Indirect electronic transitions in alkali halate crystals-I (sodium and potassium chlorate crystals)

The long wavelength tail of the fundamental absorption in NaClO₃ and KClO₃ crystals has been analysed based on the theory of band to band transitions of Bardeen et al.[8] developed in the case of semi-conducting crystals. Evidence of phonon involvement in the transitions giving an indirect band gap is observed. The energies of the phonons involved in the process are the same for both the crystals, and agree well with combinations of prinicple frequencies of ClO₃^? ion, their overtones and also lattice phonons. The indirect band gap in these crystals varies with temperature more or less linearly and the rate of variation is ?3·8 × 10^?4 eV/K and ?5·0 × 10^?4 eV/K for sodium chlorate and potassium chlorate respectively.     

Optical characteristics of particles produced using electroerosion dispersion of titanium in hydrogen peroxide

Titanium oxide particles are produced using electric-discharge dispersion of titanium in aqueous solution of hydrogen peroxide. Electron vacuum microscopy, X-ray diffraction, and diffuse reflection spectroscopy are used to study the morphology, composition, and optical characteristics of the erosion particles. It has been demonstrated that the particles consist of titanium and titanium oxides with different valences. The edge of the optical absorption is located in the UV spectral range. The band gap is 3.35 eV for indirect transitions and 3.87 eV for direct allowed transitions. The band gap decreases due to the relatively long heating in air at a temperature of 480–550°C, so that powder oxide compositions can be obtained, the optical characteristics of which are similar to optical characteristics of anatase. The erosion products are completely oxidized to rutile after annealing in air at a temperature of 1000°C.     

Fine structure in the absorption edge spectrum of NbS2Y2 (Y = Cl,Br, I)

High resolution absorption spectra of single crystals of NbS₂Y₂ (Y = Cl, Br, I), obtained at temperatures between 4.2 and 300 K revealed extensive fine structure in the absorption edge. This structure has been analysed and interpreted in terms of allowed indirect interband transitions, involving phonons corresponding to SS and NbS vibrations, followed by forbidden and by allowed direct transitions. From the shape of the absorption curve associated with the phonon branches of the indirect transitions a binding energy of 23 cm^? for indirect excitons is obtained. A binding energy of 28 cm^? for direct excitons is deduced from the exciton lines observed at the long-wavelength side of the direct transitions. A detailed interpretation of the optical transitions is given in terms of a molecular orbital diagram for the Nb₂S₄ clusters, present in these crystals.     

Optical absorption edge in SnS

Optical absorption in single crystals of tin sulfide has been studied at many temperatures between 100 and 300 °K, in the wavelength range 2·2–0·8 μ. From the interference fringe patterns the absorption coefficient, reflection coefficient and index of refraction as a function of wavelength were determined for two light polarizations (ε∥a and ε∥b). From an analysis of the data, indirect band gaps of 1·142 and 1·095 eV were found for the two directions of polarization. Also it was found that the phonon assisted transitions required the participation of two phonons at different energy thresholds with energies 0·033 or 0·038 eV and 0·082 or 0·113 eV, with reference to the two axis. The temperature dependence of the indirect band gap for each direction of light polarization is linear with a slope ?4·05 × 10^?3eV and ?4·37 × 10^?3 eV respectively.     

Absorption Spectra of Single Crystals of EuGa2S4 and EuGa2S4:Co

The optical properties of EuGa₂S₄ and EuGa₂S₄:Co single crystals in a range of temperatures from 77 to 300 K are investigated. The single crystals are obtained by the Bridgman method and are characterized by tetragonal syngony. The behavior of the optical transitions in the photon energy range 1.70–2.45 eV and the temperature range 77–300 K is determined. It is established that in the energy range 1.77–1.90 eV absorption is associated with transitions of the Co²⁺ ion, while in the range 2.20–2.40 eV, with indirect allowed optical transitions.     

The evolution of the optical spectra of LaMnO<Subscript>3</Subscript> under light <Emphasis Type="Italic">n</Emphasis>-and <Emphasis Type="Italic">p</Emphasis>-type doping and the separation of phases

The optical absorption spectra of LaMnO₃ single crystals, pure and slightly doped with Ce and Sr (7%), were studied in the middle IR region. Ellipsometry measurements were performed in the visible region to determine real and imaginary permittivity parts. The energies of the onset of indirect transitions in LaMnO₃ at 293 and 80 K were found to be 0.30 and 0.40 eV, respectively. Impurity absorption bands corresponding to transition in hole and electron clusters were observed. The anomalous temperature dependence of IR absorption was explained by the existence of conducting droplets in the insulated doped crystal matrices at temperatures below the temperature of the appearance of a ferromagnetic contribution. The existence of such a contribution was proved by studying the equatorial Kerr effect. Because of the separation of phases, manganites slightly doped by a nonisovalent admixture can be treated as optically nonuniform media whose properties are similar to those of a composite system with metallic inclusions.     

Synthesis,magneto-optics and fluorescence of the ferromagnetic semiconductor Eu3(II)SiO5

Small single crystals of the transparent ferromagnet Eu₃SiO₅ have been synthesized by high temperature chemical transport. Magnetic measurements indicate T_c = 9°K and a saturation magnetization very close to 7 μ_B/Eu ion. Crystalline samples show a very low residual optical absorption and an absorption edge near 2 eV which displays a small red shift of 20 meV on cooling below T_c. Samples, containing a small percentage of dissolved EuO clusters, show in addition an absorption band at lower energies with a temperature dependence and magnetic behavior typical for EuO. The photoluminescence of the pure compound has a single emission band near 1·9 eV with a high quantum yield. At low temperatures also the fluorescence displays a red shift similar to that of the absorption edge. The fluorescence is accompanied with photoconductivity.     

Effect of indium incorporation on the optical properties of spray pyrolyzed Cd0.22Zn0.78S thin films

In this study, effect of indium incorporation on the optical properties is investigated for the spray pyrolyzed onto glass substrates at 275°C substrate temperature undoped and indium doped Cd_0.22Zn_0.78S thin films. The average optical transmittance of all the films was over 77% in the wavelength range between 450 and 800 nm. The optical band gap energies of the thin films have been investigated by the measurement of the optical absorbance as a function of wavelength. The optical absorption studies reveal that the transitions are direct band gaps of 3.02 and 3.05 eV for undoped and doped indium Cd_0.22Zn_0.78S thin films, respectively. The Urbach tail parameter and optical constants such as refractive index, extinction coefficient, and dielectric constants were calculated for these films. The dispersion parameters such as single-oscillator energy and dispersive energy were discussed in terms oft he single-oscillator Wemple—DiDomenico model.     

Correlation of the photorefractive sensitivity in doped LiNbO3 with chemically induced changes in the optical absorption spectra

Single crystals of LiNbO₃ doped with Mn, Fe, or Cu have been reduced or oxidized at 800°C. Changes in the optical absorption spectra and in the photorefractive sensitivity have been determined, and a close correlation between the latter and a particular optical absorption band is demonstrated.     

Optical properties of Tl2In2Se3S layered single crystals

The optical properties of Tl₂In₂Se₃S layered single crystals have been analyzed using transmission and reflection measurements in the wavelength region between 500 and 1100 nm. The optical indirect transitions with a band gap energy of 1.96 eV and direct transitions with a band gap energy of 2.16 eV were determined from analysis of absorption data at room temperature. Dispersion of the refractive index is discussed in terms of the Wemple–DiDomenico single-effective-oscillator model. The refractive index dispersion parameters – oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index – were found to be 4.67 eV, 45.35 eV, 1.38 × 10¹⁴ m ^{? 2} and 3.27, respectively. Transmission measurements were also performed in the temperature range 10–300 K. As a result of temperature-dependent transmission measurements, the rate of change in the indirect band gap with temperature, i.e. γ = ?5.6 × 10^?4 eV/K, and the absolute zero value of the band gap energy, E _gi(0) = 2.09 eV, were obtained.     

Improved visible light photocatalytic activity of TiO<Subscript>2</Subscript> nano powders with metal ions doping for glazed ceramic tiles

Self-cleaning and anti-bacterial activities of the photo-catalyst titanium dioxide make it a superior compound for use in the ceramics and glass industry. In order to achieve high self-cleaning efficiency for building products, it is important that Titania is present as anatase phase. Moreover, it is desirable that the particle sizes are in Nano-range, so that a large enough surface area is available for enhanced catalytic performance. In the present paper, Cobalt and Nickel co-doped (4%mol Ni and 4%mol Co doped TiO₂) and un-doped TiO₂ Nano powders have been prepared by sol–gel technique. They were calcined at the temperatures in the range of 475–1075 °C. Ni/Co co-doped TiO₂ postponed the anatase to rutile transformation of TiO₂ by about 200–300°C, such that before calcination at 775°C, no rutile was detected for 4 mol% Ni/Co co-doped TiO₂. A systematic decreasing on crystallite size and increasing on specific surface area of Ni/Co co-doped TiO₂ were observed. Photo-catalytic activity of anatase polymorph was measured by the decomposition rate of methylene blue under visible light. The results showed enhanced catalysis under visible light for Ni/Co co-doped TiO₂ as compared to pure TiO₂. The enhanced performance was attributed to surface chemistry change associated with a slight shift in the band gap. Depending on the temperatures ranging from 475 to 1075 °C, band gap energy of Ni and Co doped TiO₂ crystals decreased. For all samples there is a general reduction of the band gap energy from 3.00 to 2.96 eV.     

The Urbach tails and optical absorption in layered semiconductor TlGaSe<Subscript>2</Subscript> and TlGaS<Subscript>2</Subscript> single crystals

TlGaSe₂ and TlGaS₂ single crystals were grown by the modified Bridgman-Stockbarger method. We report the result of an experimental study of the optical absorption of TlGaSe₂ and TlGaS₂ crystals. The absorption measurements were performed in steps of 10 K. The direct and indirect band gaps for TlGaSe₂ and TlGaS₂ samples were calculated as a function of temperature. The phonon energies in TlGaSe₂ and TlGaS₂ crystals were calculated as (39±4) and (9±4) meV at 240 K, respectively. At 10 K, direct and indirect band gaps were found as 2.294 and 2.148 eV for TlGaSe₂, 2.547 and 2.521 eV for TlGaS₂ crystals, respectively. The abrupt changes were observed in the direct and indirect band gaps in the some temperature ranges. These changes were interpreted as phase transformation temperatures. The steepness parameters and Urbach energy for TlGaSe₂ and TlGaS₂ samples increased with increasing sample temperature in the range (10–320) K.     

Optical absorption in MnIn2S4 single crystals

Optical absorption in MnIn₂S₄ single crystals has been studied. Direct and indirect optical transitions are found to occur at photon energies of 1.90?C2.16 eV in the temperature range of 80?C342 K. The temperature dependence of the band gap is determined; its temperature coefficients E _gd and E _gi are found to be ?4.84 × 10^?4 and ?6.33 × 10^?4 eV/K, respectively. The electron-phonon interaction is the main mechanism of the temperature shift of the intrinsic-absorption edge. MnIn₂S₄ single crystals exhibit anisotropy in polarized light at the absorption edge in the temperature range of 90?C190 K; the nature of this anisotropy is explained.     

Direct and indirect optical transitions in Zn3P2

Absorption measurements were made on single crystals of Zn₃P₂ at temperatures of 300, 80 and 5 K, and photo-voltage spectral responses-were measured at 300 K for Au- and In---Zn₃P₂ contacts. Interband absorption was interpreted as a process involving three mechanisms: (1) indirect transitions from the valence band at the Γ point, (2) either excitations from acceptor level to the conduction band at the Γ point, or second indirect transitions associated with the creation of excitons, and (3) band-to-band direct transitions at the Γ point. The effect of the lighting configuration on spectral PV plots is also discussed, and the origin of two peaks of PV responses is interpreted as being in accordance with optical data. The indirect energy gap has been estimated as 1.315eV at 300 K and 1.335 eV at 80 and 5 K, and the direct one as 1.505, 1.645 and 1.685 eV at 300, 80 and 5 K, respectively.     

Optical spectra of lithium niobate

The specific features revealed in the optical spectra of lithium niobate crystals at temperatures of 90 and 120–125°C can be attributed to the change in the electronic subsystem and the related isostructural transformation of the crystal lattice. In the near-IR range, the optical absorption spectra of lithium niobate crystals exhibit bands (1.43 eV) assigned to polarons of large radius with a binding energy of 0.48 eV. The decrease observed in the absorption coefficient at the maxima of these bands with an increase in the temperature to 160°C can be explained by the decay of polarons of large radius at these temperatures.     

Energy bands in stannic oxide (SnO2)

The energy band structure of stannic oxide has been calculated by a self-consistent augmented plane wave (APW) method. The calculation predicts SnO₂ to be a semiconductor with an allowed direct band gap of 3·68 eV for light polarized perpendicular to the tetragonal axis, and of 4·07 eV for parallel polarized light; these values agree well with the measured values of 3·57 and 3·93 eV. The theory also predicts indirect and direct-forbidden optical transitions which are consistent with experiment.     

Aftereffects following β-decay of181Hf1

Time differential perturbed angular correlation spectra of¹⁸¹Ta produced by beta decay of¹⁸¹Hf in monoclinic ZrO₂ have been measured over a temperature range 10K to 1300K. If times near t=0 are excluded, G₂(t) is well fitted by a standard single site model for nuclei subject to an interaction with a static electric field gradient. For high purity samples, the effective anisotropy A₂ is equal to its expected value above 200°C but decreases abruptly by approximately a factor of two for lower temperatures. This unusual decrease, which does not occur in natural crystals or in Nb-doped powders, is attributed to an aftereffect of the beta emission that populates with approximately 50% probability an electron trap located about one eV below the conduction band. At low temperatures, this trapped electron causes the Ta nucleus to relax rapidly and contributes to G₂(t) only near t=0. At high temperatures or in doped samples the electron escapes quickly enough to have negligible effect on G₂(t). Supported in part by USDOE contract DE-FG06-85ER45191     

Physical,optical and electrical properties of calcium bismuth borate glasses

The variation in physical, optical and electrical properties has been investigated as a function of Bi₂O₃ content in 20CaO?·?xBi₂O₃?·?(80???x)B₂O₃ (0?≤?x?≤?60, in mol%) glasses. The samples were prepared by normal melt-quenching process, and the optical absorption and reflection spectra were recorded in the wavelength range of 400–950 nm. The fundamental absorption edge has been identified from the optical absorption spectra. The optical band gap, E _g, for indirect allowed and indirect forbidden transitions has been determined from the available theories and its value lies between 1.80–2.37 eV and 1.08–2.19 eV, respectively. The theoretical fitting of the optical absorption indicates that the present glass system behaves as an indirect gap semiconductor. The origin of the Urbach energy, ΔE, has been associated with the phonon-assisted indirect transitions. The refractive index and optical dielectric constant have been evaluated from the reflection spectra. The density and molar volume are found to depend on the molar concentration of Bi₂O₃. The values of DC electrical conductivity have been measured from 373 to 623 K and the activation energy has been calculated. Theoretical optical basicity has been reported as a function of the Bi₂O₃ content. The variations have been discussed in terms of structural changes.     

Über den Einfluß von Zwischengitterionen auf die Absorption von Gitterlücken in KBr-Kristallen

Detailed measurements have been made of the optical properties of α-centers (anion vacancies) in KBr: KH crystals irradiated monochromatically in the U band at 58°K. The absorption band α, normally at 6·14 eV is under these conditions shifted to shorter wavelengths. In terms ofFritz's model for the infrared absorption of U₁ centers, this shift can be interpreted as the influence of the nearby interstitial hydride ions on the a absorption. The nearest observed interstitial ion, stable to 103°K, shifts the α band to 6·31 eV; the next nearest one (stable to 150°K) to 6·19 eV; heating above 150°K leaves only the normal α absorption at 6·14 eV. Data are presented on the quantum efficiency for the recombination under uv irradiation and on the oscillator strengths for the different α centers.     

Photoluminescence measurements of the 1.55 eV band of Ge doped AℓxGa1−xAs

The photoluminescence of the 1.55 eV band of Ge doped A?_xGa_1?xAs, with x=0.30–0.33, grown by liquid phase epitaxy is presented. The broad shape was found to be due to a lattice relaxation upon optical transitions. Resonant modes with ?ω_q = 35±2 meV and ?ω_q = 45±2 meV are found for the optical band, yielding a zero phonon transition energy = 1.73±0.02 eV and a Franck-Condon shift = 0.17–0.20 eV for the optical center. The activation energy of thermal quenching yields an associated donnor binding energy of 0.17±0.04 eV. Possible mechanisms for the radiative transitions are discussed.