Effects of covalency on magnetic properties of tetrahedral V4+ ion in garnets

The theory of covalency is applied to the discussion ofg-factors and hyperfine constantsA of a V⁴⁺ ion in tetrahedral sites in garnets Ca₃X₂Ge₃O₁₂ with X=Sc, Al, Lu, In, and NaCa₂. Mg₂V₃O₁₂, Ca₃Sc₂Si₃O₁₂, Sr₃Sc₂Ge₃O₁₂. The site symmetry is S₄. Coefficients in LCAO-MO wave function and the Fermi contact interaction constantx are determined by an iterative procedure from experimental data. The spin-orbit coupling coefficient _d and radial average r ^–3 are deduced from the condition of solvability of four equations: overlap and charge-transfer integrals are also determined.The author is grateful to Dr. P.Novák for many valuable discussions on the subject and Dr. J.Jokl from the Institute of Macromolecular Chemistry of Czechoslovak Academy of Sciences for optical measurements.     

Temperature dependent EPR of octahedral S-state ions in garnets

Temperature dependence of EPR of the octahedral Fe³⁺ and Mn²⁺ ions in four different garnets is studied between 77 K and the room temperature. The observed change of the spin hamiltonian parametersB ₂ ⁰ ,B ₄ ⁰ ,B ₄ ³ is of the order of several %, while theg-factor is temperature independent within the experimental errors. The role of thermal expansion and the effect of phonons on the temperature variation of B ₂ ⁰ is analyzed. It is shown that the inhomogeneous thermal expansion may account for theB ₂ ⁰ (T) dependence observed.The authors wish to thank Dr. B. V.Mill of Moscow State University (USSR) for supplying the single crystals of Na₃Sc₂V₃O₁₂, Ca₃Al₂Ge₃O₁₂ and Ca₃Ga₂Ge₃O₁₂ garnets.     

Theoretical explanation of optical absorption spectra and covalent effect of Ni2+ ions in octahedral sites of Ca3Sc2Ge3O12 crystal

The optical absorption spectra (d-d transition bands) and covalent effect of Ni²⁺ ions in octahedral sites of Ca₃Sc₂Ge₃O₁₂ crystal have been investigated by the full energy matrix based on the two spin–orbit coupling parameters model. The bond length of octahedral site is R_i?=?2.19 Å, which can be determined by the cubic crystal-field parameter and optical spectral data. The lattice distortion of the Ni²⁺ center in Ca₃Sc₂Ge₃O₁₂ crystal is also obtained from the calculations. In addition, the result has shown that the covalent effect of Ni²⁺ ion in the octahedral site of Ga₃Sc₂Ge₃O₁₂ is obvious and cannot be ignored. The calculated d-d transition bands agree well with that of the experimental findings, suggesting that the present methods can explain reasonably the optical spectral data and covalent effect of 3d⁸ ions in octahedral lattices.     

硼酸对亚微米级Ca3Sc2Si3O12:Ce绿色荧光粉的制备及发光性能的影响

采用凝胶-燃烧法合成了Ca₃Sc₂Si₃O₁₂ ∶Ce绿色LED用荧光粉,用X射线粉末衍射(XRD)、扫描电镜(SEM)、荧光光谱仪等对合成产物进行了分析和表征.结果表明:通过添加H₃BO₃做助熔剂,制得的荧光粉晶相纯正,颗粒形貌均呈现为较规则的类球形,而且所得荧光粉的粒径均小于1 μm.发射光谱呈现为一宽带,发射主峰位于505 nm,该宽峰对应于Ce³⁺关键词： 白光LED 荧光粉 3Sc₂Si₃O₁₂ ∶Ce')" href="#">Ca₃Sc₂Si₃O₁₂ ∶Ce 发光     

119Sn and 57Fe M?ssbauer study of the local structure of perovskite-type ferrites CaFe2 ? x N x O5 ( N = Sc, Al) and manganite CaMn7O12

Anion-deficient substituted ferrites Ca₂Fe_{2 − x} N _x O₅ (N = Sc³⁺, Al³⁺) and mixed manganite CaMn₇O₁₂ have been investigated by ¹¹⁹Sn and ⁵⁷Fe probe M?ssbauer spectroscopy. The mechanism of charge compensation for heterovalent impurity Sn⁴⁺ ions in the structure of the ferrite Ca₂Fe₂O₅ has been established. The presence of nonequivalent crystallographic positions of manganese cations, caused by their charge ordering in the structure of the manganite CaMn₇O₁₂, is shown. Magnetic ordering of Mn³⁺ and Mn⁴⁺ cations in the octahedral sublattice of CaMn₇O₁₂ at T < T _M2 ≈ 90 K is established. Original Russian Text ? A.V. Sobolev, I.A. Presnyakov, K.V. Pokholok, V.S. Rusakov, T.V. Gubaidulina, A.V. Baranov, G. Demazeau, 2007, published in Izvestiya Rossiiskoi Akademii Nauk Seriya Fizicheskaya, 2007, Vol. 71, No. 9, pp. 1347–1354.     

Optical spectroscopy of Ca3Sc2Si3O12, Ca3Y2Si3O12 and Ca3Lu2Si3O12 doped with Pr

The silicates Ca₃Sc₂Si₃O₁₂, Ca₃Y₂Si₃O₁₂ and Ca₃Lu₂Si₃O₁₂, both undoped and doped with Pr³⁺ ions, have been synthesized by solid-state reaction at high temperature. The luminescence spectroscopy and the excited state dynamics of the materials have been studied upon VUV and X-ray excitation using synchrotron radiation. All doped samples have shown efficient 5d-4f emission upon direct VUV excitation of 5d levels, but only Ca₃Sc₂Si₃O₁₂:Pr³⁺ shows luminescence upon interband VUV or X-ray excitation. The VUV excited emission spectra of Ca₃Y₂Si₃O₁₂:Pr³⁺ and Ca₃Lu₂Si₃O₁₂:Pr³⁺ show features attributed to emission from two distinct sites accommodating the Pr³⁺ dopant. The decay kinetics of the Pr³⁺ 5d-4f emission in Ca₃Sc₂Si₃O₁₂:Pr³⁺ upon VUV excitation across the band gap are characterized by decay times in the range 25-28 ns with no significant rise after the excitation pulse. They appear to be faster upon X-ray irradiation than for VUV excitation. Weak afterglow components are attributed to defect luminescence.     

Quadratic crystal field tensors and spin Hamiltonian tensors of Fe3+ in Li2Ge7O15 above and below the phase transition temperature

Dopant Fe³⁺ ions in tetrahedral and octahedral positions of Ge⁴⁺ in the crystal Li₂Ge₇O₁₅ were studied using EPR. Fe³⁺ substitutes for Ge⁴⁺ with a local charge compensation. The octahedral site and the tetrahedral sites significantly differ by the value of the invariant sumS(B ₄) of theB ₄ tensor of the spin Hamiltonian of Fe³⁺. The irreducible tensor products {V ₄ ? V ₄}₄ and {V ₄?V ₄}₂ theV ₄ tensor of the crystal field calculated using the point-charge model for octahedral and tetrahedral complexes provide the predominant contribution of the crystal field to theB ₄ andB ₂ tensors of the spin Hamiltonian of Fe³⁺, respectively. A comparison of the fourth-rank tensorsB ₄ of the spin Hamiltonian and {V ₄?V ₄}₄ of the crystal field determined at 300 K with those determined at 77 K supports the conclusion that the phase transition is accompanied by combined rotation of the [GeO₄] tetrahedra with the [Ge(1)O₆] octahedron almost unaltered. The spectrum lines are narrow and the variety of point defects in the vicinity of the paramagnetic impurity ions Fe³⁺, Cr³⁺ and Cu²⁺ is not detected. These facts are inconsistent with the statistically distributed model for the Li(2) atom. In specific cases at 300 K, when the wings of the two spectrum lines of theM→M+1 and theM+2 →M+3 transitions of Fe³⁺ ions belonging to one system of translationally equivalent positions overlap an extra line appears in the center between these lines. It is suggested that this effect is due to the soft phonon mode above the phase transition temperature.     

Intensities of hypersensitive transitions in garnet crystals doped with Er<Superscript>3+</Superscript> ions

We examine the oscillator strengths and the intensity parameters Ω _t (t = 2, 4, 6) of yttrium-aluminum, scandium-containing, and gallium garnet crystals doped with Er³⁺ ions. A comparative analysis of the oscillator strengths and the intensity parameters Ω _t (t = 2, 4, 6) of garnets with different contents of Al³⁺ and Sc³⁺ ions (Gd_2.4Er_0.5Sc_1.8Al_3.3O₁₂, Gd_2.4Er_0.5Sc_1.9Al_3.2O₁₂, Gd_2.4Er_0.5Sc_2.0Al_3.1O₁₂) is performed, as a result of which the oscillator strengths and the intensity parameters Ω _t (t = 2, 4, 6) of these crystals are shown to have close values. We find that Ca₃(NbGa)₅O₁₂ crystals doped with Er³⁺ ions are characterized by highest values of the oscillator strengths for hypersensitive transitions and of the intensity parameter Ω₂ of Er³⁺ ions compared to the values of these quantities in the examined garnet crystals, which is determined by the fact that the symmetry of the local environment of Er³⁺ ions in these crystals is C ₁, C ₂, or C _2ν. We reveal that, as the concentration of Er³⁺ ions in these crystals increases from 1 to 39 at %, both the oscillator strength of the hypersensitive transition ⁴ I _15/2 → ² H _11/2 of Er³⁺ ions and their intensity parameter Ω₂ tend to decrease, which can be related to an increase in the relative fraction of Er³⁺ ions with higher symmetry of the local environment.     

Superposition model analysis of the magnetocrystalline anisotropy of Ba-ferrite

Theoretical analysis of the first magnetocrystalline anisotropy constantK ₁ of BaFe₁₂O₁₉ is performed. Two contributions toK ₁ are considered — single ion anisotropy and dipolar anisotropy. ParameterD which determines the magnitude of the single ion contribution is calculated on the basis of the superposition model. It is argued that the disagreement between calculated and observed values ofK ₁ is most likely connected with the contribution of Fe³⁺ ions on bipyramidal sites, for which the value ofD is uncertain.     

Mössbauer spectroscopy of a semiconductive phosphate glass (10V2O5 30Fe2O3 60P2O5 at low temperature

In order to clarify the difference between the local structures of Fe³⁺ and those of Fe²⁺ ions in a semiconductive phosphate (10V₂O₅-30Fe₂O₃-60P₂O₅) glass, the temperature dependencies of the isomer shift (IS), quadrupole splitting (QS) and absorption area (AR) for Fe³⁺ and Fe²⁺ ions were measured. Debye temperatures ( _D) for Fe³⁺ and Fe²⁺ ions were determined to be 318 ± 29 K and 223 ± 18 K, respectively, from the temperature dependence of the absorption area (AR). From the temperature dependence of the quadrupole splitting(QS), B strength parameters for Fe³⁺ and Fe²⁺ ions, which are the coefficients for theT ^3/2 term, were deduced. It was found that in this phosphate glass, Debye temperatures _D(AR) were consistent with those obtained using the relation of the B parameter to Debye temperature described in the literature. Also from the temperature dependence of the isomer shift, the difference between the thermal effects except the second-order Doppler shift for Fe³⁺ ions and those for Fe²⁺ ions was compared.     

Bulk magnetic properties of tetravalent ion substituted Cu-ferrite

The lattice parameter,a, increases with the increase of Ti⁴⁺ substitution while it decreases with the increase of Ge⁴⁺ substitution in Cu_1+x A _x Fe_2–2x O₄ compounds [A=Ti⁴⁺, Ge⁴⁺Sn⁴⁺]. On substitution of Sn⁴⁺,a does not vary. CuFe₂O₄ exhibits single domain (SD) behaviour that changes to multi domain (MD) type on Ti⁴⁺ substitution. On Ge⁴⁺ substitution initially all the compositions exhibit SD behaviour while forx>0.2 they show MD behaviour. All Sn⁴⁺ substituted compositions exhibit SD behaviour. The Curie temperature decreases on addition of Ti⁴⁺, Sn⁴⁺ and Ge⁴⁺. The magnetic momentn _B (77 K) initially increases and then decreases with the substitution of Ti⁴⁺, Ge⁴⁺, and Sn⁴⁺. This is explained by cation distribution and reduction of Fe³⁺ ions.The authors wish to thank Prof. R. N. Patil for encouragment and useful discussion. One of the authors (B.L.P.) is grateful to UGC (New Delhi) for granting Teacher Fellowship for Ph.D. course.     

Crystal structure and luminescence properties of Lu3+ and Mg2+ incorporated silicate garnet [Ca3−(x+0.06)LuxCe0.06](Sc2−yMgy)Si3O12

The yellow-emitting phosphor [Ca_3?(x+0.06)Lu_xCe_0.06](Sc_2?yMg_y)Si₃O₁₂ obtained from Lu³⁺ and Mg²⁺ co-modified green-emitting silicate garnet Ca₃Sc₂Si₃O₁₂:Ce³⁺ (CSS:Ce³⁺) exhibits promising applications for white LEDs. In this paper, we discuss the effect of charge balance on the garnet structure formation. The changes of bond length and covalence caused by the replacement of Lu³⁺ and Mg²⁺ for Ca²⁺ and Sc³⁺ are analyzed. The shift of the Ce³⁺ emission and excitation can be attributed to the combined results from crystal field splitting effect and centroid shift of Ce³⁺ 5d levels. Thermal stability is analyzed according to configurational coordinate diagram.     

Laser action in a cw diode-pumped Nd:Ca3Sc2Ge3O12 crystal

CW diode-pumped laser operation of Nd³⁺ in Ca₃Sc₂Ge₃O₁₂ (CaSGG) single garnet crystal has been demonstrated for the first time. We measured an 18% slope efficiency and 2.7-nm tunability with two different laser cavities. A spectroscopic characterization of our sample has also been performed including the absorption cross section in the 800-nm region and the emission cross section and radiative lifetime of the upper laser level. Received: 7 December 1999 / Revised version: 6 February 2000 / Published online: 27 April 2000     

Spectra of europium-doped yttrium oxide and yttrium vanadate phosphors

The spectral distributions of the visible absorption and fluorescence emission under electron beam excitation of Eu³⁺-doped (Y₂O₃) and (YVO₄) powders have been detected and analyzed. (Y₂O₃: Eu³⁺) has a cubicC crystal structure with a unit cell dimension a=10·61 Å. Its observed transitions from⁷ F ₀ to many upper states have been recognized; the observed number of Stark components is in agreement with that based on theC ₂ site symmetry of the Eu³⁺ ion in Y₂O₃. Eu³⁺-doped yttrium vanadate has a typical zircon tetragonal crystal structure with unit cell dimensions ofc=6·29 Å anda=7·11 Å. The observed transitions in (Eu³⁺: YVO₄) have been identified and assigned in accordance with theD _2d site symmetry of the Eu³⁺ ion in this lattice.The authors would like to express their deep gratitude to Professor G. F. J.Garlick, University of Hull, England, for offering experimental facilities in his Physics Department.     

57Fe Mössbauer spectroscopy on disordered crystalline media with Ca-gallogermanate type structure. II. Below the magnetic ordering temperature

Magnetic phase transitions in disordered crystalline iron germanates of the type Sr₂LnFe₃Ge₃O₁₄ (Ln=La; Nd) were observed at low temperatures by means of⁵⁷Fe Mössbauer spectroscopy. The hyperfine interaction of Fe³⁺ magnetic moments ordered in one octahedral and two tetrahedral magnetic sublattices is discussed. A local environment computation on the octahedral sites confirms the assumption that the wide distribution found for the 4.2 K octahedral hyperfine fields is due to the random occupation by Fe³⁺ and Ge⁴⁺ ions of the nearest cation environment of the octahedral iron. The effect of substituting Ln rare earth ions in the Thompson cubes on the magnetic behaviour of these compounds is pointed out.     

X-ray,electrical conductivity,magnetic and infrared studies of the system Co2−x Ge1−x Fe2x O4

X-ray, electrical conductivity, magnetic hysteresis and IR studies for the system Co_2−x Ge_1−x Fe_2x O₄ were carried out. All the compounds, 0⩽x⩽1, showed cubic symmetry. X-ray intensity calculations, magnetic hysteresis measurements and IR studies indicated the presence of Ge⁴⁺ at tetrahedral, Co²⁺ at octahedral and Fe³⁺ at both the sites. The activation energy and threshold frequency decreased with increasing value ofx. The compounds withx⩽0.5 arep-type and those withx⩾0.75 aren-type semiconductors. Magnetic hysteresis indicated that all the compounds are ferrimagnetic except forx=0 which is antiferromagnetic. The shapes of χ/χ _i vsT plots, highH _c values andJ _R/J_s ratios showed that all the compounds exceptx=0 exhibit single-domain behaviour. Curie temperature,T _c increased with increasing Fe³⁺ ions. The probable ionic configuration for the system is suggested as Ge _1−x ⁴⁺ Fe _x ³⁺ [co _2−x ²⁺ Fe _x ³⁺ ]O ₄ ²⁻ .     

A phototropic center and distribution of the chromium impurity in rare-earth garnet crystals

Crystals of two types, namely, Gd₃Sc₂Al₃O₁₂ doped by Ca²⁺ and Cr³⁺ ions and Gd₃Sc₂Ga₃O₁₂ doped by Nd³⁺ and Cr³⁺, are studied using x-ray diffraction. In order to elucidate how an impurity is embedded in the lattice, its effect on the electron density distribution and microstrains of the crystal lattice is investigated. It is shown that chromium ions can occupy the lattice sites located predominantly in the octahedral environment of the oxygen ligands.     

Radiation-Induced Paramagnetic Centers in the Glasses Of CaO-Ga2O3-GeO2 System

The X-band (v ? 9.4 GHz) ESR spectra of the UV-, X-, and γ-irradiated glasses of CaO-Ga₂O₃-GeO₂ system with compositions similar to Ca₃Ga₂Ge₃O₁₂, Ca₃Ga₂Ge₄O₁₄, and Ca₃Ga₂O₆ crystals have been investigated at 300 and 77 K. It was shown that X- and γ-irradiation of the Ge-contained glasses induce simultaneously electron and hole paramagnetic centers, stable at room temperature. The UV-irradiation of Ge-contained glasses leads to the generation of stable centers of electron type, whereas the X- and γ-irradiation of glasses with Ca₃Ga₂O₆ composition induces stable hole centers, exclusively. The electron centers are assigned to E′ (Ge) centers with different local environments. ESR spectrum of hole centers is ascribed to O^? centers, localized on non-bridging oxygen of the glass network. The E′ (Ge) and O^? centers show high thermal stability in the CaO-Ga₂O₃-GeO₂ glass network. The obtained ESR spectra are described by spin Hamiltonian with g-factor of axial and rhombic symmetry for the E′ (Ge) and O^? centers, respectively. The nature, electron structure and some formation peculiarities of the radiation defects in CaO-Ga₂O₃-GeO₂ glasses are discussed in comparison with other glasses as well as Ca₃Ga₂Ge₃O₁₂ and Ca₃Ga₂Ge₄O₁₄ crystals.     

Luminescence spectroscopy of Eu in Ca3Sc2Si3O12

Ca₃Sc₂Si₃O₁₂ doped with 1 mol% Eu³⁺ and having a cubic garnet structure was prepared by a solid state reaction. The low temperature luminescence spectrum shows no measurable ⁵D₀→⁷F₀ band, in agreement with the location of the lanthanide dopant in a site of D₂ symmetry, i.e. with a Ca²⁺ substitution. On the other hand, the spectrum is clearly dominated by the ⁵D₀→⁷F₄ band, which is significantly stronger than that for the other transitions originating from the ⁵D₀ level. This unusual behavior is explained on the basis of a model describing the distortion of the EuO₈ coordination polyhedron from a cubic geometry to the actual D₂ one.     

EPR study of the impurity paramagnetic centres in (CaO−Ga2O3−GeO2) glasses

The X-band EPR spectra of Cr³⁺, Mn²⁺, and Fe³⁺ impurity ions in glasses of (CaO?Ga₂O₃?GeO₂) system are investigated in the 77÷300 K temperature range. The experimental data analysis yields the following results: (i) Impurity chromium ions are incorporated into the (CaO?Ga₂O₃?GeO₂) glasses network in Cr³⁺ (3d³,⁴F_3/2) paramagnetic valence state only and occupy the strong distorted oxygen coordinated octahedral sites. (ii) For all activated and non-activated (CaO?Ga₂O₃?GeO₂) glasses the iron impurity is present at concentration roughly 0.01 wt.%. Isotropic EPR signals atg _eff=4.29 andg _eff=2.00 are assigned to Fe³⁺ (3d⁵,⁶S_5/2) ions in the sites with strong rhombic distortion and in the sites with nearly cubic symmetry respectively. (iii) The manganese EPR spectrum in (CaO?Ga₂O₃?GeO₂) glasses is weakly dependent on temperature, doping procedure as well as manganese concentration. EPR spectra of impurity manganese ions in glasses with Ca₃Ga₂Ge₃O₁₂ and Ca₃Ga₂Ge₄O₁₄ compositions are virtually identical and belong to Mn²⁺ (3d⁵,⁶S_5/2) ions. Impurity manganese ions are incorporated into the (CaO?Ga₂O₃?GeO₂) glass network as isolated Mn²⁺ centres and clusters of Mn²⁺ ions.