Structural analysis of Fe3+ centers in Tl2MgF4 and Tl2ZnF4 fluorine compounds

Theoretical analysis of the Fe³⁺ centers observed in Tl₂MgF₄ fluorine crystals have been carried out by means of semi-empirical approaches. The most appropriate models are proposed by matching the theoretically predicted zero-field splitting parameters (ZFSPs) with the experimental ones obtained by EPR spectroscopy. Compression on the MF₆ octahedron of tetragonal (TE) center I is indicated in both Tl₂MgF₄ and Tl₂ZnF₄. A structural model for monoclinic (MO) center II and orthorhombic (OR) Fe³⁺ center IV in Tl₂ZnF₄ is proposed by assuming that the substitution of Fe³⁺ induces both ligand length and angular distortions.     

Theoretical investigation of zero field splitting parameters for Mn centers in ammonium tartrate

Zero-field splitting (ZFS) parameters D and E for Mn²⁺ centers in ammonium tartrate single crystal are calculated with perturbation formulae using the superposition model. The theoretically calculated ZFS parameters for Mn²⁺ at site I and site II of ammonium ion are compared with the experimental values obtained by electron paramagnetic resonance (EPR) at room temperature. The superposition model gives the ZFS parameters similar to those from experiment. The energy band positions of optical absorption spectrum of Mn²⁺in ammonium tartrate are calculated using the CFA package and crystal field parameters from superposition model. These are in good agreement with experimental energy band positions.     

Modeling techniques for analysis and interpretation of electron magnetic resonance (EMR) data for transition ions at low symmetry sites in crystals—A primer for experimentalists

Electron magnetic resonance (EMR) studies of paramagnetic centers exhibiting monoclinic and triclinic local site symmetry have gained renewed importance, since such centers occur often in various technologically important materials and biological systems. The intricate low symmetry aspects, which arise for such centers, bear on meaningful interpretation of EMR data and their correlation with structural data. This review provides a primer for experimentalists who wish to utilize efficiently the modeling techniques for analysis and interpretation of EMR data for transition ions, especially ions located at low symmetry sites in crystals. This requires proper understanding of the low symmetry effects observable in EMR spectra as well as related theoretical questions concerning, e.g., (i) existence of physically equivalent zero-field splitting (ZFS) parameter sets, (ii) clear definitions of the axis systems, (iii) proper forms of spin Hamiltonians, and (iv) distinction between apparent and actual low symmetry cases. The question (i) involves consideration of the orthorhombic standardization, which provides basis for standardization of monoclinic and triclinic ZFS parameters. Thus, the aspects pertaining to orthorhombic site symmetry are also outlined. To solve other questions several modeling techniques have been utilized and related computer packages have recently been developed in our group: (1) the superposition model calculations of the zero-field splitting parameters (ZFSPs) in arbitrary symmetry, (2) the procedure for diagonalization of the 2nd-rank ZFSPs and transformation of respective 4th- and 6th-rank ZFSPs, (3) the pseudosymmetry axes method for approximation of the 4th- and 6th-rank ZFSPs to higher symmetry, and (4) the closeness factors and norm ratios for quantitative comparisons of various ZFSP sets. These modeling techniques enable deeper analysis and interpretation of the low symmetry aspects involved in the fitted and theoretical ZFSPs. The computer packages facilitate extracting useful structural information inherent in monoclinic and triclinic ZFSP sets. Illustrative examples taken from recent studies of low symmetry ion-host systems are discussed.     

Alternative zero-field splitting (ZFS) parameter sets and standardization for Mn ions in various hosts exhibiting orthorhombic site symmetry

The intrinsic properties of orthorhombic Hamiltonians, e.g. zero-field splitting (ZFS) ones or crystal-field (CF) ones, embodied in the standardization idea may be utilized to improve reliability and comparability of experimentally determined ZFS (or CF) parameters. These properties have enabled derivation of transformation relations for Hamiltonian parameters expressed in several related axis systems defined earlier by one of us. In this paper the standardization transformations are used to generate alternative physically equivalent yet numerically distinct parameter sets for Mn²⁺ ions in various hosts exhibiting orthorhombic or lower site symmetry. One parameter set out of six alternative sets satisfies the standardization criteria. Importantly, the standardized sets should be used for direct comparisons. Several non-standard ZFS parameter (ZFSP) sets for Mn²⁺ ions in crystals, identified in our literature survey of EMR studies, are here standardized. Thus the ratio of λ=E/D or λ′=B₂²/B₂⁰ is limited to the standard range (0, ±1/3) or (0, ±1), respectively. Our considerations provide useful correlations between the distinct and non-compatible yet physically equivalent ZFS parameter sets reported in literature. All calculated alternative sets are tabularized for ten Mn²⁺ complexes considered. These ZFSP sets may serve for application of the multiple correlated fitting techniques in follow-up EMR studies. Some misinterpretations yielding ambiguous and unreliable results are clarified. The intricate low symmetry aspects are also discussed, whenever applicable. The results of this paper enable more reliable analysis, comparison, and fitting of ZFSP sets from EMR spectra for Mn²⁺ ions in various crystals.     

Effect of temperature and high pressure on Mn2+ EPR spectra in K3H(SO4)2 fast-proton conductor

The linewidth ΔH _pp and spin-Hamiltonian parameters under temperature and high hydrostatic pressure by X-band continuous wave electron paramagnetic resonance in the K₃H(SO₄)₂ crystal were studied. Spin-Hamiltonian parameters, direction cosines and coordination of Mn²⁺ ion were determined at room temperature. The pressure at 300?MPa leads to the change of hydrogen bond potential and the transition from double well to single well potential moves about 10?K towards a higher temperature.     

过渡金属TM(V,Cr,Mn)掺杂ZnS的电子结构和磁性

采用基于密度泛函理论(DFT)的第一性原理赝势平面波方法,对过渡金属V、Cr、Mn 掺杂ZnS的超晶胞体系进行了几何结构优化,计算了晶格常数、电子结构与磁学性质。研究结果表明：掺入V,Cr后,ZnS表现出明显的半金属性,而掺入Mn后,半金属性不明显;掺入过渡金属TM(V,Cr,Mn)后系统产生的磁矩主要有杂质的3d态电子贡献,且磁矩的大小与过渡金属的电子排布有关。     

Investigations of the spin-Hamiltonian parameters,optical absorption bands,and local structure for the tetragonal Cu2+ center in Cu2+-doped ZnCdO nanopowder

The spin-Hamiltonian (or EPR) parameters of tetragonal Cu²⁺ octahedral centers in ZnCdO nanopowders are calculated from the high-order perturbation formulas based on the cluster approach. In these formulas, the contributions to spin-Hamiltonian parameters due to the admixture between the d orbitals of dⁿ ion and the p orbitals of ligand ions via covalence effect are considered. The crystal field parameters are calculated from the superposition model and so the optical absorption bands (related to the crystal field energy levels) and local structure of Cu²⁺ octahedral centers in ZnCdO nanopowder are also studied. The calculated spin-Hamiltonian parameters and optical absorption bands are in reasonable agreement with the experimental values. The tetragonal elongation ΔR (=R_//−R_⊥) of Cu²⁺ octahedron in ZnCdO nanopowder due to Jahn–Teller effect is acquired from the calculations. The results are discussed.     

过渡金属TM(V,Cr,Mn)掺杂ZnS的电子结构和磁性

采用基于密度泛函理论(DFT)的第一性原理赝势平面波方法,对过渡金属V、Cr、Mn掺杂ZnS的超晶胞体系进行了几何结构优化,计算了晶格常数、电子结构与磁学性质.研究结果表明:掺入V,Cr后,ZnS表现出明显的半金属性,而掺入Mn后,半金属性不明显;掺入过渡金属TM(V,Cr,Mn)后系统产生的磁矩主要有杂质的3d态电子贡献,且磁矩的大小与过渡金属的电子排布有关.     

Theoretical investigation of the local lattice structure of Mn 2+ ion doped in tetragonal K 2ZnF 4 crystal

The complete energy matrices for a d⁵configuration ion in a tetragonal ligand-field has been constructed on the basis of the complete set of basis of d⁵configuration (252 dimension), and the relationship between the low-symmetry EPR parameters b₂ ⁰ ,b₄ ⁰ and the local distortion parameters has been established based on the complete energy matrices. As an application, we have studied the EPR parameters and the local lattice structure of Mn²⁺ ion doped in tetragonal K₂ZnF₄ system. The calculation indicated that the local lattice structure around a tetragonal Mn²⁺ ion center has an expansion distortion. Simultaneously, the local lattice structure parameters R₁ =2.0727 ?, R₂ =2.0801 ? at room temperature (295 K) and R₁ ^′ = 2.0439 ?, R₂ ^′ =2.05478 ? at low temperature (4.2 K) are determined.     

Structural characterisation of [Cu2(bptd) (H2O) Cl4] and [Ni2(bptd)2(H2O)4] Cl4·3H2O; evidence of null intramolecular exchanges by EPR and magnetic measurements

Using the 2,5-bis(2-pyridyl)-1,3,4-thiadiazole (bptd), we recently prepared [Cu₂(bptd) (H₂O) Cl₄] and [Ni₂(bptd)₂ (H₂O)₄] Cl₄, 3H₂O in which the magnetic centres are connected through one diazine+one chloro and two diazine ligand bridges, respectively. These two compounds are the first examples that show null intramolecular magnetic interactions despite M-M distances close to 3.7 Å within perfectly planar edifices:Down to , [Cu₂(bptd)Cl₄(H₂O)] is paramagnetic while, below T_t, half of the Cu²⁺ions interact, leading to residual paramagnetism of one Cu²⁺/f.u. Magnetic susceptibility measurements, EPR and pulsed EPR study indicate the original intermolecular nature of AF exchanges.[Ni₂(bptd)₂(H₂O)₄]Cl₄·3H₂O susceptibility obeys a Curie-law involving pure paramagnetism. Moreover, its EPR spectrum can be interpreted on the basis of virtual S=1 monomers. Below 70 K, Zero Field Splitting (D∼275 G) due to dipolar interactions without magnetic exchanges could be responsible for the LT spectra splitting. For both compounds, the thia role is suggested as partially responsible for the null-in-plane magnetic exchanges.     

EPR parameters and local lattice structure study of V ions in CdCl2 and CsMgCl3 crystals

The local lattice structure and EPR parameters (D, g_‖, g_⊥) have been studied systematically on the basis of the complete energy matrix for a d³ configuration ion in a trigonal ligand field. By simulating the calculated optical and EPR spectra data to the experimental results, the local distortion parameters (ΔR, Δθ) are determined for V²⁺ ions in CdCl₂ and CsMgCl₃ crystals, respectively. The results show that the local lattice structure of CdCl₂:V²⁺ system exhibits a compression distortion (ΔR=−0.0868 Å) while that of CsMgCl₃:V²⁺ system exists an elongation distortion (ΔR=0.0165 Å). The different distortion may be ascribed to the fact that the radius of V²⁺ ion is smaller than that of Cd²⁺ ion or larger than that of Mg²⁺ ion. Moreover, the relationships between EPR parameter D and local structure parameters (R, θ) as well as the orbital reduction factor k and g‐factors (g_‖, g_⊥) are discussed.     

Site symmetry of Mn(II) and Co(II) in zinc phosphate glass

Optical absorption and EPR spectra of Mn(II) and Co(II) doped zinc phosphate glasses have been investigated. Crystal filed parameters and g values are determined. For Mn(II) doped glass the values are Dq=850, B=850, and g values are around 2 at room temperature (RT). For Co(II) doped glass, Dq=890, B=700, and g=4.45 and 2.06 at liquid nitrogen temperature. The optical and EPR data has been correlated.     

Structural stability,electronic structure and mechanical properties of 4d transition metal nitrides TMN (TM=Ru,Rh, Pd)

Ab initio calculations are performed to investigate the structural stability, electronic, structural and mechanical properties of 4d transition metal nitrides TMN (TM=Ru, Rh, Pd) for five different crystal structures, namely NaCl, CsCl, zinc blende, NiAs and wurtzite. Among the considered structures, zinc blende structure is found to be the most stable one among all three nitrides at normal pressure. A structural phase transition from ZB to NiAs phase is predicted at a pressure of 104 GPa, 50.5 GPa and 56 GPa for RuN, RhN and PdN respectively. The electronic structure reveals that these nitrides are metallic. The calculated elastic constants indicate that these nitrides are mechanically stable at ambient condition.     

YAG晶体中Cr4+和Yb3+的光谱和荧光特性研究

报道了（Ｃｒ＾４＋,Ｙｂ＾３＋）：ＹＡＧ晶体的吸收光谱和荧光光谱特性。在室温下,（Ｃｒ＾４＋,Ｙｂ＾３＋）：ＹＡＧ晶体在９３７ｎｍ和９６８ｎｍ处存在两个吸收带,能与ＩｎＧａＡｓ激光二级管有效耦合：而且在１０３０ｎｍ处有一Ｃｒ＾４＋吸收峰,可以实现对Ｙｂ＾３＋的自调Ｑ激光输出。（Ｃｒ＾＋,Ｙｂ＾３＋）：ＹＡＧ 荧光光谱与Ｙｂ＾３＋：ＹＡＧ晶体一样,发光中心也是位于１０２９ｎｍ,但其强度比Ｙｂ＾３＋：     

Tetragonal distortions of the octahedral environments of Cr3+, Fe3+ and Gd3+ ions in A2CdF4 (A = Rb,Cs) and ACdF3 crystals

Abstract

The tetragonal distortions of local octahedral environments of Cr³⁺, Fe³⁺ and Gd³⁺ ions in Rb₂CdF₄, Cs₂CdF₄, RbCdF₃ and CsCdF₃ crystals have been studied by analyzing their EPR spectra. From the studies, it is found that the tetragonal distortions for Cr³⁺ and Fe³⁺ impurity ions, which substitute Cd²⁺ and have nearly the same ionic radius, are close to each other, whereas that for Gd³⁺ impurity ion, having a larger ionic radius, is larger than those for Cr³⁺ and Fe³⁺ ions in the same crystal. It appears that not only the impurity-ligand distance, but also the tetragonal distortions of impurity centres in crystals are closely related to the size of impurity.     

Superposition model (SPM) analysis of zero-field splitting parameters of Mn doped ZnAl2S4 crystal

In this study, superposition model (SPM) is employed to investigate the local environment around the different Mn²⁺ centers in ZnAl₂S₄ spinel. Using SPM and crystallographic data, the zero-field splitting (ZFS) parameters b₂⁰, b₄⁰, and b₄³ are calculated for Mn²⁺ at the B-site (with local symmetry D_3d), whereas b₄⁰ and b₄⁴ for the A-site (T_d). The lattice relaxation due to Mn²⁺ impurities is analyzed in terms of the bonding lengths and the angles between the Mn-S bond and the crystallographic axis [1 1 1]. Our SPM analysis of ZFS parameters indicates that satisfactory agreement can be achieved between the theoretical and the experimental results. Additional structural information about Mn²⁺ impurity centers is also obtained.     

NaZnLa(PO_4)_2中Ce~(3+)和Tb~(3+)的发光

采用高温固相反应合成了NaZnLa(PO4 ) 2 中掺杂Ce3 +、Tb3+的荧光体 ,对其晶体结构、发光行为进行了研究 ,并尝试对NaZnLa(PO4 ) 2 ∶Ce ,Tb荧光体进行调制。NaZnLa(PO4 ) 2 是LaPO4 的同构物 ,为单斜晶系独居石结构 ,从XRD谱数据得到NaZnLa(PO4 ) 2 基质的晶胞参数为a =0 6 82 3nm ,b =0 70 45nm ,c =0 6 497nm ,β =10 3 9° ,V =0 30 3nm3,其晶胞参数与单斜LaPO4 的晶胞参数相似。在NaZnLa(PO4 ) 2 ∶Ce ,Tb荧光体中 ,Ce3 +对Tb3+有良好的敏化作用 ,掺杂适量的BO3 -3 、Al3 +、Dy3 +,可以增强发光亮度     

EPR study of Dy 3 + ions in DyBa 2 Cu 3 O 6 + x

Concentrated polycrystalline DyBa₂Cu₃O_6+x compounds are studied by X-band EPR spectroscopy. A broad resonance line due to the highly anisotropic EPR spectrum of Dy³⁺ ions is identified on several specimens at low temperatures. Powder simulation of the EPR spectra complies with the ground Kramers doublet predicted by crystal field analysis of Dy³⁺ ions. Calculations of the second and fourth moments of the resonance lines due to the dipole-dipole interactions of Dy³⁺ indicate the presence of substantial exchange narrowing of the dipolar-broadened EPR linewidth. Received 3 June 2001