Crystal Field Effects Research Articles

Zeeman spectroscopy and crystal-field analysis of low symmetry centres in Nd3+ doped Y2SiO5

We report on infrared to visible Zeeman absorption spectroscopy and parameterised crystal-field modelling of Nd3+ centres in Y2SiO5 through the use of experimentally inferred crystal-field energy levels and Zeeman directional electronic g values. We demonstrate that good agreement between the calculated and experimental crystal-field energy levels as well as directional Zeeman g values along all three crystallographic axes can be obtained. Further, we demonstrate that the addition of correlation crystal field effects successfully account for discrepancies that arise between the calculated and experimental values relevant to the 2H(2) multiplet in a one-electron crystal field model.

Influence of high pressure on the remarkable itinerant electron behavior in Y0.7Er0.3Fe2D4.2 compound

A monoclinic Y0.7Er0.3Fe2D4.2 compound exhibits unusual magnetic properties with different field induced magnetic transitions. The deuteride is ferrimagnetic at low temperature, and the Er and Fe sublattices present magnetic transitions at different temperatures. The Er moments are ordered below TEr = 55 K, whereas the Fe moments remain ferromagnetically coupled up to TM0 = 66 K. At TM0, the Fe moments display a sharp ferromagnetic–antiferromagnetic transition (FM–AFM) through itinerant electron metamagnetic behavior very sensitive to any volume change. Y0.7Er0.3Fe2D4.2 becomes paramagnetic above TN = 125 K. The pressure dependence of TEr and TM0 has been extracted from magnetic measurements under hydrostatic pressure up to 0.49 GPa. Both temperatures decrease linearly upon applied pressure with dTEr/dP = −126 and dTM0/dP = −140 K GPa−1 for a field of B = 0.03 T. Both magnetic Er and ferromagnetic Fe orders disappear at P = 0.44(4) GPa. However, under a larger applied field B = 5 T, dTM0/dP = −156 K GPa−1, whereas dTEr/dP = −134 K GPa−1 showing weaker sensitivity to pressure and magnetic field. At 2 K, the decrease of the saturation magnetization under pressure can be attributed to a reduction of the mean Er moment due to canting and/or a crystal field effect. Above TM0, the magnetization curves display metamagnetic behavior from an AFM to FM state, which is also very sensitive to the applied pressure. The transition field Btrans, which increases linearly upon heating, is shifted to a lower temperature upon applied pressure with ΔT = −17 K between 0 and 0.11 GPa. These results show strong decoupling of the Er and Fe magnetic sublattices vs temperature, applied field, and pressure.

Dzyaloshinskii–Moriya interactions in Nd2Fe14B as the origin of spin reorientation and the rotating magnetocaloric effect

In this study, the mechanism of spin reorientation in Nd2Fe14B, which is the host crystal of prevalently employed neodymium permanent magnets, was investigated by combining first–principles calculations and Monte Carlo simulations. Spin reorientation is thought to originate from crystal field effects and has not received significant attention from researchers because it is an undesirable property in hard magnet applications. Similarly, Dzyaloshinskii–Moriya interactions are often ignored in rare–earth bulk systems, including permanent magnets such as Nd2Fe14B, because it is believed that magnetic anisotropy is more dominant than Dzyaloshinskii–Moriya interactions. However, in this study, for the first time, we found that the spin reorientation in Nd2Fe14B is caused by Dzyaloshinskii–Moriya interactions. Furthermore, we found that the spin reorientation in Nd2Fe14B results in the rotating magnetocaloric effect, which might be used for practical applications. We also found that the Dzyaloshinskii–Moriya interactions non-negligibly affect the physical properties of magnetic materials.

Diffusive excitonic bands from frustrated triangular sublattice in a singlet-ground-state system

Magnetic order in most materials occurs when magnetic ions with finite moments arrange in a particular pattern below the ordering temperature. Intriguingly, if the crystal electric field (CEF) effect results in a spin-singlet ground state, a magnetic order can still occur due to the exchange interactions between neighboring ions admixing the excited CEF levels. The magnetic excitations in such a state are spin excitons generally dispersionless in reciprocal space. Here we use neutron scattering to study stoichiometric Ni2Mo3O8, where Ni2+ ions form a bipartite honeycomb lattice comprised of two triangular lattices, with ions subject to the tetrahedral and octahedral crystalline environment, respectively. We find that in both types of ions, the CEF excitations have nonmagnetic singlet ground states, yet the material has magnetic order. Furthermore, CEF spin excitons from the tetrahedral sites form a dispersive diffusive pattern around the Brillouin zone boundary, likely due to spin entanglement and geometric frustrations.

Study of Magnetic and Magnetocaloric Properties of the Nano‐Graphene of Mixed Spins S = 3/2 and σ = 2: A Monte Carlo Simulation

The magnetic and magnetocaloric properties of the nano‐graphene monolayer structure consisting of mixed spins S = 3/2 and σ = 2 are studied by using Monte Carlo simulations. The ground‐state phase diagrams corresponding to several physical parameters are first discussed. The influence of the number of lattice sites on the total and partial magnetizations and the magnetic susceptibilities has been investigated. The critical temperature is deduced. The effect of exchange interaction, external magnetic, and crystal field on total magnetizations is studied. Magnetic hysteresis cycles are obtained. Specific heat change and magnetic entropy changes are inferred for several magnetic fields. Finally, the relative cooling power is determined.

Modifying the magnetoelectric coupling in TbMnO3 by low-level Fe3+ substitution

We report a comprehensive study of the low-level substitution of ${\mathrm{Mn}}^{3+}$ by ${\mathrm{Fe}}^{3+}$ effect on the static and dynamic magnetoelectric coupling in $\mathrm{Tb}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{3}$ ($x=0$, 0.02, and 0.04). The cationic substitution has a large impact on the balance between competitive magnetic interactions and, as a result, on the stabilization of the magnetic structures and ferroelectric phase at low temperatures. Low-lying electromagnon excitation is activated in the cycloidal modulated antiferromagnetic and ferroelectric phase in $\mathrm{TbMn}{\mathrm{O}}_{3}$, while it is observed up to ${T}_{N}$ in the Fe-substituted compounds, pointing at different mechanisms for static and dynamic magnetoelectric coupling. A second electrically active excitation near 40 $\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ is explained by means of ${\mathrm{Tb}}^{3+}$ crystal-field effects. This excitation is observed up to room temperature, and exhibits a remarkable $15\phantom{\rule{4pt}{0ex}}\text{c}{\mathrm{m}}^{\ensuremath{-}1}$ downshift on cooling in Fe-substituted compounds. Both electromagnon and crystal-field excitations are found to be coupled to the polar phonons with frequencies up to 250 $\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$. Raman spectroscopy reveals a spin-phonon coupling below ${T}_{N}$ in pure $\mathrm{TbMn}{\mathrm{O}}_{3}$, but the temperature where the coupling starts to be relevant increases with Fe concentration and reaches 100 K in $\mathrm{Tb}{\mathrm{Mn}}_{0.96}{\mathrm{Fe}}_{0.04}{\mathrm{O}}_{3}$. The anomalies in the $T$ dependence of magnetic susceptibility above ${T}_{N}$ are well accounted for by spin-phonon coupling and crystal-field excitation, coupled to oxygen motions.

Ligand-Receptor Interactions of Lamivudine: A View from Charge Density Study and QM/MM Calculations

The nature and strength of interactions for an anti-HIV drug, Lamivudine, were studied in a pure crystal form of the drug and the ligand-receptor complexes. High-resolution single-crystal X-ray diffraction studies of the tetragonal polymorph allowed the drug's experimental charge density distribution in the solid state to be obtained. The QM/MM calculations were performed for a simplified model of the Lamivudine complex with deoxycytidine kinase (two complexes with different binding modes) to reconstruct the theoretical charge density distribution. The peculiarities of intramolecular interactions were compared with previously reported data for an isolated molecule. Intermolecular interactions were revealed within the quantum theory of 'Atoms in Molecules', and their contributions to the total crystal energy or ligand-receptor binding energy were evaluated. It was demonstrated that the crystal field effect weakened the intramolecular interactions. Overall, the energies of intermolecular interactions in ligand-receptor complexes (320.1-394.8 kJ/mol) were higher than the energies of interactions in the crystal (276.9 kJ/mol) due to the larger number of hydrophilic interactions. In contrast, the sum of the energies of hydrophobic interactions was found to be unchanged. It was demonstrated by means of the Voronoi tessellation that molecular volume remained constant for different molecular conformations (250(13) Å3) and increased up to 399 Å3 and 521(30) Å3 for the Lamivudine phosphate and triphosphate.

Radiation-induced scintillation properties of Ce-doped lutetium aluminosilicate glasses doped with cerium prepared by using a xenon image furnace

Ce-doped Lu2O3–Al2O3–SiO2 glasses were prepared using a xenon image furnace and their physical, optical, and scintillation properties were investigated. The photoluminescence (PL) properties were discussed based on the variations in the coordination environment of the different glass compositions. The characteristic shifts of PL spectra were explained in terms of (i) effects of crystal field due to the presence of different coordination environments which Ce3+ can take and (ii) effects of self-absorption due to the increase in Ce4+ based on the results of Raman spectra, transmission spectra, and PL lifetimes. The PL quantum yield (QY) tends to be higher for specimens with the emission peak located on the relatively shorter wavelength side, i.e., those with significant Ce3+ in the 6-coordinated environment and low Ce4+ content. This tendency also affects scintillation properties, with both scintillation spectral intensity and light yield under γ-ray irradiation showing characteristics that correlate positively with PL QY.

Energetics and Ionic–Electronic and Geometric Variabilities of Hydroxylammonium-Based Salts

In the present work, we study the formation energy of energetic ionic salts (EISs), as well as the volumetric and electric variabilities of ions involved in EISs, by employing 25 hydroxylammonium cations (HA+) in 22 salts as examples. We identify the viability of a dispersion-corrected density functional theory method, PBE-D3(BJ), to describe the lattice parameters, with a mean relative deviation of 1.26% and a maximal relative deviation of 2.5%. Thereby, the ionization energy, formation energy, and lattice energy are predicted. The variability of volume (VHA+) is confirmed in different HA+-based salts, as well as between two HA+ of the same lattice with Z′ > 1. The largest difference of VHA+ among the 25 HA+ reaches ∼9%. This variability of VHA+ is originated from the difference in crystal field effects, or intermolecular interactions. That is, the stronger intermolecular hydrogen bond tends to exhibit a stronger drag and pull effect on HA+ and leads to a larger VHA+. In comparison, it exhibits a more significant variability of electricity, as the maximal total Mulliken charge of HA+ (QHA+) is >1.67 times the minimal one. Moreover, the observed QHA+ varying from 0.164 to 0.438 e is much less than the apparent charge of 1 e, suggesting the highly insufficient charge of HA+ in EISs. This means that the ionic bonding does not necessarily govern the intermolecular interactions in common EISs. Besides, an electrostatic potential (ESP) minimum-oxygen balance (OB) correlation is found in the anions countering HA+, as the more negative OB tends to cause a more negative ESP minimum. All of these findings are expected to richen the insight into the nature of EISs as a special group of EMs.

Rational Manipulation of Epitaxial Strains Enabled Valence Band Convergence and High Thermoelectric Performances in Mg3Sb2 Films

AbstractStrain engineering is demonstrated to effectively regulate the functionality of materials, such as thermoelectric, ferroelectric, and photovoltaic properties. As the straightforward approach of strain engineering, epitaxial strain is usually proposed for rationally manipulating the electronic structure and performances of thermoelectric materials, but has rarely been verified experimentally. In this study, tunable and large epitaxial strains are demonstrated, as well as the resulting valence band convergence can be achieved in the Mg3Sb2 epi‐films with the choice of substrates. The large epitaxial strains up to 8% in Mg3Sb2 films represent one of the most striking results in strain engineering. The angle‐resolved photoemission spectroscopy measurements and the theoretical calculations reveal the vital role of epitaxial strain in tuning the crystal field splitting and the band structure of Mg3Sb2. Benefiting from the appropriate manipulation of the crystal field effect via in‐plane compressive strain, the valence band convergence is unambiguously discovered in the strained Mg3Sb2 film grown on InP(111) substrate. As a result, a state‐of‐the‐art thermoelectric power factor of 0.94 mWm−1K−2 is achieved in the strain‐engineered Mg3Sb2 film, well exceeding that of the strain‐relaxed Mg3Sb2. The work paves the way for effectively manipulating epitaxial strain and band convergence for Mg3Sb2 and other thermoelectric films.

Universal scaling between on-site Coulomb repulsion and numbers of core and valence electrons in transition metal trichalcogenides

Electron correlations that determine a broad spectrum of the physical properties of transition metal compounds are largely manifested by the on-site Coulomb repulsion U, which so far has been mainly evaluated on a case-by-case basis. Here we employ a linear response method based on constrained local density approximation to systematically investigate U in representative classes of transition metal trichalcogenides, with the transition metals covering all the unfilled 3d, 4d, and 5d orbitals. We uncover a characteristic scaling dependence of U on two elemental physical parameters, namely, the numbers of the core and valence electrons. Such a universal scaling law reflects the intuition that more unfilled d electrons residing on a smaller spherical core will feel stronger Coulomb repulsion. Next, by using the artificial intelligence-based SISSO (Sure Independence Screening and Sparsifying Operator) approach, we identify a more sophisticated descriptor that not only further refines the scaling law, but also captures the crystal-field splitting effect as pictorially reflected by invoking an elliptical core instead of a spherical core. The approach developed in this study should find transferability in other classes of transition metal compounds.

A broad emission band of phosphor Cs2Zn3(P2O7)2:Mn2+ induced by multi-sites of Mn2+

The emitting wavelength of Mn2+-phosphors is strongly depended by the coordination environment of Mn2+ for the effect of crystal field. This paper reports a new phosphor Cs2Zn3(P2O7)2:Mn2+ with a broad emitting band from 490 to 700 nm for the three different luminescence centers of Mn2+. The emission band at 10 K can be splitted into three Gaussian peaks at 511, 582 and 662 nm, corresponding to the Mn2+ in Zn2+ sites, Mn2+ in Cs+ site and Mn2+-Mn2+ dimers, respectively. Moreover, the temperature-cycling measurements were measured to study the thermostability. The results show that 53 % of the luminescent intensity is retained at 175 °C compared to 30 °C. We think that the Cs2Zn3(P2O7)2:Mn2+ phosphor has potential applications in LED lamp field.

Synthesis, Crystal Structures, and Optical and Magnetic Properties of Samarium, Terbium, and Erbium Coordination Entities Containing Mono-Substituted Imine Silsesquioxane Ligands.

Mono-substituted cage-like silsesquioxanes of the T8-type can play the role of potential ligands in the coordination chemistry. In this paper, we report on imine derivatives as ligands for samarium, terbium, and erbium cations and discuss their efficient synthesis, crystal structures, and magnetic and optical properties. X-ray analysis of the lanthanide coordination entities [MCl3(POSS)3]·2THF [M = Er3+ (3), Tb3+ (4), Sm3+ (5)] showed that all three compounds crystallize in the same space group with similar lattice parameters. All compounds contain an octahedrally coordinated metal atom, and additionally, 3 and 5 structures are strictly isomorphous. However, surprisingly, there are two different molecules in the crystal structure of the terbium coordination entity 4, monomer (sof 65%) and dimer (sof 35%), with one and two metal centers. Absorption measurements of the investigated materials recorded at 300 K showed that regardless of the lanthanide involved, their energy band gap equals 2.7 eV. Moreover, the analogues containing Tb3+ and Sm3+ exhibit luminescence typical of these rare earth ions in the visible and infrared spectral range, while the compound with Er3+ does not generate any emission. Direct current variable-temperature magnetic susceptibility measurements on polycrystalline samples of 3-5 were performed between 1.8 and 300 K. The magnetic properties of 3 and 4 are dominated by the crystal field effect on the Er3+ and Tb3+ ions, respectively, hiding the magnetic influence between the magnetic cations of adjacent molecules. Complex 5 exhibits a nature typical for the paramagnetism of the samarium(III) cation.

Excitons and Their Fine Structure in Lead Halide Perovskite Nanocrystals from Atomistic GW/BSE Calculations

Atomistically detailed computational studies of nanocrystals, such as those derived from the promising lead-halide perovskites, are challenging due to the large number of atoms and lack of symmetries to exploit. Here, focusing on methylammonium lead iodide nanocrystals, we combine a real-space tight binding model with the GW approximation to the self-energy and obtain exciton wave functions and absorption spectra via solutions of the associated Bethe–Salpeter equation. We find that the size dependence of carrier confinement, dielectric contrast, electron–hole exchange, and exciton binding energies has a strong impact on the lowest excitation energy, which can be tuned by almost 1 eV over the diameter range of 2–6 nm. Our calculated excitation energies are about 0.2 eV higher than experimentally measured photoluminescence, and they display the same qualitative size dependence. Focusing on the fine structure of the band-edge excitons, we find that the lowest-lying exciton is spectroscopically dark and about 20–30 meV lower in energy than the higher-lying triplet of the bright states whose degeneracy is slightly broken by crystal field effects.

Tuning the interplay of spin-orbit coupling and trigonal crystal-field effect in the Ising-like spin system Ca3Co2O6

In the ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2}{\mathrm{O}}_{6}$ (CCO) system, the large contribution of the orbital moment to the magnetization and the strong magnetocrystalline anisotropy (MCA) are considered to give rise to the Ising magnetism. In this study, the dominant role of both spin-orbit-coupling (SOC) and crystal-field (CF) effects behind this Ising character of magnetism is qualitatively elucidated from the temperature and field dependence of magnetization in the presence of hydrostatic pressures up to 1.04 GPa in a CCO single crystal (SC). The local trigonal prismatic environment is compressed with the application of high pressure, resulting in higher trigonal CF as compared to ambient conditions. It reduces the effect of SOC due to the initiation of orbital quenching that finally decreases orbital moment contributions to both the magnetization and the MCA, respectively. This interplay of triagonal CF and SOC effects is further shown from the detailed quantitative analysis of the field-dependent magnetization in different orientations of CCO and ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{1.8}{\mathrm{Fe}}_{0.2}{\mathrm{O}}_{6}$ (CCFO) SCs at ambient pressure by employing a simple classical model and second-order perturbative analysis of SOC. The complete quenching of the orbital moment of ${\mathrm{Fe}}^{3+}$ ($S=5/2$) in CCFO weakens the MCA and also helps in deducing the SOC effect. Furthermore, the estimated anisotropic constants using density functional theory very well capture the Ising magnetism in CCO and deviation from it in CCFO compared to that of classical results.

Understanding electrostatics and covalency effects in highly anisotropic organometallic sandwich dysprosium complexes [Dy(CmRm)2] (where R = H, SiH3, CH3 and m = 4 to 9): a computational perspective.

In this article, we have thoroughly studied the electronic structure and 4f-ligand covalency of six mononuclear dysprosium organometallic sandwich complexes [Dy(CmRm)2]n+/- (where R = H, SiH3, CH3; m = 4 to 9; n = 1, 3) using both the scalar relativistic density functional and complete active space self-consistent field (CASSCF) and N-electron valence perturbation theory (NEVPT2) method to shed light on the ligand field effects in fine-tuning the magnetic anisotropy of these complexes. Energy decomposition analysis (EDA) and ab initio-based ligand field theory AILFT calculations predict the sizable 4f-ligand covalency in all these complexes. The analysis of CASSCF/NEVPT2 computed spin-Hamiltonian (SH) parameters indicates the stabilization of mJ |±15/2〉 for [Dy(C4(SiH3)4)2]- (1), [Dy(C5(CH3)5)2]+ (2) and [Dy(C6H6)2]3+ (3) complexes with the Ucal value of 1867.5, 1621.5 and 1070.8 cm-1, respectively. On the other hand, we observed mJ |±9/2〉 as the ground state for [Dy(C7H7)2]3- (4) and [Dy(C8H8)2]- (5) complexes with significantly smaller Ucal values of 237.1 and 38.6 cm-1 respectively. For the nine-membered ring [Dy(C9H9)2]+ (6) complex, we observed the stabilization of the mJ |±1/2〉 ground state, with the first excited state being located ∼29 cm-1 higher in energy. AILFT-NEVPT2 ligand field splitting analysis indicates that the presence of π-type 4f-ligand interactions in complexes 1-3 help generate the axial-ligand field, while the δ-type interactions in complexes 4-5 generate the equatorial ligand field despite the ligands approaching from the axial direction. As the ring size increases, φ-type interactions dominate, generating a pure equatorial ligand field stabilising mJ |±1/2〉 as the ground state for 6. Calculations suggest that the nature of the ligand field mainly governs the Ucal values in the following order: 4f-Lσ > 4f-Lπ > 4f-Lδ > 4f-Lφ. Calculations were performed by replacing ligands with CHELPG charges to access the crystal field (CF) effects which suggests the stabilization of pure mJ |±15/2〉 in all the charge-embedded models (1Q-6Q). Our findings point out that the crystal field and ligand field effects complement each other and generate a giant barrier for magnetic relaxation in the small ring complexes 1-3, while a relatively weak crystal field and adverse 4f-Lδ/4f-Lφ interactions diminish the SMM behaviour in the large ring complexes 4-6.

Magnetic susceptibility of a ternary Ising nanoisland

We investigate magnetic properties of a ternary ( 1 / 2 , 1, 3 / 2 ) bilayer Ising nanoisland with an 18-site quadratic structure. By employing exact enumeration, we study the effects of crystal fields, exchange couplings and thermal fluctuations on the magnetic susceptibility and blocking temperature. As a result, we find several scenarios in which a double peak structure in the thermal dependence of the magnetic susceptibility can be observed. In particular, the competition between core–shell antiferromagnetic interactions and the external magnetic field can drive an additional low-temperature maximum in the magnetic susceptibility. In addition, the competition between core–shell couplings and a strong crystal field in the core spins can also provide a scenario in which the magnetic susceptibility shows two maximum. From the magnetic susceptibility maximum at higher temperatures, we estimate the blocking temperature. Surprisingly, we note that an increase in the absolute value of the exchange couplings can reduce the blocking temperature. Moreover, our findings suggest that the mechanism driving the double maxima in the magnetic susceptibility also provides a context in which the increase in the strength of the exchange coupling can lead to a reduction in the blocking temperature. Therefore, our results allow us to describe different scenarios in which double-peak structures in the magnetic susceptibility of nanoparticles can be spotted, also suggesting that these scenarios can favor a reduced blocking temperature. • Magnetism of a mixed-spin (1/2,1,3/2) nanoisland with core–shell structure has been investigated. • Effects of crystal fields and exchange couplings are evaluated. • Thermal dependence of magnetic susceptibility shows a double-peak structure. • Blocking temperature shows non-monotonic dependence on the core–shell couplings.

Dual-mode luminescence of Er3+/Yb 3+ codoped LnP0.5V0.5O4 (Ln=Y, Gd, La) for highly sensitive optical nanothermometry

The luminescence properties of 2%Er3+/15%Yb3+ doped LnP0.5V0.5O4 (LnPVO4) (Ln = Y, Gd, La) phosphors, synthesized via the traditional citric-assisted Sol gel method, are studied under light excitations of 980 nm and 325 nm to generate the 2H11/2/4S3/2–4I15/2 transitions via up- and downshifting mechanisms, respectively. The phase purity of the samples was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). We present herein a comparative study of the spectral and temperature sensing properties of LnPVO4: Er3+/Yb3+ (Ln = Y, Gd, La) phosphors. The crystal field effects on the up-shifting luminescence and on the thermometric parameters are investigated by the substitution of Er3+ ions on Y3+, Gd3+ or La3+ sites in the YPVO4, GdPVO4 and LaPVO4 hosts respectively. Fluorescence intensity ratio (FIR) technique was used to study the temperature sensing behavior of the phosphors. This study showed that downshifting emission gives the highest thermal sensitivities and the greatest thermal resolution compared to downshifting emission. These outcomes indicate that these materials are preferred for use in the luminescence temperature sensing in a down-conversion process to provide the greatest performance.

An efficient LiSrGaF6:Cr3+ fluoride phosphor with broadband NIR emission towards sunlight-like full-spectrum lighting.

Sunlight-like full-spectrum phosphor-converted light-emitting diodes (pc-LEDs) require near-infrared (NIR) emission bands to fill the spectrum gap and consequently propel their widespread applications. Although fluoride NIR phosphors have been increasingly investigated, balancing high quantum efficiency (QE), high thermal stability, and wideband NIR emission to obtain excellent overall performance in a single system is still a challenge for Cr3+-doped fluoride NIR phosphor and is significant for direct utilization. Herein, a high-efficiency and thermally stable broadband NIR emission was realized in a novel LiSrGaF6:Cr3+ fluoride phosphor benefitting from a relatively weak crystal field and electron-phonon coupling effect. Upon blue light excitation, the ultra-broad NIR luminescence ranging from 650 to 1150 nm can be achieved with an FWHM of 149 nm peaking at ∼813 nm. Furthermore, this system possesses a high QE of up to 76.88% and its emission intensity at 423 K still maintains 61.62% of its initial intensity at room temperature. An NIR output power of 42.82 mW and photoelectric conversion efficiency of 14.27% of NIR pc-LED devices have also been presented based on this NIR phosphor, demonstrating its possible application in compact nonvisible light sources. In addition, a highly continuous sunlight-like vis-NIR pc-LED was further constructed by employing a blue chip with commercial cyan (BaSi2O2N2:Eu2+), yellow (Y3Al5O12:Ce3+), red (CaAlSiN3:Eu2+), and this LiSrGaF6:Cr3+ NIR phosphor. The as-obtained pc-LED exhibits an ultra-broad spectrum ranging from 400 nm to 1000 nm, exhibiting a higher color reproduction with a color rendering index (CRI) of 95.1 and luminous efficiency (LE) of 50.22 lm W-1. These results indicate that LiSrGaF6:Cr3+ phosphor can be a promising NIR phosphor candidate for high-quality sunlight-like full-spectrum lighting and infrared night vision technology.

Synthesis and luminescence enhancement of white-emitting Ca9La(VO4)7:Dy3+ phosphors through Gd3+ and Al3+ codoping for white-light-emitting diodes

The single-component Ca9La(VO4)7:Dy3+, Gd3+ and Ca9La(VO4)7:Dy3+, Al3+ white-emitting phosphors were prepared through a solid-state method, and the Gd3+/Al3+ was introduced in the structure for photoluminescence enhancement. The XRD results verified that the structure of samples was consistent with the Ca9La(VO4)7 standard phase. The as-prepared Ca9La(VO4)7:Dy3+ exhibited a characteristic white emission consisting of a yellow peak (570 nm) and a blue peak (478 nm), which originated from the electron transitions 4F9/2→6H13/2 and 4F9/2→6H15/2 of Dy3+, respectively. Through the introduction of Gd3+/Al3+ into Ca9La(VO4)7:Dy3+, the luminescence intensity could be enhanced by 1.520 and 1.522 times, respectively, which are due to the crystal-field environmental effect. By combining the Ca9La0.79(VO4)7:0.18Dy3+, 0.03Gd3+ and Ca9La0.82(VO4)6.97:0.18Dy3+, 0.03Al3+ with a n-UV LED chip (λex = 365 nm), the warm w-LEDs with suitable CCT and CRI values were realized. In summary, the Gd3+/Al3+ introduced Ca9La(VO4)7:Dy3+ samples can serve as excellent white-emitting phosphors for white-light-emitting diode applications.