Raman Spectra of υ-B₂O₃ Sodium Borate and Calcium Borate Glasses

Indian J . P h y s. 66H (4), 365-373 (1992)

Raman spectra of D-B 2 O 3 , sodium borate and calcium borate glasses

M H Rahman, B P Dwivedi, Y K A a r and B N Khanna

Spectroscopy Laboratory, Depanmcni ofFiysics. Aligarh Muslim University,

Aligarh-202 002, India. i

Received 6 July 1991, accepted 30 July I9i

Abstract : Raman spectra of pure bori^ oxide, sodium borate and calcium borate glasses in the systems (1 - x) B2O3. xNa2 0 for x = 0.20 and 0.25 and (1 - y) B2O3. yCaO for y = 0.2 0 ,0 .2 5 , 0.30, 0.35 and 0.40 have been iecorded. The analysis of v>B203 spectrum shows that its structure consists mainly of planar hexagonal B3O5 rings and BO3 triangles. Very sharp, polarized and intense Raman band at 806 cm ^ is the dominating feature of the ring structure.

Other bands have been assigned.

Addition of Na2 0 or CaO to the U-B2O3 structure causes a change in the boron atom coordination number i.c., some new structural groupings arc built up from triangular and tetrahedral distorted units, BO3 and BO4. l*or both sodium borate and calcium borate spectra, the band 806 cm ^ clearly splits at concciuration of 0 .2 0 showing the similar structure of these glasses, giving rise to a new Raman band. At concentration y ^ 0.35, only this new band remains in the spectrum of calcium boraie with the disappearance of the other splitied band. The BO4 formation reaches saturation (the network coherence saturation) aty = 0.35. Further addition of CaO (y > 0.35) brings out another mechanism of formation of non-bridging oxygens (NBO) and this number of NBO’s increases with concentration followed by the decrease of the network coherence.

Keywords : Raman spectra, glass stmciure, borate glasses.

PACS No. : 78.30. ~j

1. Introduction

The number of borate glasses idcniiricd is continuously increasing and their use is now wide spread in various fields (Pyc et al 1978). The interest in boraie glasses has grown due to exceptionally high ionic conductivities observed in these systems. Alkaline and Ag'^ ions are the species responsible for so called ‘fast ion conduction’. The structure of boraie glasses has been studied spectroscopically by many workers in the past. For glasses of the composition XR2O. (1 - x)B2 0 3, where R .stands for alkali or alkaline earths, Krogh-Moc (1965) proposed an uniformly accepted model (through spectral investigation) in which borate glasses have been described as a random network consisting of the structural groups i e., boroxol, tetraborate, diboralc and so on. Thc.se groups also occur in crystalline state of the compounds. Krogh-Moc proposed these structural units in terms of alkali oxide

© 1992 lACS

366 Af H R ahm an. B P D w iv e d i, Y K u m a r a n d B N K hanna

concentration ranges as shown below;

R2O Cone, mol %

0 - 2 0 - 3 3

Borate groups

boroxol ring, BO3 unit boroxol ring, BO3 unit tetraborate, diborate

Konijnendijk (1975) obtained Raman and Infrared spectra of alkali and alkaline earth borate glasses and came to the same conclusion as that of Krogh-Moe. Krogh-Moe model is also supported by Griscom (1978). The model essentially describes the glass former network in which B atoms are partly tricoordinated and partly teiracoordinated with O atoms. In general, the units BO3 and BO4 are included in larger structural units, whose type and distribution change with changing composition. NMR studies of Bray (1978) revealed the environments of boron atoms. The NMR study on these glasses carried out as a function of composition of modifier cations provides a relatively simple and precise method to find out the fractions of iri- and tctra-coordinated B atoms and to determine which R2O concentration gives rise to the formation of non-bridging oxygen (NBO) atoms i.e ‘O’ atoms not involved in B-O-B linkages.

As a part of the investigation on borate glasses, we have performed measurements of the Raman spectra of 0-B2O3. xNa2 0(l - jc).B2 0 3 for jt = 0.15,0.20 and 0.25 and yCaO.

(1 - y ) B2 0 3 fory = 0.20, 0.25, 0.30, 0.35 and 0.40 in order to understand the relation between the vibrational frequencies and the structure of these glasses.

2 . Experim ental

The boric oxide glass sample was made from pure boric acid powder (H3BO3) obtained from BDH Bombay, India, by heating it at 1(X)0°C for twenty four hours in an electric furnace.

The clear colourless melt was cast on to a stainless steel plate and pressed with other stainless steel plate to prepare about 1.5 mm thick glass pellet. The prepared glass was stored in a desiccator cautiously since it is a very hygroscopic material.

To prepare sodium borate and calcium borate glasses, the boric acid powder was first dehydrated by heating uplo 180®C and it was then mixed with appropriate amounts of Na2C0 3 and CaCOs (obtained from sd. Fine Chem, Bombay, India). The mixture was then heated upto 1100®C for 2 hours and using the above technique, about 1.5 mm thick glass pellets were prepared. All the glass samples were colourless except at higher concentrations in calcium borate glass which were slightly redish.

Raman spectra of all the glasses were recorded on Ramanor U-IOOO double monochromator instrument equipped with Spectra Physics model 171 agron ion laser. The

emission line of the argon ion laser at 5145A* was used as the exciting source with the laser ouQ>ut power of 400 mw. The cooled RCA photomultiplier tube coupled with spectra link |4)Oton coimdng system was used for the detection of Raman signals. A slit width of

~ 4 cm~‘ was used and the spectra were recorded in the conventional 90** scattering geometry.

3 . Results and discussion

3 .1 . v S f i j q x c t r a :

For reccuding q>ectra, rectangular shaped glass c f B2O3 was prepared and the faces of it were grinded with carborandum powder of different^neshes (300 and 8(X) grades) and polished with soft cloth. The Figures 1(a) and 1(b) show ^-8 2 0 3 spectra that have been recorded with laser powers, 400 mw (Figure la) and 800 m f (Figure lb). The important feature of the Raman spectrum of 0-B2O3 is the intense, li^hly polarized (p = O.OS) and very sharp

R a m a n sp e c tra c f v -B 2 0 j, sodiu m b o ra te e tc 367

Figure 1. Raman ipectrum of U-B2P3 wiih laserpowcr (a) 400 mw in the range of (100-1600 cm~') and (b) 800 mw in the range of (100-1600 cm *).

band at 806 cm~'. This band has been assigned by Galccner (1982) and many others to the symmeuic breathing mode o f the oxygen motions in the planar boroxol rings. The Krogh- Moe (1969) model, previously proposed by Goubcau and Keller (1953), about 0-B2O3 structure (Figure 2) shows that it consists of mainly planar hexagonal ‘boroxoT (BsOg) rings interconnected at the boron ^ m s by single oxygen atom or BO3 triangles. Within the

368 M H Rahman, B P Dwivedi, Y Kumar and B N Khanna

rings, every angle has a value of 120® (Oaleener and Thorpe 1983). The inter-connecting bridging angles (BOB) have a certain distribution around a probable value o f 130®. The dihedral angle which speciFies the relative orientation to the two neighbouring triangles or rings is randomly distributed. X-ray diffraction (Mozzi and Wanen 1970) data are ctmsistem with this geometrical structure.

atom.

The extreme narrowness of the 806 cm‘ ‘ band (FWHM is 16 cm"*) has been explained (Galeener and Thorpe 1983 and Galecner 1982) by the hfgh degree of decoupling of the boroxol ring from the rest of the amorphous network. The appreciable intensity (Table 1) of the Raman band at 806 cm"* is ascribed to the large number of boroxol rings present per unit volume. The temperature dependence study carried out by Walrafen et al (1979) confums that the position of this band remains almost invariant.

Table 1. Relative intensities* of the observed Raman bands for V-B2O3. Wave nos. (cm~^) Intensities

(arb. units)

Wave nos. (cm“*) Intensities

(arb. units)

147 2 0 1044 15

262 12 1125 35

410 25 1195 15

460 15 1245 8

505 30 1305 25

587 55 1375 30

685 35 1453 40

720 7 1534 25

755 8 1580 2 0

806 180

* Peak bight has been taken as a measure of intensity.

Cuba and Walrafen (1984) and Sinclair et al (1980) have shown that at room temperature, there is an equal concentration of boroxol rings and BO3 triangles. Table 2 shows assignmoits of the Raman bands for U-B2O3. The low frequency Raman bands below 300 cm~' are due to cooperative moUons of the whole structure, comprising a mixed BOs triangular and boroxol rings (external modes). The importance of the band near S87 cm~' is

Table 2 . Assignments of the Raman bands (jbseived for v>B2p3and comparison with those obtained by others.

R am an sp e c tra v -B 2 0 j, stxUum b o ra te e tc 369

Galecner (1980) Wave nos. (cm ')

Walfafcn(1979) Wave nos. (cm~^)

Present author |l9 9 1 ) Assignmenu Wave nos. (cmf*)

145 120 147 Ubration of boroxol rings

260 260 262 Cooperative motion of the whole structure

— 400 410 Unassigned

470 460 460 Boroxol ring bending motion

500 490 505 Rocking of BOB Linkages within the netwoik

610 585 587 Deformation of BO3 triangular motion

670 650 685 Bond bending vibration of BOB groups

— 725 720 BOB bending

750 — 755 BO stretch in complex cyclic groups

808 801 806 Totally symmetric stretch in B305 rings

1030 1040 1044 Symmetric stretch in BO4 groups

— 1125 Asymmetric stretch in BO4 groups

1210 1200 1195 BII2 defonnation (surface effect)

1260 1250 1245 Stretching vibration on boron sublattice

1325 1320 1305 Three coordinated boron motion

— — 1375 Bending and stretching o f OH groups

1475 1470 1453 B-G^ stretching in BO3 units

— 1560 1534 Motion due to boric add formed at the surface

1615 1615 1580 BOH bending

that its intensity has been found to increase with rising temperature (Walrafen et al 1979), which shows the formation of greater number of structural (triangular) groupings with rising temperature. The bands in the region 900-1130 cm"* are assigned to B-O stretching of four coordinated borons. During prq)aration o f the B2P3 glass, the OH content modifies the coordination number of boron from three to four; which also increases the rigidity of the medium (Pelous 1979).

The structure of alkali borate glasses has been studied by several authors (Lorosch et al 1984) who have shown that the addition of alkali oxide to B2O3 makes the boron atom coordination change from triangular to tetrahedral, producing the appearance of diborate, tetraborate cyclic groups,

370 M H Rahm an, B P D w iv e d i, Y K u m ar a n d B N K hanna 3 .2 . Sodium borate spectra :

O ,

, 0

B- O - + R p -

I O

- O

o-

0 1

where R stands for alkali metal. The liberated alkali cation (R'^) generates the ionic conductivity of the glass. In the case of addition of Na2 0 to the B2O3 matrix, tetraborate groups are formed (Konijnendijk and Stevels 1978) together with boroxol rings following the above scheme.

We report Raman measurements in sodium borate glasses only in (he region of interest i.e., in the range (700-850 cm”‘) with three concentrations (x) of Na2 0 (x = 0.15, 0.20 and 0.25). This range is important because the V-B2O3 matrix has the characteristic structure of planar hexagonal B3O6 rings whose vibrational motion gives rise to the totally symmetric stretching mode at 806 cm '. The sodium borate Raman spectra are shown in Figure 3. The clear splitting of the 806 cm ' band of t^B203 spectrum occurs at x = 0.20

F igu re 3. Reman spectrum of sodium horate glass in the range o f (70O-8S0 cm'*) for three concentrations.

giving rise to two bands at 772 cm'* and 803 cm'*. The band at 772 cm'* a]q>ears at concentration of x = 0.15 and rises gradually in intensity with concentration, thereby decreasing the 803 cm'* band in intensity.

The planar six membered rings with one BO4 tetrahedra (i.e., tri-, tetra - or penta - borate groups) scatter at about 780 cm'* while those containing two BO4 tetrahedra (i.e., di.

R am an sp e c tra o f t>-B2 0 3, sodium bo ra te etc 371 tri - or dipentaboratc groups) scatter at about 760 cm"*. The existence of both types of ring structures causes the appearance of a band envelope in the region 760-780 cm"* (Kamitsos el a l , 1987). The splitting shows and confirms the formation of tetraborate groups, liberating sodium cations (Na*).

The nature of the splitting of the band at 8|l6 cm * shows that the formation of cyclic groups (from BOa and boroxed units) starts with addition of NaaO and the network glass tends to reach saturation in'network coherence wift concenU'ation.

3 . 3 . " C a l c i u m b o r a t e s p e c t r a :

As a part of our Raman investigations on alkalin| earth borate glasses, we have recorded a series of spectra of calcium borate glasses in th | system (1 - y)B202. yCaO for y = 0.20, 0.25, 0.30, 0.35 and 0.40 (Figure 4). The Rama| spectrum of t>-B203 is shown in Figure

1(a). Tlhe only strong band is at 806 cm"* wh|;h is an important feature of the planar hexagonal (BaO^) ring structural matrix. The spectrum for calcium borate glass with concentration y = 0.20 is shown in the Figure 4(a). It shows a clear splitting of the band

F ig u re 4. Raman spectra o f calcium borate glasses in the range o f (400-1300 cm ) for different concentrations (y) o f CaO.

806 cm"* (of v-BgOa), giving rise to a new band at 776 cm ' togetheij with other band at 802 cm"* which is slightly displaced from 806 cm*. With gradual increase of concentration (Figures 4a-4e), the intensity of the new band, 776 cm"*, increases, causing a gradual

372 M H Rahman, B P Dwivcdi, Y Kumar and B N Khanna

decrease in iniensily of the band 802 cm \ Ai concentration of > = 0.35* (Figure 4d), the band, 802 cm ', disappears and the band, 776 cm”', shifts towards lower frequency. The wave numbers of the Raman bands for calcium borate glasses are shown in the Table 3.

Following the similar behaviour of Ag"^ ions in borate glasses (Abramo et al 1988 and Dwivedi et al 1991) we see that at lower concentration of CaO, the trigonal coordination of boron is changed into tetrahedral which gives rise to the formation of BO4 groups and increases the network coherence degree. All the oxygens begin to bridge between the boron atoms. At higher concentrations of CaO (y t 0.35) the relative number of tetrahedrally coordinated boron ions decreases, causing the formation of non-bridging oxygens (NBO): As a result a breakdown of the coherence degree occurs. The calcium (Ca**) ions become localized in the interstices of the network near BO4groups or the NBO, to ensure charge neutrality.

Tabic 3. The wave numbers of ihc observed Raman bands for calcium boraie spectra with concentration (y) changes.

Wave numbers (cm ')for

y = 0 y = 0.20 > = 0.25 > = 0.30 > = 0.35 > = 0.40

515 520 514 520 523

776 776 776 772 766

806

802 802 802 — —

1100 1115 1115 1120 1120

The Figure 4 shows .some other Raman bands near 515 cm"' and 1100 cm '.

Crystalline Zn0.2B203 gives a strong band at 1115 cm”' while Li2 0.2B2 0a gives peaks at 1170 and 1035 cm”' (Konijnendijk and Stevels 1975). The comparison of glass spectra with crystalline diborate suggests that 1100 cm”' band could be assigned to the characteristic vibration of diborate groups. The band at 515 cm ' originates from contributions from pentaborate, tetraborate and diborate groups (Konijnendijk and Stevels 1978).

4 . C o n c lu sio n s

The position of the Raman band at 806 cm'' for O-B2O3 has been confirmed, which shows a totally symmetric stretching vibration band for planar hexagonal BaO^ ring structure.

With some new bands for i>B203 spectra, the bands have been assigned and compared with those obtained by other workers. Highly intcn.se and very narrow (in comparison with other bands) band at 806 cm” indicates a large number of boroxol rings in the glass network structure and this band is a dominating pattern of the ring sturcturc.

Addition of sodium oxide or calcium oxide

matrix causes

similar modification of the strucure i.e., alkali or alkaline earth ion acts as a network modifier. The analysis indicates that the structure of these glasses arc built up from triangular BO

and

tetrahedral

(BO

) distorted units. For calcium borate glass,

network coherence saturation occurs at the concentration of y = 0.35. At higher concentrations of the metallic oxide another mechanism of non-bridging oxygen (NSD) formation starts.

Addition of metallic oxide (sodium or cjflcium) to t>-B

causes a change of the boron atom coordination number from 3 to 4 ang as a result the liberation of cations occurs

\vhich increases the conductivity of the glass nettvork.

Acknowledgments

The authors arc grateful to the Chairman of tti| Department of Physics, Aligarh Muslim University, Aligarh, India for extending all th^ facilities and encouragement. One o f the authors (MHR) would like to give thanks to the Government of India for providing scholarship and also to the Government of Bangladesh for allowing him to be on deputation for higher studies.

Raman spectra o f v-B20j, sodium borate etc 373

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