V- and V0 centers in CaO single crystals

Solid State Communications, Vol. 16, pp. 1209—12 13, 1975.

Pergamon Press.

Printed in Great Britain

V AND V°CENTERS IN CaO SINGLE CRYSTALS* -

M.M. Abraham and Y. Chen Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, U.S.A. and L.A. Boatnert Physics Department, Texas Christian University, Ft. Worth, Texas 76129, U.S.A. and R.W. Reynolds Advanced Technology Center, Dallas, Texas 75222, U.S.A. (Received 10 December 1974 by C. W. McCombie) The stable, intrinsic V center in CaO single crystals has been produced at room temperature by high-dose electron irradiation. This center, which consists of a hole trapped at an oxygen ion adjacent to a calcium vacancy, exhibits an EPR spectrum at 77 K which has (100) axial symmetry and is described byS = 1/2,g11 = 2.0021(2) andgj = 2.0697(2). At room temperature, the spectrum consists of a single thermally-averaged isotropic line atg = 2.047(1). Upon y-irradiation at 77 K, the isolated V concentration is reduced and V°centers (pairs of trapped holes at single vacancies) are produced. A room-temperature thermal anneal reverses the effect of the low-temperature y-irradiation. THE STUDY of irradiation-produced trapped-hole centers in the cubic alkaline—earth oxides was initiated by Wertz et al. who used EPR techniques to investigate X-irradiated MgO crystals. These workers observed an EPRthe spectrum which symmetry about principal (100>exhibited axes, andtetragonal they initially postulated that the corresponding defect had the following 3~where M3~ linear array: 0— [Mgvacancy] is a tripositive metaffic ion and 0—O—M represents an oxygen ion with a localized hole. In subsequent years,2’3 however, due in part to the fact that no EPR hyperfine interaction of the hole with a tripositive ion was de-

tected, the defect became known as the V center which has the configuration: 0—[Mg vacancy] —0=.

,~

______________

Research sponsored by the U.S. Atomic Energy Commission under contract with Union Carbide Corporation. t Present address: Lab. Phys. Exp., Ecole Polytechni4ue, Fédérale de Lausanne, Lausanne, Switzerland. *

1209

Recently, using the sensitve electron—nuclear 4’5 double resonance (ENDOR) technique, itcenters was found that in MgO two species of trapped-hole have the same set of g-values [g 1~= 2.0033(2) and g1centers = 2.0386(2)] at low temperatures. One of these is an Al3~charge-compensated defect called the VA! center. The other center is the intrinsic V center. These two defects can be distinguished at low temperatures either by the observation of aluminum ENDOR signals or by a careful ENDOR study of their different 25Mg superhyperfine structures.5 They may also be distinguithed by the fact that at elevated temperatures their EPR spectra differ. The magnetic field positions of the broadened VA! lines are identical to the lowtemperature positions while for the V center only a

1210

V AND V°CENTERS IN CaO SINGLE CRYSTALS

single isotropic line appears at the average position of the low-temperature anisotropic lines,6 i.e. giso = (g~ 1+ 2g1)/3. At higher temperatures, the signal amplitudes are considerably smaller than the amplitudes at low temperature and, accordingly, a high concentration of defects is needed for this type of distinction of the two centers by EPR. An additional means of disting tinguishing between7these two centers is based their At room temperature, theonVA! different center hasstabilities. a half life of hours, while the V center is more stable, and its half life is of the order of years.4’5 The formation of V centers in single crystal MgO 8 but once it is possible is strongly sample dependent,sample, then V° centers to form them in a particular (pairs of trapped holes associated with single vacancies) can also be formed. These V0 centers, which were first observed by Wertz, et a!. ,1 are unstable at room ternperature and will decay into stable V centers. Observation of V0 centers therefore provides sufficient assurance that the V center can exist in the crystal. The situation reg~dingthe formation and identification of V centers in CaO is nota as clearinasCaO it is for 9 observed center the MgO Shuskus which wascase. induced by low-temperature X-ray irradiation, and whose EPR spectrum exhibited axial syrnmetry about the (100) axes with g 11 = 2.0011(4) and g1 = 2.0710(4). Although the center was not thermally stable at room temperature, it was believed to be the CaO analogue of theable MgOto: observe V center. Henderson 1°were a highly stable and Tomlinson “V center” in CaO following only a heat treatment. They did not report any g values for this center. After low-temperature u.v. excitation of the sample, they did, however, report the observation of the EPR spectrurn of the paired V°center with g~ 2.0023, g1 =in 2.0683 and D = 0.0106 cm’. Tench1=and Duck,11 studies of proton-irradiated polycrystalhine CaO powders at 77 K observed a signal at g 1 = 2.0672 which had a half life of two days. They attributed this spectrum to the intrinsic V center. They also observed an able isotropic spectrum 300 K withg = 2.0468 and were to produce theatV°center by y-irradiation at 77 K. The D value was reported at be 110 x I 0~cm’ but no g values were reported. In addition, they observed two other centers in the CaO powder with half lives of about one minute each. One center with g 1 = 2.0705 was attributed to the VOH center, and the other with g1 = 2.0695 was designated as a

Vol. 16, Nos. 10/11

VM center which is associated with a charged impurity ion. No parallelg values were reported since resolution

between the signals from the various centers was not possible in the EPR powder spectrum. This paper reports on the room-temperature production of the intrinsic V center in CaO single crystals which were subjected to extensive irradiations of 2 MeV electrons (~ 10~e/cm2). This center is stable at room temperature (no significant change in intensity after nine months) and exhibits a 77 K EPR spectrum with (100) axial symmetry characterized by S = 1/2, g 11 = positions 2.0021(2)were andg1 = 2.0697(2). (All magnetic field measured using proton magnetic resonance). At 295 K, the spectrum consists of a single isotropic line atg = 2.047(1). The line width at 295 K is approximately 10 G while the line widths at 77 K are <0.5 G. The isotropic g value at room temperature compares very well with the average value of the 77 K g values, (g 0 + 2g1)/3 = 2.0472. [For the V center in in MgO, the isotropic room temperature g value was determined to be 2.0271(5) compared with the expected (g~= 2g1)/3 = 2.0268. See Table I.] An extensive low-temperature ENDOR was conducted on the CaO center in order to findsearch a possible association with a charge-compensating impurity similar to that found for the VA! center in MgO.4”2 No such association could be found. A subsequent y-irradiation at 77K of CaO crystals containing the V center reduced the V concentration and produced the V°center. Other trapped-hole also H formed by the unstable y-irradiation. Figure 1centers shows were that with perpendicular to the principal symmetry axis, the relative field positions of the EPR lines for the V, VF and VOD centers in CaO are the same as in MgO. The latter two centers were verified by ENDOR to be associated 13 and deuterium.14 (The VOD center is with fluorine in crystals that were intentionally doped only present with heavy water.) Figure 2 shows the complete EPR spectrum obtained after the low-temperature irradiation. The spin Hamiltonian parameters obtained for the V° centers areS = l,g~= 2.0021(2),g1 = 2.0733(2) and 1. Although no attempt D = made 114.08(5) x l0~cm was to determine the singlet—triplet splitting by monitoring the signal intensities as a function of ternperature, the absence of the V°signals at 4.2 K signifles that the S = 0 singlet is the ground state. If we assume that the fine-structure splitting D is due entirely to the dipolar interactions between the

Vol. 16, Nos. 10/11

VAND V°CENTERS IN CaO SINGLE CRYSTALS

1211

Table 1. Trapped-hole centers in MgO and CaO D g1

MgO

(i0~ cm’)

VOH VF

V

2.0033(2) 2.0032(2) 2.0033(2)

2.0398(2) 2.0390(2) 2.0386(2)

V°

2.0033(2)

2.0395(2)

13 13 4,5 [=

2.0018(2) 2.0017(2) 2.0021(2) 2.0021(2)

VOH VF

CaO

V°

Ref.

2.0729(2) 2.0719(2) 2.0697(2) 2.0733(2) [=

I

212.61(5) 227.33(5) G]

8 13 13 This work This work

114.08(5) 122.05(5) G] 2+ I

I

Mn

CaO i~9.01 GHz r 95 K ~II [100]

~ GHz

I gauSS

r-.-95K

~

“~~i~

F~

H-.-

0 (A) GAIN

10

FIG.

2. EPR spectrum of an electron-irradiated CaO after y-irradiation at 77 K.

I

I

I

~bo

VF

V

-

FIG. I. EPR spectra of trapped-hole centers in a CaO crystal with H perpendicular to the principal axis after: (a) electron irradiation at 295 K and (b) subsequent v-ray irradiation at 77 K.

value of R = 4.99 A compared with 4.20 A for the normal lattice spacing. As noted by Henderson and Tomlinson,’°the increase in deviation from the lattice constant in CaO compared with that in MgO may resuit from an exaggeration in the apparent separation of the pairs, due to the assumption of pure dipole— dipole coupling.

two positive holes, we may use the formula15 hcD

=

—

~

+ g~)~/~3

(1)

to calculate the distance R separating the holes. For CaO, we obtain a value of R = 6.16 A compared with the normal lattice spacing of 4.80 A between adjacent oxygens. A similar calculation for the V°center in MgO using the data tabulated in Table 1,8 yields a

The EPR spectra of V centers in both MgO and CaO were examined over the temperature range 77—300 K in order to determine similarities between the thermal averaging observed for these centers and the thermal averaging observed in the EPR spectra of ions displaying the Jahn—Teller effect. Significant differences were noted. In both hosts, the intensity of the

1212

VAND V°CENTERSINCaOSINGLECRYSTALS 7)

(x1o

ri——r~~

(x1017)

Isochronal annealing results (Fig. 3) show that in CaO the effects of low-temperature y-irradiation and thermal annealing are reversible. The V°,VF and VOD centers all begin to decay near 250 K, while the V concentration is enhanced. The sum of the V°and V applicable to the dynamics of the V center in MgO

-- -

--

-

concentrations indicating the the V°center initial increased isupon trapped ofannealing the a site center. at 295 other K, than a ahole V from site, only one Vpresence center will beatVformed forIf every

-—

____________

0

which cause thermal averaging in Jahn—Teller systems (i.e., rapid relaxation between vibronic states and

and CaO.population of excited vibronic states) are not thermal

15

05

Vol. 16,Nos. 10/il

~‘ ~

100

200 ANNEALING TEMPERATURE

~

0 300

FIG. 3. Concentration of trapped-hole centers in CaO as a function of annealing temperature. The concentrations were determined from the EPR signal intensities at 95 K after 5 mm. of isochronal annealing at each temperature.

averaged spectra for the V centers was independent of the orientation of the applied magnetic field and this is not the case for ions subject to the Jahn—Teller effect.’6 Moreover, the anisotropic and the isotropic (averaged) EPR spectra for the V centers were not observed to coexist at any temperature. This contrasts with the EPR spectra of ions characterized by either a dynamic or static Jahn—Teller effect at low temperature where the anisotropic and the isotropic spectra can be simultaneously observed over a certain temperature range.’6 Therefore, it appears that the mechanisms

V°-center, and the sum (V0 + V) would be conserved. On the other hand, if a hole from the V° is relocated at a V site, two V centers will result for every V° center, and the sum (2V° + V) would be conserved. If one neglects hole contributions from impurity-related hole centers, one would expect a situation bounded by these two extremes. For CaO (Fig. 3), the sum of (2 V°+ V) appears to be conserved, while for MgO, the conservation of the sum (V0 + V~)appeared to be valid.8

In summary, the intrinsic V center in CaO single crystals has been produced at room temperature by high-dose irradiation with 2 MeV electrons. The center exhibits an axial EPR spectrum at 77 K with g-values which are substantially different from the values reported previously.9’11 The arguments for identifying this defect as the V center are the same as those for MgO: (I) being negatively charged, the defect is stable at 295 K, (2) the EPR transitions seen at 77 K thermally average, producing a single isotropic line at 295 K, (3) no charge-compensating impurities could be detected using EPR and ENDOR techniques, aiid (4) y-irradiation at 77 K or CaO crystals containing V centers produces V°centers.

REFERENCES 1.

WERTZ i.E., AUZINS P., GIFFITSH J.H.E. and ORTON

J.W., Discussions Faraday Soc.

28. 136 (1959).

2.

HENDERSON B. and WERTZ J.E.,Adv. Phys. 17, 749 (1968).

3.

HUGHES A.E. and HENDERSON B.,Point Defects in Solids, (Edited by CRAWFORD J.tl., Jt. and SLIFKIN L.) Plenum, New York (1972).

4.

UNRUH W.P., CHEN Y. and ABRAHAM M.M.,Phys. Rev. Lett. 30,446(1973).

Vol. 16, Nos. 10/11

V AND V°CENTERS IN CaO SINGLE CRYSTALS

1213

5.

ABRAHAM M.M., CHEN Y. and UNRUH W.P.,Phys. Rev. B9, 1842 (1974).

6.

HALLIBURTON L.E., KAPPERS L.A., COWAN D.L., DRAVNIEKS F. and WERTZ i.E., Phys. Rev. Lett. 30, 607 (1973). CHEN Y., KOLOPUS J.L. and SIBLEY W.A.,Phys. Rev. 186, 865 (1969); TENCH A.i. and DUCK M.J., .1. Phys. C: Solid State Phys. 6, 1134 (1973).

7. 8.

CHEN Y., ABRAHAM M.M., TEMPLETON L.C. and UNRUH W.P.,Phys. Rev. B 11, 881 (1975).

9.

SHUSKUS A.J.,J. Chem. Phys. 39, 849 (1963).

10.

HENDERSON B. and TOMLINSON A.C., J. Phys. Chem. Solids 30, 1801 (1969).

11.

TENCH A.J. and DUCK M.J., Solid State Commun. 15, 333 (1974).

12. 13.

DUVARNEY R.C. and GARRISON A.K., Solid State Commun. 12, 1235 (1973). UNRUH W.P., CHEN Y. and ABRAHAM M.M.,J. Chem. Phys. 59, 3284 (1973).

14.

ABRAHAM M.M., UNRUH W.P. and CHEN Y. (to be published).

15.

ABRAGAM A. and BLEANEY B., Electron Paramagnetic Resonance of Transition Ions, p. 508, Oxford University Press (1970); See also: Reference 3, where the formula used in previous publications is corrected. HERRINGTON J.R., ESTLE T.L. and BOATNER L.A.,Phys. Rev. B3, 2933 (1971); BOATNER L.A., REYNOLDS R.W., ABRAHAM M.M., and CHEN Y.,Phys. Rev. Lett. 31,7(1973); REYNOLDS R.W., BOATNER L.A., ABRAHAM M.M. and CHEN Y., Phys. Rev. B 10,3802(1974).

16.