The distribution of ions and their valencies in magnesium manganese ferrites

A model of the distribution of cations and valencies in non-stoichiometric magnesium manganese ferrites was proposed and its validity verified by measuring the saturation magnetic moments. The results of measuring the moment in stoichiometric ferrites to a certain extent confirms the correctness of the proposed model for this type of magnesium manganese ferrites, too. In a discussion of the distribution coefficient for manganese ions the close connection between this coefficient and the average valency of the manganese was pointed out. The question of the dependence of the distribution coefficient of magnesium ions on the composition is solved by introducing the maximum values of this coefficient and assuming that a simple proportionality holds for all compositions between the actual value of this coefficient and its maximum value.

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In conclusion the author thanks Dr. S. Krupika and K. Závta for valuable discussions.     

The magnetostriction contribution of manganese ions in spinel ferrites

The assumption that the magnetostriction contribution from manganese ions in spinel ferrites arises from Mn³⁺ ions has been tested by an ab initio calculation of the theoretical values of the coefficients λ₁₀₀ and λ₁₁₁. The Mn³⁺ contribution on octahedral sites is found to be negligible while the possible contribution from Mn³⁺ ions on tetrahedral sites cannot be fitted to the experimental results. It is possible that the contribution from a small concentration of Mn⁴⁺ ions on tetrahedral sites could give a magnetostriction contribution which fitted both λ₁₀₀ and λ₁₁₁.     

The influence of Mn3+ ion clustering on electrical properties of manganese ferrites

The effect of the clustering of Mn³⁺ ions in octahedral positions of the spinel lattice on the temperature dependences of the Seebeck voltage and the electrical conductivity was studied on three single crystals of manganese-rich manganese ferrites. It was found that the conception of Mn³⁺ ion clusters may account for the deviations from the values predicted by the phenomenological theory of the hopping mechanism of electrical conductivity in these materials.     

Conductivity and cation distribution in CuxMg1−xFe2O4 system

The electrical conductivity and magnetization of Cu _x Mg_1{x Fe₂O₄ are measured. The electrical conductivity obeys the exponential behaviour characteristic of semiconducting materials. The activation energy in the paramagnetic region is found to be greater than that in the ferrimagnetic region. This is attributed to the change in magnetic ordering. It is observed that the plots of logQ vs 10³/T exhibit a linear relationship throughout the temperature region studied (room temperature to 1000 K). The slope of the plot changes at a particular temperature recognized as Curie temperature. These temperatures agree well with the experimentally obtained Curie temperatures using Laroia technique. Hopping of polarons mechanism is used to explain the conduction behaviour in ferrites. The magnetic moment (n_B) is calculated from the magnetization studies carried out at 78 K. The cation distribution is suggested on the basis of these magnetization data.The authors are thankful to Prof. R. N. Patil for encouragement.     

Absorption spectrum of the Mn2+ ions in crystals of lanthanum manganese double nitrate and praseodymium manganese double nitrate

The optical absorption spectrum of the Mn²⁺ ions in the two double nitrates La₂Mn₃(NO₃)₁₂ ? 24H₂O and Pr₂Mn₃(NO₃)₁₂ ? 24H₂O were measured up to 30000 cm^?1. Uniaxial stress was applied to the crystals to determine the multiplicity of the observed zero phonon lines belonging to the transitions from the groundstate⁶ A ₁ (⁶ S) to the excited states⁴ E(⁴ G) and⁴ T ₂(⁴ D). The Zeeman effect of the states⁴ A ₁(⁴ G) and⁴ E(⁴ G) was measured with magnetic fields up to 6.4T. The effect of covalency and trigonal field, especially to the states⁴ A ₁ (⁴ G) and⁴ E(⁴ G), is briefly discussed.     

尖晶石铁氧体TixNi1-xFe2O4中阳离子分布和Ti离子磁矩的实验研究

采用固相反应法制备了系列样品Ti_xNi_1-xFe₂O₄ (x=0.0, 0.1, 0.2, 0.3, 0.4). 室温下的X射线衍射谱表明样品全部为(A)[B]₂O₄型单相立方尖晶石结构, 属于空间群Fd3m. 样品的晶格常数随Ti掺杂量的增加而增大. 样品在10 K温度下的比饱和磁化强度σ_S随着Ti掺杂量x的增加逐渐减小. 研究发现, 当Ti掺杂量x≥ 0.2时, 磁化强度σ随温度T的变化曲线出现两个转变温度T_L和T_N. 当温度低于T_N时, 磁化强度明显减小; 当温度达到T_N时, dσ/dT具有最大值. σ-T曲线的这些特征表明, 由于Ti掺杂在样品中出现了附加的反铁磁结构. 这说明样品中的Ti离子不是无磁性的+4价离子, 而是以+2和+3价态存在, 其离子磁矩的方向与Fe和Ni离子的磁矩方向相反. 利用本课题组提出的量子力学方势垒模型拟合样品在10 K温度下的磁矩, 得到了Ti, Fe和Ni三种阳离子在(A)位和[B]位的分布情况, 并发现在所有掺杂样品中, 80%的Ti离子以+2价态占据尖晶石结构的[B]位.     

Electrical properties and cation migration in manganese ferrites II. Time effects

A careful investigation of the time dependences of the electrical resistivity in two single crystals of MnFe₂O₄ at fixed temperatures in the 300–420 °C range showed that the changes are due to the migration of cations between the tetrahedral and the octahedral sites of the spinel lattice. The migration of cations can be described as a diffusion process resulting in the equilibrium distribution characteristic for a given temperature. Time constants involved in the diffusion strongly depend on temperature with the activation energyε=2.5 eV. The importance of a well defined cooling rate through the 600-400 °C temperature region is pointed out if ferrites with definite inversion degrees are needed.     

Cation distribution in NixMg1−xFe2O4 studied by XPS and Mössbauer spectroscopy

Ni–Mg ferrites, synthesized by solid-state route, were analyzed by X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy to find the distribution of Mg²⁺ in tetrahedral (A) and octahedral (B) sites of the spinel. In Ni_xMg_1−xFe₂O₄, Ni²⁺ enters in B site resulting in a reduction in the availability of B sites for the distribution of Mg²⁺ and Fe³⁺ and thereby influences the distribution of Mg²⁺. It was observed through both techniques that higher percentage of Mg²⁺ occupied the tetrahedral sites of Ni rich spinel systems. Thermodynamic considerations showed that entropy played significant role in the distribution of Mg²⁺ in Ni_xMg_1−xFe₂O₄, in addition the difference in site preference energy between Fe³⁺ and Mg²⁺ was too small to cause any influence.     

Absorption spectrum of Mn2+ ions doped in diammonium hexaaquamagnesium(II) sulfate

Optical absorption is a tool to investigate the site symmetry of metal ion and associated distortion in doped single crystals. It provides the energy of different orbital levels of metal ion and separation among them. Mn²⁺ ions in various single crystals and glasses have been the subject of recent investigations [1–3]. We have studied optical absorption spectra of Mn²⁺ doped diammonium hexaaquamagnesium(II) sulfate in order to obtain the energy level ordering using matrices of Tanabe and Sugano [4] and to discuss the associated distortion.     

Effect of Pb2+ ions on the photoluminescence characteristics of Mn2+-doped Zn2SiO4

Zn₂SiO₄ single-doped and co-doped with Mn²⁺ ions and Pb²⁺ ions has been prepared by a sol–gel process. The luminescent properties of the samples have been investigated. From the excitation and emission spectral analysis, it was found that the emission peaks vary with the change of the excitation wavelength. An enhanced luminescent characteristic of Mn²⁺ ions (in blue and green spectral zones) has been observed, due to the energy transfer from the Pb²⁺ ion to the Mn²⁺ ion. The emission peaks originate from the d–d transitions of the Mn²⁺ ion. However, the relative intensities of the peaks show a dependence on the concentration of the Pb²⁺ ion. PACS 81.05.Je; 78.55.Hx; 61.72.Ww     

Relaxation of solvent protons and deuterons by protein-bound Mn2+ ions. Theory and experiment for Mn2+-concanavalin A

Despite the demonstrated utility of measurements of the magnetic field dependence of the magnetic relaxation rates of solvent protons in solutions of metalloproteins as an indicator of biochemical changes, it is becoming increasingly evident that quantitative comparisons of such data with the theory of relaxation, limited by the approximations and assumptions usually made, yield results for the strength of the solvent-paramagnetic ion interaction that generally do not make chemical sense. These results, when expressed as the number of solvent-donated ligands of the ions, usually give too large a value, typically by about twofold. It has been suggested by several investigators that a comparison of proton and deuteron relaxation rates could resolve the problem. Data are presented for the longitudinal relaxation rates of solvent protons and deuterons over more than four decades of magnetic field (from 0.01 to 270 MHz proton Larmor frequency) for solutions of Mn²⁺-concanavalin A, a protein for which the physical biochemistry is thoroughly documented, one that should be particularly tractable for such comparisons. The main conclusion is that, in the general case, there is no decade of magnetic field over which the mathematical criterion of best agreement of data with theory can be relied upon to yield quantitatively correct biochemical results; rather, biochemistry must still be a guide for elucidating relaxation pathways.     

Effect of Ti4+ ions doping on microstructure and dc resistivity of nickel ferrites

Nanostructured nickel ferrites (NiFe₂O₄) were prepared by doping with Ti⁴⁺ ions using solid-state reaction route. Lowest grain size of 55 nm was achieved in the specimens with 20 mole% TiO₂ doping. Magnetization in the specimens decreases with decreasing grain sizes. Lower volume fractions of ferrite phase due to dissociation of the magnetic phase into smaller particles by the disruption of super exchange interaction by the titanium substitution results a decrease in magnetizations. Coercivity showed an increasing trend. This was explained as arising due to multidomain/monodomain magnetic behavior of magnetic nanoparticles. Small polaron hopping conduction between Fe²⁺ and Fe³⁺ sites controls the dc electrical properties of the specimens. The presence of an interfacial amorphous phase between the sites is evident from Mott's analysis. Specimens containing 10 mole or more TiO₂ and sintered at 1350 °C contain NiTiO₃ as a secondary phase and show unusual dc conductivity.     

Optical spectroscopy of Mn2+ ions in CaCl2 single crystals

Abstract

In the present investigation the excitation and fluorescence spectra and lifetimes of Mn²⁺ ions in calcium chloride, for various manganese concentrations and sample temperatures have been studied for the first time. The fluorescence spectrum consists of an asymmetric broad band, which upon lowering the sample temperature, shifts its maximum from 580 nm at 300 K to 596 nm at 11 K. This luminescence band was associated with the ⁴T_lg(⁴G)→⁶A_lg(⁶S) spin-forbidden transition in the manganese ions occupying Ca-sites in the lattice of CaCl₂. The excitation spectrum of the Mn²⁺ fluorescence revealed the features of manganese ions in octahedral coordination and consisted of nine excitation peaks which were associated with Mn²⁺-crystal-field-sensitive transitions. A crystal field analysis of the wavelength positions of these transitions by means of the model developed by Curie et al. allowed us to determine the magnitude of the cubic field splitting 10Dq, the reduced Racah parameter B', the Koide-Pryce covalency parameter ε and the spin transfer coefficients f _[sgrave] and f _σ. From the measurement of the temperature dependence of the Mn²⁺ fluorescence lifetime, we have also obtained information about the different mechanisms which are involved in the relaxation of excited Mn²⁺ ions in this host crystal in the temperature range (11–300 K).     

On the role of Fe2+ ions and cation vacancies in the disaccommodation of ferrites

A study was made of the disaccommodation spectrum of ferrites Mn _x Fe _3–x O _4+y , x>1, in a temperature range above 0°C. A comparison with analogous spectra of ferrites rich in iron (x<1), which contain Fe²⁺ ions in addition to cation vacancies, shows that the disaccommodation maximum around room temperature, observed in iron rich ferrites, is subject to the presence of Fe²⁺ ions. On the other hand, the processes taking place at higher temperatures seem to be substantially common to both types of ferrites and are dependent only on the presence of vacancies.     

掺杂Mn2+的浓度对CdS纳米颗粒光致发光的影响

采用反胶束法,合成了硅土包裹的掺有不同浓度的Mn2 的CdS纳米颗粒.高分辨电镜表明这些颗粒的直径小于5 nm.仅仅改变Mn2 的掺杂浓度,研究了这些颗粒的光致发光谱和光致发光激发谱,结果表明:Mn2 浓度的大小对掺杂CdS纳米颗粒的发光产生了重要的影响.通过电子顺磁共振谱的测量和分析揭露了Mn2 浓度影响这些掺杂颗粒发光效率的原因.