Study of the electron paramagnetic resonance spectra of Zn(antipyrine)2(NO3)2:VO

In this work, a full ligand-field energy matrix (10×10) diagonalization treatment for 3d¹ ions in tetragonal symmetry is developed on the basis of the two-s.o.-coupling-parameter model. Spin Hamiltonian parameters (g factors g_∥, g_⊥ and hyperfine structure constants A_∥, A_⊥) of the tetragonal V⁴⁺ center in Zn(antipyrine)₂(NO₃)₂ are calculated from the complete energy matrix diagonalization method and the perturbation theory method. The calculated results from both methods are not only close to each other but also in good agreement with the experimental values. Furthermore, the compressed defect structure of V⁴⁺ center is discussed.     

Studies of the optical spectrum band positions and spin-Hamiltonian parameters for Pa ion in ThX4 (X=Cl, Br) crystals

The optical spectrum band positions and spin-Hamiltonian (SH) parameters (g factors g_∥ and g_⊥ and hyperfine structure constants A_∥ and A_⊥) for 5f¹ ion Pa⁴⁺ at the tetragonal Th⁴⁺ site of ThX₄ (X=Cl, Br) crystals are calculated from a complete diagonalization (of energy matrix) method (CDM). In the CDM, the magnetic (or Zeeman) interaction and hyperfine interaction terms are added to the Hamiltonian in the conventional CDM and so the optical and EPR spectra data can be studied in a unified way. The calculated results are in reasonable agreement with experimental values. The possible misprints or small errors in the experimental g factors for Pa⁴⁺ in ThX₄ crystals are pointed out. The results are discussed.     

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.     

Theoretical investigation of ESR spectra and local structure for VO in Ba2Zn(HCOO)6(H2O)

The electron spin resonance spectra (characterized by g factors g_, g_⊥ and hyperfine structure constants A_ and A_⊥) of Ba₂Zn(HCOO)₆(H₂O) (BZFA): VO²⁺ crystal are calculated from high order perturbation formulas. The calculated results are in good agreement with the observed values. The local structure parameters of [VO(H₂O)₅]²⁺ clusters are also obtained from the calculation. The magnitudes of the metal-ligand distances parallel and perpendicular to the C₄-axis are, respectively, R_≈0.163 nm and R_⊥≈0.210 nm. It is shown that the local structure around the V⁴⁺ ion possesses a compressed tetragonal distortion along C₄-axis.     

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.     

Temperature and pressure dependences of EPR spectra of Gd ion doped in the EuAl3(BO3)4 monocrystal

The ground state of Gd³⁺ ions substituting for trivalent europium in the EuAl₃(BO₃)₄ single crystal was studied by electron paramagnetic resonance (EPR) over the temperature range of 300-4.2 K and at pressures up to 9 kbar. The EPR spectra were analysed using the spin Hamiltonian of axial symmetry. The following parameters are reported: g=1.981±0.002, b₂⁰=280.18±0.12, b₄⁰=−12.95±0.08 and b₆⁰=0.61±0.12 (at Т=298 K). The distortions of the nearest environment of Gd³⁺ ion were analysed within the framework of the superposition model of crystal field.     

Luminescent properties of KGd1−x(WO4)2:Eux and KGd1−x(WO4)2−y(MoO4)y:Eux phosphors in UV-VUV regions

KGd_1−x(WO₄)_2−y(MoO₄)_y:Eu³⁺_x(0.1?x?0.75, y=0 and 0.2) phosphors are synthesized through traditional solid-state reaction and their luminescent properties in ultraviolet (UV) and vacuum ultraviolet (VUV) regions are investigated. Under 147 nm excitation, these phosphors show characteristic red emission with good color purity. In order to improve their emission intensity, the MoO₄²⁻ (20 wt%) is introduced into the anion of KGd_1−x(WO₄):Eu³⁺_x. The Mo⁶⁺ and Eu³⁺ co-doped KGd(WO₄)₂ phosphors show higher emission intensity in comparison with the singly Eu³⁺-doped KGd(WO₄)₂ in VUV region. The chromaticity coordination of KGd_0.45(WO₄):Eu³⁺_0.55 is (x=0.669, y=0.331), while that of KGd_0.45(WO₄)_1.8(MoO₄)_0.2:Eu³⁺_0.55 is (x=0.666, y=0.334) in VUV region.     

Polarized fluorescence spectra analysis of Yb:KGd(WO4)2

The average fluorescence wavelength of the laser crystal is the most important factor in the radiation-balanced laser (RBL). Polarized fluorescence spectra measurements of the anisotropic laser material ytterbium-doped potassium gadolinium tungstate, Yb³⁺:KGd(WO₄)₂, are carried out along principal refractive index directions m, p, g in three configurations in order to achieve the best design for RBL. The average fluorescence wavelength of g polarization is the shortest, so g should be in the face of fluorescence emission; m polarization should be normal to that face to avoid its strong absorption to fluorescence photons. Fluorescence re-absorption causes the average fluorescence wavelength of the directly measured spectra red-shifted at least 9 nm. Methods for depressing radiation trapping are suggested accordingly, which are high power pumping, low doping concentration, small dimensions and fusing with undoped KGd(WO₄)₂.     

Epitaxial MoOx nanostructures on TiO2(1 1 0) obtained using thermal decomposition of Mo(CO)6

Stoichiometric and highly-defective TiO₂(1 1 0) surfaces (called as yellow and blue, respectively) were exposed to Mo(CO)₆ vapours in UHV and in a reactive O₂ atmosphere. In the case of yellow-TiO₂, an O₂ reactive atmosphere was necessary to obtain the Mo(CO)₆ decomposition at 450 °C with deposition of MoO_x nanostructures where, according to core level photoemission data, the Mo⁺⁴ state is predominant. In the case of blue-TiO₂ it was possible to obtain Mo deposition both in UHV and in an O₂ atmosphere. A high dose of Mo(CO)₆ in UHV on blue-TiO₂ allowed the deposition of a thick metallic Mo layer. An air treatment of this sample at 580 °C led to the elimination of Mo as MoO₃ and to the formation of a transformed layer of stoichiometry of Ti_(1−x)Mo_xO₂ (where x is close to 0.1) which, according to photoelectron diffraction data, can be described as a substitutional near-surface alloy, where Mo⁺⁴ ions are embedded into the titania lattice. This embedding procedure results in a stabilization of the Mo⁺⁴ ions, which are capable to survive to air exposure for a rather long period of time. After exposure of the blue-TiO₂(1 1 0) substrate to Mo(CO)₆ vapours at 450 °C in an O₂ atmosphere it was possible to obtain a MoO₂ epitaxial ultrathin layer, whose photoelectron diffraction data demonstrate that is pseudomorphic to the substrate.     

Investigations on the spin Hamiltonian parameters and defect structure for the interstitial Mo centre in TiO2

The spin Hamiltonian parameters, g factors g_i (i = x, y, z) and the hyperfine structure constants A_i for the interstitial Mo⁵⁺ centre in rutile TiO₂ are quantitatively investigated from the perturbation formulas of these parameters for a 4d¹ ion in rhombically compressed octahedra. From the studies, the local compression parameter τ′ (≈0.024) and the rhombic distortion angle δ?′ (≈1.74°) around the impurity Mo⁵⁺ are smaller than the host values (≈0.091 and 3.5°). This means that the oxygen octahedron in the impurity centre has less rhombic distortion than that on the host interstitial site due to the Jahn–Teller effect and occupation of the impurity. The above local lattice distortion of the studied impurity centre is also discussed.     

Luminescent properties of HTP AgGd1−xW2O8:Eux and AgGd1−x(W1−yMoy)2O8:Eux phosphor for white LED

Intense red phosphors, AgGd_1−xEu_x(W_1−yMo_y)₂O₈ (x=0.0-1.0, y=0.0-1.0), have been synthesized through traditional solid-state reaction and characterized by X-ray diffraction (XRD) and photoluminescence (PL). XRD results reveal that AgGd_1−xEu_xW₂O₈ synthesized at 1000 °C has a tetragonal crystal structure, which is named as high temperature phase (HTP) AgGdW₂O₈. All phosphors compositions with Eu³⁺ show red and green emission on excitation either in the charge-transfer or Eu³⁺ levels. Analysis of the emission spectra with different Eu³⁺ concentrations reveal that the optimum dopant concentration for Eu³⁺ is x=0.6 in the HTP AgGd_1−xEu_xW₂O₈ (x=0.0-1.0). Studies on the AgGd_0.4Eu_0.6(W_1−yMo_y)₂O₈ (y=0.0-1.0) and AgGd_1−xEu_x(W_0.7Mo_0.3)₂O₈ (x=0.0-1.0) show that the emission intensity is maximum for compositions with y=0.3 and x=0.5, respectively, and a decrease in emission intensity is observed for higher y or x values. The Mo⁶⁺ and Eu³⁺ co-doped AgGd(WO₄)₂ phosphors show higher emission intensity in comparison with the singly Eu³⁺-doped AgGd(WO₄)₂ in UV region. The intense emission of the tungstate/molybdate phosphors under 394 and 465 nm excitations, respectively, suggests that these materials are promising candidates as red-emitting phosphors for near-UV/blue GaN-based white LED for white light generation.     

Defect model and spin-Hamiltonian parameters for the tetragonal Nd center in the tetragonal phase of SrTiO3 crystal

The spin-Hamiltonian parameters (g factors g_∥, g_⊥ and hyperfine structure constants ¹⁴³A_∥, ¹⁴³A_⊥, ¹⁴⁵A_∥ and ¹⁴⁵A_⊥) of the tetragonal Nd³⁺ center in the low-temperature (T≈4.2 K) tetragonal phase of SrTiO₃ are calculated from a diagonalization (of energy matrix) method. In the method, the Zeeman and hyperfine interaction terms are attached to the conventional Hamiltonian and a 52×52 energy matrix concerning the ground term ⁴H_J (J=9/2, 11/2, 13/2, 15/2) is constructed. The Nd³⁺ center is attributed to Nd³⁺ occupying the 12-fold coordinated Sr²⁺ site in SrTiO₃. Differing from the defect model assumed in the previous paper that the tetragonal distortion of this Nd³⁺ center is due to the association of one interstitial oxygen ion at a nearest neighborhood of Nd³⁺ and the Nd³⁺ displacement Δz along C₄ axis, we suggest that it is due to the distortion of SrTiO₃ lattice in the tetragonal phase. The calculated g factors g_∥ and g_⊥ show good agreement with the experimental values, suggesting that our defect model of Nd³⁺ center in SrTiO₃ is reasonable. The experimental hyperfine structure constants were not reported and so our calculated results remain to be checked by EPR experiment.     

Blue luminescence in Tm-doped KGd(WO4)2 single crystals

Up-conversion blue emissions of trivalent thulium ions in monoclinic KGd(WO₄)₂ single crystals at 454 and 479 nm are reported for a single pump laser source at 688 nm. We grew thulium-doped KGd(WO₄)₂ single crystals at several concentrations from 0.1% to 10%. We recorded a polarized optical absorption spectrum for the ³F₂+³F₃ energy levels of thulium at room temperature and low temperature (6 K). From the low temperature emission spectra we determined the splitting of the ³H₆ ground state. The blue emissions are characterized as a function of the dopant concentration and temperature from 10 K to room temperature. To our knowledge, this is the first time that sequential two-photon excitation process (STEP) generated blue emissions in thulium-doped single crystals with a single excitation wavelength.     

Studies of the crystal field energy levels and g factors for Ce in LiYF4 crystal

The five observed crystal field energy levels and EPR g factors g_//and g_⊥ for Ce³⁺-doped LiYF₄ crystal are calculated together from a complete diagonalization (of energy matrix) method. In the method, the contributions to g factors of ground Kramers doublet from all the rest doublets within the ground and excited manifolds ²F_5/2 and ²F_7/2 are included. The calculated results show reasonable agreement with the experimental values. The calculations suggest that the crystal field parameter B₂₀ > 0 in LiYF₄: Ce³⁺ crystal. The opinion of the parameter B₂₀ < 0 in the previous paper is not correct. Since this opinion is based on the calculation of g factors using a very simple method where only the contributions to g factors from the doublets within the ground manifold ²F_5/2 are considered, it is suggested that this simple method is not effective in the calculation of g factors for 4f¹ ions in crystals.     

Linear-chain ferromagnetism in the one-dimensional compounds CoCl2(pyrazole)2 and CoCl2(indazole)2

Two linear-chain compounds of formula CoCl₂L₂, with L = pyrazole (pz) and indazole (indz), have been investigated with the aid of IR, far-IR, ligand field and ESR spectra. Magnetic susceptibility data have been collected in the temperature region 4.2–80 K. Analysis of the susceptibility data in terms of the Ising model yielded the values of the ferromagnetic intrachain exchange constants J = 7.2(6) and 7.4(9) cm^-1 for the pz and indz compound, with corresponding g-values of g∥ = 7.9(7), g⊥ = 4.6(9) and g∥ = 10.8(9), g⊥ = 3(1), respectively. The results are discussed and compared with the similar compound CoCl₂(pyridine)₂. The influence of the bridge geometry and of the π-bonding properties of the organic ligands on the exchange constant are considered for the explanation of different J-values. It is suggested that the π-bonding properties of the organic ligand influence the magnitude of the superexchange constant to a great extent.     

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)}     

Observation of the K2 4Δg state by cw perturbation-facilitated optical-optical double resonance spectroscopy

This paper reports for the first time both, an experimental observation and theoretical calculations of the K₂ 4³Δ_g state. For the experiment we used cw perturbation-facilitated optical-optical double resonance (PFOODR) spectroscopy. A single mode Ti-sapphire laser and a dye laser served as the pump and probe lasers, respectively. A total of 55 PFOODR signals have been assigned to the 4³Δ_g ← b³Π_u transitions. Absolute vibrational numbering was determined by using quantum defect analysis combined with comparing observed intensities with calculated Franck-Condon factors (FCF). For the former we used known parameters from the 2³Δ_g state since the 2³Δ_g and the 4³Δ_g states belong to the same Rydberg series. We report here our experimental and calculated spectroscopic constants, the corresponding RKR potential energy curve, the Franck-Condon table for the 4³Δ_g ↔ b³ Π_u system, as well as a comparison with the theoretical potential energy curve. The T_e value is found to be 28408.938(52) cm⁻¹.     

Theoretical explanation of g factors for Ti ions in LiNbO3 and LiTaO3 crystals

The EPR g factors g_// and g_⊥ for Ti³⁺ ions at the trigonal octahedral Li⁺ sites of LiNbO₃ and LiTaO₃ crystals are calculated from the third-order perturbation formulas of g factors for 3d¹ ion in trigonal symmetry. In the calculations, the crystal-field parameters are obtained from the structural data by using the superposition model. The calculated values are in reasonable agreement with the observed values. The results are discussed.     

Decoupled magnetic subsystems in the random mixture FepCo1-p(C5H5NO)6(ClO4)2

Specific heat data on the random mixtures Fe_pCo_1-pL₆(ClO₄)₂, where L = C₅H₅NO, are presented. The Fe and Co magnetic atoms have competing anisotropies since the pure Fe and Co compounds are known to be good examples of the simple cubic, $\text{S =}\text{1}\text{2}$, Ising and XY magnet, respectively. The experimental data show the two magnetic subsystems in the mixtures to be almost completely decoupled, which is a consequence of the fact that the crystal field anisotropies of the Fe²⁺ and Co²⁺ ions, yielding g_∥ ? g_⊥ and g_⊥ ? g_∥, respectively, are very strong compared to the magnetic exchange interactions. Consequently the two magnetic subsystems experience one another as nonmagnetic impurities. A model is presented which explains these results, as well as those previously found for related random mixtures, in terms of two interpenetrating percolation clusters.     

Combustion synthesis and luminescence properties of NaY1−xEux(WO4)2 phosphors

The red phosphors NaY_1−xEu_x(WO₄)₂ with different concentrations of Eu³⁺ were synthesized via the combustion synthesis method. As a comparison, NaEu(WO₄)₂ was prepared by the solid-state reaction method. The phase composition and optical properties of as-synthesized samples were studied by X-ray powder diffraction and photoluminescence spectra. The results show that the red light emission intensity of the combustion synthesized samples under 394 nm excitation increases with increase in Eu³⁺ concentrations and calcination temperatures. Without Y ions doping, the emission spectra intensity of the NaEu(WO₄)₂ phosphor prepared by the combustion method fired at 900 °C is higher than that prepared by the solid-state reaction at 1100 °C. NaEu(WO₄)₂ phosphor synthesized by the combustion method at 1100 °C exhibits the strongest red emission under 394 nm excitation and appropriate CIE chromaticity coordinates (x=0.64, y=0.33) close to the NTSC standard value. Thus, its excellent luminescence properties make it a promising phosphor for near UV InGaN chip-based red-emitting LED application.