- Email: [email protected]
NMR and EPR studies on the charge transfer and formation of complexes through incompletely coordinated states
.__ I!!!!
cJs!J
SOLID
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STATE
4._
Solid State Ionics 101-103
ELSJZVIER
IONICS
(1997) 387-392
NMR and EPR studies on the charge transfer and formation of complexes through incompletely coordinated states Mamoru Senna”, Tetsuhiko of Science and Technology,
Faoul~
Keio
Universiry,
Isobe
Hiyoshi,
Yokohama
223. Japan
Abstract Changes in the state of coordination and electron distribution in a fine particulate solid mixture under mechanical stressing are studied by solid state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), coupled with more conventional analytical tools, e.g., X-ray photoelectron spectroscopy or infrared spectroscopy. Systems examined are mainly Ca(OH), + SiO, and metallic Al + hydrogel, TiO,H,O. Si-0-Ca or AI-0-Ti bridging bonds are formed either by charge transfer, i.e. an acid-base reaction in a broad definition, or by radical recombination. The charge transfer is also elucidated on the basis of electronegativity equalization, by taking low-coordination states into account. Kewords:
Solid state NMR; EPR; Charge transfer;
Complex
Materials:
Ca(OH)>;
A12Ti0,
SiOz; Casio,;
Al; Ti02H,0;
oxides: Mechanochemical
1. Introduction Being
unisolated
and
structurally
complicated,
in a fine particulate solid mixture at the contact point of dissimilar species are far from well-defined. It is therefore more reasonable to discuss the change in the chemical state of such a mixture in terms of changes in the coordination number, short range ordering and electron distribution. After we have examined incipient solid state reactions during mechanical treatments of mixtures comprising various metal oxides, hydroxides or metals in an attempt to synthesize complex oxides [ 1-IO], we are now formation
and
rupture
of chemical
*Corresponding author. Tel.: + 81 45 563 564 0950; e-mail: [email protected] 0167.2738/97/$17.00 P/I
0
SOl67-2738(97)00133-I
bonds
I 141; Fax: + 81 45
1997 Elsevier Science B.V. All rights reserved.
reaction
trying to summarize the results obtained by the versatile tools, i.e. solid state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), on the systems Ca-0-Si and Al-0-Ti during mechanical treatment to obtain precursors of complex oxides. Relevance of these analytical information about incipient chemical reactions and the subsequent solid state processes to final products on heating is also discussed.
2. Systems
and analytical
tools
Two reaction systems were employed, i.e. ( 1) Ca(OH)2 + SiOZ and (2) Al + Ti0,H20. Except titania hydrogel, which was synthesized from TiCl,, starting materials, including amorphous fumed silica (Aerosil A200, Degussa) were used as purchased.
388
Details
M. Senna. T. Isobe I Solid State Ionics IOI-
of these
materials
were
given
elsewhere
[4,61. Solid state NMR was applied in different modes: Spin echo (SE) pulse NMR for 27A1; magic angle spinning (MAS) NMR for 27A1 and 29Si; combined rotation and multiple pulse (CRAMPS) NMR for ‘H; and cross polarization MAS (CP/MAS) NMR for ‘9Si. While SE was measured by an apparatus developed in the Institute for Materials Research, Tohoku University, all other measurements were made by commercial equipment (Chemagnetics, CMX-300). EPR spectra were obtained by X-band spectrometer (JEOL, JES-RE3X). Other conventional analytical tools such as X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD), thermal analyses and vibration spectra were used as usual.
3. Polarization
103 (1997)
387-392
When basic OH-groups coexist with acidic OH, there is always an acid-base reaction, by simultaneous dehydration, leaving M-O-M’ bridging bonds, where M and M’ are two different cationic species. The process is promoted when related OH groups are polarized. We have demonstrated formation of these kind of bridging bonds in different systems as typical examples of mechanochemical reaction [1,2,.5]. This was verified by the change of surface basicity of alkaline earth hydroxides, changes in the total weight loss due to subsequent thermal dehydration or decrease in the IR absorption band due to OH groups. Mechanochemical dehydration with simultaneous formation of M-O-M’ bonds is obviously taking place during milling of a mixture of oxides at the point of contact, where plastic deformation, and hence, preferential concentration of lowcoordinated states occur.
of OH-groups 4. Mechanisms
Hydroxyl groups, either chemisorbed on the surface of anhydrous oxides or present as a part of hydroxide, are more polarized, and hence more reactive, when the substrate contains an unsaturated or low coordinate state. A direct verification of polarized OH-groups may be obtained from CRAMPS ‘H NMR, as shown in Fig. 1.
4* C
a B
*
A
;:i
I
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II
I.
5 10 Chemical shift / ppm
I
0
Fig. 1. H-CRAMPS NMR spectra for: (A) physical mixture Ca(OH!,YSiO,; (B) after milling the mixture for 3 h; and (C) after mdlmg pure SiO, for 3 h. * denotes the spinning side band of Ca(OH),. A common peak at 0.12 ppm represents a signal from a silicon rubber used as a standard. Details are given in Ref. [6].
related with radicals
There is another important mechanism involving radicals for the progress of a mechanochemical reaction [I 11. An EPR power saturation method enables us to assign the radical species appearing and disappearing during milling of a mixture. In the case of the system Ca-0-Si, we observed EPR spectra, like those given in Fig. 2. In these EPR studies, milling was carried out in vacuum and spectroscopy was carried out immediately after milling, without exposure to air, in order to examine the effect of ambient gaseous species. By careful analysis on the view point of g-values, power saturation behavior, comparison with the results of X-ray irradiation and change in the signals after exposure to air, we determined that the signals A and F in Fig. 2 are attributed to 0, radicals. They are absent in the mixture after milling. The EPR results are summarized as follows [7]: (1) on milling Ca(OH), separately, the concentration of 0, radicals increases; (2) main radical species of separately milled SiO, is a peroxide radical (POR); (3) these two radical species disappear on milling a mixture of Ca(OH)Z and SiO,. This suggests that these two species are recombined to form Ca-0-Si bridging bonds by the reaction schemes: ( 1) observed 0, radicals are formed by adsorption of an
M. Senna, T. Isobe I Solid State Ionics 101-103
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Magnetic field, H / 1 O-2T Fig. 2. EPR spectra of: (A) physical mixture Ca(OH)z-SiOz; (B) after milling the mixture for 3 h in vacuum; (C) after milling pure Ca(OH), for 3 h in vacua. Interpretations of peaks A and F are given in the text. Peaks marked by Mn are hyperfine structure of the EPR signal from Mn*‘. Details are given in Ref. [7].
molecule onto the mechanochemically oxygen formed O- radical or the radical Ca-0’ on the surface of partially decomposed Ca(OH),; (2) observed peroxide radicals, Si-0-0(-H), are formed by adsorption of an oxygen molecule to the radical species = Si.; and (3) recombination of two surface radical species to give bridging bonds as Ca-0’ + = Si’ + Ca-0-Si. n
I
I
I, 1
Milling time / h Fig. 3. milling peroxy mixture
Change in the concentration of the surface species with time: (A) 0, radical on separately milled Ca(OH)?; (B) radical on separately milled SiO,; and (C) basic site on the Ca(OH),-SiO,. Details are given in Ref. [7].
(1997) 387-392
389
An important question of the relative significance of the acid-base mechanism stated above and this radical mechanism cannot be answered confidently. It is to be noted, however, that the concentration of the basic site that remained after milling for 1 and 3 h is 10’ and 10’ times, respectively, higher than the radical concentration, as shown in Fig. 3. This does not mean, however, that the contribution of radical mechanism is by orders of magnitudes smaller, because what we can measure is the concentration of the remaining radical species on the separately milled sample. The amount of those species actually consumed could not yet be measured.
5. Change in the coordination milling of a mixture
state during
Coordination state of silicon with neighboring silicon atoms can be examined by 29Si NMR [IO]. This area has been particularly developed to elucidate the hydraulic processes in cement chemistry [ 121. The main results obtained from CPIMAS NMR of *“Si on the Ca(OH),-SiO, mixture after milling were the predominance of the peak corresponding to Q, state, i.e. with no neighboring silicon, in place of Q, peak, the latter representing surface silanol and predominating the starting mixture before milling. Examples of 29Si NMR spectra are shown in Fig. 4. This is evidence of the formation of precursor species of calcium silicate by milling of a mixture of Ca(OH), and SiO,. Whether the Q, state corresponds to C,S or C,S is not unambiguously stated, although the reported chemical shift value favors the former, as far as the anhydrous state is concerned [131. As for the AI-TiO,H,O system, milling a mixture brings about the oxidation of aluminum with the predominance of 6-coordinated AlO, units [8]. It is also to be noted, however, that the lower coordination state of aluminum, i.e. AlO,, appears in the course of milling as shown in Fig. 5. Similar phenomena were observed on milling kaolinite, having a mixed phase of Al,O, and SiOz [14]. At the same time, heterogeneity of the state of metallic aluminum after partial oxidation was also demonstrated as shown in Fig. 6, where sharper and broader peaks correspond to undistorted and distorted states
M. Senna. T. Isobe I Solid State tonics
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IOI-103
I
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Fig. 4. ‘“Si CP/MAS NMR mixture Ca(OH)*-SiO:; (B) (C) after milling the mixture peak Q,, with respect to that are given in Ref. [lo].
spectra for the mixture: (A) physical after milling the mixture for 3 h; and for 12 h. The relative intensity of the of Q, increases significantly. Details
of metallic aluminum [8]. It is surprising that the undistorted state of metallic aluminum still survives after substantial loss of the long range ordering detectable by X-ray diffractometry [S].
6. Charge transfer On milling a mixture of metal oxides, the valence state of cations also changes. In the case of the system Si-0-Ca, the Si*+ state appears as detected by XPS [6]. The divalent silicon disappeared rapidly on sputtering by Ar+ ions for only 2 s, which was estimated to correspond to 1 to 2 nm by referring the result of AFM observation on etched silica glass [6]. In the case of the system Al-0-Ti, a part of tetravalent Ti is reduced to Ti’+. This trivalent titanium ion is very sensitive to EPR as shown in Fig. 7. In this case, the reduction was compensated by the oxidation of aluminum. In the case of the system Si-0-Ca, however, some additional explanation is necessary to show how the electrons are supplied to reduce Si’+ to Si*+.
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Fig. 5. “Al MAS NMR spectra of the mixture of metallic Al and titania hydrogel, TiO,H,O, after milling for (A) 10 h; (B) 30 h; (C) 50 h; (D) 100 h; (E) 200 h; and (F) 300 h by a low energy rod mill. Details are given in Ref. [8].
A general concept of electronegativity equalization was proposed by Sanderson [ 151. An apparent electronegativity of a cation, X,, is determined from the electronegativity of a neutral atom, X,,, and the valency, i, as Xj = (1 + 2i)X,,. While the values of X, for Ca*+ and Si4’ are 5.0 and 16.2, respectively, they both become 9.0 if Si4+ is reduced to Si’+ and Ca *+ is oxidized to Ca4+. While Si’+ was actually observed as mentioned above, Ca’+ is not conceivable under a normal condition. Low coordination number could allow, however, such an unusual electronic state [ 161. This is demonstrated by referring to the DV-Xa cluster model, that the ionic charge of Mg-0 bonding in crystalline MgO decreases from 1.54 to 1.08 when the coordination number changes from 5 to 3 [ 161. Bond ionic charge and bond ionicity can reasonably be assumed to be proportional. The bond ionicity, Y, in turn, is related to the electronegativity. Under these assumptions, the electronegativity of divalent calcium in the 3 coordinated state becomes 9.1, being very close to that of
M. Senna, T. Isohe I Solid State Ionics
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103 (1997)
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391
divalent silicon, 8.9. For the calculation of apparent electronegativity of Si’+, low coordination was not taken into account, since we observed a negligibly small amount of unsaturated bonds on the surface of amorphous silica [6]. Generalization of this idea should be made with care, however, since the assumptions made are not yet fully verified.
7. Further processes
I
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Fig. 6. “Al spin echo NMR spectra of the mixture of metallic Al and titania hydrogel, TiOzH20, after milling for (A) 30 h; (B) 50 h; (C) 100 h: (D) 200 h. Details are given in Ref. 181.
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to final products
Even when the coordination state at the stage of precursor is disclosed with a considerable accuracy, we cannot predict the later stage of solid state processes to final products, which is doubtlessly more important for practical purposes. A common phenomenon which is observed repeatedly is that intimate mixing with incipient chemical interaction brings about more homogeneous phase in the product, e.g., Ca(OH), + Si02 -+ olCaSi0,; Al + TiO,H,O-+Al,TiO,. We have also obtained pure perovskite, a complicated ferroelectric material, 0.9Pb(Nb,,lMg, ,,)O,-0. IPbTiO,, from a stoichiometric mixture of PbO, TiO,, Nb,O, and Mg(OH),, preliminarily milled for 1 h by heating at temperatures as low as 850°C [ 171. While the observed homogeneity of the products on heating the starting mixture with preliminary milling should be closely related with the incipient reaction, it is also to be noted that homogeneity of the starting mixture in the nanometer scale increases with milling [4.9]. As a matter of fact, fractional conversion of some complex oxides on heating increases parallel to the extent of micro-homogeneity gained during preliminary milling [9]. It is therefore quite likely, that nucleation takes place in a more homogeneous manner, followed by a diffusion process with much shorter paths. These kind of rather particle technological aspects cannot be separated from the chemical aspects mentioned throughout this study.
I
36
10-ZT
Fig. 7. EPR spectra of the mixture of metallic Al and titania hydrogel, Ti0,H20, after milling for (A) 0 h; (B) 10 h; (C) 30 h; (D) 50 h; (E) 100 h; (F) 200 h; and (G) 300 h by a low energy rod mill. Details are given in Ref. [8].
8. Concluding
remarks
Whether and to what extent an incipient chemical process takes place in a starting fine particulate mixture for complex oxides can be examined by
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M. Senna. T. lsobe I Solid State Ionics 101-103
solid state NMR and EPR. While solid state NMR gives us information with respect to the change in the coordination state, EPR enables us to show the existence of different radical species and some special valence states. Related charge transfer could undergo the principle of electronegativity equalization. For more confident and quantitative statements from these versatile analytical tools, however, there is still much to be done, e.g., to combine with information obtained independently of these resonance spectroscopies and to compare them with the results of similar reaction systems in a systematic fashion.
Acknowledgements The authors thank many former coworkers who appear in the reference list, among others, Dr. Liao, Ms. Kojima and Mr. Watanabe for their cooperation.
References [l] J. Liao, M. Senna, Solid State Ionics 66 (1993) 313. [2] J. Liao, M. Senna, Mater. Res. Bull. 30 (1995) 385. [3] K. Takeuchi, T. Isobe, M. Senna, J. Solid State Chem. 109 (1994) 401.
(1997) 387-392
]41 Y. Kojima, M. Senna, T. Shinohara, S. Ono, K. Sumiyama, K. Suzuki, J. Alloys. Comp. 227 (1995) 97. [51 T. Watanabe, J.-F. Liao, M. Senna, J. Solid State Chem. 115 (1995) 390. f61 T. Watanabe, T. Isobe, M. Senna, J. Solid State Chem. 122 (1996) 74. ]71 T. Watanabe, T. Isobe, M. Senna, J. Solid State Chem. 122 (1996) 291. IsI Y. Kojima, M. Senna, T. Isobe, T. Shinohara, S. Ono, K. Sumiyama, K. Suzuki, J. Mater. Res. 11 (1996) 1305. [91 Y. Okamoto, T. Isobe, M. Senna, J. Non-Cryst. Solids 180 (1995) 171. [lOI T. Watanabe, T. Isobe, M. Senna, J. Solid State Chem., in press. I. Ebert, H.-P. Hennig, L.I. IllI U. Steinike, U. Kretzschmar, Barsova, T.K. Jurk, React. Solids 4 (1987) 1. 1121 G.W. Groves, A. Brough, I.G. Richardson, C.M. Dobson, J. Am. Ceram. Sot. 74 (1991) 2891; H. Ishida, K. Sasaki, Y. Okada, T. Mitsuda, J. Am. Ceram. Sot. 75 (1992) 2541; A.R. Brough, CM. Dobson, J. Am. Ceram. Sot. 77 ( 1994) 593. [I31 S. Leonardelli, L. Facchini, C. Fretigny, P. Tougne, A.P. Legrand, J. Am. Chem. Sot. 114 (1992) 6412. [ 141 T. Nakano, M. Kamitani, M. Senna, Mater. Sci. Forum 225-227 (1996) 587. [I51 R.T. Sanderson, Science 114 (1951) 670. [I61 C. Satoko, M. Tsukada, H. Adachi, J. Phys. Sot. Jpn. 45 (1978) 1333. [I71 J.-G. Baek, T. Isobe, M. Senna, J. Am. Ceram. Sot. 80 (1997) 973.
cJs!J
SOLID
__ 1
STATE
4._
Solid State Ionics 101-103
ELSJZVIER
IONICS
(1997) 387-392
NMR and EPR studies on the charge transfer and formation of complexes through incompletely coordinated states Mamoru Senna”, Tetsuhiko of Science and Technology,
Faoul~
Keio
Universiry,
Isobe
Hiyoshi,
Yokohama
223. Japan
Abstract Changes in the state of coordination and electron distribution in a fine particulate solid mixture under mechanical stressing are studied by solid state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), coupled with more conventional analytical tools, e.g., X-ray photoelectron spectroscopy or infrared spectroscopy. Systems examined are mainly Ca(OH), + SiO, and metallic Al + hydrogel, TiO,H,O. Si-0-Ca or AI-0-Ti bridging bonds are formed either by charge transfer, i.e. an acid-base reaction in a broad definition, or by radical recombination. The charge transfer is also elucidated on the basis of electronegativity equalization, by taking low-coordination states into account. Kewords:
Solid state NMR; EPR; Charge transfer;
Complex
Materials:
Ca(OH)>;
A12Ti0,
SiOz; Casio,;
Al; Ti02H,0;
oxides: Mechanochemical
1. Introduction Being
unisolated
and
structurally
complicated,
in a fine particulate solid mixture at the contact point of dissimilar species are far from well-defined. It is therefore more reasonable to discuss the change in the chemical state of such a mixture in terms of changes in the coordination number, short range ordering and electron distribution. After we have examined incipient solid state reactions during mechanical treatments of mixtures comprising various metal oxides, hydroxides or metals in an attempt to synthesize complex oxides [ 1-IO], we are now formation
and
rupture
of chemical
*Corresponding author. Tel.: + 81 45 563 564 0950; e-mail: [email protected] 0167.2738/97/$17.00 P/I
0
SOl67-2738(97)00133-I
bonds
I 141; Fax: + 81 45
1997 Elsevier Science B.V. All rights reserved.
reaction
trying to summarize the results obtained by the versatile tools, i.e. solid state nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), on the systems Ca-0-Si and Al-0-Ti during mechanical treatment to obtain precursors of complex oxides. Relevance of these analytical information about incipient chemical reactions and the subsequent solid state processes to final products on heating is also discussed.
2. Systems
and analytical
tools
Two reaction systems were employed, i.e. ( 1) Ca(OH)2 + SiOZ and (2) Al + Ti0,H20. Except titania hydrogel, which was synthesized from TiCl,, starting materials, including amorphous fumed silica (Aerosil A200, Degussa) were used as purchased.
388
Details
M. Senna. T. Isobe I Solid State Ionics IOI-
of these
materials
were
given
elsewhere
[4,61. Solid state NMR was applied in different modes: Spin echo (SE) pulse NMR for 27A1; magic angle spinning (MAS) NMR for 27A1 and 29Si; combined rotation and multiple pulse (CRAMPS) NMR for ‘H; and cross polarization MAS (CP/MAS) NMR for ‘9Si. While SE was measured by an apparatus developed in the Institute for Materials Research, Tohoku University, all other measurements were made by commercial equipment (Chemagnetics, CMX-300). EPR spectra were obtained by X-band spectrometer (JEOL, JES-RE3X). Other conventional analytical tools such as X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD), thermal analyses and vibration spectra were used as usual.
3. Polarization
103 (1997)
387-392
When basic OH-groups coexist with acidic OH, there is always an acid-base reaction, by simultaneous dehydration, leaving M-O-M’ bridging bonds, where M and M’ are two different cationic species. The process is promoted when related OH groups are polarized. We have demonstrated formation of these kind of bridging bonds in different systems as typical examples of mechanochemical reaction [1,2,.5]. This was verified by the change of surface basicity of alkaline earth hydroxides, changes in the total weight loss due to subsequent thermal dehydration or decrease in the IR absorption band due to OH groups. Mechanochemical dehydration with simultaneous formation of M-O-M’ bonds is obviously taking place during milling of a mixture of oxides at the point of contact, where plastic deformation, and hence, preferential concentration of lowcoordinated states occur.
of OH-groups 4. Mechanisms
Hydroxyl groups, either chemisorbed on the surface of anhydrous oxides or present as a part of hydroxide, are more polarized, and hence more reactive, when the substrate contains an unsaturated or low coordinate state. A direct verification of polarized OH-groups may be obtained from CRAMPS ‘H NMR, as shown in Fig. 1.
4* C
a B
*
A
;:i
I
15
II
I.
5 10 Chemical shift / ppm
I
0
Fig. 1. H-CRAMPS NMR spectra for: (A) physical mixture Ca(OH!,YSiO,; (B) after milling the mixture for 3 h; and (C) after mdlmg pure SiO, for 3 h. * denotes the spinning side band of Ca(OH),. A common peak at 0.12 ppm represents a signal from a silicon rubber used as a standard. Details are given in Ref. [6].
related with radicals
There is another important mechanism involving radicals for the progress of a mechanochemical reaction [I 11. An EPR power saturation method enables us to assign the radical species appearing and disappearing during milling of a mixture. In the case of the system Ca-0-Si, we observed EPR spectra, like those given in Fig. 2. In these EPR studies, milling was carried out in vacuum and spectroscopy was carried out immediately after milling, without exposure to air, in order to examine the effect of ambient gaseous species. By careful analysis on the view point of g-values, power saturation behavior, comparison with the results of X-ray irradiation and change in the signals after exposure to air, we determined that the signals A and F in Fig. 2 are attributed to 0, radicals. They are absent in the mixture after milling. The EPR results are summarized as follows [7]: (1) on milling Ca(OH), separately, the concentration of 0, radicals increases; (2) main radical species of separately milled SiO, is a peroxide radical (POR); (3) these two radical species disappear on milling a mixture of Ca(OH)Z and SiO,. This suggests that these two species are recombined to form Ca-0-Si bridging bonds by the reaction schemes: ( 1) observed 0, radicals are formed by adsorption of an
M. Senna, T. Isobe I Solid State Ionics 101-103
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35
Magnetic field, H / 1 O-2T Fig. 2. EPR spectra of: (A) physical mixture Ca(OH)z-SiOz; (B) after milling the mixture for 3 h in vacuum; (C) after milling pure Ca(OH), for 3 h in vacua. Interpretations of peaks A and F are given in the text. Peaks marked by Mn are hyperfine structure of the EPR signal from Mn*‘. Details are given in Ref. [7].
molecule onto the mechanochemically oxygen formed O- radical or the radical Ca-0’ on the surface of partially decomposed Ca(OH),; (2) observed peroxide radicals, Si-0-0(-H), are formed by adsorption of an oxygen molecule to the radical species = Si.; and (3) recombination of two surface radical species to give bridging bonds as Ca-0’ + = Si’ + Ca-0-Si. n
I
I
I, 1
Milling time / h Fig. 3. milling peroxy mixture
Change in the concentration of the surface species with time: (A) 0, radical on separately milled Ca(OH)?; (B) radical on separately milled SiO,; and (C) basic site on the Ca(OH),-SiO,. Details are given in Ref. [7].
(1997) 387-392
389
An important question of the relative significance of the acid-base mechanism stated above and this radical mechanism cannot be answered confidently. It is to be noted, however, that the concentration of the basic site that remained after milling for 1 and 3 h is 10’ and 10’ times, respectively, higher than the radical concentration, as shown in Fig. 3. This does not mean, however, that the contribution of radical mechanism is by orders of magnitudes smaller, because what we can measure is the concentration of the remaining radical species on the separately milled sample. The amount of those species actually consumed could not yet be measured.
5. Change in the coordination milling of a mixture
state during
Coordination state of silicon with neighboring silicon atoms can be examined by 29Si NMR [IO]. This area has been particularly developed to elucidate the hydraulic processes in cement chemistry [ 121. The main results obtained from CPIMAS NMR of *“Si on the Ca(OH),-SiO, mixture after milling were the predominance of the peak corresponding to Q, state, i.e. with no neighboring silicon, in place of Q, peak, the latter representing surface silanol and predominating the starting mixture before milling. Examples of 29Si NMR spectra are shown in Fig. 4. This is evidence of the formation of precursor species of calcium silicate by milling of a mixture of Ca(OH), and SiO,. Whether the Q, state corresponds to C,S or C,S is not unambiguously stated, although the reported chemical shift value favors the former, as far as the anhydrous state is concerned [131. As for the AI-TiO,H,O system, milling a mixture brings about the oxidation of aluminum with the predominance of 6-coordinated AlO, units [8]. It is also to be noted, however, that the lower coordination state of aluminum, i.e. AlO,, appears in the course of milling as shown in Fig. 5. Similar phenomena were observed on milling kaolinite, having a mixed phase of Al,O, and SiOz [14]. At the same time, heterogeneity of the state of metallic aluminum after partial oxidation was also demonstrated as shown in Fig. 6, where sharper and broader peaks correspond to undistorted and distorted states
M. Senna. T. Isobe I Solid State tonics
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Fig. 4. ‘“Si CP/MAS NMR mixture Ca(OH)*-SiO:; (B) (C) after milling the mixture peak Q,, with respect to that are given in Ref. [lo].
spectra for the mixture: (A) physical after milling the mixture for 3 h; and for 12 h. The relative intensity of the of Q, increases significantly. Details
of metallic aluminum [8]. It is surprising that the undistorted state of metallic aluminum still survives after substantial loss of the long range ordering detectable by X-ray diffractometry [S].
6. Charge transfer On milling a mixture of metal oxides, the valence state of cations also changes. In the case of the system Si-0-Ca, the Si*+ state appears as detected by XPS [6]. The divalent silicon disappeared rapidly on sputtering by Ar+ ions for only 2 s, which was estimated to correspond to 1 to 2 nm by referring the result of AFM observation on etched silica glass [6]. In the case of the system Al-0-Ti, a part of tetravalent Ti is reduced to Ti’+. This trivalent titanium ion is very sensitive to EPR as shown in Fig. 7. In this case, the reduction was compensated by the oxidation of aluminum. In the case of the system Si-0-Ca, however, some additional explanation is necessary to show how the electrons are supplied to reduce Si’+ to Si*+.
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Fig. 5. “Al MAS NMR spectra of the mixture of metallic Al and titania hydrogel, TiO,H,O, after milling for (A) 10 h; (B) 30 h; (C) 50 h; (D) 100 h; (E) 200 h; and (F) 300 h by a low energy rod mill. Details are given in Ref. [8].
A general concept of electronegativity equalization was proposed by Sanderson [ 151. An apparent electronegativity of a cation, X,, is determined from the electronegativity of a neutral atom, X,,, and the valency, i, as Xj = (1 + 2i)X,,. While the values of X, for Ca*+ and Si4’ are 5.0 and 16.2, respectively, they both become 9.0 if Si4+ is reduced to Si’+ and Ca *+ is oxidized to Ca4+. While Si’+ was actually observed as mentioned above, Ca’+ is not conceivable under a normal condition. Low coordination number could allow, however, such an unusual electronic state [ 161. This is demonstrated by referring to the DV-Xa cluster model, that the ionic charge of Mg-0 bonding in crystalline MgO decreases from 1.54 to 1.08 when the coordination number changes from 5 to 3 [ 161. Bond ionic charge and bond ionicity can reasonably be assumed to be proportional. The bond ionicity, Y, in turn, is related to the electronegativity. Under these assumptions, the electronegativity of divalent calcium in the 3 coordinated state becomes 9.1, being very close to that of
M. Senna, T. Isohe I Solid State Ionics
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divalent silicon, 8.9. For the calculation of apparent electronegativity of Si’+, low coordination was not taken into account, since we observed a negligibly small amount of unsaturated bonds on the surface of amorphous silica [6]. Generalization of this idea should be made with care, however, since the assumptions made are not yet fully verified.
7. Further processes
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Fig. 6. “Al spin echo NMR spectra of the mixture of metallic Al and titania hydrogel, TiOzH20, after milling for (A) 30 h; (B) 50 h; (C) 100 h: (D) 200 h. Details are given in Ref. 181.
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34 Magnetic field, H/
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to final products
Even when the coordination state at the stage of precursor is disclosed with a considerable accuracy, we cannot predict the later stage of solid state processes to final products, which is doubtlessly more important for practical purposes. A common phenomenon which is observed repeatedly is that intimate mixing with incipient chemical interaction brings about more homogeneous phase in the product, e.g., Ca(OH), + Si02 -+ olCaSi0,; Al + TiO,H,O-+Al,TiO,. We have also obtained pure perovskite, a complicated ferroelectric material, 0.9Pb(Nb,,lMg, ,,)O,-0. IPbTiO,, from a stoichiometric mixture of PbO, TiO,, Nb,O, and Mg(OH),, preliminarily milled for 1 h by heating at temperatures as low as 850°C [ 171. While the observed homogeneity of the products on heating the starting mixture with preliminary milling should be closely related with the incipient reaction, it is also to be noted that homogeneity of the starting mixture in the nanometer scale increases with milling [4.9]. As a matter of fact, fractional conversion of some complex oxides on heating increases parallel to the extent of micro-homogeneity gained during preliminary milling [9]. It is therefore quite likely, that nucleation takes place in a more homogeneous manner, followed by a diffusion process with much shorter paths. These kind of rather particle technological aspects cannot be separated from the chemical aspects mentioned throughout this study.
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Fig. 7. EPR spectra of the mixture of metallic Al and titania hydrogel, Ti0,H20, after milling for (A) 0 h; (B) 10 h; (C) 30 h; (D) 50 h; (E) 100 h; (F) 200 h; and (G) 300 h by a low energy rod mill. Details are given in Ref. [8].
8. Concluding
remarks
Whether and to what extent an incipient chemical process takes place in a starting fine particulate mixture for complex oxides can be examined by
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M. Senna. T. lsobe I Solid State Ionics 101-103
solid state NMR and EPR. While solid state NMR gives us information with respect to the change in the coordination state, EPR enables us to show the existence of different radical species and some special valence states. Related charge transfer could undergo the principle of electronegativity equalization. For more confident and quantitative statements from these versatile analytical tools, however, there is still much to be done, e.g., to combine with information obtained independently of these resonance spectroscopies and to compare them with the results of similar reaction systems in a systematic fashion.
Acknowledgements The authors thank many former coworkers who appear in the reference list, among others, Dr. Liao, Ms. Kojima and Mr. Watanabe for their cooperation.
References [l] J. Liao, M. Senna, Solid State Ionics 66 (1993) 313. [2] J. Liao, M. Senna, Mater. Res. Bull. 30 (1995) 385. [3] K. Takeuchi, T. Isobe, M. Senna, J. Solid State Chem. 109 (1994) 401.
(1997) 387-392
]41 Y. Kojima, M. Senna, T. Shinohara, S. Ono, K. Sumiyama, K. Suzuki, J. Alloys. Comp. 227 (1995) 97. [51 T. Watanabe, J.-F. Liao, M. Senna, J. Solid State Chem. 115 (1995) 390. f61 T. Watanabe, T. Isobe, M. Senna, J. Solid State Chem. 122 (1996) 74. ]71 T. Watanabe, T. Isobe, M. Senna, J. Solid State Chem. 122 (1996) 291. IsI Y. Kojima, M. Senna, T. Isobe, T. Shinohara, S. Ono, K. Sumiyama, K. Suzuki, J. Mater. Res. 11 (1996) 1305. [91 Y. Okamoto, T. Isobe, M. Senna, J. Non-Cryst. Solids 180 (1995) 171. [lOI T. Watanabe, T. Isobe, M. Senna, J. Solid State Chem., in press. I. Ebert, H.-P. Hennig, L.I. IllI U. Steinike, U. Kretzschmar, Barsova, T.K. Jurk, React. Solids 4 (1987) 1. 1121 G.W. Groves, A. Brough, I.G. Richardson, C.M. Dobson, J. Am. Ceram. Sot. 74 (1991) 2891; H. Ishida, K. Sasaki, Y. Okada, T. Mitsuda, J. Am. Ceram. Sot. 75 (1992) 2541; A.R. Brough, CM. Dobson, J. Am. Ceram. Sot. 77 ( 1994) 593. [I31 S. Leonardelli, L. Facchini, C. Fretigny, P. Tougne, A.P. Legrand, J. Am. Chem. Sot. 114 (1992) 6412. [ 141 T. Nakano, M. Kamitani, M. Senna, Mater. Sci. Forum 225-227 (1996) 587. [I51 R.T. Sanderson, Science 114 (1951) 670. [I61 C. Satoko, M. Tsukada, H. Adachi, J. Phys. Sot. Jpn. 45 (1978) 1333. [I71 J.-G. Baek, T. Isobe, M. Senna, J. Am. Ceram. Sot. 80 (1997) 973.