Magnetism in some layer molecular magnets as revealed by Mössbauer spectroscopy

⁵⁷Fe Mössbauer spectroscopy was used to explore magnetic properties of two 2D molecular ferrimagnets. In NPn₄[Fe^IIFe^III(ox)₃] (Pn = n-C₅H₁₁, (ox) =(C₂O₄)^2?), the previously reported negative magnetization is shown here by external field studies to be due to a cross-over between Fe^III and Fe^II- magnetizations. The form and parameters of the magnetic Hamiltonian describing the temperature dependence of both the Fe^III hf-field and net magnetizations has been determined. In NBu₄[Mn^IIFe^III(ox)₃] (Bu = n-C₄H₉) soft XY-magnet the low temperature relaxation spectra are interpreted in terms of slowly varying classical magnetization-evolution at low temperature.     

Magnetic field effects on electron transfer reactions involving sextet-spin (S = 5/2) intermediates generated on photoexcitation of a Cr(III)-porphyrin complex

The excited trip-sextet (⁶T₁) state of chloro-(3-methylimidazol)-(meso-tetraphenylporphyrinato) chromium(III) (Cr^IIIP) is quenched by 1,1′-dibenzyl-4,4′-bipyridinium (BV²⁺) in acetonitrile through electron transfer to give ⁵(Cr^IIIP^·+) and ²BV^·+. The intermediate is a geminate ion pair in the sextet (Sx) state ⁶[⁵(Cr^IIIP^·+) ²BV^·+], which decays through either the escape from a solvent cage to give the free ions or the spin conversion to the quartet (Qa) state followed by back electron transfer. The free ion yield (Φ_FI) increased with increasing magnetic field from 0 to 4 T and then slightly decreased from 4 T to 10 T. These magnetic field effects are explained as follows. Under low fields where the Zeeman splitting of the spin sublevels is lower than or comparable with the electron spin dipole—dipole interaction within ⁵(Cr^IIIP^·+), this interaction effectively induces the Sx → Qa conversion of [⁵(Cr^IIIP^·+)²BV⁺] to result in low Φ_FI values. Under high fields where the Zeeman splitting is larger than the dipole—dipole interaction, the Sx → Qa conversion is decreased with increasing field to cause higher Φ_FI values. The slight decrease in Φ_FI above 4 T may be due to the Δg mechanism.     

Synthesis and characterization of hexakis(acetonitrile)chromium(III) tetrafluoroborate, [Cr(NCMe)6][BF4]3. A nonaqueous Cr source

Oxidation of [Cr^II(NCMe)₄][BF₄]₂ with thianthrinium tetrafluoroborate forms [Cr^III(NCMe)₆][BF₄]₃ exhibiting two ν_CN absorptions at 2331 and 2301 cm⁻¹, and has been structurally characterized with an average Cr-N distance of 1.999 Å. From the electronic absorption spectra the ligand field splitting, Δ₀, is 20,160 cm⁻¹, which is slightly larger than [Cr^III(OH₂)₆]³⁺ in accord with the divalent chromium analogues. The 298 K ESR has a resonance at g=1.9884, and the magnetic susceptibility has a 300 K moment of 3.85μ_B characteristic of S=3/2 Cr(III). The field dependence of the magnetization can be fit to the Brillouin function also characteristic of S=3/2.     

Condensation Product of 1-Naphthaldehyde and 3-Aminophenol: Fluorescent “on” Probe for Ce3+and “off” Probe for Dichromate (Cr2O72−)

A new probe (Z)-3-((naphthalen-1-ylmethylene)amino)phenol has been synthesized by condensation reaction between 1-naphthaldehyde and 3-aminophenol for the fluorescent sensing of Ce³⁺ by “on” mode and dichromate (Cr₂O₇^2?) by “off” mode. Metal ions—Ag⁺, Al³⁺, As³⁺, Ba²⁺, Ca²⁺, Cd²⁺, Ce⁴⁺, Co²⁺, Cr³⁺, Cr⁶⁺, Cu²⁺, Fe²⁺, Fe³⁺, Hg²⁺, K⁺, La⁺, Li⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺, Zn²⁺and anions Br^?, C₂O₄^2?, CH₃COO^?, Cl^?, CO₃^2?, F^?, H₂PO₄^?, HCO₃^?, HF₂^?, HPO₄^2?, I^?, MnO₄^?, NO₃^?, OH^?, S^2?, S₂O₃^2?, SCN^?, SO₄^2? do not interfere. The limit of detection (LOD) for sensing Ce³⁺ and Cr₂O₇^2? ions are 1.286?×?10^–7 M and 6.425?×?10^–6 M, respectively.

     

Magnetic and structural studies of Cr2O5 and Cr3O8

The infrared spectra, indexed X-ray powder diffraction patterns, magnetic susceptibilities between 80 and 300 K, and electron paramagnetic resonance spectra at 80 and 300 K are reported for Cr₂O₅ and Cr₃O₈. The results indicate that both oxides are Cr³⁺/Cr⁶⁺ mixed-valence compounds which contain CrO₆ octahedra and CrO₄ tetrahedra in different ratios. The valence formula for Cr₂O₅ is Cr³⁺₂Cr⁶⁺₄O₁₅ and that of Cr₃O₈ is Cr³⁺₂Cr⁶⁺₇O₂₄. The X-ray powder data for Cr₂O₅ and Cr₃O₈ could be indexed on the basis of a monoclinic unit cell $\text{(a = 12.01(2), b = 8.52(1), c = 9.39(1)}\text{A}\text{?}\text{β = 92.0(1)°)}$ and an orthorhombic unit cell $\text{(a = 12.01(7), b = 36.60(7) and c = 3.82(1)}\text{A}\text{?}\text{)}$, respectively.     

Hydrothermal phase formation of orthorhombic LiMnO2 and its derivatives as lithium intercalation compounds

Different from a conventional solid state reaction, a hydrothermal reaction mechanism is very difficult to illuminate and sometimes it remains undisclosed. Making an attempt to understand the hydrothermal phase formation process of o-LiMnO₂ obtained between the reaction of spinel type Mn₃O₄ precursor and LiOH aqueous solution, the possible reaction route was postulated and experimentally testified. Firstly, the selective dissolution of Mn²⁺ from the tetrahedral site of [Mn^II]^tet_4a[Mn^III₂]^oct_8dO₄, which is considered as to be an ionic exchange reaction with Li⁺, and an additional Li⁺ intercalation into the host structure of precursor would give rise to the formation of meta-stable Li₂Mn₂O₄ ([Li^I]^tet_4a[Li^I]^oct_8c[Mn^III₂]^oct_8dO₄). Secondly, the phase would be simultaneously transformed to thermodynamically stable o-LiMnO₂ phase under hydrothermal state during hydrothermal reaction. Through the above reaction process, the solid solution range of o-LiCo_xMn_1−xO₂ was as large as x ≤ 0.14, and that of o-LiFe_xMn_1−xO₂ was x ≤ 0.05. Co doped o-LiMnO₂ has higher capacity and good cyclability upon cycling, being substantially more stable to cycle than the unsubstituted and Fe doped materials.     

Study of alkyl chain length dependent characteristics of imidazolium based ionic liquids [CnMIM]+[TFSA]− by Brillouin and dielectric loss spectroscopy

This study examined the acoustic phonon mode of ionic liquids consisting of 1-alkyl-3-methyl-imidazolium family (C_nMIM) cations with n values ranging from 2 to 10 and bis(trifluoromethylsulfonyl)amide (TFSA) anion in the temperature range from 300 K to 100 K. [C_nMIM]⁺[TFSA]^? showed depolarized (VH) components of Brillouin peaks at temperatures below the glass transition temperature when n is larger than 4. On the other hand, in the case of ionic liquids with different anions, such as [C₄MIM]⁺[BF₄]^?, [C₄MIM]⁺[PF₆]^? and [C₈MIM]⁺[BF₄]^?, the VH component of Brillouin peaks was not observed in the temperature range investigated. The dielectric loss spectra showed that the temperature dependence of alkyl chain domain relaxation of all ionic liquids followed the Arrhenius law and showed an increase in activation energy at the temperature where the VH component of Brillouin peak appeared. These results suggest that the observed depolarized component of Brillouin peak might originate from uniquely induced polarization in the 2nd domain composed of head groups of cations and anions.     

Synthesis and characterization of layered perovskite oxides La1 + x Sr2 − x MnCrO7 (x = 0.2, 0.5)

Intergrowth perovskite type complex oxides of composition La_1.2Sr_1.8MnCrO₇ and La_1.5Sr_1.5MnCrO₇ have been synthesized by ceramic method. Rietveld profile analysis shows that the phases crystallize with tetragonal unit cell in the space group I4/mmm. Both the phases behave as insulators in the high temperature region and the linearity of log ρ versus T ^?1/4 plot in the temperature range 150–300 K shows that the electronic conduction occurs by a 3D variable range hopping mechanism. The phases show insulator-metal transition at low temperature which could be due to the mixed valence state of Mn³⁺/Mn⁴⁺ by double exchange mechanism. The ferromagnetic interactions observed for the samples arises from double exchange interaction between Mn ³⁺ and Mn⁴⁺ and Cr³⁺ and Mn³⁺ ions.     

Behavior of Nitroxide Biradicals with Acetylene Bridges in Organic Solvents and Ionic Liquids

Intramolecular electron spin exchange (IESE) in two nitroxide biradicals, R₆–C≡C–C≡C–R₆ (1) and R₆–C≡C–p-C₆H₄–C≡C–R₆ (2), is studied as a function of temperature and solvent properties. The effect of molecular solvents and ionic liquids (ILs, [1-methyl-3-butylimidazolium]⁺[PF₆]^?, bmimPF₆, and [1-methyl-3-octylimidazolium]⁺[BF₄]^?, omimBF₄) on the IESE in magnetically diluted solutions is investigated. Changes in electron paramagnetic resonance spectra are analyzed and the thermodynamic parameters of these changes are calculated. Geometry optimization and D-tensor calculations of biradicals 1 and 2 were carried out on the DFT/UB3LYP/cc-pVdz and DFT/ROPBE/N07D levels of theory. The probable differences in biradical behavior are discussed.     

Energy transfer among three luminescent centers in full-color emitting ZnGa2O4:Mn, Cr phosphors

The ZnGa₂O₄:Mn²⁺, Cr³⁺ phosphors show three colors; the blue band of 380 nm from the charge transfer between Ga-O, the green band of 505 nm from Mn²⁺ and the red band of 705 nm from Cr³⁺. As a variation of Mn²⁺ or Cr³⁺ concentrations in ZnGa₂O₄:Mn²⁺, Cr³⁺, the relative emission intensity can be tuned. This phenomenon is explained in terms of the energy transfer based on four factors: the spectral overlap between the energy donors (Ga-O) and the energy accepters of Mn²⁺ or Cr³⁺, the absorption cross section of the energy accepters, the distance between them, and the decay time of the energy donors. ZnGa₂O₄:0.0025Mn²⁺, 0.010Cr³⁺ shows the CIE coordinates of x=0.4014, y=0.3368, which is a pure white light. The single-phased full-color emitting ZnGa₂O₄:Mn²⁺, Cr³⁺ phosphors can be applied to illumination devices.     

X-ray study of some spinels containing gallium and manganese

The ternary oxides CrMnGaO₄, NiMnGaO₄, CuMnGaO₄ and ZnMnGaO₄, crystallize in the cubic spinel structure with lattice parametera=8.41±0.02 Å, 8.34±0.02 Å, 8.36±0.02 Å and 8.32±0.02 Å, respectively. The oxidation state of manganese in these spinels was determined x-ray spectroscopically. The site distribution was determined from the structural properties and calculated site preference energies of cations in the lattice. The ionic structures were found to be Ga³⁺ [Mn²⁺ Cr³⁺] O ₄ ^2? . Ga³⁺ [Cu²⁺ Mn³⁺] O ₄ ^2? , Mn²⁺ [Ga³⁺ Ni³⁺] O ₄ ^2? and Zn²⁺ [Mn³⁺ Ga³⁺] O ₄ ^2? .     

Yellow-orange light emission from Mn2+-doped ZnS nanoparticles

ZnS nanoparticles with Mn²⁺ doping (1–2.5%) have been prepared through a simple soft chemical route, namely the chemical precipitation method. The nanostructures of the prepared undoped ZnS and Mn²⁺-doped ZnS:Mn nanoparticles have been analyzed using X-ray diffraction (XRD), Scanning electron microscope (SEM), transmission electron microscope (TEM) and UV–vis spectrophotometer. The size of the particles is found to be in 2–3 nm range. Room-temperature photoluminescence (PL) spectrum of the undoped sample only exhibits a blue-light emission peaked at ∼365 nm under UV excitation. However, from the Mn²⁺-doped samples, a yellow-orange emission from the Mn^{2+ 4}T₁–⁶A₁ transition is observed along with the blue emission. The prepared 2.5% Mn²⁺-doped sample shows efficient emission of yellow-orange light with the peak emission at ∼580 nm with the blue emission suppressed.     

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.     

Peculiarities of crystal structures and magnetic properties of Cu(II) and Ni(II) mixed-ligand complexes on the 1,3-dithiole-2-thione-4,5-dithiolate basis

Mixed-ligand Cu(II) and Ni(II) complexes, [Cu(dmit)(bpy)]₂ (I), [Ni(dmit)(phen)₂] (II) and [Ni(dmit)(phen)₂]·CH₂Cl₂ (III) (dmit=1.3-dithiole-2-thione-4.5-dithiolate, phen=1.10-phenantroline, bpy=2.2′-bipyridine) have been prepared by ligand exchange between phen or bpy and (Bu₄N)₂[M(dmit)₂] (M=Ni, Cu) and characterized by elemental analysis, IR spectroscopy, single-crystal X-ray analysis and by investigation of magnetic and resonance properties. In complex I, the monomeric units form dimers in a head-to-tail arrangement by weak coordination bonds between copper and dithiolate sulfur atoms and π–π interactions between dmit and bpy from neighboring monomers. Dimers in I are further extended into chains by weak Cu–S(thione) contacts. In crystal packing of complex II and III, there exists a weak π–π interaction between two parallel phen molecules of the adjacent complexes. As a consequence, the magnetic and resonance characteristics of copper complex may be described in approximation of exchange-coupled pairs of Cu²⁺ ions with ion spin S=1/2. The nickel complexes are described by isotropic exchange model for single-site spin S=1.     

High-spin metal-cyanide clusters: species incorporating [Mn(salen)] complexes as a source of anisotropy

The use of N,N′-ethylenebis(salycylideneiminato) (salen) complexes of Mn^III in assembling high-spin metal-cyanide coordination clusters with significant magnetic anisotropy is demonstrated. The reaction of [Mn(salen)(H₂O)₂]⁺with [Cr(CN)₆]³⁻ in aqueous solution generates {Cr[CNMn(salen)(H₂O)]₆}[Cr(CN)₆]·6H₂O (1), a previously reported compound featuring a heptanuclear cluster with a distorted octahedral geometry. A fit to the variable-temperature magnetic susceptibility data for 1 revealed the presence of weak antiferromagnetic coupling of within the cluster, giving rise to an S=21/2 ground state. In addition, variable-field magnetization data collected at low temperatures revealed the presence of magnetic anisotropy in the ground state, with the best fit yielding zero-field splitting parameters of D=+0.19 cm⁻¹ and A reaction intended to produce a direct analogue of 1 by employing [Fe(CN)₆]³⁻ in place of [Cr(CN)₆]³⁻ instead gave an unusually complex compound of formula {Fe(CN)₄[CNMn(salen)(MeOH)]₂}{[Mn(salen)(H₂O)]₂}[Mn(salen)(H₂O)(MeOH)]₂[Fe(CN)₆]·4H₂O (2). The crystal structure and magnetic properties of this compound are reported.     

Mn, Cr-co-doped MgAl2O4 phosphors for white LEDs

The Mn-, Cr-doped and Mn, Cr-co-doped MgAl₂O₄ powders have been synthesized via a gel-solid reaction method. Energy transfer from Mn²⁺ to Cr³⁺ has been observed for the first time in the co-doped MgAl₂O₄ phosphors. When excited with blue light with a wavelength of 450 nm at room temperature, both green emission from Mn²⁺ around 520 nm and red emission from Cr³⁺ around 675and 693 nm were generated. Moreover, the color of the emission can be modified by controlling the doping concentrations of Mn²⁺ and Cr³⁺. Therefore, MgAl₂O₄: Mn²⁺, Cr³⁺ could be used as a single-phased phosphor for white LED with a blue LED chip. The energy transfer in terms of Mn²⁺ to Cr³⁺ is determined by means of radiation and reabsorption.     

Magnetic properties of the compounds N(CH3)4MnIIMIII(CN)6 ·8 H2O(MIII = Mn,Fe)

In the isostructural cyanobridged chain compounds N(CH₃)₄Mn^IIM^III(CN)₆ · 8H₂O high spin Mn(II) ions couple antiferromagnetically to low spin Mn(III) of Fe(III) ions. The Mn^II–Mn^III compound orders ferrimagnetically below T_N = 28.5 ± 1 K. The tetragonal a and b axes are easy ones for the magnetic moments. In the Mn^II–Fe^III compound antiferromagnetic order occurs below T_N = 9.3 K, with spins aligned along the tetragonal c axis. The compound undergoes a meta-magnetic transition from the antiferromagnetic to a ferrimagnetic phase. This occurs at 2 K for a field H_crit ≈ 1.2 T. The temperature dependence of H_crit, which vanishes at T_N, is followed. The tricritical temperature T¹ is ～ 5 K.     

Sonochemical synthesis,characterization, thermal and semiconducting behavior of nano-sized azidopentaamminecobalt(III) complexes containing anion,CrO42− or Cr2O72−

New nano-sized cobalt(III) coordination complexes, [Co(NH₃)₅N₃]CrO₄ (1N) and [Co(NH₃)₅N₃]Cr₂O₇ (2N) were synthesized using an innovative sonochemical methodology based on reaction between [Co(NH₃)₅N₃]Cl₂ and potassium salt of CrO₄²⁻ or Cr₂O₇²⁻ in aqueous medium. These complexes were also compared with their respective bulks which were synthesized under identical conditions in the absence of sonicaion. All the complexes were characterized by elemental analysis and spectroscopic techniques (UV–visible and IR). Morphology and particle size of nano-sized complexes was determined by SEM and Zeta-sizer respectively. TGA was used for comparative thermal stability and XRD to identify the phase difference between nano structures and bulk complexes. Furthermore, the electrical property was investigated and all complexes were found to be electrical semiconducting materials and 2N shows better result than others. The single crystals X-ray structure study of new [Co(NH₃)₅N₃]Cr₂O₇ revealed the presence of discrete ions, [Co(NH₃)₅N₃]²⁺ and Cr₂O₇²⁻, crystallizes in monoclinic, space group P_c, with R = 0.0636 in the solid state.     

Transformation of Fe(II)-Fe(III) Hydroxysulphite into Hydroxysulphate Green Rusts

Fe(II)-Fe(III) hydroxysulphite Green Rust 1, GR1(SO₃ ^2?), can be obtained by oxidation of Fe(OH)₂ precipitates in aqueous solution and characterised by X-ray diffraction and Mössbauer spectroscopy. In contrast to other Green Rusts (GRs) which then oxidise directly into ferric oxyhydroxides and magnetite, GR1(SO₃ ^2?) first oxidises into a Green Rust two compound, as identified by X-ray diffraction. Mössbauer analysis reveals that this GR2 is the Fe(II)-Fe(III) hydroxysulphate, GR2(SO₄ ^2?). Such an oxidation process, GR1(SO₃ ^2?) → GR2(SO₄ ^2?) confirms that the average oxidation number of Fe increases according to the chemical formulae previously proposed, [Fe^II ₆Fe^III ₂(OH)₁₆]²⁺[SO₃ · mH₂O]^2?, and [Fe^II ₄Fe^III ₂(OH)₁₂]²⁺[SO₄ · nH₂O]^2? for GR1(SO₃ ^2?) and GR2(SO₄ ^2?) with oxidation numbers of 2.25 and 2.33, respectively. The process implies likely the presence of sulphate ions inherent to the oxidation of sulphite in the solution.     

Theory and experiment: Anion binding property of novel phenanthroline or aromatic bridged compounds

A series of artificial compounds, phenanthroline or aromatic bridged indoline derivatives, have been designed and synthesized. The interaction of these compounds with biologically important anions fluoride (F^?), acetate (AcO^?), dihydrogen phosphate (H₂PO₄^?), chloride (Cl^?), bromide (Br^?) and iodide (I^?) was determined by UV–vis, fluorescene titration and theoretical experiments. Results indicate that compound 1 (Di((1″,2″-dihydro-indol-3″-one-2″-hydrazone-1′-hydrazyl)-2′-methylene)-1,3-did-ehydebenzo) and 2 (Di((1″,2″-dihydro-indol-3″-one-2″-hydrazone-1′-hydrazyl)-2′-methylene)-1,3-didehyde-5-nitrobenzo) containing aromatic bridge do not show binding ability for various anions, and that compound 3 (Di((1″,2″-dihydro-indol-3″-one-2″-hydrazone-1′-hydrazyl)-2′-methylene)-2,9-dial-dehyde-1,10-phenanthroline) containing phenanthroline bridge shows the strongest binding ability for F^? among various anions, the moderate binding ability for AcO^? and H₂PO₄^?, and almost no binding ability for Cl^?, Br^?, I^?. The different binding ability of aromatic and phenanthroline bridged compounds may be related to the conjugative effect. What's more, the binding ability of compound 3 with F^? is not interfered by the existence of other anions. Hence, theoretical investigations explore the reasons of different binding ability between compound 3 and anions.