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Triboluminescence in single crystal alkaline earth oxides
Solid State Communications, Vol. 29, pp. 20 1—203. Pergamon Press Ltd. 1979. Printed in Great Britain. TRIBOLIJM1NESCENCE IN SINGLE CRYSTAL ALKALINE EARTH OXIDES* G.P. Williams, Jr. and T.J. Turnert Wake Forest University, Winston-Salem, NC 27109, U.S.A. (Received 2 November 1978 by S. Amelinckx) Triboluminescence has been observed in single crystals of MgO, CaO and SrO deformed by compression in a temperature range from 80 to 300 K. It appears to be a new intrinsic effect, independent of impurities or surface conditions and is of three types corresponding to three regions of the stress—strain curve. The nature of the defect emission center is open to speculation, but it is proposed that dislocation induced excitonic transitions adjacent to vacancy clusters produce the emission. TR1BOLUMINESCENCE, the emission of light caused by the application of a mechanical stress to a crystal, was observed by Francis Bacon in sugar crystals hundreds of years ago, but only in the last few years has it been satisfactorily explained [1]. The origin of the light emitted from hard sugar scraped with a knife is believed to be from molecular nitrogen on the surface. In addition to such surface effects triboluminescence (TBL) has been observed in “doped” crystals. For example, alkali halides that have been irradiated to produce F centers exhibit TBL [2], as well as those that have been doped with impurities such as manganese [2]. Finally, an “intrinsic” TBL has been reported by Reynolds and Wawner [3] who observed an orange spark upon the fracture of SiC filaments. This TBL is termed “intrinsic” in the sense that it is characteristic of the host lattice and not traceable to impurity effects, Since the TBL peaked at 575 nm and since cathodoluminescence (CDL) (emission produced by electrode bombardment by charge carriers) has been observed in SiC at 570 nm [4] it was concluded that the emission results from an electric discharge between two new faces formed as a result of fracture. Chen et aL [5] have reported photoluminescence in CaO and SrO which occurred while the crystals were being plastically deformed in compression. These authors have illustrated this effect with simultaneous stress—strain and intensity—strain curves. Figure 1 illustrates the effect for MgO obtained in this laboratory. A complete report of this work is in preparation in which it is shown that emission occurs in the pre-yield region of the stress—strain curve as well as during stress relaxation. ____________
*
t
.
.
.
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Supported in part by the National Science Foundation Grant GH 33744 Now at Stetson University, DeLand, FL 32720, U.S.A. 201
Absorption as well as emission studies indicate that point defects are produced during the deformation process. If an SrO crystal is deformed while it is being illuminated by a UV source one observed, with the unaided eye, the formation of slip (dislocation) bands. When the crystal is deformed in compression along a [100] axis one can see a band form at 450 to this axis and grow in width as the deformation proceeds. Thus, one obtains significant information about the defects produced during deformation and, hence, the deformation process itself from the spectral composition of the emission and from the location of the emission from the cystal. We wish to report here an extension of these PTL experiments in which alkaline earth oxide crystals (MgO, CaO and SrO) were deformed without UV excitation. Simultaneous stress—strain emission—strain curves result, just as in the case of PTL. Thus, TBL gives us an additional tool to study the deformation process. Characteristic and reproducible TBL emission was observed in three distinct stages of the deformation process. TBL-1 was observed at the departure from linearity of the stress—strain curve; TBL-II was observed when the crosshead was lifted from the crystal; and TBL-III occurred if and when the crystal fractured. TBL-I and TBL-II appear to be “intrinsic” of a type not previously reported. An overview of a complete run showing these three TBL emissions is presented in the top half of Fig. 2 in such a way as to allow an approximate correlation with the deformation as represented by the stress—strain curve in the bottom half. Note the luminescence of type TBL-I shown by the peaks at A and caused by the deformation and subsequent relaxation at A’~A peculiar feature of TBL.1 is shown at B and B. When the stress is reapplied up to the B’ level, the intensity immediately jumps to the B level characteristic of the emission .
202
TRLBOLIJMINESCENCE IN SINGLE CRYSTAL ALKALINE EARTH OXIDES
4—
Vol. 29, No.3
2.25~C~MPRES~3N—.
5.0
10.0
15.3
23.3 25.0 TINE (SEC I
33.3
35.0
40.3
Fig. 1. Photoluminescence (upper curve) produced by the resolved thear stress (lower curve) are plotted vs their common time scales. intensity achieved at A in the previous cycle. Furthermore, when the deformation is continued without relaxation as at C~the intensity increases approximately linearly with deformation as at C. Small emission peaks of type TBL-III occur at D and E as small cracks develop in the crystal as indicated at D’ and E’. Finally at F, one observes TBL-I1 when the crosshead is quickly lifted from the crystal immediately following continuous deformation as at F~ Figure 3 displays in greater detail the emission jump to the previous intensity level (similar to B and B’ in Fig. 2) for three stress cycles and the beginnings of a fourth during which the crystal ruptured. It is also clearly evident in this figure that TBL-I emission begins below the classical macro-yield point on the stress—strain curve, The temperature dependence and spectral distribution of these TBL emission peaks are currently being investigated. Preliminary results indicate that TBL-I increases as the temperature is decreased from room temperature, at least down to liquid nitrogen temperature. TBL-II vanished prior to reaching 80°C. Preliminary spectral distribution work has been done with filters and can probably best be compared wfth the PTL results. The emission peak in PTL in MgO is relatively broad, extending from about 325 to 525 nm. TBL-I and TBL-II appear to be narrower than this, different from one another and probably centered around 375 nm. Nonetheless, they both appear to fit —
under the profile of the FTL emission plot. This is consistent with visual observations of TBL in CaO and SrO in which the unaided eye “sees” blue and greenish yellow emission, respectively, characteristic of the photoemission (PTL) from these same deformed crystals. TBL in the plastic, as well as in the elastic region, similar to TBL-I has been reported by Aizetta et a!. [21 but only for doped ionic crystals. They were able to make a one-to-one correlation of emission peaks with steps appearing in the stress—strain curves. They concluded that the excitation results from dislocation motion produced either by the impact of moving dislocations with trapped electrons or the unpinning of dislocations from luminescent centers. The significant difference between the work of Aizetta et aL [2] and the MgO studies reported here is that our “intrinsic” TBL has its origin in defects produced by the deformation process, and not by “doping”. Although the TBL in SiC reported by Reynalds and Wawner [3] may be termed “intrinsic” it was only observed upon rupture. They concluded it could not result from dislocation motion since their observations were made at room temperature and dislocations do not move in SiC below about 1700 C. In considering the mechanism for this TBL, we must distinguish the situation in which defects are produced and excited and the situation in which they are excited, having already been produced. TBL-I
Vol. 29, No. 3
TRIBOLUMINESCENCE IN SINGLE CRYSTAL ALKALINE EARTH OXIDES TRI8Q-LUS I NESCEN CE
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F’
~
H~
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10 20
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203
0
TINE ~SECJ I~0
Fig. 3. Triboluminescence (upper curves) produced by
the compressive loads (lower curves) are plotted for Fig. 2. Triboluminescence (upper curve) produced by the compressãve load (lower curve) are plotted vs their common time scales.
three stress cycles and part of a fourth vs their common time scales.
appears to be of the first type in which defects are produced and excited while TBL-II results only from excitation of defects previously produced. TBL-III may result from a combination of these effects since defects
ative. For TBL-III donors at crack surfaces may be involved. This is supported by a striking agreement between absorption and emission from surface states in alkaline earth oxides and absorption and emission from
already present would be excited and new defects possibly of a slightly different character might be produced and excited.
deformed alkaline earth oxides. These ideas will be explored more fully in work to be published. Finally, it should be noted that the results in Fig. 3
The nature of these defects producing the emission
is open to speculation. We still believe that the most likely prospects are vacancy clusters, with the emission resulting from an excited state of a donor oxygen ion adjacent to the vacancy cluster. These oxygen ions adjacent to vacancy clusters will act as donors because the Madelung field is required in order for them to retain their second electrons. This results from the positive electron affinity of oxygen for a second elec-
are readily explained by assuming that upon reappli-
cation of a load, dislocations immediately bow out and excite donors producing a jump in emission. This then gradually increases upon nonconservative dislocation motion, which produces the vacancy clusters.
1.
tron. Thus, the impact of a dislocation sweeping by such a donor might be to excite it, thus, stimulating
2.
emission. This could explain TBL-II. The occurrence of TBL-I may result from vacancies and free electrons produced during the plastic deformation or it may result from the exciiation of donors as vacancies are produced or both of these mechanisms may be oper-
3.
REFERENCES J.I. Zink,J. Phys. Chem 80,3,248(1976). G. Aizetta eta!., Phys. Status Solidi (a) 1, 775
(1970). 4. 5.
C.L. Reynolds Jr. & F.E. Wawner,Phys. Status Solidi (a) 36, I.Kl09 (1976). L. Michalski, Swiderski & T. Niemyski,Mater. Res Bull 6 41(1971) Y. Chen. M.M. Abraham, T.J. Turner & C. Nelson, Phil. Mag. 32, 99 (1975).
*
t
.
.
.
.
Supported in part by the National Science Foundation Grant GH 33744 Now at Stetson University, DeLand, FL 32720, U.S.A. 201
Absorption as well as emission studies indicate that point defects are produced during the deformation process. If an SrO crystal is deformed while it is being illuminated by a UV source one observed, with the unaided eye, the formation of slip (dislocation) bands. When the crystal is deformed in compression along a [100] axis one can see a band form at 450 to this axis and grow in width as the deformation proceeds. Thus, one obtains significant information about the defects produced during deformation and, hence, the deformation process itself from the spectral composition of the emission and from the location of the emission from the cystal. We wish to report here an extension of these PTL experiments in which alkaline earth oxide crystals (MgO, CaO and SrO) were deformed without UV excitation. Simultaneous stress—strain emission—strain curves result, just as in the case of PTL. Thus, TBL gives us an additional tool to study the deformation process. Characteristic and reproducible TBL emission was observed in three distinct stages of the deformation process. TBL-1 was observed at the departure from linearity of the stress—strain curve; TBL-II was observed when the crosshead was lifted from the crystal; and TBL-III occurred if and when the crystal fractured. TBL-I and TBL-II appear to be “intrinsic” of a type not previously reported. An overview of a complete run showing these three TBL emissions is presented in the top half of Fig. 2 in such a way as to allow an approximate correlation with the deformation as represented by the stress—strain curve in the bottom half. Note the luminescence of type TBL-I shown by the peaks at A and caused by the deformation and subsequent relaxation at A’~A peculiar feature of TBL.1 is shown at B and B. When the stress is reapplied up to the B’ level, the intensity immediately jumps to the B level characteristic of the emission .
202
TRLBOLIJMINESCENCE IN SINGLE CRYSTAL ALKALINE EARTH OXIDES
4—
Vol. 29, No.3
2.25~C~MPRES~3N—.
5.0
10.0
15.3
23.3 25.0 TINE (SEC I
33.3
35.0
40.3
Fig. 1. Photoluminescence (upper curve) produced by the resolved thear stress (lower curve) are plotted vs their common time scales. intensity achieved at A in the previous cycle. Furthermore, when the deformation is continued without relaxation as at C~the intensity increases approximately linearly with deformation as at C. Small emission peaks of type TBL-III occur at D and E as small cracks develop in the crystal as indicated at D’ and E’. Finally at F, one observes TBL-I1 when the crosshead is quickly lifted from the crystal immediately following continuous deformation as at F~ Figure 3 displays in greater detail the emission jump to the previous intensity level (similar to B and B’ in Fig. 2) for three stress cycles and the beginnings of a fourth during which the crystal ruptured. It is also clearly evident in this figure that TBL-I emission begins below the classical macro-yield point on the stress—strain curve, The temperature dependence and spectral distribution of these TBL emission peaks are currently being investigated. Preliminary results indicate that TBL-I increases as the temperature is decreased from room temperature, at least down to liquid nitrogen temperature. TBL-II vanished prior to reaching 80°C. Preliminary spectral distribution work has been done with filters and can probably best be compared wfth the PTL results. The emission peak in PTL in MgO is relatively broad, extending from about 325 to 525 nm. TBL-I and TBL-II appear to be narrower than this, different from one another and probably centered around 375 nm. Nonetheless, they both appear to fit —
under the profile of the FTL emission plot. This is consistent with visual observations of TBL in CaO and SrO in which the unaided eye “sees” blue and greenish yellow emission, respectively, characteristic of the photoemission (PTL) from these same deformed crystals. TBL in the plastic, as well as in the elastic region, similar to TBL-I has been reported by Aizetta et a!. [21 but only for doped ionic crystals. They were able to make a one-to-one correlation of emission peaks with steps appearing in the stress—strain curves. They concluded that the excitation results from dislocation motion produced either by the impact of moving dislocations with trapped electrons or the unpinning of dislocations from luminescent centers. The significant difference between the work of Aizetta et aL [2] and the MgO studies reported here is that our “intrinsic” TBL has its origin in defects produced by the deformation process, and not by “doping”. Although the TBL in SiC reported by Reynalds and Wawner [3] may be termed “intrinsic” it was only observed upon rupture. They concluded it could not result from dislocation motion since their observations were made at room temperature and dislocations do not move in SiC below about 1700 C. In considering the mechanism for this TBL, we must distinguish the situation in which defects are produced and excited and the situation in which they are excited, having already been produced. TBL-I
Vol. 29, No. 3
TRIBOLUMINESCENCE IN SINGLE CRYSTAL ALKALINE EARTH OXIDES TRI8Q-LUS I NESCEN CE
L0~
F’
~
H~
- --
--
I
,‘
~
_____
10 20
*LCRO
203
0
TINE ~SECJ I~0
Fig. 3. Triboluminescence (upper curves) produced by
the compressive loads (lower curves) are plotted for Fig. 2. Triboluminescence (upper curve) produced by the compressãve load (lower curve) are plotted vs their common time scales.
three stress cycles and part of a fourth vs their common time scales.
appears to be of the first type in which defects are produced and excited while TBL-II results only from excitation of defects previously produced. TBL-III may result from a combination of these effects since defects
ative. For TBL-III donors at crack surfaces may be involved. This is supported by a striking agreement between absorption and emission from surface states in alkaline earth oxides and absorption and emission from
already present would be excited and new defects possibly of a slightly different character might be produced and excited.
deformed alkaline earth oxides. These ideas will be explored more fully in work to be published. Finally, it should be noted that the results in Fig. 3
The nature of these defects producing the emission
is open to speculation. We still believe that the most likely prospects are vacancy clusters, with the emission resulting from an excited state of a donor oxygen ion adjacent to the vacancy cluster. These oxygen ions adjacent to vacancy clusters will act as donors because the Madelung field is required in order for them to retain their second electrons. This results from the positive electron affinity of oxygen for a second elec-
are readily explained by assuming that upon reappli-
cation of a load, dislocations immediately bow out and excite donors producing a jump in emission. This then gradually increases upon nonconservative dislocation motion, which produces the vacancy clusters.
1.
tron. Thus, the impact of a dislocation sweeping by such a donor might be to excite it, thus, stimulating
2.
emission. This could explain TBL-II. The occurrence of TBL-I may result from vacancies and free electrons produced during the plastic deformation or it may result from the exciiation of donors as vacancies are produced or both of these mechanisms may be oper-
3.
REFERENCES J.I. Zink,J. Phys. Chem 80,3,248(1976). G. Aizetta eta!., Phys. Status Solidi (a) 1, 775
(1970). 4. 5.
C.L. Reynolds Jr. & F.E. Wawner,Phys. Status Solidi (a) 36, I.Kl09 (1976). L. Michalski, Swiderski & T. Niemyski,Mater. Res Bull 6 41(1971) Y. Chen. M.M. Abraham, T.J. Turner & C. Nelson, Phil. Mag. 32, 99 (1975).