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Magnetic resonance of the B2O3Na2OMoO3 vitreous system
Journal of Non-Crystalline Solids 57 (1983) 23-39 North-Holland Publishing Company
23
M A G N E T I C R E S O N A N C E OF THE B 2 0 3 - N a z O - M o O 3 V I T R E O U S SYSTEM
S. S I M O N and A1. N I C U L A Facul(v of Physics, University of Cluj, 3400 Cluj- Napoca, Romania Received 15 January 1982 Revised manuscript received 29 November 1982
The structure and properties of molybdenum-doped soda-borate glasses were studied by means of electron paramagnetic resonance as functions of the molybdenum oxide concentration in the range 1-25 mol% MoO3. the keeping time of the samples at the preparation temperature in the range of 15-180 rain, the preparation temperature in the range of 900-1300°C, and the sodaborate matrix composition in the range 0-34 mol% Na20. The EPR measurements gave evidence of the existence of three types of site for the Mo5+ ions, their relative fraction as well as the paramagnetic center density and the EPR lineshape symmetry being dependent on the above-mentioned factors. The evolution of the liB NMR lineshape in the studied samples also showed that the fourfold-coordinated boron atoms fraction is greatly influenced by the modification of these factors. The good agreement between the structural information obtained by HB NMR and Mo5+ EPR studies recommends the EPR spectra of Mo5+ ions as an excellent sensor for the structure of the borate glasses.
1. Introduction Electron p a r a m a g n e t i c resonance a n d the nuclear magnetic resonance have proved to be extremely sensitive methods for the investigation of the local order in different vitreous systems. The elucidation of m a n y problems regarding the i n t e r p r e t a t i o n of the resonance spectra of crystalline a n d polycrystalline materials [1-3] favored the use in the past few years of the resonance m e t h o d s in the study of glasses with a n d without p a r a m a g n e t i c ions [4-8]. Investigation of glasses with p a r a m a g n e t i c ions provided i n f o r m a t i o n o n the effect of different parameters of the p r e p a r a t i o n process on the glass structure a n d of the matrices on the state of the p a r a m a g n e t i c ions. In this way it b e c a m e possible to prepare vitreous materials with the mechanical, electrical, optical and magnetical properties required for different technological applications. There have been relatively few reports [9-11] of work on m o l y b d e n u m glasses by magnetic resonance methods. Particular a t t e n t i o n was given to the p h o s p h a t e glasses with m o l y b d e n u m , because of their special electrical properties. The studies carried out o n these glasses by electron p a r a m a g n e t i c reson a n c e have identified the presence of octahedrally coordinated Mo 5 + ions, the o c t a h e d r o n being trigonally distorted [10]. However it was not a t t e m p t e d to 0 0 2 2 - 3 0 9 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
24
S. Simon, AI. Nicula / Magnetic resonance of BzO s - N a , O - MoO 3
present any correlation between the Mo 5+ EPR lineshape and the structural units indentified in these glasses by other methods. Using NB nuclear magnetic resonance it is possible to determine the type and fraction of the structural units existing in borate glasses [12]. The purpose of this work was to attempt to correlate the results obtained from the analysis of the IIB N M R spectra with results obtained from the Mo 5+ EPR spectra of alkali borate glasses. The two methods were used to study the effect of the matrix composition, the M o O 3 concentration, the preparation temperature and the keeping time of samples at that temperature on the structural units existing in the glasses as well as on the valence state of molybdenum and on the type and symmetry of Mo 5÷ ion coordination. It was shown that the molybdenum ions occupy network-former sites, the Mo 5÷ EPR spectrum being a sensitive indicator for the structural modifications which take place in these glasses upon the modification of the above-mentioned parameters.
2. Experimental The samples were prepared by melting suitable mixtures of H 3BO 3. Na 2CO3 and M o O 3 in sintercorundum crucibles, in a furnace with superkanthal bars. in air, for 30 min at the temperatures indicated below. The melts were poured upon an inox steel plate at room temperature. Since the density of the paramagnetic centers is greatly changed by the modification of the matrix composition for the same concentration of MoO 3, two sets of samples were prepared with different concentrations of alkali oxides and the paramagnetic center density was investigated as a function of the MoO 3 concentration. The exact compositions of the matrices, prepared and kept at 1000°C for 30 min. are given in table 1. Molybdenum oxide in suitable proportions was added to these matrices to obtain samples with 0.5, 1,2,3,4,5,6,7,8,9,10,15 and 20 molCTc M o O 3. The effects of the matrix composition and of the melting temperature were investigated using samples with 10 mol% M o O 3. The composition of these samples, melted at 900, 950, 1000, 1050. 1100, 1200 and 1300°C and kept for 30 min, is given in table 2. The influence of the keeping time of the melt at the preparation temperature was investigated using the °2Mo~0 samples melted at 900°C, with the times ranging from 15 min to 3 h. Table 1 Composition of matrices Sample symbol
x % B205
y %Na20
R = y/x
°'2M 0 °'SM o
83.33 66.66
16.67 33.34
0.2 0.5
: % MoO 3
R = y/x
x %B203 y%Na20
RM z
symbol
Sample
85.7 4.3 0.05 10
o05 M ~o
Table 2 Composition of samples
81.8 8.2 0.1 10
°IMI 0
78.3 11.7 0.15 10
°ISMIo
75 15 0.2 10
°'2MIo
72 18 0.25 10
°'25MI 0
69.2 20.8 0.3 10
°'3MI O
66.6 23.4 0.35 10
°.35Mi O
64.3 25.7 0.4 10
°4Mlo
62 28 0.45 10
°45M10
60 30 0.5 10
°.SMlo
58 32 0.55 l0
°.55M10
56.2 33.8 0.6 10
°,6Mi 0
I
I
,~
e~
>,,
26
s. Simon, AL Nicula / Magnetic resonance of B20; - N a.,O - MoO 3
The EPR spectra were recorded on a standard JEOL spectrometer in the X band, at room temperature, using identical quantities of samples obtained by crushing the glasses. D P P H was used as a standard for the determination of the spin-Hamiltonian parameters. The intensity of the EPR signals was determined as the product of the square of the EPR line width and its amplitude. Equivalent values were obtained by making the product of the line width and the total area of the first derivative spectrum, determined by means of a planimeter. The ~IB N M R spectra of the powdered samples were recorded at 9.212 MHz on a JEOL spectrometer at room temperature. The effects of the matrix composition and of the melting temperature on the boron coordination were inferred from the determination of the ratio between the intensities of the two N M R lines assigned to the three fold coordinated and fourfold-coordinated boron atoms, respectively. The intensity of the two lines was determined as for the EPR spectra. The results thus obtained on s o d a - b o r a t e glasses without paramagnetic ions are in good agreement with results obtained previously by other authors using similar glasses [5]. X-ray difraction and electron microscopy were used to confirm that all the samples were in the vitreous state. without partially crystallized zones.
3. Results The shapes of the recorded EPR spectra depend on the sample composition and melting temperature. The three main line types are presented in fig. 1. The
,,g =1.92
/
/gz =1.95
/
,'g2
= 1.94
/
50Gs, H (a)
(b)
(c)
Fig. 1. Typical EPR spectra recorded from soda-borate glasses containing molybdenum prepared at 1000°C: (a) °'SMl0 sample; (b) °°SMto sample; (c) °'2Mi0 sample.
S. Simon, AL Nicula / Magnetic resonance of B.,O~-NazO-MoO 3
27
width of these lines varies from 40 to 70 Gs, with the g-factors having the values indicated in the figure. The first-derivative ~]B N M R spectra for all the glasses studied consist of two lines. A narrow central line arises from boron atoms in oxygen tetrahedre ([BO4] units) while boron atoms at the center of oxygen triangles ([BO3] units) yield a broad resonance line due to the larger interaction between the lIB nuclear quadrupole moment and the electric field gradient present at the boron site.
3.1. Effect of molybdenum oxide concentration The shape of the EPR spectra for the two sets of samples did not change essentially with the increase of molybdenum oxide concentration. The EPR spectrum of the °SM, samples remained identical in shape to that shown in Fig. l(a), while for the ° 2 M , samples one can notice a change from a line shape similar to that shown in Fig. l(b) to a more asymmetric one, similar to that shown in Fig. l(c). The resonance line intensity and implicitly the density of paramagnetic centers for the two samples exhibit different dependences on the molybdenum oxide concentration (fig. 2). For the ° 2 M , samples the paramagnetic center density increases with increased molybdenum oxide content and reaches a maximum at 4 mol% MoO 3 after which the density decreases. At concentrations higher than 9 mol% MoO 3 the effect of MoO 3 content on the paramagnetic center density is extremely small. Considering the Mo 5+ ions in the °SM: sample, the pardmagnetic centers are only observed above 5 mol% MoO 3. Between this value
lau]
o ~ o 0,2 Mz ;
I 50i
~ A
0,5Mz A
-_ A ~"
i
40)
11/I2 I
42
30~ e
I
2
4
6
8
10
15 25 Z*/o Mo 03
Fig. 2. The effect of the molybdenum oxide concentration on the intensity and asymmetry of the EPR lineshape for the °2M: and °'SM. samples prepared at 900°C.
S. Simon, AI. Nicula / Magnetic resonance of B,Oj Na20 MoO3
28
-
[a. u .] 30~-
2o~
~
,°L
~c.---o02Mlo :
:~
-
Ii/12
1
_
I
t
1
I
I
30
60
90
120
150
J
--
180 t [min"]
Fig. 3. The intensity and the asymmetry of the EPR lineshape of the °2M~0 sample versus the keeping time at the preparation temperature of 900°C.
I
[au.] o----o Tp =900"C 4£
:-
: Tp : 950"C
A
Tp = 1000"C A Tp = 1050% Tp = 1100"C .
30
Tp=I200"C
×----..~ Tp = 1300"C
20
1G
0,05
0,1
0~15 0,2
0,25 0,3
0,35 0,4
0,45 0,5
0,55 0,6
0,65
R
Fig. 4. The dependence of the EPR signal intensity on the soda-borate matrix composition for different preparation temperatures of the samples.
29
S. Simon, AI. Nicula / Magnetic resonance of B203 - NaeO - MoO+
and 8 mol% the paramagnetic center density increases slowly. Between 8 mol% and 10 mol% the increase is more rapid, reaching a value which is no longer influenced by the MoO 3 concentration. In view of these results the 10 mol% MoO3 samples will be treated in greater detail below. 3.2. Effect of keeping the samples at the preparation temperature
By increasing the keeping time of the samples at the preparation temperature, no essential modifications of the EPR lineshapes occur, as illustrated in fig. 3, where the asymmetry parameter is approximately constant. This proves that the symmetry of the Mo 5+ ions is not pronouncedly modified. One notices however, that the EPR signal intensity is considerably affected: if this time is below 30 min the paramagnetic center density increases spectacularly with the keeping time, while in the range of 30-75 min one can observe a stabilization of this density at a relatively constant value. The increase of the keeping time at the preparation temperature above 75 rain leads to a progressive diminution of the paramagnetic center density. The results of N M R measurements did not reveal essential modifications in
~= O00*C
_tLI ]21
.r.
-- Tp =950'C ~ Tp =1000"C .t i Tp: 1050"C Tp=1100'C -" -" Tp = 1200"C x----~ Tp =1300°C / •
/
/
15~
, J " ,,
jJ
3 /¸
-
! L
! 01
1 0,2
I 0,3
A .~ f , _ 0,4
L ~::~ 0,5
016
R
Fig. 5. The dependence of the line asymmetry on the alkali oxide concentration for different preparation temperatures of the samples.
30
s. Simon, AL Nicula / Magnetic resonance of B203- Na20 - MoO 3
the ratio of the fourfold and threefold-coordinated boron atom fractions. However, one notices a weak tendency of the number of fourfold-coordinated boron atoms to decrease. This observation cannot be assigned to the alkali oxide losses, because when the keeping time is shorter than 3 h, these losses are insignificant [14]. It is possible that this effect is considerable at higher preparation temperatures for keeping times above 5 h. 3.3. E f f e c t o f s o d a - b o r a t e m a t r i x c o m p o s t i o n
The examination of the EPR and M N R spectra shows that the modification of the s o d a - b o r a t e matrix composition determines important changes both in the paramagnetic center density and in the glass structure itself. Fig. 4 shows that, regardless of the preparation temperature, the increase of the alkali oxide content determines first an increase of the paramagnetic center density, leading to a maximum of the intensity of the EPR signal for R values between 0.15 and 0.25. When the alkali oxide concentration exceeds 18 mol% the paramagnetic center density decreases slowly and tends to zero for R > 0.45. The value of R
[
c-.--o Tp: 900 C
Tp.oooc
/
_, ± Tp =1050°C 0,3 ~
.------~Tp ,1100"C . Tp=1200 C
.
/ /
i "
~]
i
/
. _ .
o,~
0,2
0.3
oJ
05
0,6
Fig. 6. The influence of the sodium oxide concentration on the fraction of fourfold-coordinated boron atoms for different preparation temperatures of the samples.
s. Simon, AI. Nicula / Magnetic resonance o f Be03 - N a : O - MoO¢
31
for which the E P R signal intensity vanishes depends on the preparation temperature of the samples. C o n c o m i t a n t l y with the change of the E P R signal intensity, the lineshape itself changes markedly. At low alkali oxide content the E P R spectra are asymmetrical as shown in figs. l(b) and l(c) and one m a y consider that the Mo 5+ ions are disposed predominantly in sites of axial or rhombic symmetry. As the alkali oxide content increases, the signal becomes symmetrical, as can be seen in Fig. l(a), and this fact may be explained by the increase of the fraction of Mo 5+ ions disposed in sites of cubic symmetry (fig. 5). One notices a tendency to return to less symmetrical signals when R is greater than 0.5. The N M R measurements show that for this glass type the fraction of fourfold-coordinated b o r o n atoms is also influenced by the alkali oxide content (fig. 6). For N a 2 0 concentration smaller than 28 mol% the fraction of fourfold-coordinated b o r o n atoms increases in an approximately linear fashion with the alkali oxide content. When the N a 2 0 content exceeds this value, i.e
o---'° °zsM lo
"A/ l/
.
°o:,.
o.
°'"'°
I,
3o
o /./,'f"~
=
i /"o ~
L
~ o 3M lO ~"-°3SMlO
•
l ~
Oz'M!0
-7 10- ~
900
lob
1000
1100
1200
o
I
1300 Tp['C ]
900
J
1000
1100
1200
- - - ~
1300Tp['CI
I
la u.]
o----o0 ~SM1° c 8°5Mlo &.--~°'55M10 _, ~_05M~0
!
sV i
L
A.
900
1000
1100
1200 Tp['C]
Fig. 7. The influence of the preparation temperature on the EPR signal intensity for samples with different alkali oxide content: (a) samples with 0.05 ~
32
s. Simon, AL Nicula / Magnetic resonance of B.,O3- Na20 MoO~
for R > 0.4, the fraction of fourfold-coordinated boron atoms begins to decrease slowly. In the case of soda-borate glasses doped with molybdenum the maximum value of N4 is obtained for lower alkali oxide contents 0.4 ~< R ~< 0.5, as compared with the case of soda-borate glasses without transition metal oxides, for which this maximum occurs at R = 0.5. 3.4. Effect of the preparation temperature
As can be inferred from figs. 4-6, the preparation temperature considerably influences both the paramagnetic center density and the structure of the
N4 !
04
/M
~,~°~MI~
o . ~ o °z M10
,
~
_ _o,~,o
°"I X//xt,
0 m?o _0. 00~- / 900
LW2
~ \i,, ~ 1000
1100
..o.o
0.1i 1200
900
1300 Tp[°~" ]
1000
1100
900
1000
1100
1200
1300 Tpl'C l
1200
1300 Tp [°el
Fig. 8. The effect of the preparation temperature on the fraction of fourfold-coordinated boron atoms for samples with different alkali oxide content: (a) samples with 0.05 ~< R ~<0.2: (b) samples with 0.25 .%
S. Simon, AI. Nicula / Magnetic resonance of B20 ~- Na20-MoO~
33
glasses. The dependence of the paramagnetic center density on the preparation temperature is presented explicitely in fig. 7. One observes that this effect is extremely pronounced. The maximum value for this density corresponds for all the glasses to the samples prepared at 1000°C. As the preparation temperature exceeds this limit, the paramagnetic center density decreases and tends to zero at a temperature of 1300°C. It follows from fig. 5 that the preparation temperature does not seem to influence the lineshape obtained from the samples with a low alkali oxide content for which R < 0.2, except for the samples with R = 0.05. At alkali oxide contents higher than 15 mol%, the preparation temperature greatly affects the asymmetry of the EPR lineshape, the asymmetry decreasing with increasing temperature. The N M R results shown in fig. 6 and explained in fig. 8 demonstrate that the preparation temperature affects in particular the fraction of fourfold-coordinated boron atoms. Thus one observes that for all the samples the fraction of fourfold-coordinated boron atoms is a maximum at the temperature of 1000°C. As the preparation temperature exceeds this value, the fraction of fourfold-coordinated boron atoms decreases rapidly, so that this value at 1200°C is only about 25% of the value obtained for the samples prepared at 1000°C. One notices that by increasing the preparation temperature above 1100°C the effects of the alkali oxide content are pronouncedly diminished both as regards the EPR parameters and the fractions of the different structural units existing in these glasses. 4. Discussion
Following the manner in which the EPR lineshapes change with the studied parameters one may consider that the recorded EPR spectra arise from Mo 5+ ions and represent the superposition of the three lines presented in fig. 1. Therefore one can confirm that in these glasses there are Mo 5+ ions disposed in sites of cubic symmetry which give a symmetrical line such as that shown in fig. l(a), in sites of axial symmetry which give an asymmetrical line such as that from fig. l(b), and in sites of rhombic symmetry which give a line such as that from fig. l(c). The fraction of the Mo 5+ ions disposed in one or other of the three types of site depends on the molybdenum oxide concentration, on the matrix composition, and on the preparation temperature. Recalling the fact that in this type of glass a phase separation phenomenon has been suggested [15], one may correlate this fact with the existence of several sites for the Mo 5+ ions. The Mo 5+ ions could be disposed in a microphase consisting of droplets, richer in alkali oxide, or in the largely preponderant microphase, poorer in alkali oxide, as well as in the separation zone of two microphases.
4.1. Effect of the molybdenum oxide content The EPR spectra of the °2M: samples show that at low molybdenum oxide content most of the Mo 5 ÷ ions are disposed in sites of axial symmetry, without
34
S. Simon. AI. Nicula / Magnetic resonance of BeO3- Na,O - MoO~
excluding the other two types of site. As the molybdenum oxide content increases, a greater number of Mo 5+ ions will be found in sites of rhombical symmetry. Due to the fact that the microphase consisting of droplets richer in alkali oxide has a maximum preponderance at R = 0.25, one may suppose that this phase also occurs in the ° 2 M . samples. The fourfold-coordinated boron atoms are prevalent in this microphase and therefore it is to be expected that the Mo 5 ÷ ions are characterized by a high symmetry of the crystalline field and their EPR spectra are specific to the ions with S = ½ disposed in sites of cubic symmetry. The environmental symmetry of the Mo 5+ ions disposed in the phase with lower alkali oxide content is lower, being either axial or rhombic. The asymmetry of the lineshape increases with the increase of the MoO 3 content, which is an indication of the decrease in the fraction of fourfold-coordinated boron atoms, as confirmed by N M R measurements. The pronounced asymmetrical lineshape of the EPR signal demonstrates that the majority of the Mo 5+ ions are in sites of axial symmetry, i.e. they are in the poorer in alkali oxide microphase. Recalling the results obtained for s o d a - b o r a t e glasses doped with copper [16,17], it may be inferred that in the microphase rich in alkali oxide the molybdenum is not in the valency state V, as this would contribute to the EPR signal. The dependence of the EPR signal intensity on the molybdenum oxide content (fig. 2) shows that the paramagnetic center density increases up to 4 mol% MoO 3 and that at higher concentration the line intensity begins to decrease. This fact is due to the Mo 5+ ions which are antiparallel coupled. This assumption is plausible in view of the ability of molybdenum readily to form M o - M o bonds [18], and thus one could also explain the conversion of some paramagnetic substances containing Mo 5 + ions to diamagnetic solution as they are dissolved in HC1 [19]. In the glasses studied this process becomes apparent from 2 mol% MoO 3 and becomes prevalent at concentrations higher than 4 mol% MoO 3. Above 8 mol% MoO 3 the paramagnetic center density remains constant, a fact which suggests t h a t a t these concentrations clusters also begin to occur. A further increase of the concentration only leads to an increase of the number and of the volume of these clusters. One may not exclude the possibility that the modification of the paramagnetic center density greatly reflects the change of the molybdenum valency state. For the samples of °SM~ type the fourfold-coordinated Mo 5 + ions occupy sites of cubic symmetry, giving rise to a symmetrical line such as that from fig. l(a). The nonlinear increase of the paramagnetic center density with the MoO 3 concentration may be explained in this case by the same phenomenon of antiparallel coupling of the Mo 5+ ions, with the proviso that, generally, for this alkali oxide concentration a smaller number of Mo 6 + ions are reduced to Mo 5 ~ ions due to the less reducing character of the glasses with high alkali oxide content [20].
S. Simon, AI. Nicula / Magnetic resonance of BzO ~- N a . O - MoO¢
35
4.2. The effect of keeping the samples at the preparation temperature The increase of the paramagnetic center density during the first 30 rain of keeping the samples at the preparation temperature is assigned to the progressive dissociation of the conglomerates of molybdenum oxide originally existing in the s o d a - b o r a t e glasses. The decrease of the intensity of the EPR signal of the samples maintained at the preparation temperature for longer than 75 min is attributed to the reduction of part of the Mo 5+ ions to Mo 4+ ions, together with some of the oxygen atoms leaving the melt, a fact which confers that it has a more pronounced reducing character. The small increase of the line asymmetry with the keeping time at the preparation temperature when this time is longer than 30 min is in good agreement with the tendency of the fraction of fourfold-coordinated boron atoms to decrease, as revealed by the N M R results, and represents a new argument which supports the assumption that the molybdenum ions are disposed in equivalent sites to those of the boron atoms. In view of the fact that at higher preparation temperatures alkali oxide losses could be considerably higher and not at all negligible, and that the longer the preparation time the greater the loss is, this time was fixed at 30 min.
4.3. The effect of soda-borate matrix composition The modification of the paramagnetic center density function of the vitreous matrix composition can be explained in terms of the change of the redox equilibrium as the alkali oxide content changes. Thus, the glasses with 0.1 < R < 0.3 have a higher reducing character than the glasses with R > 0.3, which results in a greater number of Mo 6 + ions being reduced to Mo 5* ions in the former case. This explanation is also supported by the results obtained for the glasses prepared in a more reducing atmosphere. Thus, a complete series of samples was prepared at 1000°C, in a similar manner, except that equal amounts of carbon were added to the compositions given in table 2. The resulting glasses presented similar EPR intensities. In this way, the paramagnetic center density was considerably increased in the samples with R > 0.3 but decreased in the samples with 0.1 < R < 0.3. These results show that in the case of the samples with R > 0.3 a greater number of Mo 6* ions was reduced to Mo 5 +, thus causing an increase of the EPR signal, while for the samples with 0.1 < R < 0.3 part of the Mo 5÷ ions was reduced to Mo 4+ resulting in a decrease of the EPR signal intensity. In addition to the above redox mechanism, the possibility may be considered of the existence of clusters in the case of the samples with R = 0.05 as the viscosity of these samples at the preparation temperature is greater than that of the other samples [21]. It is also necessary to take into account the possibility of the antiparallel coupling of the Mo 5 + ions, esspecially for the samples with R ~< 0.3.
36
S. Simon. AI. Nicula / Magnetic resonance o f B_,O~ - N a , O - MoO~
One should also point out for this case the excellent agreement between the effect of increasing the alkali oxide content on the asymmetry of the EPR lines (fig. 5) and on the fraction of fourfold-coordinated boron atoms (fig. 6), which once again provides support for the assertion that the molybdenum ions are disposed in similar sites to those of the boron atoms. The increase of the fraction of [BO4] units is therefore reflected in the increase of the symmetry of the EPR signal.
4. 4. The effect of the preparation temperature The decrease of the paramagnetic center density as the melting temperature exceeds 1000°C, which is accompanied by a decrease of the EPR signals intensity, is due to the fact that a greater number of Mo 5 + ions are reduced to Mo 4+, it being known that the reducing character of the melts is higher at higher temperatures [22]. The relatively smaller effect of the preparation temperature on the EPR lineshape for the samples with 0.1 ~ R ~< 0.2 may be correlated with the limited range in which the fraction of fourfold-coordinated boron atoms varies in these samples when the preparation temperature is raised (Fig. 8a). As the alkali oxide content, and implicitly the fraction of fourfold-coordinated boron atoms increases, the effect of the proparation temperature on the glass structure is greater and therefore the modifications of the EPR lineshape are also more substantial. The increase of the line asymmetry occurs in conjunction with a decrease of the fraction of [BO4] units. The perfect correlation between the symmetry of the EPR lineshape and the fraction of [BOa] units confirms the assumption that the molybdenum ions are disposed in similar sites to those of the boron atoms. The considerable reduction of the differences existing between the glasses of different compositions cannot be assigned to the alkali oxide losses [17]. This and other effects observed as the preparation temperature is raised may be explained by the following mechanism of structural modifications in the glasses. As the preparation temperature is raised, the thermal agitation in the melts is also greater. As a consequence, some of the B - O bonds between the oxygens from the top of the [BO4] pyramid and the fourfold-coordinated boron atoms are broken. This assumption is based on the fact that these bonds ar considerably weaker than those between the boron atom and the oxygens from the pyramid base. The oxygens move. after the breakage of the B - O bonds, through the melt and some of them are lost. Therefore the oxidant character of the melts is diminished and this fact is reflected in a decrease of the paramagnetic center density following the reduction of the Mo 5÷ ions to Mo 4+ ions (fig. 7). At the same time a decrease of the fraction of fourfold-coordinated boron atoms takes place (fig. 8) and consequently there is an increase of the EPR signal asymmetry (fig. 5) if one accepts that the molybdenum ions are disposed in sites similar to those of the boron atoms.
S. Simon, AI. Nicula / Magnetic resonance of B:O,- Na:O MoO~
37
5. Conclusions The magnetic resonance measurements gave evidence of considerable modifiactions of the structure and properties of the s o d a - b o r a t e glasses with molybdenum oxide upon the modifications of the molybdenum oxide concentration, of the s o d a - b o r a t e matrix composition, of the preparation temperature and of the time for which the sample was kept at the preparation temperature. The following conclusions may be drawn from the investigation: (1) The EPR signals recorded from the samples studied were assigned to the Mo 5 + ions resulting from the reduction of the Mo 6+ ions originally as MoO~. Three different sites were identified for the Mo 5 + ions, for which the symmetries of the microenvironments are cubic, axial and rhombic, respectively. The paramagnetic center density i.e the density of the Mo 5+ ions, as well as the proportion of the three sites, depend on the s o d a - b o r a t e matrix composition, on the MoO 3 concentration, on the preparation temperature and on the time of keeping at this temperature. (2) The reduction of the EPR signal intensity occuring for an MoO 3 content higher than 4 mol% in the case of °2Mol0 samples was interpreted as the building up of Mo 5* ions with spin S = 0. At higher concentrations it also seems possible to have some cluster formation. (3) Increasing the time for which the samples are kept at the preparation temperature determines a slight decrease of the fraction of fourfold-coordinated boron atoms, this effect resulting in an increase of the EPR line asymmetry. When the samples are kept at 900°C for longer than 75 min, a decrease of the paramagnetic center density occurs, an effect interpreted as a reduction of some of the Mo 5+ to Mo 4+ ions caused by a number of oxygen atoms leaving the melt, which gives the melt a more pronounced reducing character. (4) The composition of the s o d a - b o r a t e matrix influences the fraction of the different structural units from these glasses and also the oxide reduction character of the matrix. Thus by increasing the alkali oxide content, the fraction of [BO4] units increases simultaneously with the EPR lineshape becoming more symmetrical. At the same time the paramagnetic center density also changes. One notices that, irrespective of the preparation temperature, the density of Mo 5* ions is a maximum for the samples with 0.15 ~ R ~< 0.25. One considers that the samples with this matrix composition have more reducing character than those with R ~ 0.1 and R >~ 0.3. In addition to this modification of the oxide-reduction equillibrium of the matrix one does not exclude the possibility of clusters building up within the samples showing the highest viscosity at the preparation temperature (R = 0.05) as well as the accumulation of molybdenum ions with spin S = 0 in the case of the less viscous samples (R > 0.3). (5) The structure and the properties of the studied glasses are to a large extent determined by the preparation temperature. The preparation temperature of 1000°C corresponds to a maximum of both the fraction of [BO4] units
38
S. Simon, A I. Nicula / Magnetic resonance o f B_,O3 - N a z O - MoO~
a n d of the p a r a m a g n e t i c center density, i.e. of the d e n s i t y of M o 5 ÷ ions. The increase of the p r e p a r a t i o n t e m p e r a t u r e a b o v e this value results in a decrease of b o t h the p a r a m a g n e t i c center d e n s i t y a n d the fraction of [BO4] units, a fact that is also reflected in the lowering of the E P R line symmetry. F o r the e x p l a n a t i o n of these effects it was a s s u m e d that as the p r e p a r a t i o n t e m p e r a t u r e increases s o m e of the w e a k e r B - O b o n d s involving the oxygens from the top of the [BOa] p y r a m i d are b r o k e n because of the increased t h e r m a l agitation, resulting in a decrease Of the fraction of these structural units. Some of the freed oxygens leave the melt, thus giving it a m o r e r e d u c i n g character, a fact which is reflected in the decrease of the E P R signal intensity, as some of the M o 5÷ ions are r e d u c e d to M o 4÷ ions. (6) The c o n s i d e r a b l e d i m i n i s h i n g of the differences existing b e t w e e n the spectral p a r a m e t e r s of the s a m p l e s with different c o m p o s i t i o n s p r e p a r e d at 1000°C, when these are p r e p a r e d at higher t e m p e r a t u r e s than 1200°C, shows that the structural m o d i f i c a t i o n s resulting from the increase of the p r e p a r a t i o n t e m p e r a t u r e are m o r e i m p o r t a n t than those caused b y the m o d i f i c a t i o n of the alkali oxide content. O n e notices that these structural m o d i f i c a t i o n s are m o r e p r o n o u n c e d in the s a m p l e s with a high s o d i u m oxide content, where the fraction of [BOa] units, for the s a m p l e s p r e p a r e d at 1000°C, is at a m a x i m u m . (7) The r e m a r k a b l e a g r e e m e n t b e t w e e n the m o d i f i c a t i o n of the fraction of [BO4] units a n d the a s y m m e t r y of the E P R lineshape versus the evolution of the m e n t i o n e d factors d e m o n s t r a t e s that the m o l y b d e n u m ions are d i s p o s e d in similar sites to those of the b o r o n atoms, a fact which w o u l d fully j u s t i f y the s t a t e m e n t that the E P R spectra of the M o 5÷ ions can be used as excellent sensors for the structure of the s o d a - b o r a t e glasses.
References [1] A. Abragam, The Principle of Nuclear Magnetism (Clarendon Press, Oxford, 1961). [2] A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions (Clarendon Press, Oxford, 1970). [3] P.C. Taylor and P.J. Bray, J. Magn. Res. 2 (1970) 305. [4] AI. Nicula, Rezonanta Magneticii (Ed. did. ~i Ped., Bucharest. 1980). [5] P.J. Bray, Borate Glasses, Eds. L.D. Pye, V.D. Frechette and N.J. Kreidl (Plenum Press. N e t York, 1978) p. 321. [6] P.C. Taylor, Resonance Effects in Glasses (Academic Press, New York and London, 1977). [7] J. Wong and C.A. Angell, Appl. Spectrosc. Rev. 4 (1971) 155. [8] D.L. Griscom, Borate Glasses, Eds. L.D. Pye, V.D. Frechette and N.J. Kreidl (Plenum Press. New York, 1978) p. 11. [9] N.S. Garif'yanov and V.N. Fedotov, 2h. Eksp. Teor. Fiz. 16 (1963) 269. [10] J.F. Sunch, M. Sayer, S.L. Segel and G. Rosenblatt, J. Appl. Phys. 42 (1971) 2587. [11] R.J. Landry, J. Chem. Phys. 48 (1968) 1422. [12] G.E. Jellison Jr. and P.J. Bray, J. Non-Crystalline Solids 29 (1978) 187. [13] S. Simon, V. Simon and AI. Nicula, Col. Nat. Fiz. Tehn. Mat. Crist. Amorfe. Ia~i (1980) p. 124. [14] M. Cable, Borate Glasses, Eds. L.D. Pye, V.D. Frechette and N.J. Kreidl (Plenum Press, New York, 1978) p. 413.
S. Simon, AL Nicula / Magnetic resonance of BeO ~- NaeO MoO~
39
[15] W. Vogel, J. Non-Crystalline Solids, 25 (1977) 171. [16] S. Simon and AI. Nicula, Studia Univ. Babe~-Bolyai, Physica, XXV, 2 (1980) 39. [17] S. Simon, V. Simon and AI. Nicula, Nat. Symp. on Physics of Amorphous Materials, Eds. AI. Nicula and S. Simon (Cluj-Napoca, 1980) p. 47. [18] A. Cotton, J. Less Common Metals 54 (1977) 3. [19] M. Ardon and A. Pernick, J. Less-Common Metals 54 (1977) 233. [20] J.A. Duffy, Phys. Chem. Glasses 11 (1970) 168. [21] C.J. Leedecke and C.G. Bergeron, Borate Glasses, Eds. L.D. Pye, V.D. Frechette and N.J. Kreidl (Plenum Press, New York, 1978) p. 413. [22] A. Paul, Amorphous Magnetism, Eds. R.A. Levy and R. Hasegawa (Plenum Press, New York, 1977) p. 597.
23
M A G N E T I C R E S O N A N C E OF THE B 2 0 3 - N a z O - M o O 3 V I T R E O U S SYSTEM
S. S I M O N and A1. N I C U L A Facul(v of Physics, University of Cluj, 3400 Cluj- Napoca, Romania Received 15 January 1982 Revised manuscript received 29 November 1982
The structure and properties of molybdenum-doped soda-borate glasses were studied by means of electron paramagnetic resonance as functions of the molybdenum oxide concentration in the range 1-25 mol% MoO3. the keeping time of the samples at the preparation temperature in the range of 15-180 rain, the preparation temperature in the range of 900-1300°C, and the sodaborate matrix composition in the range 0-34 mol% Na20. The EPR measurements gave evidence of the existence of three types of site for the Mo5+ ions, their relative fraction as well as the paramagnetic center density and the EPR lineshape symmetry being dependent on the above-mentioned factors. The evolution of the liB NMR lineshape in the studied samples also showed that the fourfold-coordinated boron atoms fraction is greatly influenced by the modification of these factors. The good agreement between the structural information obtained by HB NMR and Mo5+ EPR studies recommends the EPR spectra of Mo5+ ions as an excellent sensor for the structure of the borate glasses.
1. Introduction Electron p a r a m a g n e t i c resonance a n d the nuclear magnetic resonance have proved to be extremely sensitive methods for the investigation of the local order in different vitreous systems. The elucidation of m a n y problems regarding the i n t e r p r e t a t i o n of the resonance spectra of crystalline a n d polycrystalline materials [1-3] favored the use in the past few years of the resonance m e t h o d s in the study of glasses with a n d without p a r a m a g n e t i c ions [4-8]. Investigation of glasses with p a r a m a g n e t i c ions provided i n f o r m a t i o n o n the effect of different parameters of the p r e p a r a t i o n process on the glass structure a n d of the matrices on the state of the p a r a m a g n e t i c ions. In this way it b e c a m e possible to prepare vitreous materials with the mechanical, electrical, optical and magnetical properties required for different technological applications. There have been relatively few reports [9-11] of work on m o l y b d e n u m glasses by magnetic resonance methods. Particular a t t e n t i o n was given to the p h o s p h a t e glasses with m o l y b d e n u m , because of their special electrical properties. The studies carried out o n these glasses by electron p a r a m a g n e t i c reson a n c e have identified the presence of octahedrally coordinated Mo 5 + ions, the o c t a h e d r o n being trigonally distorted [10]. However it was not a t t e m p t e d to 0 0 2 2 - 3 0 9 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
24
S. Simon, AI. Nicula / Magnetic resonance of BzO s - N a , O - MoO 3
present any correlation between the Mo 5+ EPR lineshape and the structural units indentified in these glasses by other methods. Using NB nuclear magnetic resonance it is possible to determine the type and fraction of the structural units existing in borate glasses [12]. The purpose of this work was to attempt to correlate the results obtained from the analysis of the IIB N M R spectra with results obtained from the Mo 5+ EPR spectra of alkali borate glasses. The two methods were used to study the effect of the matrix composition, the M o O 3 concentration, the preparation temperature and the keeping time of samples at that temperature on the structural units existing in the glasses as well as on the valence state of molybdenum and on the type and symmetry of Mo 5÷ ion coordination. It was shown that the molybdenum ions occupy network-former sites, the Mo 5÷ EPR spectrum being a sensitive indicator for the structural modifications which take place in these glasses upon the modification of the above-mentioned parameters.
2. Experimental The samples were prepared by melting suitable mixtures of H 3BO 3. Na 2CO3 and M o O 3 in sintercorundum crucibles, in a furnace with superkanthal bars. in air, for 30 min at the temperatures indicated below. The melts were poured upon an inox steel plate at room temperature. Since the density of the paramagnetic centers is greatly changed by the modification of the matrix composition for the same concentration of MoO 3, two sets of samples were prepared with different concentrations of alkali oxides and the paramagnetic center density was investigated as a function of the MoO 3 concentration. The exact compositions of the matrices, prepared and kept at 1000°C for 30 min. are given in table 1. Molybdenum oxide in suitable proportions was added to these matrices to obtain samples with 0.5, 1,2,3,4,5,6,7,8,9,10,15 and 20 molCTc M o O 3. The effects of the matrix composition and of the melting temperature were investigated using samples with 10 mol% M o O 3. The composition of these samples, melted at 900, 950, 1000, 1050. 1100, 1200 and 1300°C and kept for 30 min, is given in table 2. The influence of the keeping time of the melt at the preparation temperature was investigated using the °2Mo~0 samples melted at 900°C, with the times ranging from 15 min to 3 h. Table 1 Composition of matrices Sample symbol
x % B205
y %Na20
R = y/x
°'2M 0 °'SM o
83.33 66.66
16.67 33.34
0.2 0.5
: % MoO 3
R = y/x
x %B203 y%Na20
RM z
symbol
Sample
85.7 4.3 0.05 10
o05 M ~o
Table 2 Composition of samples
81.8 8.2 0.1 10
°IMI 0
78.3 11.7 0.15 10
°ISMIo
75 15 0.2 10
°'2MIo
72 18 0.25 10
°'25MI 0
69.2 20.8 0.3 10
°'3MI O
66.6 23.4 0.35 10
°.35Mi O
64.3 25.7 0.4 10
°4Mlo
62 28 0.45 10
°45M10
60 30 0.5 10
°.SMlo
58 32 0.55 l0
°.55M10
56.2 33.8 0.6 10
°,6Mi 0
I
I
,~
e~
>,,
26
s. Simon, AL Nicula / Magnetic resonance of B20; - N a.,O - MoO 3
The EPR spectra were recorded on a standard JEOL spectrometer in the X band, at room temperature, using identical quantities of samples obtained by crushing the glasses. D P P H was used as a standard for the determination of the spin-Hamiltonian parameters. The intensity of the EPR signals was determined as the product of the square of the EPR line width and its amplitude. Equivalent values were obtained by making the product of the line width and the total area of the first derivative spectrum, determined by means of a planimeter. The ~IB N M R spectra of the powdered samples were recorded at 9.212 MHz on a JEOL spectrometer at room temperature. The effects of the matrix composition and of the melting temperature on the boron coordination were inferred from the determination of the ratio between the intensities of the two N M R lines assigned to the three fold coordinated and fourfold-coordinated boron atoms, respectively. The intensity of the two lines was determined as for the EPR spectra. The results thus obtained on s o d a - b o r a t e glasses without paramagnetic ions are in good agreement with results obtained previously by other authors using similar glasses [5]. X-ray difraction and electron microscopy were used to confirm that all the samples were in the vitreous state. without partially crystallized zones.
3. Results The shapes of the recorded EPR spectra depend on the sample composition and melting temperature. The three main line types are presented in fig. 1. The
,,g =1.92
/
/gz =1.95
/
,'g2
= 1.94
/
50Gs, H (a)
(b)
(c)
Fig. 1. Typical EPR spectra recorded from soda-borate glasses containing molybdenum prepared at 1000°C: (a) °'SMl0 sample; (b) °°SMto sample; (c) °'2Mi0 sample.
S. Simon, AL Nicula / Magnetic resonance of B.,O~-NazO-MoO 3
27
width of these lines varies from 40 to 70 Gs, with the g-factors having the values indicated in the figure. The first-derivative ~]B N M R spectra for all the glasses studied consist of two lines. A narrow central line arises from boron atoms in oxygen tetrahedre ([BO4] units) while boron atoms at the center of oxygen triangles ([BO3] units) yield a broad resonance line due to the larger interaction between the lIB nuclear quadrupole moment and the electric field gradient present at the boron site.
3.1. Effect of molybdenum oxide concentration The shape of the EPR spectra for the two sets of samples did not change essentially with the increase of molybdenum oxide concentration. The EPR spectrum of the °SM, samples remained identical in shape to that shown in Fig. l(a), while for the ° 2 M , samples one can notice a change from a line shape similar to that shown in Fig. l(b) to a more asymmetric one, similar to that shown in Fig. l(c). The resonance line intensity and implicitly the density of paramagnetic centers for the two samples exhibit different dependences on the molybdenum oxide concentration (fig. 2). For the ° 2 M , samples the paramagnetic center density increases with increased molybdenum oxide content and reaches a maximum at 4 mol% MoO 3 after which the density decreases. At concentrations higher than 9 mol% MoO 3 the effect of MoO 3 content on the paramagnetic center density is extremely small. Considering the Mo 5+ ions in the °SM: sample, the pardmagnetic centers are only observed above 5 mol% MoO 3. Between this value
lau]
o ~ o 0,2 Mz ;
I 50i
~ A
0,5Mz A
-_ A ~"
i
40)
11/I2 I
42
30~ e
I
2
4
6
8
10
15 25 Z*/o Mo 03
Fig. 2. The effect of the molybdenum oxide concentration on the intensity and asymmetry of the EPR lineshape for the °2M: and °'SM. samples prepared at 900°C.
S. Simon, AI. Nicula / Magnetic resonance of B,Oj Na20 MoO3
28
-
[a. u .] 30~-
2o~
~
,°L
~c.---o02Mlo :
:~
-
Ii/12
1
_
I
t
1
I
I
30
60
90
120
150
J
--
180 t [min"]
Fig. 3. The intensity and the asymmetry of the EPR lineshape of the °2M~0 sample versus the keeping time at the preparation temperature of 900°C.
I
[au.] o----o Tp =900"C 4£
:-
: Tp : 950"C
A
Tp = 1000"C A Tp = 1050% Tp = 1100"C .
30
Tp=I200"C
×----..~ Tp = 1300"C
20
1G
0,05
0,1
0~15 0,2
0,25 0,3
0,35 0,4
0,45 0,5
0,55 0,6
0,65
R
Fig. 4. The dependence of the EPR signal intensity on the soda-borate matrix composition for different preparation temperatures of the samples.
29
S. Simon, AI. Nicula / Magnetic resonance of B203 - NaeO - MoO+
and 8 mol% the paramagnetic center density increases slowly. Between 8 mol% and 10 mol% the increase is more rapid, reaching a value which is no longer influenced by the MoO 3 concentration. In view of these results the 10 mol% MoO3 samples will be treated in greater detail below. 3.2. Effect of keeping the samples at the preparation temperature
By increasing the keeping time of the samples at the preparation temperature, no essential modifications of the EPR lineshapes occur, as illustrated in fig. 3, where the asymmetry parameter is approximately constant. This proves that the symmetry of the Mo 5+ ions is not pronouncedly modified. One notices however, that the EPR signal intensity is considerably affected: if this time is below 30 min the paramagnetic center density increases spectacularly with the keeping time, while in the range of 30-75 min one can observe a stabilization of this density at a relatively constant value. The increase of the keeping time at the preparation temperature above 75 rain leads to a progressive diminution of the paramagnetic center density. The results of N M R measurements did not reveal essential modifications in
~= O00*C
_tLI ]21
.r.
-- Tp =950'C ~ Tp =1000"C .t i Tp: 1050"C Tp=1100'C -" -" Tp = 1200"C x----~ Tp =1300°C / •
/
/
15~
, J " ,,
jJ
3 /¸
-
! L
! 01
1 0,2
I 0,3
A .~ f , _ 0,4
L ~::~ 0,5
016
R
Fig. 5. The dependence of the line asymmetry on the alkali oxide concentration for different preparation temperatures of the samples.
30
s. Simon, AL Nicula / Magnetic resonance of B203- Na20 - MoO 3
the ratio of the fourfold and threefold-coordinated boron atom fractions. However, one notices a weak tendency of the number of fourfold-coordinated boron atoms to decrease. This observation cannot be assigned to the alkali oxide losses, because when the keeping time is shorter than 3 h, these losses are insignificant [14]. It is possible that this effect is considerable at higher preparation temperatures for keeping times above 5 h. 3.3. E f f e c t o f s o d a - b o r a t e m a t r i x c o m p o s t i o n
The examination of the EPR and M N R spectra shows that the modification of the s o d a - b o r a t e matrix composition determines important changes both in the paramagnetic center density and in the glass structure itself. Fig. 4 shows that, regardless of the preparation temperature, the increase of the alkali oxide content determines first an increase of the paramagnetic center density, leading to a maximum of the intensity of the EPR signal for R values between 0.15 and 0.25. When the alkali oxide concentration exceeds 18 mol% the paramagnetic center density decreases slowly and tends to zero for R > 0.45. The value of R
[
c-.--o Tp: 900 C
Tp.oooc
/
_, ± Tp =1050°C 0,3 ~
.------~Tp ,1100"C . Tp=1200 C
.
/ /
i "
~]
i
/
. _ .
o,~
0,2
0.3
oJ
05
0,6
Fig. 6. The influence of the sodium oxide concentration on the fraction of fourfold-coordinated boron atoms for different preparation temperatures of the samples.
s. Simon, AI. Nicula / Magnetic resonance o f Be03 - N a : O - MoO¢
31
for which the E P R signal intensity vanishes depends on the preparation temperature of the samples. C o n c o m i t a n t l y with the change of the E P R signal intensity, the lineshape itself changes markedly. At low alkali oxide content the E P R spectra are asymmetrical as shown in figs. l(b) and l(c) and one m a y consider that the Mo 5+ ions are disposed predominantly in sites of axial or rhombic symmetry. As the alkali oxide content increases, the signal becomes symmetrical, as can be seen in Fig. l(a), and this fact may be explained by the increase of the fraction of Mo 5+ ions disposed in sites of cubic symmetry (fig. 5). One notices a tendency to return to less symmetrical signals when R is greater than 0.5. The N M R measurements show that for this glass type the fraction of fourfold-coordinated b o r o n atoms is also influenced by the alkali oxide content (fig. 6). For N a 2 0 concentration smaller than 28 mol% the fraction of fourfold-coordinated b o r o n atoms increases in an approximately linear fashion with the alkali oxide content. When the N a 2 0 content exceeds this value, i.e
o---'° °zsM lo
"A/ l/
.
°o:,.
o.
°'"'°
I,
3o
o /./,'f"~
=
i /"o ~
L
~ o 3M lO ~"-°3SMlO
•
l ~
Oz'M!0
-7 10- ~
900
lob
1000
1100
1200
o
I
1300 Tp['C ]
900
J
1000
1100
1200
- - - ~
1300Tp['CI
I
la u.]
o----o0 ~SM1° c 8°5Mlo &.--~°'55M10 _, ~_05M~0
!
sV i
L
A.
900
1000
1100
1200 Tp['C]
Fig. 7. The influence of the preparation temperature on the EPR signal intensity for samples with different alkali oxide content: (a) samples with 0.05 ~
32
s. Simon, AL Nicula / Magnetic resonance of B.,O3- Na20 MoO~
for R > 0.4, the fraction of fourfold-coordinated boron atoms begins to decrease slowly. In the case of soda-borate glasses doped with molybdenum the maximum value of N4 is obtained for lower alkali oxide contents 0.4 ~< R ~< 0.5, as compared with the case of soda-borate glasses without transition metal oxides, for which this maximum occurs at R = 0.5. 3.4. Effect of the preparation temperature
As can be inferred from figs. 4-6, the preparation temperature considerably influences both the paramagnetic center density and the structure of the
N4 !
04
/M
~,~°~MI~
o . ~ o °z M10
,
~
_ _o,~,o
°"I X//xt,
0 m?o _0. 00~- / 900
LW2
~ \i,, ~ 1000
1100
..o.o
0.1i 1200
900
1300 Tp[°~" ]
1000
1100
900
1000
1100
1200
1300 Tpl'C l
1200
1300 Tp [°el
Fig. 8. The effect of the preparation temperature on the fraction of fourfold-coordinated boron atoms for samples with different alkali oxide content: (a) samples with 0.05 ~< R ~<0.2: (b) samples with 0.25 .%
S. Simon, AI. Nicula / Magnetic resonance of B20 ~- Na20-MoO~
33
glasses. The dependence of the paramagnetic center density on the preparation temperature is presented explicitely in fig. 7. One observes that this effect is extremely pronounced. The maximum value for this density corresponds for all the glasses to the samples prepared at 1000°C. As the preparation temperature exceeds this limit, the paramagnetic center density decreases and tends to zero at a temperature of 1300°C. It follows from fig. 5 that the preparation temperature does not seem to influence the lineshape obtained from the samples with a low alkali oxide content for which R < 0.2, except for the samples with R = 0.05. At alkali oxide contents higher than 15 mol%, the preparation temperature greatly affects the asymmetry of the EPR lineshape, the asymmetry decreasing with increasing temperature. The N M R results shown in fig. 6 and explained in fig. 8 demonstrate that the preparation temperature affects in particular the fraction of fourfold-coordinated boron atoms. Thus one observes that for all the samples the fraction of fourfold-coordinated boron atoms is a maximum at the temperature of 1000°C. As the preparation temperature exceeds this value, the fraction of fourfold-coordinated boron atoms decreases rapidly, so that this value at 1200°C is only about 25% of the value obtained for the samples prepared at 1000°C. One notices that by increasing the preparation temperature above 1100°C the effects of the alkali oxide content are pronouncedly diminished both as regards the EPR parameters and the fractions of the different structural units existing in these glasses. 4. Discussion
Following the manner in which the EPR lineshapes change with the studied parameters one may consider that the recorded EPR spectra arise from Mo 5+ ions and represent the superposition of the three lines presented in fig. 1. Therefore one can confirm that in these glasses there are Mo 5+ ions disposed in sites of cubic symmetry which give a symmetrical line such as that shown in fig. l(a), in sites of axial symmetry which give an asymmetrical line such as that from fig. l(b), and in sites of rhombic symmetry which give a line such as that from fig. l(c). The fraction of the Mo 5+ ions disposed in one or other of the three types of site depends on the molybdenum oxide concentration, on the matrix composition, and on the preparation temperature. Recalling the fact that in this type of glass a phase separation phenomenon has been suggested [15], one may correlate this fact with the existence of several sites for the Mo 5+ ions. The Mo 5+ ions could be disposed in a microphase consisting of droplets, richer in alkali oxide, or in the largely preponderant microphase, poorer in alkali oxide, as well as in the separation zone of two microphases.
4.1. Effect of the molybdenum oxide content The EPR spectra of the °2M: samples show that at low molybdenum oxide content most of the Mo 5 ÷ ions are disposed in sites of axial symmetry, without
34
S. Simon. AI. Nicula / Magnetic resonance of BeO3- Na,O - MoO~
excluding the other two types of site. As the molybdenum oxide content increases, a greater number of Mo 5+ ions will be found in sites of rhombical symmetry. Due to the fact that the microphase consisting of droplets richer in alkali oxide has a maximum preponderance at R = 0.25, one may suppose that this phase also occurs in the ° 2 M . samples. The fourfold-coordinated boron atoms are prevalent in this microphase and therefore it is to be expected that the Mo 5 ÷ ions are characterized by a high symmetry of the crystalline field and their EPR spectra are specific to the ions with S = ½ disposed in sites of cubic symmetry. The environmental symmetry of the Mo 5+ ions disposed in the phase with lower alkali oxide content is lower, being either axial or rhombic. The asymmetry of the lineshape increases with the increase of the MoO 3 content, which is an indication of the decrease in the fraction of fourfold-coordinated boron atoms, as confirmed by N M R measurements. The pronounced asymmetrical lineshape of the EPR signal demonstrates that the majority of the Mo 5+ ions are in sites of axial symmetry, i.e. they are in the poorer in alkali oxide microphase. Recalling the results obtained for s o d a - b o r a t e glasses doped with copper [16,17], it may be inferred that in the microphase rich in alkali oxide the molybdenum is not in the valency state V, as this would contribute to the EPR signal. The dependence of the EPR signal intensity on the molybdenum oxide content (fig. 2) shows that the paramagnetic center density increases up to 4 mol% MoO 3 and that at higher concentration the line intensity begins to decrease. This fact is due to the Mo 5+ ions which are antiparallel coupled. This assumption is plausible in view of the ability of molybdenum readily to form M o - M o bonds [18], and thus one could also explain the conversion of some paramagnetic substances containing Mo 5 + ions to diamagnetic solution as they are dissolved in HC1 [19]. In the glasses studied this process becomes apparent from 2 mol% MoO 3 and becomes prevalent at concentrations higher than 4 mol% MoO 3. Above 8 mol% MoO 3 the paramagnetic center density remains constant, a fact which suggests t h a t a t these concentrations clusters also begin to occur. A further increase of the concentration only leads to an increase of the number and of the volume of these clusters. One may not exclude the possibility that the modification of the paramagnetic center density greatly reflects the change of the molybdenum valency state. For the samples of °SM~ type the fourfold-coordinated Mo 5 + ions occupy sites of cubic symmetry, giving rise to a symmetrical line such as that from fig. l(a). The nonlinear increase of the paramagnetic center density with the MoO 3 concentration may be explained in this case by the same phenomenon of antiparallel coupling of the Mo 5+ ions, with the proviso that, generally, for this alkali oxide concentration a smaller number of Mo 6 + ions are reduced to Mo 5 ~ ions due to the less reducing character of the glasses with high alkali oxide content [20].
S. Simon, AI. Nicula / Magnetic resonance of BzO ~- N a . O - MoO¢
35
4.2. The effect of keeping the samples at the preparation temperature The increase of the paramagnetic center density during the first 30 rain of keeping the samples at the preparation temperature is assigned to the progressive dissociation of the conglomerates of molybdenum oxide originally existing in the s o d a - b o r a t e glasses. The decrease of the intensity of the EPR signal of the samples maintained at the preparation temperature for longer than 75 min is attributed to the reduction of part of the Mo 5+ ions to Mo 4+ ions, together with some of the oxygen atoms leaving the melt, a fact which confers that it has a more pronounced reducing character. The small increase of the line asymmetry with the keeping time at the preparation temperature when this time is longer than 30 min is in good agreement with the tendency of the fraction of fourfold-coordinated boron atoms to decrease, as revealed by the N M R results, and represents a new argument which supports the assumption that the molybdenum ions are disposed in equivalent sites to those of the boron atoms. In view of the fact that at higher preparation temperatures alkali oxide losses could be considerably higher and not at all negligible, and that the longer the preparation time the greater the loss is, this time was fixed at 30 min.
4.3. The effect of soda-borate matrix composition The modification of the paramagnetic center density function of the vitreous matrix composition can be explained in terms of the change of the redox equilibrium as the alkali oxide content changes. Thus, the glasses with 0.1 < R < 0.3 have a higher reducing character than the glasses with R > 0.3, which results in a greater number of Mo 6 + ions being reduced to Mo 5* ions in the former case. This explanation is also supported by the results obtained for the glasses prepared in a more reducing atmosphere. Thus, a complete series of samples was prepared at 1000°C, in a similar manner, except that equal amounts of carbon were added to the compositions given in table 2. The resulting glasses presented similar EPR intensities. In this way, the paramagnetic center density was considerably increased in the samples with R > 0.3 but decreased in the samples with 0.1 < R < 0.3. These results show that in the case of the samples with R > 0.3 a greater number of Mo 6* ions was reduced to Mo 5 +, thus causing an increase of the EPR signal, while for the samples with 0.1 < R < 0.3 part of the Mo 5÷ ions was reduced to Mo 4+ resulting in a decrease of the EPR signal intensity. In addition to the above redox mechanism, the possibility may be considered of the existence of clusters in the case of the samples with R = 0.05 as the viscosity of these samples at the preparation temperature is greater than that of the other samples [21]. It is also necessary to take into account the possibility of the antiparallel coupling of the Mo 5 + ions, esspecially for the samples with R ~< 0.3.
36
S. Simon. AI. Nicula / Magnetic resonance o f B_,O~ - N a , O - MoO~
One should also point out for this case the excellent agreement between the effect of increasing the alkali oxide content on the asymmetry of the EPR lines (fig. 5) and on the fraction of fourfold-coordinated boron atoms (fig. 6), which once again provides support for the assertion that the molybdenum ions are disposed in similar sites to those of the boron atoms. The increase of the fraction of [BO4] units is therefore reflected in the increase of the symmetry of the EPR signal.
4. 4. The effect of the preparation temperature The decrease of the paramagnetic center density as the melting temperature exceeds 1000°C, which is accompanied by a decrease of the EPR signals intensity, is due to the fact that a greater number of Mo 5 + ions are reduced to Mo 4+, it being known that the reducing character of the melts is higher at higher temperatures [22]. The relatively smaller effect of the preparation temperature on the EPR lineshape for the samples with 0.1 ~ R ~< 0.2 may be correlated with the limited range in which the fraction of fourfold-coordinated boron atoms varies in these samples when the preparation temperature is raised (Fig. 8a). As the alkali oxide content, and implicitly the fraction of fourfold-coordinated boron atoms increases, the effect of the proparation temperature on the glass structure is greater and therefore the modifications of the EPR lineshape are also more substantial. The increase of the line asymmetry occurs in conjunction with a decrease of the fraction of [BO4] units. The perfect correlation between the symmetry of the EPR lineshape and the fraction of [BOa] units confirms the assumption that the molybdenum ions are disposed in similar sites to those of the boron atoms. The considerable reduction of the differences existing between the glasses of different compositions cannot be assigned to the alkali oxide losses [17]. This and other effects observed as the preparation temperature is raised may be explained by the following mechanism of structural modifications in the glasses. As the preparation temperature is raised, the thermal agitation in the melts is also greater. As a consequence, some of the B - O bonds between the oxygens from the top of the [BO4] pyramid and the fourfold-coordinated boron atoms are broken. This assumption is based on the fact that these bonds ar considerably weaker than those between the boron atom and the oxygens from the pyramid base. The oxygens move. after the breakage of the B - O bonds, through the melt and some of them are lost. Therefore the oxidant character of the melts is diminished and this fact is reflected in a decrease of the paramagnetic center density following the reduction of the Mo 5÷ ions to Mo 4+ ions (fig. 7). At the same time a decrease of the fraction of fourfold-coordinated boron atoms takes place (fig. 8) and consequently there is an increase of the EPR signal asymmetry (fig. 5) if one accepts that the molybdenum ions are disposed in sites similar to those of the boron atoms.
S. Simon, AI. Nicula / Magnetic resonance of B:O,- Na:O MoO~
37
5. Conclusions The magnetic resonance measurements gave evidence of considerable modifiactions of the structure and properties of the s o d a - b o r a t e glasses with molybdenum oxide upon the modifications of the molybdenum oxide concentration, of the s o d a - b o r a t e matrix composition, of the preparation temperature and of the time for which the sample was kept at the preparation temperature. The following conclusions may be drawn from the investigation: (1) The EPR signals recorded from the samples studied were assigned to the Mo 5 + ions resulting from the reduction of the Mo 6+ ions originally as MoO~. Three different sites were identified for the Mo 5 + ions, for which the symmetries of the microenvironments are cubic, axial and rhombic, respectively. The paramagnetic center density i.e the density of the Mo 5+ ions, as well as the proportion of the three sites, depend on the s o d a - b o r a t e matrix composition, on the MoO 3 concentration, on the preparation temperature and on the time of keeping at this temperature. (2) The reduction of the EPR signal intensity occuring for an MoO 3 content higher than 4 mol% in the case of °2Mol0 samples was interpreted as the building up of Mo 5* ions with spin S = 0. At higher concentrations it also seems possible to have some cluster formation. (3) Increasing the time for which the samples are kept at the preparation temperature determines a slight decrease of the fraction of fourfold-coordinated boron atoms, this effect resulting in an increase of the EPR line asymmetry. When the samples are kept at 900°C for longer than 75 min, a decrease of the paramagnetic center density occurs, an effect interpreted as a reduction of some of the Mo 5+ to Mo 4+ ions caused by a number of oxygen atoms leaving the melt, which gives the melt a more pronounced reducing character. (4) The composition of the s o d a - b o r a t e matrix influences the fraction of the different structural units from these glasses and also the oxide reduction character of the matrix. Thus by increasing the alkali oxide content, the fraction of [BO4] units increases simultaneously with the EPR lineshape becoming more symmetrical. At the same time the paramagnetic center density also changes. One notices that, irrespective of the preparation temperature, the density of Mo 5* ions is a maximum for the samples with 0.15 ~ R ~< 0.25. One considers that the samples with this matrix composition have more reducing character than those with R ~ 0.1 and R >~ 0.3. In addition to this modification of the oxide-reduction equillibrium of the matrix one does not exclude the possibility of clusters building up within the samples showing the highest viscosity at the preparation temperature (R = 0.05) as well as the accumulation of molybdenum ions with spin S = 0 in the case of the less viscous samples (R > 0.3). (5) The structure and the properties of the studied glasses are to a large extent determined by the preparation temperature. The preparation temperature of 1000°C corresponds to a maximum of both the fraction of [BO4] units
38
S. Simon, A I. Nicula / Magnetic resonance o f B_,O3 - N a z O - MoO~
a n d of the p a r a m a g n e t i c center density, i.e. of the d e n s i t y of M o 5 ÷ ions. The increase of the p r e p a r a t i o n t e m p e r a t u r e a b o v e this value results in a decrease of b o t h the p a r a m a g n e t i c center d e n s i t y a n d the fraction of [BO4] units, a fact that is also reflected in the lowering of the E P R line symmetry. F o r the e x p l a n a t i o n of these effects it was a s s u m e d that as the p r e p a r a t i o n t e m p e r a t u r e increases s o m e of the w e a k e r B - O b o n d s involving the oxygens from the top of the [BOa] p y r a m i d are b r o k e n because of the increased t h e r m a l agitation, resulting in a decrease Of the fraction of these structural units. Some of the freed oxygens leave the melt, thus giving it a m o r e r e d u c i n g character, a fact which is reflected in the decrease of the E P R signal intensity, as some of the M o 5÷ ions are r e d u c e d to M o 4÷ ions. (6) The c o n s i d e r a b l e d i m i n i s h i n g of the differences existing b e t w e e n the spectral p a r a m e t e r s of the s a m p l e s with different c o m p o s i t i o n s p r e p a r e d at 1000°C, when these are p r e p a r e d at higher t e m p e r a t u r e s than 1200°C, shows that the structural m o d i f i c a t i o n s resulting from the increase of the p r e p a r a t i o n t e m p e r a t u r e are m o r e i m p o r t a n t than those caused b y the m o d i f i c a t i o n of the alkali oxide content. O n e notices that these structural m o d i f i c a t i o n s are m o r e p r o n o u n c e d in the s a m p l e s with a high s o d i u m oxide content, where the fraction of [BOa] units, for the s a m p l e s p r e p a r e d at 1000°C, is at a m a x i m u m . (7) The r e m a r k a b l e a g r e e m e n t b e t w e e n the m o d i f i c a t i o n of the fraction of [BO4] units a n d the a s y m m e t r y of the E P R lineshape versus the evolution of the m e n t i o n e d factors d e m o n s t r a t e s that the m o l y b d e n u m ions are d i s p o s e d in similar sites to those of the b o r o n atoms, a fact which w o u l d fully j u s t i f y the s t a t e m e n t that the E P R spectra of the M o 5÷ ions can be used as excellent sensors for the structure of the s o d a - b o r a t e glasses.
References [1] A. Abragam, The Principle of Nuclear Magnetism (Clarendon Press, Oxford, 1961). [2] A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions (Clarendon Press, Oxford, 1970). [3] P.C. Taylor and P.J. Bray, J. Magn. Res. 2 (1970) 305. [4] AI. Nicula, Rezonanta Magneticii (Ed. did. ~i Ped., Bucharest. 1980). [5] P.J. Bray, Borate Glasses, Eds. L.D. Pye, V.D. Frechette and N.J. Kreidl (Plenum Press. N e t York, 1978) p. 321. [6] P.C. Taylor, Resonance Effects in Glasses (Academic Press, New York and London, 1977). [7] J. Wong and C.A. Angell, Appl. Spectrosc. Rev. 4 (1971) 155. [8] D.L. Griscom, Borate Glasses, Eds. L.D. Pye, V.D. Frechette and N.J. Kreidl (Plenum Press. New York, 1978) p. 11. [9] N.S. Garif'yanov and V.N. Fedotov, 2h. Eksp. Teor. Fiz. 16 (1963) 269. [10] J.F. Sunch, M. Sayer, S.L. Segel and G. Rosenblatt, J. Appl. Phys. 42 (1971) 2587. [11] R.J. Landry, J. Chem. Phys. 48 (1968) 1422. [12] G.E. Jellison Jr. and P.J. Bray, J. Non-Crystalline Solids 29 (1978) 187. [13] S. Simon, V. Simon and AI. Nicula, Col. Nat. Fiz. Tehn. Mat. Crist. Amorfe. Ia~i (1980) p. 124. [14] M. Cable, Borate Glasses, Eds. L.D. Pye, V.D. Frechette and N.J. Kreidl (Plenum Press, New York, 1978) p. 413.
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39
[15] W. Vogel, J. Non-Crystalline Solids, 25 (1977) 171. [16] S. Simon and AI. Nicula, Studia Univ. Babe~-Bolyai, Physica, XXV, 2 (1980) 39. [17] S. Simon, V. Simon and AI. Nicula, Nat. Symp. on Physics of Amorphous Materials, Eds. AI. Nicula and S. Simon (Cluj-Napoca, 1980) p. 47. [18] A. Cotton, J. Less Common Metals 54 (1977) 3. [19] M. Ardon and A. Pernick, J. Less-Common Metals 54 (1977) 233. [20] J.A. Duffy, Phys. Chem. Glasses 11 (1970) 168. [21] C.J. Leedecke and C.G. Bergeron, Borate Glasses, Eds. L.D. Pye, V.D. Frechette and N.J. Kreidl (Plenum Press, New York, 1978) p. 413. [22] A. Paul, Amorphous Magnetism, Eds. R.A. Levy and R. Hasegawa (Plenum Press, New York, 1977) p. 597.