Yanko B. Dimitriev

Infrared-spectral investigation of tellurites

The IR-spectra are studied and the TeO-stretching vibrations are assigned of α-TeO2 and 16 tellurites, built up by TeO3-, TeO4- or combinations of these polyhedra. Bands in the 800-580 cm−1 region are assigned to stretching vibrations of the Te-O bonds on the basis of known spectral data on isolated molecules of the xy3 and xy4 types. It is shown that the presence of two bands in the 700-550 cm−1 range is due to the decrease in the symmetry of the TeO3- or of the TeO4-groups. In the first case this is the result of a removal of the degeneracy of the νdTeO3 and in the second, the intensity of νasTeO2ax increases. The results obtained may be used for a further characterization of the structure of tellurite phases.

Glass formation and structure of glasses in the V2O5MoO3Bi2O3 system

The glass formation region in the V2O5MoO3Bi2O3 system was investigated by the roller-quenching method. Low melting glasses were obtained in the MoO3- and V2O5-rich compositions. Characterization of the glasses was made by X-ray diffraction, differential thermal analysis (DTA) and IR spectroscopy. According to the DTA data, the glass transformation temperature, Tg, for the different compositions varied between 200 and 290°C and the cystallization temperature, TX, was within the interval of 225–330°C. Structural models for glasses of th MoO3Bi2O3 and V2O5Bi2O3 systems were suggested on the basis of IR spectral investigations, by comparing with known crystalline structures. It was shown that the glasses possess [MoO4], [MoO6], [VO4], and [BiO6] groups as basic structural units.

Infrared and Raman spectra of Ga2O3–P2O5 glasses

Abstract The structure of x Ga 2 O 3 (1−x) P 2 O 5 (0.15 ⩽ x ⩽ 0.4) glasses is studied by infrared (IR) and Raman spectroscopy. The results of quantitative electron microprobe analysis reveal that two sets of glasses are formed with compositions corresponding to the gallium meta and pyrophosphates. Crystalline Ga(PO 3 ) 3 metaphosphate is also obtained and its IR and Raman spectra recorded in order to examine the spectral differences glass → crystal. The IR spectra of metaphosphate glasses show bands at 1250, 930, 775 and 485 cm −1 which, are assigned to the ν as PO 2 , ν as POP, ν s POP and δPO 2 modes of the metaphosphate chain. In the Raman spectra are found patterns due to symmetric modes of PO 2 terminal and POP bridging bonds as well as features due to vibrations of GaO 4 tetrahedra and Ga–O–P bridges. Comparisons of the spectra of glasses with those of the crystal indicate that the interaction of Ga with the polyphosphate anions is more covalent in the glasses. The bands observed in the spectra of pyrophosphate glasses provide evidence for the involvement of ortho-, pyro- and polyphosphate groupings in the glass structure. The Raman spectra of both sets of glasses display intense low frequency bands, which we assigned to a symmetric bending of Ga–O–P bridging bonds involving four-coordinated gallium atoms. It is concluded that structure of Ga 2 O 3 –P 2 O 5 glasses is a highly polymerized network consisting of a variety of phosphate anions cross-linked by GaO 4 tetrahedra. This account on glass structure is consistent with the great glass forming ability of the compositions studied.

Glass formation and structure in the V2O5Bi2O3Fe2O3 glasses

Abstract Glass formation V2O5Bi2O3Fe2O3 system was investigated by the roller-quenching method and low melting V2O5- and Bi2O3-rich glasses were obtained. Glasses were characterized by X-ray diffraction, differential thermal analysis (DTA), IR and Mossbauer spectroscopy. The glass transformation temperature, Tg, varied between 270 and 442°C, and the crystallization temperature, Tx, was in the range of 342–550°C. Structural models for glasses of the V2O5Bi2O3Fe2O3 system are suggested on the basis of IR and Mossbauer spectral investigations. It was shown that the glasses posses [VO5], [VO4], [BiO6], [FeO6] and [FeO4] groups as basic structural units.

Glass formation and structure in the system MoO3–Bi2O3–Fe2O3

Abstract The glass formation region in the MoO 3 –Bi 2 O 3 –Fe 2 O 3 system was investigated by the roller-quenching method. Glasses melted at temperatures 3 -rich compositions. Analysis of the glasses was made by X-ray diffraction, differential thermal analysis (DTA), infra-red spectroscopy (IR), and Mossbauer spectroscopy. According to the DTA data, the glass transformation temperature, T g , for the different compositions varies between 360°C and 440°C and the crystallization temperature, T x , is in the range 390–470°C. Structural model for the glasses are suggested on the basis of IR and Mossbauer spectral data. In the region of MoO 3 rich compositions, the network forming units MoO 6 are connected by Mo–O–Mo bridging bonds. The presence of Me 2 O 3 (Me=Bi, Fe) leads to transformation of MoO 6 to MoO 4 . Thus, in a wide region of compositions the glass network has a scheelite-like structure containing isolated MoO 4 structural units which are surrounded by MeO 6 groups.

The structure of vitreous V2O5-TeO2

Abstract A high resolution neutron diffraction study of a sample of vitreous V2O5–TeO2, containing 5 mol% V2O5, has been performed using the LAD time-of-flight diffractometer at the ISIS spallation pulsed neutron source. Reciprocal space data were obtained to high scattering vectors, Q, and have been Fourier transformed to yield the real space correlation function, T(r). The first neighbour Te–O and O–O peaks in T(r) have been investigated using peak fitting techniques which suggest the presence of both TeO3 and TeO4 units. The shortest Te–O bond length is 1.91 A, with two further contributions at 2.10 and 2.17 A, while the average O–O distance within the TeOn structural units is 2.76 A. A comparison with the crystalline polymorphs of TeO2 indicates that the structure of the glass is nearer to α-TeO2 than to β-TeO2. The results from the present study are compared to earlier neutron diffraction data for a V2O5–TeO2 glass containing 10.2 mol% V2O5. Fourier transform infrared spectra have been recorded for a series of three V2O5–TeO2 glasses over the range 400–1000 cm−1 and analysed using deconvolution techniques. The results obtained are in good agreement with the neutron data, particularly in respect of the existence of TeO3 and TeO4 structural units.

Structure of V2O5MoO3Fe2O3 glasses

Abstract Infrared spectra have been examined in V2O5MoO3Fe2O3 glasses, and the results were compared with infrared spectra of known crystals. This study has shown that structural units formed in the glasses include [VO5] groups (bands at 1020-930 cm−1), isolated [VO4] and [MoO4] tetrahedra (a band at 840 cm−1), [FeO4] tetrahedra and [FeO6] octahedra (bands at 660 cm− and 520-480 cm−1). Glass formation is discussed on the basis of glass structure. This study has established that the appearance of isolated tetrahedral groups in the network without MOM bridging bonds decreases the glass-formation ability.

Non-isothermal crystallization kinetics of V2O5-MoO3-Bi2O3 glasses

Characteristic temperatures, such as Tg (glass transition), Tx (crystallization temperature) and Tl (liquidus temperature) of glasses from the V2O5-MoO3-Bi2O3 system were determined by means of differential thermal analysis (DTA). The higher content of MoO3 improved the thermal stability of the glasses as well as the glass forming ability. The non-isothermal crystallization was investigated and following energies of the crystal growth were obtained: glass #1 (80V2O5·20Bi2O3) EG=280 kJ mol-1, glass #2 (40V2O5·30MoO3·30Bi2O3) EG=422 kJ mol-1 and glass #3 (80MoO3·10V2O5·10Bi2O3) EG=305 kJ mol-1. The crystallization mechanism of glass #1 (n=3) is bulk, of glass #3 (n=1) is surface. Bulk and surface crystallization was supposed in glass #2. The presence of high content of a vanadium oxide acts as a nucleation agent and facilitates bulk crystallization.

Glass structure of the Ag2O-TeO2-V2O5 system

During the last years the interest to study the silvercontaining glass-forming systems has increased, due to the possibility to obtain materials exhibiting high ion conductivity [1–5]. By now mainly P2O5 and B2O3 have been used as a glass former for the ionic conducting glasses. A few publications are known on the synthesis of ionic conductive tellurite glasses [6–9] and also for V2O5 containing glasses [10–15]. In our previous investigations of the system Ag2OTeO2-V2O5 [16] a wide region of glasses (Fig. 1), which are synthesized in the central part of the system up to 40 mol.% Ag2O, using low cooling rate (≤100 degree/min) has been found. Meanwhile, many glass compositions in this system rich of V2O5 exhibit semiconducting properties [17], but with increasing Ag2O content high ion conductivity [18] is revealed. By the heat treatment of glasses in the 300–350 ◦C range, they easily crystallize with fine microstructure typical for glass-crystalline materials. The phase diagram of the Ag2O-TeO2-V2O5 system was also examined in detail [19, 20]. The fields of primary crystallization of twelve phases were outlined: 2TeO2·V2O5, Ag2O·TeO2, 3Ag2O·V2O5, 2Ag2O·V2O5, Ag2O· V2O5, Ag2O·2V2O5, Ag2O·7V2O5, TeO2, V2O5, Ag2O, Ag2O·3V2O5·6TeO2, and Ag2O·V2O5·2TeO2. In the Ag2O rich area dissociation of Ag2O to elementary silver has been noticed. This variety of crystal phases is a prerequisite for the synthesis of glasses with different structure. The glass structure of some compositions was also investigated. It is proved that the three-dimensional tellurite structure is destroyed and Te-non-bridging oxygen (NBO) bonds are formed with the increase of Agion content in the glasses of the Ag2O-TeO2 system [21]. The increase of Ag2O content up to 30 mol.% leads to the formation of TeO3 groups. S. Rossignol et al. investigate the structure of glasses from the system TeO2-AgO0.5-AgI. These glasses contain deformed TeO4 groups such as TeO3+1 groups. The structure of the tellurite network is not modified by the addition of AgI [7, 9]. Our purpose is to study the thermal stability and modifications of the structure of glasses with increase of silver ions in the glass formation range of the Ag2O-TeO2-V2O5 system. The determination of specific structural units of the glass network is made by IR-spectroscopy in accordance to the concept about the independent vibrations of separate groups in the glasses. They were discussed in detail many years ago by Tarte [22, 23] and Condrate [24, 25]. Glass compositions were chosen with a different Ag2O/TeO2/V2O5 ratio along Ag2O 2TeO2·V2O5 (A) and TeO2 Ag2O·2V2O5 (B) lines (Fig. 1). The choice was made having in mind the previous investigations of the glass forming region and the type of crystal phases, which could separate according to the phase diagram, during the thermal treatment procedure. As initial precursors for the synthesis were used reagent grade TeO2, V2O5 and Ag2O. After homogenization, the bathes were melted in porcelain crucibles at 600– 800 ◦C temperature range. The melts were quenched rapidly between two polished copper plates. A twin roller quenching technique was used for the compositions with high Ag2O content beyond the traditional glass-forming region. The samples were identified by X-ray powder diffractometer Philips APD 15 (Cu Kα radiation). The thermal behavior was investigated using Perkin-Elmer DSC 4 differential scanning calorimeter. The structure of the glasses was investigated by infrared absorption spectroscopy (IR) using SPECORD-M80 spectrometer preparing powder samples dispersed in Nujol. On Fig. 2 are presented DSC-curves and in Table I are summarized some thermal parameters of the glasses along a selected line (A). The glass-forming ability of melts and the thermal stability of glasses are interpreted using some criteria [26, 27]: (Tc− Tg), Tg/Tl,

Electron diffraction study of the short-range order in glasses of the system TeO2B2O3

Abstract The short-range order in vitreous TeO 2 and B 2 O 3 and in TeO 2 B 2 O 3 glasses has been studied using electron diffraction. The amorphous samples investigated were prepared by two methods. Powdered samples were produced either by rapid quenching using the roller technique (TeO 2 glass) or by slow cooling (B 2 O 3 and TeO 2 B 2 O 3 glasses) and amorphous TeO 2 and TeO 2 B 2 O 3 thin films were deposited by vacuum evaporation with resistive heating. According to the electron diffraction data, the TeO 4 polyhedron is shown to be the main structural unit in the TeO 2 glass. The boron atoms in the B 2 O 3 glass are found to be threefold-coordinated with respect to the oxygen and the presence of B 3 O 6 groups is indicated. In the binary glasses mainly TeO or BO distances are resolved, depending on their composition. The results for the powdered and thin film samples are compared and a change in the TeO distances for the films is established.