- Email: [email protected]
Irradiation effects in SiO2 polymorphs
Solid State Communications,
Vol. 10, pp. 127—130, 1972.
Perganion Press.
Printed in Great Britain
IRRADIATION EFFECTS IN Si02 POLYMORPHS* A.G. Revesz Communications Satellite Corporation, COMSAT Laboratories, Clarksburg, Md. 20734, USA (Received 13 September 1971 by N.B. Hannay)
Irradiation-induced changes in density and molar refraction of Si02 polymorphs as well as saturation effects result from maximizing 77-bonding and minimizing bond strain in Si—O ring configurations. Trivalent silicon and nonbridging oxygen defects are generated without significant atomic displacements. Irradiation behavior of Si02 is more similar to that of siloxane polymers than typical ionic or covalent crystals.
IRRADIATION effects have been extensively studied in two Si02 polyniorphs: vitreous silica and a-quartz.’~ of with a-quartz during irradiation,The and density saturates dose decreases at a value which is about 15 per cent less than the original one. In this process, the structure of a-quartz disappears, and the final state is noncrystalline. The same final state is reached during irradiation of vitreous silica, but in this case the density increases by 2—3 per cent. Concomitant with the change in density, the molar refraction (at 589 tim wavelength) from 7.20 3 mole~ for quartzincreases and decreases from to 7.38 7.44 to cm 7.38 cm3 mole’ for vitreous silica. The structure of the final noncrystalline silica is hardly different from vitreous Si0 2 coordination before irradiation, specifically, the tetrahedral of silicon and the Si—O bond length remain unchanged;3 but the defects Si—O—Siarebond angle is reduced Also, generated during slightly. irradiation; their saturation concentration is 10 1020 cm3 —
It is known that the Si—O bond, as occurs in the various polymorphs of Si0 2, is to a large extent covalent. substantial to rethe covalent characterA is due to thecontribution dTr—p77 bond sulting from overlap of the lone pair 2p electrons of oxygen with the (originally empty) 3d orbitals 5 It has been indicated by the author of silicon. that, among Si0 2 polymorphs of tetrahedral coordination, the iT-bond increases as the Si—O-.Si angle increases density decreases; this trend is reflectedand in the a systematic variation of several properties, especially molar refraction.6’7
.~
Recently, it has been argued that the unusual densification behavior of vitreous silica under high pressure is a manifestation of the basic trend underlying structural changes in Si0 2 polymorphs: 8 maximizing i~-bondwith minimum bond strain. irThe same trend is exhibited by quartz during radiation: the Si—O—Si angle increases(i.e. the
In this communication an explanation is provided for these phenomena by considering the *
effects of irradiation on the Si—O bond. The model proposed here should be considered as 2 complementary to the model in which, however, thephenomenological nature of the Si—O bond was not taken into account, and changes in molar refraction remained unexplained. This model also provides a new insight into the nature of irradiation-generated defects.
This paperLaboratories is based upon work performed at COMSAT under Corporate sponsorship. 127
128
Si02 POLYMORPHS
density decreases) and 77-bond increases (as revealed, for instance, by the increase of molar refraction). This change becomes increasingly difficult iT-bonding rotation about the Si—Obecause bond, and similarlyhinders to polymers,9 facilitates the distribution of deposited energy among several Si0 4/2 with units.dose. Hence, thestructural effects ofchange irradiation saturate This is similar to radiation-induced cross-linking is siloxane polymers9 in the sense that the helical (chain) like configuration of Si—O atoms in quartz is replaced by a more isotropic configuration. From these considerations, the observation,’0 that the ratio of fractional expansion perpendicular to the c-axis of quartz to that parallel with the c-axis, i.e., (~a/a 0)/ (&/c0) first increases (to a value between 5 and 6) and then decreases with irradiation dosage, can be understood. It is known that the structure of quartz is based on deformed 6 member Si04/2 rings so that Si and 0 atoms form a helix along the c-axis, Since ring formations favor delocalization of 77-electrons, 77-bonding delocalization are enhanced along the c-axis and relative to a directional perpendicular to it; consequently, the increase in the Si—O—Si bond angle, (i.e. expansion) and the concomitant increase in 77-bonding occur mostly perpendicularly to the c-axis at the beginning of irradiation. Accordingly, 77-bonding becomes more isotropic. Generation of defects makes this process easier until 77-bonding reaches such an extent that further increase becomes increasingly difficult as 77-bonds hinder rotation about the Si-.-O bonds. The irradiation behavior of vitreous Si02 is the opposite to that of quartz: density increases slightly and molar refraction decreases. The saturation effects demonstrate that at a particular density a very stable configuration (conformation) in noncrystalline Si02 reached regardless whether the starting material is quartz or vitreous Si03. Both the density (2.26 gcm~) and molar refraction (7.38 cm3mole’) values indicate that this noncrystalline Si02 is an intermediate 3 and 7.42 cm3 between mole~)and a-tridymite (2.27 g cm a-cristobalite (2.33 g cm3 and 7.39 cm3 mole’). It also closely resembles a quasi-equilibrium structure occurring during pressure densification of vitreous silica.8 However, there are no defects in pressure-densified silica, whereas irradiated SiO has a defect concentration of 10t9_ 1020cm~.
Vol. 10, No. 1
It has been pointed out that among Si02 polymorphs maximum strength of the Si—O bond (i.e. maximum bond order) is exhibited the density 3 Forindensities below range of 2.27 to 2.33Si0 gcmthis range (vitreous 3, melanoph2.20 gcm logite: 1.99 gcm3) the 2: increase in molar refrac1 tion to 7.45 (vitreous andincreased 7.71 cm~mole (melanophlogite) is notSi02) due to 77-bonding (i.e. to higher bond order) but to enhanced delocalization in Si0 1 This results 412 Si0 rings.’ from the larger size of 4,,2 rings; for instance, 2 as 4 to 8 member rings occur in vitreous Si02,’ compared with 6 member rings in a-cristobalite. The larger rings are associated with larger bond strain. The extent of bond strain is very limited .~
in a perfect crystal, disorder and of impurity effects facilitate thebut accommodation bond strain. Depending on the preparation methods, vitreous Si02 is associated with some bond strain that is released during irradiation by breaking the larger rings. In this process defects are generated. The distribution of Si04,,2 rings is, thus, shifted 3 The sizes, concomitant in 7T-electoward smaller leadingdecrease to somewhat higher density.’ tron delocalization leads to smaller values of the molar refraction. Thus, it is very likely that the stability of the final structure obtained during irradiation of Si0 2 is due to the fact that at that particular distribution of SiO~2rings (corresponding to 2.26gcm3 density) and defect density (10’s 1020 cm-s) 77-bonding (i.e. bond order) is maxi—
mized and bond strain is minimized. This may occur even at the expense of some 77-electron delocalization, as reflected in the decrease in molar refraction of vitreous Si02. Contrary to pressure, irradiation does not introduce bond strain, but rather, releases it. This is manifested in the rapid expansion of pressure-compacted vitreous silica upon irradiation. Radiation-compacted vitreous silica expands during high temperature annealing. This process is characterized by a range of activation energies extending from about 1.5—4.3eV, and by a distribution of frequency factors up to iO’~sec-1. These activation energies are much higher than those characteristic of annealing of pressure-compacted vitreous silica, that is, about 0.04—0.4eV.’4 The large difference is most likely due to the fact that
Vol. 10, No.1
Si02 POLYMORPHS
annealing of radiation-compacted vitreous silica involves annihilation of defects which requires higher activation energies than the conformational changes involved in annealing of pressure-densified silica. The high value of the frequency factor indicates that the process is not diffusion controlled. This behavior is in contrast with annealing of radiation-induced damage in covalent or ionic crystals. Another important aspect of this different behavior is that for Si0 2 there is no fundamental difference between ionizing and particle radiation: in both cases the density and defect saturate with dose. It is only the rateconcentration which is affected by the nature of irradiation and, also, by differences in the starting material (e.g., etc.). crystalline or noncrystalline, impurity, content, The reason for this is the flexibility of the Si—O—Si bridging bond angle as related to variations in the 77-character and ionicity of the Si—O bond. This allows the break up of the Si—O network at some points without creating interstitials and/or vacancies in that sense as they occur in typical covalent or ionic solids. Rather, the intrinsic defects in Si02 are the trivalent silicon and nonbridging oxygen: Si—O—Si Si 0—Si ~ irrad. .
.
where the dots represent electrons which do not participate in bonding. These defects are still part of the Si0 2 network, defects in covalent or ionic whereas crystals the (i.e.typical vacancies and interstitials) are true point defects. Since no significant displacement of atoms is involved, generation of defects in Si02 by irradiation is essentially an electronic process.
129
Irradiation changes the electron distribution in two Sj~,2 tetrahedra; this, in turn, is reflected as a change in the Si—O—Si bond angle. A relatively small change in the conformation is sufficierg to break an Si—O bond. This process is fundamentally different from generating Frenkel or Schottky defects in covalent or ionic crystals but, again, it is similar to those occurring in 9 siloxane polymers. It has already been pointed out that tI’e cxistence of oxygen and silicon vacancies, as well as of5silicon interstitials in oxygen Si02 isvacancy very unprobThe estimate of the conable.’ centrat ion in unirradiated vitreous Si0 2 (about 1020 cm-’ was based 1eV as energylow of for6 ) This value on is unrealistically (cornmation.’ pare with 7—8eV for NaCl 20eV for Al 2 03),~~ consequently, considerations based on this result ~ do not seem to be justified. ,-~.
Impurities have far reaching effects on 77-bonding between silicon and oxygen. hence, they are crucial in all properties of SiC2. These effects are discussed separately, but the important role of hydroxyl is briefly summarized. The decreased rate of defect generation during y-irradiation in 4 isnecesdue to the but lacknot of trivalent silicon; this is not some, all, OH-containing silicas sarily related to the presence of OH groups.’8 Trivalent silicon may also result from irradiation9 induced thermal dissociation of SiH groups;’ these areorimportant defects in Si0 2 films obtained by thermal or anodic oxidation of silicon.~Hydroxyl groups enhance 77-bonding in neighboring Si—O—Si bonds, thus making the silica more resistant to irradiation, whereas trivalent Si and SiH groups have the opposite effect.
REFERENCES 1.
A thorough treatment of irradiation effects is included in a recent review of vitreous silica: BRUCKNER R., J. Non-Cryst. Solids 5, 123 and 177 (1971),; this also contains an extensive list of publications. Except for special cases, no specific references are made to publications listed there.
2.
PRIMAK W. and KAMPWIRTH R., J. appi. Phys. 39, 5651 (1968), and 40, 2565 (1969); these papers also contain many references to earlier works of Priniak and co-workers.
3.
SIMON I., J. Am. Ceram. Soc. 40, 150 (1957).
4. 5.
WEEKS R.A., VU. Conference on Glass, Brussels (1965). For a review of d77—p77 see MITCHELL K.A.R., Chem. Revs. 69, 157 (1969).
6.
REVESZ A.G., J. Non-Cryst. Solids 4, 347 (1970).
130
S102 POLYMORPHS
7.
REVESZ A.G., Phys. Rev. Lett., (in press).
8.
REVESZ A:G., J. Non-Cryst. Solids, (in press).
9. 10. 11. 12. 13.
Vol. 10, No. 1
CHARLESBY A., Atomic Radiation and Polymers, Pergamon Press, New York, (1960). PRIMAK W., Phys. Rev. 110, 1240 (1958). 3, whereas the force constant and molar 77-bond becomes stronger)2.3asgcm the denThis is manifested in the reduction ofrefraction the force increase constant (i.e. as the density falls below sity decreases from 2.87 (coesite) to 2.33 gcm3 (a-cristobalite).” CARTZ L., Z. Krist. 120, 241 (1964). -~
14. 15.
This bejiavior can be compared with the observation (SKINNER B.J. and APPLEMAN D.E. Amer. Mineral 48, 854 (1963)) that grinding at room temperature is sufficient to convert melanophlogite into quartz. This effect was attributed to the release of bond strain associated with large SiO 7 412 rings around pigment inclusions originally present in melanophlogite. SAHHA S. and MACKENZIE J.D., J. Non-Cryst. Solids 1, 107 (1969). STE\JELS T.M. and KATS A., Philips Res. Repis. 11, 103 (1956).
16.
GARINO-CANINA V., C.R. hebd. sèanc. Acad. Sci., Paris 252, 3769 (1961).
17.
KROGER F.A., The Chemistry of Imperfect Crystals, p.~424,J. Wiley, New York, (1964).
18.
HETHERINGTON G., JACK K.H., and RAMSEY M.W., Phys. Chem. Glass, 6, 6 (1965).
19.
REVESZ A.G., IEEE Radiation Effects Conf. Durham, N.H. July 1971; IEEE Trans. Nucl. Sci., in press. BECKMANN K.H. and HARRICK N.J., J. Electrochem. Soc. 118, 614 (1971).
20.
Veränderungen der Dichte und Molrefraktion durch Bestrahlung von Si0 2 Polyrnorphen, sowohi auch Sättigungseffekte folgen aus der Maximalization der 7T-Bindung und Minimalization der Bindungsspannung in Si—O Ringskonfigurationen. Dreiwertiges Silizium und nichtbrückender Sauers toff Netzfehler bilden sich ohne bedeutendenden atomarischen Verschiebungen. Aus dem Standpunkt des Bestrahlungsverhaltens 1st Si02 den Siloxan Polymeren ähnlicher, als typischen ionischen oder hombopolarischen Kristallen.
Vol. 10, pp. 127—130, 1972.
Perganion Press.
Printed in Great Britain
IRRADIATION EFFECTS IN Si02 POLYMORPHS* A.G. Revesz Communications Satellite Corporation, COMSAT Laboratories, Clarksburg, Md. 20734, USA (Received 13 September 1971 by N.B. Hannay)
Irradiation-induced changes in density and molar refraction of Si02 polymorphs as well as saturation effects result from maximizing 77-bonding and minimizing bond strain in Si—O ring configurations. Trivalent silicon and nonbridging oxygen defects are generated without significant atomic displacements. Irradiation behavior of Si02 is more similar to that of siloxane polymers than typical ionic or covalent crystals.
IRRADIATION effects have been extensively studied in two Si02 polyniorphs: vitreous silica and a-quartz.’~ of with a-quartz during irradiation,The and density saturates dose decreases at a value which is about 15 per cent less than the original one. In this process, the structure of a-quartz disappears, and the final state is noncrystalline. The same final state is reached during irradiation of vitreous silica, but in this case the density increases by 2—3 per cent. Concomitant with the change in density, the molar refraction (at 589 tim wavelength) from 7.20 3 mole~ for quartzincreases and decreases from to 7.38 7.44 to cm 7.38 cm3 mole’ for vitreous silica. The structure of the final noncrystalline silica is hardly different from vitreous Si0 2 coordination before irradiation, specifically, the tetrahedral of silicon and the Si—O bond length remain unchanged;3 but the defects Si—O—Siarebond angle is reduced Also, generated during slightly. irradiation; their saturation concentration is 10 1020 cm3 —
It is known that the Si—O bond, as occurs in the various polymorphs of Si0 2, is to a large extent covalent. substantial to rethe covalent characterA is due to thecontribution dTr—p77 bond sulting from overlap of the lone pair 2p electrons of oxygen with the (originally empty) 3d orbitals 5 It has been indicated by the author of silicon. that, among Si0 2 polymorphs of tetrahedral coordination, the iT-bond increases as the Si—O-.Si angle increases density decreases; this trend is reflectedand in the a systematic variation of several properties, especially molar refraction.6’7
.~
Recently, it has been argued that the unusual densification behavior of vitreous silica under high pressure is a manifestation of the basic trend underlying structural changes in Si0 2 polymorphs: 8 maximizing i~-bondwith minimum bond strain. irThe same trend is exhibited by quartz during radiation: the Si—O—Si angle increases(i.e. the
In this communication an explanation is provided for these phenomena by considering the *
effects of irradiation on the Si—O bond. The model proposed here should be considered as 2 complementary to the model in which, however, thephenomenological nature of the Si—O bond was not taken into account, and changes in molar refraction remained unexplained. This model also provides a new insight into the nature of irradiation-generated defects.
This paperLaboratories is based upon work performed at COMSAT under Corporate sponsorship. 127
128
Si02 POLYMORPHS
density decreases) and 77-bond increases (as revealed, for instance, by the increase of molar refraction). This change becomes increasingly difficult iT-bonding rotation about the Si—Obecause bond, and similarlyhinders to polymers,9 facilitates the distribution of deposited energy among several Si0 4/2 with units.dose. Hence, thestructural effects ofchange irradiation saturate This is similar to radiation-induced cross-linking is siloxane polymers9 in the sense that the helical (chain) like configuration of Si—O atoms in quartz is replaced by a more isotropic configuration. From these considerations, the observation,’0 that the ratio of fractional expansion perpendicular to the c-axis of quartz to that parallel with the c-axis, i.e., (~a/a 0)/ (&/c0) first increases (to a value between 5 and 6) and then decreases with irradiation dosage, can be understood. It is known that the structure of quartz is based on deformed 6 member Si04/2 rings so that Si and 0 atoms form a helix along the c-axis, Since ring formations favor delocalization of 77-electrons, 77-bonding delocalization are enhanced along the c-axis and relative to a directional perpendicular to it; consequently, the increase in the Si—O—Si bond angle, (i.e. expansion) and the concomitant increase in 77-bonding occur mostly perpendicularly to the c-axis at the beginning of irradiation. Accordingly, 77-bonding becomes more isotropic. Generation of defects makes this process easier until 77-bonding reaches such an extent that further increase becomes increasingly difficult as 77-bonds hinder rotation about the Si-.-O bonds. The irradiation behavior of vitreous Si02 is the opposite to that of quartz: density increases slightly and molar refraction decreases. The saturation effects demonstrate that at a particular density a very stable configuration (conformation) in noncrystalline Si02 reached regardless whether the starting material is quartz or vitreous Si03. Both the density (2.26 gcm~) and molar refraction (7.38 cm3mole’) values indicate that this noncrystalline Si02 is an intermediate 3 and 7.42 cm3 between mole~)and a-tridymite (2.27 g cm a-cristobalite (2.33 g cm3 and 7.39 cm3 mole’). It also closely resembles a quasi-equilibrium structure occurring during pressure densification of vitreous silica.8 However, there are no defects in pressure-densified silica, whereas irradiated SiO has a defect concentration of 10t9_ 1020cm~.
Vol. 10, No. 1
It has been pointed out that among Si02 polymorphs maximum strength of the Si—O bond (i.e. maximum bond order) is exhibited the density 3 Forindensities below range of 2.27 to 2.33Si0 gcmthis range (vitreous 3, melanoph2.20 gcm logite: 1.99 gcm3) the 2: increase in molar refrac1 tion to 7.45 (vitreous andincreased 7.71 cm~mole (melanophlogite) is notSi02) due to 77-bonding (i.e. to higher bond order) but to enhanced delocalization in Si0 1 This results 412 Si0 rings.’ from the larger size of 4,,2 rings; for instance, 2 as 4 to 8 member rings occur in vitreous Si02,’ compared with 6 member rings in a-cristobalite. The larger rings are associated with larger bond strain. The extent of bond strain is very limited .~
in a perfect crystal, disorder and of impurity effects facilitate thebut accommodation bond strain. Depending on the preparation methods, vitreous Si02 is associated with some bond strain that is released during irradiation by breaking the larger rings. In this process defects are generated. The distribution of Si04,,2 rings is, thus, shifted 3 The sizes, concomitant in 7T-electoward smaller leadingdecrease to somewhat higher density.’ tron delocalization leads to smaller values of the molar refraction. Thus, it is very likely that the stability of the final structure obtained during irradiation of Si0 2 is due to the fact that at that particular distribution of SiO~2rings (corresponding to 2.26gcm3 density) and defect density (10’s 1020 cm-s) 77-bonding (i.e. bond order) is maxi—
mized and bond strain is minimized. This may occur even at the expense of some 77-electron delocalization, as reflected in the decrease in molar refraction of vitreous Si02. Contrary to pressure, irradiation does not introduce bond strain, but rather, releases it. This is manifested in the rapid expansion of pressure-compacted vitreous silica upon irradiation. Radiation-compacted vitreous silica expands during high temperature annealing. This process is characterized by a range of activation energies extending from about 1.5—4.3eV, and by a distribution of frequency factors up to iO’~sec-1. These activation energies are much higher than those characteristic of annealing of pressure-compacted vitreous silica, that is, about 0.04—0.4eV.’4 The large difference is most likely due to the fact that
Vol. 10, No.1
Si02 POLYMORPHS
annealing of radiation-compacted vitreous silica involves annihilation of defects which requires higher activation energies than the conformational changes involved in annealing of pressure-densified silica. The high value of the frequency factor indicates that the process is not diffusion controlled. This behavior is in contrast with annealing of radiation-induced damage in covalent or ionic crystals. Another important aspect of this different behavior is that for Si0 2 there is no fundamental difference between ionizing and particle radiation: in both cases the density and defect saturate with dose. It is only the rateconcentration which is affected by the nature of irradiation and, also, by differences in the starting material (e.g., etc.). crystalline or noncrystalline, impurity, content, The reason for this is the flexibility of the Si—O—Si bridging bond angle as related to variations in the 77-character and ionicity of the Si—O bond. This allows the break up of the Si—O network at some points without creating interstitials and/or vacancies in that sense as they occur in typical covalent or ionic solids. Rather, the intrinsic defects in Si02 are the trivalent silicon and nonbridging oxygen: Si—O—Si Si 0—Si ~ irrad. .
.
where the dots represent electrons which do not participate in bonding. These defects are still part of the Si0 2 network, defects in covalent or ionic whereas crystals the (i.e.typical vacancies and interstitials) are true point defects. Since no significant displacement of atoms is involved, generation of defects in Si02 by irradiation is essentially an electronic process.
129
Irradiation changes the electron distribution in two Sj~,2 tetrahedra; this, in turn, is reflected as a change in the Si—O—Si bond angle. A relatively small change in the conformation is sufficierg to break an Si—O bond. This process is fundamentally different from generating Frenkel or Schottky defects in covalent or ionic crystals but, again, it is similar to those occurring in 9 siloxane polymers. It has already been pointed out that tI’e cxistence of oxygen and silicon vacancies, as well as of5silicon interstitials in oxygen Si02 isvacancy very unprobThe estimate of the conable.’ centrat ion in unirradiated vitreous Si0 2 (about 1020 cm-’ was based 1eV as energylow of for6 ) This value on is unrealistically (cornmation.’ pare with 7—8eV for NaCl 20eV for Al 2 03),~~ consequently, considerations based on this result ~ do not seem to be justified. ,-~.
Impurities have far reaching effects on 77-bonding between silicon and oxygen. hence, they are crucial in all properties of SiC2. These effects are discussed separately, but the important role of hydroxyl is briefly summarized. The decreased rate of defect generation during y-irradiation in 4 isnecesdue to the but lacknot of trivalent silicon; this is not some, all, OH-containing silicas sarily related to the presence of OH groups.’8 Trivalent silicon may also result from irradiation9 induced thermal dissociation of SiH groups;’ these areorimportant defects in Si0 2 films obtained by thermal or anodic oxidation of silicon.~Hydroxyl groups enhance 77-bonding in neighboring Si—O—Si bonds, thus making the silica more resistant to irradiation, whereas trivalent Si and SiH groups have the opposite effect.
REFERENCES 1.
A thorough treatment of irradiation effects is included in a recent review of vitreous silica: BRUCKNER R., J. Non-Cryst. Solids 5, 123 and 177 (1971),; this also contains an extensive list of publications. Except for special cases, no specific references are made to publications listed there.
2.
PRIMAK W. and KAMPWIRTH R., J. appi. Phys. 39, 5651 (1968), and 40, 2565 (1969); these papers also contain many references to earlier works of Priniak and co-workers.
3.
SIMON I., J. Am. Ceram. Soc. 40, 150 (1957).
4. 5.
WEEKS R.A., VU. Conference on Glass, Brussels (1965). For a review of d77—p77 see MITCHELL K.A.R., Chem. Revs. 69, 157 (1969).
6.
REVESZ A.G., J. Non-Cryst. Solids 4, 347 (1970).
130
S102 POLYMORPHS
7.
REVESZ A.G., Phys. Rev. Lett., (in press).
8.
REVESZ A:G., J. Non-Cryst. Solids, (in press).
9. 10. 11. 12. 13.
Vol. 10, No. 1
CHARLESBY A., Atomic Radiation and Polymers, Pergamon Press, New York, (1960). PRIMAK W., Phys. Rev. 110, 1240 (1958). 3, whereas the force constant and molar 77-bond becomes stronger)2.3asgcm the denThis is manifested in the reduction ofrefraction the force increase constant (i.e. as the density falls below sity decreases from 2.87 (coesite) to 2.33 gcm3 (a-cristobalite).” CARTZ L., Z. Krist. 120, 241 (1964). -~
14. 15.
This bejiavior can be compared with the observation (SKINNER B.J. and APPLEMAN D.E. Amer. Mineral 48, 854 (1963)) that grinding at room temperature is sufficient to convert melanophlogite into quartz. This effect was attributed to the release of bond strain associated with large SiO 7 412 rings around pigment inclusions originally present in melanophlogite. SAHHA S. and MACKENZIE J.D., J. Non-Cryst. Solids 1, 107 (1969). STE\JELS T.M. and KATS A., Philips Res. Repis. 11, 103 (1956).
16.
GARINO-CANINA V., C.R. hebd. sèanc. Acad. Sci., Paris 252, 3769 (1961).
17.
KROGER F.A., The Chemistry of Imperfect Crystals, p.~424,J. Wiley, New York, (1964).
18.
HETHERINGTON G., JACK K.H., and RAMSEY M.W., Phys. Chem. Glass, 6, 6 (1965).
19.
REVESZ A.G., IEEE Radiation Effects Conf. Durham, N.H. July 1971; IEEE Trans. Nucl. Sci., in press. BECKMANN K.H. and HARRICK N.J., J. Electrochem. Soc. 118, 614 (1971).
20.
Veränderungen der Dichte und Molrefraktion durch Bestrahlung von Si0 2 Polyrnorphen, sowohi auch Sättigungseffekte folgen aus der Maximalization der 7T-Bindung und Minimalization der Bindungsspannung in Si—O Ringskonfigurationen. Dreiwertiges Silizium und nichtbrückender Sauers toff Netzfehler bilden sich ohne bedeutendenden atomarischen Verschiebungen. Aus dem Standpunkt des Bestrahlungsverhaltens 1st Si02 den Siloxan Polymeren ähnlicher, als typischen ionischen oder hombopolarischen Kristallen.