Semiconduction and cation valencies in manganese ferrites

doi:10.1016/0022-3697(64)90165-9

Journal of Physics and Chemistry of Solids

Volume 25, Issue 1, January 1964, Pages 95-103

https://doi.org/10.1016/0022-3697(64)90165-9 Get rights and content

Abstract

The semiconducting properties of the mixed crystals Fe₃O₄MnFe₂O₄Mn₂FeO₄ are investigated by measurements of the electrical conductivities and the Seebeck voltages on single crystals and polycrystalline materials. The experimental data are interpreted by assuming that the conductivity arises exclusively from ferric and ferrous ions on octahedral sites according to the Verwey hopping mechanism and that the reaction Mn²⁺ + Fe³⁺ → Mn³⁺ + Fe²⁺ is endotherm, involving an energy of approximately 0.30 eV.

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This study examines the role of Gd³⁺ ions in a novel spinel ferrite with a general composition of Mn_0.9Zn_0.1Ni_0.05Ti_0.05Gd_xFe_1.9−xO₄; 0.0 ≤ x ≤ 0.04, to determine the critical concentration of Gd³⁺ ions that at which the structural and dielectric properties of the ferrite show an improvement. The structure of the samples, which were obtained using the traditional ceramic method, was tested by comparing the resulting X-ray diffraction patterns with the peaks of ICDD card No. 74–2402. Scanning electron microscopy (SEM) micrographs were used to evaluate the surface morphology of the samples. The substitution of Gd³⁺ions resulted in an increase in the bulk density of the prepared samples. In the frequency range of 0.1–5 MHz, the temperature and frequency dependences of AC electrical resistivity and electrical permittivity were investigated and discussed. Maxwell-Wagner and Koops models were used to describe the frequency dependence of dielectric characteristics. Furthermore, the activation energy, AC resistivity, dielectric loss tangent, and electrical permittivity were investigated in terms of their compositional dependence. The obtained results showed also that the magnetization was found decreases as the concentration of Gd³⁺ ions increases. The final results showed that increasing the Gd concentration enhanced the physical and dielectric properties of the samples, with an increase in AC resistivity and a 68% reduction in dielectric loss. The Final results also indicated that the sample with x = 0.04 displayed the highest AC resistivity and lowest dielectric loss, which can be exploited in certain technical applications such as transformer cores.
Effects of sintering temperature on microstructure, initial permeability and electric behaviour of Ni-Mn-Zn ferrites
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However, the probability of finding Fe3+ at B sites and Mn2+/Mn3+ ions at A sites is more [58,60]. When Mn2+/Mn3+ ions are introduced at the cost of Ni2+ ions, some of the Fe3+ ions move from A to B sites, which increases the concentration of Fe3+ ions at the B sites thereby increasing the ion hopping probability and hence the conductivity [61,62]. Moreover, the increase in grain size with increasing Mn substitution further contributes to their low resistivity.
The effects of sintering temperature on microstructure, initial permeability, and electrical behaviour of Ni_0.5-xMn_xZn_0.5Fe₂O₄ (0.0 < x < 0.5) prepared using the precursor combustion method have been investigated. X-ray diffraction analysis confirmed the single phase cubic spinel structure of Ni_0.5-xMn_xZn_0.5Fe₂O₄ (0.0 < x < 0.4) compositions sintered at 1100 °C. The Mn²⁺ substituent preferentially occupies tetrahedral sites as indicated by the Rietveld analysis and the increase in the IR absorption frequency of ν₁ band with increasing Mn substitution. The SEM measurements have revealed homogeneous microstructures of the ferrites with narrow grain size distribution in the range 0.93μm–1.86 μm. Mössbauer spectra exhibit a superposition of two Zeeman sextets for the compositions x ≤ 0.2, a superposition of two Zeeman sextets, and a paramagnetic doublet for the compositions, x = 0.3 and 0.4, and one sextet and single quadrupole doublet for a composition, x = 0.5. The activation energy in the range 0.258 eV–0.612 eV suggests a polaron hopping type conduction mechanism in all the sintered ferrites. The dielectric constant in the range 0.057–55.21 and lower dielectric loss (10⁻²) imply better stoichiometry and homogeneity. The initial permeability increases at lower Mn substitution (x = 0.2) and exhibits a decrease as the Mn substitution increases. The dc resistivity, dielectric constant, and initial permeability values suggest the suitability of these ferrites for high frequency device applications.
Highly efficient removal of tetracycline hydrochloride under neutral conditions by visible photo-Fenton process using novel MnFe<inf>2</inf>O<inf>4</inf>/diatomite composite
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Studies have shown that MnFe2O4, a spinel ferrite, has excellent ferromagnetism and chemical stability, broad light absorption range and high surface hydroxyl abundance [8,9]. The electron (e−) transport of MnFe2O4 is achieved by the valence transition of Mn and Fe ions via Verway hopping [10], that is, the e− transfer of the mixed-valence of Mn2+ and Fe3+ at the octahedral position. In other words, Mn and Fe ions are believed to exert synergistic effects on the MnFe2O4 catalytic system [11].
A simple solvothermal method was used to prepare poly-porous MnFe₂O₄/diatomite (MFD) with excellent catalytic performance. The characteristics of the prepared catalysts were examined by XRD, SEM, BET, XPS, UV-DRS, PL and NH₃-TPD. BET analysis indicated that introduction of diatomite could increase the specific surface and porosity of the MFD. NH₃-TPD studies clearly revealed that loading onto diatomite increased the availability of acid sites and surface hydroxyl groups in the composite materials. Under the combination of visible light and H₂O₂, the degradation effect of 80 mg/L 100 mL tetracycline hydrochloride (TC-HCl) reached 96.7% within 60 min at 0.1 g/L MFD and initial pH value, according to the experimental results. The proposed heterogeneous photo-Fenton system with MnFe₂O₄/diatomite showed an extremely wide pH range of 3–11 for TC-HCl degradation. At pH = 7, 91.8% of the TC-HCl (80 mg/L 100 mL) could be degraded within 60 min. In addition, the removal rate remained close to 90% after 60 min of irradiation at pH = 9 and 11. Free radical capture experiment showed that superoxide radicals (•O₂⁻) and holes (h⁺) play a crucial role in the removal of TC-HCl. Moreover, the synergistic activation of H₂O₂ by Fe and Mn was confirmed to promote the degradation, leading to higher TC-HCl removal efficiency.
PH-controlled MnFe<inf>2</inf>O<inf>4</inf>@ SnS<inf>2</inf> nanocomposites for the visible-light photo-Fenton degradation
2020, Materials Research Bulletin
Citation Excerpt :
MnFe2O4, is a well-known spinel type of crystal structure as a bimetal oxides nano-photocatalyst for the degradation of the organic synthetic dyes and pigments in wastewater [7–10]. The electrical transport of MnFe2O4 arises from Mn2++Fe3+↔ Mn3++Fe2+ through Verway hopping [11], which is electron transfer between a mixed valance Mn2+ and Fe3+ on octahedral sites to enhance the efficient catalytic activity [12]. It can be separated from the catalytic reaction mixture for repeated use in the degradation process that is attributed to its distinguished magnetic properties [13–15].
The magnetically separable MnFe₂O₄ nanoparticles (15–50 nm) are synthesized onto SnS₂ microflowers (diameter 1–1.5 μm, thickness 50 nm), fabricating a direct Z-scheme MnFe₂O₄@SnS₂ heterojunction nanocomposite via two-step hydrothermal. The MnFe₂O₄@SnS₂ composites with an appropriate SnS₂ content exhibit enhanced photo-Fenton degradation performances to make efficient separation of photoexcited electron/hole pairs. In the photo-Fenton degradation tests, MnFe₂O₄@SnS₂-2 shows excellent performance that 92.08 % of methylene blue (MB) could be photocatalyzed within120 min with pH = 9 and 1.5 mL H₂O₂, which is about 2.43 times than that of bare MnFe₂O₄ nanoparticles. In general, the Mn²⁺ and Fe²⁺ in-situ regenerated through accepting electrons from the cathode as the result of the Z-scheme electron transfer. Moreover, over 88 % of MB is still removed in the 5th run is attributed that SnS₂ driven also extend the longevity of the MnFe₂O₄ catalyst. Different capture agents IPA, BQ and TEOA investigation displays that ·OH is the main active specie.
Tuning the magnetic properties of a ferrimagnet
2019, Journal of Magnetism and Magnetic Materials
Magnetic properties of manganese ferrite doped with lanthanum and gadolinium having compositional formula MnRE_xFe_2−xO₄ (where RE = La³⁺, Gd³⁺ and x = 0.02, 0.04, 0.06, 0.08, 0.10) synthesized by co-precipitation method have been investigated. The compositional variation of the magnetic properties for each rare earth ion was studied individually first. Then, the variation of the magnetic properties with the substitution of the two different ion having different magnetic moments was compared. Saturation magnetization of non-magnetic ion (La³⁺) substituted MnFe₂O₄ was found to be superior to that of magnetic ion (Gd³⁺) substituted MnFe₂O₄. The observed result was explained in terms of Neel’s theory of ferrimagnetism, distribution of cations and magnetic moment of the associated cations. Law of approach to saturation (LAS) was adopted to investigate the dependency of magnetization on the applied field. A strong dependence of magnetic property on the distribution of cations which is specific to the particular rare earth ion was revealed in the present study.
Synthesis and functionalization of MnFe<inf>2</inf>O<inf>4</inf> nano−hollow spheres for novel optical and catalytic properties
2017, Surfaces and Interfaces
Citation Excerpt :
MnFe2O4 strikingly differs from other ferrites, for example CoFe2O4 and NiFe2O4 which are highly resistive in nature. The electrical transport of cubic spinel type MnFe2O4 arises due to electron transfer between Mn and Fe ions on octahedral sites (Mn+2+Fe+3↔Mn+3+Fe+2) through Verway hopping [5], leading to its efficient catalytic activity. Recently, nano−sized spinel hollow ferrites have gained enormous attention among diversified morphologies such as thin films, nanoflakes, nanowires because of their low density, high effective surface area, large pore volume, and enhanced coercivity [6] which raise their performance in hyperthermia, drug delivery, electromagnetic wave absorption, waste water treatment, and catalysis [7] compared to their solid counterpart.
Herein, we synthesize MnFe₂O₄ nano−hollow spheres (NHSs) of average diameter 100 nm through a facile template free solvothermal process and carry out a time dependent morphological study to investigate their process of core excavation. Further, a surface engineering of as−synthesized MnFe₂O₄ NHSs has been executed with organic disodium tartrate dihydrate ligand and interestingly, the surface modified MnFe₂O₄ NHSs are found to capable of emerging multicolor fluorescence starting from blue, green to red. The magnetic measurements through vibrating sample magnetometer demonstrate that room temperature superparamagnetic nature of MnFe₂O₄ NHSs remains unaltered after surface modification. Moreover, functionalized MnFe₂O₄ NHSs are found to exhibit excellent reusable photocatalytic efficiency in the degradation of cationic dye, methylene blue with rate constant of 2.64×10⁻²min.

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Semiconduction and cation valencies in manganese ferrites

Abstract

J. Phys. Chem. Solids

J. appl. Phys.

J. appl. Phys.

Phys. Rev.

Czechoslov. J. Phys.

Phys. Rev.

Phys. Rev.

J. chem. Phys.