Optical and magneto-optical properties of the endohedral La@C82 crystal

The magneto-optical properties of solids are theoretically described by the circular dichroism (CD) and birefringence coefficient (θ). Using the Su-Schrieffer-Heeger (SSH) model in conjunction with the local field method, the optical dielectric tensor, CD and θ of the simple cubic phase of the La@C₈₂ and C₈₂ crystals are calculated. The results obtained from the La@C₈₂ and C₈₂ crystals are compared with those of the C₇₀ and C₆₀ crystals. It is shown that La@C₈₂ has a richer optical spectrum than C₈₂, C₇₀ and C₆₀. In the La@C₈₂ crystal, absorption bands are shown by spectrum in the 0.58 to 7.0?eV region with sharp structures in each band which indicate the localized molecular structure coupled with long-range crystalline order. Results show that the circular dichroism and birefringence coefficient of the La@C₈₂ crystal due to a single spin localized on the C₈₂ cage are very larger than those of the C₆₀, C₇₀ and C₈₂ crystals. Also, results show that the circular dichroism and birefringence coefficient of the C₈₂ crystal are similar to those of the C₆₀ and C₇₀ crystals.     

A femtosecond laser study of the endohedral fullerenes Li@C60 and La@C82

The endohedral fullerenes La@C₈₂ and Li@C₆₀ have been studied in the gas phase with femtosecond laser excitation. The two molecules show qualitatively the same behaviour with respect to ion yield vs. pulse energy, but markedly different fragmentation patterns, with La@C₈₂ fragmenting via the shrink-wrap mechanism and Li@C₆₀ predominantly losing the metal atom in the first fragmentation step. The ion yields and electron energy distributions of La@C₈₂ agree well with a recently developed model for fs laser ionisation of C₆₀.     

EPR study of topoisomers of La@C82 and La@C90

An analysis of the temperature dependence of the line width variation of topoisomers (I) and (II) of La@C₈₂ revealed that both molecules exhibit practically identical rotational correlation times from room temperature down to the freezing point of CS₂. Although this result indicates confinement of the metal ion, from the increased line width of¹³C satellites in isomer (II) we conclude that in this molecule there might be increased mobility within a confine volume. Three stable topoisomers of La@C₉₀ were identified for the first time. One of these topoisomers was separated by HPLC technique. We were unable, however, to detect any nuclear spin dependence of the EPR line width even at low temperatures. This might indicate that in this case the ion is no longer confined to a specific binding site, although a more trivial explanation assuming small anisotropies of all hyperfine interactions cannot yet be ruled out.     

Impregnation of Polycarbonate by Paramagnetic Probe 2,2,6,6-Tetramethyl-4-Hydroxy-Piperidine-1-Oxyl (TEMPOL) in Supercritical CO<Subscript>2</Subscript>

The aim of the research was to test the advantages of spin probe electron paramagnetic resonance approach in studying polymers impregnation with organic molecules in supercritical CO₂ (scCO₂) The impregnation of bisphenol A polycarbonate with the spin probe TEMPOL was carried out at 307–343 K and 11.6–35 MPa. The mean and local concentrations of the spin probe in the polymer were evaluated. An increase in temperature and pressure resulted in a more even distribution of the dopant in the polymer matrix. It was observed that, at 307 K and 19.6 MPa, the spin probe was located only near the surface of the sample. Local mobility of the spin probe molecules was found to be similar in polycarbonate films impregnated in scCO₂ and cast from dichloroethane solution. It was shown that changes in the structure of the surface and bulk of the polymer detected by the atomic force and optical polarization microscopy are not directly related with the distribution of the dopant molecules and their average content in the polymer.     

Synthesis of transparent BaTiO<Subscript>3</Subscript> nanoparticle/polymer hybrid

Transparent BaTiO₃ nanoparticle/polymer hybrid was synthesized by polymerization and hydrolysis of barium titanium alkoxide modified with 2-vinyloxyethoxy ligand. Barium alkoxide, titanium alkoxide and 2-vinyloxyethanol were reacted affording a BaTiO₃ precursor, which was then hydrolyzed and polymerized to form BaTiO₃ particle/polymer hybrids below 100°C. BaTiO₃ particles increased in crystallinity with increasing water amount for hydrolysis. The absorption edge of the hybrid film on silica plates shifted to shorter wavelength with decreasing crystallite size. Nano-sized BaTiO₃ particle/polymer hybrid polymerized with methyl methacrylate (MMA) was shaped into a transparent and self-standing film with a refractive index of 1.595 at 589 nm.     

Nature of EPR line asymmetry in La<Subscript>0.70</Subscript>Ca<Subscript>0.25</Subscript>Ba<Subscript>0.05</Subscript>MnO<Subscript>3</Subscript>

It was pointed out in some works that asymmetry of an electron paramagnetic resonance (EPR) line is generally caused by both the electrical conduction and the nondiagonal elements of the dynamic susceptibility of a magnetic subsystem. Direct measurements of the temperature dependences of the conductivity and the EPR line shape in a La_0.70Ca_0.25Ba_0.05MnO₃ sample showed that the conduction makes the predominant contribution to the EPR line asymmetry.     

Fullerene C<Subscript>70</Subscript> Triplet Zero-Field Splitting Parameters Revisited by Light-Induced EPR at Thermal Equilibrium

Continuous-wave (CW) electron paramagnetic resonance (EPR) and echo-detected (ED) EPR spectra of triplet state of fullerene C₇₀ in molecular glasses of decalin, o-terphenyl and toluene, and in polymethylmethacrylate polymer were obtained under continuous light illumination. Temperature was high enough so that the EPR spectra corresponded to thermal equilibrium between the spin sublevels. The comparison of CW EPR and ED EPR data has shown that pseudorotation in the ³C₇₀ frame does not remarkably affect deriving the zero-field splitting (ZFS) D and E parameters from the EPR spectra. ³C₇₀ EPR spectra were simulated at 77 K fairly well using the distribution of the ZFS D and E parameters. These distributions may be caused by the inhomogeneity of the glassy matrix surrounding, which affects the Jahn–Teller distortions of ³C₇₀ molecules (D-strain and E-strain).     

Impregnation of polymers with 2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl (TEMPONE) paramagnetic probe in sub- and supercritical CO<Subscript>2</Subscript>

The spin probe method is used to study the impregnation of polycarbonate (PC) based on bisphenol A, polyethylene oxide (PEO), and crosslinked acrylamide–acrylic acid copolymer (PAA) with organic molecules in sub- and supercritical CO₂ media. Electron spin resonance (EPR) data show that, at 196 bar and 307 K, 2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl (TEMPONE) paramagnetic spin probe molecules penetrate into the PC and PEO matrices, which are, respectively, in the glassy and elastic states under normal conditions. The degree of impregnation of PAA under these conditions is negligibly small. Estimates of the local concentration of probe molecules show that, in the PEO matrix, TEMPONE is distributed much more uniformly than in the PC matrix. Analysis of the effect of temperature on the shape of the EPR spectra of the radical in the polymer matrix shows that, under the same conditions, the mobility of TEMPONE molecules in the PEO matrix is much higher than in the PC matrix. The results suggest that the spin probe method is promising for studying the characteristics of macro- and micro-processes in polymer–supercritical fluid solvent–organic molecule ternary systems.     

Grain Boundary Diffusion in C<Subscript>60</Subscript> Thin Films

This paper focuses on the effect of grain boundaries on the diffusion processes in polycrystalline C₆₀ thin films. Electrically induced diffusion of Au was investigated by in situ measurements of the film conductivity. Electron Paramagnetic Resonance (EPR) spectroscopy was used to study diffusion of oxygen. Increase in grain sizes in polycrystalline C₆₀ thin films was found to result in the acceleration of gold and oxygen diffusion. The results are interpreted assuming that these impurities diffuse in C₆₀ films dominantly along grain boundaries.     

Detection of vortex excitations in a Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10</Subscript> crystal at temperatures above the critical temperature by EPR of the surface layer

Vortex excitations have been detected at temperatures both below and above the critical temperature when investigating local magnetic fields on the surface of a Bi₂Sr₂Ca₂Cu₃O₁₀ single crystal by means of an electron paramagnetic resonance (EPR) probe. A thin layer of a diphenyl picrylhydrazyl organic radical deposited on the crystal surface is used as the EPR probe. A narrow EPR signal makes it possible to detect weak distortions of the magnetic field appearing at T ≿ T _c. The analysis of the temperature dependences of the resonance field and the EPR linewidth is thebasis of the assumption of the vortex nature of magnetic excitations in this temperature range.     

An Electron Paramagnetic Resonance Study of the Orbital Ordering Compound MgTi<Subscript>2</Subscript>O<Subscript>4</Subscript>

We report the electron paramagnetic resonance (EPR) studies of MgTi₂O₄ in the 300–140 K range. Above the transition temperature T _t (~258 K), the EPR results indicate that MgTi₂O₄ is paramagnetic. The parameters of the EPR spectra show an anomalous change at T _t. The clear EPR lines can be observed in temperature between T _t and 220 K. Besides that the EPR intensity, g value, and EPR linewidth increase with decreasing temperature; in temperature range below 220 K, no clear EPR line can be detected. The EPR spectra results demonstrate that magnetic spin-singlet state and the orbital density wave of MgTi₂O₄ system are formed gradually with decreasing temperature at low temperature range.     

EPR Monitoring of BaTiO<Subscript>3</Subscript> Formation

BaCO₃ and anatase-type TiO₂ were adopted as initial materials to prepare BaTiO₃ powder by the solid-state reaction method at a heating rate of 350°C/h. The electron paramagnetic resonance (EPR) technique was employed to monitor the formation of BaTiO₃. TiO₂ showed a series of complicated EPR signals associated primarily with Fe impurities. The formation of BaTiO₃ can be monitored in terms of the evolution of EPR signals associated with Fe impurities with calcination and measurement temperatures. The activation of the g = 2.004 signal above the Curie point of BaTiO₃ and the disappearance of the other EPR signals in the BaCO₃/TiO₂ mixture at room temperature are characteristic of the formation of BaTiO₃.     

Electron paramagnetic resonance in a Gd<Subscript>0.14</Subscript>Si<Subscript>0.86</Subscript> amorphous film: Bottleneck regime

Electron paramagnetic resonance (EPR) in a Gd_0.14Si_0.86 amorphous film is studied over a wide temperature range from 4 to 300 K. The experimental results are analyzed with regard to the strong structural disorder in the system under study. This disorder leads to the formation of droplets, that is, regions with a high density of electronic states. It is shown that the observed EPR signal can be formed only in the double bottleneck regime, and temperature dependences are obtained for the line position and width. The spin-lattice relaxation rates for electrons and Gd ions, the second spectral moment of the line, the ferromagnetic transition temperature, the number of Gd atoms in the droplets, and the product of the electron density of states by the exchange coupling constant between electrons and Gd ions are evaluated from comparison with experimental data. The values obtained corroborate the validity of the assumptions that the double bottleneck conditions are fulfilled and structural and phase nanoscale inhomogeneities exist in the system.     

Investigation on structural characteristics of PVDF–AgCF<Subscript>3</Subscript>SO<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposite solid polymer electrolyte system

There has been an increasing interest towards the incorporation of nanosize ceramic fillers in polymer electrolytes. Solid polymer electrolytes based on polyvinylidene fluoride (PVDF), silver triflate (AgCF₃SO₃), and x wt% of aluminum oxide (Al₂O₃) nanopowders (where x = 1, 3, 5, and 10, respectively) have been prepared using solution casting technique. The structural characteristics of these thin film specimens were studied using Fourier transform infrared (FTIR) and X-ray diffraction (XRD) patterns at room temperature. The appearance of new absorption bands and gradual shifts observed in some characteristic peaks confirmed the complex formation between polyvinylidene fluoride and silver triflate. Furthermore, the addition of nanosized filler Al₂O₃ has also indicated the interaction of the filler with the polymer salt complex. The XRD patterns obtained for all these samples in the 2θ range 10° to 70° showed the amorphous nature of these samples. Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, December 7–9, 2006.     

Electron spin relaxation studies of La@C82 in the solid state by EPR

The temperature dependence of EPR spectrum of La@C₈₂ in the powder of empty C₆₀ and C₇₀ mixed crystals was studied by EPR spectroscopy employing X- and Q-band microwave frequencies. The rigid limit spectra (at 4.2 K for the X-band and at 132 K for the Q-band) could be analyzed by static spectral simulation which yielded the EPR parameters,g _‖=2.0021,g _⊥=1.9970,^La A _⊥=7.8 MHz,^La A _‖～0 MHz and an isotropic¹³C coupling value of about 3 MHz. For higher temperatures an appreciable motional averaging effect was observed and the spectra were analyzed by using dynamic spectral simulation based on the stochastic Liouville equation, where we assumed an isotropic rotational motion with the Brownian diffusion. The calculated spectra reproduced the dominant feature of the temperature dependence of the spectra almost satisfactorily for both the X-and Q-band frequencies with the appropriate rotational correlation times. The Arrhenius plots of the correlation time gave two activation energies of 0.9 kcal/mol and 2.9–3.8 kcal/mol for the temperatures below and above 200 K, respectively.     

Magnetic properties of the tetranitrosyl-iron complex Fe<Subscript>2</Subscript>(SC<Subscript>3</Subscript>H<Subscript>5</Subscript>N<Subscript>2</Subscript>)<Subscript>2</Subscript>(NO)<Subscript>4</Subscript>

The magnetic properties of the binuclear nitrosyl-iron complexes Fe₂(SC₃H₅N₂)₂(NO)₄ are investigated. It is demonstrated that several types of particles, such as dimers with a pair of spins 1/2, dimers with a pair of spins 5/2, and paramagnetic particles with spin 3/2, make a contribution to the magnetic properties of the complexes. A decrease in the temperature below 25 K leads to a change in the shape of the EPR spectra corresponding to these dimers, so that Lorentzian lines (homogeneous broadening) transform into Gaussian lines (inhomogeneous broadening). This is accompanied by a stepwise change in the EPR line width and g factors. The change in the line shape indicates that complexes become asymmetric at low temperatures, possibly, due to the decrease in the spin exchange frequency below the frequency of the microwave field of the spectrometer.     

Photoaccumulating film systems based on TiO<Subscript>2</Subscript>/MoO<Subscript>3</Subscript> and TiO<Subscript>2</Subscript>/MoO<Subscript>3</Subscript>:V<Subscript>2</Subscript>O<Subscript>5</Subscript> nanoheterostructures

The thin-film photocatalysts TiO₂/MoO₃ and TiO₂/MoO₃:V₂O₅ obtained by a combination of sol–gel and sintering techniques were studied using the photooxidation of probing dyes, EPR spectroscopy, X-ray diffraction analysis, and electron microscopy. It was shown that due to charge accumulation caused by UV irradiation, these photocatalysts retain their oxidative activity and ability for self-sterilization in the dark for a long time after irradiation was terminated (up to 5 h for TiO₂/MoO₃:V₂O₅).     

Spin-spin interaction and the EPR spectrum of KDy(WO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The EPR spectrum of a KDy(WO₄)₂ monoclinic crystal is investigated. It is found that the EPR spectrum of magnetically concentrated materials at a low frequency (9.2 GHz) undergoes a substantial transformation in addition to the well-known broadening of the EPR lines. At low Dy³⁺ concentrations (x<10^?2), the EPR spectrum of an isomorphic crystal, namely, KY_(1?x)Dy_x(WO₄)₂, is characterized by the parameters g_x=0, g_y=1.54, and g_z=14.6. For a magnetically concentrated crystal KDy(WO₄)₂, the g values are as follows: g_x=0, g_y=0.82, and g_z=2.52. It is demonstrated that the difference in the parameters is associated with the specific spin-spin interaction between Dy³⁺ ions, including the Dzyaloshinski interaction, which is not observed at high frequencies.     

The Formation of Dimeric Centers of Copper,Iron and Gadolinium in Modified Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript>

It is discovered that the electron paramagnetic resonance (EPR) spectrum of the doubly charged copper centers occurs in single crystals of Pb₅Ge₃O₁₁ doped with gadolinium or iron after annealing in an atmosphere containing chlorine and bromine. Similar annealing of the crystals doped with copper in a chlorine and fluorine atmosphere leads to redistribution of the intensities of the EPR spectra of two types of Cu²⁺ centers. The influence of annealing on the ongoing intensity of spectra of the dimeric triclinic centers Fe³⁺–A, Gd³⁺–A (where A represent Cl^?, Br^?, O^2?, F^?) was the subject of this research. Consideration is given to the mechanisms for changing the charge state and association of copper center with defects.     

Influence of K<Subscript>2</Subscript>Ti<Subscript>6</Subscript>O<Subscript>13</Subscript> on dielectric and barrier properties of polymer

Polymer composite comprising polyvinylidene fluoride (PVDF) and potassium hexatitante (K₂Ti₆O₁₃) was synthesized by solution casting. The effect of K₂Ti₆O₁₃ on surface, thermal, and electrical properties of polymer composite were investigated. The addition of K₂Ti₆O₁₃ with polymer leads to thermal degradation and transition of polymer composite from semi-crystalline to amorphous phase. The optimum results of contact angle for different loading wt% of K₂Ti₆O₁₃ were directly correlated with the surface morphology. Our experimental results confirmed the incorporation of K₂Ti₆O₁₃ in polymer by SEM micrographs. The evaluated dielectric properties (ε' = 424; tan δ = 2.14 at 130 °C and 100 Hz frequency for 20 wt% loading of K₂Ti₆O₁₃) for polymer composite are higher in compared to pure polymer. The enhancement in dielectric constant and changing the surface properties of polymer composite can be used for the development of electrochemical storage device applications.