Theoretical calculations of the optical band positions and spin-Hamiltonian parameters for Cr ions in Y2Ti2O7 crystal

The optical band positions and spin-Hamiltonian parameters (g factors g_g_? and zero-field splitting D) for the trigonal Cr³⁺ centers in Y₂Ti₂O₇ crystal are calculated from the complete diagonalization (of energy matrix) method based on the two-spin-orbit-parameter model. In the calculations, the contributions to spectral data from both the spin-orbit parameter of central dⁿ ion and that of ligand ion are considered and the crystal field parameters used are estimated from the superposition model. The calculated results are in reasonable agreement with the experimental values. The defect structures of Cr³⁺ center is suggested.     

立方对称晶场中6S(3d5)态离子的磁相互作用及其自旋哈密顿参量的微观起源

基于完全对角化方法(complete diagonalization method, CDM), 研究了⁶S(3d⁵)态离子在立方对称晶场中的磁相互作用,分析了自旋哈密顿参量(a, g,Δg)的微观起源.研究中除了考虑研究者通常考虑的SO(spin-orbit)磁相互作用外,同时考虑了SS(spin-spin),SOO(spin-other-orbit),OO(orbit-orbit)磁相互作用.研究表明：⁶S(3d⁵)态离子在立方对称晶场中的自旋哈密顿参量起源于五种机理,即SO机理,SS机理,SOO机理,OO机理以及SO-SS-SOO-OO联合作用机理.文中研究了五种机理的相对重要性,结果表明：SO机理与SO-SS-SOO-OO联合作用机理在五种机理中最为重要.尽管SS,SOO,OO磁相互作用单独作用时对自旋哈密顿参量的贡献很小,但它们的联合作用SO-SS-SOO-OO机理对自旋哈密顿参量的贡献非常可观.此外研究表明：零场分裂参量a主要来自纯自旋四重态及自旋二重态与自旋四重态联合作用的贡献,而Zeemang(或者Δg)因子主要来自纯自旋四重态的贡献.纯自旋二重态对自旋哈密顿参量a与g(或者Δg)的贡献为零.在我们所选择的晶场区域,发现下列关系始终成立：a>0,a(-|Dq|)<a(|Dq|),g(-Dq)=g(Dq),a(-Dq,-ξ_d,B,C)=a(Dq,ξ_d, B,C),Δg(-Dq,-ξ_d, B, C)=Δg(Dq,ξ_d, B, C).作为本文理论的应用,研究了四种典型的Mn²⁺掺杂晶体材料,即Mn²⁺:KZnF₃,Mn²⁺: RbCdF₃,Mn²⁺: MgO,Mn²⁺: CaO,理论与实验测量符合很好. 关键词： 自旋哈密顿参量 6S(3d⁵)态离子')" href="#">⁶S(3d⁵)态离子 磁相互作用 完全对角化方法(CDM)     

Combustion synthesized MgAl2O4:Cr phosphors—An EPR and optical study

Magnesium aluminate (MgAl₂O₄) doped with trivalent chromium (Cr³⁺) was synthesized by the combustion method. The prepared sample was characterized by X-ray powder diffraction, Brunauer-Emmet-Teller (BET) adsorption isotherms and diffuse-reflectance UV-vis spectroscopy techniques. Electron paramagnetic resonance (EPR) and photoluminescence (PL) studies have been performed at room temperature and at 110 K. The EPR spectrum exhibit resonance signals at g=5.37, 4.53, 3.82, 2.26 and 1.96 characteristic of Cr³⁺ ions. The luminescence of Cr³⁺-activated MgAl₂O₄ exhibits a red emission peak around 686 nm from the synthesized phosphor particles upon 551 nm excitation. The luminescence is assigned to a transition from the upper ²E_g→⁴A_2g ground state of Cr³⁺ ions. By correlating EPR and optical data the crystal field splitting parameter (D_q), Racah inter-electronic repulsion parameter (B) and the bonding parameters have been evaluated and discussed. The bonding parameters suggests that the ionic nature of Cr³⁺ ions with the ligands and the Cr³⁺ ions are in distorted octrahedral environment.     

Theoretical study of the dependence of EPR spectra on local structure for octahedral Cr3+ centres in oxides and fluorides series

A theoretical method for investigating the inter-relation between the EPR parameters and local structure has been established on the basis of the complete energy matrices for 3d³ configuration ions in both the trigonal and tetragonal ligand fields. By means of this method, the local structure of the octahedral Cr³⁺ centres in double molybdates series and spinels series as well as the perovskite-type fluorides series has been studied systematically. Furthermore, the dependence of the EPR zero-field splitting parameter D on the local structure parameters in both trigonal and tetragonal ligand-fields has been revealed, simultaneously. The inter-relation between the EPR parameters D and Δg(g _// ??g _⊥) is also elucidated.     

Theoretical calculations of the optical band positions and spin-Hamiltonian parameters for Yb at the tetragonal Y site of KY3F10 crystal

The six optical band positions and six spin-Hamiltonian parameters [g factors g_∥, g_⊥ and hyperfine structure constants A_∥(¹⁷¹Yb³⁺), A_⊥(¹⁷¹Yb³⁺), A_∥(¹⁷³Yb³⁺), A_⊥(¹⁷³Yb³⁺)] for Yb³⁺ ion at the tetragonal Y³⁺ site of KY₃F₁₀ crystal are calculated from a diagonalization (of energy matrix) method. In the method, the Hamiltonian of energy matrix contains the free-ion, crystal-field interaction, Zeeman (or magnetic) interaction and hyperfine interaction terms and so a 14×14 complete energy matrix for 4f¹³ ion in tetragonal crystal-field and under an external magnetic field is constructed. Diagonalizing the energy matrix, these optical and EPR spectral data are calculated together and the calculated results are in reasonable agreement with the experimental values. The signs of hyperfine structure constants A_∥, A_⊥ for the isotopes ¹⁷¹Yb³⁺ and ¹⁷³Yb³⁺ in KY₃F₁₀ are suggested. The results are discussed.     

四角对称晶场中4B1(3d3)态离子的磁相互作用及其自旋哈密顿参量研究

基于完全对角化方法，研究了⁴B₁(3d³)态 离子在四角对称晶场中的磁相互作用，分析了自旋哈密顿参量(b⁰₂, g_∥, g_⊥ , Δg)的微观起源.结果表明 ：在被考虑的大部分晶场区域，人们通常考虑的SO(spin-orbit)磁相互作用的贡献最为重要 ；然而，对于零场分裂参量b⁰₂而言，来自其他机理(包 括SS(spin-orbit)，SOO(sp in-other-orbit)，SO-SS-SOO)的贡献在大部分晶场区域超过了20％；在部分晶场区域，其 他机理的贡献甚至超过SO机理的贡献.详细地分析了Macfarlane 零场分裂参量b^{0₂ 近似三阶微扰理论的收敛性，结果表明：该理论在大部分晶场区域收敛性较差.讨论了3d3}态离子第一激发态²E_g分裂的微观起源.并利用 群论方法解 释了在C_4v和C_3v对称晶场中²E_{g< /sub>态分裂的不同机理. 关键词： 4B1(3d³)态离子')" href="#">⁴B1(3d³)态离子 磁相互作用 自旋哈密 顿参量 完全对角化方法(CDM) 微扰理论方法(PTM)}     

Spectral studies on Cr ions doped in sodium-lead borophosphate glasses

Electron paramagnetic resonance (EPR), optical absorption and emission spectra of Cr³⁺ ions doped in (30−x) (NaPO₃)₆+30PbO+40B₂O₃+xCr₂O₃ (x=0.5, 2.0, 3.0, 4.0 and 5.0 mol%) glasses have been studied. The EPR spectra exhibit resonance signals with effective g values at g≈4.55 and g≈1.97. The EPR spectra of x=3.0 mol% of Cr₂O₃ in sodium-lead borophosphate glass sample were studied at various temperatures (295-123 K). The intensity of the resonance signals increases with decrease in temperature. The optical absorption spectrum exhibits four bands characteristic of Cr³⁺ ions in octahedral symmetry. From the analysis of the bands, the crystal-field parameter Dq and the Racah interelectronic repulsion parameters B and C have been evaluated. The emission spectrum exhibit one broad band characteristic of Cr³⁺ ions in octahedral symmetry. This band has been assigned to the transition ⁴T_2g (F)→⁴A_2g (F). Correlating EPR and optical data, the molecular bonding coefficient (α) has been evaluated.     

Comparative analysis of crystal field effects and energy level scheme of six-fold coordinated Cr ion in the pyrochlores, Y2B2O7 (B=Ti, Sn)

The electronic energy levels of the six-fold coordinated Cr⁴⁺ ion in the pyrochlores Y₂B₂O₇ (B=Sn⁴⁺, Ti⁴⁺), have been computed using the exchange charge model of crystal field theory. The calculated Cr⁴⁺ energy levels and their trigonal splitting are in good agreement with experimental spectra. Calculations of the crystal field parameters show that the higher crystal field strength in Y₂Sn₂O₇ (in comparison with Y₂Ti₂O₇) arises from increased orbital overlap effects between the Cr⁴⁺ ion and the nearest oxygen ions, which are located at the 48f crystallographic position of the pyrochlore lattice. The increased overlap in Y₂Sn₂O₇ occurs despite the fact that the Cr⁴⁺-O^2- bond distance in Y₂Sn₂O₇ is longer than in Y₂Ti₂O₇. This is attributed to a lack of hybridization (covalent bonding) between the filled 2p orbital of oxygen ion occupying the 48f site of the pyrochlore lattice and the filled Sn⁴⁺ 4d¹⁰ orbital. As a result, a stronger crystal field is experienced by Cr⁴⁺ ions in Y₂Sn₂O₇, even if the Cr⁴⁺-O²⁻ distances are greater in this case, when compared to those in Y₂Ti₂O₇.     

Preparation and optical properties of ZnGa2O4:Cr thin films derived by sol-gel process

ZnGa₂O₄:Cr³⁺ thin films with bright red emission were synthesized using a sol-gel process, characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Auger electron spectroscopy (AES) and UV-vis and fluorescence spectrophotometry measurements. Effects of calcining temperature, film thickness, calcining duration and substrates on the crystal structure and photoluminescent property have been investigated. It is found that the crystallinity, Ga/Zn ratio and band gap energy (E_g) are significant factors influencing optical characteristics, while the nature of substrates affect the surface morphologies of ZnGa₂O₄:Cr³⁺ thin films.     

Spectroscopic properties of Fe ions at tetragonal sites—Crystal field effects and microscopic modeling of spin Hamiltonian parameters for Fe (S=2) ions in K2FeF4 and K2ZnF4

Magnetic and spectroscopic properties of the planar antiferromagnet K₂FeF₄ are determined by the Fe²⁺ ions at tetragonal sites. The two-dimensional easy-plane anisotropy exhibited by K₂FeF₄ is due to the zero field splitting (ZFS) terms arising from the orbital singlet ground state of Fe²⁺ ions with the spin S=2. To provide insight into the single-ion magnetic anisotropy of K₂FeF₄, the crystal field theory and the microscopic spin Hamiltonian (MSH) approach based on the tensor method is adopted. Survey of available experimental data on the crystal field energy levels and free-ion parameters for Fe²⁺ ions in K₂FeF₄ and related compounds is carried out to provide input for microscopic modeling of the ZFS parameters and the Zeeman electronic ones. The ZFS parameters are expressed in the extended Stevens notation and include contributions up to the fourth-order using as perturbation the spin-orbit and electronic spin-spin couplings within the tetragonal crystal field states of the ground ⁵D multiplet. Modeling of the ZFS parameters and the Zeeman electronic ones is carried out. Variation of these parameters is studied taking into account reasonable ranges of the microscopic ones, i.e. the spin-orbit and spin-spin coupling constants, and the energy level splittings, suitable for Fe²⁺ ions in K₂FeF₄ and Fe²⁺:K₂ZnF₄. Conversions between the ZFS parameters in the extended Stevens notation and the conventional ones are considered to enable comparison with the data of others. Comparative analysis of the MSH formulas derived earlier and our more complete ones indicates the importance of terms omitted earlier as well as the fourth-order ZFS parameters and the spin-spin coupling related contributions. The results may be useful also for Fe²⁺ ions at axial symmetry sites in related systems, i.e. Fe:K₂MnF₄, Rb₂Co_1−xFe_xF₄, Fe²⁺:Rb₂CrCl₄, and Fe²⁺:Rb₂ZnCl₄.     

Enhanced luminescence of Ca3B2O6:Dy phosphors by Na-codoping for LED applications

The Ca_2.95−yDy_0.05B₂O₆:yNa⁺ (0≤y≤0.20) phosphors were synthesized at 1100 °C in air by the solid-state reaction route. The as-synthesized phosphors were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), photoluminescence excitation (PLE), photoluminescence (PL) spectra and thermoluminescence (TL) spectra. The PLE spectra show the excitation peaks from 300 to 400 nm due to the 4f-4f transitions of Dy³⁺. This mercury-free excitation is useful for solid-state lighting and light-emitting diodes (LEDs). The emission of Dy³⁺ ions on 350 nm excitation was observed at 480 nm (blue) due to the ⁴F_9/2→⁶H_15/2 transitions, 575 nm (yellow) due to ⁴F_9/2→⁶H_13/2 transitions and 660 nm (red) due to weak ⁴F_9/2→⁶H_11/2 emissions. The PL results from the investigated Ca_2.95−yDy_0.05B₂O₆:yNa⁺ phosphors show that Dy³⁺ emissions increase with the increase of the Na⁺ codoping ions. The integral intensity of yellow to blue (Y/B) can be tuned by controlling Na⁺ content. By the simulation of white light, the optimal CIE value (0.328, 0.334) can be achieved when the content of Na⁺-codoping ions is y=0.2. The results imply that the Ca_2.95−yDy_0.05B₂O₆:yNa⁺ phosphors could be potentially used as white LEDs.     

Luminescence of La3F3[Si3O9]:Ce

The luminescence properties of Ce³⁺ in La₃F₃[Si₃O₉] are reported. Excitation and emission bands corresponding to 4f¹→5d¹ transitions of Ce³⁺ were identified. The center of gravity of the 5d states lies at remarkable high energy (43.2×10³ cm⁻¹) for Ce³⁺ in a silicate compound. This high value is attributed to the combined oxygen/fluoride coordination of the Ce³⁺ ion. Emission from the lowest 4f5d level to the ²F_5/2 and ²F_7/2 levels was found at 32.4×10³ and 30.4×10³ cm⁻¹. These results are compared with literature data on silicates and fluorides. From the values found for Ce³⁺, predictions are made for the positions of the 4f5d bands of Pr³⁺ and Er³⁺ in La₃F₃[Si₃O₉]. For both ions, it is concluded that in this host lattice emission is expected from high lying 4fⁿ energy levels.     

Synthesis and ion conductivity of (Bi2O3)0.75(Dy2O3)0.25 ceramics with grain sizes from the nano to the micro scale

Using (Bi₂O₃)_0.75(Dy₂O₃)_0.25 nano-powder synthesized by reverse titration co-precipitation method as raw material, dense ceramics were sintered by both Spark Plasma Sintering (SPS) and pressureless sintering. According to the predominance area diagram of Bi-O binary system, the sintering conditions under SPS were optimized. (Bi₂O₃)_0.75(Dy₂O₃)_0.25 ceramics with relative density higher than 95% and an average grain size of 20 nm were sintered in only 10 min up to 500 °C. During the pressureless sintering process, the grain growth behavior of (Bi₂O₃)_0.75(Dy₂O₃)_0.25 followed a parabolic trend, expressed as D² − D₀² = Kt, and the apparent activation energy of grain growth was found to be 284 kJ mol^− 1. Dense (Bi₂O₃)_0.75(Dy₂O₃)_0.25 ceramics with different grain sizes were obtained, and the effect of grain size on ion conductivity was investigated by impedance spectroscopy. It was shown that the total ion conductivity was not affected by the grain size down to 100 nm, however lower conductivity was measured for the sample with the smallest grain size (20 nm). But, although only the δ phase was evidenced by X-ray diffraction for this sample, a closer inspection by Raman spectroscopy revealed traces of α-Bi₂O₃.     

Studies of the g factors and the local structure of the orthorhombic Ni center in KTaO3 crystal

The local structure and the g factor (g_x, g_y, and g_z) of the Ni⁺ center in KTaO₃ are theoretically studied using the perturbation formulas of the g factors for a 3d⁹ ion in orthorhombically elongated octahedra. The orthorhombic field parameters are determined from the superposition model and the local geometry of the system. In view of the covalency, the contributions from the ligand orbital and spin–orbit coupling interactions are taken into account from the cluster approach. In the calculations, the orthorhombic center is attributed to Ni⁺ occupying the host Ta⁵⁺ site, associated with the nearest-neighboring oxygen vacancy V_O along the c-axis. Furthermore, the planar Ni⁺–O²⁻ bonds are found to experience the relative variation ΔR (≈0.076 Å) along the a- and b-axis, respectively, due to the Jahn–Teller effect and the size mismatching substitution of Ta⁵⁺ by Ni⁺. Meanwhile, the effectively positive V_O can make the central Ni⁺ displace away from V_O along the c-axis by about 0.20 Å. The calculated g factors based on the above local distortions show good agreement with the experimental data.     

Luminescence properties of Y2WO6:Eu incorporated with Mo or Bi ions as red phosphors for light-emitting diode applications

In this study, the red phosphors, Y₂W_1−xMo_xO₆:Eu³⁺ and Y₂WO₆:Eu³⁺,Bi³⁺, have been investigated for light-emitting diode (LED) applications. In Y₂WO₆:Eu³⁺, the excitation band edge shifts to longer wavelength with the incorporation of Mo⁶⁺ or Bi³⁺ ions. The emission spectra exhibit ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transition of Eu³⁺ ion at 588, 593, and 610 nm, respectively. Moreover, the bluish-green luminescence of the WO₆⁶⁻ at about 460 nm is observed to decrease with the incorporation of Mo⁶⁺, which results in pure red color. Thus, this study shows that the red phosphor, Y₂WO₆:Eu³⁺, incorporated with Mo⁶⁺ or Bi³⁺ ions is advantageous for LEDs applications.     

An interpretation of the g factors for the tetragonally-compressed Cr centers in YVO4 and YPO4 crystals

The g factors g_∥ and g_⊥ for the tetragonally-compressed (CrO₄)³⁻ clusters in YMO₄ (M=V, P) crystals are calculated from the high-order perturbation formulas based on the two-mechanism model for the compressed d¹ tetrahedra with the ground state |d_z²〉. From the calculated values and by considering a small admixture of the first excited state |d_x²−y²〉 to the ground state |d_z²〉 due to the vibrational motion of ligands (which leads a twinkling compressed tetrahedron to become an elongated one), the observed g_∥ and g_⊥ for Cr⁵⁺ centers in YMO₄ crystals are explained reasonably. The difficulty of the large deviations of g_∥ from g_e (≈2.0023) in the two systems is therefore removed and the above dynamic effect may be the cause which results in the large deviation of g_∥ from g_e for some (CrO₄)³⁻ clusters in crystals.     

Photoluminescence properties of Eu in Y(PO3)3 under VUV excitation

The photoluminescence properties of Y_1−x(PO₃)₃:xEu³⁺ (0<x≤0.2) are investigated. The excitation spectrum of Y_0.85(PO₃)₃:0.15Eu₃⁺ shows that both the (PO₃)₃³⁻ groups and the CT bands of O²⁻-Y³⁺ can efficiently absorb the excitation energy in the region of 120-250 nm. Under 147 nm excitation, the optimal emissive intensity of Y_1−x(PO₃)₃:xEu³⁺ (0<x≤0.2) is about 36% of the commercial phosphor (Y,Gd)BO₃:Eu³⁺, which hints that the absorbed energy by the host matrix could be efficiently transferred to Eu³⁺. We try to study the concentration quenching mechanism of Y_1−x(PO₃)₃:xEu³⁺ (0<x≤0.2) under 147 and 172 nm excitation.     

Exchange charge model and analysis of the microscopic crystal field effects in KAl(MoO4)2:Cr

In the present paper, we report on consistent crystal field calculations of the Cr³⁺ ions energy levels in KAl(MoO₄)₂ using actual D_3d site symmetry of the Cr³⁺ position and employing the exchange charge model (ECM) of the crystal field. In addition to the energy level calculations, the Huang-Rhys factor S=5.7 and effective phonon energy ?ω=268 cm^-1 were evaluated in the single configurational coordinate model. Detailed treatment of the microscopic crystal field effects in the ECM framework allowed to obtain analytical dependence of the crystal field strength 10Dq on the Cr-O interionic distance and extracting from it the values of some parameters of the electron-vibrational interaction (EVI) in the KAl(MoO₄)₂:Cr³⁺ system. All obtained results are compared with experimental data and discussed; agreement between the calculated and experimental parameters is good.     

Local structure of the trigonal defect center for Cr3+ ions in KMgF3 crystals

Abstract

The zero-field splitting D, the anisotropic g-factors g _∥, Δg(=g _∥ ? g _⊥) and the first excited state splitting Δ(² E) for the trigonal Cr³⁺–V_K center in KMgF₃: Cr³⁺ crystals have been studied from Macfarlane's high-order perturbation formulas. From the studies, the local structure of the trigonal center is obtained. The local lattice distortions (i.e., the displacement directions of the ions in the center) are consistent with the expectation based on the electrostatic interaction.     

Emission analysis of Pr and Tm: Ca2Gd2W3O14 phosphors

5 mol% of Pr³⁺ and Tm³⁺ ions activated calcium gadolinium tungstate (Ca₂Gd₂W₃O₁₄₎ phosphors were synthesized by traditional solid state reaction method. Crystalline phase structure was identified from the X-ray diffraction (XRD) profiles. From the scanning electron microscopy (SEM) images, we have observed the agglomeration of the particles, and average grain size is around 40-300 nm. Using the energy dispersive X-ray analysis (EDAX) and Fourier transform infrared (FTIR) spectra, identified the elements and functional groups present in the prepared phosphors. The emission spectrum of Pr³⁺: Ca₂Gd₂W₃O₁₄ powder phosphors have shown an intense red emission at 615 nm with the excitation wavelength λ_exci=450 nm and thus these red color emitting powder phosphors are used as one of the components in the preparation of WLEDs. The excitation spectrum of Tm³⁺: Ca₂Gd₂W₃O₁₄ powder phosphor has shown a ligand to metal charge transfer (W-O) band (LMCT) within the WO₄²⁻ group. Emission spectrum of Tm³⁺: Ca₂Gd₂W₃O₁₄ phosphors have shown blue emissions at 453 nm (¹D₂→³F₄).