Raman microprobe investigation of sulphur-doped alkali borate glasses

Journal of Non-Crystalline Solids 210 Ž1997. 59–69

Raman microprobe investigation of sulphur-doped alkali borate glasses A.A. Ahmed a , N.A. Sharaf b, R.A. Condrate, Sr

c,)

a

c

Glass Research Laboratory, National Research Center Dokki, Cairo 12622, Egypt b Department of Physics, Faculty of Science, Tanta UniÕersity, Tanta, Egypt UniÕersityr Industry Center for Glass Research, New York State College of Ceramics, Alfred, NY 14802, USA Received 6 March 1995; revised 27 June 1996

Abstract Coloured as well as colourless binary sodium and potassium borate glasses Ž20–30 mol% Na 2 0 and 5–35 mol% K 2 O. doped with sulphur in various concentrations Ž0.3–6 wt%. were prepared by melting together the appropriate weights of sodium and potassium carbonates, boric acid and sulphur. Melting was carried out in platinum–2% rhodium crucibles in an electrically heated furnace at 10008" 108C for 2 h. The glasses were annealed at, depending on alkali content, temperatures in the range 300–3508C for 12 h and slowly cooled at the rate of 28Crmin to room temperature to release strain. The Raman microprobe spectra of the glasses prepared were measured in the range of 200 to 1600 cmy1 using the green line of an argon laser Ž514.5 nm. for Raman spectral excitation. The Raman data obtained in this study were compared with Raman spectral data of known sulphur states stable in sulphur-containing solids and solutions to identify the different states of sulphur formed in the glasses studied. A comparison was also made with published data for similar glass compositions using other spectral techniques. The Raman data obtained were found to support and complement published results of the UV and visible absorption and Raman spectra of sulphur doped glasses of similar compositions.

1. Introduction Sulphur is an important inorganic colouring agent which is capable of producing a wide range of colours in solids and solutions w1–3x. It is commonly used in the chemical industry for the production of coloured solutions and solids as well as sulphur dyes w1,2x. The same situation holds true for glass. Sulphur, either alone or in conjunction with other elements, is known to develop many colours; the most extensively commercially produced are the brown, yellow, orange and red w1,3–6x. Other colours in-

)

Corresponding author. E-mail: [email protected].

clude the green, violet and blue w1,5,7,8x. Sulphurcontaining glasses may also be colourless w9x. The colours developed by sulphur in glass are essentially dependent on glass composition, type of alkali, sulphur concentration and batch ingredients as well as the melting conditions w1,4,7,8x. Although the relation of some colours of sulphur-containing glasses to the glass batch ingredients and melting conditions is fairly understood, there has been controversy about the identity of sulphur species responsible for each colour w10–20x. This may be due to the fact that sulphur exists in unusually large number of molecular and ionic forms w1,10–21x. Many of these sulphur states form widely different coloured solutions and solids w10–21x. The abundance of the stable forms of

0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 6 . 0 0 5 8 1 - 9

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A.A. Ahmed et al.r Journal of Non-Crystalline Solids 210 (1997) 59–69

sulphur is due to the ability of the sulphur atom to bond to as many as five other sulphur atoms, besides its marked tendency to form rings and chains of linked sulphur atoms in elemental sulphur, polysul. phides ŽS 2y y , y s 2–6 and polythiothianates, as well . as sulphur radical anions ŽSy x , x s 2–4 , sulphate and many others w22x. The difficulty in identifying the sulphur species responsible for each colour is also attributed to the fact that sulphur colours are amorphous coloured powders which are insoluble in water, ether and many other solvents, and therefore, cannot be isolated in pure form for determination of their structure. Moreover, a mixture of dye molecules that cannot be separated is usually obtained from the reactions leading to form a sulphur dye. One of the useful analytical tools employed to determine the states of sulphur responsible for colour development in glass is ultraviolet ŽUV. and visible absorption spectroscopy. It has been used by Weyl w1x, Giggenbach w12x, Paul et al. w15x, Bamford w16x, Douglas and Zaman w17x, Karlsson w18,19x, Chivers w23x and Ahmed et al. w7,9x to determine the states of sulphur predominating in glasses of different compositions by comparison with spectra of well defined states of sulphur. This technique has made it possible to identify several states of sulphur, particularly those with absorption bands in the visible. No conclusive evidence, however, could be achieved, particularly for states that have absorptions only in the UV w7,12,15,24x. Raman spectra have unique characteristics, i.e. well defined limited number of bands which are markedly dependent on glass composition and impurities w3,25–28x. It was used to identify the various states of sulphur formed in sulphur-containing alkali borate glasses w24,26x. The results obtained supported the presence of some states deduced from earlier UV and visible spectral studies w7,8,24,26x and raised some doubts about others w9,13x. The glass samples used for such studies were in the powder form. As the glass compositions studied were alkali borate, which are hygroscopic, the states identified from the Raman spectra could not be taken as a true representation of the states in the bulk samples, as absorbed water may interact with the different states of oxidation of sulphur existing in glass leading to the formation of new species. The Raman microprobe w25x is a variation of a

Raman spectrometer designed to scan samples spatially and to probe spatial volumes. In essence, the microprobe couples an optical telescope to a conventional Raman spectrometer. The purpose of this work is to measure the microRaman spectra of some selected, coloured and colourless, sulphur-containing sodium and potassium borate glasses in the bulk form to avoid possible modifications in the states of sulphur induced by absorbed moisture. The Raman spectra were used to identify the dominant states of sulphur in these compositions. The compositional ranges of stability of these states will be considered. A comparison of the results obtained with that published on the UV and visible absorption, as well as Raman spectra of sulphur containing glasses of similar compositions, will be made.

2. Experimental methods 2.1. Preparation of glass samples Sulphur-containing sodium and potassium borate glasses of different alkali oxide to B 2 O 3 ratios and different sulphur concentrations were prepared. The nominal compositions of the different glasses prepared are given in Table 1. B 2 O 3 , Na 2 O and K 2 O were introduced in the form of boric acid and sodium and potassium carbonates respectively whereas sulphur was introduced either as such or as sodium sulphide. The appropriate amounts of powders of high purity chemicals were dry mixed. Melting was carried out in platinum–2% rhodium crucibles in an electrically heated furnace at 1000 " 1O8C. Melting was continued for 2 h, under normal atmospheric pressure, after the last traces of batch materials had disappeared. The melts were occasionally swirled about to obtain a bubble-free glass and to ensure homogeneity. The molten glass was poured onto a stainless plate in the form of rectangular slabs 4 = 1 = O.8 cm which were subsequently annealed at, depending on glass composition, temperatures in the range 300–3508C for 12 h and left to cool inside the furnace at an average rate of 28Crmin to room temperature to release strain. The rough surfaces of the glass were smoothed by free lapping silicon carbide grains. The smoothed surfaces were polished using cerium oxide polishing powder embedded in

A.A. Ahmed et al.r Journal of Non-Crystalline Solids 210 (1997) 59–69

61

felt polisher. The details of the technique are given elsewhere w29x. 2.2. MicroRaman measurements The microRaman spectra of the different glasses were measured in the range 200 to 1600 cmy1 using a double grating spectrophotometer ŽISA Ul000.. The spectra were measured using Raman microprobe optics with a backscattering geometry. The magnification of the objective lens in the microscope component was 50 = . The excitation source for the Raman spectra was an argon-ion laser ŽInnova 90.. The green line Ž514.5 nm. was used for Raman spectral excitation with a power of about 1 W.

3. Results The glasses obtained were classified into colourless and coloured types Žsee Table 1.. The colourless glasses are limited to those containing 5 to 15 mol% K 2 O. Potassium and sodium borate glasses of K 2 O or Na 2 O contents higher than 15 mol% are either blue, yellow orange or faint green according to the alkali content, type of the alkali and sulphur concentration Žsee Table 1.. The main features of the Raman spectra of both types are given in the following.

Fig. 1. Raman spectra of colourless sulphur-containing potassium borate glasses.

develop at 540 and 1540 cmy1 in the spectra of the glass containing 10 mol% K 2 O and increases in intensity with further increase of K 2 O ŽFig. 1B and C.. Also, a weak shoulder develops at 920 cmy1 in the spectra of the glass containing 15 mol% K 2 O Žsee Fig. 1C..

3.1. Colourless glasses 3.2. Coloured glasses The Raman spectra of the colourless glasses, those containing 5–15 mol% K 2 O, are shown in Fig. 1. The spectrum of the glass containing 5 mol% K 2 O shows a principal band at 807 cmy1 together with two additional weak and broad bands at 658 and 462 cmy1 . A weak shoulder is also observed at 771 cmy1 Žsee Fig. 1A.. The same bands are also observed in the spectra of glasses containing up to 15 mol% K 2 O. However, with increasing K 2 O content, the following differences are observed. The intensity of the principal band at 807 cmy1 gradually decreases. The weak shoulder at 770 cmy1 becomes well defined and increases in intensity so that it becomes equal to that of the principal band at 807 cmy1 in the spectra of the glass containing 15 mol% K 2 O. Also, the weak shoulders at 658 and 460 cmy1 slightly increase in intensity. Weak bands

All sulphur-containing borate glasses of ) 15 mol% of either Ka 2 O or K 2 O are either blue, yellow, orange or faint green Žsee Table 1.. The main characteristic features of the spectra of each group of glasses are given in the following subsections. 3.2.1. Blue glasses The blue colour is acquired by glasses containing 20 to 25 mol% K 2 O or Na 2 O. The spectra Žsee Fig. 2. reveal the following features. 3.2.1.1. Glass containing 20 mol% Na2 O. The Raman spectrum Žsee Fig. 2A., has several well defined as well as weak bands and shoulders. The most intense bands are observed at 775 and 537 cmy1 . Weak bands are observed at 580, 665, 933, 1077,

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A.A. Ahmed et al.r Journal of Non-Crystalline Solids 210 (1997) 59–69

1102 and 1540 cmy1 . A few shoulders at 256, 490 and 799 cmy1 are observed.

Table 1 Nominal compositions and colours of the sulphur-doped sodium and potassium borate glasses prepared in this work

3.2.1.2. Glass containing 25 mol% Na2 O. The intense band at 772 cmy1 retains its large amplitude. The bands at 500 and 1540 cmy1 remain almost unchanged. The intensity of the bands at 537, 580, 665, 1077 and 1102 cmy1 decrease Žsee Fig. 2B.. The bands at 256, 799 and 930 cmy1 disappear. A new weak band develops at 970 cmy1 .

Glass No.

3.2.1.3. Glass containing 20 mol% K 2 O. The band at 771 cmy1 remains intense. The intensity of the bands at 537, 579, 665, 803, 950 and 1500 cmy1 decreases. New weak bands at 275, 470 and 635 cmy1 appear Žsee Fig. 2C.. 3.2.2. Yellow and orange glasses Yellow and orange colours develop in both sodium and potassium borate glasses of alkali contents ) 25 mol% Žsee Table 1.. The colour deepens with increasing sulphur content. The following spectral observations are noted. 3.2.2.1. Glass containing 30 mol% Na2 O. The Raman spectrum of the glass shows strong bands at

Fig. 2. Raman spectra of blue sulphur-containing potassium and sodium borate glasses.

Glass composition mol%

Glass colour wt%

B2 O3

R 2O

S

K 2O 1 2 3 4 5 6 7

95 90 85 80 70 70 65

5 10 15 20 30 30 35

3.0 3.0 3.0 3.0 0.3 1.2 3.0

colourless colourless colourless blue faint yellow faint green orange

Na 2 O 8 9 10 11 12

80 75 70 70 70

20 25 30 30 30

3.0 3.0 1.5 3.0 6.0

blue deep blue faint yellow yellow orange

772, 500 and 986 cmy1 Žsee Fig. 3A., and weak bands at 533, 570, 633, 940, 1010, 1110, 1400 and 1500 cmy1 . With increasing sulphur content, the

Fig. 3. Raman spectra of sulphur-containing yellow and orange sodium borate glasses.

A.A. Ahmed et al.r Journal of Non-Crystalline Solids 210 (1997) 59–69

63

glass turns orange and the different bands increase in intensity, particularly those at 500, 570 and 985 cmy1 . 3.2.2.2. Glasses containing 30–35 mol% K 2 O. Potassium borate glasses of 30 mol% K 2 O are yellow when sulphur content is kept below 1.2 wt% whereas the 35 mol% K 2 O glass is orange Žsee Table 1.. In general, the spectra are characterized by bands of relatively low intensity. The main characteristics are as follows. The spectrum of the glass containing 30 mol% K 2 O shows a well defined band at 764 cmy1 in addition to weak bands and shoulders at 256, 460 480, 537, 580, 630, 764, 975, 1080, 1400 and 1500 cmy1 . The spectrum of the glass containing 35 mol% K 2 O Žsee Fig. 4., shows two well defined bands at 579 and 765 cmy1 as well as weak bands and shoulders at 345, 400, 468, 505, 570, 636, 965, 1120, 1400 and 1500 cmy1 . 3.2.3. Green glass The potassium borate glass containing 30 mol% K 2 O acquired a faint green colour only when sulphur was added in concentrations ) 1.2 wt%. The spectrum shows a set of weak bands and shoulders at

Fig. 5. Raman spectrum of Ž1.2 wt%. sulphur-containing green potassium borate glass.

430, 482, 537, 630, 968, 1124, 1400 and 1500 cmy1 as well as a strong band at 767 cmy1 Žsee Fig. 5..

4. Discussion

Fig. 4. Raman spectra of sulphur-containing yellow and orange potassium borate glasses.

The identification of the sulphur states predominating in sulphur-containing binary alkali borate glasses was reported previously by Ahmed et al. w7–9,24x who studied the UV and visible absorption spectra of such glasses in the range 200 to 700 nm. The absorption spectra of sulphur-containing potassium and sodium borate glasses are reproduced in Figs. 6 and 7 respectively w8,24x. The interpretation of their data was based on a comparison of the position of the absorption bands they obtained, with that of specific sulphur species. They were able to y 2y identify the species S 2 , Sy 3 , S 2 and S x . Formation of other sulphur species which absorb in regions outside 200 to 700 nm, e.g. sulphate and sulphide ions, could not be deduced. The stability of the different sulphur states as a function of alkali content of the glass was also reported. They showed that glasses containing 15 mol% alkali oxide, ŽR 2 O., could not retain sulphur. The S 2 molecule was stable

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A.A. Ahmed et al.r Journal of Non-Crystalline Solids 210 (1997) 59–69

in glasses containing at least 15 mol% R 2 O. The Sy 3 and Sy 2 states were found to predominate in glasses containing 20 to 30 mol% R 2 O. The polysulphide ions S 2y x , which they believed to be the tetra– and pentasulphide ions, predominated in glasses containing 30 to 35 mol% R 2 O. The identification of the origin of vibrations observed in the spectra of glass can usually be performed by using the spectra of crystalline compounds and solutions which contain definite structural units and groups as a fingerprint to demonstrate the presence of certain structural units in the glass. Accordingly, the frequencies of the different Raman bands observed in this work will be compared with that of definite and established states of sulphur formed in solutions, crystalline solids and molten salts and also with that of borate units and groups formed in both crystalline solids and glass. The Raman spectra obtained for the various glasses prepared in this work show a great number of bands. The number, intensity and shape of these bands are dependent on glass composition and type of alkali as well as the sulphur concentration. All the observed bands can be classified into two groups according to their origin. The first group of bands includes those which are associated with vibrations of the different

Fig. 7. UV and visible absorption spectra of sulphur-containing sodium borate glasses, taken from Ahmed et al. w24x.

borate units and groups existing in the base glass and have nothing to do with sulphur. The other group involves bands that are related to the different states of sulphur. Before dealing with identification of sulphur states formed in the glasses studied, the structural borate groups formed in alkali borate glasses as a function of alkali content and their corresponding Raman bands will be considered. 4.1. Raman bands and the structure of alkali borate glasses

Fig. 6. UV and visible absorption spectra of sulphur-containing potassium borate glasses, taken from Ahmed et al. w8x.

Detailed studies w30–36x of the structure of alkali borate glasses have revealed that vitreous boron oxide is built from a random network of planar oxygen triangles centered by boron atoms with most of the triangles arranged in the form of boroxol groups w30,31x. Addition of up to 20 mol% alkali oxide to boron oxide leads to gradual conversion of boroxol groups mainly to tetraborate groups which consist of penta- and triborate groups. At 20 mol% alkali oxide, the borate glass structure consists of tetraborate groups, a small number of loose BO4 groups and minor numbers of boroxol groups and loose BO 3 triangles. With increasing alkali content in the composition range 20–35 mol% alkali oxide, the tetraborate

A.A. Ahmed et al.r Journal of Non-Crystalline Solids 210 (1997) 59–69 Table 2 Raman spectra of borate groups Borate group

Raman spectra Žcmy1 .

Ref.

NI) Ring type metaborate Diborate Triborate Pentaborate Boroxol Pyroborate Isolated orhoborate Tetraborate NI) NI) NI)

480–495 630–655 720–755 770 785 803–807 820–850 905 950–960 1120 1245 1540

w33x w33,36x w33x w33x w33,34x w34x w33–35x w34x w33x w33x w33x w33x

NI): Bands of unidentified or no definite origin.

groups are gradually replaced by diborate groups. At 33 mol% alkali oxide, the network is built of diborate groups and a small number of loose B0 4 tetrahedra and ring type metaborate groups, as well as a minor number of loose BO 3 triangles. The abundance of these groups is slightly affected by the type of alkali ion, whether sodium or potassium. The Raman spectra w33–35x of alkali borate glasses show the bands corresponding to the vibrations of these borate groups, as well as the change in their concentration with changes in alkali content and type of the alkali ion. The frequencies of the Raman bands associated with the different borate groups are given in Table 2. The table also shows frequencies of other bands observed in the spectra of crystalline and glassy borates but which could not be assigned to known borate units or groups w33,34x. 4.2. Raman spectra of the different states of sulphur The frequencies of the Raman bands associated with the different states of sulphur stable in sulphurcontaining solutions and solids are given in Table 3 w37–39x. The table makes it clear that most of the strong bands corresponding to singly and doubly charged sulphur ions occur in the range 400 to 600 cmy1 , a range which is outside the important range of frequencies of the principal borate groups Žsee Table 2.. The other states of sulphur proposed to exist in sulphur-containing borate and silicate glasses and which have Raman bands outside this range are

65

limited to the S 2 molecule and the sulphate ion. Accordingly, our emphasis will be on identifying the Raman bands that occur in the range 400 to 600 cmy1 , as well as around 716, 990 and 1077 cmy1 and attributing them to the most probable states of sulphur. 4.3. Identification of the sulphur states formed in the glasses studied A summary of the frequencies of the Raman bands observed in the spectra of the various colourless and coloured glasses studied in this work, as well as a semi-quantitative estimation of their amplitudes are given in Table 4. 4.3.1. Colourless glasses The Raman spectrum of the glass containing 5 mol% K 2 O shows only the bands at 807, 771, 658 and 462 cmy1 , associated with borate groups. No bands could be detected which are associated with sulphur. The spectra of the glasses containing 10 and 15 mol% have additional Raman bands; two at f 1540 and 920 cmy1 due to borate groups and one around 540 cmy1 associated with Sy 3. Table 3 Raman spectra of the different states of sulphur State of sulphur

Raman spectra Žcmy1 .

Ref.

S2 S3 S4

713–716 583 674m, 653w, 352s 668,601,440 585–595s 1077w, 535s, 239 518w, 439m, 384s 451 466s, 238m, 476s, 458m, 238w 482s, 445m 496m, 432s, 252s 453m, 373s, 358m 1130m, 994vs, 630, 470 1150w, 1110w, 984vs, 625m, 460m 1064 1151 470, 261, 152

w37x w37x w37x w37x w37x w3,37x w37x w37x w37x w37x w37x w37x w37x w39x

Sy 2 Sy 3 S4 2y Ž S2 b-Na 2 S 2 . S32y ŽK 2 S 3 . ŽNa 2 S 3 . S42y S52y S62y q S 6 ŽNa 2 SO4 . ŽK 2 SO4 . SO 3 SO 2 Elemental noncrystalline S

m s medium, s sstrong, w s weak, v s very.

w39x w38x w38x w38x

A.A. Ahmed et al.r Journal of Non-Crystalline Solids 210 (1997) 59–69

66

Table 4 Positions Žcmy1 . and relative intensities of Raman bands observed in the spectra of sulphur-containing binary sodium and potassium borate glasses Band No.

Colourless

Blue

K 2O 5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

10

15

Yellow and orange

K 2O

Na 2 O

20

20

25

Na 2 O

K 2O

30

30

256wsh

Green K 2O 35

30

256w

275w 345 400w 430 462vw

462vw

642w

470w 490wsh

540?vw

658vw 771wsh 807s

658w 770w 807s

540w

658w 770m 807m 920vs

537m 579w 635w 665w 771s 803w

490wsh

537vs 580w

537w 580w

665w 775vs 799wsh

665w 772s

933w 950w

970w 1077w 1102w

1077w 1102w

1500w 1540w

1540w

1540w

460w 480w

468w 482w

500s 533w 570ms 633w

505w 537w 580w 630w

579w 636w

630w

772vs

764m

765m

767s

940w 986s 1010w

975w

965w

968w

1120w 1400w 500w

1124 1400w 1500w

537w

1080w 1110w 1400w 1500w

1400w 1500w

1540w

m s medium, s s strong, w s weak, v s very, sh s shoulder, ? s existence is not certain. Full details of composition of all glasses are given in Table 1.

It is known that the Sy 3 state is associated with a visible absorption band around 610 nm which is responsible for the blue colour of sulphur-containing solutions, crystalline solids, molten salts and glasses w3,7,8,13,23,24,26,27 x. The intensity of the blue colouration is proportional to the concentration of the Sy 3 . We conclude that the concentration of the Sy 3 state, in these glasses, is too low to develop the blue colour which explains why the visible absorption of glasses of similar compositions w7–9x failed to detect its presence. Previous UV absorption of sulphur-containing alkali borate glasses of 15 to 17.5 mol% Na 2 O or K 2 O has an absorption band around 280 nm which was suggested to be due to the S 2 molecule w7,9x. Such state of sulphur has a Raman band at 716 cmy1 which could not be detected in w14,24x as well as the present work Žsee Fig. 1.. Accordingly, there is no evidence from the Raman spectra for the existence of

the S 2 molecule in sulphur-containing binary alkali borate glasses of 5–15 mol% K 2 O or Na 2 O. 4.3.2. Blue glasses The absorption spectrum of the glass containing 20 mol% Na 2 O has Raman bands at 665, 775, 799, 1102 and 1540 cmy1 Žsee Fig. 2. which are associated with different borate groups Žsee Table 2.. Besides, some other bands are observed which could be assigned to several sulphur states; bands at 537 y1 and 1077 cmy1 to Sy to Sy 3 , band at 580 cm 2 and y1 2y bands at 256 cm to S 5 . Another band at 490 cmy1 is also observed. Although it may be related to borate groups Žsee Table 2., it would be appropriate to relate it to S 52y. The presence of the band at 256 cmy1 which in conjunction with that at 490 cmy1 is due to S 52y, may lend support to this assignment. The y1 high intensity of the blue Sy 3 band at 537 cm produces the intense blue colouration of the glass.

A.A. Ahmed et al.r Journal of Non-Crystalline Solids 210 (1997) 59–69

The spectrum of the glass containing 25 mol% Na 2 O Žsee Fig. 2. has fewer borate Raman bands at 665, 772, 970, 1102 and 1500 cmy1 . Also, bands of significantly lower intensity are observed at 500 cmy1 assigned to S 52y, at 537 and 1077 cmy1 asy1 signed to Sy assigned to Sy 3 and at 580 cm 2. Still fewer bands are observed at 635, 665, 771, 803, 950 and 1500 cmy1 for the glass containing 20 mol% K 2 O. Bands at 537 and 579 cmy1 due to Sy 3 and Sy 2 respectively are also observed. Moreover, a band at 470 cmy1 is noticed which may be due to y1 S 2y or S 2y is 3 4 . The origin of the band at 275 cm not known. The states inferred from the Raman spectra agree well with recent UV and visible absorption spectra, as well as earlier Raman findings w7,8,24,27x. 4.3.3. Yellow and orange glasses 4.3.3.1. Sodium borate glasses containing 30 mol% Na2 O. The spectrum of the glass containing 30 mol% Na 2 O Žsee Fig. 3. has borate Raman bands at 633, 772, 940, 986, 1110, 1400 and 1500 cmy1 . A notable feature is the existence of a strong and broad Raman band around 500 cmy1 which may be an envelope to other bands in the range of 400–500 cmy1 . These bands could be assigned to different 2y 2y polysulphide ions; S 2y and S 62y. Addi3 , S 4 , S5 y1 tional weak bands at 535 cm assigned to Sy 3 and at 570 cmy1 assigned to Sy 2 are also observed. With increasing sulphur content, the intensity of the polysulphide bands gradually increases. The significant increase in intensity of the yellow polysulphide bands relative to that of the blue bands is responsible for the development of the yellow and orange colour in agreement with the UV and visible absorption studies w8x. Another band at 1110 cmy1 of unidentified origin is also observed. 4.3.3.2. Potassium borate glasses containing 30–35 mol% K 2 O. In general, the Raman bands Žsee Fig. 4. are weak compared to the spectra of other coloured glasses. The spectrum of the glass containing 30 mol% K 2 O has Raman bands at 630, 764, 975, 1400 and 1500 cmy1 associated with borate groups. Other bands are observed at 256, 460 and 480 cmy1 asand signed to different polysulphide ions S 52y, S 2y 3 S 2y and at 537 and 1080 cmy1 assigned to Sy 4 3 as

67

well as at 570 cmy1 due to Sy 2 . The intensity of the polysulphide bands are much higher than that of the blue bands as observed in the spectra of sodium borate glasses of 30 mol% Na 2 O Žsee Fig. 3.. The spectrum of the glass containing 35 mol% K 2 O has sulphur bands at 345, 400, 468 and 505 cmy1 assigned to different polysulphides and at 579 cmy1 Ž . assigned to Sy 2 as well as borate groups see Fig. 4 . Interpretation of the obtained Raman data is in agreement with that previously derived from UV, visible and Raman spectral studies w8,24x. 4.3.4. Green glass The green colour is obtained only in potassium borate glass of 30 mol% K 2 O when sulphur is added in amounts ) 1 wt%. The intensity of the Raman bands remains weak Žsee Fig. 4.. The bands due to borate groups are observed at 630, 767, 985, 1120, 1400 and 1500 cmy1 . Weak bands associated with sulphur are observed at 430 and 480 cmy1 , which may involve other bands of lower intensity in between, assigned to different polysulphide ions S 2y 4 , S 52y, S 32y and S 62y and at 537 cmy1 assigned to Sy 3. The presence of the Sy is responsible for the devel3 opment of faint green colouration. The interpretation of the Raman data obtained agrees with that published on the UV and visible absorption, as well as earlier-obtained Raman spectra w7,8,24x. 4.4. Ranges of stability of the different sulphur states Results obtained revealed that: ŽA. No bands that could be assigned to any of the states of sulphur were observed in the Raman spectra of glasses that contain up to 10 mol% alkali oxide. The existence of the Sy 3 in the glass containing 10 mol% K 2 O is not definite as the intensity of its Raman band is very low. Such observation indicates that these glasses could not retain sulphur. This can be attributed to their extremely acidic nature which would lead to the formation of H 2 S that can easily escape from the glass melt. ŽB. The blue Sy 3 is definitely formed in potassium borate glasses containing 15 to 30 mol% K 2 O. It reaches maximum concentration in glasses containing 20 mol% K 2 O. It was not possible to detect its formation in the glass containing 15 mol% K 2 O from the UV spectra w7,8x. The development of the

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blue colour associated with this band depends on the concentration of the Sy 3 ion. Accordingly the glass containing 15 mol% K 2 O was colourless as its content of the Sy 3 ion is very low. The range of formation of the blue colour is in agreement with that stated by Volf w40x. ŽC. The Sy 2 state is stable in glasses containing 20 to 35 mol% alkali oxide. ŽD. The polysulphides are formed in the range 20 to 35 mol% alkali oxides. Glasses containing 25 mol% Na 2 O contain only the S 52y, whereas a mixture of several polysulphide ions is formed in other glasses of higher R 2 O content. The maximum concentration of polysulphides is reached in glasses containing the highest alkali content, which is a consequence of formation of non-bridging oxygens w8,30x. ŽE. Only the glasses containing 10–15 mol% alkali oxide contain only a single sulphur state, the Sy 3 state, whereas glasses of higher alkali content contain a mixture of several states of sulphur. ŽF. The sequence of formation of the different states of sulphur with increasing alkali content is in conformity with the increase of basicity of glass.

5. Conclusions The following states of sulphur could be identiy 2y 2y 2y 2y fied: Sy and S 62y, and their 3 , S 2 , S 2 , S 3 , S 4 , S5 ranges of stability defined. No evidence could be achieved from the data obtained for the formation of any of the molecular forms of elemental sulphur, particularly the S 2 molecule. Also, the sulphide and sulphate anions could not be detected, as they usually need reducing and oxidizing conditions of melting w41x respectively. In general, the Raman spectral measurements present more refined data which support the UV and visible absorption data and provide additional information needed for better understanding of the relationship between the nature of the sulphur state and the colour developed in sulphur-containing glasses.

Acknowledgements The measurements of the microRaman spectra were accomplished in the Spectroscopy Laboratory,

at Alfred University, during the supported Academic Leave ŽAugust 1990–July 1991., granted by the Egyptian government to Dr N.A. Sharaf. The authors wish to thank Dr R.P. Hapanowiz, UniversityrIndustry Center for Glass Research, Alfred University, for experimental assistance in the microRaman spectral measurements.

References w1x W.A. Weyl, Coloured Glasses ŽSociety of Glass Technology, Sheffield, 1959.. w2x Kirk–Othmer, Encycl. Chem. Technol. 13 Ž1963. 358; 19 Ž1969. 337. w3x T. Chivers, in: Homoatomic Rings, Chains and Macromolecules of Main Group Elements, ed. A.L. Rheingold ŽElsevier, Amsterdam, 1977. p. 499, and references therein. w4x A.A. Ahmed, T.M. ElShamy and N.A. Sharaf, in: Proc. 1st Arab Regional Conf. on Sulphur and its Usage in the Arab World ŽKuwait April 3–6, 1982. ŽKuwait Institute for Scientific Research and Kuwait Foundation for the Advancement of Sience, 1983. p. 133. w5x C. Genisson, A. Michaud and J. Herbert, in: Proc. Symp. on Coloured Glass ŽAug. 17–20, Res. Inst. for Glass and Imitation Jewelry, Jablonec, Czechoslovakia 1965. p. 136. w6x N.J. Kreidl, in: Handbook of Glass Manufacture ŽOptical Properties., ed. F.V. Tooley, Vol. II ŽOgden, New York, 1960. p. 1. w7x A.A. Ahmed, T.M. ElShamy and N.A. Sharaf, J. Non-Cryst. Solids 33 Ž1979. 159. w8x A.A. Ahmed, T.M. ElShamy and N.A. Sharaf, J. Am. Ceram. Soc. 63 Ž1980. 537. w9x A.A. Ahmed, T.M. ElShamy and N.A. Sharaf, J. Non-Cryst. Solids 30 Ž1978. 225. w10x H. Lux and H. Anslinger, Chem. Ber. 94 Ž1961. 1161. w11x F.G. Bodewing and J.A. Plambeck, J. Electrochem. Soc. 116 Ž1959. 607; 117 Ž1970. 904. w12x W. Giggenbach, J. Inorg. Nucl. Chem. 30 Ž1968. 3189. w13x T. Chivers and I. Drummond, Inorg. Chem. 11 Ž1972. 2525. w14x T. Chivers, J. Non-Cryst. Solids 41 Ž1980. 143. w15x A. Paul, A. Ward and S. Gomolka, J. Mater. Sci. 9 Ž1974. 1133. w16x C.R. Bamford, Phys. Chem. Glasses 2 Ž1961. 163. w17x R.W. Douglas and M.S. Zaman, Phys. Chem. Glasses 10 Ž1969. 125. w18x K.H. Karlsson, Glastek. Tidskr. 24 Ž1969. 13. w19x K.H. Karlsson, thesis, Doctor of Tech., Helsinki University of Technology Ž1971.. w20x D.M. Gruen, R.L. McBeth and A.L. Zielen, J. Am. Chem. Soc. 93 Ž1971. 6691. w21x B. Meyer, Chem. Rev. 76 Ž1976. 367. w22x G. Nickless, ed., Inorganic Sulphur Chemistry ŽElsevier, Amsterdam, 1968.. w23x T. Chivers and I. Drummond, Chem. Soc. Rev. 2 Ž1973. 233.

A.A. Ahmed et al.r Journal of Non-Crystalline Solids 210 (1997) 59–69 w24x A.A. Ahmed, T.M. ElShamy and N.A. Sharaf, in: Proc. 2nd Conf. Opt. Specroscopy, Laser and Applications, Cairo, Nov. 1985, p. 459. w25x B. Bendow, in: Experimental Techniques of Glass Science, ed. C.J. Simmons and O.H. El–Bayoumi ŽAmerican Ceramic Society, OH, 1993. p. 37. w26x T. Chivers, J. Mater. Sci. 12 Ž1977. 417. w27x T. Chivers, E. Gilmour and R.A. Kydd, J. Mater. Sci. 13 Ž1978. 1585. w28x P. Dhamelincourt et al., Anal. Chem. 51 Ž1979. 414A. w29x F. Twyman, Optical Glass Working ŽHilger and Watts, London, 1955.. w30x D.L. Griscom, in: Borate Glasses: Structure, Properties & Applications, ed. L.D. Pye, V.D. Frechette and N.J. Kreidl ŽPlenum, New York, 1978. p. 11. w31x P.E. Stallworth and P.J. Bray, in: Glass Science and Technology, ed. D.R. Uhlmann and N.J. Kreidl, Vol. 4B ŽAcademic Press, New York, 1990. p. 77. w32x N.J. Kreidl, in: Glass Science & Technology, ed. D.R.

w33x w34x w35x w36x w37x w38x w39x w40x w41x

69

Uhlmann and N.J. Kreidl, Vol. 3 ŽAcademic Press, New York, 1983. p. 105. W.L. Konijnendijk, Philips Res. Repts. Suppl. Ž1975. No. 1. T.W. Bril, thesis, Technological University Eindhoven Ž1975.. T. Furukawa and W.B. White, Phys. Chem. Glasses 21 Ž1980. 85. J. Yifen and C. Xiangsheng, in: Proc. 13th Int. Cong. Glass ŽGermany, 1983.; Glastech. Ber. 56K, Bd. 2 Ž1983. 910. T. Chivers and C. Lau, Inorg. Chem. 21 Ž1982. 453. G.J. Rosasco and J.H. Simmons, Am. Ceram. Soc. Bull. 53 Ž1974. 626; 54 Ž1975. 590. W.L. Konijnendijk and J.H.J.M. Buster, J. Non-Cryst. Solids 23 Ž1977. 401. M.B. Volf, Chemical Approach to Glass, Glass Science and Technology, Vol. 7 ŽElsevier, Amsterdam, 1984.. C.J.B. Fincham and F.D. Richardson, in: Proc. Roy. Soc. London, Ser. A223 w1152x Ž1954. 40.