Showing papers in "Journal of Non-crystalline Solids in 1982"

Sol-gel transition in simple silicates II☆

Abstract: Silicate gels were prepared under a range of conditions in which the rate of hydrolysis was varied from fast to slow with respect to the rate of condensation. When hydrolysis was fast, larger, more highly condensed polymers were formed during gelation. Conversely, for slow hydrolysis, smaller, less highly condensed polymers were formed. These gels dried to low density coarse textured and high density fine textured gels, respectively. High temperatures, (>800°C) were required to densify the coarse gels by viscous sintering. Lower temperatures were sufficient to densify fine gels by a process which was postulated to consist of polymer relaxation followed by condensation and pore collapse.

Raman and infrared spectra on silica gel evolving toward glass

Abstract: Infrared and Raman spectra of gels obtained from specially prepared solutions of Si(OC2H5)4 (TEOS) which have been thermally treated in the 40–800°C temperature range, are reported and discussed with reference to the spectra of fused quartz. The results show that the gel to glass transformation is an hydrolytic polycondensation process, which takes plase gradually and is practically completed in the samples treated at 800°C. Particularly revealing is the behaviour of the bands due to OH (associated and/or unassociated H-bonded) stretching modes, including those of water, which are weakened by increasing temperature; the behaviour of the band due to the SiOH stretching mode; the behaviour of the network bands, some of which become more intense with increasing temperature, due to asymmetric and symmetric stretching and the bending SiOSi modes. In the Raman spectra of gels heated at different temperatures two peaks also appear at ≈600 and ≈490 cm−1, which are also present in the Raman spectrum of fused quartz, they are related to network defects of the glass structure, for which no definite interpretation has yet been given in the literature.

The size of cooperatively rearranging regions at the glass transition

Abstract: For six glass forming substances of very different kinds the volumes of cooperatively rearranging regions (CRR) and the corresponding correlation lengths are calculated using parameters obtained by Moynihan et al. from thermograms and formulas from a fluctuation theory of glass transition. The correlation lengths range from 0.7 nm for a crown glass up to 2.2 nm for a lubricant oil. The CRR contains between 35 and 290 strongly interacting particles. The consequences of such wide correlations are discussed with respect to the relations between the dynamic glass transition, the freezing-in process, and the structure of glasses.

The sol-gel transition in the hydrolysis of metal alkoxides in relation to the formation of glass fibers and films

Abstract: The sol-gel transition of Si(OC 2 H 5 ) 4 was investigated by measuring the volume, the density, the SiO 2 content, the viscosity and the specific and intrinsic viscosities of the solution. The transition behavior of Si(OC 2 H 5 ) 4 solutions with different amounts of water and different catalysts was compared. The measurements indicated that solutions with a small water content were comprised of linear polymer when catalyzed by HCl. Fibers could be easily drawn from such solutions immediately before gelation. Solutions catalyzed by HCl and containing a large amount of water behaved similarly to solutions catalyzed by NH 4 OH, suggesting that a high water content causes the formation of non-linear or network colloidal polymers in hydrolysis-polycondensation. Such solutions did not exhibit spinnability but gelled into elastic bulk masses which were easily converted to bulk glasses.

Planar rings in vitreous silica

Abstract: An energy minimization argument shows that 2- and 3- fold rings of SiO bonds in vitreous silica can be expected to be quite planar. Evidence supporting the existence of planar 3-fold rings is found in the sharp line at 606 cm−1 in the Raman spectra. The method of argument is general, and suggests that similar sharp Raman lines reported in several other glasses may signify the existence of small planar rings in concentrations sometimes greater than 1%.

The structures and vibrational spectra of crystals and glasses in the silica-alumina system

Abstract: Solar furnace melting and fast-quench techniques have been used to prepare SiO2Al2O3 glasses to high alumina content (near 60 mol% Al2O3), which have been studied by Raman spectroscopy. These spectra may not be simply interpreted. The structures of crystalline compounds in the SiO2Al2O3 system are discussed in relation to their vibrational spectra. On the basis of this discussion and other considerations, a structural model for the silica-alumina glass system is proposed, which is consistent with the stable or metastable immiscibility suggested along this join. The essential features of this model include a modified silica structure at low alumina content, and “structure-broken” regions at high alumina compositions, with silicon in tetrahedral coordination, but aluminium assuming a variety of bonding geometries. These are proposed to include aluminate tetrahedra with higher polymerization than simple corner-sharing, and less well-defined polyhedra of higher average coordination number.

History and principles of the sol-gel process, and some new multicomponent oxide coatings

Abstract: Certain historical facts are discussed regarding the principles of the sol-gel process and some new applications are mentioned. Forty years ago Geffcken and Bergen prepared single oxide coatings by the sol-gel process and Schroeder developed a thin film physics. The first products appeared on the market in 1953 and large scale production in 1959 with automotive rear-view mirrors, later anti-reflection coatings and sunshielding windows were introduced using TiO 2 and SiO 2 . Dislich and Hinz elaborated the chemical basis for the preparation of multicomponent oxides, glasses, glass ceramics and crystalline substances in 1969 and published their results in 1971. So from 1971 on it was known that any type of multicomponent oxide can be synthesized using the alkoxides of the different elements by the sol-gel process. Of the three steps; complexation, hydrolysis, polycondensation, it is the first step that has been newly introduced. The first product on the market now for some years is a silicate phosphate layer which improves the hydrolytic stability of optical glasses, used in laser protective filters manufactured by the Deutsche Spezial Glas AG. An 8-component glass ceramic (Si, Al, P, Li, Mg, Na, Ti, Zr) and a crystalline cadmium stannate layer (Cd 2 SnO 4 have been prepared.

Glasses from melts and glasses from gels, a comparison

Abstract: In recent years, an increasing number of glasses have been prepared by the sol-gel method. Many of these glasses have identical chemical compositions to glasses prepared by the conventional method of cooling the melts. The sol-gel glasses do not involve the melting of crystalline mixtures. It is of both scientific and technical interest to make comparisons between the two different processes of preparing glasses which have the same chemical compositions. It is important to inquire in the structure and microstructure of these glasses are similar, and if not, whether some important properties for certain applications are then different. In this paper, published data on one-component and polycomponent sol-gel glasses are utilized for comparison. The advantages and disadvantages of the sol-gel method as compared with the conventional melting method are discussed.

DC conduvtivity of V2O5TeO2 glasses

Abstract: The dc conductivity of semiconducting vanadium tellurite glasses of compositions in the range 50 to 80 mol% V2O5 has been measured in the temperature region 77 to 400 K. Measurements have been made on annealed samples at different annealing temperatures. Annealing the samples at temperature of about 250°C causes the appearance of a complex crystalline phase resulting in an increase of conductivity. Results are reported for amorphous samples of different compositions. The conductivity of tellurite glasses is slightly higher than the corresponding composition of phosphate glasses, but the general trend of the increase of conductivity and decrease of high temperature activation energy with increasing V2O5 content is similar in the two systems. The data have been analysed in the light of existing models of polaronic hopping conduction. A definite conclusion about the mechanics of conduction (adiabatic or nonadiabatic) is difficult in the absence of a precise knowledge of the characteristic phonon frequency v0. Adiabatic hopping is indicated for v0∼1011 Hz, however this value leads to unreasonably low value for the Debye temperature θD, and higher values for v0∼1013 hz satifiies the conditions for nonadiabatic hopping which appears to be the likely mechanism of conduction in V2O5TeO2 glasses. The low temperature data (< 100 K) can be fitted to Mott's variable range hopping, which when combined with ac conductivity data gives reasonable values of α, but a high value for the disorder energy.

Chemical durability of glasses

Abstract: Glass has many excellent properties, e.g. good chemical resistance. This is particularly evident in natural glasses but also in glasses made by human hand. The first period in this field of research is characterized by obtaining a great deal of empirical experience and the collection of data. The main dependences are to be found in the second period, while in the third period the physico-mathematical treatment begins, especially the interpretation of the chemical durability as a diffusion process. The present or fourth period is characterized by the use of new methods for analysing the leached glasses. This new knowledge and the critical judgement of the hitherto known results show that it is necessary to correct some existing conceptions. This is true especially with regard to the leached layers where the glass structure and H2O molecules seem to play an important role for the chemical durability. During leaching the structure of the glass is changed and the moving particles are altered, which means that some fundamentals for applying the usual diffusion equations are not valid. Therefore the fifth period of the chemical durability claims new ideas.

A neutron diffraction study of the structure of evaporated amorphous germanium

Abstract: Neutron diffraction techniques have been employed to obtain an accurate, high resolution correlation function for evaporated amorphous Ge. The structure of amorphous Ge is unlike that of any of the related crystalline polymorphs, but is much more accurately described in terms of a tetrahedrally bonded random network. The best fitting random network models, however, all predict too much structure in the correlation function at high r and severa; exhibit too narrow a distribution of bond angles.

Characterisation of models of multicomponent amorphous metals: The radical alternative to the Voronoi polyhedron

Abstract: A generalisation of the Voronoi polyhedron procedure for examining the detailed local structures in multicomponent metallic glass models is described. The resulting radical plane polyhedra depend upon the relative sizes of the model atoms and form an assembly of polyhedra which completely fill space. Application of the method to two slightly different models of amorphous Pd 4 Si illustrates the sensitivity of the technique to changes in local structure, and how such changes in polyhedron topology may relate to the changes in real space The method may be particularly useful in examining local changes on structural relaxation.

Optical properties of chalcogenide glasses

Abstract: Over the past two decades chalcogenide glasses have been researched in order to assess their suitability as passive bulk optical component materials for 3–5μm and 8–12μm infrared applications. This research has led to a greater understanding of the physical properties of these materials, and the present paper concentrates on the optical properties and applications of these bulk chalcogenide glasses. The various factors influencing the intrinsic and extrinsic optical loss mechanisms in materials are discussed, numerical data on the basic optical properties of chalcogenide glasses is presented and applications are discussed.

A neutron diffraction investigation of the structure of vitreous boron trioxide

Abstract: An extended review is given of previous structural work on vitreous B2O3 which comes to the conclusion that the best structural model comprises a mixed random network of boroxol groups and independent BO3 triangles. The neutron scattering length for 11B is slightly greater than that for oxygen so that neutrons should be more sensitive than X-rays to the presence of boroxol groups. Results are presented from a neutron diffraction investigation of vitreous 11B2O3, using the Harwell linac pulsed neutron source, in which data have been obtained to high scattering vector values ( Q max =40 A −1 ) yielding good real space resolution. It is concluded that these data are consistent with a mixed boroxol group - independent BO3 triangle network having 60% of the boron atoms in boroxol rings.

Introduction and effect of structural variations in inorganic polymers and glass networks

Abstract: Oxide glasses, being thermoplastic polymers, should be subject to considerable property variations as a result of differences in the network structure. Melting point, viscosity, and strength are among the properties that are closely related to network connectivity and distribution of the network bonds in the polymer. Glass formation by melting, however, does not allow a significant variation in network parameters, except by compositional changes. Chemical polymerization, on the other hand, permits the introduction of structural variations into the polymer network, and provides preparation of modified oxide glasses without compositional alterations. Silica network formation from silicon alkoxides is used as a model in driving relationships between physical and structural parameters, and supporting experimental evidence is presented. Some general effects of these parameters in several glass systems are included.

10B NMR studies of lithium borate glasses

Abstract: 10B NMR studies have been carried out on lithium borate glasses over the entire glass-forming region of the system. Using the ideas of Krogh-Moe, the relative abundances of the various structural groupings are inferred from the data by fitting computer-simulated lineshapes to the experimental spectra. Structural model are proposed which are consistent with the data: for 0.4 R < 1.0, where R = mol% Li2O/mol% B2O3, the model states that diborate and tetraborate units are proportionately destroyed to form metaborate units and loose BO4 units; for 1.0 < R < 1.86, the model states that metaborate units and loose BO4 units are destroyed linearly but not proportionately to form pyroborate and orthoborate units. These results are compared with earlier 11B NMR studies from this laboratory.

Insights into the structure of alkali borate glasses

Abstract: In the light of extensive new glass transition temperature (Tg) and density (ϱ) data obtained for the ternary glass system Li2O(LiCl)2B2O3 we have re-examined the well accepted structural models developed for borate glasses by Krogh-Moe and co-workers. We find that, in contrast to the conclusions of NMR, Raman and IR spectroscopy, glasses in the binary and in the ternary systems with common O/B ratios do not exhibit identical boron-oxygen networks. Rather, we find that as LiCl in the ternary is increased the network is systematically weakened and expanded to accommodate the large Cl− anion. On the basis of these observations we question the ability of spectroscopic techniques such as Raman and NMR to confirm the uniqueness of structural models based on combinations of intermediate-range units. We propose a tentative model which is qualitatively in agreement with the observed changes in Tg and ϱ upon addition of LiCl, and explains how the large Cl− ions can be accommodated without large increases in energy due to strain and electrostatic effects while maintaining BO4/BO3 constant. Recent transport results on some fast ion conducting glasses in this system are also discussed in terms of the proposed model.

A neutron diffraction investigation of the structure of vitreous zinc chloride

Abstract: A survey is presented of the AX 2 glasses and the corresponding crystalline polymorphs. The relationship between ZnCl 2 and the other members of the series is discussed and it is predicted that the number of shortest path seven and higher-membered rings in the non-chalcogenide AX 2 glasses is likely to be small. A neutron diffraction investigation of vitreous ZnCl 2 using the D4 diffractometer at ILL Grenoble shows that the structure of vitreous ZnCl 2 comprises a distorted random close packed array of Cl − ions with the Zn 2+ ions in tetrahedral holes, arranged in such a way as to maximise corner sharing of the resulting ZnCl 4 2− tetrahedra at the expense of edge and face sharing. The inability of molecular dynamics simulations, using purely ionic potentials, to predict the ZnZn component correlation function is taken as an indication of covalent character to the ZnCl bond. The crystalline polymorph formed on devitrification of dry ZnCl 2 glass is δ-ZnCl 2 and not the α-form as previously reported.

Molecular dynamic calculations of glass structure and diffusion in glass

Abstract: During the past few years several studies have been made of the structure of network forming glasses using the method of molecular dynamics (MD). In general, the MD structures closely resemble the random network structures first proposed by Zachariasen. For example, silica glass has a cage-like structure resembling a randomly distorted high-cristobalite. In simulated sodium silicate glasses, Na ions create non-bridging oxygen atoms and occupy interstitial sites with no well-defined coordination but a most probable coordination number of five. Vitreous B 2 O 3 contains planar regular triangles linked at apices to form a random network of ribbons and sheets. The MD structure of vitreous B 2 O 3 does not contain boroxyl rings. In sodium borosilicate glasses the trigonal to tetrahedral conversion of boron with the addition of sodium was observed to agree with the NMR results. In sodium alumino-silicate glass simulations, aluminum prefers four coordination, regardless of the concentration of Na 2 O. The mechanism is discussed. The MD structure can be used to interpret many physical properties. All of the simulated glasses undergo a kinetic glass transition as they are cooled. Glass formation occurs when the system reaches a temperature at which the network forming cations and anions cease to diffuse on the time scale of the MD runs. This occurs at temperatures corresponding to viscosities of 1 P or less diffusion constants of around 10 −5 cm 2 /s. Hence, the MD glasses have very high fictive temperatures. Sodium and other alkali ions, however, can be observed to diffuse at much lower temperatures. “Experiments” can be performed on the computer simulated network glasses and liquids.

Structure of alkali germanate glasses studied by spectroscopic techniques

Abstract: EXAFS spectra and X-ray radial distribution curves were determined for Li2OGeO2, Na2OGeO2 and K2OGeO2 glasses to elucidate the glass structure. The formation of 6-coordinated Ge atoms on the addition of an alkali oxide R2O to GeO2 glass was confirmed. The concentration of 6-coordinated Ge atoms N6 increased with an increase of the R2O content, exhibited a maximum around 20 mol% R2O and decreased again with a further increase of the R2O content. No essential dicrepancies between the techniques employed and between the glass systems investigated were found in the N6 value and the R2O content of glass at the maximum. The small discrepancies were attributed to the different assumptions made for the calculations. It was concluded that the concentration of 6-coordinated Ge atoms N6 follows the formula N6=x/(100-x), where x is mol% R2O. The maximum value of N6 was about 25% for the composition of about 20 mol% R2O.

Vibrational spectra of oxygen- and boron-isotopically substituted B2O3 glasses

Abstract: The Raman spectra of 10B, 11B, 16O, 18O, and mixed isotopic compositions of B2O3 (gl) are reported. The 808 cm−1 band assigned to the boroxol ring structure is found to shift upon oxygen isotope substitution, and not to depend on the boron mass. It is replaced by four band, in the intensity ratio 1 : 3 : 3 : 1, in the case of the 50% 16O-50% 18O-containing material. On the other hand, the frequencies of other principal bands depend on the masses of both boron and oxygen. The about 1260 cm−1 band, in particular, depends on the masses of both, and shifts without broadening upon partial oxygen substitution. The analysis of these data shows that both a local unit, behaving as predicted for the boroxol ring, and a continuous random network of connecting bonds are manifested in the spectra.

Glass formation of the SiO2B2O3 system by the gel process from metal alkoxides

Abstract: Glasses of the SiO 2 B 2 O 3 system containing up to about 33 mol% B 2 O 3 were prepared from Si(OC 2 H 5 ) 4 and B(OCH 3 ) 3 through hydrolysis, gelling and heating. The transparent monolithic gels were obtained by hydrolyzing the solution of B(OCH 3 ) 3 , reacted with the partially hydrolyzed Si(OC 2 H 5 ) 4 . On heating the gels, the network structure was strengthened accompanying the formation of SiOB bonds. The gels were converted into the glasses by heating above the critical temperature which corresponded to about 0.7 to 0.8 of the liquidus temperature. The glasses thus obtained had the glass transition temperatures. The glass transition temperature, Vickers hardness and density of the glasses prepared by the gel process were almost the same as those of the conventional glasses. The BOB bonds were not found in the glasses which contained B 2 O 3 below about 33 mol%.

Short range order and glass transition in AgIAg2OB2O3 vitreous electrolytes

Abstract: Several experimental techniques are used to study the short range order, the dynamics and the glass transition in AgIAg2B2O3 compounds. Addition of Ag2O to B2O3, up to [Ag2O]/[B2O3] ⩽0.5 modifies the borate network by creating a BO4 unit for each silver added. Addition of AgI decreases the glass transition temperature (Tg) but has only minor effects on the short range structure of the borate network. Silver iodide is partially accomodated in the interstices of the glass network. The relationship among a tentative structural picture, the ion transport phenomena and the low temperature dynamics are discussed. An investigation of the dynamics in the AgI·Ag2O·2B2O3 glass near and above Tg is presented. With NMR techniques, we monitor the onset of tumbling of the borate units and the dynamical effects of crystallization and/or aging of the glass. Hysteresis effects in the ionic conductivity (σ) temperature dependence and the non-Arrhenian behavior of σT near Tg are interpreted in terms of structural modifications occurring at elevated temperatures in the glass.

Crystallization of gels and glasses made from hot-pressed gels

Abstract: Silica gels were prepared by two different methods: (1) destabilization of a silica hydrosol (gel 1); (2) hydrolysis and polycondensation of a tetra-methoxysilane (gel 2). The crystallization of the gels was then studied as a function of the temperature by means of X-ray diffraction. It was evident that the crystallization was strongly influenced by the amount of alkali oxides present in the gel. It is effectively the concentration of akali impurities which could explain the lower temperature of crystallization necessary for gel 1 compared with that of gel 2. During the crystallization of the gel containing Na2O the crystalline phase of silica which appears first is the cristobalite; with Li2O it is quartz. The effect of additives such as boric anhydride was studied. This oxide was found to reduce the tendency of the gels to crystallize. The glasses of the system SiO2B2O3 obtained by the hot-pressing of the gels confirmed this phenomenon. Above 10 mol% B2O3 it was impossible to crystallize the gels and the glasses of this system under two hours.

Silica and silica-titania glasses prepared by the sol-gel process

Abstract: Clear monolithic samples of silica and silica-titania glasses were prepared by the sol-gel process from alkoxides as starting materials. The effects of the composition of the initial alcoholic solutions on the gelation of the silica materials and the effects of using different titanium compounds on the formation of silica-titania gels and glasses were investigated. DTA and TGA revealed losses of water and organic volatiles during heat treatment of the gels at lower temperatures (up to 400°C) and the glass transformation and crystallization behaviour at higher temperatures (up to 1500°C). The effects of using atmospheres with varying oxygen contents on the DTA peaks caused by oxidation reactions were also studied. Structural changes occurring during heat treatment were monitored by infra-red spectroscopy which indicated that the water contents of the glasses after heat treatment to 900°C were about 1000 ppm. Transmission electron microscopy of ion beam thinned foils of a 80 SiO220TiO2 composition showed a microstructure of extremely fine pores for heat treatments up to 1000°C. However, after extended heat treatments above 950–1000°C, the porosity appeared to decrease and a high concentration of fine crystallites of anatase (approximately 100 A in diameter) embedded in a silica-rich glass matrix were obtained.

On the short range atomic structure of non-crystalline carbon

Abstract: Neutron diffraction data show there isllittle tetrahedral bonding in glassy carbon, and correspond to the Stenhouse-Grout structure factor model for amorphous carbon taking the degenerate case in which the amount of tetrahedral bonding is negligible. This is in contrast to the X-ray results of Noda and Inagaki. Further analysis of the radial distribution function from the neutron diffraction data is presented. A brief summary of other X-ray diffraction measurements on a variety of amorphous carbons is given, all of which confirm the predominantly trigonal coordination. The disagreement of the earlier X-ray and electron diffraction measurements is due to the poor resolution and normalization of the data, and also perhaps the method of preparation. The amount of tetrahedral bonding present in amorphous carbon requires careful diffraction measurements at large scattering vectors to enable better resolution of the peaks in the radial distribution function.

Compositional dependence of nonradiative decay rate in Nd laser glasses

Abstract: Variations of nonradiative decay rate from 4 F 3 2 of a neodymium ion with the change of network modifier were determined from the fluorescence lifetime and radiative decay rate, calculated on the basis of Judd-Ofelt theory, for silicate and phosphate glasses. While, on the basis of Miyakawa-Dexter's theory, the nonradiative decay rate due to the multiphonon relaxation process was estimated from the electron-phonon coupling strength and phonon energy which were in turn determined from phonon sideband measurements. The latter did not always give the same modifier dependence of the nonradiative decay rate as the former, which suggested that the Judd-Ofelt theory did not always give the precise value of the radiative decay rate. The opposite variations of nonradiative decay rate with alkali modifier were found for silicate and phosphate glasses. This was explained as a result of the difference in the contribution of the electron-phonon coupling strength to the nonradiative decay rate due to the multiphonon relaxation process.

New organic molecular materials for nonlinear optics

Abstract: We present four molecular organic crystals which were recently proposed for applications in nonlinear optics (three-wave-mixing and electrooptic effects) : methyl - (2,4 - dinitrophenyl) - aminopropanoate (MAP), 2 - methyl - 4 - nitroaniline (MNA), 3 - methyl - 4 - nitropyridine - 1 - oxyde (POM) and urea. The nonlinear efficiency of molecular organic crystals is seen to depend on the molecular hyperpolarizability and on the crystalline geometry. Both features are discussed and shown to lead, for crystal point groups of lower symmetry, to new structural relations between crystalline non linear coefficients. Each of the four molecular crystals discussed here are shown to have specific properties connected to different types of applications.

Raman spectroscopic studies of fluorophosphate glasses

Abstract: Some fluorophosphate glasses obtained by fluorination of NaPO3, xNaPO3−yAlF3 or tNaPO3−wAlF3−vCaF2 have been studied by Raman spectroscopy in VV and VH polarisations. The observed bands have been assigned by comparison with those vitreous NaPO3 and Na4P2O7, as well as crystallised K2PO3F and KPO2F2, and molten cryolite. Structural models are proposed for the various compositions. They are built up with arrangements of following species: (PO3)2n−n chains; (P2O7)4− dimers, (AlF6)3− octahedra, P(O, F)4, (AlF4)− and (AlOF3)2− tetrahedral groups. For glasses rich in phosphate −OPO2F− and −OPOF2 terminal groups are displayed; the latter being favoured for a higher fluorine concentration.

Structure of amorphous and glassy Sb2S3 and its connection with the structure of As2X3 arsenic chalcogenide glasses

Abstract: Two types of amorphous and glassy Sb 2 S 3 were studied: amorphous orange-red powder obtained by precipitating a SbCl 3 solution using a stream of hydrogen sulphide in a bath of boiling water and a glassy material in the form of thin platelets obtained by the method of rotating cylinders. It was found by DTA that there is a great difference in the crystallization entropy and temperature between the powder and glass. Radial electron density distribution curves were measured for both modifications and a difference analysis showed that they can be considered to be much the same. From the distribution curves it follows that the basic unit is a trigonal SbS 3 pyramid which forms in the case of Sb 2 S 3 glass, a covalent random network while for the amorphous powder a random ordering of crystalline-like and band-like (Sb 2 S 3 ) n molecules can be considered as an appropriate model. A comparison of radial electron density distribution curves for As 2 X ( X =S, Se, Te and Sb 2 S 3 shows that Sb 2 S 3 belongs to the class of glassy As 2 X 3 structures. A rule is found that in this class of glasses the mutual ratios of diameters of corresponding coordination spheres are constant. On the basis of coordination numbers and of broadening functions for individual coordination spheres it is shown that in the series As 2 S 3 :As 2 Se 3 As 2 Te 3 a gradual transition from a layer-like to a random network takes place and that glassy Sb 2 S 3 can be put beside As 2 Te 3 . This process is a result of the difference between orpiment-like configurations and the As 2 X 5 ones, which evoke chain branching and interconnecting. This leads finally for As 2 Te 3 and Sb 2 S 3 to a random continuous covalent network. This process is confirmed by increase of both occupation numbers and the widths of broadening functions corresponding to individual coordination spheres.