Electronic Spectra and Crystal-Field Analysis of Europium in Hexanitritolanthanate Systems

The luminescence spectra of Eu³⁺ at a <i>T</i>_<i>h</i> point-group site in the hexanitritolanthanate systems Cs₂NaEu­(¹⁴NO₂)₆, Cs₂NaEu­(¹⁵NO₂)₆, Rb₂NaEu­(¹⁴NO₂)₆, Cs₂LiEu­(¹⁴NO₂)₆, and Cs₂NaY­(¹⁴NO₂)₆:Eu³⁺ have been recorded between 19 500 and 10 500 cm^–1 at temperatures down to 3 K. The spectra comprise magnetic-dipole-allowed zero phonon lines, odd-parity metal–ligand vibrations, internal anion vibrations, and lattice modes, with some weak vibrational progressions based upon vibronic origins. With the aid of density functional theory calculations, the vibrational modes in the vibronic sidebands of transitions have been assigned. The two-center transitions involving NO₂^– stretching and scissoring modes are most prominent for the ⁵D₀ → ⁷F₂ hypersensitive transition. The onset of NO₂^– triplet absorption above 20 000 cm^–1 restricts the derived Eu³⁺ energy-level data set to the ⁷F_<i>J</i> (<i>J</i> = 0–6) and ⁵D_0,1 multiplets. A total of 21 levels have been included in crystal-field energy-level calculations of Eu³⁺ in Cs₂NaEu­(NO₂)₆, using seven adjustable parameters, resulting in a mean deviation of ∼20 cm^–1. The comparison of our results is made with Eu³⁺ in the double nitrate salt. In both cases, the fourth-rank crystal field is comparatively weaker than that in europium hexahaloelpasolites

What Factors Affect the ⁵D₀ Energy of Eu³⁺? An Investigation of Nephelauxetic Effects

Relationships involving the interelectronic repulsion parameters, <i>F</i>^<i>k</i> (<i>k</i> = 2, 4, 6), the spin–orbit coupling constant, ζ_f, and <i>J</i>-mixing, with the ⁵D₀–⁷F₀ energy, <i>E</i>, have been investigated for Eu³⁺ using various approaches. First, the linear relationship between <i>E</i> and the ⁷F₁ splitting (or the second rank crystal field parameter) is shown to be applicable not only to glasses but also to solid-state crystalline systems with Eu³⁺ site symmetry of <i>C</i>₂, <i>C</i>_2<i>v</i>, or lower. In these cases, the change in ⁵D₀–⁷F₀ energy is mainly due to the <i>J</i>-mixing effect of ⁷F_<i>J</i> (<i>J</i> = 2, 4, 6: most notably <i>J</i> = 2) which depresses ⁷F₀, whereas the ⁵D₀ energy is relatively constant. The ⁵D₀–⁷F₀ energy also depends upon certain energy parameters in the Hamiltonian, in particular, <i>F</i>^<i>k</i> and ζ_f. Model calculations show that increase in <i>F</i>⁴ or <i>F</i>⁶ produces an increase in <i>E</i>, whereas increase in <i>F</i>² produces a decrease in <i>E</i>. An increase in ζ_f produces a decrease in <i>E</i>. These findings are rationalized. Most previous 4f⁶ crystal field calculations have only considered the F and D terms of Eu³⁺ so that the Slater parameters are not well-determined. More reliable energy level data sets and crystal field calculations for Eu³⁺ with fluoride, oxide, or chloride ligands have been selected, and certain of these have been repeated since most previous calculations have errors in matrix elements. The fitted Slater parameters have been corrected for the effects of three-body Coulomb interactions. Some systems do not follow the ligand trend F ∼ O > Cl for Slater and spin–orbit parameters. From the limited data available, the average values of the corrected Slater parameters are greater for fluoride compared with chloride ligands, but the differences are comparable with the standard deviations of the parameters. There is no clear nephelauxetic series for these three types of ligands, with respect to spin–orbit coupling. Previous correlations of <i>E</i> with various parameters are of limited value because the ⁵D₀–⁷F₀ energy difference not only depends upon the <i>F</i>^<i>k</i> and ζ_f parameters but in addition is sensitive to the importance of <i>J</i>-mixing for low symmetry systems

Ce–O Covalence in Silicate Oxyapatites and Its Influence on Luminescence Dynamics

Cerium substituting gadolinium in Ca₂Gd₈(SiO₄)₆O₂ occupies two intrinsic sites of distinct coordination. The coexistence of an ionic bonding at a 4F site and an ionic–covalent mixed bonding at a 6H site in the same crystalline compound provides an ideal system for comparative studies of ion–ligand interactions. Experimentally, the spectroscopic properties and photoluminescence dynamics of this white-phosphor are investigated. An anomalous thermal quenching of the photoluminescence of Ce³⁺ at the 6H site is analyzed. Theoretically, ab initio calculations are conducted to reveal the distinctive properties of the Ce–O coordination at the two Ce³⁺ sites. The calculated eigenstates of Ce³⁺ at the 6H site suggest a weak Ce–O covalent bond formed between Ce³⁺ and one of the coordinated oxygen ions not bonded with Si⁴⁺. The electronic energy levels and frequencies of local vibrational modes are correlated with specific Ce–O pairs to provide a comparative understanding of the site-resolved experimental results. On the basis of the calculated results, we propose a model of charge transfer and vibronic coupling for interpretation of the anomalous thermal quenching of the Ce³⁺ luminescence. The combination of experimental and theoretical studies in the present work provides a comprehensive understanding of the spectroscopy and luminescence dynamics of Ce³⁺ in crystals of ionic–covalent coordination

Energetic, Optical, and Electronic Properties of Intrinsic Electron-Trapping Defects in YAlO₃: A Hybrid DFT Study

The formation energies of cation antisite defects (Y_Al and Al_Y), oxygen vacancies (V_O), and nearest-neighbor defect complexes (Y_Al–Al_Y and Y_Al–V_O) in various charge states in the YAlO₃ crystal are calculated using density functional theory (DFT) with a modified PBE0 hybrid functional containing 32% Hartree–Fock (HF) exchange. It is found that the formation of Y_Al is more energetically favorable than Al_Y under oxygen-poor condition, consistent with the fact that the latter was not observed in experiments. On the basis of calculated optical transition energies associated with the excitons trapped at Y_Al, V_O, and Y_Al–V_O, the two emission bands observed under excitonic excitation at low temperature are identified. Electronic properties of Y_Al–V_O complexes in the neutral and singly negative charge states are finally investigated. It shows that the extra electron added into the negative charge state is mainly localized at 4d orbitals of Y_Al with a two-component feature of its density distribution extending axially along the Y_Al–V_O direction

Electronic Structure and Site Occupancy of Lanthanide-Doped (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂ Garnets: A Spectroscopic and First-Principles Study

Photoluminescence excitation (PLE) and emission spectra (PL) of undoped (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂ as well as Eu³⁺- and Ce³⁺-doped samples have been investigated. The PL spectra show that Eu³⁺ enters into both dodecahedral (Ca, Sr) and octahedral (Y, Lu) sites. Ce³⁺ gives two broad excitation bands in the range of 200–450 nm. First-principle calculations for Ce³⁺ on both dodecahedral and octahedral sites provide sets of 5d excited level energies that are consistent with the experimental results. Then the vacuum referred binding energy diagrams for (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂ have been constructed with the lanthanide dopant energy levels by utilizing spectroscopic data. The Ce³⁺ 5d excited states are calculated by first-principles calculations. Thermoluminescence (TL) glow curves of (Ce³⁺, Sm³⁺)-codoped (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂ samples show a good agreement with the prediction of lanthanide trapping depths derived from the energy level diagram. Finally, the energy level diagram is used to explain the low thermal quenching temperature of Ce³⁺ and the absence of afterglow in (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂

Temperature and Eu²⁺-Doping Induced Phase Selection in NaAlSiO₄ Polymorphs and the Controlled Yellow/Blue Emission

The union of temperature-dependent phase transition and relating structural transformation via modification of chemical compositions is of fundamental importance for the discovery of new materials or their functional properties optimization. Herein, the synthesis temperature and Eu²⁺-doping content induced phase selection and variations of the local structures in nepheline, low-carnegieite and high-carnegieite types of NaAlSiO₄ polymorphs were studied in detail. The luminescence of Eu²⁺ in low-carnegieite and nepheline phases shows blue (460 nm) and yellow (540 nm) broad-band emissions, respectively, under near-ultraviolet excitation. The photoluminescence evolution can be triggered by the different synthesis temperatures in relation to the Eu²⁺-doping concentration, as corroborated by density functional theory calculations on the local coordination structures and corresponding mechanical stabilities in terms of the Debye temperature. The fabricated white light-emitting diode device with high color rendering index demonstrates that the multicolor phosphors from one system provides a new gateway for the photoluminescence tuning

Mechanical Properties, Electronic Structures, and Potential Applications in Lithium Ion Batteries: A First-Principles Study toward SnSe₂ Nanotubes

First-principles calculations were carried out to investigate the mechanical and electronic properties as well as the potential application of SnSe₂ nanotubes. It was found that the mechanical properties are closely dependent on diameter and chirality: the Young’s modulus (<i>Y</i>) increases with the enlargement of diameter and converges to the monolayer limit when the diameter reaches a certain degree; with a comparable diameter, the armchair nanotube has a larger Young’s modulus than the zigzag one. The significantly higher Young’s modulus of SnSe₂ nanotubes with the larger diameter demonstrates that the deformation does not easily occur, which is beneficial to the application as anode materials in lithium ion batteries because a large volume expansion during charge–discharge cycling will result in serious pulverization of the electrodes and thus rapid capacity degradation. On the other hand, band structure calculations unveiled that SnSe₂ nanotubes display a diversity of electronic properties, which are also diameter- and chirality-dependent: armchair nanotubes (ANTs) are indirect bandgap semiconductors, and the energy gaps increase monotonously with the increase of tube diameter, while zigzag nanotubes (ZNTs) are metals. The metallic SnSe₂ ZNTs exhibit terrific performance for the adsorption and diffusion of Li atom, thus they are very promising as anode materials in the Li-ion batteries

Eu²⁺ Site Preferences in the Mixed Cation K₂BaCa(PO₄)₂ and Thermally Stable Luminescence

Site preferences of dopant Eu²⁺ on the locations of K⁺, Ba²⁺, and Ca²⁺ in the mixed cation phosphate K₂BaCa­(PO₄)₂ (KBCP) are quantitatively analyzed via a combined experimental and theoretical method to develop a blue-emitting phosphor with thermally stable luminescence. Eu²⁺ ions are located at K2 (M2) and K3 (M3) sites of KBCP, with the latter occupation relatively more stable than the former, corresponding to emissions at 438 and 465 nm, respectively. KBCP:Eu²⁺ phosphor exhibits highly thermal stable luminescence even up to 200 °C, which is interpreted as due to a balance between thermal ionization and recombination of Eu²⁺ 5d excited-state centers with the involvement of electrons trapped at crystal defect levels. Our results can initiate more exploration of activator site engineering in phosphors and therefore allow predictive control of photoluminescence tuning and thermally stable luminescence for emerging applications in white LEDs

Eu²⁺ Site Preferences in the Mixed Cation K₂BaCa(PO₄)₂ and Thermally Stable Luminescence

Site preferences of dopant Eu²⁺ on the locations of K⁺, Ba²⁺, and Ca²⁺ in the mixed cation phosphate K₂BaCa­(PO₄)₂ (KBCP) are quantitatively analyzed via a combined experimental and theoretical method to develop a blue-emitting phosphor with thermally stable luminescence. Eu²⁺ ions are located at K2 (M2) and K3 (M3) sites of KBCP, with the latter occupation relatively more stable than the former, corresponding to emissions at 438 and 465 nm, respectively. KBCP:Eu²⁺ phosphor exhibits highly thermal stable luminescence even up to 200 °C, which is interpreted as due to a balance between thermal ionization and recombination of Eu²⁺ 5d excited-state centers with the involvement of electrons trapped at crystal defect levels. Our results can initiate more exploration of activator site engineering in phosphors and therefore allow predictive control of photoluminescence tuning and thermally stable luminescence for emerging applications in white LEDs

A Theoretical Study on the Structural and Energy Spectral Properties of Ce³⁺ Ions Doped in Various Fluoride Compounds

Geometry optimization and wave function-based complete-active-space self-consistent field-embedded cluster calculations have been performed for a series of Ce³⁺-doped fluoride compounds (CaF₂, YF₃, LaF₃, KMgF₃, LiYF₄, K₂YF₅, and KY₃F₁₀) to investigate local coordination structures, crystal field parameters, and 5d¹ energy-level structures of doping Ce³⁺ ions. The crystal-field parameters of Ce³⁺ are extracted from the calculated energies and wave functions. The calculated crystal-field parameters and 5d¹ energy-level structures show excellent consistency with the experimental results. Our calculations show that the onset of 4f → 5d absorption, which is important in phosphors and scintillators, can be well-predicted. Apart from that, the distortion of local structure due to doping, the wave functions, and the crystal-field parameters of 4f¹ and 5d¹ states of Ce³⁺ in the hosts can be obtained. Those can seldom be obtained by fitting empirical crystal-field Hamiltonian to experimental data but are required by some detailed theoretical analysis, such as the calculation of transition intensities and hyperfine splittings. The obtained crystal-field parameters of Ce³⁺ may also be useful for other lanthanide ions in the same hosts