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Glass formation and structure of glasses in the WO3–ZnO–Nd2O3–Al2O3 system
Journal of Non-Crystalline Solids 414 (2015) 42–50
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Glass formation and structure of glasses in the WO3–ZnO–Nd2O3–Al2O3 system R. Iordanova a, M. Ataalla b, M. Milanova a,⁎, L. Aleksandrov a, A. Staneva b, Y. Dimitriev b a b
Institute of General and Inorganic Chemistry, BAS, Acad. G. Bonchev Str. bld. 11, 1113 Sofia, Bulgaria University of Chemical Technology and Metallurgy, 8 “Kl. Ohridski” blvd., 1756 Sofia, Bulgaria
a r t i c l e
i n f o
Article history: Received 9 December 2014 Received in revised form 11 February 2015 Accepted 12 February 2015 Available online xxxx Keywords: Tungstate glasses; Glass structure; IR; DR-UV–Vis
a b s t r a c t Glass formation ability in three different sections: WO3–ZnO–Nd2O3; (100-x) (0.8WO3·0.1ZnO·0.1Nd2O3)·xAl2O3, x = 5–30 mol% and 90(xWO3·yZnO·zNd2O3)·10Al2O3 from WO3–ZnO-Nd2O3–Al2O3 system is investigated by melt quenching technique. Glasses are obtained from the compositions with a high WO3 content (65–75 mol%). According to DTA data the glass transition temperature (Tg) is in the range 508–525 °C and the glass crystallisation temperature (Tx) is around 600 °C. Neodymium tungstates, Al2(WO4)3, ZnWO4 and WO3 are identified in the XRD patterns of quenched samples beyond the glass formation region. The structure of glasses is studied by IR, Raman and DR-UV–Vis spectroscopies. It is established that the amorphous network is built up mainly by WO6 octahedra, connected by W–O–W bridges. This structure does not satisfy the classical concept for the formation of an amorphous network which is built up by MeO4 tetrahedra. © 2015 Elsevier B.V. All rights reserved.
1. Introduction 1.1. Glass formation in tungstate systems WO3 is known as a conditional glass former that is not able to form glasses itself with standard methods of vitrification. Its glass formation ability is significantly enhanced by the addition of other compounds. The glass formation regions between 23 and 70 mol% WO3 in binary R2O–WO3; R = Li, K, Na, Rb, and Cs systems, have been determined applying rapid quenching techniques [1–3]. The high critical cooling rates (5 × 105–106 K/s) have been applied for the amorphization of compositions in WO3–Nd2O3, WO3–MxOy (M = Al, Sm, Eu, Gd, Dy, Ho) and BaO– Nd2O3–WO3 systems [4,5]. Binary WO3–Pb3O4 glasses within the concentration range of 5–30 mol% WO3 have been also reported [6]. In several papers, special attention has been paid to the glass-formation in tungstate systems containing simultaneously and other nontraditional glass formers such as V2O5, MoO3, and Bi2O3 [7–14]. It has been proven that V2O5 is a better glass-former in comparison with MoO3 and WO3 while MoO3 is better than WO3. Among the classical glass forming oxides, P2O5 allows the preparation of stable glasses in combination with WO3 over a wide concentration range [15]. Other more intensively studied glass compositions are with participation of TeO2 [16]. These glass systems are not the object of our investigation and will not be discussed below. The structure of WO3-based glasses and especially the coordination state of tungsten atoms is not well understood. The first structural ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (M. Milanova).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.02.010 0022-3093/© 2015 Elsevier B.V. All rights reserved.
studies [17,18] have shown that amorphous WO3 thin films are built up by strongly distorted, but well defined octahedra joined by vertices. The possibility to construct amorphous network containing only WO6 octahedra has been demonstrated by modelling [19,20]. Participation of both WO6 and WO4 polyhedra in the structure of binary glasses in the R2O–WO3, R = Li, K, Na, Rb, and Cs systems has been proven by IR spectroscopy [1,3]. Some authors agree that tungsten oxide units in many component glasses are present in the network as WO6 octahedra together with WO4 tetrahedra [21–23]. Other authors [24–29] using different complimentary techniques suggested that only six-fold coordinated (WO6) tungsten atoms exist in the glasses and they are enough for the formation of the amorphous network. However, up to now there is not enough data for the glass formation in many component tungstate glass systems where WO3 is a main glass former. Having in mind the above short survey, it can be concluded that WO3 is an appropriate compound for the solving of general fundamental structural problems concerning the amorphous state and as well as for the preparation of new type of oxide glasses for technology. The present paper is a continuation of our fundamental investigations on the glass formation tendency in complex oxide systems containing high WO3 content [11–13]. The aim is to study the glass forming ability of the compositions rich in WO3 (60–80 mol%) in the ZnO–WO3–Nd2O3–Al2O3 system and to elucidate the main building units determining the glass network stability (short range order). 1.2. Selection of the compositions The object of this investigation is a complex WO3-containing system without participation of typical modifiers such as alkali or alkali-earth
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Fig. 2. Fast quenched compositions in the ZnO–WO3–Nd2O3 section (see Table 1, Section 1).
Fig. 1. Investigated sections for glass formation ability in the ZnO–WO3–Nd2O3–Al2O3 system: 1) ZnO–WO3–Nd2O3; 2) (100-x)(0.8WO3·0.1Nd2O3·0.1ZnO)·xAl2O3, x = 5– 30 mol%; 3) 90(xZnO·yWO3·zNd2O3)·10Al2O3.
oxides. The other selected oxides were ZnO, Al2O3 and Nd2O3 because of their specific behaviour established in systems with participation of other transition metal oxides. The motivation for their selection will be summarised below. Recently X-ray amorphous or glass-crystalline materials from multicomponent oxide melts containing 70–75 mol% ZnO have been obtained by us [30]. Traditionally ZnO is used as a modifier in industrial glass compositions in order to improve their chemical and thermal durability and as well as their optical properties (see for
example the monograph of Vogel [31]). Few examples for the structural role of ZnO in model glass systems have been published applying suitable structural methods. It has been suggested by Calas et al. [32] that ZnO4 tetrahedra are co-polymerised with the silicate network. Hoppe et al. [33,34] have established a change of ZnO6 to ZnO4 groups in zinc–phosphate glasses containing 70–80 mol% ZnO. Sakida et al. [35] have found that in the structure of ZnO–TeO2 glasses, the coordination number of Zn atoms against oxygen decreases from 5.3 to 3.9 as ZnO content increases from 10 to 40 mol%. Nevertheless, the existence of connectivity between ZnO4 polyhedra in different glass networks and the formation of Zn–O–Zn linkages are still questionable. On the other hand, the participation of ZnO together with V2O5 and WO3 does not stimulate the formation of four-fold coordinated zinc atoms (ZnO4) and the behaviour of ZnO in this case may be considered as a modifier. Al2O3 according to the classical classifications [31] is considered as an
Table 1 Investigated compositions. Sections
Nominal composition (mol%)
Method of quenching
Classification according XRD
1. ZnO–WO3–Nd2O3
80WO3·20Nd2O3 10ZnO·80WO3·10Nd2O3 10ZnO·70WO3·20Nd2O3 20ZnO·65WO3·15Nd2O3 80WO3·20Al2O3 60WO3·40Al2O3 9.5ZnO·76WO3·9.5Nd2O3·5Al2O3 9ZnO·72WO3·9Nd2O3·10Al2O3 8ZnO·64WO3·8Nd2O3·20Al2O3 7.5ZnO·60WO3·7.5Nd2O3·25Al2O3 7ZnO·56WO3·7Nd2O3·30Al2O3 80WO3·10Nd2O3·10Al2O3 70WO3·20Nd2O3·10Al2O3 2.5ZnO·85WO3·2.5Nd2O3·10Al2O3 5ZnO·80WO3·5Nd2O3·10Al2O3 10ZnO·75WO3·5Nd2O3·10Al2O3 5ZnO·75WO3·10Nd2O3·10Al2O3 5ZnO·70WO3·15Nd2O3·10Al2O3 15ZnO·70WO3·5Nd2O3·10Al2O3 20ZnO·65WO3·5Nd2O3·10Al2O3 12.5ZnO·65WO3·12.5Nd2O3·10Al2O3 5ZnO·65WO3·20Nd2O3·10Al2O3 15ZnO·60WO3·15Nd2O3·10Al2O3 25ZnO·60WO3·5Nd2O3·10Al2O3 5ZnO·60WO3·25Nd2O3·10Al2O3 17.5ZnO·55WO3·17.5Nd2O3·10Al2O3 20ZnO·50WO3·20Nd2O3·10Al2O3
Roller technique Roller technique Press quenching Press quenching Roller technique Roller technique Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching
Nd10W22O81 WO3; Nd6W3O18; Nd2W2O9; ZnWO4; Nd2(WO4)3 WO3; Nd2W4O15; Nd6W3O18 WO3; Nd2W4O15; Nd6W3O18; ZnWO4; Nd2(WO4)3 Al2(WO4)3 Al2(WO4)3 Glass + crystals-W19O49 Glass Glass Glass + crystals-Nd10W22O81 Glass + crystals-Nd10W22O81 Glass + crystals-W19O49 Glass + crystals-Nd2(WO4)3; WO3; WO3 WO3 Glass Glass Glass Glass Glass Glass Nd2(WO4)3; Nd10W22O81 Glass + crystals-ZnAl2O4; Nd2WO6 Glass + crystals-ZnAl2O4 Nd2(WO4)3; Nd10W22O81; Nd2(WO4)3; Nd10W22O81 ZnWO4; Nd2W2O9
2. (100-x)(0.1ZnO·0.8WO3·0.1Nd2O3)·xAl2O3; x = 5–30 mol%
3. 90(xZnO·yWO3·zNd2O3)·10Al2O3
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Fig. 4. XRD patterns of (100-x)(0.8 WO3·0.1Nd2O3·0.1ZnO)·xAl2O3, x = 5–30 mol% press quenched samples (see Table 1, Section 2). Fig. 3. XRD patterns of crystallised samples in the ZnO–WO3–Nd2O3 section obtained by: (a) and (b) press quenching of the melts (cooling rates 101–102 K/s); (c) roller quenching technique (cooling rates 103–104 K/s).
intermediate, because it is possible to participate in the glass network in tetrahedral (network former) or octahedral (modifier) oxygen coordination, depending on the other components. The third component chosen (Nd2O3) is very attractive because of its optical properties. The absorption and emission characteristics of Nd3 + (f3) ions have been the subject of extensive investigations for many years in a number of crystalline and glass materials especially for applications in laser technology [31,36–39]. Some newly investigated systems have been published recently [40–43]. Generally speaking, inorganic glasses such as silicates, phosphates, borates, germanates, and tellurites, have been proven to be an excellent host for rare earth ions, but little attention has been paid to non-traditional oxide glasses as host materials. This is a new field for fundamental studies and as well as for investigations connected with potential practical applications. 2. Experimental The investigated sections in ZnO–WO3–Nd2O3–Al2O3 are present in Fig. 1. Table 1 lists the corresponding nominal compositions which are situated near the WO3 corner because the main purpose is to verify the role of WO3 as a network former. All specimens were prepared using reagent grade WO3, ZnO, Nd2O3 and Al2O3. The homogenised batches were melted for 20 min at 1240 °C in air atmosphere. According to the electron microprobe analysis (EMRA) 8–10 mol% Al2O3 was dissolved in the ternary ZnO–WO3–Nd2O3 glasses after melting of the batches for 30 min in corundum crucibles at the maximum temperature. That is why all samples were melted in platinum crucibles. The glass forming ability of the compositions was investigated by a melt quenching technique applying two different cooling rates: 101–102 K/s
by pouring of the melt between two copper plates and 103–104 K/s by use of a roller-quenching technique. The phase formation of the samples was established by X-ray phase analysis with a Bruker D8 Advance diffractometer, using Cu Kα radiation in the 10 b 2 b 60 range. The differential thermal analysis (DTA) was carried out on LABSYS™ EVO apparatus with Pt/Pt Rh thermocouples with a heating rate of 10 K/s in Argon flow, using Al2O3 as a reference material. The IR spectra of the glasses were recorded in the 1400–400 cm−1 region, using the KBr pellet technique (Varian 660 FTIR spectrometer). The room temperature Raman measurement was performed in the range 200–1300 cm−1 on a micro-Raman system from Jobin-Yvon Horiba (LABRAM HR-800) spectrometer with a green laser (wavelength: 532 nm). Scanning electron microscopy (JEOL, JEM-200, CX) and electron microprobe analysis EMRA (JEOL, Superprobe 733) were performed. The optical spectra of powder samples at room temperature were recorded with a spectrometer (Evolution 300 UV–Vis Spectrophotometer) employing the integration sphere diffuse reflectance attachment. The samples were measured in the wavelength (λ) range of 200–1100 nm with a magnesium oxide reflectance standard used as the baseline. The uncertainty in the observed wavelength is about ± 1 nm. The Kubelka–Munk function (F(R∞)) was calculated from the UV–Vis diffuse reflectance spectra. The band gap energy (Eg) was determined by plot (F(R∞)hν)n, n = 2 versus hν (incident photon energy). 3. Results 3.1. Glass formation and crystallisation 3.1.1. ZnO–WO3–Nd2O3 (Section 1) Binary WO3–ZnO, WO3–Nd2O3 and WO3–Al2O3 compositions do not form glasses under experimental conditions applied. Investigated
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Fig. 7. Triangular diagram of the glass formation region in the 90(xZnO·yWO3· zNd2O3)·10Al2O3 section: (○) glasses; (●) crystals; ( ) glass + crystals (see Table. 1, Section 3).
conditions of quenching (WO3-JCPDS-00-020-1323; Nd2(WO4)3JCPDS-00-019-0829; ZnWO4-JCPDS-01-073-0554; Nd2W4O15-JCPDS00-035-0161; Nd6W3O18-JCPDS-00-032-0688, see Table 1). Single phase Nd10W22O81 (JCPDS-01-070-2755) was indentified in a eutectic composition (80WO3·20Nd2O3) of WO3–Nd2O3 system.
Fig. 5. DTA curves of (100-x)(0.8WO3·0.1Nd2O3·0.1ZnO)·xAl2O3, x = 5–30 mol% press quenched samples (see Table 1, Section 2).
samples in the ternary system ZnO–WO3–Nd2O3 are indicated in the triangular diagram present in Fig. 2. According to the XRD data shown in Fig. 3 there are no amorphous samples after quenching of the melts. Several phases were separated under applied non-equilibrium
Fig. 6. XRD patterns of a) 8ZnO·64WO3·8Nd2O3·20Al2O3 parent glass; b) glass, heat treated at 600 °C for 5 h.
Fig. 8. XRD patterns of glasses in the 90(xZnO·yWO3·zNd2O3)·10Al2O3 section (see Table 1, Section 3).
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3.1.2. (100-x)(0.1ZnO·0.8WO3·0.1Nd2O3)·xAl2O3, x = 5–30 mol% (Section 2) Four compositions with a constant ZnO/WO3/Nd2O3 ratio and varying Al2O3 content from 5 to 30 mol% are investigated (Fig. 1). The introduction of a small amount of Al2O3 significantly enhances the glass forming ability of the samples. As it is seen from the XRD data obtained (Fig. 4) the diffraction pattern of the specimen containing 5 mol% of Al2O3 exhibits an amorphous hallo and diffraction peaks corresponding to W18O49 (JCPDS-01-073-2177). X-ray amorphous samples were obtained after the introduction of 10 to 25 mol% Al2O3. Further increase in the Al2O3 content deteriorates the glass formation. Nd10W22O81 compound (JCPDS-01-070-2755) was detected in the composition containing 30 mol% Al2O3, irrespective that a significant part of the quenched sample remains amorphous. The thermal parameters obtained from DTA curves of quenched samples in this section are presented in Fig. 5. They are characterised with the glass transition temperature (Tg) in the range of 508-525 °C and glass crystallisation temperatures (Tx) above 590 °C. The calculated ΔT = Tx − Tg which is in the range 60–80 °C determines a good thermal stability of the amorphous samples. Selected glass compositions were subjected to heat treatment at temperatures near the glass crystallisation temperatures in order to identify the products of crystallisation. Mainly Al2(WO4)3 (JCPDS-01081-2415), together with Nd2(WO4)3 (JCPDS-00-019-0829) crystalline phase were detected (Fig. 6). 3.1.3. 90(xZnO·yWO3·zNd2O3)·10Al2O3 (Section 3) The glass formation region in this section determined visually and by XRD analysis is presented in a triangular diagram shown in Fig. 7.
Fig. 10. IR spectra of (100-x)(0.8WO3·0.1Nd2O3·0.1ZnO)·xAl2O3, x = 5–30 mol% press quenched samples (see Table 1, Section 2).
Typical diffraction patterns of all glasses obtained are shown in Fig. 8. The glasses are situated near the WO3 corner. Several crystalline phases (WO3-JCPDS-00-020-1323 and JCPDS-83-049; ZnWO4-JCPDS-01-0730554; Nd2(WO4)3-JCPDS-00-019-0829; Nd2W2O9-JCPDS-00-023-1268 and Nd10W22O81-JCPDS-01-070-2755) were detected in the XRD patterns of the crystallised sample situated outside of the glass formation region (Fig. 9). As it is seen from Fig. 9 the neodymium tungstates are the predominant phases crystallised after quenching of the melts.
3.2. IR, Raman and DR-UV–Vis spectra
Fig. 9. XRD patterns of crystallised samples obtained by melt quenching in the 90(xZnO·yWO3·zNd2O3·10Al2O3) section: 1—65WO3·5ZnO·20Nd2O3·10Al2O3; 2— 60WO3·5ZnO·25Nd2O3·10Al2O3; 3—55WO3·17.5ZnO·17.5Nd2O3·10Al2O3; 4—85WO3· 2.5ZnO·2.5Nd2O3·10Al2O3; 5—50WO3·20ZnO·20Nd2O3·10Al2O3; 6—80WO3·5ZnO· 5Nd2O3·10Al2O3.
The structure of glasses is studied by IR, Raman and DR-UV–Vis spectroscopies. A strong band at 870 cm−1 and as well as an increasing absorption in the range of 610–700 cm−1 are ascertained in all IR glass spectra (Figs. 10 and 11). Low frequency band at 460 cm−1, growing in intensity with the increase in Al2O3 content is also observed in the IR spectra (Fig. 10). A new band at 550 cm−1 appeared in the absorption spectra of glasses containing a higher amount of Al2O3 (25 mol% and 30 mol%). In some spectra it is possible to detect increasing absorption near 1000–1020 cm−1. IR spectra of reference crystalline m-WO3, Al2(WO4)3 and ZnWO4 are also given (Fig. 12). Raman spectra of 90(xZnO·yWO3·zNd2O3)·10Al2O3 glasses are shown in Fig. 13. The broad peaks being typical for glass materials are observed at 330, 840, and 985 cm−1. Fig. 14, a) compares the UV–Vis absorption spectra in the reflection regime of selected glass compositions from the ZnO–WO3–Nd2O3– Al2O3 system, pure WO3 and Al2(WO4)3. All spectra exhibit absorption maximum in the range of 220–330 nm and several peaks in the visible part of the spectral region (525, 583, 745 and 803 nm). The band gap
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attributed to the vibration of significantly shorter W–O bonds for some apical oxygen atoms [51]. Raman spectrum of Bi2W2O9 phase does not contain bands above 900 cm−1 [51]. Raman spectra of tungsten oxide clusters composed of polymeric WO6 units [52] contain high frequency bands between 950 and 1015 cm−1, (νs (W_O)), bands in the 825–930 cm−1 range (νas (W_O)) and W–O–W modes at 700–500 and 330–190 cm−1. The compounds M2W2O7 (M = K, Na) that are composed of alternating WO4 and WO6 units joined in infinite polymeric chain with bridging W–O–W bonds, are characterised by stretching modes in 800–500 cm−1 region and bands above 900 cm−1 connected with the symmetric vibration of shorter double (W_O) bonds [52]. Orthotungstate compounds possessing significantly distorted, isolated WO4 groups are characterised spectroscopically with the absence of bands in 700–500 cm− 1, but strong bands in range of 800–920 cm−1[53,54]. Table 2 lists the spectral data for the reference crystalline tungstates mentioned above. Taking into account the above spectral data, the most intensive band at 870 cm−1 together with the shoulder in the 610–660 cm−1 region observed in the IR spectra of all glass samples (Figs. 10, 11) can be assigned to the stretching vibrations of WO6 units connected by bridging W–O–W bonds. Many authors observed similar bands in the IR spectra of glasses rich in WO3 and attributed them to the vibration of WO6 octahedra [25,26,29,55,56]. The presence of a high-frequency shoulder about 1020 cm−1 in our IR spectra (Figs. 10, 11) is less pronounced relatively to the base line. This may be as a result of the widening of the distribution of bond lengths in WO6. The vibrations of WO4 units are difficult to identify, because of the overlapping with the bands due to WO6 species (see Table 2). The absence of a band around 920 cm−1 in the IR spectra of investigated glasses which is typical for the vibration of WO4 tetrahedra [53,54] does not allow us to ignore the WO4 formation. Raman analysis also gave useful information for the glass structure. Taking into account the structural and Fig. 11. IR spectra of glasses in the 90(xZnO·yWO3·zNd2O3)·10Al2O3 section (see Table 1, Section 3): 1—65WO3·20ZnO·5Nd2O3·10Al2O3; 2—70WO3·15ZnO·5Nd2O3·10Al2O3; 3—70WO3·5ZnO·15Nd2O3·10Al2O3; 4—75WO3·5ZnO·10Nd2O3·10Al2O3; 5—75WO3· 10ZnO·5Nd2O3·10Al2O3.
energy (Eg) (Fig. 14, b)) of the same samples is calculated following the above-mentioned procedure (see the Experimental Section). 4. Discussion 4.1. Structural analysis From the structural point of view it is interesting to verify the main structural units and the transformation of tungstate network with the compositions. Infrared and Raman spectroscopies were used to reveal the short range order, the type of polyhedra and their transformation with the composition. The assignments of the infrared bands are made in the framework of the local point symmetry approach for inorganic compounds [13,44–46] taking into account the spectral and structural data for WO3-containing crystalline and amorphous phases existing in the literature. It is well known that the structure of m-WO3 consists of corner-sheared distorted WO6 octahedra [47,48]. According to Guery et al. [49] the IR spectrum of m-WO3 shows a large band with two maxima at 740 cm−1 and 810 cm−1 corresponding to stretching WO6 vibrations. The structure of hydrates WO3·xH2O involving structural water is built up of layers in which WO6 octahedra shear their four equatorial oxygen forming W–O–W bonds with a length of about 1.9 Å. Two other oxygen atoms are implicated in a longer W–OH2 bond and in a shorter axial double (W_O) bond respectively [49]. The short (W_O) bonds are characterised with absorption peaks in the range of 920–1007 cm− 1 [49,50]. Mazcka et al. have reported IR and Raman spectra of layered Aurivilius type compound Bi2W2O9 [51]. IR spectrum of Bi2W2O9 is characterised by band at 850 cm−1 assigned to the asymmetric stretching modes of WO6 octahedra, and band at 936 cm− 1
Fig. 12. IR spectra of the reference crystalline phases in the WO3–ZnO–Nd2O3–Al2O3 system.
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containing 65–75 mol% WO3 are more symmetrical in comparison with tungsten octahedra in the pure crystalline WO3 [65]. The absence of any absorption below 260 nm in the UV–Vis glass spectra is evidence that the amorphous network is formed by the participation of octahedrally coordinated W6+ mainly. Optical band gap values (Eg) evaluated from the UV–Vis spectra can also give information about the structural arrangement of the glasses under investigation. A correlation between the Eg energy and the number of covalent bridging W–O–W bonds around the central W6+ cations was found. As it was shown by Barton et al. [61] and Ross-Medgaarden and Wachs [52] Eg linearly decreases with an increasing number of bridging W–O–W bonds. The band gap energy values of studied glasses are 3.6–3.7 eV and these values are compatible with the other tungstate glasses and highly disordered tungsten oxide films [66,67]. As it is seen from Fig. 14, b), the Eg of both glasses is much higher as compared with the band gap energy of the pure WO3 oxide apical oxygen atoms, but it is lower than Eg value of isolated WO4 tetrahedra in Al2(WO4)3. This modification in band gap energy values can be related with the changes in the bonding during the vitrification, which is a result of some polymerisation or clustering of WO6 polyhedra [52,61]. The last part of this study is connected with the absorption in the visible part of the spectrum above 400 nm. The absorption bands at 525 nm, 583 nm, 745 nm, 803 nm and 875 nm, observed in the glass spectra can be attributed to the electron transitions from the ground state 4I9/2 to the excited states of neodymium [31,41,42,68]. The
Fig. 13. Raman spectra of glasses in the 90(xZnO·yWO3·zNd2O3)·10Al2O3 section (see Table. 1, Section 3): 1—65WO3·20ZnO·5Nd2O3·10Al2O3; 2—70WO3·15ZnO·5Nd2O3· 10Al2O3; 3—70WO3·5ZnO·15Nd2O3·10Al2O3; 4—75WO3·5ZnO·10Nd2O3·10Al2O3; 5—75WO3·10ZnO·5Nd2O3·10Al2O3.
spectral data of tungsten oxide clusters composed of polymeric WO6 units, the observed Raman bands at 985 cm−1, 840 cm−1 and 330 cm−1 (Fig. 13) can be related with WO6 species possessing short (terminal) W_O bond and W–O–W bridges [52,57]. The bands in the 460–580 cm−1 range which rise in intensity with increasing Al2O3 content can be related with the presence of AlO6 units [58–60]. The vibrations of Nd- and Zn-oxygen polyhedral are also difficult to identify because they are at the boundary of the investigated spectral region. UV–Vis diffuse spectroscopy is used to obtain information about the local structure of glasses via ligand to metal charge transfer (LMCT) band position and the corresponding edge energy (Eg) values. The UV–Vis diffuse reflectance spectra of various tungstate reference compounds show that the absorption bands due to oxygen–tungsten charge transfer (O2− → W6+) between 218 and 260 nm can be assigned to tetrahedral WO4(Td) groups, and the bands between 250 and 360 nm can be attributed to octahedral WO6(Oh) units [52,61,62]. The exact location of these bands maxima depends on ligand field symmetry surrounding the W site. More symmetrical WOn (n = 4 or 6) possess bands at a lower wavenumber, while more distorted ones exhibit bands at a higher wavenumber. On this base the observed bands at 260 and 330 nm in the optical spectrum of pure WO3 shown in Fig. 14, a) are due to the presence of WO6 units with different symmetry. The same figure displays also the spectrum of Al2(WO4)3 as a reference for the tetrahedrally coordinated W atoms against oxygen. Its UV–Vis spectrum contains only one maximum typical for the presence of isolated tetrahedra (WO4) [52,63,64]. As it is seen from Fig. 14, a) two absorption bands are observed in the optical absorption spectra of investigated glasses. Following the above analysis they could be connected with the presence of WO6 octahedra. The shift of the band at 330 nm present in the optical spectrum of crystalline WO3 to lower wavenumber (300 nm) in the glass spectra indicates that WO6 units in the glasses
Fig. 14. a) DR-UV–Vis spectra and LMCT bands of 1) Al2(WO4)3, 2) 65WO3· 5Nd2O3·20ZnO·10Al2O3 glass, 3) 765WO3·5Nd2O3·10ZnO·10Al2O3 glass, 4) WO3; b) DR-UV–Vis spectra and Eg values of the same samples.
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Table 2 Absorption frequencies of reference crystalline tungsten oxygen compounds. Crystal
Raman band positions (cm−1)
IR band positions (cm−1)
Vibration assignments
Refs.
m-WO3 Bi2W2O9
807, 715 851 νas (W–Oap) 799 νs (W–Oap) 739, 699 νas (W–O) 945–960 ν (W_O) 800, 645–685 (O–W–O) 934–933 νs (W_O) 888–876 νs (W_O) ~500–800 ν (W–O–W), ~200–300 δ(W–O–W) ~976 νs (W_O) 958, 930, 834 νas (W_O) 700–500 ν (W–O–W); 330–190 δ (W–O–W) 1023–974 ν1 (WO4) 959–800 ν3 (WO4) 390–314ν2+ ν4(WO4)
940, 870, 810, 760, 740, 460 963, 855 νas (W–Oap) 799 νs (W–Oap) 759, 709, 647 νas (W–O) 920–1007 ν (W_O) 600–695, 700–880 ν (O–W–O)
WO6 WO6
49, 50 51
WO6
49, 50
WO4 WO6
52
WO6
52, 57
WO4
54, 58
WO3.xH2O M2W2O7; M_K, Na
(NH4)10H2W12O42
A2(WO4)3; A_Sc, In, Al
neodymium gives a strong red-violet colouration of investigated glasses. Our experiments also revealed a good solubility of neodymium in the tungstate network. The obtained complex glasses are interesting objects for optical applications and as coloured art glasses. These results could be also a base for future experiments over synthesis of nontraditional WO3-based glasses with simultaneous participation of several rare earth ions.
1010–960 ν1 (WO4) 921–799 ν3 (WO4) 420–300 ν2+ ν4(WO4)
that from the compositions containing above 30 mol% Al2O3 mainly Al2(WO4)3 derived after quenching of the melt. Its structure contains isolated WO4 tetrahedra. It is interesting to note that AlO4 and AlO5 units are identified by NMR in the structure of the amorphous Al2(WO4)3 obtained after precipitation from solutions [54,58]. This fact could be the starting point for more detailed investigation and development of a plausible hypothesis about the role of Al2O3 for the amorphisation of multicomponent tungstate compositions.
4.2. A hypothesis for the amorphous network formation Having in mind the above results and as well as the classical concepts for the glass formation in the traditional glass forming systems, some basic questions related to the glass formation in the nontraditional glasses containing WO3 have emerged: a) What are the main structural units forming the amorphous network (MeO4, MeO5, MeO6) and do they change with the compositions? b) What is the connectivity between the polyhedra (corners, edges, clusters)?
In our previous studies [13,69,70] a model for the non-traditional glass forming systems containing MoO3 have been proposed. According to this model, the process of structural transformation is realised in different concentration ranges depending on the nature of the other components. A partial MoO6 → MoO4 transformation and breaking of the edge sharing between the polyhedra existing in the crystalline MoO3 have been established. The formation of layered structures with participation of WO6 octahedra has been suggested for tungstate glasses [13]. This model is different from the proposed one for molybdate glasses. In the case of tungstate glasses, studied in this paper, we assume, that the amorphous network is dominated by the presence of deformed WO6, participating in the polymeric clusters. Another assumption is that except corner shared WO6 and formation of W–O–W bridges, shorter W–O bonds varying in length in wide ranges are formed as well. It is still questionable why Al2O3 in a limited concentration (5– 30 mol%) plays an essential role for the amorphisation of the complex tungstate compositions. Concerning this, some ideas based on the structure of the crystalline phases that have been identified in the quenched composition outside of the glass formation and have also been obtained after crystallisation of glasses by heat treatment could be drawn. Having in mind the classical criteria for amorphisation mentioned above, the short range order elements of glasses and crystalline phases will be compared. Obviously Nd2(WO4)3 [71] W18O49 [72] Nd10W22O81 [73] Al2(WO4)3 [63] and ZnWO4 [74] will deteriorate the glass formation ability of the compositions. Their structure consists of edge shared WO6 and WO6 forming clusters or isolated WO4 tetrahedra without corner W–O–W bonds. This assumption is supported as well by the fact
5. Conclusions The glass formation in the ZnO–WO3–Nd2O3–Al2O3 system was investigated for the first time and it was proved that WO3 is the main glass former. Glasses were obtained from the compositions containing a high content of WO3 (65–75 mol%). It was established that the addition of Al2O3 (from 5 to 30 mol%) facilitates the amorphous state. It was proved that in the system under investigation, ZnO and Nd2O3 play the role of modifiers. The good solubility of Nd2O3 into tungstate glasses was confirmed. Using IR, Raman and UV–Vis spectroscopies it was established that the amorphous network is built up mainly by WO6 octahedra, participating in the formation of partially polymerised network with bridge W–O–W bonds and terminal tungsten–oxygen bonds. The structure of the obtained glasses does not satisfy the classical criteria for the glass formation. References [1] R.G. Gossink, Properties of Vitreous and Molten Alkali Molybdates and Tungstates, Philips' Gloeilampenfabrieken, Eindhoven, Netherlands, 1971. [2] M. Imaoka, N. Nesegawa, Yogyo-Kyokai-Shi 84 (8) (1976) 389–396. [3] T. Minami, M. Tatsumisago, Fusion and Processing of Glass, 1987. 167–174. [4] K. Gamayunov, T. Maslova, V. Osiko, V. Tatarintsev, Neorgan. Mater. 23 (2) (1987) 264–267 (in Russian). [5] A. Blinov, M. Borik, V. Voronov, K. Gamayunov, A. Ivanov, V. Osiko, V. Tatarincev, A. Ustin, Neorgan. Mater. 23 (6) (1987) 964–966 (in Russian). [6] A. Bahgat, M. El-Samanoudy, A. Sabry, J. Phys. Chem. Solids 60 (1999) 1921–1931. [7] M. Tatsumisago, A. Taniguchi, A. Minami, J. Am. Ceram. Soc. 76 (1) (1993) 235–237. [8] B. Vaidhyanathan, S. Asokan, K.J. Rao, Bull. Mater. Sci. 18 (3) (1995) 301–307. [9] I.Z. Hager, R. El-Mallawany, M. Poulain, J. Mater. Sci. 34 (1999) 5163–5168. [10] R. Agrawal, M. Verma, R. Gupta, Solid State Ionics 171 (2004) 199–205. [11] Y. Dimitriev, Phys. Chem. Glasses 43C (2002) 380–386. [12] Y. Dimitriev, R. Iordanova, D. Klissurski, M. Milanova, Phys. Chem. Glasses 43C (2002) 387–391. [13] M. Milanova, R. Iordanova, L. Aleksandrov, M. Hassan, Y. Dimitriev, J. Non-Cryst. Solids 357 (2011) 2713–2718. [14] S. Kubuki, K. Matsuda, K. Akiyama, Z. Homonnay, K. Sinko, E. Kuzmann, T. Nishida, J. Non-Cryst. Solids 378 (2013) 227–233. [15] S. Muthupari, G.U. Kulkarni, K.J. RaO, Bull. Mater. Sci. 17 (6) (1994) 1029–1037. [16] R.A.H. El Mallawany, Tellurite Glasses Handbook: Physical Properties and Data, CRC Press LLC, USA, 2002. [17] A. Balerna, E. Bernieri, E. Burattini, A. Kuzmin, A. Lusis, J. Purans, P. Cikmach, Nucl. Inst. Methods Phys. Res. A 308 (1991) 240–242. [18] E. Burattini, J. Purans, A. Kuzmin, Jpn. J. Phys. 32 (1993) 655–657. [19] J. Greneche, J. Teillet, J. Coey, J. Phys. 48 (1987) 1709–1714.
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Glass formation and structure of glasses in the WO3–ZnO–Nd2O3–Al2O3 system R. Iordanova a, M. Ataalla b, M. Milanova a,⁎, L. Aleksandrov a, A. Staneva b, Y. Dimitriev b a b
Institute of General and Inorganic Chemistry, BAS, Acad. G. Bonchev Str. bld. 11, 1113 Sofia, Bulgaria University of Chemical Technology and Metallurgy, 8 “Kl. Ohridski” blvd., 1756 Sofia, Bulgaria
a r t i c l e
i n f o
Article history: Received 9 December 2014 Received in revised form 11 February 2015 Accepted 12 February 2015 Available online xxxx Keywords: Tungstate glasses; Glass structure; IR; DR-UV–Vis
a b s t r a c t Glass formation ability in three different sections: WO3–ZnO–Nd2O3; (100-x) (0.8WO3·0.1ZnO·0.1Nd2O3)·xAl2O3, x = 5–30 mol% and 90(xWO3·yZnO·zNd2O3)·10Al2O3 from WO3–ZnO-Nd2O3–Al2O3 system is investigated by melt quenching technique. Glasses are obtained from the compositions with a high WO3 content (65–75 mol%). According to DTA data the glass transition temperature (Tg) is in the range 508–525 °C and the glass crystallisation temperature (Tx) is around 600 °C. Neodymium tungstates, Al2(WO4)3, ZnWO4 and WO3 are identified in the XRD patterns of quenched samples beyond the glass formation region. The structure of glasses is studied by IR, Raman and DR-UV–Vis spectroscopies. It is established that the amorphous network is built up mainly by WO6 octahedra, connected by W–O–W bridges. This structure does not satisfy the classical concept for the formation of an amorphous network which is built up by MeO4 tetrahedra. © 2015 Elsevier B.V. All rights reserved.
1. Introduction 1.1. Glass formation in tungstate systems WO3 is known as a conditional glass former that is not able to form glasses itself with standard methods of vitrification. Its glass formation ability is significantly enhanced by the addition of other compounds. The glass formation regions between 23 and 70 mol% WO3 in binary R2O–WO3; R = Li, K, Na, Rb, and Cs systems, have been determined applying rapid quenching techniques [1–3]. The high critical cooling rates (5 × 105–106 K/s) have been applied for the amorphization of compositions in WO3–Nd2O3, WO3–MxOy (M = Al, Sm, Eu, Gd, Dy, Ho) and BaO– Nd2O3–WO3 systems [4,5]. Binary WO3–Pb3O4 glasses within the concentration range of 5–30 mol% WO3 have been also reported [6]. In several papers, special attention has been paid to the glass-formation in tungstate systems containing simultaneously and other nontraditional glass formers such as V2O5, MoO3, and Bi2O3 [7–14]. It has been proven that V2O5 is a better glass-former in comparison with MoO3 and WO3 while MoO3 is better than WO3. Among the classical glass forming oxides, P2O5 allows the preparation of stable glasses in combination with WO3 over a wide concentration range [15]. Other more intensively studied glass compositions are with participation of TeO2 [16]. These glass systems are not the object of our investigation and will not be discussed below. The structure of WO3-based glasses and especially the coordination state of tungsten atoms is not well understood. The first structural ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (M. Milanova).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.02.010 0022-3093/© 2015 Elsevier B.V. All rights reserved.
studies [17,18] have shown that amorphous WO3 thin films are built up by strongly distorted, but well defined octahedra joined by vertices. The possibility to construct amorphous network containing only WO6 octahedra has been demonstrated by modelling [19,20]. Participation of both WO6 and WO4 polyhedra in the structure of binary glasses in the R2O–WO3, R = Li, K, Na, Rb, and Cs systems has been proven by IR spectroscopy [1,3]. Some authors agree that tungsten oxide units in many component glasses are present in the network as WO6 octahedra together with WO4 tetrahedra [21–23]. Other authors [24–29] using different complimentary techniques suggested that only six-fold coordinated (WO6) tungsten atoms exist in the glasses and they are enough for the formation of the amorphous network. However, up to now there is not enough data for the glass formation in many component tungstate glass systems where WO3 is a main glass former. Having in mind the above short survey, it can be concluded that WO3 is an appropriate compound for the solving of general fundamental structural problems concerning the amorphous state and as well as for the preparation of new type of oxide glasses for technology. The present paper is a continuation of our fundamental investigations on the glass formation tendency in complex oxide systems containing high WO3 content [11–13]. The aim is to study the glass forming ability of the compositions rich in WO3 (60–80 mol%) in the ZnO–WO3–Nd2O3–Al2O3 system and to elucidate the main building units determining the glass network stability (short range order). 1.2. Selection of the compositions The object of this investigation is a complex WO3-containing system without participation of typical modifiers such as alkali or alkali-earth
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Fig. 2. Fast quenched compositions in the ZnO–WO3–Nd2O3 section (see Table 1, Section 1).
Fig. 1. Investigated sections for glass formation ability in the ZnO–WO3–Nd2O3–Al2O3 system: 1) ZnO–WO3–Nd2O3; 2) (100-x)(0.8WO3·0.1Nd2O3·0.1ZnO)·xAl2O3, x = 5– 30 mol%; 3) 90(xZnO·yWO3·zNd2O3)·10Al2O3.
oxides. The other selected oxides were ZnO, Al2O3 and Nd2O3 because of their specific behaviour established in systems with participation of other transition metal oxides. The motivation for their selection will be summarised below. Recently X-ray amorphous or glass-crystalline materials from multicomponent oxide melts containing 70–75 mol% ZnO have been obtained by us [30]. Traditionally ZnO is used as a modifier in industrial glass compositions in order to improve their chemical and thermal durability and as well as their optical properties (see for
example the monograph of Vogel [31]). Few examples for the structural role of ZnO in model glass systems have been published applying suitable structural methods. It has been suggested by Calas et al. [32] that ZnO4 tetrahedra are co-polymerised with the silicate network. Hoppe et al. [33,34] have established a change of ZnO6 to ZnO4 groups in zinc–phosphate glasses containing 70–80 mol% ZnO. Sakida et al. [35] have found that in the structure of ZnO–TeO2 glasses, the coordination number of Zn atoms against oxygen decreases from 5.3 to 3.9 as ZnO content increases from 10 to 40 mol%. Nevertheless, the existence of connectivity between ZnO4 polyhedra in different glass networks and the formation of Zn–O–Zn linkages are still questionable. On the other hand, the participation of ZnO together with V2O5 and WO3 does not stimulate the formation of four-fold coordinated zinc atoms (ZnO4) and the behaviour of ZnO in this case may be considered as a modifier. Al2O3 according to the classical classifications [31] is considered as an
Table 1 Investigated compositions. Sections
Nominal composition (mol%)
Method of quenching
Classification according XRD
1. ZnO–WO3–Nd2O3
80WO3·20Nd2O3 10ZnO·80WO3·10Nd2O3 10ZnO·70WO3·20Nd2O3 20ZnO·65WO3·15Nd2O3 80WO3·20Al2O3 60WO3·40Al2O3 9.5ZnO·76WO3·9.5Nd2O3·5Al2O3 9ZnO·72WO3·9Nd2O3·10Al2O3 8ZnO·64WO3·8Nd2O3·20Al2O3 7.5ZnO·60WO3·7.5Nd2O3·25Al2O3 7ZnO·56WO3·7Nd2O3·30Al2O3 80WO3·10Nd2O3·10Al2O3 70WO3·20Nd2O3·10Al2O3 2.5ZnO·85WO3·2.5Nd2O3·10Al2O3 5ZnO·80WO3·5Nd2O3·10Al2O3 10ZnO·75WO3·5Nd2O3·10Al2O3 5ZnO·75WO3·10Nd2O3·10Al2O3 5ZnO·70WO3·15Nd2O3·10Al2O3 15ZnO·70WO3·5Nd2O3·10Al2O3 20ZnO·65WO3·5Nd2O3·10Al2O3 12.5ZnO·65WO3·12.5Nd2O3·10Al2O3 5ZnO·65WO3·20Nd2O3·10Al2O3 15ZnO·60WO3·15Nd2O3·10Al2O3 25ZnO·60WO3·5Nd2O3·10Al2O3 5ZnO·60WO3·25Nd2O3·10Al2O3 17.5ZnO·55WO3·17.5Nd2O3·10Al2O3 20ZnO·50WO3·20Nd2O3·10Al2O3
Roller technique Roller technique Press quenching Press quenching Roller technique Roller technique Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching Press quenching
Nd10W22O81 WO3; Nd6W3O18; Nd2W2O9; ZnWO4; Nd2(WO4)3 WO3; Nd2W4O15; Nd6W3O18 WO3; Nd2W4O15; Nd6W3O18; ZnWO4; Nd2(WO4)3 Al2(WO4)3 Al2(WO4)3 Glass + crystals-W19O49 Glass Glass Glass + crystals-Nd10W22O81 Glass + crystals-Nd10W22O81 Glass + crystals-W19O49 Glass + crystals-Nd2(WO4)3; WO3; WO3 WO3 Glass Glass Glass Glass Glass Glass Nd2(WO4)3; Nd10W22O81 Glass + crystals-ZnAl2O4; Nd2WO6 Glass + crystals-ZnAl2O4 Nd2(WO4)3; Nd10W22O81; Nd2(WO4)3; Nd10W22O81 ZnWO4; Nd2W2O9
2. (100-x)(0.1ZnO·0.8WO3·0.1Nd2O3)·xAl2O3; x = 5–30 mol%
3. 90(xZnO·yWO3·zNd2O3)·10Al2O3
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Fig. 4. XRD patterns of (100-x)(0.8 WO3·0.1Nd2O3·0.1ZnO)·xAl2O3, x = 5–30 mol% press quenched samples (see Table 1, Section 2). Fig. 3. XRD patterns of crystallised samples in the ZnO–WO3–Nd2O3 section obtained by: (a) and (b) press quenching of the melts (cooling rates 101–102 K/s); (c) roller quenching technique (cooling rates 103–104 K/s).
intermediate, because it is possible to participate in the glass network in tetrahedral (network former) or octahedral (modifier) oxygen coordination, depending on the other components. The third component chosen (Nd2O3) is very attractive because of its optical properties. The absorption and emission characteristics of Nd3 + (f3) ions have been the subject of extensive investigations for many years in a number of crystalline and glass materials especially for applications in laser technology [31,36–39]. Some newly investigated systems have been published recently [40–43]. Generally speaking, inorganic glasses such as silicates, phosphates, borates, germanates, and tellurites, have been proven to be an excellent host for rare earth ions, but little attention has been paid to non-traditional oxide glasses as host materials. This is a new field for fundamental studies and as well as for investigations connected with potential practical applications. 2. Experimental The investigated sections in ZnO–WO3–Nd2O3–Al2O3 are present in Fig. 1. Table 1 lists the corresponding nominal compositions which are situated near the WO3 corner because the main purpose is to verify the role of WO3 as a network former. All specimens were prepared using reagent grade WO3, ZnO, Nd2O3 and Al2O3. The homogenised batches were melted for 20 min at 1240 °C in air atmosphere. According to the electron microprobe analysis (EMRA) 8–10 mol% Al2O3 was dissolved in the ternary ZnO–WO3–Nd2O3 glasses after melting of the batches for 30 min in corundum crucibles at the maximum temperature. That is why all samples were melted in platinum crucibles. The glass forming ability of the compositions was investigated by a melt quenching technique applying two different cooling rates: 101–102 K/s
by pouring of the melt between two copper plates and 103–104 K/s by use of a roller-quenching technique. The phase formation of the samples was established by X-ray phase analysis with a Bruker D8 Advance diffractometer, using Cu Kα radiation in the 10 b 2 b 60 range. The differential thermal analysis (DTA) was carried out on LABSYS™ EVO apparatus with Pt/Pt Rh thermocouples with a heating rate of 10 K/s in Argon flow, using Al2O3 as a reference material. The IR spectra of the glasses were recorded in the 1400–400 cm−1 region, using the KBr pellet technique (Varian 660 FTIR spectrometer). The room temperature Raman measurement was performed in the range 200–1300 cm−1 on a micro-Raman system from Jobin-Yvon Horiba (LABRAM HR-800) spectrometer with a green laser (wavelength: 532 nm). Scanning electron microscopy (JEOL, JEM-200, CX) and electron microprobe analysis EMRA (JEOL, Superprobe 733) were performed. The optical spectra of powder samples at room temperature were recorded with a spectrometer (Evolution 300 UV–Vis Spectrophotometer) employing the integration sphere diffuse reflectance attachment. The samples were measured in the wavelength (λ) range of 200–1100 nm with a magnesium oxide reflectance standard used as the baseline. The uncertainty in the observed wavelength is about ± 1 nm. The Kubelka–Munk function (F(R∞)) was calculated from the UV–Vis diffuse reflectance spectra. The band gap energy (Eg) was determined by plot (F(R∞)hν)n, n = 2 versus hν (incident photon energy). 3. Results 3.1. Glass formation and crystallisation 3.1.1. ZnO–WO3–Nd2O3 (Section 1) Binary WO3–ZnO, WO3–Nd2O3 and WO3–Al2O3 compositions do not form glasses under experimental conditions applied. Investigated
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Fig. 7. Triangular diagram of the glass formation region in the 90(xZnO·yWO3· zNd2O3)·10Al2O3 section: (○) glasses; (●) crystals; ( ) glass + crystals (see Table. 1, Section 3).
conditions of quenching (WO3-JCPDS-00-020-1323; Nd2(WO4)3JCPDS-00-019-0829; ZnWO4-JCPDS-01-073-0554; Nd2W4O15-JCPDS00-035-0161; Nd6W3O18-JCPDS-00-032-0688, see Table 1). Single phase Nd10W22O81 (JCPDS-01-070-2755) was indentified in a eutectic composition (80WO3·20Nd2O3) of WO3–Nd2O3 system.
Fig. 5. DTA curves of (100-x)(0.8WO3·0.1Nd2O3·0.1ZnO)·xAl2O3, x = 5–30 mol% press quenched samples (see Table 1, Section 2).
samples in the ternary system ZnO–WO3–Nd2O3 are indicated in the triangular diagram present in Fig. 2. According to the XRD data shown in Fig. 3 there are no amorphous samples after quenching of the melts. Several phases were separated under applied non-equilibrium
Fig. 6. XRD patterns of a) 8ZnO·64WO3·8Nd2O3·20Al2O3 parent glass; b) glass, heat treated at 600 °C for 5 h.
Fig. 8. XRD patterns of glasses in the 90(xZnO·yWO3·zNd2O3)·10Al2O3 section (see Table 1, Section 3).
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3.1.2. (100-x)(0.1ZnO·0.8WO3·0.1Nd2O3)·xAl2O3, x = 5–30 mol% (Section 2) Four compositions with a constant ZnO/WO3/Nd2O3 ratio and varying Al2O3 content from 5 to 30 mol% are investigated (Fig. 1). The introduction of a small amount of Al2O3 significantly enhances the glass forming ability of the samples. As it is seen from the XRD data obtained (Fig. 4) the diffraction pattern of the specimen containing 5 mol% of Al2O3 exhibits an amorphous hallo and diffraction peaks corresponding to W18O49 (JCPDS-01-073-2177). X-ray amorphous samples were obtained after the introduction of 10 to 25 mol% Al2O3. Further increase in the Al2O3 content deteriorates the glass formation. Nd10W22O81 compound (JCPDS-01-070-2755) was detected in the composition containing 30 mol% Al2O3, irrespective that a significant part of the quenched sample remains amorphous. The thermal parameters obtained from DTA curves of quenched samples in this section are presented in Fig. 5. They are characterised with the glass transition temperature (Tg) in the range of 508-525 °C and glass crystallisation temperatures (Tx) above 590 °C. The calculated ΔT = Tx − Tg which is in the range 60–80 °C determines a good thermal stability of the amorphous samples. Selected glass compositions were subjected to heat treatment at temperatures near the glass crystallisation temperatures in order to identify the products of crystallisation. Mainly Al2(WO4)3 (JCPDS-01081-2415), together with Nd2(WO4)3 (JCPDS-00-019-0829) crystalline phase were detected (Fig. 6). 3.1.3. 90(xZnO·yWO3·zNd2O3)·10Al2O3 (Section 3) The glass formation region in this section determined visually and by XRD analysis is presented in a triangular diagram shown in Fig. 7.
Fig. 10. IR spectra of (100-x)(0.8WO3·0.1Nd2O3·0.1ZnO)·xAl2O3, x = 5–30 mol% press quenched samples (see Table 1, Section 2).
Typical diffraction patterns of all glasses obtained are shown in Fig. 8. The glasses are situated near the WO3 corner. Several crystalline phases (WO3-JCPDS-00-020-1323 and JCPDS-83-049; ZnWO4-JCPDS-01-0730554; Nd2(WO4)3-JCPDS-00-019-0829; Nd2W2O9-JCPDS-00-023-1268 and Nd10W22O81-JCPDS-01-070-2755) were detected in the XRD patterns of the crystallised sample situated outside of the glass formation region (Fig. 9). As it is seen from Fig. 9 the neodymium tungstates are the predominant phases crystallised after quenching of the melts.
3.2. IR, Raman and DR-UV–Vis spectra
Fig. 9. XRD patterns of crystallised samples obtained by melt quenching in the 90(xZnO·yWO3·zNd2O3·10Al2O3) section: 1—65WO3·5ZnO·20Nd2O3·10Al2O3; 2— 60WO3·5ZnO·25Nd2O3·10Al2O3; 3—55WO3·17.5ZnO·17.5Nd2O3·10Al2O3; 4—85WO3· 2.5ZnO·2.5Nd2O3·10Al2O3; 5—50WO3·20ZnO·20Nd2O3·10Al2O3; 6—80WO3·5ZnO· 5Nd2O3·10Al2O3.
The structure of glasses is studied by IR, Raman and DR-UV–Vis spectroscopies. A strong band at 870 cm−1 and as well as an increasing absorption in the range of 610–700 cm−1 are ascertained in all IR glass spectra (Figs. 10 and 11). Low frequency band at 460 cm−1, growing in intensity with the increase in Al2O3 content is also observed in the IR spectra (Fig. 10). A new band at 550 cm−1 appeared in the absorption spectra of glasses containing a higher amount of Al2O3 (25 mol% and 30 mol%). In some spectra it is possible to detect increasing absorption near 1000–1020 cm−1. IR spectra of reference crystalline m-WO3, Al2(WO4)3 and ZnWO4 are also given (Fig. 12). Raman spectra of 90(xZnO·yWO3·zNd2O3)·10Al2O3 glasses are shown in Fig. 13. The broad peaks being typical for glass materials are observed at 330, 840, and 985 cm−1. Fig. 14, a) compares the UV–Vis absorption spectra in the reflection regime of selected glass compositions from the ZnO–WO3–Nd2O3– Al2O3 system, pure WO3 and Al2(WO4)3. All spectra exhibit absorption maximum in the range of 220–330 nm and several peaks in the visible part of the spectral region (525, 583, 745 and 803 nm). The band gap
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attributed to the vibration of significantly shorter W–O bonds for some apical oxygen atoms [51]. Raman spectrum of Bi2W2O9 phase does not contain bands above 900 cm−1 [51]. Raman spectra of tungsten oxide clusters composed of polymeric WO6 units [52] contain high frequency bands between 950 and 1015 cm−1, (νs (W_O)), bands in the 825–930 cm−1 range (νas (W_O)) and W–O–W modes at 700–500 and 330–190 cm−1. The compounds M2W2O7 (M = K, Na) that are composed of alternating WO4 and WO6 units joined in infinite polymeric chain with bridging W–O–W bonds, are characterised by stretching modes in 800–500 cm−1 region and bands above 900 cm−1 connected with the symmetric vibration of shorter double (W_O) bonds [52]. Orthotungstate compounds possessing significantly distorted, isolated WO4 groups are characterised spectroscopically with the absence of bands in 700–500 cm− 1, but strong bands in range of 800–920 cm−1[53,54]. Table 2 lists the spectral data for the reference crystalline tungstates mentioned above. Taking into account the above spectral data, the most intensive band at 870 cm−1 together with the shoulder in the 610–660 cm−1 region observed in the IR spectra of all glass samples (Figs. 10, 11) can be assigned to the stretching vibrations of WO6 units connected by bridging W–O–W bonds. Many authors observed similar bands in the IR spectra of glasses rich in WO3 and attributed them to the vibration of WO6 octahedra [25,26,29,55,56]. The presence of a high-frequency shoulder about 1020 cm−1 in our IR spectra (Figs. 10, 11) is less pronounced relatively to the base line. This may be as a result of the widening of the distribution of bond lengths in WO6. The vibrations of WO4 units are difficult to identify, because of the overlapping with the bands due to WO6 species (see Table 2). The absence of a band around 920 cm−1 in the IR spectra of investigated glasses which is typical for the vibration of WO4 tetrahedra [53,54] does not allow us to ignore the WO4 formation. Raman analysis also gave useful information for the glass structure. Taking into account the structural and Fig. 11. IR spectra of glasses in the 90(xZnO·yWO3·zNd2O3)·10Al2O3 section (see Table 1, Section 3): 1—65WO3·20ZnO·5Nd2O3·10Al2O3; 2—70WO3·15ZnO·5Nd2O3·10Al2O3; 3—70WO3·5ZnO·15Nd2O3·10Al2O3; 4—75WO3·5ZnO·10Nd2O3·10Al2O3; 5—75WO3· 10ZnO·5Nd2O3·10Al2O3.
energy (Eg) (Fig. 14, b)) of the same samples is calculated following the above-mentioned procedure (see the Experimental Section). 4. Discussion 4.1. Structural analysis From the structural point of view it is interesting to verify the main structural units and the transformation of tungstate network with the compositions. Infrared and Raman spectroscopies were used to reveal the short range order, the type of polyhedra and their transformation with the composition. The assignments of the infrared bands are made in the framework of the local point symmetry approach for inorganic compounds [13,44–46] taking into account the spectral and structural data for WO3-containing crystalline and amorphous phases existing in the literature. It is well known that the structure of m-WO3 consists of corner-sheared distorted WO6 octahedra [47,48]. According to Guery et al. [49] the IR spectrum of m-WO3 shows a large band with two maxima at 740 cm−1 and 810 cm−1 corresponding to stretching WO6 vibrations. The structure of hydrates WO3·xH2O involving structural water is built up of layers in which WO6 octahedra shear their four equatorial oxygen forming W–O–W bonds with a length of about 1.9 Å. Two other oxygen atoms are implicated in a longer W–OH2 bond and in a shorter axial double (W_O) bond respectively [49]. The short (W_O) bonds are characterised with absorption peaks in the range of 920–1007 cm− 1 [49,50]. Mazcka et al. have reported IR and Raman spectra of layered Aurivilius type compound Bi2W2O9 [51]. IR spectrum of Bi2W2O9 is characterised by band at 850 cm−1 assigned to the asymmetric stretching modes of WO6 octahedra, and band at 936 cm− 1
Fig. 12. IR spectra of the reference crystalline phases in the WO3–ZnO–Nd2O3–Al2O3 system.
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containing 65–75 mol% WO3 are more symmetrical in comparison with tungsten octahedra in the pure crystalline WO3 [65]. The absence of any absorption below 260 nm in the UV–Vis glass spectra is evidence that the amorphous network is formed by the participation of octahedrally coordinated W6+ mainly. Optical band gap values (Eg) evaluated from the UV–Vis spectra can also give information about the structural arrangement of the glasses under investigation. A correlation between the Eg energy and the number of covalent bridging W–O–W bonds around the central W6+ cations was found. As it was shown by Barton et al. [61] and Ross-Medgaarden and Wachs [52] Eg linearly decreases with an increasing number of bridging W–O–W bonds. The band gap energy values of studied glasses are 3.6–3.7 eV and these values are compatible with the other tungstate glasses and highly disordered tungsten oxide films [66,67]. As it is seen from Fig. 14, b), the Eg of both glasses is much higher as compared with the band gap energy of the pure WO3 oxide apical oxygen atoms, but it is lower than Eg value of isolated WO4 tetrahedra in Al2(WO4)3. This modification in band gap energy values can be related with the changes in the bonding during the vitrification, which is a result of some polymerisation or clustering of WO6 polyhedra [52,61]. The last part of this study is connected with the absorption in the visible part of the spectrum above 400 nm. The absorption bands at 525 nm, 583 nm, 745 nm, 803 nm and 875 nm, observed in the glass spectra can be attributed to the electron transitions from the ground state 4I9/2 to the excited states of neodymium [31,41,42,68]. The
Fig. 13. Raman spectra of glasses in the 90(xZnO·yWO3·zNd2O3)·10Al2O3 section (see Table. 1, Section 3): 1—65WO3·20ZnO·5Nd2O3·10Al2O3; 2—70WO3·15ZnO·5Nd2O3· 10Al2O3; 3—70WO3·5ZnO·15Nd2O3·10Al2O3; 4—75WO3·5ZnO·10Nd2O3·10Al2O3; 5—75WO3·10ZnO·5Nd2O3·10Al2O3.
spectral data of tungsten oxide clusters composed of polymeric WO6 units, the observed Raman bands at 985 cm−1, 840 cm−1 and 330 cm−1 (Fig. 13) can be related with WO6 species possessing short (terminal) W_O bond and W–O–W bridges [52,57]. The bands in the 460–580 cm−1 range which rise in intensity with increasing Al2O3 content can be related with the presence of AlO6 units [58–60]. The vibrations of Nd- and Zn-oxygen polyhedral are also difficult to identify because they are at the boundary of the investigated spectral region. UV–Vis diffuse spectroscopy is used to obtain information about the local structure of glasses via ligand to metal charge transfer (LMCT) band position and the corresponding edge energy (Eg) values. The UV–Vis diffuse reflectance spectra of various tungstate reference compounds show that the absorption bands due to oxygen–tungsten charge transfer (O2− → W6+) between 218 and 260 nm can be assigned to tetrahedral WO4(Td) groups, and the bands between 250 and 360 nm can be attributed to octahedral WO6(Oh) units [52,61,62]. The exact location of these bands maxima depends on ligand field symmetry surrounding the W site. More symmetrical WOn (n = 4 or 6) possess bands at a lower wavenumber, while more distorted ones exhibit bands at a higher wavenumber. On this base the observed bands at 260 and 330 nm in the optical spectrum of pure WO3 shown in Fig. 14, a) are due to the presence of WO6 units with different symmetry. The same figure displays also the spectrum of Al2(WO4)3 as a reference for the tetrahedrally coordinated W atoms against oxygen. Its UV–Vis spectrum contains only one maximum typical for the presence of isolated tetrahedra (WO4) [52,63,64]. As it is seen from Fig. 14, a) two absorption bands are observed in the optical absorption spectra of investigated glasses. Following the above analysis they could be connected with the presence of WO6 octahedra. The shift of the band at 330 nm present in the optical spectrum of crystalline WO3 to lower wavenumber (300 nm) in the glass spectra indicates that WO6 units in the glasses
Fig. 14. a) DR-UV–Vis spectra and LMCT bands of 1) Al2(WO4)3, 2) 65WO3· 5Nd2O3·20ZnO·10Al2O3 glass, 3) 765WO3·5Nd2O3·10ZnO·10Al2O3 glass, 4) WO3; b) DR-UV–Vis spectra and Eg values of the same samples.
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Table 2 Absorption frequencies of reference crystalline tungsten oxygen compounds. Crystal
Raman band positions (cm−1)
IR band positions (cm−1)
Vibration assignments
Refs.
m-WO3 Bi2W2O9
807, 715 851 νas (W–Oap) 799 νs (W–Oap) 739, 699 νas (W–O) 945–960 ν (W_O) 800, 645–685 (O–W–O) 934–933 νs (W_O) 888–876 νs (W_O) ~500–800 ν (W–O–W), ~200–300 δ(W–O–W) ~976 νs (W_O) 958, 930, 834 νas (W_O) 700–500 ν (W–O–W); 330–190 δ (W–O–W) 1023–974 ν1 (WO4) 959–800 ν3 (WO4) 390–314ν2+ ν4(WO4)
940, 870, 810, 760, 740, 460 963, 855 νas (W–Oap) 799 νs (W–Oap) 759, 709, 647 νas (W–O) 920–1007 ν (W_O) 600–695, 700–880 ν (O–W–O)
WO6 WO6
49, 50 51
WO6
49, 50
WO4 WO6
52
WO6
52, 57
WO4
54, 58
WO3.xH2O M2W2O7; M_K, Na
(NH4)10H2W12O42
A2(WO4)3; A_Sc, In, Al
neodymium gives a strong red-violet colouration of investigated glasses. Our experiments also revealed a good solubility of neodymium in the tungstate network. The obtained complex glasses are interesting objects for optical applications and as coloured art glasses. These results could be also a base for future experiments over synthesis of nontraditional WO3-based glasses with simultaneous participation of several rare earth ions.
1010–960 ν1 (WO4) 921–799 ν3 (WO4) 420–300 ν2+ ν4(WO4)
that from the compositions containing above 30 mol% Al2O3 mainly Al2(WO4)3 derived after quenching of the melt. Its structure contains isolated WO4 tetrahedra. It is interesting to note that AlO4 and AlO5 units are identified by NMR in the structure of the amorphous Al2(WO4)3 obtained after precipitation from solutions [54,58]. This fact could be the starting point for more detailed investigation and development of a plausible hypothesis about the role of Al2O3 for the amorphisation of multicomponent tungstate compositions.
4.2. A hypothesis for the amorphous network formation Having in mind the above results and as well as the classical concepts for the glass formation in the traditional glass forming systems, some basic questions related to the glass formation in the nontraditional glasses containing WO3 have emerged: a) What are the main structural units forming the amorphous network (MeO4, MeO5, MeO6) and do they change with the compositions? b) What is the connectivity between the polyhedra (corners, edges, clusters)?
In our previous studies [13,69,70] a model for the non-traditional glass forming systems containing MoO3 have been proposed. According to this model, the process of structural transformation is realised in different concentration ranges depending on the nature of the other components. A partial MoO6 → MoO4 transformation and breaking of the edge sharing between the polyhedra existing in the crystalline MoO3 have been established. The formation of layered structures with participation of WO6 octahedra has been suggested for tungstate glasses [13]. This model is different from the proposed one for molybdate glasses. In the case of tungstate glasses, studied in this paper, we assume, that the amorphous network is dominated by the presence of deformed WO6, participating in the polymeric clusters. Another assumption is that except corner shared WO6 and formation of W–O–W bridges, shorter W–O bonds varying in length in wide ranges are formed as well. It is still questionable why Al2O3 in a limited concentration (5– 30 mol%) plays an essential role for the amorphisation of the complex tungstate compositions. Concerning this, some ideas based on the structure of the crystalline phases that have been identified in the quenched composition outside of the glass formation and have also been obtained after crystallisation of glasses by heat treatment could be drawn. Having in mind the classical criteria for amorphisation mentioned above, the short range order elements of glasses and crystalline phases will be compared. Obviously Nd2(WO4)3 [71] W18O49 [72] Nd10W22O81 [73] Al2(WO4)3 [63] and ZnWO4 [74] will deteriorate the glass formation ability of the compositions. Their structure consists of edge shared WO6 and WO6 forming clusters or isolated WO4 tetrahedra without corner W–O–W bonds. This assumption is supported as well by the fact
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