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.     

Magnetic excitations in the two dimensional planar antiferromagnets K2FeF4 and Rb2FeF4

The magnetic excitation spectrum of K₂FeF₄ and Rb₂FeF₄, two K₂NiF₄-structure planar antiferromagnets with rather large anisotropy and spins perpendicular to the c-axis, has been measured by Raman and FIR-spectroscopy. One of the two predicted one-magnon transitions and the two-magnon mode have been observed in K₂FeF₄ (Rb₂FeF₄) at 48.5 cm^-1 (37.6 cm^-1) and 182.0 cm^-1 (160.5 cm^-1) respectively. The magnetic field and temperature dependence of the spectra are reported too. The data are discussed on the basis of an easy plane spin model Hamiltonian. In K₂FeF₄: Mn²⁺ a low lying magnetic impurity mode is observed at 40.5 cm^-1.     

Fourth-order spin Hamiltonian terms for Fe2+ in rutile structure antiferromagnet FeF2

The paper deals with the spectroscopic and magnetic properties of Fe²⁺ ions in FeF₂. The microscopic spin Hamiltonian theory for Fe²⁺ in crystalline environments with second-kind orthorhombic symmetry is considered. Explicit formulas for the parameters B₀⁽²⁾(D),B₂⁽²⁾(E), g_x, g_y, g_z and, for the first time in the literature, the fourth-order parameters B₀⁽⁴⁾, B₂⁽⁴⁾ and B₄⁽⁴⁾, are derived. Using semi-empirical data for the ⁵D-term energy levels of Fe²⁺ ion in FeF₂, the pressure dependence of the parameters B_q^(k) in the region from 0 to 133 kbar is discussed. The relative role of the fourth-order parameters with respect to the second-order ones is found to increase strongly with pressure (e.g. in the region studied, D increases only by a factor of 3, whereas B₀⁽⁴⁾ increases by a factor of nearly 20). The magnetocrystalline anisotropy of FeF₂; is considered in the strong anisotropy model taking into account the fourth-order spin Hamiltonian terms. The uniaxial anisotropy constants K₁ and K₂ are derived theoretically and their pressure dependence is discussed quantitatively. The theory and numerical results of this paper are useful with regard to Fe²⁺ in other isomorphic fluorides, namely: MgF₂, ZnF₂, VF₂ and MnF₂. It is found that the fourth-order spin Hamiltonian parameters are accessible to experimental detection from spectroscopic studies on Fe²⁺ in non-magnetic fluorides and magnetic studies on Fe²⁺ : MnF₂ and FeF₂, preferably under high pressure.     

Mössbauer study of an amorphous iron III flouride: FeF3, xHF (0 ⩽ x ⩽ 1)

A ferric amorphous flouride FeF₃, xHF with 0 ⩽ x ⩽ 1 synthesized by a soft chemistry reaction was studied by Mössbauer spectroscopy. The hyperfine characteristics are very close to those of deposited variety previously published: they show that the Fe³⁺ ions are surrounded by weakly distorted octahedra of flourine atoms. The study under external magnetic field indicates that the spins are randomly distributed: FeF₃, xHF has a speromagnetic behaviour like deposited amorphous FeF₃. Elsewhere the similarity between the two amorphous forms is confirmed by susceptibility measurements.     

The size and strain effects on the Raman spectra of Ce1−xNdxO2−δ (0≤x≤0.25) nanopowders

Ultrafine Ce_1−xNd_xO_2−δ (x=0-0.25) powders were synthesized by self-propagating room temperature synthesis. Raman spectra were measured at room temperature in the 300-700 cm⁻¹ spectral range. The shift and asymmetric broadening of the Raman F_2g mode at about 454 cm⁻¹ in pure and doped ceria samples could be explained with combined size and inhomogenous strain effects. Increased concentration of O²⁻ vacancies with doping is followed by an appearance of new Raman feature at about 545 cm⁻¹.     

Far-infrared absorption of MgO: Fe2+ in magnetic fields

We have measured the far-infrared absorption of iron-doped MgO in the wavenumber region 10–200 cm^?1 and in magnetic fields up to 6 T. Absorption peaks found at 107.0 and 110.5 cm^?1 are assigned to magnetic dipole transitions between the spin-orbit Г_5g groundstate (J = 1) and the Г_3g, Г_4g excited states (J = 2) of the Fe²⁺ -ion at a cubic site. The observed magnetic field dependence shows that Г_4g is the higher excited level, so that the crystal field order of the levels is not changed by the reduction of the spin-orbit splitting attributed to a dynamic Jahn-Teller effect. An additional absorption peak at 33.4 cm^?1 is found to split in magnetic field.In iron-doped KMgF₃ absorption peaks at 52 and 87 cm^?1 that have previously been attributed to the same transitions of Fe²⁺ are found to remain unshifted and unsplit in magnetic fields up to 6 T.     

Raman scattering in FeF3

Peaks in the FeF₃ phonon Raman spectrum are assigned to the A_1g(312 cm^-1) and E_g(105, 187, 450 cm^-1) representations of the D⁶_3d group. Anomalous temperature dependence of the frequency, intensity and line width of the 187 cm^-1 line has been observed.     

Structural and electrical transport properties of ZnxFe3−xO4 thin film deposited on Si (1 1 1) by pulsed-laser deposition

Polycrystalline thin films of Fe_3−xZn_xO₄ (x = 0.0, 0.01 and 0.02) were prepared by pulsed-laser deposition technique on Si (1 1 1) substrate. X-ray diffraction studies of parent as well as Zn doped magnetite show the spinel cubic structure of film with (1 1 1) orientation. The order–disorder transition temperature for Fe₃O₄ thin film with thickness of 150 nm are at 123 K (Si). Zn doping leads to enhancement of resistivity by Zn²⁺ substitution originates from a decrease of the carrier concentration, which do not show the Verwey transition. The Raman spectra for parent Fe₃O₄ on Si (1 1 1) substrate shows all Raman active modes for thin films at energies of T_2g¹, T_2g³, T_2g², and A_1g at 193, 304, 531 and 668 cm⁻¹. It is noticed that the frequency positions of the strongest A_1g mode are at 668.3 cm⁻¹, for all parent Fe₃O₄ thin film shifted at lower wave number as 663.7 for Fe_2.98Zn_0.02O₄ thin film on Si (1 1 1) substrate. The integral intensity at 668 cm⁻¹ increased significantly with decreasing doping concentration and highest for the parent sample, which is due to residual stress stored in the surface.     

α-Al2O3单晶中Fe3+离子的电子顺磁共振

本文对α-Al₂O₃单晶体中Fe³⁺离子在室温下,X波段进行了电子顺磁共振研究,发现Fe³⁺离子实际上占据四种磁性不等价晶位。在同一氧离子层间的两种晶位上的Fe³⁺离子具有相同的自旋哈密顿参量,而不同氧离子层间的晶位上的Fe³⁺离子具有不同的自旋哈密顿参量,两种自旋哈密顿参量为:(1)g_‖=2.001,g_⊥=2.003,D=1679 关键词：     

Observation of electronic Raman transitions from the transition metal ions Ti3+ and V4+ highly diluted in α-Al2O3

α-Al₂O₃ : Ti³⁺ (300 mass ppm concentration) at 10 K shows two electronic Raman transitions at 38 cm^-1 and 109 cm^-1. The E symmetry of the scattering matrix agrees with the selection rules for transitions between the ground state B_{$\text{3}\text{2}$} and the E_{$\text{1}\text{2}$} of the ground state manifold ²t_2g.α-Al₂o₃ : V⁴⁺ shows two very weak electronic Raman transitions at about 30 cm^-1 and 56 cm^-1 respectively.     

Excitation spectra and fluorescent lifetime measurements of Mn2+ in CaF2 and CdF2

The excitation spectrum of the Mn²⁺ emission has been measured in CaF₂ and CdF₂. The observed excitation bands have been assigned to transitions of the Mn²⁺ ions in a cubic environment. The calculated values for the crystal field (Dq) and Racah parameters (B,C) are Dq = 425 cm^-1 for CaF₂, Dq = 500 cm^-1 for CdF₂ and, B = 770 cm^-1 and C / B = 4.48 for both compounds. The lifetime of the fluorescent level ⁴T_1g(⁴G) has been measured in both compounds at different temperatures in the range from 10 to 500 K. The lifetime thermal dependence is explained taking into account different mechanisms (purely radiative, phonon assisted, and radiationless transitions) for the decay of excited Mn²⁺ ions.     

Indirect measurements of the Jahn-Teller quenched spin-orbit splitting in the 5T2g-state of KMgF3:Fe2+

The energy separation of the first excited spin-orbit States Γ_3g, Γ_4g from the Γ_5g ground state, in the orbital ⁵T_2g triplet state of Fe²⁺ in KMgF₃, has been estimated from temperature dependence measurements on the 7860 cm^-1 zero-phonon-line (ZPL) transition from Γ_5g to the orbital ⁵E_g doublet state. Using a simplified crystal field energy level model, we find the Γ_5g Γ_3g, Γ_4g separation to be ～30 cm^-1, indicating that the vibronic Jahn-Teller coupling is considerably stronger in KMgF₃:Fe²⁺ than in MgO:Fe²⁺. Far infrared absorption data on KMgF₃:Fe²⁺ in magnetic fields up to 6T, are found to be consistent with this interpretation.     

Mixed alkali effect in borate glasses—EPR and optical absorption studies in xNa2O–(30−x)K2O–70B2O3 glasses doped with Mn2+

The mixed alkali borate xNa₂O–(30−x)K₂O–70B₂O₃ (5≤x≤25) glasses doped with 1 mol% of manganese ions were investigated using EPR and optical absorption techniques as a function of alkali content to look for ‘mixed alkali effect’ (MAE) on the spectral properties of the glasses. The EPR spectra of all the investigated samples exhibit resonance signals which are characteristic of the Mn²⁺ ions. The resonance signal at g≅2.02 exhibits a six line hyperfine structure. In addition to this, a prominent peak with g≅4.64, with a shoulder around g≅4.05 and 2.98, was also observed. From the observed EPR spectrum, the spin-Hamiltonian parameters g and A have been evaluated. It is interesting to note that some of the EPR parameters do show MAE. It is found that the ionic character increases with x and reaches a maximum around x=20 and thereafter it decreases showing the MAE. The number of spins participating in resonance (N) at g≅2.02 decreases with x and reaches a minimum around x=20 and thereafter it increases showing the MAE. It is also observed that the zero-field splitting parameter (D) increases with x, reaches a maximum around x=15 and thereafter decreases showing the MAE. The optical absorption spectrum exhibits a broad band around ∼20,000 cm⁻¹ which has been assigned to the transition ⁶A_1g(S)→⁴T_1g(G). From ultraviolet absorption edges, the optical bandgap energies and Urbach energies were evaluated. It is interesting to note that the Urbach energies for these glasses decrease with x and reach a minimum around x=15. The optical band gaps obtained in the present work lie in the range 3.28–3.40 eV for both the direct and indirect transitions. The physical parameters of all the glasses were also evaluated with respect to the composition.     

X-ray photoelectron and mössbauer spectroscopy studies of the valence state of transition metal ions in Co1–x Fe x Cr2O4 (x = 0.1, 0.2, 0.5) ceramics

The valence state of transition metal ions in the Co_1–x Fe _x Cr₂O₄ (x = 0.1, 0.2, 0.5) system has been investigated using X-ray photoelectron and Mössbauer spectroscopy. It has been shown that, in this system, there are Fe²⁺ and Fe³⁺ ions. The relative Fe³⁺/Fe²⁺ contents have been determined by fitting the experimental Fe 2p photoelectron spectra by a superposition of theoretical spectra of the Fe²⁺ and Fe³⁺ ions, as well as using Mössbauer spectroscopy.     

EPR and magnetic susceptibility studies of iron ions in ZnO-Fe2O3-SiO2-CaO-P2O5-Na2O glasses

Room temperature electron paramagnetic resonance (EPR) spectra and temperature dependent magnetic susceptibility data have been obtained on bulk x(ZnO,Fe₂O₃)(65−x)SiO₂20(CaO, P₂O₅)15Na₂O (6≤x≤21 mole%) glasses prepared by melt quenching method. EPR spectra of the glasses revealed absorptions centered at g≈2.1 and 4.3. The variations of the intensity and line width of these absorption lines with composition have been interpreted in terms of the variation in the concentration of the Fe²⁺ and Fe³⁺ ions in the glass and the interaction between the iron ions. EPR and magnetic susceptibility data of the glasses reveal that both Fe²⁺ and Fe³⁺ ions are present in the glasses, with their relative concentration being dependent on the glass composition. The studies reveal superexchange type interactions in these glasses, which are strongly dependent on their iron content.     

Effects of nickel doping on structural and optical properties of spinel lithium manganate thin films

Spinel LiNi_xMn_2−xO₄ (x≤0.9) thin films were synthesized by a sol-gel method employing spin-coating. The Ni-doped films were found to maintain cubic structure at low x but to exhibit a phase transition to tetragonal structure for x≥0.6. Such cubic-tetragonal phase transition can be explained in terms of Ni³⁺(d⁷) ions with low-spin (t_2g⁶,e_g¹) configuration occupying the octahedral sites of the compound, thus being subject to the Jahn-Teller effect. By X-ray photoelectron spectroscopy both Ni³⁺ and Ni²⁺ ions were detected where Ni²⁺ is more populated than Ni³⁺. Optical properties of the LiNi_xMn_2−xO₄ films were investigated by spectroscopic ellipsometry in the visible-ultraviolet range. The measured dielectric function spectra mainly consist of broad absorption structures attributed to charge-transfer transitions, O²⁻(2p)→Mn⁴⁺(3d) for 1.9 (t_2g) and 2.8-3.0 eV (e_g) structures and O²⁻(2p)→Mn³⁺(3d) for 2.3 (t_2g) and 3.4-3.6 eV (e_g) structures. Also, sharp absorption structures were observed at about 1.6, 1.7, and 1.9 eV, interpreted as being due to d-d crystal-field transitions within the octahedral Mn³⁺ ion. In terms of these transitions, the evolution of the optical absorption spectrum of LiMn₂O₄ by Ni doping could be explained and the related electronic structure parameters were obtained.     

Studies on the complex behavior of optical phonon modes in wurtzite (ZnO)1−x (Cr2O3) x

This paper reports on the use of phonon spectra obtained with laser Raman spectroscopy for the uncertainty concerned to the optical phonon modes in pure and composite ZnO_1?x (Cr₂O₃) _x . Particularly, in previous literature, the two modes at 514 and 640 cm^?1 have been assigned to ZnO are not found for pure ZnO in our present study. The systems investigated for the typical behavior of phonon modes with 442 nm as excitation wavelength are the representative semiconductor (ZnO)_1?x (Cr₂O₃) _x (x = 0, 5, 10 and 15 %). Room temperature Raman spectroscopy has been demonstrated polycrystalline wurtzite structure of ZnO with no structural transition from wurtzite to cubic with Cr₂O₃. The incorporation of Cr³⁺ at most likely on the Zn sub-lattice sites is confirmed. The uncertainty of complex phonon bands is explained by disorder-activated Raman scattering due to the relaxation of Raman selection rules produced by the breakdown of translational symmetry of the crystal lattice and dopant material. The energy of the E ₂ (high) peak located at energy 53.90 meV (435 cm^?1) due to phonon–phonon anharmonic interaction increases to 54.55 meV (441 cm^?1). A clear picture of the dopant-induced phonon modes along with the B ₁ silent mode of ZnO is presented and has been explained explicitly. Moreover, anharmonic line width and effect of dislocation density on these phonon modes have also been illustrated for the system. The study will have a significant impact on the application where thermal conductivity and electrical properties of the materials are more pronounced.     

Production and Laser Characteristics of Fe<Superscript>2+</Superscript>:ZnS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> Polycrystals

A method is developed for producing active laser elements (spectrum range 4 to 5 μm) based on polycrystalline solid solutions ZnS_xSe_x-1 doped with iron ions. Bilateral diffusion doping of the elements by Fe²⁺ ions is performed during hot isostatic pressing. Spectral and energy characteristics of the laser are investigated with the Fe²⁺:ZnS_0.1Se_0.9 active element kept at room temperature. It is found that the absorption band of the Fe²⁺:ZnS_0.1Se_0.9 crystal is blueshifted with respect to the Fe²⁺:ZnSe absorption band, while the lasing spectra of the Fe²⁺:ZnSe and Fe²⁺:ZnS_0.1Se_0.9 lasers and their energy parameters are almost identical. The lasing energy of 580 mJ is obtained at the slope efficiency with respect to the absorbed energy of 46%. Further increase in the lasing energy is limited by development of transversal parasitic oscillation at a large size of the pump beam spot.     

Cation distribution of the system Zn1−xCoxFeMnO4 by x-ray,electrical conductivity and Mössbauer studies

Structural, electrical and Mössbauer studies were carried out for the system Zn_1?xCo _x Fe MnO₄. It is observed that forx?0.6, the ionic configuration of the system is Zn _1?x ²⁺ Mn _x ²⁺ [Co _x ³⁺ Mn _1?x ³⁺ Fe³⁺]O ₄ ^2? and forx?0.8 Fe³⁺ ions occupy tetrahedral site also. On the basis of electrical and Mössbauer studies a probable valence distribution of CoMnFeO₄ has been suggested. All the compounds showed positive values of thermoelectric coefficient and electrical conduction takes place by a hopping mechanism. Activation energy and thermoelectric coefficient values decreased with decrease in concentration of Zn²⁺ ions. The compounds possess low mobility values varying between 10^?7 and 10^?9 cm²/V sec.     

Mn-doped (Ba,Y)Fe12O19 hexaferrites: Crystal structure and oxidation states of Mn and Fe

We have fabricated Ba_0.95Y_0.05Fe_12-xMn_xO₁₉ samples with large Mn-doping amounts of x = 4 and 6, using the mechanical milling and heat treatment. X-ray diffraction analysis indicated the samples crystallized in the M-type hexaferrite structure. The Mn doping caused the modification, shift and broadening of some characteristic phonon-vibration modes, which were recorded by Raman spectroscopy. This is due to an incorporation of Mn ions into the M-type structure that disorders the periodic lattice and changes symmetry. Basing on X-ray absorption spectroscopy, we have found Fe in all samples stable with an oxidation state 3+ (Fe³⁺). Though Mn²⁺ and Mn³⁺ ions coexist, the concentration of Mn²⁺ in x = 4 is larger than that in x = 6. The analysis of Fourier-transform spectra have demonstrated the replacement of Mn^2+,3+ ions for Fe³⁺ in the M-type structure. The sites of Mn^2+,3+ ions in this structure have been discussed.