- Email: [email protected]
Intrinsic V- and Vo centers in single crystal SrO
Solid State Communications, Vol. 21, pp. 883—885, 1977.
Pergamon Press.
Printed in Great Britain
0 CENTERS IN SINGLE CRYSTAL SrO
INTRINSIC V~AND V
Lawrence A. Kappers Department of Physics and Institute of Materials Science, University of Connecticut, Storrs, Conn.
06268
(Received 3 November 1976; in revised form 9 December 1976 by R.H. Silsbee)
The intrinsic V~center (a positive hole trapped at a cation vacancy) has been observed in single crystals of SrO following x—irradiation at 77 K. The ESR spectrum corresponds to a spin—l/2 defect having <100> axial symmetry with g; 1 = 2.0012(3) and g1 = 2.0703(3). The concentra0 centers (two holes trapped tion of V centers was enhanced significantly by heating crystals in at a cation vacancy) were alsoquenching. observed after x—irradiatlon at low oxygen at 1000°Cand rapidly V temperature, with g 11 = 2.0011(3), gj = 2.0751(3), and D = 380.4(5) MHz.
Introduction Vacancy—associated hole centers may be formed in the cubic alkaline—earth oxides by ionizing radiation at low temperature. The intrinsic V— center is produced when a single hole is trapped adjacent to a cation vacancy which acts as a site of effective negative charge in the host lattice. The V center~-is a paramagnetic (S=l/2) defect which possesses tetragonal symmetry about a [l00]—typedirec— tion. The simple behavior of the intrinsic V center is often complicated by the tendency of the cation vacancy to associate with impuri— ties. Therefore, a number of related types of trapped—hole centers are produced in many crystals. VF, VA1 and In V°centers MgO for example, have all the been VOH, observed. VOD, (In this notation the subscripts refer to charge—compensating impurities located in neighboring positions.) The intrinsic V0 cen— ter consists of two holes trapped by a positive— ion vacancy, and its properties have been exa— mined thoroughly)-3 Background The information available for trapped—hole centers in SrO is less extensive than for MgO and CaO. The VOH center in SrO has b 2en identified with ENDOR by Blake et al. with 5 haveRecent identified g11 = 2.0010 and gj = 2.0750. ENDOR the VF studies center by in Abraham SrO with et al. g 11 2.0014 and g~ 2.0736. They also performed measurements for the V0~center and their results agree with intrinsic V—centers is more Assignment questionableofinthe those previously reported. SrO, and only limited information is available. Culvahouse et al.6 have observed a hole—like center in neutron—irradiated crystals described by g 1 = 2.0703 and g11 = 2.0010. They tenta— tively assigned the defect to the V center, however, the signals decayed shortly after the 7’8 have made irradiation and an extensive investigation was not an investigation carried out. of Tench polycrystalline and Duck SrO which
was irradiated with 20—MeV protons. Four ESR signals were observed in their samples having gj values of 2.0816, 2.0751, 2.0734 and 2.0667. The individual g11 values could not be resolved in their powder spectra due to overlap and interfering signals from other centers. The center with gj = 2.0734 was attributed to a VN center, where H refers to a charge—compensating impurity, and the center having g1 = 2.0751 was assigned to the VOH center. The ESR signal at g1 = 2.0667 was attributed to the intrinsic V~ center. The assignment was based on its much higher stability at room temperature (‘v.5 mm.) compared with the other V—type centers (several seconds). Additional evidence supporting this assignment was provided by the observance of V° 0 centeratare centers following y—irradiation 77 gj K. = Their reported 2.0713 andvalues D = 368 forMHz the in V the proton—irradiated SrO powder. Results and Discussion In the present study single crystals of SrO were obtained from W. & C. Spicer, Ltd., St. Mary’s Winchcombe, England. Defects were produced at 77 K by irradiation from a 50—kV X—ray source. The ESR spectrometers were Varian E—3 and E—12 systems operating with 100—kllz modulation, and all g—values were measured using proton magnetic resonance. Several strong paramagnetic resonance signals ing irradiation 77 K. The strongest were observed in at as—grown SrO two crystals follow— signals were observed to have gj values of 2.0751(3) and 2.0735(3), which agree well with 4’5 A much weaker the reported values for the VOH resonance and VF centers, exhibiting V—like character was observed with respectively. gj = 2.0703(3) in the as—grown crystals. For SrO crystals which had been quenched from high temperature, the ESR results were strikingly different. The concentration of centers with g1 2.0703 was enhanced significantly by heating crystals in vacuum and rapidly quench— fold achieved by heating 1mg. enhancement, An even morewas dramatic effect, about 30— crystals at 1000°Cin oxygen (1 atm) and
883
884
INTRINSIC V
AND V° CENTERS IN SINGLE CRYSTAL SrO
quenching by direct immersion in liquid nitro— gen. Figure 1(a) shows the ESR spectrum for 2 exhibits such a crystal after x—irradiation for 1 <111> hr at symmetry a detailed discussion will be 77 K. Theanddefect labeled 0H presented in another manuscript.9
Vol. 21, No. 9
center are g
11 = 2.0011(3), g1 = 2.0751(3) and D = 380.4(5) MHz. A measure of the separation byl° R between the two trapped holes may be obtained hcD = —(2g 2 ÷ g 2)p~/2R3, (1) if one assumes 11 the zero—field splitting is due 1 entirely to a dipolar interaction. From the experimental values R equals 5.96 ~ for the V0 center compared with 5.08 ~ for the normal
V
oxygen—oxygen spacing in SrO. Similar increases of R from the normal lattice spacing have been observed for MgO and Ca0J~- Figure 2 shows the ESR angular variation of the intrinsic V and V° centers for rotation in the (100) plane.
0~
v; OH’
_Th~&
s4._____4~._..I1.,~
3400
~
I
I
I
I
0,
3300 ~50G.I
(a)
—J Ui
C-)
~ 3200
___________________________________
Iii z 4
v; v; 3100 —.‘
s—
(b)
Fig. 1(a) ESR spectrum at 90 K of single crystal SrO which has been heated (1000°C) in oxygen for 1 hr and quenched in liquid nitrogen. Measurement was obtained after a 1—hr x—irradia— tion at 77 K for V = 9.15 GHz and [001]. The line marked (V) 11 is due to coincident lines from those VOH, VF and V centers having their tetragonal axes parallel to the magnetic field. (b) ESR spectrum at 90 K of the same crystal after a 10—mm anneal in the dark at 270 K.
~~II
In this paper the center having gj = 2.0703(3) is attributed to the intrinsic V center. The center is described by the array: 0 — [Vac] — 0, and possesses <100> axial symmetry. A value for g11 could not be extrac— ted from Fig. 1(a) because of overlapping lines fron the VON and VF centers in this orientation, however, a value of gIl = 2.0012(3) was obtained for the V center by annealing at 270 K for 10 mm. At this temperature the VOH and VF centers are unstable in SrO, and the annealing treatment left an isolated spectrum of the intrinsic V— center as shown in Fig. 1(b). X—irradiation of the quenched samples also produced centers which have trapped a 0 center and is The described pair of holes at a cation vacancy. defect by linear is the referred to array: as a V 0 — [Vac] — 0, where the holes are trapped on opposite sides of the vacancy. The measured values for this spin—l
I
I
-________________
I
I
ANGLE (dsg) 0 (solid line, circle) dependence and V (dashed line, open Fig. 2 closed ESR angular of the V circle) centers in SrO for rotation of the magnetic field in the (100) plane. __________________________________________ 1.5
1’-~ / \
-
/
-
z
\
I
0
\
/ -.
.
.
1
\.—S44_~_I_~&-J_-1-’(’
,~,
a
k
o Ui N
-
U
-
‘4
\
~0.5 0 z
~OH
I
\ I
I
I
I
I
0 100
ANNEAUNG
200 TEMPERATURE (K)
300
Fig. 3 Decay of paramagnetic centers as a function of temperature for 5—minute isochronal anneals at successively higher temperature. Measurements were taken at 90 K and all inten— sities were initially normalized to unity at that temperature. studyThe are results shown in 3. Measurements of Fig. an isochronal pulsed were anneal taken in 10—degree intervals and reveal that the VOH, VF and V°centers become thermally unstable in the same region near 240 K. The decay of
Vol. 21, No. 9
INTRINSIC V
AND V°CENTERS IN SINGLE CRYSTAL SrO
these centers is accompanied by an increase in the concentration of V centers. These results are satisfying and consistent with the assign— ment of the V~center. The greatly enhanced thermal stability is a characteristic feature of V center and in the Mg&- ~ntrimsic ,13 and CaO)-~-,~-4 This has is been due toobserved the deeper potential well for the trapped hole; all of the other known V—type centers already have partial charge compensation. The increase in for the decay of V—type centers. the by concentration of the V other centers is accounted For example, the loss of a single hole from the V°center results in its conversion to a V center. For MgO, where the V center is stable at room temperature, the sum of the concentra— tions of V°and V centers was observed to remain constant at 295 K.3 At elevated temper— atures (~28O K) the ESR spectrum of the V center in SrO consists of a single broad iso— tropic line at Ni~o = 2.047. The isotropic line is indicative of motional averaging and arises from the rapid hopping of the hole among the six oxygen ions about the vacancy.1-2 The g value of the isotropic line should be giso = (2g 1+ g11)/3 = 2.0473 in good agreement with observation. In MgO, the isotropic line was measured at g~so = 2.027, compared with (2g1 + 12 g11)/3 = 2.0267, and was shown to be a charac— Notional been observed for teristic averaging feature of has the not intrinsic V— center. any of the other vacancy type centers mentiSned in the introduction. Even the V~j~ center~ ,15 which differs only by a substitutional Al~~+ ion located two lattice spaces from the vacancy~ does not exhibit motional averaging in MgO.-’-2 The intrinsic V and V°centers studied in this report were produced by trapping one or two holes adjacent to an isolated strontium vacancy. The assignments are based on the following: (1) the relative persistence of the V center to resist thermal decay, (2) the ob— servation of a single isotropic line at 280 K which indicates notional averaging, (3) the production of V°centers which are formed only when isolated vacancies exist, and (4) a VO±V center conversion which accompanies the thermal decay of the V°center. In crystals (as— received) the number of V centers observed
following x—irradiation at 77 K was indeed small. The success of this investigation and the enhancement in V~concentration may be attributed to the rapid quenching treatments from high temperatures. Similar results have 12 and CaO.~-4 In SrO, heat been reported Mg0 appear mere efficient than treatments in for oxygen vacuum treatments. The intrinsic V center studied herein appears to be the same 6center 7’8 to The be the reported earlier by Culvahouse et al. V and V°centers were not and observed centers reported by Tench Duck in this in— vestigation. In fact, no defects were observed in these single crystals which show the spin— Hamiltonian values they report for these cen— ters. Another difference is in the stability of the V0 center, for they report that the V center increases while the V0 center decays, but the other V—type centers are unaffected. In this study the VOH, VF and V°all decayed near 240 K. Similar differences in g values for the intrinsic V centers were also reported in CaO between the single crystal results of Abraham et al.11 and the polycrystallmne data of Tench and Duck.7’8 An explanation for these discre— pancies is unavailable at the present time. After completing this work it came to the author’s attention that a completely independent and co—workers’6 the Oak Ridge investigation wasatperformed in SrONational by Rubio Lab— 0. oratory which yielded similar results. It should be emphasized, however, that distinctly different means were used to produce the in— trinsic centers. In their study fast—particle or high dose y—ray irradiations were used to create the defects. In this study, the centers were produced by x—irradiation in crystals which were heated in oxygen and rapidly quenched. It is satisfying that their results for the intrinsic centers are in agreement with the data reported herein, however, the discre— pancies with the work of Tench and Duck7’8 remain unexplained.
Acknowledgement—The author wishes to express his appreciation to Professor L. E. Halliburton and Drs. Y. Chen and N. M. Abraham for helpful comments and discussion.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
885
WERTZ, J.E., AUZINS, P., GRIFFITHS, J.H.E. and ORTON, J.W., Disc. Far. Soc. 28, 136 (1959). ROSE, B.H. and HALLIBURTON, L.E., J. Phys. C: Solid State Phys. 7, 3981 (1974). KAPPERS, L.A., DRAVNIEKS, F. and WERTZ, J.E., J. Phys. C: Solid State Phys. 7, 1387 (1974). BLAKE, W.B.J., GITELSON, H.A. and WERTZ, J.E., J. Phys. C. 4, 261 (1971). ABRAHAM, N.M., CHEN, Y. and RUBIO O.,J., Phys. Rev. B14, 2603 (1976). CULVAHOUSE, J.W., HOLROYD, L.V. and KOLOPUS, J.L., Phys. Rev. 140, 1181 (1965). TENCH, A.J. and DUCK, N.J., Solid State Comnun. 15, 333 (1974). TENCH, A.J. and DUCK, M.J., J. Phys. C: Solid State Phys. 8, 257 (1975). HALLIBURTON, L.E., NORMAN, C.D., SARA, K. and RAPPERS, L.A., to be published. ABRAGAN, A. and BLEANEY, B., Electron Paramagnetic Resonance of Transition Ions, p. 508, Oxford University Press (1970). ABRAHAM, N.M., CHEN, Y., BOATNER, L.A. and REYNOLDS, R.W., Solid State Commun. 16, 1209 (1975). HALLIBURTON, L.E., RAPPERS, L.A., COWAN, D.L., DRAVNIEKS, F. and WERTZ, J.E., Phys. Rev. Lett. 30, 607 (1973). UNROll, W.P., dEN, Y. and ABRAHAM, N.M., Phys. Rev. Lett. 30, 446 (1973). HENDERSON, B. and TONLINSON, A.C., 3. Phys. Chem. Solids 30, 1801 (1969). DuVARNEY, R.C. and GARRISON, A.K., Solid State Commun. 12, 1235 (1973). RUBIO 0., J., TOHVER, H.T., CHEN, Y. and ABRAHAM, N.M., to be published in Phys. Rev. B.
Pergamon Press.
Printed in Great Britain
0 CENTERS IN SINGLE CRYSTAL SrO
INTRINSIC V~AND V
Lawrence A. Kappers Department of Physics and Institute of Materials Science, University of Connecticut, Storrs, Conn.
06268
(Received 3 November 1976; in revised form 9 December 1976 by R.H. Silsbee)
The intrinsic V~center (a positive hole trapped at a cation vacancy) has been observed in single crystals of SrO following x—irradiation at 77 K. The ESR spectrum corresponds to a spin—l/2 defect having <100> axial symmetry with g; 1 = 2.0012(3) and g1 = 2.0703(3). The concentra0 centers (two holes trapped tion of V centers was enhanced significantly by heating crystals in at a cation vacancy) were alsoquenching. observed after x—irradiatlon at low oxygen at 1000°Cand rapidly V temperature, with g 11 = 2.0011(3), gj = 2.0751(3), and D = 380.4(5) MHz.
Introduction Vacancy—associated hole centers may be formed in the cubic alkaline—earth oxides by ionizing radiation at low temperature. The intrinsic V— center is produced when a single hole is trapped adjacent to a cation vacancy which acts as a site of effective negative charge in the host lattice. The V center~-is a paramagnetic (S=l/2) defect which possesses tetragonal symmetry about a [l00]—typedirec— tion. The simple behavior of the intrinsic V center is often complicated by the tendency of the cation vacancy to associate with impuri— ties. Therefore, a number of related types of trapped—hole centers are produced in many crystals. VF, VA1 and In V°centers MgO for example, have all the been VOH, observed. VOD, (In this notation the subscripts refer to charge—compensating impurities located in neighboring positions.) The intrinsic V0 cen— ter consists of two holes trapped by a positive— ion vacancy, and its properties have been exa— mined thoroughly)-3 Background The information available for trapped—hole centers in SrO is less extensive than for MgO and CaO. The VOH center in SrO has b 2en identified with ENDOR by Blake et al. with 5 haveRecent identified g11 = 2.0010 and gj = 2.0750. ENDOR the VF studies center by in Abraham SrO with et al. g 11 2.0014 and g~ 2.0736. They also performed measurements for the V0~center and their results agree with intrinsic V—centers is more Assignment questionableofinthe those previously reported. SrO, and only limited information is available. Culvahouse et al.6 have observed a hole—like center in neutron—irradiated crystals described by g 1 = 2.0703 and g11 = 2.0010. They tenta— tively assigned the defect to the V center, however, the signals decayed shortly after the 7’8 have made irradiation and an extensive investigation was not an investigation carried out. of Tench polycrystalline and Duck SrO which
was irradiated with 20—MeV protons. Four ESR signals were observed in their samples having gj values of 2.0816, 2.0751, 2.0734 and 2.0667. The individual g11 values could not be resolved in their powder spectra due to overlap and interfering signals from other centers. The center with gj = 2.0734 was attributed to a VN center, where H refers to a charge—compensating impurity, and the center having g1 = 2.0751 was assigned to the VOH center. The ESR signal at g1 = 2.0667 was attributed to the intrinsic V~ center. The assignment was based on its much higher stability at room temperature (‘v.5 mm.) compared with the other V—type centers (several seconds). Additional evidence supporting this assignment was provided by the observance of V° 0 centeratare centers following y—irradiation 77 gj K. = Their reported 2.0713 andvalues D = 368 forMHz the in V the proton—irradiated SrO powder. Results and Discussion In the present study single crystals of SrO were obtained from W. & C. Spicer, Ltd., St. Mary’s Winchcombe, England. Defects were produced at 77 K by irradiation from a 50—kV X—ray source. The ESR spectrometers were Varian E—3 and E—12 systems operating with 100—kllz modulation, and all g—values were measured using proton magnetic resonance. Several strong paramagnetic resonance signals ing irradiation 77 K. The strongest were observed in at as—grown SrO two crystals follow— signals were observed to have gj values of 2.0751(3) and 2.0735(3), which agree well with 4’5 A much weaker the reported values for the VOH resonance and VF centers, exhibiting V—like character was observed with respectively. gj = 2.0703(3) in the as—grown crystals. For SrO crystals which had been quenched from high temperature, the ESR results were strikingly different. The concentration of centers with g1 2.0703 was enhanced significantly by heating crystals in vacuum and rapidly quench— fold achieved by heating 1mg. enhancement, An even morewas dramatic effect, about 30— crystals at 1000°Cin oxygen (1 atm) and
883
884
INTRINSIC V
AND V° CENTERS IN SINGLE CRYSTAL SrO
quenching by direct immersion in liquid nitro— gen. Figure 1(a) shows the ESR spectrum for 2 exhibits such a crystal after x—irradiation for 1 <111> hr at symmetry a detailed discussion will be 77 K. Theanddefect labeled 0H presented in another manuscript.9
Vol. 21, No. 9
center are g
11 = 2.0011(3), g1 = 2.0751(3) and D = 380.4(5) MHz. A measure of the separation byl° R between the two trapped holes may be obtained hcD = —(2g 2 ÷ g 2)p~/2R3, (1) if one assumes 11 the zero—field splitting is due 1 entirely to a dipolar interaction. From the experimental values R equals 5.96 ~ for the V0 center compared with 5.08 ~ for the normal
V
oxygen—oxygen spacing in SrO. Similar increases of R from the normal lattice spacing have been observed for MgO and Ca0J~- Figure 2 shows the ESR angular variation of the intrinsic V and V° centers for rotation in the (100) plane.
0~
v; OH’
_Th~&
s4._____4~._..I1.,~
3400
~
I
I
I
I
0,
3300 ~50G.I
(a)
—J Ui
C-)
~ 3200
___________________________________
Iii z 4
v; v; 3100 —.‘
s—
(b)
Fig. 1(a) ESR spectrum at 90 K of single crystal SrO which has been heated (1000°C) in oxygen for 1 hr and quenched in liquid nitrogen. Measurement was obtained after a 1—hr x—irradia— tion at 77 K for V = 9.15 GHz and [001]. The line marked (V) 11 is due to coincident lines from those VOH, VF and V centers having their tetragonal axes parallel to the magnetic field. (b) ESR spectrum at 90 K of the same crystal after a 10—mm anneal in the dark at 270 K.
~~II
In this paper the center having gj = 2.0703(3) is attributed to the intrinsic V center. The center is described by the array: 0 — [Vac] — 0, and possesses <100> axial symmetry. A value for g11 could not be extrac— ted from Fig. 1(a) because of overlapping lines fron the VON and VF centers in this orientation, however, a value of gIl = 2.0012(3) was obtained for the V center by annealing at 270 K for 10 mm. At this temperature the VOH and VF centers are unstable in SrO, and the annealing treatment left an isolated spectrum of the intrinsic V— center as shown in Fig. 1(b). X—irradiation of the quenched samples also produced centers which have trapped a 0 center and is The described pair of holes at a cation vacancy. defect by linear is the referred to array: as a V 0 — [Vac] — 0, where the holes are trapped on opposite sides of the vacancy. The measured values for this spin—l
I
I
-________________
I
I
ANGLE (dsg) 0 (solid line, circle) dependence and V (dashed line, open Fig. 2 closed ESR angular of the V circle) centers in SrO for rotation of the magnetic field in the (100) plane. __________________________________________ 1.5
1’-~ / \
-
/
-
z
\
I
0
\
/ -.
.
.
1
\.—S44_~_I_~&-J_-1-’(’
,~,
a
k
o Ui N
-
U
-
‘4
\
~0.5 0 z
~OH
I
\ I
I
I
I
I
0 100
ANNEAUNG
200 TEMPERATURE (K)
300
Fig. 3 Decay of paramagnetic centers as a function of temperature for 5—minute isochronal anneals at successively higher temperature. Measurements were taken at 90 K and all inten— sities were initially normalized to unity at that temperature. studyThe are results shown in 3. Measurements of Fig. an isochronal pulsed were anneal taken in 10—degree intervals and reveal that the VOH, VF and V°centers become thermally unstable in the same region near 240 K. The decay of
Vol. 21, No. 9
INTRINSIC V
AND V°CENTERS IN SINGLE CRYSTAL SrO
these centers is accompanied by an increase in the concentration of V centers. These results are satisfying and consistent with the assign— ment of the V~center. The greatly enhanced thermal stability is a characteristic feature of V center and in the Mg&- ~ntrimsic ,13 and CaO)-~-,~-4 This has is been due toobserved the deeper potential well for the trapped hole; all of the other known V—type centers already have partial charge compensation. The increase in for the decay of V—type centers. the by concentration of the V other centers is accounted For example, the loss of a single hole from the V°center results in its conversion to a V center. For MgO, where the V center is stable at room temperature, the sum of the concentra— tions of V°and V centers was observed to remain constant at 295 K.3 At elevated temper— atures (~28O K) the ESR spectrum of the V center in SrO consists of a single broad iso— tropic line at Ni~o = 2.047. The isotropic line is indicative of motional averaging and arises from the rapid hopping of the hole among the six oxygen ions about the vacancy.1-2 The g value of the isotropic line should be giso = (2g 1+ g11)/3 = 2.0473 in good agreement with observation. In MgO, the isotropic line was measured at g~so = 2.027, compared with (2g1 + 12 g11)/3 = 2.0267, and was shown to be a charac— Notional been observed for teristic averaging feature of has the not intrinsic V— center. any of the other vacancy type centers mentiSned in the introduction. Even the V~j~ center~ ,15 which differs only by a substitutional Al~~+ ion located two lattice spaces from the vacancy~ does not exhibit motional averaging in MgO.-’-2 The intrinsic V and V°centers studied in this report were produced by trapping one or two holes adjacent to an isolated strontium vacancy. The assignments are based on the following: (1) the relative persistence of the V center to resist thermal decay, (2) the ob— servation of a single isotropic line at 280 K which indicates notional averaging, (3) the production of V°centers which are formed only when isolated vacancies exist, and (4) a VO±V center conversion which accompanies the thermal decay of the V°center. In crystals (as— received) the number of V centers observed
following x—irradiation at 77 K was indeed small. The success of this investigation and the enhancement in V~concentration may be attributed to the rapid quenching treatments from high temperatures. Similar results have 12 and CaO.~-4 In SrO, heat been reported Mg0 appear mere efficient than treatments in for oxygen vacuum treatments. The intrinsic V center studied herein appears to be the same 6center 7’8 to The be the reported earlier by Culvahouse et al. V and V°centers were not and observed centers reported by Tench Duck in this in— vestigation. In fact, no defects were observed in these single crystals which show the spin— Hamiltonian values they report for these cen— ters. Another difference is in the stability of the V0 center, for they report that the V center increases while the V0 center decays, but the other V—type centers are unaffected. In this study the VOH, VF and V°all decayed near 240 K. Similar differences in g values for the intrinsic V centers were also reported in CaO between the single crystal results of Abraham et al.11 and the polycrystallmne data of Tench and Duck.7’8 An explanation for these discre— pancies is unavailable at the present time. After completing this work it came to the author’s attention that a completely independent and co—workers’6 the Oak Ridge investigation wasatperformed in SrONational by Rubio Lab— 0. oratory which yielded similar results. It should be emphasized, however, that distinctly different means were used to produce the in— trinsic centers. In their study fast—particle or high dose y—ray irradiations were used to create the defects. In this study, the centers were produced by x—irradiation in crystals which were heated in oxygen and rapidly quenched. It is satisfying that their results for the intrinsic centers are in agreement with the data reported herein, however, the discre— pancies with the work of Tench and Duck7’8 remain unexplained.
Acknowledgement—The author wishes to express his appreciation to Professor L. E. Halliburton and Drs. Y. Chen and N. M. Abraham for helpful comments and discussion.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
885
WERTZ, J.E., AUZINS, P., GRIFFITHS, J.H.E. and ORTON, J.W., Disc. Far. Soc. 28, 136 (1959). ROSE, B.H. and HALLIBURTON, L.E., J. Phys. C: Solid State Phys. 7, 3981 (1974). KAPPERS, L.A., DRAVNIEKS, F. and WERTZ, J.E., J. Phys. C: Solid State Phys. 7, 1387 (1974). BLAKE, W.B.J., GITELSON, H.A. and WERTZ, J.E., J. Phys. C. 4, 261 (1971). ABRAHAM, N.M., CHEN, Y. and RUBIO O.,J., Phys. Rev. B14, 2603 (1976). CULVAHOUSE, J.W., HOLROYD, L.V. and KOLOPUS, J.L., Phys. Rev. 140, 1181 (1965). TENCH, A.J. and DUCK, N.J., Solid State Comnun. 15, 333 (1974). TENCH, A.J. and DUCK, M.J., J. Phys. C: Solid State Phys. 8, 257 (1975). HALLIBURTON, L.E., NORMAN, C.D., SARA, K. and RAPPERS, L.A., to be published. ABRAGAN, A. and BLEANEY, B., Electron Paramagnetic Resonance of Transition Ions, p. 508, Oxford University Press (1970). ABRAHAM, N.M., CHEN, Y., BOATNER, L.A. and REYNOLDS, R.W., Solid State Commun. 16, 1209 (1975). HALLIBURTON, L.E., RAPPERS, L.A., COWAN, D.L., DRAVNIEKS, F. and WERTZ, J.E., Phys. Rev. Lett. 30, 607 (1973). UNROll, W.P., dEN, Y. and ABRAHAM, N.M., Phys. Rev. Lett. 30, 446 (1973). HENDERSON, B. and TONLINSON, A.C., 3. Phys. Chem. Solids 30, 1801 (1969). DuVARNEY, R.C. and GARRISON, A.K., Solid State Commun. 12, 1235 (1973). RUBIO 0., J., TOHVER, H.T., CHEN, Y. and ABRAHAM, N.M., to be published in Phys. Rev. B.