Network structure of sodium and potassium borosilicate glass systems

Journal of Non-Crystalline Solids 55 (1983) 15-26 North-Holland Publishing Company

NETWORK STRUCTURE BOROSILICATE

OF SODIUM GLASS SYSTEMS

K. TAKAHASHI

a n d A. O S A K A

15

AND POTASSIUM

Department of Industrial Chemistry, School of Engineering, Okayama Universi(v, 3 - 1 - 1, Tsushima Naka, Okayama, 700 Japan R. F U R U N O Ntppon Soda Co. Ltd., Ichihara, Chiba, 299-01 Japan Received 23 May 1981 Revised manuscript received 25 September 1982

Molar volumes and optical absorption spectra of Ni 2+ ions were measured for sodium and potassium borosilicate glasses of the compositions xRzO- B203. rSiO2(0.01 < x < 2.0; r = 1 and 2), where parameters x and r represent respectively the molar ratios R 20/I3203 and SiO2/1~203. The presence of structural groups was discussed from the results. It was confirmed that addition of an alkali oxide changed BO 3 units to RBO4 units in the range x < 0.55 for r = 1 and x < 0.6 for r = 2, while it created ROSiO 3 units in the range 0.55 < x < 0.8 for r = 1 and 0.6 < x < 0.9 for r = 2. In the range x > 0.8 for r = 1 and x > 0.9 for r = 2, ROSiO 3 and ROBO 2 units were created by the alkali addition. Here ROBO 2 units were also formed as a result of self-decomposition of RBO4 units. The volume of void associated with a BOa unit was calculated and compared with the size of sodium and potassium ions. It was shown that sodium ions were small enough to be accommodated in the void whereas potassium ions were too large. This could explain the composition dependence of the molar volume of the sodium and potassium borosilicate glasses in the composition range x < 0.55 for r = 1 and x < 0.6 for r = 2.

1. Introduction There have been many reports on the constitution of borate and borosilicate g l a s s e s , s i n c e t h o s e g l a s s e s e x h i b i t e d t h e b o r o n o x i d e a n o m a l y , t h a t is, a n a b r u p t c h a n g e i n p r o p e r t y v e r s u s c o m p o s i t i o n r e l a t i o n s at a m o l a r r a t i o R20/B203

= 0.2. A b e [1] e x p l a i n e d t h i s p h e n o m e n o n i n t e r m s o f t h e c h a n g e in

c o o r d i n a t i o n n u m b e r o f b o r o n a t o m s , a s s u m i n g t h a t a d d i t i o n o f R 2° c h a n g e d two BO a units into two four coordinated borate units RBO 4 and that the f r a c t i o n o f R B O 4 u n i t s , N 4, b e c a m e a m a x i m u m at R 2 0 / B 2 0 3 = 0.2. B r a y et al. [2] s h o w e d f r o m t h e 11B N M R m e a s u r e m e n t s t h a t N 4 i n alkali b o r a t e g l a s s e s c o n t i n u e d t o i n c r e a s e u p t o R E O / / B 2 0 3 = 0.5, a c c o r d i n g t o eq. (1). N4 = R 20/B203

( 1)

Y u n a n d B r a y [3] m e a s u r e d N 4 o f t h e g l a s s e s i n t h e s y s t e m N a 2 0 - B 2 0 3 - S i O i n d i c a t i n g t h a t eq. (1) w a s v a l i d i n t h e r a n g e N a 2 0 / B 2 0 3 0022-3093/83/0000-0000/$03.00

© 1983 N o r t h - H o l l a n d

2,


16

K. Takahashi et al. / Sodium and potassium borosilicate glass systems

represented the composition where N 4 was a m a x i m u m and was in the vicinity of 0.6 depending on the ratio SiO2/B203. No data of N 4 have been reported for other alkali borosilicate glasses than the sodium borosilicate glasses. On further addition of sodium oxide beyond x . . . . N 4 deviated from the values given by eq. (1) [2,3], indicating that the role of sodium oxide changed from converting BO 3 units to RBO 4 units to creating the non-bridging oxygens (NBOs). It is still not clear which units should be formed at the first stage of creating NBOs, the ROSiO 3 unit or the ROBO 2 unit, or both. In this work it was intended to clarify the role of potassium oxide in the borosilicate glasses and to examine the constitution of the glasses in the ranges where NBOs are created. The molar volumes of sodium and potassium borosilicate glasses have been measured and plotted as a function of molar ratio RzO/B203. The composition dependence of the volume has been interpreted based on the additivity rule, that is, the linear relation between the volume and the composition. Moreover, in order to suggest the mechanisms to create NBOs, optical absorption spectra of Ni 2 + ions have been measured in a series of sodium borosilicate glasses in the range 350-2600 nm. Ni 2+ ions are assumed to play the role of modifier ions such as Na + and K + and may act as the indicator for the oxygen environment.

2. Experimental procedures Compositions of the sodium and potassium borosilicate glasses prepared in this experiment are expressed as x R 2 0 . 8 2 0 3 • rSiO 2 where the parameters x and r represent the molar ratios REO/B203 and SIO2/B203, respectively, x varies from 0.01 to 2.0 and r is 1 or 2. The series of glasses with r = 1 are referred to as series A and those with r = 2 as series B. Reagent grade alkali carbonate, 13203 and silica sand were mixed together. The batch was placed in a platinum crucible and melted in an electric furnace at temperatures ranging from 1100°C to 1450°C depending on the glass composition. The homogenized melt was cast into a graphite mold of 1.5 × 1.5 × 1.5 cm 3 in size. The sodium glasses of series B containing 0.03 mol NiO per liter of glass were also prepared. The compositions of some glasses are located in the immiscible region in the sodium [4] and potassium [5] borosilicate systems. In order to determine the temperature for annealing the glasses without causing phase separation, the deformation temperature TO and the glass transition temperature T~ were measured for all glasses. Td and Tg for the sodium glasses of series B are shown as examples together with the immiscible region [4] in fig. 1. Phase separation was checked by taking replicated electron micrographs of glass samples heated below the immiscible region for about five days. No phase separated particles could be detected except in glasses x < 0.1. Preliminary studies on phase separation [6] of 9.5 Na20.30.5B203 • 60.0SiO2(wt% ) glass revealed that the particles grew with time but the total volume fraction of phase separated

K. Takahashi et a L / Sodium and potassium borosilicate glass systems

17

800 I ~ ~\

\

/

\\

iI

700

series B x Na20.B203.25i 02 \ \

!

\ A

I u

~'2--immiscibte boundary (after HaiLeret at.)

6oo!

50O o/

40%

b--annealing ~ temperature

o'.5

i'.o

1.5

X, Na20/B203

Fig. 1. Temperaturefor annealing the sodium borosilicateglasses of series B, x Na 20-B203. 2SiO2. Tg and Td represent the glass transition temperature and deformation temperature, respectively. particles remained constant. Utsumi et al. [7] reported similar results on 2 L i 2 0 . 3 3 B z O 3- 65SiO2(wt% ) glass. Therefore, the fact that no particles are detected after an elongated heat treatment can be interpreted to indicate that no phase separation will occur on heating at the temperatures indicated in fig. 1. Similar results were also obtained for the other series of glasses. Thus it was confirmed that all of the glasses prepared were homogeneous. Densities were measured by the Archimedes method using kerosene as immersing liquid. Optical absorption spectra of Ni 2+ ions in sodium glasses of series B were measured by a Hitachi spectrophotometer EPS-3 in the range 350-2600 nm (29 000-4000 cm-1). The profiles, positions and assignment of the bands were the same as those in the previous reports [8,9].

3. Results and discussion 3.1. Molar volume versus molar ratio R 20 / B203

The molar volumes of glasses x R 2 0 . B203 • rSiO 2 were calculated by eq. (2). molar volume = [ x M w ( R 2 0 ) + Mw(B203)

+

rMw(Si02 )]/d,

(2)

18

K. Takahashi et al. / Sodium and potassium borosilicate glass systems

where M w ' s and d represent the molecular weight of an oxide and the density of the glass, respectively. They are plotted in fig. 2 for sodium and potassium borosilicate glasses of series A as a function of the molar ratio R z O / B 2 0 3 = x. The plots for series B are shown in fig. 3. Dependence of the molar volume upon x can be summarized as follows. (i) The molar volume versus x relation can be approximated by segments of line, assuming the additivity. (ii) Two bending points are found; one at x = 0.55 and the other at x = 0.8 for series A and one at x = 0.6 and the other at x = 0.9 for series B. These values of x are identical in sodium and potassium glasses. (iii) The whole composition range is divided by those bending points into three ranges, I, II and III in the order of increasing x as indicated in the figures. (iv) In range I the molar volumes of sodium glasses of series A and B remain constant, not being affected by addition of sodium oxide. It has already been shown in alkali borate glasses [2] that all kinds of alkali oxides act equivalently when they change BO 3 units to RBO 4 units and bring NBOs depending on the glass composition. Item (i) described above strongly suggests that potassium oxide plays the same role as sodium oxide in alkali borosilicate glasses as well as in alkali borate glasses. Then the following discussions might be effective for all of the alkali species. Since eq. (1) is valid only in range I and N 4 in ranges II and III is less than that given by eq. (1) [3], NBOs start to be created at the first bending point in each series of borosilicate glasses. NBO increases the molar volume. Therefore, the bending points at x = 0.8 for series A and x = 0.9 for series B in figs. 2 and 3 can reasonably be attributed to a discontinuous increase in the rate of formation of NBOs. Another mechanism is effective in creating NBOs in range I I I in addition to the mechanism which is effective in range II. There are three possible mechanisms to create NBOs in alkali borosilicate glasses. (a) -= S i - O - S i - + R 2 0 ~ 2( = Si-OR), (b) = B - O - B = + R 2 0 ~ 2 ( = B - O R ) , (c)RBO 4 --~ = B - O R . Here the formation of RBO 4 units with an NBO, RB(O3/2)-OR, was not taken into consideration, although the possibility could not be completely ruled out. The last mechanism represents the self-decomposition of an RBO 4 unit into a = B - O R unit and is estimated from N 4 data [2,3]. Mechanisms (b) and (c) give identical borate units. The borate and silicate units with more than one NBOs can be obtained similarly by (a) and (b).

3.2. Crystal field strength Dq of Ni e + ions In order to examine the kinds of unit with NBOs created in ranges II and III, optical absorption spectra of Ni 2+ ions in the sodium borosilicate glasses of series B were measured. An absorption band at the near-infrared region has been assigned to d - d transition 3Azg ~ 3T2g of Ni 2+ ions in octahedral crystal

K. Takahashi et al. / Sodium and potassium borosilicate glass systems

19

160

150

110

series B xR20"B2032SiO 2

~ 140

series A xR20"B203SiO 2 0

E 100

R=K

R=K

E .2u

~1301

ID

composition range z

~ 90

composition r a n g , /

5

120 -

I ~lI

~l~

--llr

R=Na

-

1101 R=Na

70 (

1001

6O 0

015

L_

901

110

X

1.5

2.0

0

0.5

1.0

1.5

X

Fig. 2. Molar volumes of glasses of series A, x R 20. B203. SiO2, plotted as a function of x, molar ratio R20/B203. Fig. 3. Molar volumes of glasses of series B, x R 20. B:O3•2SIO2, plotted as a function of x, molar ratio R20/B203.

field [8,9]. This b a n d directly gives the crystal field strength Dq. The values of Dq are plotted in fig. 4 as a function of x, molar ratio N a 2 0 / B 2 0 3 . Those of Dq for sodium borate and silicate glasses are also plotted in the figure against N a 2 0 / B 2 0 3 or N a 2 0 / S i O 2. In order to enlarge the glass formation range, 0.03 mol SiO 2 was added to some of the sodium borate glasses which are represented by full squares in fig. 4. Almost no difference in Dq was observed for 0 . 5 N a 2 0 - B 2 0 3 glass with and without SiO 2, indicating that addition of SiO 2 causes no more effect than vitrifying the sodium borate glasses with high alkali contents. Composition ranges I and II, derived from molar volume measurements, are also indicated in fig. 4. The values of Dq for sodium borate glasses decrease with increasing x until they become constant in the composition range 0.5 < x. Those for the borosilicate glasses decrease in a similar way in range I and exhibit an abrupt decrease at x = 0.6 to remain constant in range II. The values of Dq for sodium

2.0

K. Takahashi et al. / Sodium and potassium borosilicate glass systems

20

silicate glasses are independent of the alkali content in ranges I and II, and agree well with the results reported by Sakka and Nishiyuki [10]. Fig. 5 schematically shows o- and 7r-molecular orbital levels for the interaction between a transition metal ion-oxygen ligands when the metal is surrounded octahedrally by six oxygens. When d electrons of the metal ion occupy t ~ and eg, the greater extent of 7r-interaction between the metal ion and oxygens brings lower values of Dq. On the contrary, a-interaction results in higher values of Dq. It is sure that these 7r- and o-interactions between the metal ion and ligand oxygens are dependent upon the nature of ligand p-electrons which is affected by the kind of network former or network modifier cations bonded to the oxygens. Then it might be possible to suggest the units such as BO a, BO3, O2B-OR, SiO4 and O3Si-OR with which Ni 2+ ions are associated in the borosilicate network. On comparing the values of Dq for the borate, silicate and borosilicate glasses, it is clear that in range I the borate and borosilicate glasses exhibit nearly equal values of Dq. This fact suggests that nickel ions change the B O 3 units to BO 4 units and are present in the neighborhood of borate units such as BO 3 and BO a. The negative formal charge on BO 4 is neutralized by the nickel ion. However, in range II where NBOs are created the values of Dq for the borosilicate glasses are closer to those for the sodium silicate glasses rather than for the borate glasses. This can be explained by either of the following possibilities: (i) no borate units with an NBO or NBOs are formed but silicate units (O3Si-OR) in the borosilicate glasses in range II and two of the 6 ligand 800

,

,

, ~.~---

Crystal field strength Dq of Ni 2÷ ion

700 ' ~ Na20B203 "~ 600

\~.

composition range

series _B _7

u-

n 500

~ xNa20B2032SiO2 " ' ~ "O-Oll

-

~

_ • .....

•--O

xNa20.SiO2 o

z~O

02

04,

06

08

1.0

12

x Fig. 4. Crystal field strength Dq of the Ni 2+ ion in sodium borosilicate glasses of series B, sodium silicate glasses and sodium borate glasses as a function of x, molar ratio N a 2 0 / B 2 0 3 for borate and borosilicate glasses and N a 2 0 / S i O 2 for silicate glasses.

K. Takahashi et al. / Sodium and potassium borosilicate glass systems

metal o--MO orbitals

a%

liqand oFbitals

21

metal rc-MO ligand orbitats orbitals

II°Dq/ -X 3d'2g'° 3p t l u ~ 3s alg

(a) G-interaction

°q 3d t2g'et
2p

(b)Tz-interaction

Fig. 5. Schematic representation of energy levels of the molecular orbitals of the complex between a 3d transition metal ion and ligand oxygens in octahedral symmetry. The energy gap corresponding to crystal field splitting is indicated for each case of (a) o-interaction and (b) ~r-interaction.

oxygens a r o u n d Ni 2÷ ions are N B O s of the silicate units; and (ii) both borate and silicate units with an N B O are formed but Ni 2÷ ions have higher affinity to S i - O - than to B - O - . This problem will be examined on the basis of N a data [3]. 3.3. N 4 versus x relation in sodium borosilicate glasses

N 4 data of x N a 2 0 • B203 • rSiO z glasses published so far are plotted in fig. 6 as a function of molar ratio x = N a 2 0 / B 2 0 3 . The data of Y u n and Bray [3] are indicated by open circles and the data by others are shown by full circles [ 11], full squares [12], open triangles [13] and open squares [14]. Y u n and Bray interpreted the results as showing that N 4 starts to decrease with x at the composition where N 4 deviates from eq. (1). However, there seems to be a plateau stage in the N 4 versus x plot before N 4 decreases, that is, in the range 0.6 < x < 0.9 for r = l, 0.6 < x < 0.9 for r--- 2 and 0.7 < x < 1.0 for r = 3. Such a trend cannot be seen in the borosilicate glasses with r = 0.5 and borate glasses (r = 0). Furthermore, the composition ranges showing constant values of N 4 are almost identical with range II for each of series A and B. The presence of those plateau stages means the formation of N B O s by mechanisms (a) and (b) described in section 3.1. According to Yun and Bray [3] the a m o u n t of asymmetric BO 3 units [ = B - O R , - B = (OR)2 ] becomes significant only after x exceeds 0.8 in series A and 1.0 in series B, that is, in range III. The above discussion suggests strongly that in range II the silicate units with an N B O are mainly created by mechanism (a). Discussions in sections 3.1 through 3.3 on the role of alkali oxide in the borosilicate glasses can be summarized in fig. 7. In range I the alkali oxide changes BO 3 units to R B O 4 units. In range II where N 4 remains constant, O 3 S i - O R units are formed by mechanism (a). In range III, the self-decomposi-

K. Takahashi et al. / Sodium and potassium borosilicate glass systems

22

T0

-'9

'

'

/ x Na20B203

rSiO2_/

z,.s

r -5

•

,,2 O~--"------ r = 2 ..~_._ 3 ~

:2~0-5

-

0

0.5

3, _3

~---°2-"I ~ ~ 3 -

1.0

1.5

2.0

2.5

X Fig. 6. Fraction o f four coordinated boron atoms N 4 of the glasses in the system N a 2 0 - B 2 0 3 - S i O 2 as a function o f x, molar ratio N a 2 0 / B 2 0 3 . The numbers in the figure represent the molar ratio r = SiO2/B203. Data cited: ref. 3 ( © ) ; ref. II ( I ) ; ref. 12 ( I ) ; ref. 13 (z~); ref. 14 ([3).

tion mechanism (c) takes part in the formation of N B 0 s as well as mechanisms (a) and (b). 3.4. Molar volume of the borate and borosilicate glasses in range I The molar volumes of the sodium borosilicate glasses of series A and B remain constant in range I as described previously, while those of sodium borate glasses x N a 2 0 . ]3203, calculated from density data [15-17], increase linearly with x as indicated in fig. 8. The relation between the molar volume V of the sodium borate glasses and x can be represented by eq. (3) in terms of the partial volumes of BO 3 and NaBO 4 units which are represented by v ( B O 3 ) and v(NaBO4), respectively.

V = v (B03) + 2[v (NAB04) - v(B03) ] x.

(3)

Here the effect of the formation of any borate groups on the partial volumes of BO 3 and NaBO 4 units is not taken into consideration but eq. (3) seems valid as Molar ratlo R20/B203 =x 0 xR20'B203'SI02 (series A)

0,55 1

I BO3 --->RBO4

xR20'B203'2SI02 0" (serles B)

0,8 II SiO4 -~>ROSIO

0,6

llI BO3--~ROB02 RBO4 --> ROBO2 0.9Si04 -> ROSi03

Fig. 7. The role of alkali oxide in alkali borosilicate glasses x R 2 0 - B 2 0 3 , rSiO 2 ( r = 1 and 2) in composition ranges I, II and III.

K, Takahashi et aL / Sodium and potassium borosilicate glass systems

23

55

x R20.B203

E u

/,0 ~

l

-

35[~ '~'L:

,

,

,

,

,

0

0-1

0.2

03

0./,

0.5

x

0.6

Fig. 8. The molar volume of alkali borate glasses xR20.B203 (R = Na and K) plotted as a function of x, molar ratio R20/B203. Density data cited: ref. 15 ((3, o); ref. 16 ([3, II); ref. 17 (,x A).

the first order approximation. Application of eq. (3) to the sodium borate glasses gives v(BO3)= 18.0 and v(NaBO4)-- 24.2 cm3/mol. Assuming that the borate units take the same configurations in the borosilicate glasses as in the borate glasses, the molar volumes of the glasses 0.5 N a 2 0 . B203 • rSiO 2 are calculated as 69.5 and 99.5 cm3/mol for r = 1 and 2, respectively, while the measured volumes are 66.2 and 96.8 cm3/mol. The differences between the calculated and measured volumes, 3.3 and 4.3 cm3/mol, respectively, can be attributed to changes in the partial volume of the NaBO 4 unit, because the density data [18] showed that the partial volumes of BO 3 and SiO 4 units were constant in glasses in the system B203-SiO 2. Some borate groups such as boroxol and triborate groups or free BO 3 and RBO 4 units may constitute a part of the borosilicate glass networks with different arrangements from that in the borate glasses. The three dimensional networks of the borosilicate glasses produce voids large enough to accommodate the sodium ions, whereas this is not likely in the sodium borate glasses. If it is not true, the molar volume of the sodium borosilicate glasses must increase with. the addition of sodium oxide similarly as in the sodium borate glasses. In order to calculate the decrease in the partial volume of the NaBO a unit due to the formation of the borosilicate network, the volume of voids associated with SiO4 and BO 4 units will be evaluated and the size of the void will be compared with those of sodium and potassium ions. Silica glass is probably constructed from Si10012 cage skeletons like cristobalite. The molar volumes of silica glass and cristobalite are almost equal, being 27.6 and 27.3 cm3/mol, respectively. One of the S i - O - S i bonds of the cage is shared by six cages at the

24

K. Takahashi et al. / Sodium and potassium borosilicate glass systems

same time so that the cage corresponds stoichiometrically to a SiO 2 molecule. Taking the S i - O bond length as 1.62 ,~ and the adjacent Si-Si distance as 3.9 •~ [19], the radius r(R) of the largest sphere which can be accommodated in the void of the cage is given as r ( R ) + r ( O ) = 5.03/31/2, where r(O) is the radius of a bridging oxygen and 5.03 ,~ is the longest O - O distance in the cage. Letting r(O) = 1.35 A [20], r(R) = 1.56 A,. It is shown that the alkali ions with ionic radius less than 1.56 A can be accommodated in the void associated with SiO 4 units without expanding the S i - O skeleton. The average B - O bond length for a BO 4 tetrahedron is 1.47 ,~ in some of the borate crystals [21]. Since the volume is proportional to third power of the bond length, a BO 4 tetrahedron may occupy a volume as large as 20.4 cm3/mol [ = 27.3 × (1.47/1.62) 3] when the BO4 units are substituted for SiO 4 in the borosilicate networks. The largest ionic radius of the alkali ion which can be accomodated in the void associated with a BO 4 unit is 1.29 A when r(0) = 1.35 ,~. Shannon and Prewitt [20] have reported that the effective ionic radius of sodium ion is 1.16 ,~ and 1.32 ,~ for eight- and nine-fold coordination, respectively, and that of potassium ion is 1.38 ,h, and 1.55 A for six- and nine-fold coordination, respectively. It is clear that sodium ions are small enough to be accommodated in the void associated with BO 4 units, whereas potassium ions are somewhat larger and require voids as large as those associated with SiO 4 units. Consequently, the partial volume of a NaBO4 unit in the borosilicate glass networks can be taken as 20.4 cm3/mol, while that in the borate glass networks is 24.2 cm3/mol. The difference between them, 3.8 cm3/mol, is very close to the difference in the calculated and measured molar volumes of 0.5Na20 • B203 • rSiO 2 glasses. Moreover, the difference AV between the sodium and potassium glasses shown in table 1 are attributable to the effect of the size of the alkali ions. It is very interesting that the value of AV for the alkali borates is equivalent to that for the borosilicates with r = 1. The sodium borate glass network, proposed as an aggregate of some borate groups constructed by BO 3 and BO 4, seems expanded to produce just enough interstitial sites to accommodate the sodium ions but not for potassium ions. Thus in both glasses an excess volume should be necessary over the volume of sodium glasses to accommodate potassium ions. The value of AV for the borosilicate glasses with r = 2 is smaller than that with r = 1 and closer to that for the alkali silicates. This may be due to the fact that the fraction of the cage-type skeletons becomes large and the excess volume due to bulk potassium ions associated with BO 4 units is absorbed to some extent by the adjacent silica cage skeletons.

4. Summary Two series of sodium and potassium borosilicate glasses x R 2 0 . B203 • rSiO2 were prepared where x represents the molar ratio R 2 0 / B 2 0 3 (0.01 < x < 2.0)

K. Takahashi et al. / Sodium and potassium borosilicate glass systems

25

Table 1 The molar volumes of alkali borate, silicate and borosilicate glasses. The difference between the molar volumes of sodium and potassium glasses is compared (see text) Glass

0.5R20. B203 0.5R 2° . B203 - SiO 2 0.5 R 20" B203 • 2SiO 2 0.SR 2O- SiO 2

Molar volume (cm3/mol) R = Na

R= K

A V = V K - VN~

42.3 66.2 92.5 36.4

50.3 74.5 100.0 43.4

8.2 8.3 7.5 7.0

and r represents the molar ratio SiO2/B203 (r = 1 and 2). The molar volumes of sodium and potassium borosilicate glasses of both series A (r = 1) and B (r = 2) were calculated from density data, measured by the Archimedes method. Optical absorption spectra of Ni 2+ ions were measured for the sodium borosilicate glasses of series B, sodium borate glasses and sodium silicate glasses in order to suggest the mechanism of the creation of non-bridging oxygens (NBOs). In the molar volume versus x relations, represented by segments of line, two bending points were observed irrespective of the alkali species; one at x = 0.55 and 0.6, other at x = 0.8 and 0.9 for series A and B, respectively. Those bendings divided the whole composition range into three ranges; I: x < 0.55 for series A and x < 0.6 for series B, II: 0.55 < x < 0.8 for A and 0.6 < x < 0.9 for B, and III: 0.8 < x for A and 0.9 < x for B. The values of Dq of the sodium borosilicate glasses were almost equal to those of the sodium borate glasses in range I, whereas in range II the Dq values of the borosilicate glasses were closer to those of the sodium silicate glasses. Examination of N 4 data for sodium borosilicate glasses by Yun and Bray [3] revealed that formation of asymmetric BO 3 units became significant only after x exceeded 0.8 in series A and 0.9 in series B. These discussions indicated that in range II the silicate units with an N B O ( O 3 S i - O R ) were created and that the borate units with an N B O ( O z B - O R ) were created as well as O3Si-OR units in range III where self-decomposition of RBO 4 units was also effective. Furthermore, potassium or other alkali oxides were suggested to play the same role as sodium oxide in borosilicate glasses. Calculation of the volume of the void associated with a BO 4 unit using silica glass as the reference material showed that a sodium ion was small enough to be accommodated in the void, while a potassium ion was too large. This is the reason why the molar volumes of the sodium borosilicate glasses of series A and B remain constant and those of potassium glasses increase on addition of R 2 0 t o alkali free borosilicate glasses in range I. The authors wish to thank Professor S. Sakka, Mie University for valuable discussions.

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K. Takahashi et al. / Sodium and potassium borosilicate glass systems

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