Local instability in BaF2:Mn2+: Analysis of lattice relaxation

Abstract

The distortion undergone by BaF₂ : Mn²⁺ below 50 K is examined here through the analysis of the experimental isotropic (A_s) and anisotropic (A_p) superhyperfne constants. This quantitative analysis support that the “short” Mn²⁺-F^? distance experiences a decrement of 8(4) pm with respect to the distance R= 232(2) existing in the O_h MnF₈ ^6- complex in BaF₂ at room temperature. By contrast the “long” Mn²⁺-F^? distance becomes 250(3) pm.     

Nonradiative energy transfer from Mn2+ to Eu3+ in K2CaP2O7:Mn2+,Eu3+ phosphor

A series of color tunable phosphors K₂Ca_1?x?yP₂O₇:xMn²⁺, yEu³⁺ are synthesized by solid state reaction method. The energy transfer phenomenon from Mn²⁺ to Eu³⁺ has been observed in the Mn²⁺/Eu³⁺ codoped non-magnetic K₂CaP₂O₇ host, which was confirmed by PL spectra and decay curves. The Mn²⁺→Eu³⁺ energy transfer is controlled by quadrupole–quadrupole interaction between sensitizer and activator. The maximum efficiency of energy transfer is estimated to be 33% with x=0.125 and y=0.03 in K₂Ca_1?x?yP₂O₇:xMn²⁺, yEu³⁺ phosphor. The phosphors can emit light from green to yellow and eventually to orange under 400 nm excitation by changing the Mn²⁺/Eu³⁺ content ratio, indicating that K₂CaP₂O₇: Mn²⁺, Eu³⁺ would be potential candidates for use in lighting and displays applications.     

Oxygen vacancy-transition metal-ion impurity association in SrTiO3

The progressive reduction of SrTiO₃:Mn has been monitored by EPR. The results show the conversion of Mn⁴⁺ to Mn²⁺ and Mn²⁺ to Mn²⁺-V₀ by oxygen-vacancy capture. The latter defect association takes place with the copious oxygen vacancies produced during high-temperature reduction between 1100 - 1300 K. At still higher reduction temperatures, other defects are formed. Analysis of the temperature dependence of the defect association gives a value of 0.7 eV for the enthalpy of association.     

EPR study of Fe3+ and Mn2+ in CeO2 and ThO2

Attempts were made to grow CeO₂ and ThO₂ single crystals doped with transition metal ions. Only Fe³⁺ and Mn²⁺ could be detected by the EPR technique. The EPR spectrum of Fe³⁺ in CeO₂ exhibits the well-known fine structure in cubic fields. The parameters areg=2.0044(1) anda=15.6(1)·10^?4 cm^?1. The hyperfine constantA for⁵⁷Fe in hexahedral coordination was found to be 8.9(1)·10^?4 cm^?1. The EPR spectrum of Mn²⁺ in CeO₂ reveals two cubic Mn²⁺ centers. The parameters for center 1 areg=1.9999(1) andA=86.9(1)·10^?4 cm^?1 and for center 2g=1.9984(1) andA=87.0(1)·10^?4 cm^?1. Heating the Mn doped CeO₂ samples in hydrogen, the Mn²⁺ centers transform from cubic into trigonal centers with approximate values ofg=1.9988(2),A=84.5(6)·10^?4 cm^?1 andD=203(1)·10^?4 cm^?1. The two observed Mn²⁺ centers in ThO₂ exhibita priori axial symmetry with approximate values ofg=2.0006(2),A=88.9(4)·10^?4 cm^?1 andD=33(3)·10^?4 cm^?1.     

Multiphonon radiationless phenomena in Mn2+-Doped Ca1-x Sr x F2 and BaF2 Fluorites

This work investigates the surprising disappearance of the Mn²⁺ photoluminescence on passing from CaF₂:Mn²⁺ to SrF₂:Mn²⁺ or BaF₂:Mn²⁺ using pressure spectroscopy. We show that the loss of luminescence in these systems is associated with non-radiative thermally activated processes whose activation energy and pre-exponential rates strongly depend on the crystal volume irrespective of the chemical composition. A salient feature is the PL enhancement observed in the cotunnite high-pressure phase along the series. This enhancement is explained in terms of the large volume reduction at the phase transition, as well as by the presence of low-symmetry crystal fields attained at the cation sites leading to an increase of the radiative transition rate by the electric-dipole mechanism.     

Alkane oxidation by the system ‘tert‐butyl hydroperoxide–[Mn2L2O3][PF6]2 (L = 1,4,7‐trimethyl‐1,4,7‐triazacyclononane)–carboxylic acid’

The kinetics of cyclohexane (CyH) oxygenation with tert‐butyl hydroperoxide (TBHP) in acetonitrile at 50 °C catalysed by a dinuclear manganese(IV) complex 1 containing 1,4,7‐trimethyl‐1,4,7‐triazacyclononane and co‐catalysed by oxalic acid have been studied. It has been shown that an active form of the catalyst (mixed‐valent dimeric species ‘Mn^IIIMn^IV’) is generated only in the interaction between complex 1 and TBHP and oxalic acid in the presence of water. The formation of this active form is assumed to be due to the hydrolysis of the Mn? O? Mn bonds in starting compound 1 and reduction of one Mn^IV to Mn^III. A species which induces the CyH oxidation is radical tert‐BuO ^. generated by the decomposition of a monoperoxo derivative of the active form. The constants of the equilibrium formation and the decomposition of the intermediate adduct between TBHP and 1 have been measured: K = 7.4 mol^?1 dm³ and k = 8.4 × 10^?2 s^?1, respectively, at [H₂O] = 1.5 mol dm^?3 and [oxalic acid] = 10^?2 mol dm^?3. The constant ratio for reactions of the monomolecular decomposition of tert‐butoxy radical (tert‐BuO ^. → CH₃COCH₃ + CH) and its interaction with the CyH (tert‐BuO ^. + CyH → tert‐BuOH + Cy ^. ) was calculated: 0.26 mol dm^?3. One of the reasons why oxalic acid accelerates the oxidation is due to the formation of an adduct between oxalic acid and 1 (K ≈ 10³ mol^?1 dm³). Copyright © 2007 John Wiley & Sons, Ltd.     

Na2SrMg(PO4)2: Ce3+, Mn2+荧光粉的发光性质及其能量传递机理

采用高温固相法制备了Na₂SrMg(PO₄)₂: Ce³⁺, Mn²⁺ (NSMP: Ce³⁺, Mn²⁺) 荧光粉, 并对其发光性质及Ce³⁺ 对Mn²⁺ 的能量传递机理进行了研究. Ce³⁺ 和Mn²⁺ 在334 nm 和617 nm 的发射峰分别为Ce³⁺ 的5d→4f 跃迁和Mn²⁺ 的⁴T₁(⁴G)→⁶A₁(⁶S) 跃迁产生. Ce³⁺ 对Mn²⁺ 的发光有较强的敏化作用, 根据Dexter能量传递效率公式判断Na₂SrMg(PO₄)₂ 中Ce³⁺ 对Mn²⁺ 的能量传递属于电偶极-电四极相互作用引起的共振能量传递.     

High-field EPR Study of the Effect of Chloride on Mn2+ Ions in Frozen Aqueous Solutions

The effect of chloride concentration on Mn²⁺ (S = 5/2, I = 5/2) ions in frozen aqueous solutions is studied by high-field high-frequency electron paramagnetic resonance (HFEPR). The usually six sharp lines characteristic of Mn²⁺ ions, arising from the m _s  = ?1/2 → 1/2 transition, is modified by the addition of Cl^? anions and the six resonances become much broader and more complex. This new feature likely arises from the ligation of one Cl^? anion to a hydrated Mn²⁺ ion forming a [Mn(H₂O)₅Cl]^? complex. This complex increases linearly with Cl^? concentration with an association constant of K _{a, apparent} = 61 M^?1. The structure of the putative chloride complex was studied using density functional theory calculations and the expected zero-field interaction of such a manganese center was calculated using the superposition model. The predicted values were similar to those determined from the simulation of the spectrum of the m _s  = ?5/3 → ?3/2 transition of the chloride complex. This effect of Cl^? anions occurs at biologically relevant concentration and can be used to probe the Mn²⁺ ions in cellular and protein environments.     

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.     

Studies on the synthesis and characterization of Zn1− x Cd x S and Zn1− x Cd x S:Mn2+ semiconductor quantum dots

Quantum dots (3–4?nm) of Zn_{1? x} Cd _x S (both free of Mn²⁺ and with Mn²⁺ incorporated) were synthesized through a novel solvothermal-microwave irradiation technique. Detailed structural analysis of the Zn_{1? x} Cd _x S and Zn_{1? x} Cd _x S:Mn²⁺ (x?=?0, 0.25, 0.5, 0.75 and 1) materials was carried out using powder X-ray diffraction technique. For all the compositions, the crystallite size was controlled to less than 1.5?nm. The optical energy gap for Zn_{1? x} Cd _x S was found to vary from 3.878 to 2.519?eV and for Zn_1?x Cd _x S:Mn²⁺ it varies from 3.830 to 2.442?eV when x is increased from 0 to 1. Overall, the optical energy gap could be tuned from a minimum of 2.442?eV to a maximum of 3.878?eV. DC conductivity analysis (from 40°C to 150°C) and electrical energy gap analysis for all the compositions were also performed. The dc conductivity for Zn_{1? x} Cd _x S solid solutions varies from 0.3840?×?10^?10 to 8.7782?×?10^?10?mho/m at 150°C and for Zn_{1? x} Cd _x S:Mn²⁺ it varies from 0.5751?×?10^?10 to 9.8078?×?10^?10 mho /m at 150°C (for x?=?0 to x?=?1). The method of synthesis and the results observed in this investigation may assist in the fabrication of optical devices when the required operational performance falls under the range observed in the study.     

ESR study of Fe3+ and Mn2+ ions on trigonal sites in ilmenite structure MgTiO3

Electron spin resonance has been observed for Fe³⁺ and Mn²⁺ ions occupying sites with trigonal symmetry in undoped and doped Verneuil-grown crystals of the ilmenite type compound MgTiO₃. At 300 K, the fine structure parameters in the spin Hamiltonian are (in 10^?4cm^?1) D = +844 (± 1), (a? F) = +118 (± 1), a = 69 (± 7) for Fe³⁺ and D = +164 (± 1), (a ? F) = +10.2 (± l), a = 7.0 (± 1) for Mn²⁺. These values are compared with literature data for Fe³⁺ and Mn²⁺ in other oxides, especially Al₂o₃, with particular reference to the recent “superposition” theory of the effect of a trigonal distortion. From the orientation of the axes of cubic pseudosymmetry of the spin Hamiltonian, and with the assumption that a has the same sign for both ions, it is proposed that Fe³⁺ and Mn²⁺ occupy the same octahedral site, namely the Mg²⁺ site. Anomalous line splittings observed for one sample were attributed to twinning on (0001) or {1120} planes.     

Lattice relaxation and charge-transfer optical transitions due to self-trapped holes in nonstoichiometric LaMnO3 crystal

We explore the role of electronic and ionic polarization energies in the physics of “colossal” magnetoresistive (CMR) materials. We use the Mott-Littleton approach to evaluate polarization energies in the LaMnO₃ lattice associated with holes localized on both the Mn³⁺ cation and the O^2?anion. The full (electronic and ionic) lattice relaxation energy for a hole localized at the O site is estimated at 2.4 eV, which is appreciably greater than that of 0.8 eV for a hole localized at the Mn site, indicating a strong electron-phonon interaction in the former case. The ionic relaxation around the localized holes differs for anion and cation holes. The relaxation associated with Mn⁴⁺ is approximately isotropic, whereas ionic displacements around O^? holes show axial symmetry with the axis directed towards the apical oxygens. Using the Born-Haber cycle, we examine thermal and optical energies of the hole formation associated with the electron ionization from Mn³⁺, O^2?, and La³⁺ions in the LaMnO₃ lattice. For these calculations, we derive a phenomenological value for the second electron affinity of oxygen in the LaMnO₃ lattice by matching the optical energies of the La⁴⁺ and O^? hole formation with maxima of binding energies in the experimental photoemission spectra. The calculated thermal energies predict that the electronic hole is marginally more stable in the Mn⁴⁺ state in the LaMnO₃ host lattice, but the energy of a hole in the O^? state is only higher by a small amount, 0.75 eV, suggesting that both possibilities should be treated seriously. We examine the energies of a number of fundamental optical transitions, as well as those involving self-trapped holes of Mn⁴⁺ and O^? in the LaMnO₃ lattice. The reasonable agreement of our predicted energies, linewidths, and oscillator strengths with experimental data leads us to plausible assignments of the optical bands observed. We deduce that the optical band near 5 eV is associated with the O(2p)-Mn(3d) transition of a charge-transfer character, whereas the band near 2.3 eV is rather associated with the presence of Mn⁴⁺ and/or O^? self-trapped holes in the nonstoichiometric LaMnO₃ compound.     

Inverse bottleneck in Eu-Mn energy transfer

The energy transfer (ET) between Eu²⁺ and Mn²⁺ in Ca₅(PO₄)₃Cl has been investigated. At low intensities under 405 nm excitation, time-resolved experiments provide microscopic parameters for the energy transfer between adjacent Eu²⁺ and Mn²⁺ ions. At high intensities, we observe a non-linear component in the energy transfer process due to ground state depletion of the Mn²⁺ ions, leading not to a reduction, but to an increase in the energy transfer rate between Eu²⁺ and Mn²⁺ ions and also energy transfer between excited Mn²⁺ ions. This results in a sub-linear response of both Eu²⁺ and Mn²⁺ luminescence as a function of intensity. Our observations are quantitatively described by a model using energy transfer to Mn²⁺ ions in both the ground and the excited state.     

Photoluminescence properties of Eu and Mn-activated BaMgP2O7 as a potential red phosphor for white-emission

A phosphate compound, BaMgP₂O₇ was co-doped with Eu²⁺ and Mn²⁺ for making a red-emitting phosphor. The phosphor was prepared by a solid-state reaction at high temperature. The photoluminescence properties were investigated under ultraviolet (UV) ray excitation. From a powder X-ray diffraction (XRD) analysis, the formation of single-phased BaMgP₂O₇ with a monoclinic structure was confirmed. In the photoluminescence spectra, the BaMgP₂O₇:Eu,Mn phosphor emits two distinctive colors: a blue band centered at 409 nm originating from Eu²⁺ and a red band at 615 nm caused by Mn²⁺. Also, efficient energy transfer from Eu²⁺ to Mn²⁺ in the BaMgP₂O₇:Eu,Mn system was verified by observing that the excitation spectra of BaMgP₂O₇:Eu,Mn emitted at 409 and 615 nm by Eu²⁺ emission and Mn²⁺ emission, respectively, are almost the same as that of BaMgP₂O₇:Eu monitored at 409 nm. The optimum concentration of Eu²⁺ ions in BaMgP₂O₇:0.015Eu excited at 309 nm wavelength is 1.5 mol%. With an increase of Mn²⁺ content up to 17.5 mol%, a systematic decline in the intensity of the excitation spectrum by Eu²⁺ and a gradual growth in the intensity of emission band by Mn²⁺ were observed. Accordingly, the optimum concentration of Mn²⁺ in BaMgP₂O₇:0.015Eu,Mn is 17.5 mol%. The maximum spectral overlap between emission of Eu²⁺ and excitation of Mn²⁺ is achieved in a composition of BaMgP₂O₇:0.015Eu,0.175Mn, resulting in considerable red-emission at 615 nm.     

Local structure distortion and spin Hamiltonian parameters of oxide-diluted magnetic semiconductor Mn-doped ZnO

The local structure distortion, the spin Hamiltonian (SH) parameters, and the electric fine structure of the ground state for Mn²⁺(3d⁵) ion in ZnO crystals are systematically investigated, where spin--spin (SS), spin--other--orbit (SOO) and orbit--orbit (OO) magnetic interactions, besides the well-known spin--orbit (SO) coupling, are taken into account for the first time, by using the complete diagonalization method. The theoretical results of the second-order zero-field splitting (ZFS) parameter D, the fourth-order ZFS parameter (a-F), the Zeeman g-factors: g_// and g_⊥, and the energy differences of the ground state: \delta₁ and \delta₂ for Mn²⁺ in Mn²⁺: ZnO are in good agreement with experimental measurements when the three O^2- ions below the Mn²⁺ ion rotate by 1.085^o away from the [111]-axis. Hence, the local structure distortion effect plays an important role in explaining the spectroscopic properties of Mn²⁺ ions in Mn²⁺: ZnO crystals. It is found for Mn²⁺ ions in Mn²⁺: ZnO crystals that although the SO mechanism is the most important one, the contributions to the SH parameters, made by other four mechanisms, i.e. SS, SOO, OO, and SO～SS～SOO～OO mechanisms, are significant and should not be omitted, especially for calculating ZFS parameter D.     

Phase dependent photoluminescence and energy transfer in Ca2P2O7: Eu2+, Mn2+ phosphors for white LEDs

α- and β-Ca₂P₂O₇: Eu²⁺, Mn²⁺ phosphors were prepared by solid-state reaction. Phase transition from tetragonal (β-phase) to monoclinic (α-phase) is performed. A strong orange emission of Mn²⁺ is observed in both α-and β-Ca₂P₂O₇: Eu²⁺, Mn²⁺ upon near ultraviolet (UV) excitation through energy transfer from Eu²⁺ to Mn²⁺. The transfer efficiencies for various Mn²⁺ concentrations are estimated based on lifetime measurements of the fluorescence of Eu²⁺ in the two phases. The photoluminescence excitation spectra of α-Ca₂P₂O₇: Eu²⁺, Mn²⁺ can cover 400 nm of the near-UV range, denoting its potential use as a phosphor with intense orange component for white light emitting diodes (LEDs).     

Optical properties of annealed Mn-doped ZnS nanoparticles

Cysteine stabilized ZnS and Mn²⁺-doped ZnS nanoparticles were synthesized by a wet chemical route. Using the ZnS:Mn²⁺ nanoparticles as seeds, silica-coated ZnS (ZnS@Si) and ZnS:Mn²⁺ (ZnS:Mn²⁺@Si) nanocomposites were formed in water by hydrolysis and condensation of tetramethoxyorthosilicate (TMOS). The influence of annealing in air, formier gas, and argon at 200-1000 °C on the chemical stability of ZnS@Si and ZnS:Mn²⁺@Si nanoparticles with and without silica shell was examined. Silica-coated nanoparticles showed an improved thermal stability over uncoated particles, which underwent a thermal combustion at 400 °C. The emission of the ZnS@Si and ZnS:Mn²⁺@Si passed through a minimum in photoluminescence intensity when annealed at 600 °C. Upon annealing at higher temperatures, ZnS@Si conserved the typical emission centered at 450 nm (blue). ZnS:Mn²⁺@Si yielded different high intensity emissions when heated to 800 °C depending on the gas employed. Emissions due to the Mn²⁺ at 530 nm (green; Zn₂SiO₄:Mn²⁺), 580 nm (orange; ZnS:Mn²⁺@Si), and 630 nm (red; ZnS:Mn²⁺@Si) were obtained. Therefore, with a single starting product a set of different colors was produced by adjusting the atmosphere wherein the powder is heated.     

Red persistent luminescent silicate nanoparticles

Red persistent luminescent diopside nanoparticles doped with Mn²⁺ and codoped with RE³⁺ (Eu²⁺, Dy³⁺) have been obtained by sol-gel method. The influence of codoping rare earth ions on the persistent luminescence was studied by wavelength-resolved thermally stimulated luminescence (TSL) measurements from 30 to 650 K after X-ray irradiation. From these first results, a mechanism of persistent luminescence is proposed. In this mechanism Mn²⁺ and Eu²⁺ act as TSL recombination centers, Mn³⁺ and Eu³⁺ being stable hole centers, whereas Dy³⁺ acts as a good electron trap giving rise to a TSL peak at high temperature. Finally, persistent luminescence was measured. Intensity and persistence of the red luminescence of CaMgSi₂O₆: Mn²⁺–Dy³⁺ are better than those of CaMgSi₂O₆: Mn²⁺ and CaMgSi₂O₆: Mn²⁺–Eu²⁺, which are in agreement with the results of TSL.     

Diluted ferromagnetic semiconductor (LaCa)(ZnMn)SbO isostructural to “1111” type iron pnictide superconductors

We report discovery of ferromagnetism in(LaCa)(ZnMn)SbO isostructural to the well-studied iron-based superconductor LaFeAs(O1 xFx).Spin is induced by partial substitution of Mn2+for Zn2+,while charge is induced by substitution of Ca2+for La3+within the parent compound LaZnSbO.Ferromagnetism with Curie temperature(TC)is observed up to 40 K at the spin doping 0.15 by introducing Mn2+into the Zn2+sites for(La0.95Ca0.05)(Zn1 xMnx)SbO.The Hall coefficient measurement indicates p-type carrier for(La0.95Ca0.05)(Zn0.9Mn0.1)SbO with concentration of n~1020cm 3showing anomalous Hall effect below TC.     

BaAl12O19:Mn green emitting nanophosphor for PDP application synthesized by solution combustion method and its Vacuum Ultra-Violet Photoluminescence Characteristics

Nanostructured BaAl₁₂O₁₉:Mn²⁺ phosphor particles of nano-rod morphology with diameter 40-100 nm and length up to 200-600 nm has been synthesized by solution combustion method and its photoluminescence characteristics have been studied by Vacuum Ultra-Violet Photoluminescence spectrometer (VUVPL) under 147 nm excitation. The crystallographic phase purity of BaAl₁₂O₁₉:Mn²⁺ nanostructured phosphor particle synthesized by solution combustion approach is confirmed by X-ray diffraction (XRD). The broadening of XRD diffraction peaks indicates nanocrystalline nature of particles present in powder. The emission spectrum of BaAl₁₂O₁₉:Mn²⁺ nanophosphor on 147 nm excitation consists of a wide green band with a peak at about 515 nm, which is due to a 3d⁵ (⁴T_1g)-3d⁵ (⁶A_1g) transition corresponds of Mn²⁺ ions. It is found that the concentration quenching is obtained when Mn²⁺ content (x) is 0.05 in BaAl₁₂O₁₉:xMn²⁺ nanophosphor on 147 nm excitation. The decay time of 3d⁵ (⁴T_1 g)-3d⁵ (⁶A_1 g) transition of Mn²⁺ ions at 147 nm excitation is about 23 ms for BaAl₁₂O₁₉:Mn²⁺ nanophosphor. This nanostructured green emitting BaAl₁₂O₁₉:Mn²⁺ phosphor can find potential application in Plasma Display Panels (PDPs) and mercury-free fluorescent lamps.