EPR observation of phase transitions in calcium cadmium acetate hexahydrate: Using Mn2+ and Cu2+ ions as probe

Results of temperature dependence of EPR spectra of Mn²⁺ and Cu²⁺ ions doped calcium cadmium acetate hexahydrate (CaCd(CH₃COO)₄·6H₂O) have been reported. The investigation has been carried out in the temperature range between room temperature (～ 300 K) and liquid nitrogen temperature. A I-order phase transition at 146 ± 0.5 K has been confirmed. In addition a new II-order phase transition at 128 ± 1 K has been detected for the first time. There is evidence of large amplitude hindered rotations of CH₃ groups which become frozen at ～ 128 K. The incorporation of Cu²⁺ and Mn²⁺ probes at Ca²⁺ and Cd²⁺ sites respectively provide evidence that the phase transitions are caused by the molecular rearrangements of the common coordinating acetate groups between Ca²⁺ and Cd²⁺ sites. In contradiction to the previous reports of a change of symmetry from tetragonal to orthorhombic below 140 K, the symmetry of the host is concluded to remain tetragonal in all the three observed phases between room temperature and liquid nitrogen temperature.     

Solvothermal synthesis of nanocrystalline zinc oxide doped with Mn2+, Ni2+, Co2+ and Cr3+ ions

ZnO nanopowders doped with Mn²⁺, Ni²⁺, Co²⁺ and Cr³⁺ ions have been synthesised for the first time using a solvothermal reaction with microwave heating. The nanopowders were produced from a solution of zinc acetate and manganese (II), chromium (III), nickel (II) and cobalt (II) acetates, using ethylene glycol as a solvent. The content of Ni²⁺, Co²⁺ and Cr³⁺ ions in the solution and in the solid phase were close to each other up to 5 mol%. The doping level of Mn²⁺ ions in the solid is about 50% of that in the solution. No phases or compounds other than ZnO were detected by X-ray diffraction with Mn²⁺, Co²⁺ and Ni²⁺ doping. With Cr³⁺ ions a small amount of chromium oxide was found. None of the powders displayed any luminescence after doping. The Mn²⁺-doped powder displayed a paramagnetic behaviour. ESR and magnetisation investigations have revealed that no clustering of Mn²⁺ ions occurred up to a doping level of 3.9 mol%. The average grain size of powders doped with Ni²⁺, Cr³⁺, Co²⁺ and Mn²⁺ for a 10 mol% ion content in the solution was about 20 nm and the grain size dispersion 30%. With increasing dopant content the grain size decreased. It appears that the solvothermal process employed allows relatively high doping levels of the transition metal ions to be achieved without any dopant clustering or oxide precipitation.     

Substitutional site of Co2+ ion in RbMgF3 crystal

The spin-Hamiltonian (SH) parameters (g factors g _//, g _⊥ and hyperfine structure constants A _//, A _⊥) for Co²⁺ ions at the trigonal Mg²⁺ (I) and Mg²⁺ (II) sites of RbMgF₃ crystal are calculated from the second-order perturbation formulas based on the cluster approach for 3d⁷ ions in trigonal symmetry. From the calculations, it is found that the calculated SH parameters for Co²⁺ ion at the Mg²⁺ (I) site are in poor agreement with, but those for Co²⁺ at the Mg²⁺ (II) site are close to, the experimental values. Therefore, we suggest that Co²⁺ in RbMgF₃ crystal substitutes for Mg²⁺ (II) ion. The results are discussed.     

Polarized absorption spectra of Co2+ in Na2Cd3Cl8 single crystals

Summary The polarized optical absorption spectra of Na₂Cd₃Cl₈: Co²⁺ in the range of 15 000 to 40 000 cm⁻¹ down to 15 K are reported. The Co²⁺ ion is found to occupy the Cd²⁺ sites in octahedral geometry and the spectra are interpreted satisfactorily in terms of a cubic ligand field model including spin-orbit coupling. The observed crystal field spectra are well reproduced withB=745 cm⁻¹,C=3410 cm⁻¹,Dq=700 cm⁻¹ and ζ (spin-orbit interaction) =520 cm⁻¹. No spectral evidence for tetragonal distortion is observed.     

A New Highly Sensitive and Selective Fluorescent Cadmium Sensor

Condensation product (L) of salicylaldehyde and semicarbazide behaves as a fluorescent sensor for Cd²⁺ ion, in 1:1 DMSO:H₂O, over Mn²⁺, Fe²⁺, Ni²⁺, Co²⁺, Cu²⁺, Pb²⁺ and Hg²⁺ ions. The emission peak of L at λ_max = 520 nm, on excitation with 420 nm wavelength photons, showed an enhancement in intensity of ca 60-fold when interacted with Cd²⁺ ion. The intensity was however found to remain unaltered when interacted with metal ions—Mn²⁺, Fe²⁺, Ni²⁺, Co²⁺, Cu²⁺, Pb²⁺ and Hg²⁺. The intensity increases by approximately 20 fold on interaction with Zn²⁺ ion. The increase in the fluorescent peak can be explained on the basis of photo induced electron transfer (PET) mechanism. A 1:1 complexation between Cd²⁺ and L with log β = 4.25 has been proved.     

EPR study of exchange interactions of manganese ions in a CdGeAs<Subscript>2</Subscript> matrix

The results of an EPR study of the CdGeAs₂ compound doped with 6 at % Mn are reported. The experimental data are analyzed under the assumption that magnetic centers of the following two types are formed in the system: Mn_Cd, Mn ions that replace Cd²⁺ and have spin S = 5/2, and Mn_Ge, Mn ions that replace Ge⁴⁺ and form the Mn²⁺ + 2p complex with two “heavy” holes with spin S = 1/2. The absence of signals from isolated centers and the Lorentzian shape of an absorption curve suggest a strong exchange narrowing of the spectrum and the extension of an isotropic exchange interaction involving Mn_Ge to distances much longer than the lattice parameter. It is found that the exchange interaction between Mn²⁺ + 2p complexes is ferromagnetic, and it is stronger than the characteristic superexchange interaction involving Mn_Cd centers by three to four orders of magnitude. The form of the temperature dependence of susceptibility obtained by the double integration of spectra is indicative of the formation of nanoscale regions, which weakly interact with each other, with ferromagnetically ordered Mn²⁺ + 2p complexes at ～250 K.     

Preferential ion location and structural stability in CsCdCl3

On the basis of experimental observation of the opposite preferential occupation of the two symmetrically different Cd²⁺ sites by Co²⁺ and Mn²⁺ ions in CsCdCl₃, a rule taking into account the symmetry of both the electronic configuration of the substituting ion and the local receiving site is derived. The chief parameter is the orbital degeneracy of the ground state; a singlet leads to a preferential occupation of cubic octahedral sites, whereas a triplet leads to a settlement in the axially distorted sites. Doublet states unsplit by trigonal symmetry seem to give rise to spontaneous Jahn-Teller effects. This rule could be checked for most transition metal ions of the fourth row of the periodic table.     

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.     

Luminescent Cu2+ Probes Based on Rare-Earth (Eu3+ and Tb3+) Emissive Transparent Cellulose Hydrogels

Cellulose hydrogels are biodegradable materials that can be applied as accommodating hosts for various species. Here we report the preparation of novel thin films based on luminescent cellulose hydrogels. The spectroscopic behavior of these soft materials and their sensing effects are investigated. Interestingly, we found that these films only give selective signal changes in the presence of Cu²⁺ in water in comparison with Na⁺, K⁺, Ag⁺, Mn²⁺, Co²⁺, Ca²⁺, Cd²⁺, Hg²⁺, Pd²⁺, Mg²⁺, Ni²⁺, Fe²⁺, and Fe³⁺. High visible-light transmittance and good flexibility for these films can be observed. More importantly, the thermal stability of rare-earth complexes could be significantly enhanced in aqueous solution as result of the protection by hydrogel matrix.     

Magnetic and magnetotransport properties of Co-doped manganites with perovskite structure

A systematic study of the doping of the Mn-sites by cobalt in three series of manganites — La_0.76Ba_0.24(Mn_1−xCo_x)O₃ single crystals, La_2/3Ba_1/3(Mn_1−xCo_x)O₃ and La(Mn_1−xCo_x)O₃ ceramics has been performed. It was found that La(Mn_1−xCo_x)O₃ annealed at 800°C in the range 0.4x0.9 is a mixture of ferromagnetic domains with ordered Mn and Co ions and ionically disordered spin-glass domains. In the quenched samples the fraction of spin-glass-type component increases strongly. The La_2/3Ba_1/3(Mn_1−xCo_x)O₃ solid solutions exhibit also an evidence for phase separation in the range 0.5x0.8. All the La(Mn_1−xCo_x)O₃ samples show an insulating behavior, however, magnetoresistance reduces strongly when the cobalt content rises to x=0.5. The La_0.76Ba_0.24(Mn_1−xCo_x)O₃ single crystals show first-order phase transition below their Curie points associated with a change of ground state of the Co²⁺ ions. The magnetic phase diagrams are depicted. The results are discussed in terms of positive Mn³⁺–O–Mn⁴⁺, Mn³⁺–O–Mn³⁺, Mn⁴⁺–O–Co²⁺ and negative Mn⁴⁺–O–Mn⁴⁺, Co²⁺–O–Co²⁺, Co²⁺–O–Mn³⁺ superexchange interactions as well as Co²⁺ and Mn⁴⁺ ionic ordering.     

SUPERCONDUCTING PROPERTIES AND STRUCTURAL PERFECTION OF EPITAXIAL YBa2Cu3O7-x THIN FILMS

The photoconductivity spectra of semimagnetic semiconductor Cd_0.7Mn_0.3Se have been measured at 50-300 K and in the wavelength range of 5500? to 8000?. It is found that the energy gap of Cd_0.7Mn_0.3Se changes with the temperature linearly, and the temperature coefficient (dE_g/dT) is about -7×10^-4 eV/K. A photoconductivity peak which is related to Mn²⁺ is also found. This peak is located around 1.85 eV, nearly unshifted with variation of the temperature. The possible transition mechanisms in Cd_1-xMn_xSe have been discussed in the light of group theory and crystal field theory, and the results are in good agreement with experiments.     

Luminescence of CdMgTe with CdMnTe ultrathin nanolayers

The luminescence spectra of Cd_0.75Mn_0.25Te/Cd_{1 ? y} Mg _y Te superlattices with narrow-band-gap Cd_0.75Mn_0.25Te nanolayers having nominal thicknesses of 0.5, 1.5, and 3.0 monolayers (MLs) exhibit exciton emission bands of Cd_{1 ? y} Mg _y Te barriers and nanolayers, as well as intracenter luminescence of Mn²⁺ ions. For all samples, the luminescence spectra of Cd_0.75Mn_0.25Te nanolayers consist of two bands. From analyzing the dependences of the luminescence intensity on the nanolayer thickness and temperature, these bands are assigned to excitons localized at two-and zero-dimensional potentials. The intracenter emission of Mn²⁺ in 3.0-ML-thick Cd_0.75Mn_0.25Te clearly reveals excitation migration.     

Eu2+,Mn2+在BaZnP2O7中的发光及Eu2+→Mn2+能量传递

采用高温固相法合成了BaZnP₂O₇:Eu²⁺,Mn²⁺荧光粉,并对其发光性质及Eu²⁺对Mn²⁺的能量传递机理进行了研究.Eu²⁺和Mn²⁺在380 nm和670nm的发射峰分别由Eu²⁺的5d—4f跃迁和Mn²⁺的⁴T₁(^{4关键词： 磷酸盐 2+}')" href="#">Eu²⁺ 2+')" href="#">Mn²⁺ 能量传递     

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.     

Effects of Mn substitution of Co and Fe in spinel CoFe2O4 thin films

Effects of Mn substitution for Co and Fe on the structural and magnetic properties of inverse-spinel CoFe₂O₄ have been investigated. Mn_xCo_1−xFe₂O₄ and Mn_yCoFe_2−yO₄ thin films were prepared by a sol–gel method. The observed increase of the lattice constant of Mn_xCo_1−xFe₂O₄ indicates that Mn²⁺ ions substitute the octahedral Co²⁺ sites. Conversion electron Mössbauer spectroscopy data indicate that a fraction of octahedral Co²⁺ ions exchange sites with tetrahedral Fe³⁺ ions through Mn doping. Vibrating-sample magnetometry data exhibit a large increase of saturation magnetization for both Mn_xCo_1−xFe₂O₄ and Mn_yCoFe_2−yO₄ films compared to that of the CoFe₂O₄ film. Such enhancement of magnetization can be explained in terms of a breaking of ferrimagnetic order induced by the Co²⁺ migration.     

Causes of the Metamagnetism in a Disordered EuMn<Subscript>0.5</Subscript>Co<Subscript>0.5</Subscript>O<Subscript>3</Subscript> Perovskite

The magnetic properties of the EuMn_0.5Co_0.5O₃ perovskite synthesized under various conditions are studied in fields up to 140 kOe. The sample synthesized at T = 1500°C is shown to exhibit a metamagnetic phase transition, which is irreversible below T = 40 K, and the sample synthesized at T = 1200°C demonstrates the field dependence of magnetization that is typical of a ferromagnet. Both samples have T_C = 123 K and approximately the same magnetization in high magnetic fields. The metamagnetism is assumed to be related to a transition from a noncollinear ferromagnetic phase to a collinear phase, and the presence of clusters with ordered Co²⁺ and Mn⁴⁺ ions leads to ferromagnetism. The noncollinear phase is formed due to the competition between positive Co²⁺–Mn⁴⁺ and negative Mn⁴⁺–Mn⁴⁺ and Co²⁺–Co²⁺ interactions, which make almost the same contributions, and to the existence of a high magnetic anisotropy.     

A New Fluorescent Chemosensor Detecting Co<Superscript>2+</Superscript> and K<Superscript>+</Superscript> in DMF Buffered Solution

A new fluorescent chemosensor 2-(2-thiophene)imidazo [4,5,f]-1,10-phenanthroline (L) was prepared and characterized. By adding univalent or divalent metal ions such as Na⁺, K⁺, Mg²⁺, Ba²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Zn²⁺, Cd²⁺ and Hg²⁺ ions into the solution of L in DMF under buffered conditions with the working pH ranging from 7.0 to 8.0, we found that L could be used to detect K⁺ ratiometricly and it could also be applied to sense Co²⁺ with the phenomenon of fluorescence quenching of L. While the response behavior of L was not discernibly affected by other examined metal ions.     

Evidence for the spin-dependent scattering of conduction electrons on Mn2+ ions in Hg1−xMnxTe and Cd1−xMnxSe mixed crystals

Changes in the resistivity of Hg_1?xMn_xTe and Cd_1?xMn_xSe mixed crystals associated with paramagnetic resonance of the Mn²⁺ ions have been observed at liquid helium temperature in a strong magnetic field. The effect was recorded by monitoring the submillimeter radiation induced photoconductivity in a swept magnetic field. An increase in the resistivity associated with EPR of the Mn²⁺ ions is interpreted in terms of the spin- dependent scattering of electrons on magnetic impurities, the spins of which are selectively depolarised by means of paramagnetic resonance. Some additional effects influencing the experiments are also discussed.     

Luminescence spectroscopy and energy transfer in Eu2+/Mn2+-doped KCaPO4 phosphors

Eu²⁺/Mn²⁺-doped KCaPO₄ phosphors were prepared by conventional solid-state reaction. X-ray powder diffraction (XRD), SEM, photoluminescence excitation, and emission spectra, and the luminescence decay curves were measured. Mn²⁺ singly doped KCaPO₄ shows the weak origin-red luminescence band peaked at about 590 nm. The Eu²⁺/Mn²⁺ co-doped phosphors emit two distinctive luminescence bands: a blue one centered at 480 nm originating from Eu²⁺ ions and a broad red-emitting one peaked at 590 nm from Mn²⁺ ions. The luminescence intensity from Mn²⁺ ions can be greatly enhanced with the co-doping of Eu²⁺ ions. The efficient energy transfer from Eu²⁺ to Mn²⁺ was verified by the photoluminescence spectra together with the luminescence decay curves. The resonance-type energy transfer via a dipole–quadrupole interaction mechanism was supported by the decay lifetimes. The emission colors could be tuned by changing the Mn²⁺-doping concentration.     

Emission properties of Eu, Mn in MAl2Si2O8 (M=Sr, Ba)

The emission properties of Eu²⁺ and Mn²⁺ in monoclinic SrAl₂Si₂O₈ (M-SAS) and hexagonal BaAl₂Si₂O₈ (H-BAS), both of which have only one alkaline-earth site, were studied. The emission peaks of both Eu²⁺ (405 nm) and Mn²⁺ (564 nm) in SrAl₂Si₂O₈, are located at longer wavelengths, compared with those in H-BAS (373 nm for Eu²⁺ and 518 nm for Mn²⁺), because of the stronger crystal field strength at the Sr site. EPR spectra showed that the g values of Mn²⁺ are 4.5065 in M-SAS:Mn and 2.0247 in H-BAS:Mn. Magnetic measurements proved that Mn²⁺ was at high-spin state in both hosts. The large g value of Mn²⁺ in M-SAS was ascribed to the mixing of the first excitation state to the ground state, both of which have lower d orbital degeneracy due to the lower symmetry of Mn²⁺ site. The transfer efficiency from Eu²⁺ to Mn²⁺was about 10% in M-SAS, higher than that in H-BAS (5%). This was probably because Eu²⁺ emission overlaps the relatively low excitation level of Mn²⁺ in M-SAS. In order to obtain high transfer efficiency, it was necessary for the Eu²⁺ emission to overlap the lowest excitation level of Mn²⁺. The results obtained in this work may be helpful to design the new white or red phosphors for white-light emitting diode (w-LED) applications.