Annealing and photobleaching effects on the absorption spectra of gamma-irradiated BaFI crystals

We have studied processes induced in gamma-irradiated BaFI crystals by illumination and thermal annealing and identified defect centers responsible for the observed absorption bands. We assign the 205-nm absorption band to α(F⁻) centers, the 230-nm band to α(I⁻) centers, the 270-nm band to F_i ⁰ centers, the 370-nm band to I_i⁰ centers, the bands around 480 nm to F(F⁻) centers, and the 610-nm band to F(I⁻) centers. The proposed assignment accounts well for the radiation-induced processes identified in the crystals.     

Gamma-induced defects in BaFI crystals

We have studied gamma-induced defects in BaFI crystals. To gain insight into the origin of the observed induced absorption bands, we analyzed the dose dependence of the concentration of color centers. The results lead us to assign the 205-nm absorption band to α(F⁻) centers, the 270-nm band to F_i⁰ centers, the 370-nm band to I_i⁰ centers, the bands around 480 nm to F(F⁻) centers, and the 610-nm band to F(I⁻) centers.     

Photoluminescence properties of RE-activated Na3GdP2O8 (RE = Tb, Dy, Eu, Sm) under VUV excitation

RE³⁺-activated monoclinic Na₃GdP₂O₈ (RE³⁺ = Tb³⁺, Dy³⁺, Eu³⁺, Sm³⁺) phosphors have been synthesized by a solid-state reaction method. Their photoluminescence properties in the vacuum ultraviolet (VUV) region were investigated. By analyzing their excitation spectra, the host-related absorption band was determined to be around 166 nm. The f-d transition bands and the charge transfer bands for Na₃GdP₂O₈:RE³⁺ (RE³⁺ = Tb³⁺, Dy³⁺, Eu³⁺, Sm³⁺) were assigned and corroborated. For the sample Na₃GdP₂O₈:5%Tb³⁺, the strong bands at around 202 and 221 nm are assigned to the 4f-5d spin-allowed transitions and the weak band at 266 nm is related to the spin-forbidden transition of Tb³⁺. For Na₃GdP₂O₈:5%Dy³⁺, the broad band at 176 nm could be related to the f-d transitions of Dy³⁺ and the O²⁻ → Dy³⁺ charge transfer band (CTB) besides the host-related absorption. In the excitation spectrum of Eu³⁺ doped sample, the O²⁻ → Eu³⁺ CTB is observed to be at 245 nm. For the Sm³⁺ doped sample, the O²⁻ → Sm³⁺ CTB is not distinguished obviously and is overlapped with the host-related absorption band.     

Measurement of Specific Fracture Energy and Surface Tension of Brittle Materials in Powder Form

Studies of the influence of specimen geometry and size–effect on the K _R –curves and the related fracture parameters were carried out by the authors (Kumar and Barai 2008b). The present paper is a supplementary contribution and reports interesting results related to the effect of the loading condition and size–effect studies on the K _R –curves associated with the cohesive stress distribution for complete fracture process, the double–K fracture parameters, the CTOD–curves and the process zone length using two different loading conditions (i.e., three–point bending test and four–point bending test). The laboratory size specimen with initial–notch length/depth ratios 0.3 and 0.5 are considered in the work. The load–crack opening displacement curves for these loading conditions are obtained using well known version of fictitious crack model.     

Ab initio calculations of electronic structures of LiYF4 crystals containing F-type color centers

The electronic structures of LiYF₄ crystals containing F, F⁻, and M color centers (F₂ center) with the lattice structure optimized are studied within the framework of the density functional theory. The calculation indicated that F, F⁻, and M color centers have donor energy levels in the forbidden band. The electronic transition energies from the donor level to the bottom of the conduction band are 3.74 eV, 2.85 eV, and 2.42 eV, respectively, which correspond to the 331 nm, 436 nm, and 513 nm absorption bands. It is predicted that the absorption bands observed at 330 nm, 440 nm, and 505 nm could arise from the F, F⁻, and M centers, respectively, in LiYF₄ crystals.     

X-ray spectroscopy and local structures in glasses

X-ray spectroscopy which is a combination of two techniques, namely x-ray absorption near edge structure (xanes) and extended x-ray absorption fine structure (EXAFS) analyses, is a unique technique for the study of local structures in glasses. Availability of synchrotron radiation sources has made the technique quite attractive and useful because the photon flux from synchrotrons is very intense and polarized. In this article, a brief summary ofxanes andEXAFS techniques is given along with a few applications to the study of local structures in glasses. Communication No. 323 from Solid State and Structural Chemistry Unit.     

Optical absorption and electron spin resonance studies of Cu2+ in Li2O-Na2O-B2O3-As2O3 glasses

The local structure around Cu²⁺ ion has been examined by means of electron spin resonance and optical absorption measurements in xLi₂O-(40-x)Na₂O-50B₂O₃-10As₂O₃ glasses. The site symmetry around Cu²⁺ ions is tetragonally distorted octahedral. The ground state of Cu²⁺ isd _x ²−y ².The glass exhibited broad absorption band near infrared region and small absorption band around 548 nm, which was assigned to the ²B_1g →²E_g transition.     

Raman Investigations of C-, Li- and Mn-Doped MgB2

A systematic micro-Raman study of the atomic substitution effects has been carried out on single crystals of MgB₂, including C-doping for B and Li- or Mn-doping for Mg. The spectra of C (electron)-doped compounds exhibit remarkable differentiations with respect to pure MgB₂, and new bands of E_2g and A_1g/B_1g symmetry play the dominant role in the substituted spectra. We have observed a two-mode behavior apparently driven by the electron–phonon coupling strength and associated with an electronic topological transition of the σ bands. On the other hand, Li (hole) and Mn (magnetic impurity) doping do not seem to affect the well-defined broad band at 600 cm⁻¹. Combining Raman investigations and T _c -interdependence shows that this band is not related with the transition temperature, questioning its relationship to superconductivity in diborides.      

First-principles calculations on surface hydroxyl impurities in BaF2

OH⁻ impurities located near the (1 1 1) BaF₂ surface have been studied by using density functional theory (DFT) with hybrid exchange potentials, namely DFT-B3PW. Twenty surface OH⁻ configurations were studied, and the hydroxyls located on the first surface layer are the energetically most favorable configurations. For the (1 1 1) BaF₂ surface atomic layers, the surface hydroxyls lead to a remarkable XY-translation and a dilating effect in the Z-direction, overcoming the surface shrinking effect in the perfect slab. Bond population analysis shows that the surface effect strengthens the covalency of surface OH⁻ impurities. The studies on band structures and density of states (DOS) of the surface OH⁻-impurity systems demonstrate that there are two defect levels induced by OH⁻ impurities. The O p_x and p_y orbitals form two superposed occupied O bands, located above the valence bands (VB), and the H s orbitals do the major contribution to an empty H band, located below the conduction bands (CB). Because of the surface effect, the O bands move downward, toward the VB with respect to these bands in the bulk case, and this leads to narrowing of the VB → O gap and widening of the O → H gap which corresponds to the first optical absorption.     

High and low spin energy states of the Tb 4f5d configuration in BaF2

In this paper we present results of detailed spectroscopic studies of Tb³⁺ luminescence from BaF₂:0.075 mol% Tb performed at Superlumi station of Hasylab, DESY, Hamburg and at Institute of Physics, N. Copernicus University, Torun, Poland (IF UMK Torun).We have measured UV/VUV excitation spectra of the dominant blue Tb³⁺ luminescence and emission spectra under excitation into various bands found from the excitation spectra. The excitation spectra are dominated by the two well known broad bands due to the parity and spin-allowed (SA) 4f⁸ → 4f⁷5d transitions. The higher energy triple t-band and unresolved single e-band at lower energies are widely separated (10 Dq about 16,500 cm⁻¹) reflecting the single d-electron energy in the high symmetry cubic crystal field with a relatively low contribution from the low symmetry component.In addition to these bands we have also detected a number of weaker bands at lower energy sides of both dominant SA bands. While the structured band with components at 252.8, 252.3, 250.4 and 248.4 nm clearly corresponds to the SF (spin-forbidden) band found earlier in LiYF₄:Tb, the bands at 183, 178 and 172 nm have been never, to the best of our knowledge, reported before. We present arguments that these new bands are the SF counterparts of the triple SA t-band at about 158 nm. The d-f exchange energies for d(e) and d(t) electrons are different at 7779 and 8245 cm⁻¹, respectively.Although the energy levels corresponding to the identified high and low spin (HS and LS) states of the 4f⁷5d configuration are, in BaF₂, well separated, there are no SA nor SF transitions generating d-f emission in BaF₂:Tb. This is because the numerous 4f⁸ energy levels intercept the excitation energy from 4f⁷5d levels leading to the well known blue and green Tb³⁺ 4f⁸ emissions.     

Radiation-induced modification of polyurethane with hydroxyethyl methacrylate: blood compatibility

An attempt is made to develop the hydrophilic grafting of polyether urethane urea with hydroxyethyl methacrylate (hema) using⁶⁰Coγ-irradiation for achieving optimum hydrophilic/hydrophobic property needed towards blood compatibility. Contact angle method and platelet adhesion from calf’s blood are used to determine the suitability of these modified surfaces.     

Electronic structure and photoluminescence properties of Eu-activated Ca4Si2O7F2

The blue-emitting phosphors Ca_(4−x)Eu_xSi₂O₇F₂ (0 < x ? 0.05) have been prepared by solid-state reaction and the photoluminescence properties have been studied systematically. The electronic structure of calcium fluoride silicate Ca₄Si₂O₇F₂ was calculated using the CASTEP code. The calculation results of electronic structure show that Ca₄Si₂O₇F₂ has an indirect band gap with 5 eV. The top of the valence band is dominated by O 2p and Si 3p states, while the bottom of the conduction band is mainly composed of Ca 3d states. Under the 350 nm excitation, the obtained sample shows a broad emission band in the wavelength range of 400-500 nm with peaks of 413 nm and 460 nm from two different luminescence centers, respectively. The relative intensity of the two peaks changes with the alteration of the Eu²⁺ concentration. The strong excitation bands of the powder in the wavelength range of 200-420 nm are favorable properties for the application as lighting-emitting-diode conversion phosphor.     

Synthesis and luminescence properties of Eu<Superscript>2+</Superscript>-doped Li<Subscript>2</Subscript>SrSiO<Subscript>4</Subscript>

We have studied the luminescence spectra of Li₂Sr_{1 − x} Eu _x SiO₄ (x = 0.0001–0.01) solid solutions prepared by solid-state reactions and a sol-gel process in a reducing atmosphere. The spectra show a broad band in the range 500–700 nm, centered at 578 nm, which is due to the 4f ⁶5d → 4f ⁷ transition. The luminescence excitation spectrum shows, in addition to bands due to Eu²⁺ 4f ⁷ → 4f ⁶5d transitions, a strong band centered at 174 nm, attributable to absorption in the SiO₄⁴⁻ group.     

Luminescence and scintillation properties of Cs3BiCl6 crystals

The optical and scintillation properties of Cs₃BiCl₆ single crystals were characterized, and absorption properties were investigated for thin films. The thin films showed two absorption bands at ∼220 nm and 330 nm; these bands can be attributed to the ¹A_1g → ¹T_1u and ¹A_1g → ³T_1u transitions of Bi³⁺ ions, respectively. A luminescence band was observed at ∼390 nm in the photoluminescence spectra; this band can be attributed to the ³T_1u → ¹A_1g transition of Bi³⁺ ions. In the X-ray-induced radioluminescence spectrum, in addition to the luminescence band at 390 nm, another band was observed at 600–700 nm; this band was tentatively attributed to the radiative recombination of self-trapped excitons. The decay-time constants of photoluminescence and scintillation decay were of the order of nanoseconds. The scintillation light yield was 800 photons/MeV. The results indicate that Cs₃BiCl₆ has a fast scintillation decay and a relatively poor light yield.     

Optical absorption and fluorescence properties of Er3+ in sodium borate glass

Spectroscopic properties of Er³⁺ ions in sodium borate glass have been studied. The indirect and direct optical band gaps (E_opt) and energy level parameters (RacahE ¹, E² and E³), spin-orbit (ξ_4f) and configurational interaction (α)) are evaluated. Spectral intensities for various absorption bands of Er₃₊ doped sodium borate glass are calculated. Using Judd-Ofelt intensity parametersΩ ₂, Ω₄, Ω₆, radiative transition probabilities (A), branching ratios (β) and integrated absorption cross sections (Σ) are reported for certain transitions. The radiative lifetimes (τ_R) for different excited states are estimated. From the fluorescence spectra, the emission cross section (σ_P) for the transition,⁴I_13/2 → ⁴I_15/2 is reported.     

Photoelectrochemical response and characterization of p-CulnSe2 electrodeposited with different citrate ion concentrations

The photoelectrochemical response of electrodeposited p-CulnSe₂ thin films from sulfate bath and different citrate ion concentrations is reported. The best cathodic photoresponse was obtained with thin films deposited in 0.06 m sodium citrate. The film characteristics show a preferred orientation with <112> direction and sphalerite structure. Dark capacitance measurements in 0.5 m H₂SO₄ solution at 50 KHz showed a linear behaviour from which flat band potential, V _fb ∼ − 0.275 V versus saturated calomel electrode (SCE), and a doping level density, N _A ∼ 1.2 10¹⁶ cm⁻³, were determined.     

Synthesis and Optical Studies of Superconducting MgB<Subscript>2</Subscript> Thin Films

Synthesis and optical transmission of MgB₂ thin films on optically transparent glass are reported. In the 400–1000 nm regime as deposited films show high metallic reflectivity and very little transmission. After deposition, the films were annealedex situ and rendered superconducting withT _c of 38 K, approaching that of the bulk material. The reaction conditions where quite soft ∼ 10 min at 550°C. The optical absorption coefficient,α and photon energy,E followed a Tauc-type behavior, (αE)^1/2=β _T(E −E _g). The band gap (E _g) was observed to peak at 2.5 eV; but, the slope parameterβ _Tbehaved monotonically with reaction temperature. Our results indicate that an intermediate semiconducting phase is produced before the formation of the superconducting phase; also optical measurements provide valuable information in monitoring the synthesis of MgB₂ from its metallic constituents. In addition these films have interesting optical properties that may be integrated into optoelectronics.     

Luminescence properties of Ca4Y6(SiO4)6O:RE (RE = Eu, Tb, Dy, Sm and Tm) under vacuum ultraviolet excitation

The vacuum ultraviolet excited luminescent properties of Eu³⁺, Tb³⁺, Dy³⁺, Sm³⁺ and Tm³⁺ in the matrices of Ca₄Y₆(SiO₄)₆O were investigated. The bands at about 173 nm in the vacuum ultraviolet excited spectra were attributed to host lattice absorption of the matrix Ca₄Y₆(SiO₄)₆O. For Eu³⁺-doped samples, the O²⁻ → Eu³⁺ CTB was identified at 258 nm. Typical 4f-5d absorption bands in the region of 195-300 nm were observed in Tb³⁺-doped samples. For Dy³⁺-doped and Sm³⁺-doped samples, the broad excitation bands consisted of host absorptions, CTB and f-d transition. For Tm³⁺-doped samples, the O²⁻ → Tm³⁺ CTB was located at 191 nm. About the color purity and emission intensity, Ca₄Y₆(SiO₄)₆O:Tb³⁺ is an attractive candidate of green light PDP phosphor, and Ca₄Y₆(SiO₄)₆O:Dy³⁺ has potential application in the field of mercury-free lamps.     

Magnetic properties of RRh3B2(R=La,Ce, Nd,Gd) compounds

The¹¹Bnmr results on RRh₃B₂(R=La, Ce, Nd and Gd) are reported. For CeRh₃B₂, specific heat and electrical resistivity are reported. From a comparative study of the¹¹Bnmr Knight shifts and the spin lattice relaxation times of these compounds it is shown that in CeRh₃B₂, there is strong hybridization of 4f states with the conduction electrons. A local moment on Ce with admixture of 4f and 5d–6s orbitals is suggested.     

Thermal properties of some α-amino acids in aqueous solutions

The thermal properties (thermal conductivity, thermal diffusivity, and volumetric heat capacity) of aqueous solutions of -amino acids [R-CH(NH₂)COOH]—glycine, dl-alanine, l-valine, and dl-leucine, were measured in the temperature range 20–90C using the hot-wire technique. The results show that the values of the thermal properties depend mainly on the amino acid concentration in the medium, the substituted alkyl group of the substances under investigation, and the temperatures. The mechanism of heat transfer is discussed, and the roles of both radiation and convection are taken into consideration.