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The Yb3+-Mn2+codoped zinc metaphosphate glass gives rise to several processes such as upconversion, downshifting and double ion absorptions producing light from the visible to the IR wavelengths. These processes are possible since the Mn2+ and the Yb3+ ions replaces the Zn2+ ion in nanoparticles of the α phase of the compound Zn(PO3)2. An important result is that the α structure presence allows the formation of Yb3+-Mn2+ dimers, that gives rise to a superexchange coupling that allows an upconversion process from the IR to the red region of the electromagnetic spectrum. The experimental results also show that these dimers can couple to produce Yb3+ ion pairs that led to a cooperative emission in the green, around 500nm, coupling also the Mn2+ ions that in turn allows to produce a double absorption of this ions in the red region of the electromagnetic spectrum and as well a downshifting process conducting to the Yb3+ emission in the infrared. If the manganese ion concentration is higher than 10% most of these effects are masked. All these results make the material an effective option for different applications, due to the large wavelength variety that can be selected for excitation or emission

This survey reports on topics that were presented at the workshop on “Challenges with Rare Earth Elements. The Periodic Table at work for new Science & Technology” hold at the Academia dei Lincei in November 2019. The herein reported materials refer to presentations dealing with studies and applications of rare earth elements (REE) in several areas of Biology and Medicine. All together they show the tremendous impact REE have in relevant fields of living systems and highlight, on one hand, the still existing knowledge gap for an in-depth understanding of their function in natural systems as well as the very important role they already have in providing innovative scientific and technological solutions in a number of bio-medical areas and in fields related to the assessment of the origin of food and on their manufacturing processes. On the basis of the to-date achievements one expects that new initiatives will bring, in a not too far future, to a dramatic increase of our understanding of the REE involvement in living organisms as well as a ramp-up in the exploitation of the peculiar properties of REE for the design of novel applications in diagnostic procedures and in the set-up of powerful medical devices. This scenario calls the governmental authorities for new responsibilities to guarantee a continuous availability of REE to industry and research labs together with providing support to activities devoted to their recovery/recycling.

In this work, we investigated from a theoretical point of view the dynamics of the energy transfer process from the ligand to Eu(III) ion for 12 isomeric species originating from six different complexes differing by nature of the ligand and the total charge. The cationic complexes present the general formula [Eu(L)(H2O)2] + (where L = bpcd2− = N,N′-bis(2-pyridylmethyl)- trans-1,2-diaminocyclohexane N,N′-diacetate; bQcd2− = N,N′- bis(2-quinolinmethyl)-trans-1,2-diaminocyclohexane N,N′-diacetate; and bisoQcd2− = N,N′-bis(2-isoquinolinmethyl)-trans-1,2- diaminocyclohexane N,N′-diacetate), while the neutral complexes present the Eu(L)(H2O)2 formula (where L = PyC3A3− = Npicolyl-N,N′,N′-trans-1,2-cyclohexylenediaminetriacetate; QC3A3− = N-quinolyl-N,N′,N′-trans-1,2-cyclohexylenediaminetriacetate; and isoQC3A3− = N-isoquinolyl-N,N′,N′-trans-1,2-cyclohexylenediaminetriacetate). Time-dependent density functional theory (TD-DFT) calculations provided the energy of the ligand excited donor states, distances between donor and acceptor orbitals involved in the energy transfer mechanism (RL), spin-orbit coupling matrix elements, and excited-state reorganization energies. The intramolecular energy transfer (IET) rates for both singlet-triplet intersystem crossing and ligand-to-metal (and vice versa) involving a multitude of ligand and Eu(III) levels and the theoretical overall quantum yields (ϕovl) were calculated (the latter for the first time without the introduction of experimental parameters). This was achieved using a blend of DFT, Judd−Ofelt theory, IET theory, and rate equation modeling. Thanks to this study, for each isomeric species, the most efficient IET process feeding the Eu(III) excited state, its related physical mechanism (exchange interaction), and the reasons for a better or worse overall energy transfer efficiency (ηsens) in the different complexes were determined. The spectroscopically measured ϕovl values are in good agreement with the ones obtained theoretically in this work. published

We present a structural and optical characterization of magnetoelastic zircon-type TmVO4 at ambient pressure and under high pressure. The properties under high pressure have been determined experimentally under hydrostatic conditions and theoretically using density functional theory. By powder X-ray diffraction we show that TmVO4 undergoes a first-order irreversible phase transition to a scheelite structure above 6 GPa. We have also determined (from powder and single-crystal X-ray diffraction) the bulk moduli of both phases and found that their compressibilities are anisotropic. The band gap of TmVO4 is found to be Eg = 3.7(2) eV. Under compression the band gap opens linearly, until it undergoes a huge collapse following the structural phase transition (ΔEg = 1.15 eV). Ab initio structural and free energy calculations support our findings. Moreover, calculations of the band structure and density of states reveal that for both zircon and scheelite TmVO4 the band gap is entirely determined by the V 3d and O 2p states of the VO43– ion. The behavior of the band gap can thus be understood entirely in terms of the structural modifications of the VO4 units under compression. Additionally, we have calculated the evolution of the infrared and Raman phonons of both phases upon compression. The presence of soft modes is related to the dynamic instability of the low-pressure phase and to the phase transition.

The luminescent characteristics of K3LuSi2O7:Pr3+ (1%), which is a promising optical material for the use as a scintillator, have been studied using a set of techniques. The luminescence spectra of K3LuSi2O7:Pr3+ (1%) contain two bands in the UV-range with peaks at 284 and 330 nm, which correspond intraconfigurational 5d → 4f transitions in the Pr3+ ions. The radiation in the visible and near IR range (480–850 nm) has been represented by the intraconfigurational 4f → 4f transitions. The kinetics of 5d → 4f-luminescence contains a build-up stage (τrise ∼ 7–12 ns), nonexponential decay stage (τ1/2 ∼ 60 ns), and a slow component of the μs-range when excited by high-frequency (∼8 MHz) synchrotron radiation of the X-ray range. The fast component of the decay (τ = 54 ns) dominates in the decay kinetics of luminescence along with the build-up stage while the contribution of the μs decay component is less than 0.5% at the excitation by an pulse electron beam (5Hz). The excitation spectra of d–f- and f–f-photoluminescence in the ultraviolet and vacuum ultraviolet range that are measured using synchrotron radiation reveal features that are caused by both intracenter transitions and processes related to the energy transfer from the intrinsic electronic excitations to the impurity center.

The environmental and economic benefits of phosphor-converted white-light-emitting diodes (pc-WLEDs) have been increasingly appreciated in recent years. However, a significant challenge in this fie...

Eu3+ (1 mol %)-doped Ca2LnSbO6 (replacing Ln3+; Ln = Lu, Y, Gd, and La) and Ca2EuSbO6 were synthesized and structurally characterized by means of X-ray powder diffraction. The Eu3+ luminescence spectroscopy of the doped samples and of Ca2EuSbO6 has been carefully investigated upon collection of the excitation/emission spectra and luminescence decay curves of the main excited states. Surprisingly, apart from the dominant red emission from 5D0, all the doped samples show an uncommon blue and green emission contribution from 5DJ (J = 1, 2, and 3). This is made possible thanks to both multiphonon and cross-relaxation mechanism inefficiencies. However, the emission from 5D3 is more efficient and the decay kinetics of the 5DJ (J = 0, 1, and 2) levels is slower in the case of Y- and Lu-based doped samples. This evidence can find a possible explanation in the crystal chemistry of this family of double perovskites: our structural investigation suggests an uneven distribution of the Eu3+ dopant ions in Ca2YSbO6 and Ca2LuSbO6 hosts of the general A2BB′O6 formula. The luminescent center is mainly located in the A crystal site, and on average, the Eu–Eu distances are longer than in the case of the Gd- and La-based matrix. These longer distances can further reduce the efficiency of the cross-relaxation mechanism and, consequently, the radiative transitions are more efficient. The slower depopulation of Eu3+ 5D2 and 5D1 levels in Ca2YSbO6 and Ca2LuSbO6 hosts is reflected in the longer rise observed in the 5D1 and 5D0 decay curves, respectively. Finally, in Ca2EuSbO6, the high Eu3+ concentration gives rise to an efficient cross-relaxation within the subset of the lanthanide ions so that no emission from 5DJ (J = 1, 2, and 3) is possible and the 5D0 decay kinetics is faster than for the doped samples., Eu3+ as a dopant ion in the series of double perovskite Ca2LnSbO6 (Ln = La, Gd, Y, and Lu) hosts shows a different occupation of the available A and B crystal sites; this strongly affects the luminescence behavior in the visible spectral region.

[EN] YBO3 is used in photonics applications as a host for red phosphors due to its desirable chemical stability, high quantum efficiency and luminescence intensity. Despite its fundamental thermodynamic nature, the isothermal bulk modulus of YBO3 has remained a contentious issue due to a lack of comprehensive experimental and theoretical data and its vibrational modes are far from being understood. Here, we present an experimental-theoretical structural and vibrational study of YBO3. From structural data obtained from synchrotron X-ray diffraction data and ab initio calculations, we have determined the YBO3 bulk modulus, isothermal compressibility tensor and pressure-volume (P-V) equation of state (EoS). The isothermal compressibility tensor reveals that the compressibility of YBO3 is highly anisotropic, with the principal compression axis lying perpendicular to the ab-plane being approximately twice as stiff as the two axes perpendicular to it. From the vibrational data obtained from Raman scattering measurements and ab initio calculations, the experimental and calculated pressure response of the YBO3 Raman modes is also determined with the corresponding Grüneisen parameters and the symmetry of the experimental modes has been tentatively assigned and discussed. No evidence for a pressure-induced phase transition in YBO3 is observed up to 27 GPa, however we note that an apparent discontinuity in the compressibility at 8 GPa, likely due to the onset of non-hydrostaticity, could lead to the misinterpretation of an atypically high bulk modulus., The authors thank the financial support from the Spanish Ministerio de Ciencia, Innovacion y Universidades, Spanish Research Agency (AEI), Generalitat Valenciana, and European Fund for Regional Development (ERDF, FEDER) under grants no. F15201783295-P, MAT2016-75586-C4-1/2/3-P, RTI2018-101020-BI00, PID2019-106383 GB-C41/C42/C43, RED2018-102612-T (MALTA Consolier Team), and Prometeo/2018/123 (EFIMAT). R.T. acknowledges funding from the Spanish MINECO via the Juan de la Cierva Formacion program (FJC2018-036185-I), and J.A.S. acknowledges funding from the Ramon y Cajal fellowship program (RYC-201517482). The authors gratefully thank Erica Viviani (Univ. Verona) for expert technical assistance. The authors also acknowledge ALBA synchrotron (Barcelona, Spain) for provision of synchrotron radiation facilities at the MSPD-BL04 beamline. Parts of this research were carried out under proposal 2016021648.

We present a spectroscopic study of Mn-doped Mg2TiO4 as a function of pressure and temperature to check its viability as a red-emitting phosphor. The synthesis following a solid-state reaction route yields not only the formation of Mn4+ but also small traces of Mn3+. Although we show that Mn4+ photoluminescence is not appreciably affected by the presence of Mn3+, its local structure at the substituted Ti4+ host site causes a reduction of the Mn4+ pumping efficiency yielding a drastic quantum-yield reduction at room temperature. By combining Raman and time-resolved emission and excitation spectroscopies, we propose a model for explaining the puzzling nonradiative and inefficient pumping processes attained in this material. In addition, we unveil a structural phase transition above 14 GPa that worsens their photoluminescence capabilities. The decrease of emission intensity and lifetime with increasing temperature following different thermally activated de-excitation pathways is mostly related to relatively small activation energies and the electric−dipole transition mechanism associated with coupling to odd-parity vibrational modes. A thorough model based on the configurational energy level diagram to the A1g normal mode fairly accounts for the observed excitation and emissionthe quantum yieldof this material.

In the pursuit of a systematic characterization of rare-earth vanadates under compression, in this work we present a multifaceted study of the phase behavior of zircon-type orthovanadate PrVO

Despite the outstanding optoelectronic properties of MoS2 and its analogues, synthesis of such materials with desired features including fewer layers, arbitrary hollow structures, and particularly specifically customized morphologies, via inorganic reactions has always been challenging. Herein, using predesigned lanthanide‐doped upconversion luminescent materials (e.g., NaYF4:Ln) as templates, arbitrary MoS2 hollow structures with precisely defined morphologies, widely variable dimensions, and very small shell thickness (≈2.5 nm) are readily constructed. Most importantly, integration of the near‐infrared‐responsive template significantly improves the photoresponse of up to 600 fold in device made of NaYF4:Yb/Er@MoS2 compared with that of MoS2 nanosheets under 980 nm laser illumination. Multichannel optoelectronic device is further fabricated by simply changing luminescent ions in the template, e.g., NaYF4:Er@MoS2, operating at 1532 nm light excitation with a 276‐fold photoresponse enhancement. The simple chemistry, easy operation, high reliability, variable morphologies, and wide universality represent the most important advantages of this novel strategy that has not been accessed before., A facile template‐based strategy for reliable construction of arbitrary MoS2 hollow architectures with widely varied morphologies and dimensions is reported. An up to 600‐fold improvement of photoresponse to 980 nm light illumination is obtained in the representative optoelectronic device made of NaYF4:Yb/Er@MoS2 composite, compared with that of MoS2 nanosheets.

The study deals with the investigation of luminescence properties and energy transfer processes in LiSrPO4 doped with Pr 3+ and co-doped with Dy 3+ or Sm 3+. Ultraviolet (UV) and vacuum ultraviolet (VUV) excited emission and excitation spectra are studied, as well as pulsed cathodoluminescence spectra in wide temperature range 5–295 K. In both compounds, Pr 3+ plays a role of a sensitizer of co-dopant ion emission in visible range via energy transfer processes. Studies of the photoluminescence excitation spectra show rather complex energy transfer mechanisms and defect emission influence on the emission spectra. The use of Pr 3+ in pair with Dy3 + , Sm 3+ and other ions may enhance the luminescence properties of a compound via increased absorption in UV range, enhancing its suitability to be used as possible white light emitting diodes (W-LEDs) phosphors.

International audience; The paper discusses the origin of emission quenching and the formation of metal-to-metal charge-transfer states in yttrium orthovanadate-arsenates doped with Tb3+ ions. For that purpose, a series of highly crystalline yttrium orthovanadate-arsenate nanoparticles (type YVxAs1–xO4, where x = 2–65 mol %) doped with Tb3+ ions were prepared using the co-precipitation method. The structural and morphological properties of the particles were studied by means of X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) techniques. The photoluminescence spectral and dynamical properties of YVxAs1–xO4 doped with 1 mol % Tb3+ ions were investigated in detail. It was found that the emission quenching is related to changes in the band gap and Tb3+-to-V5+ metal-to-metal charge transfer. Further, the Tb3+-to-As5+ metal-to-metal charge transfer was identified and rationalized based on different models.

In pursue of a systematic characterization of rare-earth vanadates under compression, in this work we present a multifaceted study of the phase behavior of zircon-type orthovanadate PrVO$_4$ under high pressure conditions, up until 24 GPa. We have found that PrVO$_4$ undergoes a zircon to monazite transition at around 6 GPa, confirming previous results found by Raman experiments. A second transition takes place above 14 GPa, to a BaWO$_4$-I--type structure. The zircon to monazite structural sequence is an irreversible first-order transition, accompanied by a volume collapse of about 9.6%. Monazite phase is thus a metastable polymorph of PrVO$_4$. The monazite-BaWO$_4$-II transition is found to be reversible instead and occurs with a similar volume change. Here we report and discuss the axial and bulk compressibility of all phases. We also compare our results with those for other rare-earth orthovanadates. Finally, by means of optical-absorption experiments and resistivity measurements we determined the effect of pressure on the electronic properties of PrVO$_4$. We found that the zircon-monazite transition produces a collapse of the band gap and an abrupt decrease of the resistivity. The physical reasons for this behavior are discussed. Density-functional-theory simulations support our conclusions., 34 pages,19 Figures (Including Supporting Information). Supporting Information. Submitted to Inorganic Chemistry

We have studied the high-pressure behavior of FeVO

We report on the synthesis and comprehensive characterization of Ce3+ doped LiLa9(SiO4)6O2 (LLSO) crystals, grown using the slow cooling flux method. The luminescence and optical spectroscopic properties and luminescence decay kinetics were studied upon excitation with UV, VUV, X-ray photons and cathode rays. Two impurity Ce3+ centres substituting for La3+- ions in two nonequivalent crystal positions are observed in LLSO:Ce3+. They are well distinguished spectroscopically in the UV range both in emission and excitation spectra. Apart from emission associated with Ce3+ 5 d→4f interconfigurational transitions, a low-temperature defect-related emission has been observed upon UV-VUV and higher energy X-ray excitation. Processes lying behind the temperature quenching and host-to-impurity energy transfer are analyzed and discussed.

We studied the electronic and vibrational properties of monazite-type SrCrO4 under compression. The study extended the pressure range of previous studies from 26 to 58 GPa. The existence of two previously reported phase transitions was confirmed at 9 and 14 GPa, and two new phase transitions were found at 35 and 48 GPa. These transitions involve several changes in the vibrational and transport properties with the new high-pressure phases having a conductivity lower than that of the previously known phases. No evidence of chemical decomposition or metallization of SrCrO4 was detected. A tentative explanation for the reported observations is discussed.

In this work the optical spectroscopy and the energy transfer processes involving the Tb3+and Tm3+ ions, have been studied in eulytite double phosphate hosts of the type Sr3Y(PO4)3 doped with various amounts of the two Ln ions. It has been found that several energy transfer and cross-relaxation processes are active in this class of materials, upon excitation in the 5D4 level of Tb, and in the 1G4 one of Tm. In particular, a Tb→Tm transfer of excitation has been found to quench strongly the 5D4 level of Tb. This process occurs with a transfer efficiency increasing from 0.08 to 0.62, for a donor concentration of 2 mol%, and an acceptor concentration increasing from 2 to 15 mol%. The emission spectra are strongly affected by the presence of Tb⟷Tm energy transfer, and Tm→Tm cross relaxation processes.

The synthesis of the new chiral ligand N, N′-bis[(2-quinolylmethyl]-cyclohexanediamine-N-tert-butylacetate N′-acetic acid and its Eu complex is presented. The spectroscopic investigation of the complex has been performed both in the solid state and in solution of ethanol. Thanks to the heteroaromatic quinoline ring, an efficient ligand to europium energy transfer (antenna effect) is possible. The low symmetry of the ligand (C1 point symmetry) guarantees a strongly distorted Eu(III) geometric environment and the predominance of the 5D0→7F2 hypersensitive band, in the Eu(III) luminescence emission spectrum. As for the majority of the Eu(III)-based coordination compounds, the observed lifetime of the 5D0 excited state of the complex under investigation falls in the range of ms. Further, its good solubility in polar protic solvent (ethanol) and the excitation around 320 nm, which minimizes interfering excitation of chromophores in biological media, candidates this molecule for applications in the biomedical field.

Zircon-type NdVO4 and scheelite-type PrVO4 have been studied by means of Raman spectroscopy up to approximately 20 GPa. In the first compound, zircon-scheelite and scheelite-fergusonite phase transitions are reported at 6.4(3) and 19.6(4) GPa, respectively. In the case of scheelite-type PrVO4, a reversible phase transition to a PbWO4-III structure is observed at 16.8(5) GPa. In both cases, a scheelite-type structure is recovered in a metastable state at low pressures. The pressure evolution of the Raman modes is also reported. Our experimental findings are supported by ab initio calculations, which allowed us to discuss the role of mechanic and dynamical instabilities in the phase transition mechanisms.

In this work the structural and spectroscopic properties of several materials belonging to the K 3 Lu 1− x Y x (PO 4 ) 2 family and activated with 1% of Pr 3+ have been studied in detail. The structural characterization has been performed by X-ray diffraction. The crystal structure of the host has been determined by means of Rietveld refinement of the powder diffraction pattern, in the case of K 3 Y(PO 4 ) 2 doped with 1%of Pr 3+ .The optical emission upon UV direct excitation into the dopant ion is dominated by the Pr 3+ 4f 1 5d 1 →4f 2 interconfigurational transitions; the substitution of Lu by Y causes the blue shift of the 4f 1 5d 1 →4f 2 emission band. This is due to an important change of the Pr 3+ local environment. Upon X-ray excitation it has been found that the ratio between the 4f 1 5d 1 →4f 2 and 4f 2 →4f 2 emission intensities varies progressively when Y 3+ concentration is increased. This behavior is explained in terms of the favorable overlap between the defect-related luminescence of the host and the Pr 3+ 4f 2 →4f 2 intraconfigurational transitions. No thermal quenching of the Pr 3+ luminescence has been observed up to 500 K.

This review addresses the energy transfer processes involving lanthanide ions, analysing in particular the case of the Tb 3+ →Eu 3+ energy transfer processes in terbium-based materials containing Eu 3+ as a dopant. These examples are used as case studies for the processes and mechanisms concerning energy transfer generally involving Ln 3+ ions, pointing out the differences caused by the structural characteristics of the host. The review introduces the theoretical background of the energy transfer processes, providing a brief description of the possible regimes for the donor–acceptor transfer depending on the donor concentration, together with the general considerations on the specific case of Tb 3+ →Eu 3+ transfer. The review is completed by a brief survey of the literature published on this topic from the 60 s to date, including some of the most recent papers.

The luminescence spectroscopy of Tb3+ and Eu3+ has been studied in the Ca9Tb(PO4)7, Ca9Eu(PO4)7, and Ca9Tb0.9Eu0.1(PO4)7 materials having a whitlockite structure, by using excitation in the near UV, vacuum UV and X-ray regions. The Eu3+ ion in Ca9Eu(PO4)7 is located mainly in two cationic sites, as evidenced by the fine structure of the 5D0 → 7F0 transition at 5 K. In the case of Ca9Tb0.9Eu0.1(PO4)7, weak Tb3+ → Eu3+ energy transfer is observed upon excitation in the UV bands of Tb3+. The low efficiency of the transfer appears to be due to slow energy migration in the 5D4 subset of the Tb3+ ions. The overall behavior is strongly affected by the multisite and disordered nature of the Tb-based whitlockite host.

Controlling luminescence in phosphors able to produce several emissions from different stable excited states determines their use in optical devices. We investigate the color control mechanism that quenches the greenish-blue emission in favor of the red one in the archetype phosphor CaTiO3:Pr3+. State-of-the-art ab initio calculations indicate that direct host-to-dopant energy transfer (released by electron–hole recombination following the interband excitation and structural reorganization) selectively populates the 1D2 red luminescent state of Pr3+ and bypasses the 3P0 greenish-blue emitter. Local defects can modulate the electron–hole recombination energy and therefore increase the red emission efficiency, as experimentally observed. The selection of red emission does not happen in CaZrO3:Pr3+ because the electron–hole recombination energy is much higher. The calculations could not support the widely accepted color control mechanism based on metal-to-metal charge transfer states. The conclusion sets new...

In this study the optical spectroscopy, the excited state dynamics, and in particular the Tb3+ → Eu3+ energy transfer, have been investigated in detail both from the theoretical and experimental po...

This paper reports the spectroscopic properties of praseodymium-doped phosphates, borates and silicates. Polycrystalline samples of Pr3+-doped KLuP2O7, Li6Y(BO3)3, LiY6O5(BO3)3, LiSrPO4, Sr9Sc(PO4)7, K3Lu(PO4)2, K3LuSi2O7 were synthesized using a solid state reaction. Spectra of X-ray excited luminescence and its decay kinetics were measured. Studied samples demonstrate interconfigurational 5d-4f, intraconfigurational 4f-4f transitions and defect-related luminescence. Comparative analysis of spectroscopic properties and application perspectives is presented. © 2020 American Institute of Physics Inc. All rights reserved. The work was partially supported by the Ministry of Science and Higher Education of the Russian Federation (through the basic part of the government mandate, project No. FEUZ-2020-0060), Act 211 Government of the Russian Federation (contract № 02.A03.21.0006) and RFMEFI62117X0012 project. The time-resolved X-ray excited measurements were performed at the Shared research center SSTRC based on the NovoFEL/VEPP-4 -VEPP-2000 facilities at Budker Institute of Nuclear Physics (Siberian Branch of Russian Academy of Sciences, Novosibirsk, Russia) while using experimental equipment funded by RFMEFI62119X0022 project. The authors are grateful to S. Omelkov (University of Tartu, Estonia) for assisting in the measurement using a pulsed electron beam.

The spectroscopy of nitrate complexes of Eu(III) and Tb(III) with chiral and racemic imine-based [L1 = (N,N’-bis(2-pyridylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine) and L3 = N, N’-bis(2-quinolylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine] and amine-based [L2 = N,N’-bis(2-pyridylmethyl)-1,2-(R,R + S,S)-cyclohexanediamine) and L4 = N,N’-bis(2-quinolylmethyl)-1,2-(R,R + S,S)-cyclohexanediamine] ligands has been studied under high hydrostatic pressure (above 100 kbar). With the increasing pressure, a reduction of the Tb(III) and Eu(III) luminescence intensity is detected for all the complexes, whilst a significant reduction of the Tb(III) and Eu(III) excited state lifetimes has been observed for all Tb-based complexes [L1Tb(NO3)3 → L4Tb(NO3)3] and only for the Eu(III) complexes containing the imine-based ligands [L1Eu(NO3)3 and L3Eu(NO3)3]. This behavior has been rationalized taking into account two main aspects: i) the relative position of the energy levels of the ligands and the metal ions and ii) the change of these position upon compression DFT calculations have been also performed to elucidate the nature of the orbitals involved in the UV electronic absorption transitions (NTO orbitals) upstream of the energy transfer process to the metal ion.

We present a study of the different high-pressure polymorphs of $\mathrm{GdV}{\mathrm{O}}_{4}$ and its stability. Powder x-ray diffraction and Raman experiments show a phase transition from a zircon- to a scheelite-type structure taking place at 6.8(4) GPa. Ab initio density functional theory calculations support this conclusion. The equations of state of these two phases are reported. In addition, we studied the pressure evolution of the Raman modes for the zircon and scheelite phases, showing good agreement between calculations and experiments. For the sake of completeness, we performed optical-absorption measurements up to 16 GPa, showing a band-gap collapse at the transition point. Beyond 20 GPa a second phase transition to a monoclinic fergusonite structure takes place as a consequence of a mechanical instability. A third transition is observed at around 29.3 GPa in Raman experiments. According to our calculations, this fourth polymorph corresponds to an orthorhombic structure described by space group Cmca. This phase involves an increase of the atomic coordination number of vanadium and gadolinium. The results are compared to those reported on isomorphic compounds.

Photoluminescence (PL) studies of yttrium oxyfluoride (YOF) doped with bismuth (Bi3+) revealed a broad ultraviolet emission band centered at 31 800 cm−1 that can be applied for silicon solar cells. YOF doped with different Bi3+ concentrations were synthesized by using the reported pyrolysis technique and annealed at 900 °C. X-ray photoelectron spectroscopy showed two doublet peaks for the Bi 4f region which were attributed to Bi3+ ions and Bi metal. The PL excitation band was a symmetric broad band centered at 37 500 cm−1 nm and corresponded to the 1S0 → 3P1 A band with a Stokes shift of 5600 cm−1. This excitation overlapped with a metal to metal charge transfer band of Bi3+. The optimum concentration was 0.4 mol % of Bi3+ and the quantum yield of this sample was about 60%.

The synthesis of lanthanide-activated phosphors is pertinent to many emerging applications, ranging from high-resolution luminescence imaging to next-generation volumetric full-color display. In particular, the optical processes governed by the 4f-5d transitions of divalent and trivalent lanthanides have been the key to enabling precisely tuned color emission. The fundamental importance of lanthanide-activated phosphors for the physical and biomedical sciences has led to rapid development of novel synthetic methodologies and relevant tools that allow for probing the dynamics of energy transfer processes. Here, we review recent progress in developing methods for preparing lanthanide-activated phosphors, especially those featuring 4f-5d optical transitions. Particular attention will be devoted to two widely studied dopants, Ce3+ and Eu2+. The nature of the 4f-5d transition is examined by combining phenomenological theories with quantum mechanical calculations. An emphasis is placed on the correlation of hos...

A spectroscopic investigation of Zn(PO3)2 glass activated with Eu3+ and Dy3+ ions is carried out from photoluminescence data. White light emission is obtained in the glass phosphor, and it is due mainly to europium 5D0→7F2 and dysprosium 4F9/2→6H15/2,13/2 emissions, being Eu3+ sensitized by Dy3+ through a non-radiative energy transfer. Non-radiative energy transfer from Eu3+ to Dy3+ is also observed upon Eu3+ excitation at 414 nm. An electric quadrupole–quadrupole interaction might be the dominant mechanism in the energy transfer among Dy3+ and Eu3+ ions, as it was revealed from decay time data. The tonality of the white phosphor can be shifted from neutral white (0.364, 0.387) of 4522 K, upon 348 nm excitation, to warm white (0.393, 0.394) of 3811 K, upon 445 nm excitation. Excitations at 348–445 nm range can be attained with InGaN based LED chips, so that Eu3+ and Dy3+ codoped Zn(PO3)2 glasses excited by these LED chips could be appropriated for solid state lighting technology as neutral and warm white light sources.

The synthesis, the structural characterization and the luminescence spectroscopy of new Ln(III) nitrate complexes (Ln = La, Eu, Gd, Tb and Lu) with the quinoline-based N, N’-bis(2-quinolylmethylidene)- 1,2-(R,R+S,S)-cyclohexanediamine, L1 and N, N’-bis(2-quinolylmethyl)- 1,2-(R,R+S,S)-cyclohexanediamine, L2 are presented. All the L1Ln(NO3)3 complexes and L2Ln(NO3)3 with Ln= La, Eu and Tb are isostructural. Metal ions such as La(III), Gd(III) and Lu(III) are able to promote intra-ligand (IL) fluorescence from L1 and L2. Gd(III) and Lu(III) in addition, are also able to stimulate IL phosphorescence at room temperature from the L2 ligand, by means of “paramagnetic effect” and/or “heavy-atom effect”. Both ligands are able to sensitize the typical f-f Eu(III) luminescence and only L2 the one of Tb(III) (green emission). The Eu(III) emission features for both complexes [L1Eu(NO3)3 and L2Eu(NO3)3] are in agreement with their structural properties. All the complexes under investigation reveal promising application in the field of luminescent devices and biomedicine. In particular, excitation at 360 nm, in the case of L1-based complexes, enable the use of glass optics and minimize interfering excitation by chromophores in biological media.

Luminescent characteristics of K3Lu(PO4)2:Pr3+ (1 and 5 mol.%) microcrystalline powders, a promising optical material for scintillation applications, were investigated using various experimental techniques. The material shows emission features connected with both high intensity interconfigurational 4f15d→4f2 transitions (broad UV emission bands) and intraconfigurational 4f2→4f2 transitions (weak emission lines in the visible range). The output of X-ray excited 4f15d→4f2 emission of Pr3+ increases with a temperature rise from 90 K to room temperature and higher depending on the Pr3+ ions concentration. The high 5% concentration of Pr3+ ions is found to be favourable for the stabilization of a monoclinic phase (P21/m space group) over a trigonal one (P3‾ space group) while emission properties of the material reveal that a phase transition occurs at higher temperatures. Decay kinetics of Pr3+ 4f15d→4f2 emission are recorded upon excitation with high repetition rate X-ray synchrotron excitation and pulse cathode ray excitation. Issues related to a non-exponential decay of luminescence and presence of slow decay components are discussed in terms of energy transfer dynamics. The presence of defects was revealed with thermoluminescence measurements and these are suggested to be the mainly responsible for delayed recombination of charge carriers on the Pr3+ 4f15d states. Some peculiarities of host-to-impurity energy transfer are discussed. © 2020 Elsevier B.V. The work was partially supported by the Ministry of Science and Higher Education of the Russian Federation (through the basic part of the government mandate, project No. FEUZ-2020-0060), Act 211 Government of the Russian Federation (contract № 02.A03.21.0006), STSM grant from COST Action TD1401 ″FAST” as well as by Estonian Research Council (project PRG629) and Estonian Center of Excellence in Research “Advanced materials and high-technology devices for sustainable energetics, sensorics and nanoelectronics” TK141 (project No. 2014-2020.4.01.15-0011) by the ERDF funding in Estonia. The time-resolved X-ray excited measurements were performed at the Shared research center SSTRC based on the NovoFEL/VEPP-4 - VEPP-2000 facilities at Budker Institute of Nuclear Physics (Siberian Branch of Russian Academy of Sciences, Novosibirsk, Russia) while using experimental equipment funded by RFMEFI62119X0022 project. Authors thank Erica Viviani (University of Verona) for assistance in the synthesis of the samples and Yulya Khatchenko (UrFU, Ekaterinburg) for assistance in the processing the experimental data. F. P. and M. B. thank the Facility “Centro Piattaforme Tecnologiche” of the University of Verona for access to the Thermo ARL X’TRA powder diffractometer.

The luminescent characteristics of the double phosphate KLuP2O7 doped with Pr3+ ions, a promising optical material for scintillator applications, were investigated using various experimental spectroscopic techniques. KLuP2O7:Pr3+ shows Pr3+ emission associated with interconfigurational 4f15d1→4f2 transitions located in the UV range (250–320 nm), intraconfigurational 4f2→4f2 transitions (weak lines in the visible spectral range) and defect-related luminescence. The light output of X-ray or cathode ray excited Pr3+ 4f15d1→4f2 luminescence is found to be nearly independent of temperature in the range of 90–400 K. The output of pulsed cathode luminescence shows thermal quenching above 450 K characterized by an activation energy of about 0.5 eV. Upon excitation with pulsed cathode rays or high-frequency (~8 MHz) X-ray synchrotron radiation, the luminescence decay kinetics of Pr3+ 4f15d1→4f2 transitions are dominated by a decay component with lifetime of about 20 ns. An additional slower decay component with lifetime of 75 ns was observed upon cathode ray excitation and was shown to result from delayed host-to-impurity energy transfer. The latter, in turn, originates from re-trapping of charge carriers at defect-related traps whose presence was demonstrated by thermoluminescence measurements. Parameters of the trapping centers were estimated. Peculiarities of host-to-impurity energy transfer are analysed and discussed.

It is well known that water density varies anomalously with temperature. However, nowadays there is still a lack of agreement on the causes of this phenomenon. In this work, we use Eu3+ luminescent ion as a probe to investigate the changes in the arrangement of water molecules around the temperature of maximum density. The slight changes in the arrangement of water molecules in the first coordination sphere of Eu3+ ions due to the water density anomaly affect the variation with temperature of Eu3+ ions emission spectra and intensity decay time. This is the first time, to the best of our knowledge, that the effect of the anomaly in the density of water is detected by a luminescent material. From these experimental results we can infer that possibly the anomaly could be caused by variation in the length of hydrogen bonds between molecules with temperature for both water and heavy water.