Valence State Of Manganese Ions Research Articles

Exploring the low-temperature polarization and the multifield-manipulated dielectric tunability behavior of NiMn2O4

Manganese oxides with spinel structure could exhibit abundant physical effects due to the complex sublattice structure, variable valence states and the intricate roles of manganese cations in magnetic interaction. In this work, NiMn2O4 ceramic with space group of Fd3‾m is synthesized. The low-temperature polarization behavior is investigated accompanied by revealing the multi-stage dielectric relaxation. Moreover, the relaxation mechanism is confirmed through microstructure characterization, multiple-condition pyroelectric measurement and the dielectric tunability properties under electric/magnetic fields. With increasing measuring temperature, different relaxation phenomena are respectively induced by lattice distortion because of the competition among magnetic coupling interactions (below ∼135 K), electron hopping relating to the coexistence of different valence states of manganese ions (∼135–230 K), and the synergistic contribution of oxygen vacancies and domain wall effect (above ∼230 K). These results provide additional evidence for comprehensive understanding the low-temperature polarization in NiMn2O4, guiding the method design for exploring the physical origin within a wide temperature range.

Regularities of Manganese Charge State Formation and Luminescent Properties of Mn-Doped Al2O3, YAlO3, and Y3Al5O12 Single Crystalline Films

In this work, three sets of single crystalline films (SCF) of Al2O3:Mn sapphire, YAlO3:Mn perovskite (YAP:Mn), and Y3Al5O12:Mn garnet (YAG:Mn), with a nominal Mn content of 0.1%, 1%, and 10 atomic percent (at.%) in the melt-solutions, were crystallized by the liquid phase epitaxy (LPE) method onto sapphire, YAP and YAG substrates, respectively. We have also calculated the average segregation coefficient of Mn ions for Al2O3:Mn, YAP:Mn and YAG:Mn SCFs with Mn content in the melt-solution in the 0.1–10% concentration range, which was equal to 0.1, 0.14 and 0.2, respectively. The main goal of the conducted research was the spectroscopic determination of the preferable valence states of manganese ions which were realized in the SCFs of sapphire, perovskite and garnet depending on the Mn content. For this purpose, the absorption, cathodoluminescence (CL), photoluminescence (PL) emission/excitation spectra and PL decay kinetics of Al2O3:Mn, YAP:Mn and YAG:Mn SCFs with different Mn concentrations were studied. Based on the CL and PL spectra, we showed that Mn ions, depending on the Mn content in the melt-solution, are incorporated in Al2O3:Mn, YAP:Mn and YAG:Mn SCFs in the different charged states and are located in the different crystallographic positions of the mentioned oxide lattices. We have observed the presence of the luminescence of Mn4+, Mn3+ and Mn2+ valence states of manganese ions in CL spectra in all SCFs under study with 0.1 and 1% Mn concentrations. Namely, the Mn4+ ion valence state with the main sharp emission bands peaked at 642 and 672 nm, related to the 2E → 4A2 transitions, was found in the luminescence spectra of the all studied Al2O3:Mn SCFs. The luminescence of the Mn2+ valence state was found only in YAP:Mn and YAG:Mn SCFs, grown from melt solution with 1% Mn content, in the emission bands peaked at 525 and 560 nm, respectively, related to the 4T1 → 6A1 transitions. The PL and CL spectra of YAP:Mn and YAG:Mn SCFs with the Mn content in the 0.1–1% range show that the main valence state of manganese ions in these films is Mn3+ with the main emission bands peaking at 655 and 608 nm, respectively, related to the 1T2 → 5E transitions. Meanwhile, higher than 1% Mn content in the melt solution causes a strong concentration quenching of luminescence of all Mn centers in Al2O3:Mn, YAP:Mn and YAG:Mn SCFs.

Magnetic transitions and different valence states of manganese ions in solid solutions (Ni,Cu,Mn)-=SUB=-3-=/SUB=-BO-=SUB=-5-=/SUB=-

The content of metal ions and their valence state for three NixCuyMn3-x-yBO5 compositions were estimated by EXAFS and XANES spectroscopy methods. In all three compositions, the copper y content did not exceed 0.25; in the third composition, some manganese ions entered the compound not only in the trivalent, but also in the divalent state. A detailed study of the magnetic properties showed that two magnetic transitions are observed in all compositions: one in the region of 60-75 K, and the second in the region of 10 K. Based on the exchange interactions calculated within the framework of the empirical model, the magnetic ordering temperatures for two- and three-lattice magnets were determined and it was assumed that the magnetic transition in the region of 60-75 K is associated with the ordering of magnetic moments in positions 2 and 3. Keywords: ludwigites, magnetic phase transition, indirect exchange interactions.

White emitting 5ZnO-(5–2.5x)Ga2O3- 2.5xAl2O3–90SiO2 glass ceramic embedded with Mn doped Zn(Ga,Al)2O4 nanocrystals

Dual-emitting manganese ions singly doped 5ZnO-(5–2.5x)Ga 2 O 3 -2.5 xAl 2 O 3 –90SiO 2 glass-ceramic (GCs) samples, containing the alloyed Zn(Ga, Al) 2 O 4 nanocrystals with a size of about 5 nm, were successfully prepared by a sol-gel method under different heat treatment temperatures, demonstrating an excellent white light emission. We found that the formation of the ZnGa 2 O 4 nanocrystals began at 650 °C (absence of Al 2 O 3 , x = 0) while that of the ZnAl 2 O 4 crystals required a higher temperature of about 900 °C (presence of Al 2 O 3 , x = 2) in SiO 2 glasses. A nanostructured Zn(Ga, Al) 2 O 4 :Mn 2+ , Mn 4+ in the glass-ceramic was synthesized by Al 3+ replacing part of Ga 3+ . The doping of Al 3+ has effectively contributed to the existence of Mn 2+ , thus realizing the regulation of the valence state of manganese ions. The glass ceramic samples showed red emission arrived from the 2E g → 4A 2g transitions of Mn 4+ , green emission originated from the 4T 1 →6A 1 transition of Mn 2+ , and blue emission derived from the self-activation center of ZnGa 2 O 4 nanocrystalline. In particular, the 0.1%Mn doped the 5ZnO-(5–2.5x)Ga 2 O 3 -2.5xAl 2 O 3 –90SiO 2 glass-ceramic samples can provide white color emission depending on the Al doping concentration (x), x = 0.4–0.9 and the annealing temperature, 650–900 °C.

Unraveling the valence states of manganese ions and the effects of composition variation and post-processing in YGG1-x-LuGGx:Mn garnet optical sensor

Transition metal manganese ions possess multiple valence states, and the change of local coordination environments would seriously influence their energy-level schemes and luminescence properties. Herein, YGG1-x-LuGGx:Mn garnet phosphors were proposed, the valence states of manganese ions and the effects of composition variation and post-processing were investigated systematically. Based on the temperature-dependent photoluminescence (PL) spectral analysis and XPS survey, the existing manganese ions are verified as Mn3+ and Mn4+. And both co-doping of Zr4+ ions and reduction processing can enhance and weaken the emissions of Mn3+ and Mn4+, respectively. Besides, introducing lutetium ions causes a PL blueshift of the two kinds of manganese ions and raises the resistance to thermally induced fluorescence quenching of Mn3+. Moreover, as for LuGG:Mn, the reduction treatment gives rise to a new garnet phase with a larger lattice parameter than that of the LuGG phase because of the existence of the LuGa antisite-defects. Finally, the combination of Mn3+ and Mn4+ ions demonstrates outstanding temperature sensing performance, further verifying the application potential of YGG1-x-LuGGx:Mn phosphors in optical thermometry.

ZnGa2–yAlyO4:Mn2+,Mn4+ Thermochromic Phosphors: Valence State Control and Optical Temperature Sensing

Dual-emitting and thermochromic manganese ion single doped ZnGa2-yAlyO4 phosphors were prepared by solid-state reaction. The regulation of the valence state and the luminescent properties, especially the luminescent thermal stability of manganese ions in ZnGa2-yAlyO4, are discussed in detail. When excited by ultraviolet (UV) light, the emission spectra of ZnGa2O4:Mn2+,Mn4+ present an ultranarrow green emission band at 503 nm with a fwhm of 22 nm, which derives from the Mn2+ ions formed by the self-reduction of doped Mn4+, and a red emission band of the Mn4+ ions at 669 nm. In addition, a ZnGa2-yAlyO4:Mn2+,Mn4+ solid solution was designed and synthesized by Al3+ replacing Ga3+. The doping of Al3+ effectively inhibited the degree of Mn4+ self-reduction to Mn2+, thus realizing the regulation of valence state of manganese ions. Interestingly, the thermal stability of luminescence shows that the response of Mn2+ and Mn4+ to temperature is obviously different in ZnGa2-yAlyO4, implying the potential of the prepared phosphors as optical thermometers. Subsequently, three kinds of optical thermometers with superior color discrimination and high relative sensitivity (Sr) based on the fluorescence intensity ratio (FIR) technique were realized in 100-475 K. The Sr value of ZGO:0.005Mn/ZGA0.5O:0.005Mn/ZGAO:0.005Mn phosphors can be as high as 4.345%/4.001%/3.488% K-1 (at 350/325/400 K), reflecting the great potential of ZnGa2O4:Mn2+,Mn4+ for optical thermometry applications.

Luminescence of Fluorochlorozirconate Glasses Doped with Manganese Ions

A series of fluorochlorozirconate glasses doped with manganese ions is synthesized and investigated as possible red‐emitting phosphors in comparison with conventional green‐emitting fluorozirconate glasses doped with Mn2+. It is found that the replacement of fluorine by chlorine ions results in broadening and a long‐wavelength shift of the manganese luminescence band into the red region (peaked at 615 nm) although the emission spectrum still has some contribution of the green sub‐band. It turns out also that decay kinetics of luminescence from fluorochlorozirconate glasses becomes nonexponential in contrast to single‐exponential decay observed for green luminescence from Mn2+‐doped fluorozirconate glasses. The observation of two emission components in the luminescence spectrum of fluorochlorozirconate glasses is interpreted as being due to the presence of two Mn2+ emission centers in different local environments. The presence of 2+ valence states of manganese ions in both types of glasses is confirmed by recording X‐ray absorption near edge structure (XANES) spectra at Mn L2,3 edge. Substitution of F− by Cl− ions in anion sites neighboring to Mn2+, leads to a significant rearrangement of the local environment of Mn2+ ions, which results in the long‐wavelength shift of Mn2+ emission spectrum.

A remarkably tunable emission from red to yellow to green in Mn4+-activated CaAl12O19 phosphor via co-doping Bi3+

In this study, Bi3+ and Mn4+ co-doped CaAl12O19 phosphors were prepared via high-temperature solid-state reaction method, and their crystal structure, particle morphologies, luminescence properties, charge compensation mechanism and crystal field effect were investigated in detail. A most remarkable tuning effect can be achieved when the Bi3+ concentration is 5% in CaAl12O19:Bi3+, 0.05% Mn4+ and the electric multipolar interaction is inferred to be the main mechanism for concentration quenching effect. It is also found that, as charge compensators rather than sensitizers, Bi3+ ions can effectively tailor the emissions in a wide range through partial reducing Mn4+ to Mn2+. The charge compensation physics presumably involves the shift of Fermi energy level (EF) and variations of valence states of manganese ions after Bi3+ introduction. Moreover, the authors calculated the splitting energy (Δ) and Racah parameters (B and C) for Mn4+ in octahedral sites and Mn2+ in tetrahedral sites by combining excitation spectra with Tanabe-Sugano diagram. The experimental results will be of certain helpfulness in developing phosphors used in white LED.

Shape-controlled synthesis of MnOx–CeO2 oxides and their catalytic performance in NO oxidation

MnOx–CeO2 nanocubes and nanorods with the pure fluorite-type ceria crystalline structure have been synthesized, in order to investigate the effect of catalysts shape in NO oxidation. The results indicated that both of the nanomaterials growing from Ce(OH)3 nanorods nuclei, and higher hydrothermal temperature and alkali concentration facilitated the conversion of nanorods to nanocubes. During the synthesis process, manganese ions could incorporate into the ceria lattice without affecting its crystalline structure and growth mechanism. The morphology of this binary oxide determines NO oxidation rate, while the specific reaction rate on the reactive {100} planes was two times higher than that on the {111} planes, due to co-effect of the valence state of manganese ions, surface energy and the ratio of surface lattice oxygen species. The results could provide guidance for understanding of the importance of catalyst design in NO oxidation.

Local Structure of Core–Shell MnFe2O4+δ-Based Nanocrystals: Cation Distribution and Valence States of Manganese Ions

We investigate the local structure of nanoparticles based on a manganese ferrite core surrounded or not by a maghemite layer obtained after hydrothermal surface treatment. Results of X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) measurements are crossed with those of in-field Mossbauer spectroscopy and X-ray absorption spectroscopy (XANES/EXAFS) to study the valence state of Mn ions and the cation distribution at interstitial sites of the core–shell nanoparticle structure. Linear combination fitting of XANES data clearly indicates the existence of mixed valence states of Mn cations in the Mn ferrite phase. As a direct consequence, it induces nonequilibrium cation distributions in the nanoparticle core with the presence of a large amount of Mn cations at octahedral sites. The quantitative results of the inversion degree given by NPD, Mossbauer spectroscopy measurements, and EXAFS are in good accordance. It is also shown that both the proportions of each oxidation degree of Mn ions and...

Determining the valence state of manganese ions in complex oxides La1–x Ca x MnO3 (x = 0.5, 0.7, 0.85, and 0.9) based on Mn2p and Mn3s X-ray photoelectron spectra

The valence state of manganese ions in complex oxides La1–xCaxMnO3 (x = 0.5, 0.7, 0.85, and 0.9) is studied on the basis of Mn2p and Mn3s X-ray photoelectron spectra. It is shown that manganese ions have the valence of Mn3+ and Mn4+ in the studied compounds. The relative Mn3+/Mn4+ content is determined.

Phase Stability of Lithium Manganese Oxide Stored at High Temperatures

Capacity degradation of the LiMn2O4 cathode during high-temperature storage was investigated for various lithium contents. The capacity recovery rate was determined after the storage test, and the phases and valence state of manganese ions were investigated by X-ray diffraction and X-ray absorption fine structure. The storage characteristics depend on the state of charge (SOC); a significantly low recovery rate was found for the shallow SOC region of LiMn2O4. The phase changes determined after the deterioration were as follows. (i) The lattice parameters decrease with an increase in manganese valence, caused by the formation of defects at the 16d site due to manganese dissolution. (ii) A new phase forms as a second phase. This phase has higher manganese valence and a larger amount of manganese vacancies. The same type of phase was produced by a high-temperature treatment of the shallow SOC phase at 150°C, which indicates that the phase was stable at high temperatures. (iii) A lower manganese valence state was observed at the spinel surface, which is related to manganese dissolution. Deterioration in the shallow SOC region during high-temperature storage is related to the phase stability of the de-intercalated spinel phases.

Photo-induced degradation and thermal decomposition in ZnSnF6·6H2O:Mn4+ red-emitting phosphor

ZnSnF6·6H2O:Mn4+ red-emitting hexahydrate phosphor is synthesized by the chemical reaction method, and its unique structural and optical properties are investigated using X-ray diffraction measurement, photoluminescence (PL) analysis, PL excitation spectroscopy, and electron spin resonance (ESR) spectroscopy. The Mn4+-activated phosphor exhibits remarkable degradation in the PL intensity under Xe lamp exposure and also by the coherent laser beam irradiation with the following order of degradation: Ar+ (488nm)>He–Cd (325nm)>He–Ne laser (632.8nm). The mechanism of degradation is considered to be due to a change in the valence state of manganese ions from Mn4+ to Mn5+ by the photooxidation and/or disproportionation. The ESR measurement supports decreased Mn4+ spin density in the Xe-lamp exposed, degraded sample. The changes in the structural and PL properties are also observed after thermal annealing above ∼100°C and explained by the thermal-annealing-induced dehydration/decomposition of the host material.

Optical spectroscopy and degradation behavior of ZnGeF6·6H2O:Mn4+ red-emitting phosphor

We report on a method of synthesizing Mn4+-activated ZnGeF6·6H2O hexahydrate red phosphor by the chemical reaction and its photo-induced degradation behavior. The structural and optical properties of ZnGeF6·6H2O:Mn4+ are investigated using X-ray diffraction measurement, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, PL decay measurement, diffuse reflectance measurement, and electron spin resonance (ESR) spectroscopy. The present hexahydrate phosphor exhibits the sharp red emission lines typically observed in various Mn4+-activated phosphors. However, the phosphor shows remarkable PL intensity degradation under Xe lamp exposure for a few minutes or coherent laser beam irradiation at different wavelengths. Its degree is in the order of Ar+ (488nm)>He–Cd (325nm)>He–Ne laser (632.8nm). The photo-induced degradation mechanism is explained by a change in the valence state of manganese ions from Mn4+ to Mn5+ by the photooxidation (Mn4+→Mn5+) or disproportionation reaction (2Mn4+→Mn3++Mn5+). The ESR measurement confirms the decreased Mn4+ spin density in the sample exposed with Xe lamp after Xe lamp exposure for 5min. The photo-induced degradation is concluded to universally occur in AIIBIVF6·6H2O:Mn4+ hexahydrate phosphors.

Unique light-induced degradation in yellow-emitting K2SiF6:Mn2+ phosphor

Photo-induced luminescence intensity degradation in yellow-emitting K2SiF6:Mn2+ phosphor is studied using x-ray diffraction measurement, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, PL decay analysis, and electron spin resonance (ESR) measurement. The yellow-emitting K2SiF6:Mn2+ phosphor exhibits remarkable degradation in the PL intensity under Xe lamp exposure. Coherent laser irradiation also induces degradation and its degree is in the order of He–Cd (λ = 325 nm) > Ar+ (488 nm) > He–Ne laser (632.8 nm). The degradation mechanism is proposed to be due to change in the valence state of manganese ions from Mn2+ to Mn3+ by the photooxidation (Mn2+ → Mn3+) or disproportionation reaction (2Mn2+ → Mn+ + Mn3+). The ESR measurement confirms the decreased Mn2+ spin density in the sample exposed with Xe lamp. The PLE spectrum suggests that the excitation of Mn3+ ions occurs through energy transfer upon absorption of exciting radiation by the Mn2+ ions. Thermal annealing of the degraded samples at ≥200 °C causes a blueshift in the PL emission band with an appearance of the Mn4+-related sharp red emission lines.

Luminescent properties of Mn-doped Y3Al5O12 single crystalline films

The absorption, cathodoluminescence and photoluminescence (PL) spectra as well as PL decay kinetics of Mn doped Y3Al5O12 (YAG:Mn) single crystalline films (SCF) with manganese concentration in the 0.002–0.2at.% range, grown by the LPE method from PbO to B2O3 based flux onto YAG substrates, were analyzed in this work. The special goal was spectroscopic determination of valence states of manganese ions which are realized in these SCF depending on Mn content. In SCF with Mn content (0.002–0.02at.%), the incorporation of Mn4+ and Mn2+ ions was found. The absorption and emission spectra of YAG:Mn SCF at higher (0.02–0.2at.%) content confirmed that the main valence state of manganese ions is Mn3+ state.

Light-Induced Degradation in Red-Emitting ZnSiF6·6H2O:Mn4+ Hydrate Phosphor

Photo-induced degradation behaviors of the red-emitting ZnSiF6·6H2O:Mn4+ hydrate phosphor are studied using X-ray diffraction measurement, photoluminescence (PL) analysis, PL excitation spectroscopy, PL decay analysis, diffuse reflectance spectroscopy, and electron spin resonance (ESR) measurement. The ZnSiF6·6H2O:Mn4+ hydrate phosphor exhibits remarkable degradation in the PL intensity under Xe lamp exposure for a few minutes. Coherent laser beams also induce degradation and its degree is in the order of Ar+ (488 nm) > He−Cd (325 nm) > He−Ne laser (632.8 nm). The degradation mechanism is found to be due to change in the valence state of manganese ions from Mn4+ to Mn5+ by the photooxidation (Mn4+ → Mn5+) or disproportionation reaction (2Mn4+ → Mn3+ + Mn5+). This is supported by the facts that Mn4+ ion has larger absorbance at 488 nm than at 325 or 632.8 nm and that the Mn5+-related absorption structures appear in the diffuse reflectance spectra at wavelengths below ∼700 nm. The ESR measurement also confirms the decreased Mn4+ spin density in the sample exposed with Xe lamp. A comparative discussion is given on the PL properties of the two different hydrate phosphors, ZnSiF6·6H2O:Mn4+ and K2SnF6·H2O:Mn4+.

On the structural integrity and electrochemical activity of a 0.5Li2MnO3·0.5LiCoO2 cathode material for lithium-ion batteries

Structural changes in a 0.5Li2MnO3·0.5LiCoO2 cathode material were investigated by X-ray absorption spectroscopy. It is observed that both Li2MnO3 and LiCoO2 components of the material exist as separate domains, however, with some exchange of transition metal (TM) ions in their slab layers. A large irreversible capacity observed during activation of the material in the 1st cycle can be attributed to an irreversible oxygen release from Li2MnO3 domains during lithium extraction. The average valence state of manganese ions remains unchanged at 4+ during charge and discharge. In the absence of conventional redox processes, lithium extraction/reinsertion from/into Li2MnO3 domains occurs with the participation of oxygen anions in redox reactions and most likely involves the ion-exchange process. In contrast, lithium deintercalation/intercalation from/into LiCoO2 domains occurs topotactically, involving a conventional Co3+/Co4+ redox reaction. The presence of Li2MnO3 domains and their unusual participation in electrochemical processes enable LiCoO2 domains of the material to sustain a higher cut-off voltage without undergoing irreversible structural changes.

Luminescent properties of YAlO3:Mn single crystalline films

The YAP:Mn single crystalline films (SCF) have been crystallized by liquid phase epitaxy (LPE) method onto YAP substrates. The cathode- (CL) and photo-luminescence (PL) spectra of the YAP:Mn SCF were analyzed for determination of the preferable valence states of manganese ions which are realized in these SCF depending on Mn content in the 0.01–0.81at.% range. The thermoluminescence (TL) properties of YAP:Mn SCF with the different Mn content above the RT range were also examined in comparison with the properties of YAP:Mn single crystal counterpart. We show that YAP:Mn (0.01at.%) SCF possesses effective TL properties both under α-particle and γ-quanta excitation with main TSL peaks at 130 and 195°C. We assume that the different valence states of Mn ions are responsible for their TL properties, e.g. both emission and trapping centers in YAP:Mn are formed mainly by the different valence states of Mn ions.

Electrical, dielectric and spectroscopic studies on MnO doped LiI–AgI–B2O3 glasses

LiI–AgI–B2O3 glasses doped with different concentrations of MnO (ranging from 0 to 0.8 mol%) were prepared. Electrical and dielectric properties have been studied over a wide frequency range of 10−2 – 106 Hz and in the temperature range from 173 to 523 K. The valence states of manganese ions and their coordination in the glass network have been investigated using optical absorption, luminescence, and ESR spectroscopy. The analysis of the spectroscopic results has indicated that the manganese ions exist in both Mn2+ and Mn3+ states and occupy octahedral and tetrahedral positions. With increasing MnO concentration there is a gradual increase in the tetrahedral occupancy of Mn2+ ions at the expense of octahedral occupancy in the glass network. The results of dc conductivity have indicated that when T > θD/2, the small polaron hopping model is appropriate and the conduction is adiabatic in the nature. Further, the analysis of experimental data indicates that there is a mixed, ionic and electronic, conduction. It has been observed that the electrical conductivity decreases as the concentration of MnO increases suggesting the electronic conduction controlled by polaron hopping between manganese ions. In the low temperature region, up to 250 K, the ac conductivity is nearly temperature independent and varies linearly with frequency, which can be explained by the quantum mechanical tunneling (QMT) model. The dielectric properties have been analyzed in the framework of complex dielectric permittivity and complex electrical modulus formalisms. The evolution of the complex permittivity as a function of frequency and temperature has been investigated.