Correlating Structure and Luminescence Properties of Undoped and Eu3+-Doped La2Hf2O7 Nanoparticles Prepared with Different Coprecipitating pH Values through Experimental and Theoretical Studies

Composition-dependent photoluminescence in nanocrystalline La2Hf2-xZrxO7:Eu phosphor: role of chemical twin Zr/Hf environments around a luminescent center

Based on chemical intuition, linear trends are anticipated in Eu3+ photoluminescence performance inside a pyrochlore matrix of the chemical twins, Hf and Zr, owing to probable geometrical and chemical similarity around the luminescent center. The present work reports the drastically fluctuating result of doping Eu3+ in nanocrystalline pyrochlore, La2Hf2-xZrxO7 (LHZO), matrix on composition variation; the variation is counter to the anticipation-based chemical brotherhood of Hf and Zr. Zirconium-enriched samples of LHZO improve asymmetry around Eu3+ ion leading to enhanced photoluminescence quantum yield (PLQY). The samples with compositions 0.7Hf and 1.3Zr depict the lowest non-radiative channels with the highest theoretically calculated PLQY of ∼71% and excellent thermal stability (∼91%). Synergistic experimental and theoretical analysis reveals that Eu does not unbiasedly occupy La-sites in the pyrochlore LHZO matrix towards chemical twins of Hf and Zr; rather, it energetically prefers to occupy Zr-rich vicinal sites. When the composition with Zr is in the low-medium range, Eu has a higher probability of occupying Zr-rich vicinal sites depicting higher lifetime and PLQY. When Zr-content goes beyond 70-80%, the other site occupancies start contributing leading to a reduction in both lifetime and quantum yield. This work paves a great strategy and provides a futuristic potential to utilize europium luminescence in separating chemically close Hf-Zr for various technological applications.

Understanding the excited state dynamics and redox behavior of highly luminescent and electrochemically active Eu(III)-DES complex

Deep eutectic solvents (DES) are considered a novel class of environmentally benign molecular solvents that are considered as potential solvents for nuclear fuel reprocessing, material recycling, and many other technological applications in both research and industry. However, there is a complete dearth of understanding pertaining to the behavior of metal ions in DES. Herein, we have investigated the speciation, complexation behavior, photochemistry, and redox properties and tried to obtain insight into the chemical aspects of the europium ion in DES (synthesized from heptyltriphenylphosphonium bromide and decanoic acid). The same has been probed using time-resolved photoluminescence (TRPL), cyclic voltammetry (CV), synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) calculations. TRPL indicated the stabilization of europium in the +3 oxidation state, favoring the potential of the Eu(III)-DES complex to emit red light under near UV excitation and the existence of inefficient energy transfer between DES and Eu3+. EXAFS analysis revealed the presence of Eu-O and Eu-Br, which represent the local surroundings of Eu3+ in the Eu(III)-DES complex. TRPL measurement has also suggested two distinct local environments of europium ions in the complex. DFT calculations supported the EXAFS findings, confirming that the Eu(III)-DES structure involves not only the oxygen atom of decanoic acid but also the oxygen atoms from the nitrate ions, contributing to the local coordination of Eu(III). Electrochemical studies demonstrated that the redox reaction of Eu(III)/Eu(II) in DES displays quasi-reversible behavior. The reaction rate was observed to increase with higher temperatures. The findings of this study can contribute to the understanding of the fundamental properties and potential applications of this luminescent and electrochemically active complex and pave the way for further studies and the development of novel materials with enhanced luminescent and electrochemical properties.

Metal ion induced dual switchable dielectric and luminescent properties in hybrid halides

A new multi-functional organic-inorganic hybrid compound was successfully obtained by regulating metal halides. Apart from excellent luminescence properties, in particular, the introduction of a Mn halide successfully achieved a dual-switchable dielectric property, which could lead to very interesting exploration in sensors.

Spin-Dependent First-Principles Study on Optoelectronic Properties of Neodymium Zirconates Pyrochlores Nd2Zr2O7 in Fd-3m and Pmma Phases

Rare-earth zirconate pyrochlores (RE₂Zr₂O₇) are of much fundamental and technological interest as optoelectronic, scintillator and thermal barrier coating materials. For the first time, we report the detailed optoelectronic properties of rare-earth zirconates Nd₂Zr₂O₇ in both, i.e., for spin up and spin down states, via the use of first-principles density functional theory (DFT) procedure. To obtain the desired optoelectronic properties, we used a highly accurate method called full-potential linearized augmented plane wave (FPLAPW) within the generalized gradient approximation (GGA), parametrized with Hubbard potential U as an exchange-correlation function. The band gaps predicted for Nd₂Zr₂O₇ were of the order 2.4 eV and 2.5 eV in Fd-3m and Pmma symmetrical phases, respectively. For both the phases, our research involved a complete examination of the optical properties of Nd₂Zr₂O₇, including extinction coefficient, absorption coefficient, energy loss, function, reflectivity, refractive index, and real optical conductivity, analyzed in the spectral range from 0.0 eV to 14 eV. The calculated optical properties in both phases showed a considerable spin-dependent effect. The electronic bonding characteristics of different species in Nd₂Zr₂O₇ within the two crystal symmetries were explored via the density distribution mapping of charge.

Excitation-Dependent Photoluminescence of BaZrO3:Eu3+ Crystals

The elucidation of local structure, excitation-dependent spectroscopy, and defect engineering in lanthanide ion-doped phosphors was a focal point of research. In this work, we have studied Eu³⁺-doped BaZrO₃ (BZOE) submicron crystals that were synthesized by a molten salt method. The BZOE crystals show orange-red emission tunability under the host and dopant excitations at 279 nm and 395 nm, respectively, and the difference is determined in terms of the asymmetry ratio, Stark splitting, and intensity of the uncommon ⁵D₀ → ⁷F₀ transition. These distinct spectral features remain unaltered under different excitations for the BZOE crystals with Eu³⁺ concentrations of 0-10.0%. The 2.0% Eu³⁺-doped BZOE crystals display the best optical performance in terms of excitation/emission intensity, lifetime, and quantum yield. The X-ray absorption near the edge structure spectral data suggest europium, barium, and zirconium ions to be stabilized in +3, +2, and +4 oxidation states, respectively. The extended X-ray absorption fine structure spectral analysis confirms that, below 2.0% doping, the Eu³⁺ ions occupy the six-coordinated Zr⁴⁺ sites. This work gives complete information about the BZOE phosphor in terms of the dopant oxidation state, the local structure, the excitation-dependent photoluminescence (PL), the concentration-dependent PL, and the origin of PL. Such a complete photophysical analysis opens up a new pathway in perovskite research in the area of phosphors and scintillators with tunable properties.

Solid-State Reaction Synthesis of Nanoscale Materials: Strategies and Applications

Nanomaterials (NMs) with unique structures and compositions can give rise to exotic physicochemical properties and applications. Despite the advancement in solution-based methods, scalable access to a wide range of crystal phases and intricate compositions is still challenging. Solid-state reaction (SSR) syntheses have high potential owing to their flexibility toward multielemental phases under feasibly high temperatures and solvent-free conditions as well as their scalability and simplicity. Controlling the nanoscale features through SSRs demands a strategic nanospace-confinement approach due to the risk of heat-induced reshaping and sintering. Here, we describe advanced SSR strategies for NM synthesis, focusing on mechanistic insights, novel nanoscale phenomena, and underlying principles using a series of examples under different categories. After introducing the history of classical SSRs, key theories, and definitions central to the topic, we categorize various modern SSR strategies based on the surrounding solid-state media used for nanostructure growth, conversion, and migration under nanospace or dimensional confinement. This comprehensive review will advance the quest for new materials design, synthesis, and applications.

Modulating Optical and Electrical Properties of Oxygen Vacancy Enriched La2Ce2O7:Sm3+ Pyrochlore: Role of Dopant Local Structure and Concentration

This work projected the important role of defects in achieving efficient luminescence and electrical conduction in pyrochlore materials. Undoped and Sm3+ doped La2Ce2O7 (LCO and LCOS) stabilize in defect fluorite...

Chromium Doped UO2-Based Ceramics: Synthesis and Characterization of Model Materials for Modern Nuclear Fuels

Cr-doped UO₂ as a modern nuclear fuel type has been demonstrated to increase the in-reactor fuel performance compared to conventional nuclear fuels. Little is known about the long-term stability of spent Cr-doped UO₂ nuclear fuels in a deep geological disposal facility. The investigation of suitable model materials in a step wise bottom-up approach can provide insights into the corrosion behavior of spent Cr-doped nuclear fuels. Here, we present new wet chemical approaches providing the basis for such model systems, namely co-precipitation and wet coating. Both were successfully tested and optimized, based on detailed analyses of all synthesis steps and parameters: Cr-doping method, thermal treatment, reduction of U₃O₈ to UO₂, green body production, and pellet sintering. Both methods enable the production of suitable model systems with a similar microstructure and density as a reference sample from AREVA. In comparison with results from the classical powder route, similar trends upon grain size and lattice parameter were determined. The results of this investigation highlight the significance of subtly different synthesis routes on the properties of Cr-doped UO₂ ceramics. They enable a reproducible tailor-made well-defined microstructure, a homogeneous doping, for example, with lanthanides or alpha sources, the introduction of metallic particles, and a dust-free preparation.

White light emission from co-doped La2Hf2O7 nanoparticles with suppressed host → Eu3+ energy transfer via a U6+ co-dopant

This work highlights white light emission from La₂Hf₂O₇:Eu³⁺ nanoparticles assisted by uranium co-doping to restrict energy transfer from the host oxygen vacancy to the Eu³⁺ dopant.

Recent advances, challenges, and opportunities of inorganic nanoscintillators

This review article highlights the exploration of inorganic nanoscintillators for various scientific and technological applications in the fields of radiation detection, bioimaging, and medical theranostics. Various aspects of nanoscintillators pertaining to their fundamental principles, mechanism, structure, applications are briefly discussed. The mechanisms of inorganic nanoscintillators are explained based on the fundamental principles, instrumentation involved, and associated physical and chemical phenomena, etc. Subsequently, the promise of nanoscintillators over the existing single-crystal scintillators and other types of scintillators is presented, enabling their development for multifunctional applications. The processes governing the scintillation mechanisms in nanodomains, such as surface, structure, quantum, and dielectric confinement, are explained to reveal the underlying nanoscale scintillation phenomena. Additionally, suitable examples are provided to explain these processes based on the published data. Furthermore, we attempt to explain the different types of inorganic nanoscintillators in terms of the powder nanoparticles, thin films, nanoceramics, and glasses to ensure that the effect of nanoscience in different nanoscintillator domains can be appreciated. The limitations of nanoscintillators are also highlighted in this review article. The advantages of nanostructured scintillators, including their property-driven applications, are also explained. This review article presents the considerable application potential of nanostructured scintillators with respect to important aspects as well as their physical and application significance in a concise manner.

Remarkable enhancement of photoluminescence and persistent luminescence of NIR emitting ZnGa2O4:Cr3+ nanoparticles

A modified hydrothermal route with slow precipitation kinetics and core-shell strategies to synthesize ZnGa₂O₄:Cr³⁺ nanoparticles which demonstrate much-improved photoluminescence and persistent luminescence for bioimaging applications.

Controlling the luminescence in K2Th(PO4)2:Eu3+ by energy transfer and excitation photon: a multicolor emitting phosphor

This work highlighted green, red, and white light emission from a single K₂Th(PO₄)₂ compound consisting of actinide and an alkali ion through defect, doping, excitation, and energy transfer manipulation.

Optical properties of undoped, Eu3+ doped and Li+ co-doped Y2Hf2O7 nanoparticles and polymer nanocomposite films

Li⁺ co-doping of Y₂Hf₂O₇:Eu³⁺ nanoparticles improve their quenching concentration, asymmetry ratio, quantum yield, and radioluminescence intensity due to the enhanced covalent character of Eu³⁺–O²⁻ bonding.

High pressure induced local ordering and tunable luminescence of La2Hf2O7:Eu3+ nanoparticles

This work highlights the high-pressure induced site swapping and improved ordering of Eu³⁺ in La₂Hf₂O₇:Eu³⁺ nanocrystals which leads to red-orange-yellow tunable emission at low-moderate-high pressure regime and enhanced correlated color temperature.

Samarium-Activated La2Hf2O7 Nanoparticles as Multifunctional Phosphors

Recent developments in the field of designing novel nanostructures with various functionalities have pushed the scientific world to design and develop high-quality nanomaterials with multifunctional applications. Here, we propose a new kind of doped metal oxide pyrochlore nanostructure for solid-state phosphor, X-ray scintillator, and optical thermometry. The developed samarium-activated La₂Hf₂O₇ (LHOS) nanoparticles (NPs) emit a narrow and stable red emission with lower color temperature and adequate critical distance under near-UV and X-ray excitations. When the LHOS NPs are exposed to an energetic X-ray beam, the Sm³⁺ ions situated at the symmetric environment get excited along with those located at the asymmetric environment, which results in a low asymmetry ratio of Sm³⁺ under radioluminescence compared to photoluminescence. High activation energy and adequate thermal sensitivity of the LHOS NPs highlight their potential as a thermal sensor. Our results indicate that these Sm³⁺-activated La₂Hf₂O₇ NPs can serve as a multifunctional UV, X-ray, and thermographic phosphor.

Rare-Earth Zirconate Ln2Zr2O7 (Ln: La, Nd, Gd, and Dy) Powders, Xerogels, and Aerogels: Preparation, Structure, and Properties

The physicochemical properties of rare-earth zirconates can be tuned by the rational modification of their structures and phase compositions. In the present work, La³⁺-, Nd³⁺-, Gd³⁺-, and Dy³⁺-zirconate nanostructured materials were prepared by different synthetic protocols, leading to powders, xerogels, and, for the first time, monolithic aerogels. Powders were synthesized by the co-precipitation method, while xerogels and aerogels were synthesized by the sol-gel technique, followed by ambient and supercritical drying, respectively. Their microstructures, thermogravimetric profiles, textural properties, and crystallographic structures are reported. The co-precipitation method led to dense powders (S_BET < 1 m² g^-1), while the sol-gel technique resulted in large surface area xerogels (S_BET = 144 m² g^-1) and aerogels (S_BET = 168 m² g^-1). In addition, the incorporation of lanthanide ions into the zirconia lattice altered the crystal structures of the powders, xerogels, and aerogels. Single-phase pyrochlores were obtained for La₂Zr₂O₇ and Nd₂Zr₂O₇ powders and xerogels, while defect fluorite structures formed in the case of Gd₂Zr₂O₇ and Dy₂Zr₂O₇. All aerogels contain a mixture of cubic and tetragonal ZrO₂ phases. Thus, a direct effect is shown between the drying conditions and the resulting crystalline phases of the nanostructured rare-earth zirconates.

Phase Control in Hafnia: New Synthesis Approach and Convergence of Average and Local Structure Properties

Technologically relevant tetragonal/cubic phases of HfO₂ can be stabilized at room temperature by doping with trivalent rare earths using various approaches denoted generically as bulk coprecipitation. Using in situ/ex situ X-ray diffraction (XRD), Raman spectroscopy, high-resolution transmission electron microscopy, and in situ/ex situ site-selective, time-gated luminescence spectroscopy, we show that wet impregnation of hafnia nanoparticles with 10% Eu oxide followed by mild calcination in air at 500 °C produces an efficient stabilization of the cubic phase, comparable to that obtained by bulk precipitation. The physical reasons behind the apparently conflictual data concerning the actual crystallographic phase and the local symmetry around the Eu stabilizer and how these can be mediated by luminescence analysis are also discussed. Apparently, the cubic crystal structure symmetry determined by XRD results in a pseudocubic/tetragonal local structure around Eu determined by luminescence. Considering the recent findings on wet impregnated CeO₂ and ZrO₂, it is concluded that CeO₂, ZrO₂, and HfO₂ represent a unique case of a family of oxides that is extremely tolerant to heavy doping by wet impregnation. In this way, the same batch of preformed nanoparticles can be doped with different lanthanide concentrations or with various lanthanides at a fixed concentration, allowing a systematic and reliable investigation of the effect of doping, lanthanide type, and lanthanide concentration on the various functionalities of these technologically relevant oxides.

Thermally Induced Disorder-Order Phase Transition of Gd2Hf2O7:Eu3+ Nanoparticles and Its Implication on Photo- and Radioluminescence

Crystal structure has a strong influence on the luminescence properties of lanthanide-doped materials. In this work, we have investigated the thermally induced structural transition in Gd₂Hf₂O₇ (GHO) using Eu³⁺ ions as the spectroscopic probe. It was found that complete phase transition from the disordered fluorite phase (DFP) to the ordered pyrochlore phase (OPP) can be achieved in GHO with the increase of annealing temperature from 650 → 1100 → 1300 °C. OPP is the more stable structural form for the GHOE nanoparticles (NPs) annealed at a higher temperature based on the energy calculation by density functional theory (DFT). The asymmetry ratio of the GHOE-650 NPs was the highest, whereas the quantum yield, luminescence intensity, and lifetime values of the GHOE-1300 NPs were the highest. Emission intensity of Eu³⁺ ions increases significantly with the phase transition from the DFP to OPP phase and is attributed to the higher radiative transition rate (281 s^-1) of the ⁵D₀ level of the Eu³⁺ ion in the environment with relatively lower symmetry (C _2v ) because of the increase of crystal size. As the structure changes from DFP to OPP, radioluminescence showed tunable color change from red to orange. The Eu³⁺ local structure obtained from DFT calculation confirmed the absence of inversion symmetry in the DFP structure, which is consistent with the experimental emission spectra and Stark components. We also elucidated the host to dopant optical energy transfer through density of states calculations. Overall, our current studies present important observations for the GHOE NPs: (i) thermally induced order-disorder phase transition, (ii) change of point group symmetry around Eu³⁺ ions in the two phases, (iii) high thermal stability, and (iv) tunability of radioluminescent color. This work provides fundamental understanding of the relationship between the crystal structure and photophysical properties of lanthanide-doped materials and helps design a strategy for advanced optoelectronic materials.

Pyrochlore Rare-Earth Hafnate RE2Hf2O7 (RE = La and Pr) Nanoparticles Stabilized by Molten-Salt Synthesis at Low Temperature

Complex oxides of the RE₂Hf₂O₇ series are functional materials that exist in the fluorite or pyrochlore phase depending on synthesis method and calcination temperature. In this study, we investigate the process of synthesis, crystal structure stabilization, and phase transition in a series of RE hafnate compounds, synthesized by the coprecipitation process of a single-source complex hydroxide precursor followed with direct calcination or molten-salt synthesis (MSS) method. Phase pure RE₂Hf₂O₇ (RE = Y, La, Pr, Gd, Er, and Lu) ultrafine nanostructured powders were obtained after calcinating the precursor in a molten salt at 650 °C for 6 h. Moreover, we demonstrate that the MSS method can successfully stabilize ideal pyrochlore structures for La₂Hf₂O₇ and Pr₂Hf₂O₇ in the nanodomain, which is not possible to achieve by direct calcination of the coprecipitated precursor at 650 °C. We propose mechanisms to elucidate the differences in these two synthesis methods and highlight the superiority of the MSS method for the production of RE hafnate nanoparticles.