Coordination number of Ge atoms in Na2OGeO2 glasses studied by chemical shift measurements

Journal of Non-Crystalline Solids 69 (1984) 97-103 North-Holland, Amsterdam

97

C O O R D I N A T I O N N U M B E R OF Ge A T O M S IN N a 2 0 - G e O 2 G L A S S E S S T U D I E D BY C H E M I C A L S H I F T M E A S U R E M E N T S C.D. Y I N , H. M O R I K A W A and F. M A R U M O Research Laboratory of Engineering Materials, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 227, Japan

Y. G O H S H I , Y.Z. BAI and S. F U K U S H I M A Department of Industrial Chemistry, University of Tokyo, Hongo 7- 3 - 1, Bunkyo - ku, Tokyo 113, Japan

Received 19 January 1984 Revised manuscript received 7 March 1984

The coordination number of Ge atoms in Na20-GeO 2 glasses was estimated from chemical shift of GeKa 1. As the concentration of Na20 increases, the chemical shift tends to increase, reaching a maximum at a concentration between 15-20 tool.% Na20, and then to decrease. The concentration of six-coordinated Ge atoms was found to be about 25% at its maximum and zero at 35 mol.% Na20.

1. Introduction Alkali germanate glasses exhibit maxima in refractive index versus composition and density versus composition curves; in sodium germanate glasses the m a x i m u m was observed at 15 mol.% N a 2 0 [1] or 16 mol.% N a 2 0 [2]. The p h e n o m e n o n has been tentatively explained as being due to the occurrence of a change in the coordination n u m b e r of Ge atoms from four to six. Sakka et al. [3] analyzed the shift of the infrared absorption b a n d around 800 c m - 1 in alkali germanate glasses and concluded that the concentration of six-coordinated G e atoms, N 6, reached the m a x i m u m value of 10-28% for the composition containing about 20 mol.% alkali oxide; in sodium germanate glasses N 6 reached about 20% at about 20 mol.% N a 2 0 . Sakka and K a m i y a [4] have also conducted an X-ray radial distribution analysis of alkali germanate glasses and found that the m a x i m u m value of N 6 is about 25% for the composition of about 20 mol.% alkali oxide. T a d a et al. [5] have investigated the coordination n u m b e r of G e atoms in 2 0 N a 2 0 • 80GeO 2 glass by means of G e E X A F S spectroscopy and concluded that N 6 is about 23%. Verweij and Buster [6] conducted a R a m a n spectroscopic study of alkali germanate glasses and suggested that six-coordinated Ge atoms occur in a network structure which resembles the structures occurring in crystals of Li4Ge9020(N6 = 22%) and Na4GeqOE0(N6 = 44%). They also suggested that 0022-3093/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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C.D. Yin et al. / Coordination number o f Ge atoms in N a 2 0 - GeO 2 glasses

GeO4 tetrahedra with one non-bridging O atom are formed in the region between 18 and 33 mol.% alkali oxide. Smets and Lommen [7] have pointed out from an X-ray photoelectron spectroscopic study that non-bridging O atoms in alkali germanate glasses are observed only when the amount of alkali oxide exceeds 18 mol.%. Recently, Ueno et al. [8] have reported a high resolution observation of the short-range structure in Na20-GeO 2 glasses obtained by a TOF pulse neutron total scattering method and found that one GeO4-tetrahedron structural unit is modified into one GeO6-octahedron structural unit without forming non-bridging O atoms up to the composition of 20 mol.% Na20. Chemical shiffts of AIKa have been successfully applied to estimate the coordinate number of A1 atoms in glasses [9] and in non-crystalline materials [10]. Kunugi et al. [11] have measured the Ka, Kfl, La and Lfl spectra of Ge atoms in Na20-GeO 2 glasses by using an electron probe X-ray microanalyzer. GeKa and GeKfl were analyzed by an LiF crystal and the others by an RAP crystal. They observed that the chemical shifts reach maxima at about 25 mol.% Na20, but failed to quantitatively determine the average coordination number of Ge atoms in the glasses from the shifts. The chemical shift of the 25 mol.% Na20 glass was as large as that of futile-type GeO 2 which contains only octahedrally coordinated Ge atoms. Sakka and Kamiya [3] have measured GeKa, GeKfl, GeLet and GeLfl spectra by means of a one-crystal fluorescence X-ray spectrometer. The largest chemical shift was, however, within the limit of the experimental error. To provide additional information on coordination number of Ge atoms in Na20-GeO 2 glasses, we measured the chemical shifts of GeKa 1 with a two-crystal X-ray spectrometer [12].

2. Experimental 2.1. Sample preparation Quartz-type GeO2(99.999%) was supplied by the Soekawa Chemicals Co. and was converted to rutile-type GeO2 by adding 0.5% Li2CO3 and heating the mixture at 950 °C for 20 h. Crystals of NaA1Ge308 were prepared from GeO2, Na 2CO3 and A1203 by crystallizing the glass at 850 o C. Crystals of Na4GegO20 were prepared from Na2CO 3 and GeO2 by crystallizing the glass at 650°C. Crystals of Na2Ge409 were prepared by cooling the melt. Crystals of GeP207 were prepared from GeO 2 and H3PO4. The mixture in excess of H3PO 4 was slowly heated up to 1300°C in a silica crucible and kept for 2 h to evaporate the excess P205 component [13]. The crystalline compounds were confirmed by the X-ray diffraction method and used for reference materials. Na2CO 3 and GeO 2 were used to prepare the glasses with the compositions given in table 1. The glass batch was melted in a platinum crucible at temperatures from 1200 to 1400°C and quenched in air.

C.D. Yin et al. / Coordination number of Ge atoms in N a 2 0 - GeO 2 glasses

99

Table 1 GeKa~ energy shift and coordination number Compound

Shift

Width at half

(eV) a~

maximum (eV)

0.000 0.250 0.250 0.275 0.285 0.320 0.345

0.250 0.265 0.270 0.275 0.275 0.255 0.250

Coordination number b) 4-fold (%)

6-fold(%)

3.35 3.35 3.35 3.35 3.35 3.35 3.35

100 100 75 56 0 0

0 0 25 44 100 100

3.35 3.35 3.35 3.40 3.35 3.35 3.30

100 85 80 75 75 95 100

0 15 20 25 25 5 0

Crystalline compound Ge metal Quartz-type GeO 2 NaAIGe308 Na 2Ge409 Na4GegO20 Rutile-type GeO 2 GeP20 7

Glassy compound GeO 2 5Na 2 0 . 9 5 G e O 2 10Na 20- 90GeO z 15Na20.85GeO 2 20Na 2° . 80GeO 2 30Na 2° . 70GeO 2 35Na 2° . 65GeO 2

a) Shift a E = E s a m p l e - Emeta I. b) Coordination number for glasses was estimated by a proportional allotment between the shifts of quartz-type GeO 2 and Na2Ge409.

2.2. Measurement of the G e K a t spectrum G e K a I spectra were taken by the fluorescence method, where the samples were excited with X-rays from a Rh target tube which was operated at 35 k V - 3 0 mA. The apparatus used was a two-crystal fluorescent X-ray spectrometer (Toshiba AFV-701). The powder sample was fixed flatly on an A1 plate using grease. The G e K a 1 spectrum was analyzed by the two Si(220) crystals and counted by a gas flow proportional counter of 90% Ar and 10% CH 4 at the flow rate of 100 m l / m i n . The scanning was continuously performed from 38.072 ° to 38.172 ° in 20 for 1536 s and the counts accumulated for every 6 s were stored in 256 channels of a multichannel analyzer in that order. The output of the multichannel analyzer was numerically smoothed 15 times with the nine-point cubic least-squares method [14]. The peak position of the G e K a l spectrum was calculated as the middle point of the width at half maximum intensity. A standard Ge metal plate and a sample were alternately measured seven times. A shift of G e K a I was determined as a difference between the value of the sample and that of the Ge metal plate. The values of the Ge metal plate which were obtained before and after the sample measurements were averaged and this was used for calcultion. An average value over seven measurements was used for further analysis.

100

C.D. Yin et aL / Coordination number of Ge atoms in N a 2 0 - GeO. glasses

3. Results Fig. 1 shows G e K a I spectra for G e metal, q u a r t z - t y p e G e O 2 a n d rutile-type G e O 2. A l t h o u g h energy shifts of G e K a 1 d u e to c o o r d i n a t i o n are clearly observed, they are small, c o m p a r e d with those of A I K a [9,10]. The shift between four- a n d s i x - c o o r d i n a t e d G e a t o m s was a b o u t 2% of the full width at half m a x i m u m intensity. N o t i c e a b l e changes in peak profile as a function of N 6 were n o t observed. T h e results for G e K a 1 width a n d the energy shift are s u m m a r i z e d in table 1. The energy shifts of G e K a I for the reference samples are shown in fig. 2 as a function of N 6. Q u a r t z - t y p e G e O 2 a n d G e a n a l o g u e f e l d s p a r N a A 1 G e 3 0 s

~.

/GeOe[8]

r

10.00

(+9874.:39 eV)

20.00

Fig. 1. G e K a I spectra for Ge metal, quartz-type GeO 2 [4] and futile-type GeO 2 [6].

0.35

,

GeP207

-.

0.3(

~'~2(vl:

""

~

Na2Ge409

0.25 ~GeO2(IV ) NaAIGe308 0

I 20

I 40

I 60 GeO6

I 80

concentration

100 ('/. }

Fig. 2. Plots of energy shift of GeKa] versus concentration of six-coordinated Ge atoms for reference crystals. The solid line shows the calibration curve for the glass samples.

C.D. }fin et al. / Coordination number of Ge atoms in Na20 - GeO2 glasses

101

consist of four-coordinated Ge atoms, and their plots are located at the same place. The structure of Na2Ge409 [15] is similar to that of K2Ge409 [16] which contains isolated G e O 6 octahedra connected by Ge309 rings consisting of three G e O 4 tetrahedra to form a three-dimensional network. Na4Ge9020 contains chains of G e O 4 tetrahedra, connected by Ge4012 groups which consist of edge-shared G e O 6 octahedra [17]. The O atoms in Na2Ge409 and Na4GegO20 are all of the bridging type. Rutile-type GeO 2 and GeP207 consist of six-coordinated Ge atoms. The energy shift of G e K a I for GeP207 was larger than that for futile-type GeO2, which is probably due to the bonding nature of G e - O . The G e - O bonds in GeP207 seem to be more ionic than those in rutile-type GeO2, since the P atom itself acts as network former. It is observed in fig. 2 that the energy shift increases linearly with increasing N6 for the compounds in the N a 2 0 - G e O 2 system as shown by a broken line. The energy shifts of G e K a l for N a 2 0 - G e O 2 glasses are shown in fig. 3 as a function of N a 2 0 content. A calibration scale to estimate N 6 was obtained by a proportional allotment between the energy shifts of quartz-type G e O 2 and Na2Ge409 (solid line in fig. 2) because the energy shifts of glass samples were not larger than that of Na2Ge409. When the concentration of N a 2 0 increases, the energy shifts tend to increase, reaching a maximum at the composition between 15 and 20 mol.% N a 2 0 , and then decrease. The energy shifft of GeKcq for vitreous G e O 2 was exactly the same as those for quartz-type G e O 2 and Ge analogue feldspar which were used as reference samples for four-coordinated Ge atoms. The maximum N6 was estimated to be about 25%. If the broken line in fig. 2 is used for a calibration scale, the maximum N 6 is about 30%. At 35 mol.% N a 2 0 composition, N 6 was nearly equal to zero.

0.300

•"

/

.°

/

40

v

°.°

/ A v

0.275

20

¢ L9

0.250

0.225 0

I 10

I 20

I 30 Na20 ( m o t % )

Fig. 3. P l o t s o f e n e r g y shift o f G e K a 1 v e r s u s N a 2 0 c o n t e n t for N a 2 0 - G e O d a s h e d lines s h o w N 6 = x / ( l O 0 - x ) a n d N 6 = 2 x / ( 1 0 0 x), respectively.

2 glasses. B r o k e n a n d

102

C.D. Yin et al. / Coordination number o f Ge atoms in N a 2 0 - GeO, glasses

4. Discussion The occurrence of a maximum on the density versus composition curve for N a 2 0 - G e O 2 glasses can be explained by the change of the coordination number of Ge atoms from four to six with the addition of N a 2 0 to G e O 2 glass. The formation of six-coordinated Ge atoms in the glasses was confirmed by the present study as well as the previous ones [3-8]. Sakka and Kamiya [4] found that N6 follows the formula N6 = x / ( 1 0 0 - x) op to x = 20, where x is mol.% Na20. The formula means that addition of N a 2 0 produces an equivalent amount of six-coordinated Ge atoms, and is supported by the result obtained by the neutron diffraction study [8]. They suggested that the structure of the glass at maximum N 6 might be similar to that of Na2Ge409 crystals, in which N6 satisfies the formula exactly. A curve N 6 = x / ( 1 0 0 - x) is shown by a broken line in fig. 3. At the maximum density of x = 16, N 6 is calculated to be 19%. On the other hand, Verweij and Buster [6] observed the polarized band at 870 cm-~ in the R a m a n spectra for the glasses containing 18-33 mol.% Na20. The band is assignable to v G e - O - (non-bridging oxygen) of alkali digermanates. They suggested that a network structure resembling the crystal structure of Na4Ge9020 occurred in the glasses containing 0-18 mol.% N a 2 0 , where addition of N a 2 0 produces a double amount of six-coordinated Ge atoms. A curve N6 = 2 x / ( 1 0 0 - x) is shown by a dotted line in fig. 3. The glass containing 16 mol.% N a 2 0 has the maximum density of 4.06 g / c m 3 [18]. The calculated densities of crystalline Na4Ge9020 and Na2Ge409 are 4.27 [17] and 4.44 g / c m 3 [15], respectively. The latter is denser than the former, though N6 is higher in the former. If the Na2Ge409 type model is true for the glass structure, the increase in density is probably due to the formation of Ge309 three-membered rings, where an average G e - G e distance is 3.19 .~. If the Na4Ge9020 type model is true, the increase in density is probably due to the formation of edge-shared Ge40~2 groups, where an average G e - G e distance is 2.97 .~. In the range of 0 - 1 0 mol.% N a 2 0 , the estimated N 6 for the glasses in the present study is as high as the calculated N6 for the Na4Ge9020 type model, even if one takes account of the experimental error of _+7.5%. Then, at the composition of 15 mol.% N a 2 0 , the estimated N6 is lower than the N 6 for the Na4Ge9020 type model and higher than the N 6 for the Na2Ge409 type model. At the composition of 20 tool.% N a 2 0 , the estimated N 6 is as high as the calculated N6 for the Na2Ge409 type model. One of the possible interpretations for the third peak at about 3.1 .A on the radial distribution curve of 1 6 N a 2 0 - 84GeO 2 glass at the maximum density in fig. 2 of ref. [8] is as follows; since the N a - O interactions at about 2.4 ,~ do not form a clear peak on the radial distribution curve, neither the N a - N a nor N a - G e interactions seem to form clear peaks, and the third peak at about 3.1 ,~ seems to be assignabble to the G e - G e interactions. The estimated G e - G e distance is shorter than the G e - G e distance for Ge309 rings found in N a 2Ge409

C.D. Yin el al. / Coordination number of Ge atoms in Na20 - GeO, glasses

103

crystals, but longer than the G e - G e distance for Ge4012 groups found in Na4GegOz0 crystals. By the way, the average G e - G e distance between tetrahedral and octahedral coordinations in NazGe409 crystals is 3.25 ,~ and the G e - G e distances in Na4GegO20 crystals are shorter than 3.10 ,~. The average G e - O distances for tetrahedral and octahedral coordinations in the glass [8] are 1.78 and 1.95 ,~, respectively, while those in K2Ge409 crystals [16] are 1.76 and 1.89 ,~, respectively. This fact suggests the existence of polymerized GeO 6 octahedra in the glass at the maximum density. According to the Na2Ge409 para-crystal model, the framework of the glasses in the range of 0-16 mol.% N a 2 0 consists of two structural units; three-dimensional GeO 2 type and Na 2Ge409 type units. The present study proposes the existence of polymerized GeO 6 octahedra as the third structural unit. The fraction of the polymerized GeO 6 octahedra decreases with increasing N a 2 0 content. The polymerized GeO 6 octahedra may be replaced by Ge309 rings when the N a 2 0 content increases. The local existence of Ge4012 groups in the glass may be possible and it works as nuclei for crystallization of the stable Na4Ge9020 phase. It was pointed out by the Raman spectroscopic study [6] that a network structure of 3 3 N a 2 0 . 6 7 G e O 2 glass is probably similar to that of crystalline Na2G%O5 which contains GeO 4 tetrahedra only. It was also reported that N6 in 4 0 N a 2 0 . 6 0 G e O 2 glass estimated by radial distribution analysis [8] is zero. The estimated N6 for 3 5 N a 2 0 - 6 5 G e O 2 glass in the present study is nearly equal to zero, which is consistent with the previous results.

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