XANES, EXAFS and photoluminescence investigations on the amorphous Eu:HfO2

Amplification of blue emission of Tm3+ ions in Li2 O-HfO2 -SiO2 glass system by means of Au0 metallic particles

Li₂ O-HfO₂ -SiO₂ -Tm₂ O₃ :Au₂ O₃ glass samples (containing fixed content of Tm₂ O₃ and different concentration of Au₂ O₃ ) were prepared and characterized. Bearing of Au⁰ metallic particles (MPs) on improving blue emission of thulium ions (Tm³⁺ ) ions was explored. Optical absorption (OA) spectra exhibited multiple bands excited from ³ H₆ of Tm³⁺ . Additionally, a broad peak in the wavelength range 500-600 nm due to surface plasmon resonance (SPR) of Au⁰ MPs was noticed in the spectra. Photoluminescence (PL) spectra (of thulium free glasses) indicated a peak in the visible range due to sp → d electronic transition of Au⁰ MPs. Luminescence spectra of Tm³⁺ and Au₂ O₃ co-doped glasses exhibited intense blue emission with substantial increase of intensity with increase of Au₂ O₃ content. Bearing of Au⁰ MPs on the reinforcement of blue emission of Tm³⁺ was discussed in detail with kinetic rate equations.

Charge Transfer-Triggered Bi3+ Near-Infrared Emission in Y2Ti2O7 for Dual-Mode Temperature Sensing

Trivalent bismuth is a popular main group ion showing versatile luminescent behaviors in a broad spectral range from ultraviolet to visible, but barely in the near-infrared (NIR) region. In this study, we have observed unexpected NIR emission at ∼744 nm in a Bi³⁺-doped pyrochlore, Y₂Ti₂O₇ (YTOB). Our first-principles electronic structure calculation and analysis of the Bi local structure via extended X-ray absorption fine structure indicate that only Bi³⁺ species appears in YTOB and it has a similar local environment to that of Y³⁺. The NIR emission is assigned to a Ti⁴⁺ → Bi³⁺ metal-to-metal charge transfer process. Moreover, we have demonstrated dual-mode luminescence thermometry based on the luminescence intensity ratio (LIR) and lifetime (τ) in 0.5% Bi³⁺ and 0.5% Pr³⁺ co-doped Y₂Ti₂O₇ (YTOB0.5P0.5). It exhibits high thermometric sensitivity simultaneously in the cryogenic temperature range from 78 to 298 K based on τ of the NIR emission of Bi³⁺ at 748 nm and in the temperature range of 278-378 K based on the LIR of Bi³⁺ to Pr³⁺ emissions (I₇₄₈/I₆₁₅). As a novel LIR-τ dual-mode thermometric material over a wide temperature range, the maximum relative sensitivities of the YTOB0.5P0.5 reach 3.53% K^-1 at 298 K from the τ mode and 3.52% K^-1 at 318 K based on the LIR mode. The dual-mode luminescence thermometry with high responsivity from our Bi³⁺-based pyrochlore Y₂Ti₂O₇ phosphor opens a new avenue for more luminescent materials toward multi-mode thermometry applied in complex temperature-sensing conditions.

Effect of Oxide Ion Distribution on a Uranium Structure in Highly U-Doped RE2Hf2O7 (RE = La and Gd) Nanoparticles

Rare-earth based A₂B₂O₇ compounds have been considered as potential host materials for nuclear waste due to their exceptional chemical, physical, capability of accommodating high concentration of actinides at both A- and B-sites, negligible leaching, tendency to form antisite defects, and radiation stabilities. In this work, La₂Hf₂O₇ (LHO) and Gd₂Hf₂O₇ (GHO) nanoparticles (NPs) were chosen as the RE-based hafnates to study the structural changes and the formation of different U molecular structures upon doping (or alloying) at high concentration (up to 30 mol %) using a combined coprecipitation and molten-salt synthesis. These compounds form similar crystal structures, i.e., ordered pyrochlore (LHO) and disordered fluorite (GHO), but are expected to show different phase transformations at high U doping concentration. X-ray diffraction (XRD) and Rietveld refinement results show that the LHO:U NPs have high structural stability, whereas the GHO:U NPs exhibit a highly disordered structure at high U concentration. Alternatively, the vibrational spectra show an increasingly random oxygen distribution with U doping, driving the LHO:U NPs to the disordered fluorite phase. X-ray spectroscopy indicates that U is stabilized as different U⁶⁺ species in both LHO and GHO hosts, resulting in the formation of oxygen vacancies stemming from the U local coordination and different phase transformation. Interestingly, the disordered fluorite phase has been reported to have increased radiation tolerance, suggesting multiple benefits associated with the LHO host. These results demonstrate the importance of the structural and chemical effect of actinide dopants on similar host matrices which are important for the development of RE-based hafnates for nuclear waste hosts, sensors, thermal barrier coatings, and scintillator applications.

Characteristics and local structure of hafnia-silicate-zirconate ceramic nanomixtures

Zirconate systems having the composition 3HfO₂·15SiO₂·xY₂O₃·(82 - x)ZrO₂, where x = 2, 7 and 12 mol% Y₂O₃, were synthesized by a sol-gel method. The analysis of X-ray diffraction data showed the presence of the t-ZrO₂, m-ZrO₂, m-HfO₂, Y₂SiO₅ and Y₂Si₂O₇ crystalline phases in a ceramic nanomixture. Spectroscopic data show that the increase of the Y₂O₃ content of samples determines the increase of the t-ZrO₂, m-HfO₂ and silicate crystalline phases. Gap energy values decrease almost linearly with increasing Y₂O₃ content of samples. A detailed study of XANES data does not show a significant difference with increasing Y₂O₃ content of the samples suggesting an appreciable stability of the hafnium ions +4 oxidation state and their microvicinity. EXAFS results show that the local structure around the Hf cation is similar to that from the monoclinic crystalline HfO₂ where the Hf-O coordination number tends to 7. The bond lengths of Hf-O shells show small deviations from ∼2.12 Å and the Hf-metal paths become more structured by increasing the Y₂O₃ content of the samples.

Electronic structure and luminescence assets in white-light emitting Ca2V2O7, Sr2V2O7 and Ba2V2O7 pyro-vanadates: X-ray absorption spectroscopy investigations

Ca₂V₂O₇, Sr₂V₂O₇, and Ba₂V₂O₇ pyro-vanadates were synthesized using a modified chemical precipitation method and annealing. Detailed crystal structure, morphology, electronic structure and optical properties were investigated by XRD, UV-visible absorption, FTIR, Raman, FE-SEM, XANES, and photoluminescence spectroscopy. Rietveld refinement on the XRD patterns of Ca₂V₂O₇, Sr₂V₂O₇, and Ba₂V₂O₇ has confirmed the triclinic structure (space group; P1̄(2)) of the pyro-vanadates. The band gap energy of Ca₂V₂O₇, Sr₂V₂O₇, and Ba₂V₂O₇ is estimated to be ∼2.67 eV, ∼2.97 eV and ∼3.09 eV, respectively. XANES spectra at the Ca L-edge, Sr K-edge and Ba L-edge have confirmed the Ca²⁺, Sr²⁺ and Ba²⁺ ions in the Ca₂V₂O₇, Sr₂V₂O₇ and Ba₂V₂O₇ compounds, respectively. V K-edge XANES spectra have strengthened the presence of sub-pentavalent V ions in all of the pyro-vanadates. O K-edge XANES spectra of Ca₂V₂O₇, Sr₂V₂O₇ and Ba₂V₂O₇ have shown dominating tetrahedral symmetry of the V ions which is also corroborated with the V K-edge XANES. Broad-band emission spectra, ranging from 400 nm to 700 nm, have been observed from the charge-transfer transitions of VO₄ tetrahedra. ³T₁ → ¹A₁ and ³T₂ → ¹A₁ transitions, from the VO₄ tetrahedra, have provided two distinct emission peaks from the compounds which exhibit a red-shift with the decreasing ionic-radii of alkali-earth metal ions. The mixed compounds, with equal weight proportions, have shown remarkable emission characteristics towards the realization of rare-earth element free white-light-emitting devices.

Chemically synthesized pyro-vanadates exhibit sub-pentavalent V ions and white-light emission via charge transfer transitions of VO₄ tetrahedra.

Investigation on cation distribution and luminescence in spinel phase γ-Ga3−δO4 : Sm nanostructures using X-ray absorption spectroscopy

In this study, spectroscopic investigations are employed to quantify the Ga distribution over the tetrahedral/octahedral sites and to assimilate the luminescence properties in the barely reported γ-Ga_2.67O₄ : Sm nanoparticles.

Mechanistic insights on the electronic properties and electronic/atomic structure aspects in orthorhombic SrVO3 thin films: XANES–EXAFS study

Distortion in the V–O6 octahedra is observed in SrVO₃ thin films and leads to larder separation between bonding and anti-bonding d_‖ orbitals.