Crystal structure and ferromagnetism of (n-C3H7)4N[CoFe(dto)3] (dto=C2O2S2)

Recently, we have discovered a new type of first order phase transition around 120 K for (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] (dto=C₂O₂S₂), where the charge transfer transition between Fe^II and Fe^III occurs reversibly. In order to elucidate the origin of this peculiar first order phase transition. Detailed information about the crystal structure is indispensable. We have synthesized the single crystal of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃] whose crystal structure is isomorphous to that of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃], and determined its detailed crystal structure. Crystal data: space group P6₃, a=b=10.044(2) Å, c=15.960(6) Å, α=β=90°, γ=120°, Z=2 (C₁₈H₂₈NS₆O₆FeCo). In this complex, we found a ferromagnetic transition at T_c=3.5 K. Moreover, on the basis of the crystal data of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃], we determined the crystal structure of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] by simulation of powder X-ray diffraction results.     

Optical absorption, EPR, infrared and Raman spectral studies of clinochlore mineral

Optical absorption, EPR, Infrared and Raman spectral studies have been carried out on natural clinochlore mineral. The optical absorption spectrum exhibits bands characteristic of Fe²⁺ and Fe³⁺ ions. A band observed in the NIR region is attributed to an intervalence charge transfer (Fe²⁺-Fe³⁺) band. The room temperature EPR spectrum of single crystal of clinochlore mineral reveals the dominance of Fe³⁺ ion exhibiting resonance signals at g=2.66; 3.68 and 4.31 besides one isotropic resonance signal at g=2.0. The EPR studies have been carried out for a polycrystalline sample in the temperature range from 103 to 443 K and for a single crystal of clinochlore mineral in the temperature range 123-297 K. The number of spins (N) participating in resonance at g=4.3 signal of the single crystal of clinochlore mineral has been calculated at different temperatures. The paramagnetic susceptibility (χ) is calculated from the EPR data at different temperatures for single crystal of clinochlore mineral. The Curie constant and Curie temperature values are evaluated from 1/χ versus T graph. The infrared spectral studies reveal the formation of Fe³⁺-OH complexes due to the presence of higher amount of iron in this mineral. The Raman spectrum exhibits bands characteristic of Si-O-Si stretching and Si-O bending modes.     

Laser frequency stabilization at 1.5 microns using ultranarrow inhomogeneous absorption profiles in Er:LiYF4

Single-frequency diode lasers have been frequency stabilized to 1.5 kHz Allan deviation over 0.05-50 s integration times, with laser frequency drift reduced to less than 1.4 kHz/min, using the frequency reference provided by an ultranarrow inhomogeneously broadened Er³⁺:⁴I_15/2→⁴I_13/2 optical absorption transition at a vacuum wavelength of 1530.40 nm in a low-strain LiYF₄ crystal. The 130 MHz full-width at half-maximum (FWHM) inhomogeneous line width of this reference transition is the narrowest reported for a solid at 1.5 μm. Strain-induced inhomogeneous broadening was reduced by using the single isotope ⁷Li and by the very similar radii of Er³⁺ and the Y³⁺ ions for which it substitutes. To show the practicability of cryogen-free cooling, this laser stability was obtained with the reference crystal at 5 K; moreover, this performance did not require vibrational isolation of either the laser or crystal frequency reference. Stabilization is feasible up to T=25 K where the Er³⁺ absorption thermally broadens to ∼500 MHz. This stabilized laser system provides a tool for interferometry, high-resolution spectroscopy, real-time optical signal processing based on spatial spectral holography and accumulated photon echoes, secondary frequency standards, and other applications such as quantum information science requiring narrow-band light sources or coherent detection.     

Mössbauer study of Na0.82Co0.99Fe0.01O2

We studied by Mössbauer spectroscopy the Na_0.82CoO₂ compound using 1% ⁵⁷Fe as a local probe which substitutes for the Co ions. Mössbauer spectra at T=300 K revealed two sites which correspond to Fe³⁺ and Fe⁴⁺. The existence of two distinct values of the quadrupole splitting instead of a continuous distribution should be related with the charge ordering of Co⁺³, Co⁺⁴ ions and ion ordering of Na(1) and Na(2). Below T=10 K part of the spectrum area, corresponding to Fe⁴⁺ and all of Fe³⁺, displays broad magnetically split spectra arising either from short-range magnetic correlations or from slow electronic spin relaxation.     

Crystal structure and structural transition caused by charge-transfer phase transition for iron mixed-valence complex (n-C3H7)4N[FeFe(dto)3] (dto=C2O2S2)

(n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] shows a new type of first order phase transition called charge-transfer phase transition around 120 K, where the charge transfer between Fe^II and Fe^III occurs reversibly. Recently, we have succeeded in obtaining single crystals of the title complex and determined the crystal structure at room temperature. Crystal data: space group P6₃, Z=2. Moreover, we have investigated the structural transition caused by the charge-transfer phase transition by means of powder X-ray diffraction measurement. When the temperature is decreased, the a-axis, which corresponds to the hexagonal ring size in two-dimensional honeycomb network structure of [Fe^IIFe^III(dto)₃]_∞, contracts by 0.1 Å at the charge-transfer transition temperature (T_CT), while the c-axis, perpendicular to the honeycomb network layer, elongates by 0.1 Å at T_CT. Consequently, when the temperature is decreased, the unit cell volume decreases without noticeable anomaly around T_CT, which is responsible for the quite small vibrational contribution to the entropy change, compared with usual spin crossover transition. Thus, the charge-transfer phase transition around 120 K for (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] is regarded as spin entropy driven phase transition.     

Structural and electrical transport properties of ZnxFe3−xO4 thin film deposited on Si (1 1 1) by pulsed-laser deposition

Polycrystalline thin films of Fe_3−xZn_xO₄ (x = 0.0, 0.01 and 0.02) were prepared by pulsed-laser deposition technique on Si (1 1 1) substrate. X-ray diffraction studies of parent as well as Zn doped magnetite show the spinel cubic structure of film with (1 1 1) orientation. The order–disorder transition temperature for Fe₃O₄ thin film with thickness of 150 nm are at 123 K (Si). Zn doping leads to enhancement of resistivity by Zn²⁺ substitution originates from a decrease of the carrier concentration, which do not show the Verwey transition. The Raman spectra for parent Fe₃O₄ on Si (1 1 1) substrate shows all Raman active modes for thin films at energies of T_2g¹, T_2g³, T_2g², and A_1g at 193, 304, 531 and 668 cm⁻¹. It is noticed that the frequency positions of the strongest A_1g mode are at 668.3 cm⁻¹, for all parent Fe₃O₄ thin film shifted at lower wave number as 663.7 for Fe_2.98Zn_0.02O₄ thin film on Si (1 1 1) substrate. The integral intensity at 668 cm⁻¹ increased significantly with decreasing doping concentration and highest for the parent sample, which is due to residual stress stored in the surface.     

An alternative interpretation of the optical spectra for Fe-doped KTaO3 crystals

In this paper, the excitation spectrum and luminescence at 14 569, 17 225, 18 829 and 14 659 cm^-1 for Fe³⁺ ion at the K⁺ site of KTaO₃ crystals are assigned, respectively, to the ⁶A₁(S)→⁴T₁(G), ⁴T₂(G), ⁴E₁(G)[⁴A₁(G)] and ⁴T₁(G)→⁶A₁(S) transitions rather than to the ⁶A₁(S)→⁴T₁(G), ²T₂(I), ⁴T₂(G) and ⁴T₁(G)→⁶A₁(S) transitions given in a previous paper [Bryknar et al., Radiat. Eff. Def. Solids 149(1999)51]. On the basis of this assignment, the reasonable optical spectrum parameters (in particular, the cubic field parameter Dq≈−640 cm⁻¹) are obtained. The validity of this assignment is discussed.     

Effect of thermal treatment in vacuum on Fe-doped SnO2 powders

A sample of 10 at% Fe-doped SnO₂ powder was prepared by mechanical alloying and then thermally treated at 773 K in vacuum. The fit of the diffraction patterns and X-ray absorption spectroscopy measurements revealed that the as milled sample was pure doped rutile. Fe dissolved into SnO₂ was found in Fe²⁺/Fe³⁺ ionic valence with mainly paramagnetic behavior. After the thermal treatment all techniques indicate the formation of the ternary Sn_0.36Fe_2.64O₄ spinel phase, which is responsible for the observed ferromagnetism.     

Magnetic,electrical transport and electron spin resonance studies of Fe-doped manganite LaMn0.7Fe0.3O3+δ

We have investigated the magnetic, electrical transport and electron spin resonance (ESR) properties of polycrystalline Fe-doped manganite LaMn_0.7Fe_0.3O_3+δ prepared by sol–gel method. A typical cluster-glass feature is presented by DC magnetization and AC susceptibility measurements and a sharp but shallow memory effect was observed. Symmetrical Lorentzian lines of the Mn/Fe spectra were detected above 120 K, where the sample is a paramagnetic (PM) insulator. When the temperature decreases from 120 K, magnetic clusters contributed from ferromagnetic (FM) interaction between Mn³⁺ and Mn³⁺/Fe³⁺ ions develop and coexist with PM phase. At lower temperature, these FM clusters compete with antiferromagnetic (AFM) ones between Fe³⁺ ions, which are associated with a distinct field-cooled (FC) effect in characteristic of cluster-glass state.     

Optical and electrical characterization of cadmium selenide doped with cobalt

The optical and electrical properties of Co²⁺ ions in CdSe have been investigated. Absorption, photoluminescence, electron paramagnetic resonance, and Hall measurements were used to characterize a cobalt-doped (1×10¹⁹ cm⁻³) single crystal. Infrared absorption and emission spectra associated with transitions between the ⁴A₂(F) ground state and the ⁴T₁(F) and ⁴T₂(F) excited states are described. At 10 K, spin-orbit splittings result in three structured absorption bands associated with the ⁴A₂(F) to ⁴T₁(F) transition having zero-phonon lines at 4926, 5101, and 5724 cm⁻¹. The ⁴A₂(F) to ⁴T₂(F) transition shows two zero-phonon lines at 2874 and 3286 cm⁻¹, also accompanied by vibronic structure. Intrinsic lattice modes explain most of the sharp-line structure observed at low temperature, except for a subset of peaks where local modes (25-30 cm⁻¹) are invoked. Using below-band-gap light, selective excitation allows detection of the ⁴T₁(F) to ⁴A₂(F) recombination at liquid-helium temperatures. The activation of free carriers in our n-type material containing shallow donors is affected by the presence of cobalt, and we find the Co^+/++ level to be about 34 meV below the conduction band of CdSe.     

Very high field magnetization and AC susceptibility of native horse spleen ferritin

The magnetization of native horse spleen ferritin protein is measured in pulsed magnetic fields to 55 T at T=1.52 K. The magnetization rises smoothly with negative curvature due to uncompensated Fe³⁺ spins and with a large high field slope due to the underlying antiferromagnetic ferritin core. Even at highest fields the magnetic moment is only ∼4% of the saturation moment of the full complement of Fe³⁺ in the ferritin molecule. The AC magnetic susceptibility, χ_AC(T,f), responding to the uncompensated spins, reaches a maximum near the superparamagnetic blocking temperature with the temperature of the maximum, T_M, varying with excitation frequency, T_M⁻¹ α log f for 10?f?10⁴ Hz.     

Antiferromagnetism and spin-glass transition in the FeInxCr2−xSe4 series of selenides

The magnetic behavior of the FeIn_xCr_2−xSe₄ system (with x=0.0, 0.2 and 0.4) has been investigated by magnetic and Mössbauer spectroscopy. Hyperfine parameters indicate that iron is in the Fe²⁺ oxidation state, with a minor (∼9%) Fe³⁺ fraction, located at different layers in the structure. Low-field magnetization curves as a function of temperature showed that the antiferromagnetic (AFM) order temperature is T_N=208(2) K for FeCr₂Se₄ and decreases to 174(3) K for FeIn_0.4Cr_1.6Se₄. The effective magnetic moment μ_eff decreases with increasing In contents, and shows agreement with the expected values from the contribution of Fe²⁺ (⁵D) and Cr³⁺ (⁴F) electronic states. A second, low-temperature transition is observed at T_G∼13 K, which has been assigned to the onset of a glassy state.     

Crystal structure,NMR study,dielectric relaxation and AC conductivity of a new compound [Cd3(SCN)2Br6(C2H9N2)2]n

The crystal structure, the ¹³C NMR spectroscopy and the complex impedance have been carried out on [Cd₃(SCN)₂Br₆(C₂H₉N₂)₂]_n. Crystal structure shows a 2D polymeric network built up of two crystallographically independent cadmium atoms with two different octahedral coordinations. This compound exhibits a phase transition at (T=355±2 K) which has been characterized by differential scanning calorimetry (DSC), X-rays powder diffraction, AC conductivity and dielectric measurements. Examination of ¹³C CP/MAS line shapes shows indirect spin–spin coupling (¹⁴N and ¹³C) with a dipolar coupling constant of 1339 Hz. The AC conductivity of this compound has been carried out in the temperature range 325–376 K and the frequency range from 10⁻² Hz to 10 MHz. The impedance data were well fitted to two equivalent electrical circuits. The results of the modulus study reveal the presence of two distinct relaxation processes. One, at low frequency side, is thermally activated due to the ionic conduction of the crystal and the other, at higher frequency side, gradually disappears when temperature reaches 355 K which is attributed to the localized dipoles in the crystal. Moreover, the temperature dependence of DC-conductivity in both phases follows the Arrhenius law and the frequency dependence of σ(ω,T) follows Jonscher's universal law. The near values of activation energies obtained from the conductivity data and impedance confirm that the transport is through the ion hopping mechanism.     

Phase transitions and molecular motions in [Zn(NH3)4](BF4)2 studied by nuclear magnetic resonance, infrared and Raman spectroscopy

Middle infrared absorption, Raman scattering and proton magnetic resonance relaxation measurements were performed for [Zn(NH₃)₄](BF₄) in order to establish relationship between the observed phase transitions and reorientational motions of the NH₃ ligands and BF₄⁻ anions. The temperature dependence of spin-lattice relaxation time (T₁(¹H)) and of the full width at half maximum (FWHM) of the bands connected with ρ_r(NH₃), ν₂(BF₄) and ν₄(BF₄) modes in the infrared and in the Raman spectra have shown that in the high temperature phase of [Zn(NH₃)₄](BF₄)₂ all molecular groups perform the following stochastic reorientational motions: fast (τ_R≈10⁻¹² s) 120° flips of NH₃ ligands about three-fold axis, fast isotropic reorientation of BF₄⁻ anions and slow (τ_R≈10⁻⁴ s) isotropic reorientation (“tumbling”) of the whole [Zn(NH₃)₄]²⁺ cation. Mean values of the activation energies for uniaxial reorientation of NH₃ and isotropic reorientation of BF₄⁻ at phases I and II are ca. 3 kJ mol⁻¹ and ca. 5 kJ mol⁻¹, respectively. At phases III and IV the activation energies values for uniaxial reorientation of both NH₃ and of BF₄⁻ equal to ca. 7 kJ mol⁻¹. Nearly the same values of the activation energies, as well as of the reorientational correlation times, at phases III and IV well explain existence of the coupling between reorientational motions of NH₃ and BF₄⁻. Splitting some of the infrared bands at T_C2=117 K suggests reducing of crystal symmetry at this phase transition. Sudden narrowing of the bands connected with ν₂(BF₄), ν₄(BF₄) and ρ_r(NH₃) modes at T_C3=101 K implies slowing down (τ_R?10⁻¹⁰ s) of the fast uniaxial reorientational motions of the BF₄⁻ anions and NH₃ ligands at this phase transition.     

EPR and optical absorption studies of Fe ions in sodium borophosphate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectral investigations have been carried out on Fe³⁺ ions doped sodium borophosphate glasses (NaH₂PO₄-B₂O₃-Fe₂O₃). The EPR spectra exhibit resonance signals with effective g values at g=2.02, g=4.2 and g=6.4. The resonance signal at g=4.2 is due to isolated Fe³⁺ ions in site with rhombic symmetry whereas the g=2.02 resonance is due to Fe³⁺ ions coupled by exchange interaction in a distorted octahedral environment. The EPR spectra at different temperatures (123-295 K) have also been studied. The intensity of the resonance signals decreases with increase in temperature whereas linewidth is found to be independent of temperature. The paramagnetic susceptibility (χ) was calculated from the EPR data at various temperatures and the Curie constant (C) and paramagnetic Curie temperature (θ_p) have been evaluated from the 1/χ versus T graph. The optical absorption spectrum exhibits bands characteristic of Fe³⁺ ions in octahedral symmetry. The crystal field parameter (Dq) and the Racah interelectronic repulsion parameters (B and C) have also been evaluated and discussed.     

Mössbauer spectroscopic studies of the disintegration of hexavalent iron compounds (BaFeO4 and K2FeO4)

Mössbauer spectroscopic studies of BaFeO₄ and K₂FeO₄ as prepared, then either sealed, or exposed to air, or exposed to moist air for a period up to more than one year, were performed at room temperature as a function of time. Some of the samples were studied as a function of temperature down to 4.2 K. K₂FeO₄ and BaFeO₄ after preparation, exhibit a pure Fe⁶⁺ spectrum. K₂FeO₄ shows low stability. After a period of 14 months in a sealed sample holder, the spectrum exhibits 83% noncrystalline Fe³⁺, as Fe₂O₃ nanoparticles, and only 17% of the original Fe⁶⁺. BaFeO₄ sealed, or exposed to dry air disintegrates slowly, exhibiting a spectrum composed of three subspectra. In addition to the original Fe⁶⁺ and final Fe³⁺ subspectra, a subspectrum, of an intermediate stage of a crystalline Fe⁴⁺ system, is present. In the first month the increase of the Fe³⁺ subspectrum is 15%, and that of the Fe⁴⁺ is 8%. BaFeO₄ exposed to moist air, disintegrates at a very fast rate. The Fe³⁺ subspectrum, due to Fe₂O₃ nanoparticles, increases in the first days at the rapid rate of ∼13%/day, and there is no evidence for Fe⁴⁺ in the spectrum. The Fe⁶⁺ in BaFeO₄, Fe³⁺ and Fe⁴⁺ in the disintegrated systems are all magnetically ordered at 4.2 K. Above 90 K the Fe³⁺ subspectra exhibit a superposition of a paramagnetic doublet and a diffuse magnetic sextet, with relative intensities changing with temperature, and changing from sample to sample according to their blocking temperatures, which are determined by the distribution in size of the nanoparticles.     

YGG:Cr3+晶体的光谱特性

本文通过实验研究了YGG:Cr³⁺晶体的光谱特性,报道了室温下的吸收谱,10,133,300K的荧光谱,以及荧光寿命、无辐射跃迁几率、辐射量子效率与温度之间的依赖关系。从吸收谱及荧光谱中确定在C_3i(S₆)低对称场微扰下,Cr³⁺离子在基质YGG中²T₁能级分裂的子能级及基态⁺A₂→²E零声子跃迁R线的位置。 关键词：     

Magnetic phases in lanthanum-strontium manganite-cobaltite La1.25Sr0.75MnCoO6

Two methods—the solid-phase high-temperature (1300 °C) and the liquid-phase low-temperature (750 °C) routes—were used to synthesize the complex oxide La_1.25Sr_0.75MnCoO₆, which has the structure of rhombohedral perovskite and is characterized by a disordered distribution of Mn and Co in structural sites. It was found by means of X-ray absorption near edge spectroscopy (XANES) at the K-edge that mixed valence states of Co²⁺/Co³⁺ and Mn³⁺/Mn⁴⁺, exist in both phases. Measurements of dc magnetization and real (χ′) and imaginary (χ″) parts of the ac susceptibility showed that the magnetic properties of these oxides are determined by a ferromagnetic transition at T_C=217 K and a frequency-dependent transition at T_g<100 K. The high frequency dependence of T_g is indicative of the cluster-glass behavior of La_1.25Sr_0.75MnCoO₆ (7 5 0) at T<T_C within the ferromagnetic state.     

Charge order melting and magnetic transition in Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 system

The magnetic property of double doped manganite Nd_0.5(1+x)Ca_0.5(1−x)Mn_(1−x)Cr_xO₃ with a fixed ratio of Mn³⁺:Mn⁴⁺=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr³⁺ for Mn³⁺ ions causes the increase of magnetization and the rise of T_c. As the Cr³⁺ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn³⁺ and Cr³⁺ ions causes a magnetic upturn, namely, the second FM phase transition.     

Electron hopping mechanism in hematite (α-Fe2O3)

The frequency dependence of the real (?′) and imaginary (?″) parts of the dielectric constant of polycrystalline hematite (α-Fe₂O₃) has been investigated in the frequency range 0-100 kHz and the temperature range 190-350 K, in order to reveal experimentally the electron hopping mechanism that takes place during the Morin transition of spin-flip process. The dielectric behaviour is described well by the Debye-type relaxation (α-dispersion) in the temperature regions T<233 K and T>338 K. In the intermediate temperature range 233 K<T<338 K a charge carrier mechanism takes place (electron jump from the O²⁻ ion into one of the magnetic ions Fe³⁺) which gives rise to the low frequency conductivity and to the Ω-dispersion. The temperature dependence of relaxation time (τ) in the −ln τ vs 10³/T plot shows two linear regions. In the first, T<238 K, τ increases with increasing T implying a negative activation energy −0.01 eV, and in the second region T>318 K τ decreases as the temperature increases implying a positive activation energy 0.12 eV. The total reorganization energy (0.12-0.01) 0.11 eV is in agreement with the adiabatic activation energy 0.11 eV given by an ab initio model in the literature. The temperature dependence of the phase shift in the frequencies 1, 5, 10 kHz applied shows clearly an average Morin temperature T_Mo=284±1 K that is higher than the value of 263 K corresponding to a single crystal due to the size and shape of material grains.