Spinel-type solid solutions involving Mn and Ti:Crystal chemistry, magnetic and electrochemical properties

This study reports the structural and magnetic properties of spinel systems Li₄Mn_5−xTi_xO₁₂ (“4-5-12” series) and LiNi_0.5Mn_1.5−xTi_xO₄ (“LNMTO” series), both based on Mn⁴⁺ substitution by Ti⁴⁺. Intermediate compositions covering the whole range of compositions (0≤x≤5 and 0≤x≤1.5, respectively) were prepared by solid state reaction. The 4-5-12 system forms a continuous spinel solid solution, whereas the spinel phase range in LNMTO stops before the end member “LiNi_0.5Ti_1.5O₄”, which is multi-phased with a major hexagonal phase component. Cell parameters and (Mn,Ti)-O distances increase monotonically with titanium content in both series. In the LNMTO series, the end member LiNi_0.5Mn_1.5O₄ is known to form a superstructure with Ni/Mn cation ordering. Neutron diffraction and Raman spectroscopy show that this order is lost when Ti is substituted, even at low level (x=0.15). The LNMTO crystal chemistry is also complicated by the presence of partial cation inversion, and the presence of a secondary rocksalt-type phase that modifies the spinel stoichiometry. Magnetic properties are characterized by a competition between ferromagnetic and antiferromagnetic interactions; no magnetic ordering is achieved, in agreement with B-site cation frustration and disorder. Electrochemical measurements show that the Ti^3+/4+ and Mn^3+/4+ redox couples behave independently in the 4-5-12 series, and that titanium decreases the high-potential electrochemical redox activity of LNMTO because of its blocking character for electron transfer to and from the nickel sites in the spinel structure.     

Control of luminescence and conductivity of delafossite-type CuYO2 by substitution of rare earth cation (Eu, Tb) and/or Ca cation for Y cation

Delafossite-type oxides of CuTb_yY_1−yO₂, CuEu_yY_1−yO₂, CuCa_xTb_yY_1−x−yO₂ and CuCa_xEu_yY_1−x−yO₂ have been prepared by solid state reactions. The lattice-parameter dependence on the composition implies substitution of the Tb³⁺, Eu³⁺ and Ca²⁺ cations for the Y³⁺ site. Noticeable sharp emission lines due to the f-f transitions (⁵D₄→⁷F_J, J=3-6) of Tb³⁺ or due to the f-f transitions (⁵D₀→⁷F_J, J=0-4) of Eu³⁺ are observed at room temperature. Electrical conductivities of CuCa_xTb_yY_1−x−yO₂ and CuCa_xEu_yY_1−x−yO₂ are larger than those of CuTb_yY_1−yO₂ and CuEu_yY_1−yO₂, indicating the increase of the hole concentration caused by the substitution of Ca²⁺ for the Y³⁺ site. These results indicate the controllability of the luminescence and conductivity in CuCa_xTb_yY_1−x−yO₂ and CuCa_xEu_yY_1−x−yO₂ delafossite-type oxides by simultaneous substitution of the rare earth Tb³⁺ or Eu³⁺ cation and the Ca²⁺ cation for the Y³⁺ site.     

Reactivity and XAFS study on (1−x)CeO2-xNiO (x=0.025-0.3) system in the two-step water-splitting reaction for solar H2 production

The redox reaction of Ce⁴⁺-Ce³⁺ promoted by the catalytic function of nickel ions in a (1−x)CeO₂-xNiO solid solution was investigated for solar H₂ production by the two-step water-splitting reaction. By irradiation using an infrared imaging lamp as a solar simulator, the O₂-releasing reaction with (1−x)CeO₂-xNiO solid solution proceeded at 1673-1873 K, and its reduced form was produced. The amounts of H₂ gas evolved by the reduced form were 1.2-2.5 cm³/g and the evolved gases amounts ratio of H₂/O₂ was nearly 2, which is equal to the stoichiometric value of the water-splitting reaction (H₂O=H₂+1/2O₂). The maximum amounts of evolved H₂ and O₂ gases were obtained at the Ce:Ni mole ratio of 0.95:0.05 (x=0.05) in the (1−x)CeO₂-xNiO system. The X-ray absorption fine structure (XAFS) measurement showed that the O₂-releasing and H₂-generation reactions with (1−x)CeO₂-xNiO solid solution were repeatable with the redox system of Ce⁴⁺-Ce³⁺, which was enhanced by the catalytic function of Ni²⁺-Ni⁰.     

Electronic structure and magnetic properties of BaTi1-xMnxO3

The electronic structure and magnetic properties of polycrystalline BaTi_1-xMn_xO₃ (x = 0–0.1) compounds prepared by solid-state reactions were studied. The results revealed that the increase in Mn content (x) did not change the oxidation numbers of Ba (+2) and Ti (+4) in BaTi_1-xMn_xO₃. However, there is the change in Mn valence that Mn^3+,4+ ions coexist in the samples with x = 0.01–0.04 while Mn⁴⁺ ions are almost dominant in the samples with x = 0.06–0.1. We also point out that Mn³⁺ and Mn⁴⁺ ions substitute for Ti⁴⁺ and prefer locating in the tetragonal and hexagonal BaTiO₃ structures, respectively, in which the hexagonal phase constitutes soon as x = 0.01. Particularly, all the samples exhibit room-temperature ferromagnetism. Ferromagnetic order increases with increasing x from 0 to 0.02, but decreases as x ≥ 0.04. We think that ferromagnetism in BaTi_1-xMn_xO₃ is related to lattice defects and/or exchange interactions between Mn³⁺ and Mn⁴⁺ ions.     

Structure and magnetic properties of Ca1−x−yMgxCu2+yO3 influenced by isoelectronic substitution x

Phases of the composition Ca_1−x−yMg_xCu_2+yO₃ have been prepared for the first time. The compounds are isostructural with the known end-members CaCu₂O₃ and MgCu₂O₃ showing a two-leg spin-ladder-like connection of copper and oxygen atoms within the Cu₂O₃-layer. Opposite the spin ladders this layer is folded, which results in a long-range antiferromagnetic ordering of these phases. The Néel temperature can be adapted by variation of x in Ca_1−x−yMg_xCu_2+yO₃ between 24 and 80 K. Several structural features, which influence the magnetic ordering, are discussed.     

Effect of interface states associated with transitional nanophase precipitates in the enhancement of red emission from SrAl12O19:Pr by Ti incorporation

SrAl₁₂O₁₉:Pr³⁺, Ti⁴⁺ phosphor suitable for field emission displays is prepared by the wet chemical gel-carbonate method and the mechanism of enhancement in red photoluminescence (PL) intensity with Ti⁴⁺ therein has been investigated. The PL spectra of Pr³⁺ show both ¹D₂-³H₄ and ³P₀-³H₆ emission in the red region with very weak intensity when excited at 355 nm. The emission intensity has increased by about 100 times at room temperature in the compositional range SrAl_12−xTi_xO_19+x/2:Pr³⁺, with 0.1≤x≤0.3 in comparison to Ti-free SrAl₁₂O₁₉:Pr³⁺. TEM investigations show the presence of exsolved nanophase of SrAl₈Ti₃O₁₉, the precipitation of which is preceded by the presence of defect centers at the interfacial regions between the semicoherent transient phase and the parent SrAl₁₂O₁₉ matrix. The presence of transitional nanophase and the associated defects modify the excitation-emission process by way of formation of electronic sub-levels at lower energy (3.5 eV) than the band gap of SrAl₁₂O₁₉ (∼7 eV) followed by non-resonance energy transfer to Pr³⁺ level, leading to magnetic-dipole related red emission with enhanced intensity. The PL intensity of Pr³⁺ decreases at high Ti⁴⁺ concentrations (x>0.3) due to higher extent of segregation of non-emissive SrAl₈Ti₃O₁₉:Pr³⁺ phase.     

Blue and red long lasting phosphorescence (LLP) in β-Zn3(PO4)2:Mn,Zr

Multi-color long lasting phosphorescent (LLP) phenomenon in β-Zn₃(PO₄)₂:Mn²⁺,Zr⁴⁺ was systematically investigated. It is found that the red (λ_Em=616 nm) LLP performance of Mn²⁺ such as brightness and duration is largely improved, and that the blue (λ_Em=475 nm) LLP of Zr⁴⁺ with lower intensity appears when Zr⁴⁺ ions are co-doped into the matrix. The fluorescence, phosphorescence and thermoluminescence (TL) spectra show that Mn²⁺ ion is solely expected as a luminescent center, while Zr⁴⁺ ion not only acts as a luminescent center, but also induces an electron trap (Trap_Zr) associated with a TL peak at 344 K. The trap depth for Trap_Zr is 0.25 eV, while that for the intrinsic trap is 0.38 eV, associated with a dominant peak at 385 K for Zn₃(PO₄)₂:Mn²⁺. The Zr⁴⁺-induced trap with suitable depth is responsible for the improvement of the red LLP of Mn²⁺ ion and the appearance of the blue LLP of Zr⁴⁺ ion. The LLP mechanism is also investigated.     

Effects of nickel doping on structural and optical properties of spinel lithium manganate thin films

Spinel LiNi_xMn_2−xO₄ (x≤0.9) thin films were synthesized by a sol-gel method employing spin-coating. The Ni-doped films were found to maintain cubic structure at low x but to exhibit a phase transition to tetragonal structure for x≥0.6. Such cubic-tetragonal phase transition can be explained in terms of Ni³⁺(d⁷) ions with low-spin (t_2g⁶,e_g¹) configuration occupying the octahedral sites of the compound, thus being subject to the Jahn-Teller effect. By X-ray photoelectron spectroscopy both Ni³⁺ and Ni²⁺ ions were detected where Ni²⁺ is more populated than Ni³⁺. Optical properties of the LiNi_xMn_2−xO₄ films were investigated by spectroscopic ellipsometry in the visible-ultraviolet range. The measured dielectric function spectra mainly consist of broad absorption structures attributed to charge-transfer transitions, O²⁻(2p)→Mn⁴⁺(3d) for 1.9 (t_2g) and 2.8-3.0 eV (e_g) structures and O²⁻(2p)→Mn³⁺(3d) for 2.3 (t_2g) and 3.4-3.6 eV (e_g) structures. Also, sharp absorption structures were observed at about 1.6, 1.7, and 1.9 eV, interpreted as being due to d-d crystal-field transitions within the octahedral Mn³⁺ ion. In terms of these transitions, the evolution of the optical absorption spectrum of LiMn₂O₄ by Ni doping could be explained and the related electronic structure parameters were obtained.     

Effect of hole doping and antiferromagnetic coupling on the itinerant ferromagnetism of through Cu substitution at Ru site

Systematic studies on the structural, transport and magnetic properties of SrRu_1−xCu_xO₃ (x=0.0–0.2) compounds have been performed. SrRu_0.8Cu_0.2O₃ shows a tetragonal structure unlike the other compositions which exhibit a pseudo-cubic structure. Low temperature powder X-ray diffraction data of SrRu_0.8Cu_0.2O₃ collected at a synchrotron beam line reveals that the tetragonal structure is stable down to 8 K. Ferromagnetic transition temperatures (T_c) are significantly reduced from 160 to 34 K with Cu doping. All the compositions exhibit irreversibilities in M_ZFC(T) and M_FC(T) curves ascribable to the presence of domain structures. Magnetic susceptibility measurements show that the copper ions are anti-ferromagnetically coupled for concentrations higher than x=0.16. The antiparallel arrangement of Ru⁵⁺ ions with its neighboring cations also contributes to the large reduction in the observed magnetic moment. X-ray photoemission spectroscopy measurements show evidence for both tetravalent and pentavalent Ru ions while copper is in a divalent state. We conclude from our resistivity data that Cu²⁺ substitution promotes a polaronic type of conductivity.     

Temperature dependence of the luminescence of nanocrystalline CdS/Mn

The temperature dependence of the luminescence properties of nanocrystalline CdS/Mn²⁺ particles is investigated. In addition to an orange Mn²⁺ emission around 585 nm a red defect related emission around 700 nm is observed. The temperature quenching of both emissions is similar (T_q≈100 K). For the defect emission the reduction in the lifetime follows the temperature dependence of the intensity. For the Mn²⁺ emission however, the intensity decreases more rapidly than the lifetime with increasing temperature. To explain these observations a model is proposed in which the Mn²⁺ ions are excited via an intermediate state involving shallowly trapped (≈40 meV) charge carriers.     

Phase transformation behavior and dielectric characteristics of BaTi1−x(Al1/2Nb1/2)xO3 (0.01≤x≤0.40) ceramics

Microstructure, phase transformation behavior and dielectric properties of BaTi_1−x(Al_1/2Nb_1/2)_xO₃ (0.01≤x≤0.40) ceramics were investigated. A high level of (Al_1/2Nb_1/2)⁴⁺ substitution for Ti⁴⁺ ions was not conducive to the stability of the perovskite structure and resulted in the formation of BaAl₂O₄. As x was increased, lattice constants and unit cell volume decreased, reached a minimum at x=0.10 and then increased. The BaTi_1−x(Al_1/2Nb_1/2)_xO₃ ceramics at room temperature experienced a transformation from ferroelectric to paraelectric phase with increasing (Al_1/2Nb_1/2)⁴⁺ concentration. Meanwhile, permittivity of the BaTi_1−x(Al_1/2Nb_1/2)_xO₃ ceramics was markedly reduced, while Q value was slightly increased. Frequency dispersion of dielectric peak was obviously increased as x was increased from 0.01 to 0.10. It is of great interest that a dielectric abnormity represented by a broad dielectric peak at 200-400 K was observed for the composition with x=0.40.     

Low-temperature synthesis of nanocrystalline Ca1−xHoxMnO3−δ (0?x?0.3) powders

Nanocrystalline Ca_1−xHo_xMnO_3−δ (0?x?0.3) manganites were synthesized as phase-pure by a simple and instantaneous solution autogel combustion method, which is a low temperature initiated synthetic route to obtain fine grain size. All the samples, heated at 800 °C for 18 h, can be produced as phase-pure; the polycrystalline powders are homogeneous and possess ultrafine particle size. The holmium-doped calcium manganites retain the orthorhombic phase of the undoped sample. The scanning electron microscope (SEM) images revealed that the combustion-derived compounds exhibit particle size that decreases with holmium content from 300 to 80 nm. All manganites show two active IR vibrational modes near 400 and 600 cm⁻¹. The high temperature dependence of resistivity was measured using a standard four-probe method in the range 25-600 °C. All the samples exhibit semiconductor behaviour and holmium induces a marked decrease in the electrical resistivity when compared with the parent CaMnO₃. The results can be well attributed to the Mn⁴⁺/Mn³⁺ ratio and to the particle grain size.     

Phase relationships and magnetic phase diagram in the system Mn1−xCuxCr2S4

Ternary spinel compounds have been found in the system Mn_1?xCu_xCr₂S₄ for x < 0.4 and x ? 0.8. The unit cell parameter does not follow a Vegard law. Magnetization measurements up to 150 kOe for Mn_0.95Cu_0.05Cr₂S₄ show three magnetic structures: Néel ferrimagnet, Yafet-Kittel and tridimensional. The magnetic phase diagram has been calculated within the molecular field approximation. There is strong evidence for the non-existence of Mn³⁺. Our findings are in conformity with a model proposed by Goodenough.     

Superposition model (SPM) analysis of zero-field splitting parameters of Mn doped ZnAl2S4 crystal

In this study, superposition model (SPM) is employed to investigate the local environment around the different Mn²⁺ centers in ZnAl₂S₄ spinel. Using SPM and crystallographic data, the zero-field splitting (ZFS) parameters b₂⁰, b₄⁰, and b₄³ are calculated for Mn²⁺ at the B-site (with local symmetry D_3d), whereas b₄⁰ and b₄⁴ for the A-site (T_d). The lattice relaxation due to Mn²⁺ impurities is analyzed in terms of the bonding lengths and the angles between the Mn-S bond and the crystallographic axis [1 1 1]. Our SPM analysis of ZFS parameters indicates that satisfactory agreement can be achieved between the theoretical and the experimental results. Additional structural information about Mn²⁺ impurity centers is also obtained.     

Dielectric properties of lanthanum-doped CaCu3Ti4O12 synthesized by semi-wet route

Effect of La³⁺ doping at Ca²⁺ site in CaCu₃Ti₄O₁₂ has been examined. Compositions with x=0.10, 0.20 and 0.30 were synthesized in the system Ca_(1−3x/2)La_xCu₃Ti₄O₁₂ by semi-wet method. Powder X-ray diffraction confirmed the formation of monophasic compounds. The structure remains cubic similar to CaCu₃Ti₄O₁₂. Lattice parameter increases slightly with increasing La³⁺ concentration. Microstructure has been studied using scanning electron microscopy (SEM). Average grain size is in the range 2-4 μm for various compositions. Energy-dispersive spectrometer (EDS) studies confirm the stoichiometry of the synthesized materials. Dielectric constant, dielectric loss and conductivity of the samples decrease with increasing lanthanum concentrations.     

Chiral optical absorption bands in [ ]Cd1−x Mnx Ga2S4 semiconducting compound

[ ]Cd_1−x Mn_x Ga₂S₄ is a semimagnetic semiconductor and it has revealed an exceptional property namely ‘optical activity‘. Therefore, a spectroscopic investigation of chiral absorption bands has been carried out with the view to examine the role of d*-d states of manganese atoms. It has been found that inner transitions of Mn⁺⁺ dominate the spectral region with a special feature, indicating that these transitions show the presence of a substantial contribution from the magnetic dipole moment which rotates the electric vector of the incident polarized radiation. The origin is associated to the lack of a symmetry center caused by the ordered vacancies in this defect compound.     

CaCuMn<Subscript>6</Subscript>O<Subscript>12</Subscript> vs. CaCu<Subscript>2</Subscript>Mn<Subscript>5</Subscript>O<Subscript>12</Subscript>: A comparative study

The ferrimagnetic compounds Ca(Cu_xMn_3?x)Mn₄O₁₂ of the double distorted perovskites AC₃B₄O₁₂ family exhibit a rapid increase of the ferromagnetic component in magnetization at partial substitution of square coordinated (Mn³⁺)_C for (Cu²⁺)_C. In the transport properties, this is seen as a change of the semiconducting type of resistivity for the metallic one. The evolution of magnetic properties of Ca(Cu_xMn_3?x)Mn₄O₁₂ is driven by strong antiferromagnetic exchange interaction of (Cu²⁺)_C with (Mn³⁺/Mn⁴⁺)_B coordinated octahedra. The competing interactions of (Mn³⁺)_C with (Mn³⁺/Mn⁴⁺)_B lead to the formation of noncollinear magnetic structures that can be aligned by magnetic fields.     

The suppression of structural phase transformation in LaVO3 and La1−xSrxVO3 thin films fabricated by pulsed laser deposition

Highly oriented (100) thin films of LaVO₃ and La_1−xSr_xVO₃ have been fabricated by pulsed laser deposition in a reducing atmosphere. The films show a transition from insulating to metallic behaviour in the composition region of x, 0.175<x<0.200. In the single crystals of the antiferromagnetic insulating phase, a first-order structural phase transition is observed few degrees below the magnetic transition, which manifests itself as a kink in the temperature dependence of resistivity. In the highly oriented thin films of LaVO₃ and La_1−xSr_xVO₃ fabricated on lattice matched substrates in this study, the structural phase transformation in the insulating phase has been suppressed. The electrical conduction is found to take place via hopping through localized states at low temperatures. The metallic compositions show a non-linear (T^1.5) behaviour in the temperature dependence of resistivity. V (2p) core level spectra of these films show a gradual change in the relative intensities of V³⁺ and V⁴⁺ ions as the value of x increases.     

Electrochemical behavior of Li intercalation processes into a Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 cathode

Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (X=0.17, 0.25, 0.33, 0.5) compounds are prepared by a simple combustion method. The Rietvelt analysis shows that these compounds could be classified as having the α-NaFeO₂ structure. The initial charge-discharge and irreversible capacity increases with the decrease of x in Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂. Indeed, Li[Ni_0.50Mn_0.50]O₂ compound shows relatively low initial discharge capacity of 200 mAh/g and large capacity loss during cycling, with Li[Ni_0.17Li_0.22Mn_0.61]O₂ and Li[Ni_0.25Li_0.17Mn₀.₅₈]O₂ compounds exhibit high initial discharge capacity over 245 mAh/g and stable cycle performance in the voltage range of 4.8 -2.0 V. On the other hand, XANES analysis shows that the oxidation state of Ni ion reversibly changes between Ni²⁺ and about Ni³⁺, while the oxidation state of Mn ion sustains Mn⁴⁺ during charge-discharge process. This result does not agree with the previously reported ‘electrochemistry model’ of Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂, in which Ni ion changes between Ni²⁺ and NI⁴⁺. Based on these results, we modified oxidation-state change of Mn and Ni ion during charge-discharge process.     

Defect formation and transport in mixed-conducting La0.90Sr0.10Al0.85−xFexMg0.15O3−δ

The redox behavior of perovskite-type La_0.90Sr_0.10Al_0.85−xFe_xMg_0.15O_3−δ (x=0.20-0.40) mixed conductors was analyzed by the Mössbauer spectroscopy and measurements of the total conductivity and Seebeck coefficient in the oxygen partial pressure range from 10⁻²⁰ to 0.5 atm at 1023-1223 K. The results combined with oxygen-ion transference numbers determined by the faradaic efficiency technique in air, were used to calculate defect concentrations, mobilities, and partial ionic and p- and n-type electronic conductivities as a function of oxygen pressure. The redox and transport processes can be adequately described in terms of oxygen intercalation and iron disproportionation reactions, with the thermodynamic functions independent of defect concentrations. No essential delocalization of the electronic charge carriers was found. The oxygen non-stoichiometry values estimated from the conductivity vs. p(O₂) dependencies, coincide with those evaluated from the Mössbauer spectra.