EPR Study of Fe3+- and Ni2+-Doped Macroporous CaSiO3 Ceramics

Thermally stable macroporous CaSiO₃, Fe³⁺- and Ni²⁺-doped (0.5 to 5 mol%) ceramics have been prepared by solution combustion process by mixing respective metal nitrates (oxidizers), fumed silica. Diformol hydrazine is used as a fuel. The combustion products were identified by their X-ray diffraction and thermal gravimetry/differential thermal analysis. Single phases of β-CaSiO₃ and α-CaSiO₃ were observed at 950 and 1200 °C, respectively. The phase transition temperatures of combustion-derived CaSiO₃ were found to be lower compared to those obtained via solid-state reaction method. It is interesting to note that with an increase in the calcination temperature the samples become more porous with an increase in the pore diameter from 0.2 to 8 μm. The electron paramagnetic resonance (EPR) spectrum of Fe³⁺ ions in CaSiO₃ exhibits a weak signal at g = 4.20 ± 0.1 followed by an intense signal at g = 2.0 ± 0.1. The signal at g = 4.20 is ascribed to isolated Fe³⁺ ions at rhombic site. The signal at g = 2.0 is due to Fe³⁺ coupled together with dipolar interaction. In Ni²⁺-doped CaSiO₃ ceramics the EPR spectrum exhibits a symmetric absorption at g = 2.23 ± 0.1. This deviation from the free electron g-value is ascribed to octahedrally coordinated Ni²⁺ ions with moderately high spin–orbit coupling. The number of spins participating in resonance and the paramagnetic susceptibilities have been evaluated from EPR data as a function of Fe³⁺ as well as Ni²⁺ content. The effect of alkali ions (Li, Na and K) on the EPR spectra of these ceramics has also been studied. Authors' address: R. P. Sreekanth Chakradhar, Glass Technology Laboratory, Central Glass and Ceramic Research Institute, Kolkata 700032, India     

Spectral dependence of photoinduced optical absorption in doped yttrium iron garnet single crystals

The photoinduced optical absorption α of doped yttrium iron garnets (YIG) is investigated. It is found that the optical absorption α at a wavelength of 1.1 μm depends on the wavelength of irradiating light in the range 0.6–1.9 μm. It is demonstrated that, in the Y₃Fe₅O₁₂ crystal with an acceptor Ba impurity, the photoinduced increase in α is due to the formation of Fe⁴⁺ ions in octahedral sites. The charge transfer occurs through photoexcitation of the ⁶ A _1g (⁶ S) → ⁴ T _1g (⁴ G) and ⁶ A _1g (⁶ S) → ⁴ T _2g (⁴ G) transitions of octahedral Fe³⁺ ions. In the crystal with a donor Si impurity, the increase in α is caused by the formation of Fe²⁺ ions upon photoionization of silicon. __________ Translated from Fizika Tverdogo Tela, Vol. 43, No. 7, 2001, pp. 1233–1235. Original Russian Text Copyright ? 2001 by Doroshenko, Nadezhdin.     

Optical parameters of Nd3＋：Er3＋：yb3＋ co-doped borosilicate glasses and their energy transfers at high temperature

This paper reports that a series of Nd³⁺:Er³⁺:Yb³⁺ co-doped borosilicate glasses have been prepared and their absorption spectra measured. The J--O intensity parameters Ω_k (k=2, 4, 6), spontaneous radiative lifetime τ_rad, spontaneous transition probability A, fluorescence branching ratio β and oscillator strength f_ed of the Nd³⁺ ions at room temperature are calculated based on Judd--Ofelt (J--O) theory. The temperature dependence of the up-conversion photoluminescence characteristics in a Nd³⁺:Er³⁺:Yb³⁺ co-doped sample is studied under a 978 nm semiconductor laser excitation, and the energy transfer mechanisms among Yb³⁺, Er³⁺ and Nd³⁺ ions are analysed. The results show that the J--O intensity parameters Ω₂ increase when the Nd³⁺ concentration of the Nd³⁺:Er³⁺:Yb³⁺ co-doped borosilicate glasses increases. The possibility of spontaneous transition is small and lifetimes are long at levels of ⁴F_5/2 and ⁴F_3/2. The intensity of Nd³⁺ emissions at 595, 691, 753, 813 and 887 nm are markedly enhanced when the sample temperature exceeds 400 K. The reasons being the cooperation of the secondary sensitization from Er³⁺ to Nd³⁺ and the contribution of a multi-phonon.     

Structures and luminescence properties of Yb3＋ in the double perovskites Ba2YB＇O6 （B＇=Ta5＋, Nb5＋） phosphors

The Yb³⁺ doped Ba₂YB'O₆ (B'=Ta⁵⁺, Nb⁵⁺) were prepared by high temperature solid-state reaction method, their structures were determined by x-ray diffraction and refined by Rietveld method. The diffuse reflection absorption, excitation and emission spectra of Yb³⁺:Ba₂YB'O₆ (B'=,Ta⁵⁺, Nb⁵⁺) were measured at room temperature. Under the excitation of ultraviolet light, these phosphors exhibit broad charge transfer band emissions of TaO₆ or NbO₆ centre with large Stokes shift. The Yb³⁺ doped into these hosts are situated at Y³⁺ sites of cubic symmetry (O_h). The experimental energy levels of Yb³⁺ in Ba₂YTaO₆ and Ba₂YNbO₆ were determined by photoluminescence and diffuse reflection absorption spectra. Their wavefunctions and theoretical energy levels were obtained by diagonalising the Hamiltonian matrix. The experimental energy levels were fitted by Levenberg--Marquardt iteration algorithm to determine crystal field parameters. Then, the magnetic-pole transition line strengths of Yb³⁺:Ba₂YB'O₆(B'=Ta⁵⁺, Nb⁵⁺) from (²F_5/2)Γ₈^- to the low-energy states were calculated.     

New expressions for g—factors of the mixed ground state of 3d^9 ions with a compressional tetragonal symmetry and its application to NaCl：Ni^＋（I）

We deduce new expressions for g-factors of the 3d⁹ ions with a compressional tetragonal symmetry, in which we consider not only the contribution from the spin-orbit coupling of the central transition metal ion but also that of the ligand orbits as well as the admixture of ²B_1g into ²A_1g (ground state). By using the new formulae, the electron paramagnetic resonance g-factors for NaCl:Ni⁺(I) are studied. Thus, the puzzle that the g_‖-shift is positive is reasonably explained.     

CRYSTAL STRUCTURE AND MAGNETIC PROPERTIES OF La1.2Sr1.8-xCaxMn2O7

The effect of divalent cation substitution on the structure and magnetic properties of La_1.2Sr_1.8-xCa_xMn₂O₇ (x = 0－0.900) is investigated in this paper. Partly replacing divalent cation Sr²⁺ by Ca²⁺ ions results in the weakening and then disappearance of long-range ferromagnetic ordering, and the formation of spin canting and low-temperature spin-glass. Based on structural analysis by Rietveld profile fitting, we suggest that this variation of magnetic property be related to a Jahn－Teller-type attice distortion of MnO₆ octahedra due to the introduction of the smaller sized Ca²⁺ ions.     

Stability of the positively charged manganese centre in GaAs heterostructures examined theoretically by the effective mass approximation calculation near the Γ critical point

This paper describes an n-i-p-i-n model heterostructure with a manganese (Mn)-doped p-type base region to check the stability of a positively charged manganese A_Mn⁺ centre with two holes weakly bound by a negatively charged 3d⁵(Mn) core of a local spin S=5/2 in the framework of the effective mass approximation near the Γ critical point (k～0). By including the carrier screening effect, the ground state energy and the binding energy of the second hole in the positively charged centre A_Mn⁺ are calculated within a hole concentration range from 1 × 10¹⁶ cm^-3 to 1 × 10¹⁷ cm^-3, which is achievable by biasing the structure under photo-excitation. For comparison, the ground-state energy of a single hole in the neutral A_Mn⁰ centre is calculated in the same concentration range. It turns out that the binding energy of the second hole in the A_Mn⁺ centre varies from 9.27 meV to 4.57 meV. We propose that the presence of the A_Mn⁺ centre can be examined by measuring the photoluminescence from recombination of electrons in the conduction band with the bound holes in the A_Mn⁺ centre since a high frequency dielectric constant of varepsilon _∞ =10.66 can be safely adopted in this case. The novel feature of the ability to tune the impurity level of the A_Mn⁺ centre makes it attractive for optically and electrically manipulating local magnetic spins in semiconductors.     

Crystal electric field next to a hydrogen-like interstitial— µ+ in PrNi5

Muon spin rotation measurements of the temperature dependence and the anisotropy of the μ⁺ Knight shift in single crystals of the crystal electric field singlet ground state system PrNi₅ reveal pronounced deviations from a linear scaling of the Knight shift with the bulk magnetic susceptibility atT≤50 K. They are explained by a μ⁺ induced modification of the atomic susceptibility of neighboring Pr³⁺ ions. From the Knight shift anisotropy atT> 50 K it is determined that the implanted μ⁺ occupy a single intersitial site, namely the 6i site (0.5, 0, 0.21±0.02). Using this site information, good model fits to the measured data are obtained assuming a μ⁺ induced perturbation of the crystal electric field at the Pr³⁺ ions next to the μ⁺. Apparently, the presence of the μ⁺ leads to a lowering of the local symmetry, causing a lifting of the degeneracy and a pronounced rearrangement of the low lying crystal electric field levels for these ions.     

Spin current and its heat effect in a multichannel quantum wire with Rashba spinben orbit coupling

Using the perturbation method, we theoretically study the spin current and its heat effect in a multichannel quantum wire with Rashba spin—orbit coupling. The heat generated by the spin current is calculated. With the increase of the width of the quantum wire, the spin current and the heat generated both exhibit period oscillations with equal amplitudes. When the quantum-channel number is doubled, the oscillation periods of the spin current and of the heat generated both decrease by a factor of 2. For the spin current j_s,xy, the amplitude increases with the decrease of the quantum channel; while the amplitude of the spin current j_s,yx remains the same. Therefore we conclude that the effect of the quantum-channel number on the spin current j_s,xy is greater than that on the spin current j_s,yx. The strength of the Rashba spin—orbit coupling is tunable by the gate voltage, and the gate voltage can be varied experimentally, which implies a new method of detecting the spin current. In addition, we can control the amplitude and the oscillation period of the spin current by controlling the number of the quantum channels. All these characteristics of the spin current will be very important for detecting and controlling the spin current, and especially for designing new spintronic devices in the future.     

Theoretical investigations of the local distortion and electron paramagnetic resonance parameter for CdCl2：V^2＋ and CsMgX3：V^2＋（X=Cl,Br） systems

This paper systematically investigates the local distortion and electron paramagnetic resonance (EPR) parameter for CdCl 2 :V 2+ and CsMgX 3 :V 2+ (X=Cl, Br) systems on the basis of the complete energy matrix, in which not only the contributions due to the spin–orbit coupling of the central ions but also that of the ligands are considered. To describe the difference of overlapping between d-orbits and p orbit, two spin–orbit coupling coefficients are introduced. By simulating the crystal field parameter and EPR parameter, the local distortion parameters are studied and the relationships between the EPR parameter and the spin–orbit coupling coefficients as well as divergent parameter are discussed. These results show that the local structures exhibit compression distortion for CdCl 2 :V 2+ and elongation distortions for CsMgX 3 :V 2+ (X=Cl, Br), respectively. It notes that the empirical formula R ≈ R H + (r i-r h )/2 is not suitable for CdCl 2 :V 2+ and CsMgX 3 :V 2+ (X=Cl, Br) systems. The contributions of ligand to spin–orbit coupling interaction cannot be neglected for strong covalent systems, especially for V 2+ doped in CsMgBr 3 :V 2+ .     

Local structure and the electron paramagnetic resonance parameters for the Cr3+ ions at K+-vacancy site in Cr3+:KZnF3 laser crystal

The quantitative relationship between the electron paramagnetic resonance (EPR) parameters D,g_∥,g_⊥ and the local structure parameters of Cr³⁺ ion in KZnF₃ crystals is established. The local structure for Cr³⁺ paramagnetic center in KZnF₃:Cr³⁺ crystal has been determined from EPR parameters of Cr³⁺ ion. This work shows that the trigonal crystal field of Cr³⁺ ion in KZnF₃ crystals comes from following two origins: (1) the nearest-neighbor K⁺ vacancy caused by the charge compensation in the [1 1 1]-axis direction; and (2) the lattice distortions of the nearest-neighbor fluorine coordination caused by the K⁺ vacancy and the differences in mass, charge, and radius between Cr³⁺ ion and Zn²⁺ ion. The unified calculation of the EPR zero-field splitting and g factors, taking into account the K⁺ vacancy and the lattice distortions, has been carried out on the basis of the complete diagonalization procedure and the superposition crystal-field model, all calculation results are in excellent agreement with the experimental data. Although the main source of the trigonal crystal field comes from the K⁺ vacancy caused by the charge compensation, the contribution of the lattice distortion cannot be neglected.     

Microstructure of pair centers of Cr3+-Cr2+ ions in the KZnF3 crystal

Studies involving the piezospectroscopy method have shown that the symmetry of the pair centers of Cr³⁺-Cr²⁺ ions in the KZnF₃ crystal is tetragonal. In this paper we develop a microscopic model of a pair center. We use the temperature dependence of the integrated intensity of the absorption line to find the effective hopping integral for an e _g electron, t _σσ=205 ± 10 cm⁻¹, and the polaron reduction factor, equal to 0.11. By analyzing the selection rules for exchange-induced electric dipole transitions under double-exchange conditions we identify all the absorption lines of Cr³⁺-Cr²⁺ pairs. Zh. éksp. Teor. Fiz. 114, 1421–1429 (October 1998)     

KZnF3:Cr3+和KMgF3:Cr3+四角对称电子顺磁共振参量与晶格缺陷的研究

用Newman叠加模型研究了KZnF₃:Cr³⁺四角对称基态的零场分裂,证实了Zn²⁺空位和畸变的存在;并指出,空位对晶场的贡献不可忽略。计算得到:KZnF₃:cr³⁺晶体[0,01]方向的一个F^-配体向Cr³⁺移动Δ(KZnF₃)=0.0029-0.0043nm。还研究了KMgF₃:Cr^{3+关键词：}     

Investigation of local lattice structure around Cr3+ ions at the trigonal and tetragonal sites in RbCdF3: Cr3+ crystal

The local lattice structure and EPR, optical spectra for Cr³⁺ doped in RbCdF₃ crystal have been studied by diagonalizing the complete energy matrices. The results show that the local structure of the Cr³⁺ ions in RbCdF₃ exhibits a compressed distortion at the trigonal and tetragonal sites. The compressed distortion can be ascribed to the fact that the radius of Cr³⁺ ion is smaller than that of Cd²⁺ ion, and therefore Cr³⁺ ion will draw the fluorin ligands inwards. The variational ranges of the local structural parameters for Cr³⁺ doped in RbCdF₃ crystal R =?1.9491 Å ～?1.9814 Å, θ?= 55.234° ～?55.286° at the trigonal site and R ₁ =?1.8617 Å ～?1.8928 Å, R ₂ =?1.9527 Å ～?1.9851 Å at tetragonal site are obtained respectively, and the EPR and optical spectra agree well with the experimental results.     

EPR and optical spectroscopy of neodymium ions in KMgF3 and KZnF3 crystals

KMgF₃ and KZnF₃ crystals doped with Nd³⁺ ions were studied using EPR and optical spectroscopy methods. Several types of paramagnetic centers of Nd³⁺: KMgF₃ — two of tetragonal and one of rhombic symmetry; KZnF₃ — one of tetragonal and one of trigonal symmetry were found. Parameters of the corresponding spin Hamiltonians were determined. Using optical spectroscopy paramagnetic centers Nd²⁺ and Nd⁴⁺ in KMgF₃ were found.     

The fine structure levels and spin-singlet contributions to zero-field-splitting parameters of Cr2+ ion in CdGa2S4

A theoretical investigation is reported of the fine structure levels and the spin-singlet contributions to zero-field-splitting (ZFS) parameters for Cr²⁺ ion in CdGa₂S₄ crystals. Firstly, the complete energy matrix including all spin states for a 3d ⁴ ion in tetrahedral D _2d symmetry is constructed according the double-group chain in the strong-field scheme. Then, by diagonalizing the complete energy of electron–electron interactions, the crystal field and the spin–orbit coupling for the Cr²⁺ (3d ⁴) ion in CdGa₂S₄ crystal, the fine structure levels and the spin-singlet contributions to ZFS parameters a, D and F are calculated. The results show that the spin-singlet contribution to D is negligible, but the contributions to a and F are very important. So, to obtain more accurate ZFS parameters for 3d ⁴ ions in the tetrahedral crystals, all spin states should be considered.     

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 investigations of the local structure distortion and the spin Hamiltonian parameters of Cr ions at tetragonal charge-compensation defect CrF5O site in Cr: KMgF3 crystals

The local structure distortion and the spin Hamiltonian (SH) parameters, including the zero-field splitting (ZFS) parameter D and the Zeeman g-factors g_‖ and g_⊥, are theoretically investigated by means of complete diagonalization method (CDM) and the microscopic spin Hamiltonian theory for tetragonal charge compensation CrF₅O defect center in Cr³⁺:KMgF₃ crystals. The superposition model (SPM) calculations are carried out to provide the crystal field (CF) parameters. This investigation reveals that the replacement of O²⁻ for F⁻ and its induced lattice relaxation Δ₁(O²⁻) combined with an inward relaxation of the nearest five fluorine Δ₂(F⁻) give rise to a strong tetragonal crystal field, which in turn results in the large ZFS and large anisotropic g-factor Δg. The experimental SH parameters D and Δg can be reproduced well by assuming that O²⁻ moves towards the central ion Cr³⁺ by Δ₁(O²⁻)=0.172R₀ and the five F⁻ ions towards the central ion Cr³⁺ by Δ₂(F⁻)=0.022R₀. Our approach takes into account the spin-orbit (SO) interaction as well as the spin-spin (SS), spin-other-orbit (SOO), and orbit-orbit (OO) interactions omitted in previous studies. This shows that although the SO interaction is the most important one, the contributions to the SH parameters from other three magnetic interactions are appreciable and should not be omitted, especially for the ZFS parameter D.     

High-frequency tunable EPR spectroscopy of Cr3+ in synthetic forsterite

The electron paramagnetic resonance spectra of isolated and dimer impurity centers of trivalent chromium ions in the octahedral Ml sites in synthetic forsterite are studied in the frequency range of 65–90 GHz. The measurements are performed at 4.2 K in magnetic field from ?0.04 to 0.3 T. The zero-field splitting between spin doublets of the isolated Cr³⁺ ion Δ_s = 66.7 GHz and between spin sublevels of the Cr³⁺-Cr³⁺ dimer Δ_d1 = 71.5 GHz and Δ_d2 = 73.0 GHz is measured directly at zero field. The analysis of the spin Hamiltonian parameters shows that the dimer center consists of a pair of Cr³⁺ ions with an Mg²⁺ vacancy between them replacing three Mg²⁺ ions situated in a quasi-one-dimensional chain aligned parallel to the crystal c-axis. It is found that the exchange interaction in the dimer is ferromagnetic with parameters J_z = 0.47 GHz and J_t = 0.79 GHz.     

Investigations of the optical band positions,EPR parameters and defect structure for the trigonal Cr3+–Li+ centre in CsMgCl3 crystal

A unified calculation of optical band positions and electron paramagnetic resonance (EPR) (or spin-Hamiltonian) parameters for the trigonal Cr³⁺–Li⁺ centre in CsMgCl₃ crystal is made by using the complete diagonalisation (of energy matrix) method based on the two-spin–orbit-parameter model. In this model, the contributions from the spin–orbit parameter of central dⁿ ion and that of ligand ion are included. From the calculation, the 11 observed optical and EPR data (eight optical band positions and three EPR parameters g_//, g_⊥, D) are reasonably explained with five adjustable parameters and the defect structure of the Cr³⁺–Li⁺ centre (which is consistent with the expectation based on the electrostatic interaction) in CsMgCl₃ is acquired. The results are discussed.