Synthesis and studies on photochromic properties of vanadium doped TiO2 nanoparticles

Pure and (0.5–3 at%) vanadium doped TiO₂ nanoparticles have been synthesized by wet chemical method. The as synthesized materials have been characterized by using XRD, atomic force microscope (AFM), Raman, EPR and UV–vis spectroscopy techniques. From XRD studies, both pure as well as vanadium doped TiO₂ have been found to show pure anatase phase. The value of lattice constant c is smaller in doped TiO₂ as compared to undoped and has been found to decrease with increase in vanadium concentration. AFM studies show formation of spherical particles with particle size ～23 nm in all the samples. Photochromic behavior of these materials has been studied by making their films in alkyd resin. Vanadium doped TiO₂ films show reversible change in color from beige-yellow to brownish violet on exposure to UV light. The mechanism of coloration and bleaching process has been discussed.     

Effect of the Different Sr Dopant Contents on NiO Ceramic

Sr - doped NiO ceramic was studied. The effect of composition variation of Ni_(1-x)Sr_xO where x = 0, 0.01, 0.02, 0.03, 0.05 and 0.10 mole % was prepared by using solid state method. The calcination temperature used at 950 °C for 4 hours and the sintering temperature used at 1200 °C for 3 hours. The results depict the microstructures increase in grains size (0.43 - 3.30 μm) by increase of Sr dopant contents. The density and porosity testing support the result of microstructures analysis. The larger grains size led to increase in density and lower in porosity. The dielectric properties is observed in a wide frequency range of (1 - 1 000 MHz). The increase of dielectric constant is associated with the decrease of dielectric loss. The optimum composition was obtained for the x = 0.03 mole % sample with highest dielectric constant (3.24 x 10³) and lowest dielectric loss (1.42) at 1 MHz.     

Spin-flop and spin-Peierls transition in doped CuGeO3

Ni⁺⁺ ions doped inorganic CuGeO₃ sample has been studied by using electron spin resonance (ESR) technique in the temperature range of 3–300 K. The ESR spectrum of Cu⁺⁺ ion has been observed to be strongly temperature dependent for inorganic spin-Peierls (SP) Cu_0.96Ni_0.04GeO₃ samples. The ESR line width and ESR amplitude exponentially vanishes below a critical temperature, T_sp = 14 K. The one-dimensional (1D) antiferromagnetic (AF) spin chain formed of Cu⁺⁺ is broken by Ni⁺⁺ (spin-1) ion, giving uncoupled spins at the end of the chains that give extra contribution to the spectra at lower temperature and stabilizes a Néel state. The g-factor is much smaller than the expected value for isolated Cu⁺⁺ and Ni⁺⁺ ions and is much more anisotropic than for undoped samples [O. Yalçın, B. Aktaş, J. Magn. Magn. Mater. 258/259 (2003) 137 (reference therein)]. It is shown that the ground state of dimerized spins is singlet. The spin-flop (SF) phenomenon is obtained from AF state mixed condition and then ferromagnetic (FM) state. The spin-flop field slightly increases when increasing temperature in the temperature range 100–300 K. The SF transition is showed almost AF order for Cu_0.96Ni_0.04GeO₃.     

Effect of Mn-substitution on structural and dielectric properties of Pb(Zr0.65−xMnxTi0.35)O3 ceramics

The polycrystalline samples of Pb(Zr_0.65?xMn_xTi_0.35)O₃ (x = 0, 0.05, 0.10, 0.15) (PZMT) were prepared by a high-temperature solid-state reaction technique. Structural properties of the compounds were examined using an X-ray diffraction (XRD) technique to confirm the formation of single-phase compounds (with perovskite structure) at room temperature. Microstructural analysis of the surface of the compounds by scanning electron microscopy (SEM) exhibits that there is a significant change in grain size on introduction, at the Zr-site, of Mn. Detailed studies of the dielectric properties of PZMT show a measurable shift in T_c, change in dielectric constant, and ac conductivity.     

Substitution of Sr2+ by Eu3+ in Bi-2201 ceramics,effects on structure and physical properties

In this paper, we report the effects of the substitution of Sr by Eu on the properties of Bi-2201 ceramics. Samples with nominal compositions of Bi₂Sr_2?xEu_xCuO_y (x = 0–0.4) are elaborated in air by solid state reaction. They are characterized by means of X ray diffraction (XRD), scanning electron microscopy (SEM), magnetic and resistivity measurements. The undoped sample (x = 0) is monophasic and its structure belongs to phase A. No trace of superconductivity is observed down to 2 K for this sample and the variation of resistivity with temperature shows a semiconducting behaviour. As Eu is added and for x ≥ 0.2, the samples convert totally to B or Raveau phase and become superconducting. The highest T_c, obtained from both magnetic and resistivity measurements, is observed for x = 0.3. In the normal state, all the samples exhibit a semiconducting character which decreases as well as resistivity when the Eu content increases. The refinement of cell parameters is done with considering the structural modulation. The study shows that the substitution of Sr²⁺ by Eu³⁺ leads to an increase of a and b parameters, while c decreases similarly to those of the La doped phases. The a axis component of the modulation is observed to be independent of Eu content, while the c axis one increases slightly as this content increases. The XRD analysis has also revealed that the limit solubility of the used Eu₂O₃ oxide is situated between x = 0.3 and 0.4 of Eu content. The SEM micrographs show that the undoped sample consists of poorly connected grains with a random distribution. A quite different microstructure is obtained for the doped samples. The grains are more connected and have a flat shape which is characteristic of the Bi-based superconductors.     

Luminescence and dielectric properties of nano-structured Eu3+:K2O–Nb2O5–SiO2 glass-ceramics

In the present study, results concerning luminescence and dielectric properties of Eu₂O₃ (0.5 wt% in excess) doped nano-crystallized KNbO₃ containing transparent glass-ceramics obtained from glass of composition 25K₂O–25Nb₂O₅–50SiO₂ (mol%) by varied heat-treatment duration at 800 °C have been analyzed and reported. The formed crystallization phase, crystallite size and morphology have been examined through XRD, FESEM, TEM and FTIRRS measurements. The observed steep increase in the dielectric constant (?) of glass-ceramics over the as-prepared glass is attributed to the formation of ferroelectric nano-crystalline KNbO₃ in glass matrix. The absorption spectra of all the samples have revealed the characteristic 4f–4f intraband absorption transitions of Eu³⁺ ions. The measured photoluminescence spectra have exhibited emission transitions ⁵D_{0, 1} → ⁷F_j (j = 0, 1, 2, 3 and 4) of Eu³⁺ ions. The excited level lifetimes have been determined from measured fluorescence decay curves. The rare earth ion site symmetry (nearly C_v) has been understood based on the nature of the Stark splittings of emission bands detected in both Eu³⁺: glass and Eu³⁺: glass-ceramics.     

Effects of NiO addition on the structure and electric properties of Dy3−xNixFe5O12 garnet ferrite

Polycrystalline garnet ferrites Dy_3?xNi_xFe₅O₁₂ with varying Ni substitutions (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) have been prepared by the standard ceramic technique and their crystalline structures were investigated by using X-ray diffraction and IR spectroscopy. The X-ray diffraction analysis showed that all samples have a single cubic garnet phase. The materials prepared are identified by infrared rays which indicate the presence of three absorption bands ν₂, ν₃ and ν₄ which represent the tetrahedral, octahedral and dodecahedral sites respectively which characterize the garnet ferrite.The dielectric constant (?), and dielectric loss (tan δ) of the prepared samples were measured at 1 kHz in the temperature range 300–700 K. The dielectric constant (?), and dielectric loss (tan δ) are functions of temperature.The initial magnetic permeability has been studied at different temperatures. The initial magnetic permeability (μ_i) increases gradually with increasing temperature and then drops suddenly at a certain temperature T_c.     

Influence of the RE2O3 (RE = Y,Gd) and CaO nanoadditives on the electromagnetic properties of nanocrystalline Co0.2Ni0.3Zn0.5Fe2O4

The present work reports the influence of the nanoadditives Y₂O₃, Gd₂O₃, and CaO on the magnetic, electrical and dielectric properties of sintered nanoferrites Co_0.2Ni_0.3Zn_0.5Fe₂O₄. All powders were synthesized via the polyol method. XRD analysis showed that except the nanoferrite which was obtained by in an one-post procedure, subsequent calcinations of the as-produced additives were necessary to obtain nanocrystals of the desired phases. The mean particle size inferred from TEM images of the nanoadditives sintered at 1000 °C ranges from 87 nm for Y₂O₃ to 126 nm for CaO. IR spectroscopy provided useful information on the nature of the core and the surface chemistry of the as-produced additives and their associated annealed powders. Upon sintering, it was found that the incorporation of 5 wt.% additives remarkably increased the densification of the doped materials. The most important increase in densification was observed with CaO due to its larger particles. dc M-H hysteresis loops taken at 300 K revealed a superparamagnetic behavior of the produced ferrite/nanoadditives. Additionally, as expected, the ferrite/nanoadditives showed reasonable saturation magnetization and high Curie temperature. The electrical and dielectric properties, namely the resistivity, the loss factor, and the relation frequency were found to be clearly affected by doping. The resistivity decreased with increasing temperature indicating a semiconducting behavior. Further, at room temperature, the highest resistivity was observed with Y₂O₃. The major role was attributed to the high fraction of insulating Y₂O₃ owing to its smallest particles combined with the low Fe²⁺ concentration in the ferrite nanoparticles taking advantages of the moderate sintering temperature. In addition, the dc conductivity was found to follow the Arrhenius law with a slope change observed at the Curie temperature. Further, all the additives clearly affected the ac conductivities of the pure ferrite. The variation of the dielectric permittivity with frequency and temperature was explained on the basis of M-W type of interfacial polarization. Additionally, at high frequencies, the lower dielectric loss was found with Y₂O₃ doping. It was found to be of about 10 times lower than the undoped material and much larger than reported for similar undoped bulk ferrites.     

Influence of redox behavior of copper ions on dielectric and spectroscopic properties of Li2O–MoO3–B2O3: CuO glass system

Li₂O–MoO₃–B₂O₃ glasses mixed with different concentrations of CuO (ranging from 0 to 1.2 mol%) were prepared. The samples were characterized by X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. Optical absorption, luminescence, ESR, IR and dielectric properties (viz., dielectric constant ?′, loss tan δ and a.c. conductivity σ_ac, over a wide range of frequency and temperature) of these glass materials have been investigated. The results of differential scanning calorimetric studies suggest that the glass forming ability is higher for the glasses containing CuO beyond 0.6 mol%. The analysis of results of the dielectric properties has revealed that the glasses possess high insulating strength when the concentration of CuO is >0.6 mol%. The variation of a.c. conductivity with the concentration of CuO passes through a maximum at 0.6 mol%. In the high-temperature region, the a.c. conduction seems to be connected with the mixed conduction viz., electronic conduction and ionic conduction. The optical absorption spectra of these glasses exhibited bands due to Cu⁺ ions in the UV region in addition to the conventional band due to Cu²⁺ ions in the visible region. The ESR spectral studies have indicated that there is a gradual adoption of Cu²⁺ ions from ionic environment to covalent environment as the concentration of CuO increases beyond 0.6 mol% in the glass matrix. The luminescence spectra excited at 271 nm have exhibited an intense yellow emission band centered at about 550 nm and a relatively broad blue emission band at about 450 nm; these bands have been attributed to the ³D₁ → ¹S₀ transition of isolated Cu⁺ ions and ³D₁ → ¹S₀ transition of (Cu⁺)₂ pairs, respectively. The quantitative analysis of the results of all these studies has indicated that as the concentration of CuO is increased beyond 0.6 mol% in the glass matrix, a part of Cu²⁺ ions have been reduced to Cu⁺ ions that have influenced the physical properties of these glasses to a substantial extent.     

Synthesis,structure and magnetic properties of {Mn5(OC(O)CH3)6(OC(O)C6H5)4}∞

The ligand exchange reaction between Mn(OC(O)CH₃)₂ and benzoic acid under solvothermal conditions in toluene at 110 °C yields colorless crystals of {Mn₅(OC(O)CH₃)₆(OC(O)C₆H₅)₄}_∞ (1). The asymmetric unit of this complex is Mn_2.5(OC(O)CH₃)₃(OC(O)C₆H₅)₂ with each of the three different Mn(II) atoms in 6-fold coordination and one of the benzoate ligands exhibiting the rare μ₃-symmetric bridging mode (O–Mn–O angle = 57°). The structure consists of edge-shared Mn₁₂ loops arranged in a honeycomb-like 2D sheet with the acetate ligands displaced slightly out of the plane. The sheets are spaced at 12 Å and linked into a 3D network via weak intersheet interactions. Magnetic susceptibility characterization of 1 indicates antiferromagnetic exchange with a Weiss constant of −165 K and a transition toward ferromagnetic exchange below 10 K corroborated with a finite imaginary component in the variable temperature susceptibility data.     

Dielectric characteristics of a Ce3+-doped Sr0.61Ba0.39Nb2O6 single crystal with Cole–Cole plots technique

In this paper, we have investigated relaxation mechanisms and dielectric characteristics of an Sr_0.61−xBa_0.39Nb₂O₆Ce_x (abbreviated as SBN61 and x=0.0066) single crystal with dielectric spectroscopy measurements. The crystal undergoes a ferroelectric phase transition at 340 K. The temperature dependence of the real and imaginary part of the complex dielectric susceptibility in the vicinity of ferroelectric–paraelectric phase transition has been studied in the frequency region 100 Hz–10 mHz. The measurement of the dielectric constants of the real and imaginary parts shows strong frequency dependence. The investigations of the dielectric constant using Cole–Cole plots revealed a non-Debye-type dielectric relaxation for Ce⁺³-doped SBN61. It reveals the coexistence of the two dielectric relaxators in the vicinity of the phase transition.     

Optical and Raman studies of Zn1-xMgxO ceramic pellets

The undoped and Mg-doped ZnO ceramics have been successfully synthesized using the conventional solid state sintering method. The doping effect of MgO content on the structural properties of ZnO/MgO composites has been investigated by X-ray diffraction (XRD) and Raman spectroscopy. The XRD patterns reveal that all the samples are polycrystalline and have a prominent hexagonal crystalline structure with (002) and (101) as preferred growth directions. The formation of the hexagonal ZnMgO alloy phase and the segregation of MgO-cubic phase took place for an MgO composition x ≥ 20 wt%. This finding is in good agreement with the Raman spectroscopy measurements which prove the existence of multiple-order Raman peaks originating from ZnO-like and MgO phonons. The band gap energy and the carrier concentration of ZnO pellets were found to be dependent upon the Mg doping whose values vary from 3.287 to 3.827 eV and from 1.6 × 10¹⁷ to 5.2 × 10²⁰ cm⁻³, respectively.     

Fe-doped SnO2 transparent semi-conducting thin films deposited by spray pyrolysis technique: Thermoelectric and p-type conductivity properties

In this paper, we report structural, electrical, optical, and especially thermoelectrical characterization of iron (Fe) doped tin oxide films, which have been deposited by spray pyrolysis technique. The doping level has changed from 0 to 10 wt% in solution ([Fe]/[Sn] = 0–40 at% in solution). The thermoelectric response versus temperature difference has exhibited a nonlinear behavior, and the Seebeck coefficient has been calculated from its slope in temperature range of 300–500 K. The Hall effect and thermoelectric measurements have shown p-type conductivity in SnO₂:Fe films with [Fe]/[Sn] ≥ 7.8 at%. In doping levels lower than 7.8 at%, SnO₂:Fe films have been n-type with a negative thermoelectric coefficient. The Seebeck coefficient for SnO₂:Fe films with 7.8 at% doping level has been obtained to be as high as +1850 μV/K. The analysis of as-deposited samples with thicknesses ～350 nm by X-ray diffraction (XRD) and scanning electron microscopy (SEM) has shown polycrystalline structure with clear characteristic peak of SnO₂ cassiterite phase in all films. The optical transparency (T%) of SnO₂:Fe films in visible spectra decreases from 90% to 75% and electrical resistivity (ρ) increases from 1.2 × 10^?2 to 3 × 10³ Ω cm for Fe-doping in the range 0–40 at%.     

Structural characterization,molecular dynamics,dielectric and spectroscopic properties of tetrakis(pyrazolium) bis(μ2-bromo-tetrabromobismuthate(III)) dihydrate, [C3N2H5]4[Bi2Br10]·2H2O

The structure of [C₃N₂H₅]₄[Bi₂Br₁₀]·2H₂O, (PBB) was determined by single crystal X-ray diffraction at 100 K. It crystallizes in the monoclinic space group C2/m, with a = 12.992(4) Å, b = 16.326(5) Å, c = 8.255(3) Å, β = 108.56°(3), V = 1659.9(9) Å³ and Z = 2. The structure consists of discrete binuclear [Bi₂Br₁₀]⁴⁻ anions, ordered pyrazolium cations and water molecules. The crystal packing is governed by strong N–H⋯O and weak O–H⋯Br hydrogen bonds. A sequence of structural phase transitions in PBB was established on the basis of differential scanning calorimetry and dilatometric studies. Two reversible first-order phase transitions were found: (I → II) at 381/371 K (on heating/cooling) and (II → III) at 348/338 K. Dielectric response near both phase transitions is characteristic of crystals with the “plastic-like” phases. Over the phase III a low frequency dielectric relaxator is disclosed. The possible molecular motions in the PBB compound are characterized by the ¹H NMR studies. The infrared spectra of polycrystalline compound in the temperature range 300–380 K are reported for the region 4000–400 cm⁻¹. The observed spectral changes through the structural phase transition III → II are attributed to an onset of motion both of the pyrazolium cations and water molecules.     

Studies of dielectric characteristics of BaBi2Nb2O9 ferroelectrics prepared by chemical precursor decomposition method

BaBiNb₂O₉ (BBN) powders in the nanometer range were prepared by chemical precursor decomposition method (CPD). TG–DTA showed that precursor sample got freed from organic contaminants at 575 °C. XRD showed that a single phase with the layered perovskite structure of BBN was formed after calcining at 600 °C. No intermediate phase was found during heat treatment at and above 600 °C. The crystallite size (D) and the effective strain (η) were found to be 26 nm and 0.000867, respectively, while the particle size obtained from TEM was 28 ± 2 nm. SEM revealed that the average grain size after sintering at 900 °C for 4 h was ∼1.67 μm. A relative density of ∼93% was obtained using a two-step sintering process at moderate pressure. Dielectric and ferroelectric properties were investigated in the temperature range 50–500 °C and frequencies from 1 kHz to 5 MHz. Strong dispersion of the complex relative dielectric constant was observed including typical relaxor features such as shift of permittivity maximum with frequency and broadening of the peak maximum. The high dielectric constant of 545 measured at 100 kHz and other properties of BBN ceramics were compared to that of BBN prepared by other conventional methods and found to be superior.     

The low temperature magnetic response of a layered manganese acetate benzoate complex

We report the low temperature magnetic behavior of a complex with nominal formula “Mn(OAc)(OBz)(μ-O)_0.1”. It is presumed that the phase responsible for the interesting magnetic phenomena is related to {Mn₅(OAc)₄(OBz)₆}_∞ – a recently reported complex comprised of two dimensional sheets of Mn₁₂ loops arranged in a honeycomb-like fashion via carboxylate bridging in the plane. Temperature and field dependent magnetic studies show exchange transitions between 1.7 and 12 K which are similar to those found in other Mn carboxylate complexes. The point of interest we wish to explore in this paper, that differs from similar manganese carboxylate-based complexes, is the rather strong two-dimensional antiferromagnetic exchange and the possible role of interplanar exchange on ordering.     

XAFS characterization of La1−xSrxMnO3±δ catalysts prepared by Pechini’s method

XAFS experiments at the Mn and Sr K-edges were carried out in order to investigate the short-range arrangement of Mn and Sr sites on La_1−xSr_xMnO_3±δ highly doped perovskites (x = 0, 0.2, 0.4 and 0.6). The Mn K-edge EXAFS spectra show a static Jahn–Teller distortion of the MnO₆ for x = 0 and 0.2, which is drastically reduced as x increases. The distortion of perovskite, characterized by the Mn–O–Mn tilt angle, progressively decreases with increasing Sr contents. Sr K-edge results indicated a decrease on the Sr–Mn coordination number upon Sr doping. Based on this and TPD results, a charge compensating mechanism is proposed suggesting a partial Mn oxidation and formation of Mn defect vacancies due to the introduction of Sr.     

Photo-luminescence studies of strontium barium niobate crystals doped with Cr3+ ions

This paper reports the photo-luminescence spectroscopic results of Strontium–Barium–Niobate, Sr_x,Ba_1−xNb₂O₅ (SBN, x = 0.61 for near congruent composition) crystals doped with Cr₂O, at cryogenic temperature (20 K). The experimental results reveal the need of re-assignment of the Cr³⁺ ions defect centres in this material. For first time, a broad emission band in the near infrared region centred at ca. 950 nm is reported. This emission band has micro-seconds decaytime constant and a FWHM band-width > 1700 cm⁻¹ and has been ascribed to the vibronically assisted ⁴T₂ → ⁴A₂ transition. A much narrower emission band centred at ca. 764 nm with milli-seconds decaytime constant and a FWHM band-width of ca. 170 cm⁻¹ is correlated to the ²E → ⁴A₂ radiative transition (R-line).     

Magneto-structural correlation and low temperature heat capacity of a Mn (III) quadridentate Schiff-base coordination compound

A new Mn (III) Schiff-base coordination compound, [Mn(L)(NCS)]₂ (H₂L = N,N′-bis(5-chlorosalicylidene)-1,2-diaminoethane), has been synthesized and characterized structurally and magnetically. The target compound is a phenoxo-bridged dimeric compound with the isothiocyanate coordinating in a usual bent mode. A magnetic susceptibility study reveals that the target compound exhibits antiferromagnetic intra-dimer coupling between Mn (III) ions. The low temperature heat capacity of the compound over the temperature range (2 to 300) K has been measured using the heat capacity option of a Quantum Design Physical Property Measurement System (PPMS). The thermodynamic functions in the experimental temperature range have been determined by curve fitting. The standard entropy and enthalpy of the as-prepared compound at T = 298.15 K have been calculated to be (924.52 ± 10.17) J · K⁻¹ · mol⁻¹ and (133.47 ± 1.47) kJ · mol⁻¹, respectively.     

Infrared studies of phase transitions in ferroelectric (C5H5NH)5Bi2Br11

Infrared spectra (3500–500 cm⁻¹) of polycrystalline (C₅H₅NH)₅Bi₂Br₁₁ samples were investigated within the temperature range 27–456 K. The assignments of the observed bands in the spectra measured at 27, 310 and 456 K are proposed. A temperature dependence of the wavenumbers and full width at half maximum (FWHM) of the bands arising from some internal vibrations of pyridinium cations are analysed in order to explain the role of cations in the mechanism of the phase transition at 118 (paraelectric–ferroelectric) and 403 K. It was found that numerous bands arising from the internal modes of the cations exhibit the splitting in the vicinity of both phase transitions, that indicates a distinct changes in the motional state of the pyridinium moieties.