanoparticles is a promising material for application in the medical area.

Prediction of feasibility of Li and Fe doped natural olivine Mg2SiO4 for Li ion battery applications using Density Functional Theory

Li and Fe doped olivine Mg2SiO4 was studied using density functional theory. It was found that Li atom prefers to occupy M1 site to M2 site of Pbnm space group symmetry. Also, probability of Fe occupying M1 site over M2 site was also investigated and was observed, like Li, Fe also preferably occupies M1 site to M2 site. Two samples like Li2RLi2iMg6Si4O16 (R stands for replacement and i stands for interstitial site) and Li2RLi2iFe1Mg5Si4O16 were formulated by inserting Li at replacement site and interstitial site to maintain charge neutrality while Fe atom was inserted at replacement site. The probability of oxygen evolution during charging was minimized by incorporating redox active Fe+2 ion in Li doped Mg2SiO4. It was found that extraction of all the Li ions from Li2RLi2iMg6Si4O16 and Li2RLi2iFe1Mg5Si4O16 could be done at 3.68 V and 3.29 V with specific capacities of 198 mAh/g and 187 mAh/g respectively.

Frontiers of Deep-Red Emission of Mn4+ Ions with Ruddlesden-Popper Perovskites.

It is well-known that the chemical composition of the host material significantly affects the spectroscopic performance of transition metal ions. However, it is worth noting that also the structure and symmetry of crystallographic sites play significant roles in transition metal ion luminescence. In this study, we demonstrate three perovskite structures of strontium titanate forming so-called Ruddlesden-Popper phases doped with Mn4+ ions. The observed reduction in the average Ti4+-O2- distance in the series SrTiO3-Sr2TiO4-Sr3Ti2O7 allowed for a record-breaking shift in the spectral position of Mn4+ emission band with a maximum of around 734 nm and led to an improvement of the already impressive thermometric performance of SrTiO3:Mn4+ in ratiometric and lifetime-based approaches. This research encourages a further search for structures that, with the help of the developed correlations between structural and optical properties, could lead to the discovery of phosphors beyond the limits established so far.

Stark splitting of intense red emission for Er3+ in Oh symmetry sites realizing optical temperature sensing in biological applications

Stark splitting of intense red emission for Er3+ in Oh symmetry sites realizing optical temperature sensing in biological applications

Structural ordering studies of Nd3+ ion in eulytite-type M3Y(PO4)3 (M=Sr2+ or Ba2+) phosphates. First translucent ceramics from micro-crystalline cubic powders

Cubic phosphates of M3Y(PO4)3 (M = Sr2+ or Ba2+) from the eulytite family were studied as potential candidates for new optical polycrystalline ceramics showing transparency. Two series of Nd3+-doped Ba3Y(PO4)3 and Sr3Y(PO4)3 micro-powders were synthesized via solid-state reaction method. The correlation between their structure and spectroscopic properties and the effect of divalent Sr2+/Ba2+ cation creating a network was analyzed in detail. The crystal structure of Ba3Y(PO4)3 and Sr3Y(PO4)3 was solved ab initio based on their single crystals. Y3+ and Sr2+/Ba2+ ions do not share the same symmetry site and some slight distortion over crystallographic positions occupied by divalent and trivalent ions in the cationic sublattice was visible. Nd3+ reside at the Y3+ site but partial substitution of Ba2+ or Sr2+ cannot be excluded. The cationic and anionic disorder is reflected in spectroscopic properties. Nd3+ ion used as a structural probe shows multisite character and inhomogeneous distribution in both types of material, as determined using absorption spectroscopy at 4.2 K and laser-excited site-selective luminescence spectroscopy at 77 K. This is also clearly seen in excited state dynamics of 4F3/2 doublet analysis. A greater order for barium host lattice was observed. Also, the first un-doped translucent ceramics were obtained from micro-crystalline powders.

Remarkable water-mediated proton conductivity of two porous zirconium(IV)/hafnium(IV) metal-organic frameworks bearing porphyrinlcarboxylate ligands

Obtaining crystalline materials with high structural stability as well as super proton conductivity is a challenging task in the field of energy and material chemistry. Therefore, two highly stable metal-organic frameworks (MOFs) with macro-ring structures and carboxylate groups, Zr-TCPP (1) and Hf-TCPP (2) assembled from low-toxicity as well as highly coordination-capable Zr(IV)/Hf(IV) cations and the multifunctional linkage, meso-tetra(4-carboxyphenyl)porphine (TCPP) have attracted our strong interest. Note that TCPP as a large-size rigid ligand with high symmetry and multiple coordination sites contributes to the formation of the two stable MOFs. Moreover, the pores with large sizes in the two MOFs favor the entry of more guest water molecules and thus result in high H2O-assisted proton conductivity. First, their distinguished structural stabilities covering water, thermal and chemical stabilities were verified by various determination approaches. Second, the dependence of the proton conductivity of the two MOFs on temperature and relative humidity (RH) is explored in depth. Impressively, MOFs 1 and 2 demonstrated the optimal proton conductivities of 4.5 × 10−4 and 0.78 × 10−3 S·cm−1 at 100 °C/98 % RH, respectively. Logically, based on the structural information, gas adsorption/desorption features, and activation energy values, their proton conduction mechanism was deduced and highlighted.

Highly Bright and Stable Lead-Free Double Perovskite White Light-Emitting Diodes.

Lead-free double perovskites (DPs) are emerging highly stable emitters with efficient broadband self-trapped exciton (STE) photoluminescence (PL), but their low electroluminescent (EL) efficiency is a critical shortcoming. This work promotes the external quantum efficiency (EQE) and luminance of DP-based white light-emitting diode (wLED) with a normal device structure to 0.76% and 2793cd m-2 via two modifications: This work prevents the formation of adverse metallic silver, spatially confined STE, and lowers local site symmetry in Cs2 Na0.4 Ag0.6 In0.97 Bi0.03 Cl6 DP by terbium doping; and this work develops a guest-host strategy to improve film morphology, reduce defect density, and increase carrier mobility. These alterations cause substantial increase in STE radiative recombination and charge injection efficiency of perovskite layer. Finally, pure white EL with ideal color coordinates of (0.328, 0.329) and a record-breaking optoelectronic performance is achieved by introducing additional green carbon dots in LED to fill the deficient green component.

Correlated structural and optical properties of crystal-engineered Eu3+-doped gadolinium oxyfluoride polymorphs compatible for lighting and display applications

Functional materials exhibiting polymorphism serve as excellent candidates for various optical applications since the properties can be tuned to the desired target/application by modifying the structural features. Herein, we explored the synthesis of Eu3+ doped gadolinium oxyfluoride systems via a green microwave-assisted hydrothermal method, and phase controllability was established by varying the O2− to F- ratio. The phase evolution and crystal structure of the Eu3+ doped oxyfluoride analogs were examined using Rietveld refinement studies, and the results were consistent with the site symmetries unveiled by the photoluminescence spectra of Eu3+. The low phonon energy of all the phosphors, calculated from the Raman spectra, suggested the possibility of these groups of materials as excellent host matrices for optical applications. Further, a drastic variation in the morphology of studied oxyfluorides at different phases is evidenced by the electron microscopic analysis. The highly asymmetric rhombohedral sites enhanced the color purity, lifetime, and quantum yield of the red phosphor, making it an excellent red component for the fabrication of UV-excitable red as well as white light-emitting diodes for lighting and display device functionalities.

Role of site symmetry in photoluminescence spectrum of Eu2+ doped phosphors - A review

The role of site symmetry is one of the important parameters in the emission spectrum of the phosphor. The emission from the phosphor can be tuned due to alteration in the site symmetry by selecting appropriate synthesis procedure. The spectral shift in the emission spectrum can be achieved by variation in the coordination number (CN) or by tuning symmetry in a coordination environment. Few reported phosphors such as Eu2+ doped Ca3Mg3(PO4)4, Ba2CaMg2Si6O17 and Ca3Al2O6 where the spectral shift is noted are used in this review for the analysis.

Effect of A-Cation Radius on the Structure, Luminescence, and Temperature Sensing of Double Perovskites A2MgWO6 Doped with Dy3+ (A = Ca, Sr, Ba).

Herein, we report a case of A2MgWO6 (A = Ca, Sr, Ba) doped with 2%Dy3+, 2%Li+, in which the influences of the cation substitution are exhibited through the crystal structure, the charge transfer band (O2--W6+), the emission spectrum, the color of the luminescence, and the luminescent decay time. The substitution of Ca2+ and Sr2+ ions for larger ions (Ba2+) led to the crystal structure alteration from cubic to monoclinic and tetragonal, respectively. These structure changes also lowered the crystallography symmetry site of Dy3+, tuned the color of the emitted light from the whitish to yellowish region, and caused a blue shift of the CTB. Furthermore, a significant decline in the lifetime of the 4F9/2 → 6H13/2,15/2 transitions was noticed, from 748, 199, to 146 μs corresponding to Ba, Sr, Ca sample owing to the reduction in the local symmetry of Dy3+. Moreover, the thermal sensing properties of 2%Dy3+-doped samples were investigated based on the fluorescence intensity ratio technique in the range of 80-325 K. Under 266 nm excitation wavelength, the maximum relative sensitivity of the investigated samples was remarkably enhanced from 2.26%/K, 3.04%/K, to 4.38%/K corresponding to Ba, Ca, and Sr samples, respectively. In addition to providing a comprehensive understanding of the effects of compositional modifications on the optical properties, the results also present a viable pathway to manipulate the temperature sensing performance.

Striking Change in Radiative Lifetime and Asymmetry Ratio with Temperature for the Double Perovskite Cs2NaEuCl6

AbstractThe measurement of quantum yield (QY) is an important process for a phosphor. This is often accomplished by the comparison of the low‐temperature lifetime with the room temperature (RT) lifetime. It is shown that this procedure provides a considerable underestimation of QY in certain cases. Additionally, the use of the europium ion as an indicator of changes in site symmetry with temperature has severe limitations. This work reveals that a decrease in measured lifetime with increasing temperature of a system cannot always be associated with a descent in site symmetry and may be due to increasing radiative rate.

Optical properties of Tb3+- and Cr3+-doped MgAl2O4 ceramics prepared by capsule- and carbon-free hot isostatic pressing

Transparent spinel with high real in-line transmittance (RIT up to 79%) was prepared from commercial spinel powder by capsule-free hot isostatic pressing (HIP). Optimal pre-sintering conditions were found in order to enhance the RIT values and doping with Cr3+ (0.01–0.04 at%) and Tb3+ (0.05–0.125 at%) secured photoluminescence. The incorporation of the dopants in tested range does not noticeably reduced the RIT. The Cr3+-doped MgAl2O4 shows efficient narrow red emission at about 685 nm (R-line) due to the 2Eg→4A2g transition of Cr3+ ions located in octahedral sites of D3d local symmetry with a strong crystal field. The Tb3+-doped MgAl2O4 ceramics have shown emissions from blue to red spectral range (5D4→7F3–6 transitions), with prominent green emission centered at 545 nm. The maximal emission intensity was achieved at 0.075 at% of Tb3+, Further increase of Tb3+ content led to the concentration quenching caused by Tb3+ ions segregation at the grain boundaries.

Efficient NMR measurement and data analysis supported by the Bayesian inference: The case of the heavy fermion compound YbCo[formula omitted]Zn20

We propose a data-driven technique to infer microscopic physical quantities from nuclear magnetic resonance (NMR) spectra, in which the data size and quality required for the Bayesian inference are investigated. The 59Co-NMR measurement of YbCo2Zn20 single crystal generates complex spectra with 28 peaks. By exploiting the site symmetry in the crystal structure, the isotropic Knight shift Kiso and nuclear quadrupole resonance (NQR) frequency νQ were respectively estimated to be Kiso=0.7822±0.0090% and νQ=2.008±0.016 MHz (T=20 K and H≃10.2 T) by analyzing only 30 data points from one spectrum. The estimated νQ is consistent with the precise value obtained in the NQR experiment. Our method can significantly reduce the measurement time and the computational cost of data analysis in NMR experiments.

Synthesis, structural, morphology, spectroscopic and optical study of new metal orthophosphate MII(Ga0.5Sb0.5)(PO4)2 (MII= Sr, Pb, Ba) compounds

This paper reports the synthesis, structure, BFDH crystal morphology, Raman and Infrared spectroscopic studies of three novel metal orthophosphate salts MII(Ga0.5Sb0.5)(PO4)2 (MII= Sr, Pb, Ba), abbreviated [Sr], [Pb], [Ba]. The structures of these phases are refined from X-ray powder diffraction data by the Rietveld method. The yavapaiite [Ba] phase is monoclinic of C2/m space group [Z=2; a= 8.106(4) Å; b= 5.178(3) Å c= 7.806(1) Å, β=94.79(4)°, V= 327(1) Å3]. Its structure consists of layers, running along the (a, b) plane, and built up of corner-connected Ga(Sb)O6 octahedra and PO43− tetrahedra. The two isotypic phases [Sr]/[Pb] of distorted yavapaiite structure crystallize in the monoclinic C2/c space group (Z=4) with parameters: a=16.455(4)/16.622(7) Å; b=5.158(1)/5.163(2) Å c= 8.005(1)/8.067(3) Å; β=115.49(1)/114.85(2)°, V= 613(1)/628(5) Å3. Their structures consist of MO10 (M= Sr, Pb), PO43− tetrahedra and Sb(Ga)O6 octahedra. In [Pb] phase, the Pb is surrounded by eight oxygen atoms (Pb-O: 2.65-2.77 Å) and two oxygen atoms forming abnormal bond distances Pb-O(4) [3.447(3)Å]. The BFDH crystal morphology predicts hight (h k l) facets for each compound, the center-to-plane distances dhkl are slightly strong in the distorted Yavapaiite [Pb/Sr] rather than in [Ba]. The IR and Raman study is performed based on theoretical group analysis considering Cs site symmetry of PO43− groups in [Ba], and Ci symmetry in [Pb]/[Sr] phases. The assignment and discussion of the observed bands were done by comparison with other yavapaiite materials. The UV–visible investigation reveals direct optical gap energies of values 4.16 eV for [Ba] and 4.24 eV for [Sr], indicating a semi-conducting nature of the prepared materials.

Pathways of hydrogen atom diffusion at fcc Cu: Σ9 and Σ5 grain boundaries vs single crystal

The diffusion of H-atoms is relevant for innumerous physical–chemical processes in metals. A detailed understanding of diffusion in a polycrystalline material requires the knowledge of the activation energies (ΔEa’s) for diffusion at different defects. Here, we report a study of the diffusion of H-atoms at the Σ9 and Σ5 grain boundaries (GBs) of fcc Cu that are relevant for practical applications of the material. The complete set of possible diffusion pathways was determined for each GB and we compared the ΔEa at bulk fcc Cu with the landscape of ΔEa’s at these defects. We found that while a number of diffusion pathways at the GBs have high tortuosity, there are also many paths with very low tortuosity because of specific structural features of the interstitial GB sites. These data show that the diffusion of H-atoms at these GBs is highly directional but can be fast because at certain paths the ΔEa can be as low as 0.05 eV. The lowest energy paths for diffusion of H-atoms through the whole GB models are ΔEa = 0.05 eV for the Σ9 and ΔEa = 0.20 eV at Σ5 which compare with ΔEa = 0.42 eV for the bulk fcc crystal. This shows that H-atoms will be able to diffuse very fast at these defects. With the Laguerre–Voronoi tessellation method, we studied how the local atomic structure of the interstitial sites of the GBs leads to different ΔEa’s for diffusion of H-atoms. We found that the volume expansions and the coordination numbers alone cannot account for the magnitude of the ΔEa’s. Hence, we developed a symmetry quantifying parameter that measures the deviation of symmetry of the GB sites from that of the bulk octahedral site and hence accounts for the distortion at the GB site. Only when this parameter is introduced together with the volume expansions and the coordination numbers, it is possible to correlate the local structure with the ΔEa’s and to obtain descriptors of diffusion. The complete set of data shows that the extrapolation of diffusion data for H-atoms between different types of GBs is non-trivial and should be done with care.

Selective Laser Spectroscopy of the Bixbyite-Type Yttrium Scandate Doped by Rare-Earth Ions.

Yttrium scandate crystals doped by Nd3+ and Tm3+ ions have been successfully grown in the form of fibers using the laser-heated pedestal growth (LHPG) technique. The selective laser spectroscopy methods have identified and distinguished three distinct types of optically active centers associated with Nd3+ and Tm3+ ions. The substitution of Y3+ and Sc3+ for rare-earth ions in the C2 structural site leads to the formation of two distinct basic long-time centers. In Nd3+:YScO3, another type of center (a short-lifetime one) is formed known as the Nd3+-Nd3+ aggregate pair. This center arises from the substitution of Y3+ or Sc3+ for Nd3+ cation in the adjacent MO6 polyhedra that share an edge. In Tm3+:YScO3, the third optical center is formed as a result of the substitution of Y3+ or Sc3+ for Tm3+ in the MO6 octahedra with the C3i site symmetry. The fluorescence decay lifetimes of Nd3+ and Tm3+ ions in the YScO3 crystal structure have been accurately measured and estimated. A Stark level diagram illustrating the splitting of 4F3/2, 4I11/2, and 4I9/2 multiplets of Nd3+ ions has been constructed to show features of the active optical centers with the C2 site symmetry.

Rerefinement of the crystal structure of α-ThBr4.

Single crystals of α-ThBr4, thorium(IV) tetra-bromide, were obtained as a side product from the reaction of CuBr with β-ThBr4 at 753 K. In the crystal structure, the Th atom (site symmetry ..) is surrounded by eight Br atoms in the form of a tetra-gonal-disphenoidal coordination polyhedron. The connectivity of these polyhedra is 3 ∞[ThBr4/2Br4/2]. In comparison with the previous crystal structure refinement [Mason et al. (1974 ▸). J. Less-Common Met. 35, 331-338], the current rerefinement resulted in much higher preciscion of the lattice parameters and the atomic coordinates.