LOCALISED ELECTRONIC STATES OF DEFECT AGGREGATES IN NEUTRON IRRADIATED MAGNESIUM OXIDE

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LOCALISED ELECTRONIC STATES OF DEFECT

AGGREGATES IN NEUTRON IRRADIATED

MAGNESIUM OXIDE

B. Henderson, R. King

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C 4 , supplkment au no 8-9, Tome 28, Aozit-Septembre 1967, page C 4.75

LOCALISED ELECTRONIC STATES OF DEFECT AGGREGATES

IN N E U T ~ O N IRRADIATED MAGNESIUM

OXIDE

by B. HENDERSON

Physics Department, University of Keele, Keele, Staffordshire (England) and R. D. KING

Ceramics Centre, A. E. R. E. (Harwell) England.

Rksumk.

-

Les oxydes de terres alcalines ont la mgme structure cristalline que les halogknures alcalins, et par consequent on observe des defauts d'ktats electroniques et des proprietes de symetrie analogues aux centres M et R apres une irradiation aux neutrons. La divalence du rQeau rkcep- teur offre aussi un potentiel assez grand pour pieger les Bectrons : a cet Cgard les centres type R ont et6 particulikrement intkressants. En utilisant la technique de forces uniaxales, on a confirm6 que les raies optiques B zero phonons de 5 248

A,

6 419

A

et 6 490

A

sont associkes a ces centres-18. La raie de 5 248

A

est fortement dichro'ique sous charge B 4 OK, et les resultats indiquent une transition E -+ A. Le fait que les raies de 6 419

A

et 6 490

A

ne sont pas dichrofques confirme que ces raies sont associkes A. des transitions A +E. Le calcul des etats 6ectroniques de ces defauts trigo- naux suggkre que les raies de 5 248

A,

6 419

A

et 6 490

A

sont dues a des faisceaux de trois places vacantes d'anions dans un plan (1 1 l), contenant respectivement 5, 6 et 7 Bectrons.

En plus des defauts que l'on reconnait aisement par leurs caracteristiques type M et R, il y en a qui ne sont analogues a aucun dbfaut dej8 identifie parmi les halog&nures alcalins. Deux de ces dkfauts ont des 6tats triplets de spin (S = 1) a 300 OK, mais des etats singulets de spin a 4 OK. Les modeles pour ces defauts comprennent des paires couplBes de centres F e n Cchange en association alternee dans les directions [l001 (symetrie tetragonale) et [l 101 (symetrie orthorhombique). Un troisieme defaut, que Wertz nomme le centre Fz, a des propriCtCs optiques en accord avec celles que l'on calcule pour un seul electron lib a une paire de places vacantes anion-cation. Plus recemment, le modele propod pour ce defaut a ete fortement corrobore par des mesures de resonance parama- gnktique dans la bande Q.

Abstract.

-

The alkaline earth oxides have the same crystal structure as the alkali halides and consequently defects with electronic states and symmetry properties analogous with the M-and R-centers are observed after neutron irradiation. The divalent nature of the host lattice also offers additional potential for electron trapping : in this respect the R-like centres have been especially interesting. The application of uniaxial stress techniques has confirmed that optical zero-phonon lines at 5 248

A,

6 419

A

and 6 490

A

are associated with such centres. The line at 5 248

A

is strongly dichroic under stress at 4 OK and the results are indicative of E + A tansitions. The lack of such dichroic behaviour in the 6 419

A

and 6 490

A

lines confirm that these lines are associated wiht A -t E transitions. Calculation of the electronic states of theses trigonal defects suggest that the 5 248

A,

6 419

A

and 6 490

A

lines are due to clusters of three anion vacancies lying in a (111) plane and containing 5, 6 and 7 electrons respectively.

In addition to defects which are readily recognised on account of their M- and R-like characte- ristics, there are defects that are not analogous with any defects so far identified in the alkali halides. Two such defects have spin triplet states (S = 1) at 300 O K but have spin singlet states at 4 OK.

Models for these defects involve exchange coupled pairs of F-centres in next nearest neighbour association along the [l001 axes (tetragonal symmetry) and [l101 axes (orthorhombic symmetry). A third defect, designated the F2-centre by Wertz, has optical properties in accord with those expected for a single electron bound to an anion-cation vacancy pair. More recent paramagnetic

resonance measurements at Q-band strongly support the proposed model for this defect.

I. Introduction. - The alkaline earth oxides have expected t o be analogous with those observed in the the simple rock-salt crystal structure. Consequently alkali halides. However, the poor quality of commer- intrinsic lattice defects produced by irradiation .are cially available single crystals of these oxides, together

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C 4 - 7 6 B, HENDERSON AND R. D. KING

with the ineffectiveness of polarized bleaching techniques and the resistance of the oxides to damage by ionizing radiation have considerably hindered the progress towards understanding the electronic states of the intrinsic lattice defects. Detectable concentration of intrinsic lattice defects are in fact observed only in crystals exposed to fast neutron irradiation. Under such conditions almost equal numbers of anions and cations are displaced in magnesium oxide. Conse- quently, numerous complex defects are readily identified by their electron spin resonance spectrum in addition to the copious numbers of F-centres [l, 21.

The optical absorption spectrum of neutron irradiated magnesium oxide is not yet completely understood. The F-band is known to be centred at 2 500

A

[3,4, 51 :

however, a peak observed at 5 740

A

has not yet been assigned to any particular defect species. Bands at 9 500

A

and 3 500

a

show well resolved zero-phonon lines and phonon-assisted structure at temperatures below 77 OK. Ludlow [6] has suggested that these bands are due to transitions within the same centre :

the centre has orthorhombic symmetry and [l101 orientation with linear electric dipole oscillators parallel '

to the [l 101 direction. If magnesium oxide is irradiated at elevated temperatures (6000C) or annealed at temperatures > 3000C after irradiation, then the optical absorption spectrum is much more complex consisting of many overlapping bands [2, 4, 71. Uni- axial stress measurements on the zero-phonon lines observed at low temperatures to be associated with these bands, have indicated that defects are present in the crystals which may be analogous with the M, R and N-centres observed in the alkali halides. Some of the properties of the M and R-like centres are discussed in this paper.

In addition to the defects which have optical properties analogous with the F-aggregate defects in the alkali halides, a number of centres may be produced for which no such analogy exists. Evidence supporting the models proposed for these defects is discussed briefly.

11. Experimental procedure.

-

Magnesium oxide

single crystals were obtained from the Norton Com- pany, Worcester, Massachussetts, U. S. A. Specimens cleaved from large single crystal blocks were irradiated at temperatures in the range 150-600 OC in the Harwell D I D 0 reactor. Optical absorption measurements were made using either a Cary 14 recording spectrophoto- meter or a Hilger Universal 3 metre spectrograph. The uniaxial stress apparatus used at 4 OK with both the spectrometer and spectrograph have been described in the literature [7, 81 :

Electron spin resonance measurements were made at temperatures between 40-293 OK using a Varian super- heterodyne spectrometer ; 100 kc/s field modulation and a Fieldial regulated 9" rotatable magnet were used. Where necessary, illumination of the sample with visible or ultra-violet light was carried out in situ in the TE 102 microwave cavity via precisely machined radiation slots in the end wall of the cavity. An Osram 250 W high pressure mercury lamp was used together with a Hilger and Watts grating monochromater to select particular bleaching wavelengths.

111. Results and discussion.

(i) STUDIES OF OPTICAL ZERO-PHONON LINES. - The general features of the optical absorption spectrum of magnesium oxide after neutron irradiation at 600 OC are shown in figure 1. The spectrum was recorded at 77 OK and the presence of many zero-phonon lines in the range 5 000-7 000

A

indicates that the very strong absorption in this range is due to the presence of numerous overlapping broad bands. Observation of the zero-phonon lines shown in figure 1 was first reported by Wertz et al. [2]. The half-widths of these lines are strongly sensitive to radiation dose and to thermal treatment [7]. Such variations in the half- widths are apparently associated with changes in the magnitude of microscopic strains due to the presence of dislocations and point defect in the irradiated crystals. Under conditions of high resolution phonon-assisted transitions can be seen on the high energy side of the zero-phonon line. Detailed comparisons of the phonon-assisted structure with the phonon dispersion curves obtained from slow neutron scattering measurements have been made by numerous authors [6,7, S].

I

I I I I

3 000 40CO 5000 7040 10,000

WAVELENGTH (X)

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LOCALISED ELECTRONIC STATES OF DEFECT AGGREGATES C 4

-

77

(ii) UNIAXIAL STRESS MEASUREMENTS.

-

The Zero-

phonon lines at 4 684 and 4 700

A

behave similarly under an applied uniaxial stress (Fig. 2). Analysis of the stress splitting patterns confirms that these lines are associated with defects having [l101 orientations and orthorhombic symmetry with linear electric dipole oscillators along a [l 101 direction. Such symmetry properties are identical with those expected for M- centres ; recent evidence however, suggests that the production of these defects is impurity controlled [9].

/ \

lT P O L A R I S E D

C P O L A R I S E D

&A

-- 1 4

4 6 8 0 46FO 4 7 0 0 4 6 8 0 4 6 9 0 4 7 0 0 8

FIG. 2.

-

Stress-induced splittings of the 4684 d and 4700 A

lines at 77 OK in neutron irradiated MgO.

a) [l001 stress of 46 kg. mm-2.

6 ) [l 101 stress of 41.5 kg. mm -2 with light along [I l 0] (z-refers

to E parallel to stress and a refers to E-perpendicular to stress).

Similar analysis of the stress splitting patterns of the lines at 5 248 A, 6 419

A

and 6 490

A

show that the lines are due to A +--+ E transitions within defects having trigonal symmetries and [ l 1 l ] symmetry axes [7,10]. At 40K the 5248

A

line is strongly dichroic under uniaxial stress (Fig. 3). Such behaviour is expected only if the E-state is the ground state since the applied stress removes the orbital degeneracy of the E-ground s t a ~ e and the intensity ratio between the two optical transitions will be proportional to

exp

-

AEIkT, AE being the stress induced ground state

splitting. The compared intensities under stress at

77 OK and 4 O K shown in figure 3 are consistent with

those expected for a trigonal defect with optical dipole moments along the C1 101 and [l 121 directions in the plane perpendicular to the trigonal axis. Ground state splittings of 15 cm-' have been achieved under stress and for this splitting a population ratio between the two levels will be of order 11300 at 4 OK. The absence

of one component at 4 OK is in accord with these ideas and hence it is concluded that the defect associated with the 5 248

A

line has trigonal symmetry and the transition is from an E-ground state to an A-excited state. No such dichroism is observed for the 6 419

W

and 6 490

A

lines and the defects associated with these lines presumably have A-ground states.

THEORETICAL EXPERIMENTAL

I

FIG. 3.

-

a) Dichroism expected for an E - + A transition of a trigonal centre with [l101 and [l121 dipoles and stress along [loo].

b) Experimentally observed behaviour of 5248 A zkrophonon line under [l001 stress at 50 kg. mm-2.

Models for the centres associated with these three zero-phonon lines consist of three anion vacancies forming an equilateral triangle on the (1 11) plane and containing different numbers of trapped electrons : six are required for electrostatic neutrality. It is now instructive to use a molecular orbital approach to construct the most likely set of one electron levels for defects with appropriate symmetry containing five, six or seven electrons. The author has shown that for a five electron centre the ground level is

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C 4 - 7 8 B. HENDERSON AND R. D. KING

occur to a 2 Al[(l a,) (1 e)4] excited state. On the accumulated evidence for the proposed model of this basis of similar molecular orbital calculations, the defect is quite compelling.

ground states of defects containing six and seven elec- trons may be shown to be 'A, and ' A , respectively :

the expected excited states are then ' E and 2E. The

optical zero-phonon lines at 6 419

A

and 6 490

A

have been assigned to R-like centres containing six and seven electrons respectively [l l].

The discussion given above suggests that the defect responsible for the zero-phonon line at 5 248

A

should be paramagnetic. Since S =

3

for this five electron defect, the electron spin resonance spectrum for such a defect is expected to overlap the Pcentre spectrum. As a result, and since the F-centre concentration is nor- mally much larger than that of other defects, this spectrum has not previously been reported. However, in crystals irradiated at 600 OC the concentration of aggregate centres is approximately equal ot the F- centre concentration. Recent measurement at X-band have shown that in such crystals, defects are present which have anisotropic g-tensors and which might be associated with the defect in question. The lines occur at higher g-falues than the F-centre, but at X-band the separation is such that the detailed g-tensor anisotropy cannot be fully investigated. At G-band our investiga- tions are not complets, since the orientation dependence of the g-tensor has only been investigated for H, parallel to [l001 and to [l101 axes. The Q-band spectrum along the [ l 101 axis is shown in figure 4 : the results are in accord with centres with trigonal symmetry. Preli- minary g-values for this defect are g,, = 2.000 and g, = 2.018 : further examination of this spectrum are currently in progress. An enormous paramagnetic Faraday rotation pattern has also been associated with the 5 248

A

zero-phonon line. Although much remains to be done both theoretically and experimentally, the

I

&centre lines

F- centre l~ne

FIG. 4.

-

The Q-band electron spin resonance spectrum of

an R-like centre in magnesium oxide at 4 OK. The spectrum was

measured with Ho along [110].

(iii) ELECTRON SPIN RESONANCE STUDIES. - After long periods of irradiation numerous trapped hole centres may be recognized by their paramagnetic resonance spectrum [l]. In addition, the authors have recently shown that paramagnetic trapped electron centres may also be present. One such centre [13], Ft-centre (*), is believed to consist of two next nearest neighbour F-centres separated by a Mg2' vacancy : the defect therefore has a tetragonal axis along [loo]. The F-centres are coupled by an isotropic exchange interaction of the form ;

where S is the total spin and can take values S = 0 or S = 1. The triplet state is empty at 4 OK but almost

fully occupied at 300 OK and comparison of the

experimental signal intensities -give a value for

J/K = 80 f 10 OK. The paramagnetic resonance spec-

trum, shown in figure 5, may be described by a spin- Hamiltonian of the form

X = gPH.S

+

D(S:

-

113 S(S

+

l)} : the measured parameters are g = 2 0030

+

.000 5 and

Assuming that the anisotropic interaction parameter D is accounted for entirely by dipole-dipole coupling the estimated separation between the F-centers is 4.07

A

since D = 11 1 P2/(1 0 rz.). This value is consistent with the oxygen-oxygen separation along [l001 of 4.2

A

in magesium oxide.

A second triplet state resonance has been observed for a defect with orthorhombic symmetry and [l101 axes of symmetry. The exchange interaction is smaller than that of the Ft-centre. Due to overlap with other lines, it has not been possible to measure accdrately the values of the Hamiltonian parameters. Approximate values available at present are g = 2, D = 140 Gs and E = - 30 Gs. Assuming weak dipole-dipole coupling we find r l j = 6.7

A

which suggests that the defect consists of two exchange coupled F-centres in next-nearest neighbour association along the [l 101 direction. For purposes of identification we designated this centre the F, centre where the subscript R is intended to refer to the rhombic symmetry of the defect. The two defects referred to above are extremely unstable to annealing treatment : a few minutes at

(*) This nomenclature is that used by Tanimoto et al. [l31 to

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LOCALISED ELECTRONIC STATES OF DEFECT AGGREGATES

F- CENTRE

"0

-

F, - SPECTRUM.

--fMt

FIG. 5. - The electron spin resonance spectrum of Ft-centres at 300 OK and 20 O K .

temperatures of 4000C is sufficient to reduce the electron spin resonance signal intensities to undetec- tably low levels. In addition, such treatment results in the formation of F,-centres at the expense of F-centres. The model proposed by Wertz et al. 1121 for this defect was an electron trapped at a cation-anion vacancy pair. The only evidence for this was the g-value of the F2- centre and the line shape of the e. S. r. line in powders.

Recent measurements by the author at Q-band confirm that the centre has an axially symmetric g-tensor : the appropriate values are g,, = 2.000 4 and g, = 2.001 7. It is to be noted that Ag, =

+

.001 7 indicating that the spin orbit coupling constant is negative. This is similar to the situation in the magnesium oxide F-centre [5, 141. A complete analysis of the hyperfine structure of this centre has not yet been possible although there is reason to believe that the magnitude of the isotropic component of the hyperfine interaction is larger than in the F-centre. This evidence strongly supports the original model proposed bu Wertz for this defect.

Recent observations by King and Henderson [l01 that the F,-centre may be optically bleached has clarified the controversial ideas related to the position of the F,-band [2,4]. The efficiency of bleaching of the F,- centre as a function of photon wavelength is shown in figure 6. This curve was obtained by monitoring the decay of the F,-centre concentration as a function of the bleaching time with monochromatic radiation of wavelengths in the range 2 500

A

- 6 500

A.

For this purpose the amplitude of the F,-centre resonance line

E W E R G I I e Y I

BLEACHING E F F l C l E H C l OF F2 CENTQE IN NEUTRON I R W D I A T E D MAGNESlUY

OXlDE

FIG. 6. - Efficiency of bleaching of Fz-centre electron spin resonance signal as a function of photon energy.

was compared with that of the Mn2' lines since the latter do not change during the in situ )) experiments.

The curve shows that a peak bleaching efficiency is obtained at 3.7 eV. As discussed by King and Hen- derson [l01 this behaviour is consistent with the removal of the F,-centre electron from the ground state into the conduction band. Such behaviour is predicted by simple calculations of the energy levels of the F,-centre electron which show that only one bound state exists below the top of the conduction [l 51.

IV. Conclusions. - The defect structures discus-

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B. HENDERSON AND R. D. KING

M;;likz centres

4684 it line 4 7 0 0 2 line

P-

P

P--

R centres - like

L<[

.+--. -.._

L-+,i

.

-

- . - - - -

d)

,$- -. ... -

.

f

.

+

5248 lino 6419 it bne 6490 ii line

OoOoO

F; centre C ~t centre

0 M$+

0

0'- F, centre

FIG. 7. - Defect models for centres in magnesium oxide. For electrostatic neutrality, M-centres and R-centres need to

trap four and six electrons respectively. The F2-centre is elec-

trically neutral whereas the F2-centre has a single excess nega-

tive charge.

uniaxial stress measurements convincingly demonstrate that centres analogous with the M, R and N centres in the alkali halides may exist in the alkaline earth oxides. The lines at 4 684

A

and 4 700

A

have extremely similar properties and the most reasonable models for these two defects are shown in figures 7a and 7b. The results suggest that one R-like centre containing 5 trapped electrons should be paramagnetic : preliminary experimental evidence confirms this suggestion.

In addition, centres exist for which no analogue exists in the alkali halides. Three of these centres, the

F,, FR and F2-centres have simple symmetry properties that are readily adduced from the paramagnetic resonance spectra. The optical absorption in the F2-centre occurs with maximum efficiency at 3.7 eV :

the transition apparently occurs between a 1 S-Like ground state and the conduction band.

Acknowledgements. - We have benefited from

the many fruitful discussions with numerous members of the Harwell staff : in this respect we are especially indebted to Dr. D. H. Bowen, Dr. A. E. Hughes, Dr. D. Pooley and Dr. M. A. Stoneham.

References

[l] WERTZ (J. E.), ORTON (J. W.) and AUZINS (P.), Disc. Far. Soc., 1961, 31, 140.

[21 WERTZ (J. E.), SAVILLE (G.), AUZINS (P.), and ORTON

(J. W.), J. Phys. Soc. Jap. (Supp. Z), 1963,18,305. [3] HENDERSON (B.) and KING (R. D.), Phil. Mug., 1966,

13, 1149.

[4] WERTS (J. E.), SAVILLE (G.), HALL (L.), and AUZINS

(P.), Proc. Brit. Ceram. Soc., 1964, 1, 59. [S] KEMP (J. C.), ZINIKER (W. M.) and GLAZE (J. A.),

Proc. Brit. Ceram. Soc., to be published.

[6] LUDLOW (I. K.), Proc. Phys. Soc., 1966, 88, 763.

[7] KING (R. D.) and HENDERSON (B.), Proc. Phys. SOC., 1966, 89, 153.

[g] RUNCIMAN (W. A.), Proc. Phys. Soc., 1965, 86, 629. [9] KING (R. D.), unpublished data.

[l01 KING (R. D.) and HENDERSON (B.), Proc. Brit. Ceram. Soc., to be published.

[l11 LUDLOW (I. K.), unpublished data.

[l21 HENDERSON (B.), Brit. J. App. Phys., 1966, 17, 851. [l31 TANIMOTO (D. H.), ZINIKER (W. M.), and KEMP (J. C.),

Phys. Rev. Lett., 1965, 14, 645.

[l41 KEMP (J. C.), ZINIKER (W. M*), and GLAZE (J. A.),

Phys., Lett., 1966, 22, 37.