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1. Much more to explore with an oxidation state of nearly four: Pr valence instability in intermetallic m -Pr 2 Co 3 Ge 5

   https://doi.org/10.1126/sciadv.adl2818  

   Kyrk, Trent M. ; Kennedy, Ellis R. ; Galeano-Cabral, Jorge ; McCandless, Gregory T. ; Scott, Mary C. ; Baumbach, Ryan E. ; Chan, Julia Y. ( January 2024 , Science Advances)

   For some intermetallic compounds containing lanthanides, structural transitions can result in intermediate electronic states between trivalency and tetravalency; however, this is rarely observed for praseodymium compounds. The dominant trivalency of praseodymium limits potential discoveries of emergent quantum states in itinerant 4f¹systems accessible using Pr⁴⁺-based compounds. Here, we use in situ powder x-ray diffraction and in situ electron energy-loss spectroscopy (EELS) to identify an intermetallic example of a dominantly Pr⁴⁺state in the polymorphic system Pr₂Co₃Ge₅. The structure-valence transition from a nearly full Pr⁴⁺electronic state to a typical Pr³⁺state shows the potential of Pr-based intermetallic compounds to host valence-unstable states and provides an opportunity to discover previously unknown quantum phenomena. In addition, this work emphasizes the need for complementary techniques like EELS when evaluating the magnetic and electronic properties of Pr intermetallic systems to reveal details easily overlooked when relying on bulk magnetic measurements alone.

    

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2. Why are the Elemental Nonmetals (F 2 , Cl 2 , Br 2 , I 2 , S 8 , P 4 ) of so Many Hues or of Any Hues and Where is the Chromophore? Insight into Intera ‐X–X Bonds

   https://doi.org/10.1111/php.13270  

   Jabeen, Shakeela ; Greer, Alexander ; Edwards, Kathleen F. ; Liebman, Joel F. ( May 2020 , Photochemistry and Photobiology)

   A unique approach is used to relate the HOMO‐LUMO energy difference to the difference between the ionization potential (IP) and electron affinity (EA) to assist in deducing not only the colors, but also chromophores in elemental nonmetals. Our analysis focuses on compounds with lone pair electrons and σ electrons, namely X₂(X = F, Cl, Br, I), S₈and P₄. For the dihalogens, the [IP – EA] energies are found to be: F₂(12.58 eV), Cl₂(8.98 eV), Br₂(7.90 eV), I₂(6.78 eV). We suggest that theinterahalogen X–X bond itself is the chromophore for these dihalogens, in which the light absorbed by the F₂, Cl₂, Br₂, I₂leads to longer wavelengths in the visible by a π → σ* transition. Trace impurities are a likely case of cyclic S₈which contains amounts of selenium leading to a yellow color, where the [IP – EA] energy of S₈is found to be 7.02 eV. Elemental P₄with an [IP – EA] energy of 9.09 eV contains a tetrahedral and σ aromatic structure. In future work, refinement of the analysis will be required for compounds with π electrons and σ electrons, such as polycyclic aromatic hydrocarbons (PAHs).

    

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3. Visible emission studies of melt-grown Dy-doped CsPbCl 3 and KPb 2 Cl 5 crystals

   https://doi.org/10.1364/OME.398498  

   Hommerich, Uwe ; Uba, Samuel ; Kabir, A. ; Trivedi, Sudhir B. ; Yang, Clayton ; Brown, Ei Ei ( July 2020 , Optical Materials Express)

   We report results of the optical properties of Dy-doped CsPbCl₃and KPb₂Cl₅bulk crystals for potential applications in yellow solid-state laser development. The crystals were synthesized from purified starting materials and melt-grown by vertical Bridgman technique. Optical transmission measurements revealed characteristic absorption bands from intra-4f transitions of Dy³⁺ions. Direct optical excitation at 455 nm (⁶H_15/2→⁴I_15/2) resulted in dominant yellow emission bands at ∼575 nm from the⁴F_9/2excited state of Dy³⁺ions. In addition, both crystals exhibited weaker emission lines in the blue (∼483 nm) and red (∼670 nm) regions. The peak emission-cross sections for the yellow transition (⁴F_9/2→⁶H_13/2) were determined to be ∼0.22 × 10⁻²⁰cm²for Dy: CsPbCl₃(λ_peak= 576.5 nm) and ∼0.59 × 10⁻²⁰cm²for Dy: KPb₂Cl₅(λ_peak= 574.5 nm). The spectral properties and decay dynamics of the⁴F_9/2excited state were evaluated within the Judd-Ofelt theory to predict total radiative decay rates, branching ratios, and emission quantum efficiencies.

    

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4. Resolving the F 2 bond energy discrepancy using coincidence ion pair production (cipp) spectroscopy

   https://doi.org/10.1039/D1CP00140J  

   Matthíasson, Kristján ; Kvaran, Ágúst ; Garcia, Gustavo A. ; Weidner, Peter ; Sztáray, Bálint ( April 2021 , Physical Chemistry Chemical Physics)

   null (Ed.)

   Coincidence ion pair production (cipp) spectra of F 2 were recorded on the DELICIOUS III coincidence spectrometer in the one-photon excitation region of 125 975–126 210 cm −1 . The F + + F − signal shows a rotational band head structure, corresponding to F 2 Rydberg states crossing over to the ion pair production surface. Spectral simulation and quantum defect analysis allowed the characterization of five new molecular Rydberg states (F 2 **): one Π and four Σ states. The lowest-energy Rydberg state spectrum observed ( T 0 = 125 999 cm −1 ) lacked some of the predicted rotational structure, which allowed an accurate determination of the ion pair production threshold of 15.6229 4 ± 0.0004 3 eV. Using the well-known atomic fluorine ionization energy and electron affinity, this number leads to a ground state F–F dissociation energy of 1.6012 9 ± 0.0004 4 eV. Photoelectron photoion coincidence (PEPICO) experiments were also carried out on F 2 and the dissociative photoionization threshold to F + + F was determined as 19.0242 ± 0.0006 eV. Using the atomic fluorine ionization energy, this can be converted to an F 2 dissociation energy of 1.6013 2 ± 0.0006 2 eV, further confirming the cipp-derived value above. Because the two experiments were independently energy-calibrated, they can be averaged to 1.6013 0 ± 0.0003 6 eV and this value can be used to derive the fluorine atom's 0 K heat of formation as 77.25 1 ± 0.01 7 kJ mol −1 . This latter is in excellent agreement with the latest Active Thermochemical Table (ATcT) value but improves its accuracy by almost a factor of three. 

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5. Local environment rigidity and the evolution of optical properties in the green-emitting phosphor Ba 1−x Sr x ScO 2 F:Eu 2+

   https://doi.org/10.1039/D1TC05411B  

   Hariyani, Shruti ; Amachraa, Mahdi ; Khan, Mariam ; Ong, Shyue Ping ; Brgoch, Jakoah ( February 2022 , Journal of Materials Chemistry C)

   Developing chemically and thermally stable, highly efficient green-emitting inorganic phosphors is a significant challenge in solid-state lighting. One accessible pathway for achieving green emission is by forming a solid solution with superior blue-emitting materials. In this work, we demonstrate that the cyan-emission ( λ em = 481 nm) of the BaScO 2 F:Eu 2+ perovskite can be red-shifted by forming a solid solution following (Ba 1− x Sr x ) 0.98 Eu 0.02 ScO 2 F ( x = 0, 0.075, 0.15, 0.25, 0.33, 0.40). Although green emission is achieved ( λ em = 516 nm) as desired, the thermal quenching (TQ) resistance is reduced, and the photoluminescence quantum yield (PLQY) drops by 65%. Computation reveals the source of these changes. Surprisingly, a basic density functional theory analysis shows the gradual Sr Ba substitution has negligible effects on the band gap ( E g ) energy, suggesting the activation energy barrier for the thermal ionization quenching remains unchanged, while the nearly constant Debye temperature indicates no loss of average structural rigidity to explain the decrease in the PLQY. Instead, temperature-dependent ab initio molecular dynamics (AIMD) simulations show that gradual changes of the Eu 2+ ion's local coordination environment rigidity are responsible for the drop in the observed TQ and PLQY. These results express the need to computationally analyze the local rare-earth environment as a function of temperature to understand the fundamental origin of optical properties in new inorganic phosphors. 

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