A comparison between the Raman modes of the tetrahedral network in amorphous and layered crystalline GeSe2

The characteristic A₁ peak at 199 cm⁻¹ in the Raman spectrum of amorphous GeSe₂ were compared with the peaks at 211 and 216 cm⁻¹ in the spectrum of crystalline GeSe₂. It was proved that the crystalline 216 cm⁻¹ peak is an intrinsic mode which is enhanced by the bulk exciton transition. From a model calculation using a valence force field and bond polarizability, the 211 cm⁻¹ peak was assigned to in-phase breathing vibrations extended along the GeSe₄ tetrahedral chain structure, while the 216 cm⁻¹ peak was attributed to in-phase breathing vibrations quasi-localized at the GeSe₄ edge-sharing tetrahedra. The phonon density of states in the crystal has a doublet peak similar to the amorphous Raman spectrum. A correspondence between the amorphous and the crystalline Raman spectra was proposed.     

Photo-enhanced crystallization by laser irradiation and thermal annealing in amorphous GeSe2

The laser irradiated crystallization process in amorphous GeSe₂ was studied by Raman scattering to discriminate between thermal and photo-induced contributions. Amorphous GeSe₂ crystallizes into a low-temperature (LT) form, LT-GeSe₂, and a high-temperature (HT) form, HT-GeSe₂. An amorphous sample was annealed at various temperatures and times in an electric furnace with and without laser irradiation. The laser power was adjusted to about 20 mW having a 0.4 mm spot size to determine the photo-induced effect. The difference between the thermal and laser irradiated crystallization process was discussed by considering the activation energy of crystallization. The activation energy due to laser irradiation above the band-gap energy was found to be smaller than that for the pure thermal crystallization process. We present a model for the photo-enhanced effect in which the lone-pair electrons of Se atoms are excited by the photons and the bond switching or atomic rearrangement occurs when the electrons relax through an electron–phonon interaction.     

Structure and properties of the pure and Pr-doped Ge25Ga5Se70 and Ge30Ga5Se65 glasses

The Ge₂₅Ga₅Se₇₀ and Ge₃₀Ga₅Se₆₅ pure and Pr³⁺-doped glasses were prepared by direct synthesis from elements and PrCl₃. It was found that up to 1 mol% PrCl₃ can be introduced in the Ge₂₅Ga₅Se₇₀ and Ge₃₀Ga₅Se₆₅ glasses. Both types of glasses with overstoichiometric and substoichiometric content of Se were homogeneous and of black color. The optical energy gap is E^opt_g=2.10 eV, and the glass transition temperature is T_g=543 K for Ge₂₅Ga₅Se₇₀ and T_g=633 K for Ge₃₀Ga₅Se₆₅. The long-wavelength absorption edge is near 14 μm and it corresponds to multiphonon processes. Doping by Pr³⁺ ions creates absorption bands in transmission spectra, which can be assigned to the electron transitions from the ground ³H₄ level to the higher energy levels of Pr³⁺ ions ³H₅, ³H₆, ³F₂, ³F₃ and ³F₄, respectively. By excitation with YAG:Nd laser line (1064 nm), two intense luminescence bands (1343 and 1601 nm) were excited. The first band can be ascribed to electron transitions between ¹G₄ and ³H₅ energy levels of Pr³⁺ ions. Full width at half of maximum (FWHM) of the intensity of luminescence was found to be 70 nm for (Ge₂₅Ga₅Se₇₀)_1 − x(PrCl₃)_x and (Ge₃₀Ga₅Se₆₅)_1 − x(PrCl₃)_x glasses. The FWHM in selenide glasses is lower than in halide and sulphide glasses. The second luminescence band (1601 nm) can be probably ascribed to the transitions between ³F₃ and ³H₄ energy levels of Pr³⁺ ions. The absorption and luminescence spectra of Pr³⁺ ions in studied glasses are slightly influenced by stoichiometry of glassy matrix. The Raman spectra of studied glasses were deconvoluted and assignment of Raman bands to individual vibration modes of basic structural units was suggested. The structure of studied glasses is mainly formed by corner-sharing and edge-sharing GeSe₄ tetrahedra. The vibration modes of Ga-containing structural units were not found, they are apparently overlapping with Ge-containing structural units due to small difference between atomic weights of Ge and Ga. In the glasses with substoichiometry of Se, the Ge–Ge bonds of Ge₂Se₆ structural units were found. In Se-rich glasses the Se–Se vibration modes were found. In all studied glasses also ‘wrong' bonds between like atoms were found in small amounts. Maximum phonon energy of studied glasses is 320 cm⁻¹.     

Vibrational spectroscopy of LaBSiO5 glass and glass–crystal composites

Infrared (IR) reflectivity and Raman scattering spectra of LaBSiO₅ glass and glass–crystal composites were studied in the temperature range 25–260 °C. Using an analogy with the LaBGeO₅ crystal it was possible to assign the main spectral features of the LaBSiO₅ glass and glass–crystal composites. The BO₄ chain arrangement and the bending vibrations of SiO₄ are influenced by the loss of the long-range order in the glass whereas the stretching vibrations of the SiO₄ groups are practically unaffected. The structural disorder in LaBSiO₅ crystallites is caused by rotation of BO₄ tetrahedra.     

A comparison of the medium-range order in glassy GeS2 and Sb2S3

In amorphous (a-) GeS₂ there is only one medium-range order (MRO) dependent X-ray scattering feature, i.e., the ‘first sharp diffraction peak’ (FSDP), whereas in a-Sb₂S₃ all scattering features in the interval 1–2.4 Å⁻¹ are in fact MRO-dependent. The basic element of MRO in a-GeS₂ is a part of a chain of corner bound tetrahedra extracted from the high temperature form of c-GeS₂; in a-Sb₂S₃ it is a part of a crystalline-like (Sb₂S₃)_n band. Ordering of parts of these bands is characterized by a greater interband separation than in the crystal. By contrast with a-GeS₂, the MRO-dependent medium-angle scattering features in a-Sb₂S₃ are less sensitive to fluctuations of the interband (intercluster) ordering distance.     

Phonons in TiO2---SiO2 glasses

We report the infrared and Raman studies of titanium doped vitreous silica glasses for a number of titanium concentrations. The vibrational modes associated with the randomly oriented chains of SiO₄ tetrahedra show broadenings and shifts. The LO-TO splitting of some Raman active modes decreases with increasing titanium concentration. This is attributed to the decrease in long-range coulomb fields associated with the chains of SiO₄ tetrahedra which are broken by the titanium atoms. The results are discussed in the context of random network models. An increase in the average intertetrahedral angle of the SiO₄ network is calculated from the data. This explains the anomalous decrease in the density of TiO₂---SiO₂ glasses with increasing titanium content. We identify two new modes associated with the distorted titanium tetrahedra. A polarized Raman mode at 1115 cm⁻¹ which is infrared inactive and an unpolarized Raman mode at 945 cm⁻¹ which is infrared active are observed.     

Preparation and characterization of Ni-incorporated FeS2 single crystals

Nickel-incorporated FeS₂ single crystals with various Ni compositions of Fe_0.99S₂:Ni_0.01, Fe_0.98S₂:Ni_0.02, Fe_0.96S₂:Ni_0.04, and Fe_0.9S₂:Ni_0.1 were grown by chemical vapor transport (CVT) method using ICl₃ as a transport agent. Physical properties of the Ni-incorporated FeS₂ single crystals were characterized using X-ray diffraction, Raman spectroscopy, electrical conductivity, and photoconductivity (PC) measurements. By means of the analyses of the X-ray diffraction patterns, the whole series of Ni-doped FeS₂ single crystals were determined to be single-phase and isostructural. Raman spectroscopy of the Ni-doped FeS₂ crystals was carried out at room temperature. Raman resonant peaks of the Ni-doped FeS₂ crystals demonstrate an energy red-shift behavior with respect to the increase of the dopant densities. Conductivity measurements show the resistivity of the Ni-doped FeS₂ decreased as the doping concentration of Ni is increased. Nickel is an n-type dopant, which behaves like a donor level existed near the conduction band edge of the synthetic FeS₂. On the other hand, dopant effect of nickel on the synthetic FeS₂ also destroys the photoconductive sensitivity in the photoconductivity measurements.     

Raman spectrum studies of the glasses in the system Na2O---Al2O3---P2O5

Raman spectra of ternary sodium aluminosphosphate glasses indicate that for glasses with Al₂O₃/P₂O₅<0.63, the glass network is mainly built up of (PO₃)_n^n- chains and rings or different kinds of phosphate groups and AlO₄ tetrahedra; for glasses with Al₂O₃/P₂O₅>0.63, the glass network is mainly built up of AlPO₄ groups.     

Raman spectra of Ag- and Cu-photo-doped As40S60−xSex films

Raman spectra of the chalcogenide vitreous layers (As₄₀S₆₀, As₄₀S₄₀Se₂₀, As₄₀S₂₀Se₄₀, As₄₀Se₆₀) non-doped and photo-doped by Ag, Cu were measured. The spectra were analyzed in terms of a molecular model. It was ascertained that, for the spectra of photo-doped As₄₀S_60−xSe_x layers, the shift of the main bands to the high frequency side and the appearance of the additional scattering bands in the low frequency spectral range are characteristic features. Variations in the Raman spectra with photodoping by Ag or Cu are consistent with the supposition concerning normal covalent and coordinative bond formation between metal additives and chalcogen atoms, which results in the different activation energy values for the silver photo-stimulated diffusion into chalcogenide layers.     

Identification of gaseous oxygen and nitrogen in bubble inclusions in Bi4(GeO4)3 (BGO) crystals by means of Raman spectroscopy

Bubble inclusions in BGO crystals have been studied by means of Raman spectoscopy. Since the crystals were grown under ambient atmospheric conditions, we focused our attention on oxygen and nitrogen. Both species are diatomic homonuclear molecules which are vibrationally Raman active. Concerning the vibrational part of the O₂ and N₂ Raman spectra, no evidence was found for the presence of both gases. However, in the spectrum below the lowest 64 cm^-1 energy mode of BGO three faint peaks were found which show the characteristic features of the rotational spectra of O₂ and N₂. It was found that the composition of the inclusion atmosphere was enriched by oxygen compared to the composition of the air.     

Solution growth of CuGaS2 and CuGaSe2 using CuI solvent

The phase diagrams of the CuGaS₂-CuI and CuGaSe₂-CuI pseudobinary systems are investigated. Both systems show eutectic behavior; liquids along the liquids curves for solute concentrations of more than 15 mol% are shown to be single phase and solids in equilibrium with the liquids are demonstrated to be CuGaS₂ and CuGaSe₂ chalcopyrite compounds. On the basis of the results, bulk crystals of CuGaS₂ and CuGaSe₂ are prepared from CuI solutions using the Bridgman technique. The photoluminescence spectra of these crystals are nearly the same as those of the crystals grown by iodine transport method.     

Photoluminescence from E band centers in amorphous and crystalline SiO2

Photoluminescence (PL) originating from intrinsic defects in neutron-irradiated crystalline (x-) SiO₂ is observed and compared with that from amorphous (a-) SiO₂. The PL and photoluminescence excitation (PLE) spectra of x-SiO₂ are qualitatively similar to those of a-SiO₂. This suggests that PL centers, similar to those found in amorphous systems, are present in ordered systems. In addition, the PL and PLE spectra a-SiO₂ and a-As₂S₃ scale with energy band gap, suggesting that the radiative processes in a-SiO₂ and semiconducting chalcogenide glasses are alike. The similarity of the PL from x-SiO₂ to that from a-SiO₂ and the scaling of the PL from a-SiO₂ and a-As₂S₃ with energy band gap strongly suggest that PL centers are unique bonding configurations which primarily depend on the chemistry of the system rather than on the degree of long range order.     

Electronic structure and charge transport properties of amorphous Ta2O5 films

Amorphous Ta₂O₅ films were deposited by sputtering Ta onto silicon substrates with reactive ion beam. Electron energy loss spectroscopy measurements on the film found that the plasma oscillation energy is 23.1 eV. The refractive index and the extinction coefficient were measured with spectroscopic ellipsometry over the spectral range of 1.9–4.9 eV. The optical band gap is found to be 4.2 ± 0.05 eV. The valence band consists of three bands separated by ionic gaps. The values of electron effective masses were estimated with DFT quantum-chemical calculation. Experiments on injection of minority carriers from silicon into oxide were also conducted and we found that the electron component of conduction current governed by the electron current in the amorphous Ta₂O₅.     

Crystal growth of NdAl3(BO3)4 from K2O/MoO3/Nd2O3/B2O3/KF flux

NdAl₃(BO₃)₄ single crystals were grown by the flux method and the TSSG technique using a K₂O/3MoO₃/B₂O₃/0.5Nd₂O₃/KF flux system. Light-violet clear crystals could be obtained. The effects of fluoride on the growth of NAB crystals were investigated. As the content of KF was gradually increased, the growth form of NAB was changed from the equant to the columnar and the primary crystalline region of NAB was shrinked. At the ratio of KF/K₂O = 0.75, NAB crystals could not be grown.     

Structural analysis of CaSiO3 glass by X-ray diffraction and Raman spectroscopy

The short-range structure of CaSiO₃ glass was studied by a combination of radial distribution analysis with an intensity comparison method on the basis of X-ray diffraction data. Three main peaks appeared at 1.64, 2.43 and 3.64 Å in the radial distribution function curve. The Raman bands were observed at 367, 642, 878 and 972 cm⁻¹ and the Raman spectrum was found to be similar to that observed in pyroxenes: single chains consisting of SiO₄ tetrahedra are linked by calcium ions laterally. The calcium ion is surrounded by six oxygen atoms at an average distance of 2.34 Å and the remainder Ca---O pairs at a distance of 2.54 Å. The calculated intensity curve based on this model was in good agreement with the observed one.     

Raman spectroscopic study on the structure of ZnCl2---ZnX2 and ZnCl2---KX (X = Br, I) glasses

Raman spectra have been measured for ZnCl₂---ZnX₂ and ZnCl₂---KX (X = Br, I) glasses to investigate the structure of the glasses with varying composition. The assignment of each band was made, and the change of the spectra with composition was explained in terms of the bridging and non-bridging states of halide ions and the change of the tetrahedral units, ZnX_nCl_4−n²⁻ (n = 0–4), formed in the glasses. As the content of ZnX₂ in ZnCl₂---ZnX₂ glasses increases (20 → 80 mol%), the peak frequency of the Zn---Cl stretching mode increases (238 → 248 cm⁻¹ in X = I glasses, 238 → 259 cm⁻¹ in X = Br glasses) while the Zn---I and Zn---Br stretching frequencies decrease (173 → 120 cm⁻¹ for Zn---I, 196 → 157 cm⁻¹ for Zn---Br). The decrease of the Zn---I and Zn---Br band frequencies was attributed to the increase of the number n of the ZnX_nCl_4−n²⁻ tetrahedra. The increase of the Zn---Cl frequency suggests the existence of the bonding state of Cl⁻ ions which is intermediate between the bridging and the non-bridging states. In ZnCl₂---KX glasses, the Zn---Cl_non-bridging band at about 300 cm⁻¹ was observed in addition to the bands observed in ZnCl₂---ZnX₂ glasses. The addition of KX produces non-bridging anions while the tetrahedral units, ZnX_nCl_4−n²⁻ are also formed.     

Short-range order in ultrafine powder of an amorphous (Fe7Ni3)69B31 alloy

The short-range order structure of ultrafine powder of an amorphous (Fe₇Ni₃)₆₉B₃₁ alloy has been studied by ZAFS. The particles with diameters 1.7–15.3 nm are prepared by chemical reduction. The fact that the local environments around Fe or Ni are not the same suggests that there is chemical short-range order in this material. The mean distances of near neighbors surrounding an Fe and a Ni atom are shorter than those in amorphous Fe₈₀B₂₀ ribbon. The XANES spectra also show some differences between these two amorphous alloys. These indicate that the interaction between near neighbors is stronger in this amorphous ultrafine powder than in bulk amorphous material.     

Growth and properties of Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 single crystals directly from melt

Pb[(Zn_1/3Nb_2/3)_0.91Ti_0.09]O₃ (PZNT91/9) single crystals were grown by a modified Bridgman method directly from melt using an allomeric Pb[(Mg_1/3Nb_2/3)_0.69Ti_0.31]O₃ (PMNT69/31) single crystal as a seed. X-ray diffraction (XRD) measurement confirmed that the as-grown PZNT91/9 single crystals are of pure perovskite structure. Electrical properties and thermal stabilization of PZNT91/9 crystals grown directly from melt exhibit different characters from those of PZNT91/9 crystals grown from flux, although segregation and the variation of chemical composition are not seriously confirmed by X-ray fluorescence analysis (XPS). The [0 0 1]-oriented PZNT91/9 crystals cut from the middle part of the as-grown crystal boules exhibit broad dielectric-response peaks at around 105 °C, accompanied by apparent frequency dispersion. The values of piezoelectric constant d₃₃, remnant polarization P_r, and induced strain are about 1800–2200 pC/N, 38.8 μC/cm², and 0.3%, respectively, indicating that the quality of PZNT crystals grown directly from melt can be comparable to those of PZNT91/9 single crystals grown from flux. However, further work deserves attention to improve the dielectric properties of PZNT crystals grown directly from melt. Such unusual characterizations of dielectric properties of PZNT crystals grown directly from melt are considered as correlating with defects, microinhomogeneities, and polar regions.     

Czochralski grown Ga2O3 crystals

We report on the successful growth of β-Ga₂O₃ single crystals using the Czochralski method. Model calculations show that the gas phase consists of Ga₂O, GaO or Ga independent of the ratio of oxygen and Ar or N₂. We find that for growing single crystals the evaporation has to be suppressed by a finite amount of oxygen. A CO₂/Ar gas atmosphere was found to meet this requirement.     

A new technique for the growth of superconducting YBa2Cu3O6 + δ crystals completely separated from flux

Growth of completely flux-separated YBa₂Cu₃O_{6 + δ} (referred to as 123 phase) crystals using a novel technique is described. The technique employs a modification of the seed pulling method commonly used in crystal growth. The crystals are grown in the temperature range of 960–1000°C using a BaCuO₂ flux. A 123 flux ratio of 1:5 is maintained. Photographs of the crystals and photomicrograph of the surfaces are presented to show complete flux-separation of the crystals measuring 6 mm × 3 mm × 1 mm. The Raman spectra recorded on the as-grown crystals show that they are in the tetragonal phase. Magnetic susceptibility measurements on crystals annealed in an oxygen atmosphere show a superconducting transition starting at 71 K. The present technique offers a possibility of growing large, completely flux-separated crystals of 123 for superconductivity research.