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PROPERTIES OF THE FA CENTRE IN
MAGNESIUM-DOPED CALCIUM OXIDE
L. Welch, A. Hughes, G. Pells, A. Schoenberg
To cite this version:
L. Welch, A. Hughes, G. Pells, A. Schoenberg. PROPERTIES OF THE FA CENTRE IN
C7-198 JOURNAL DE PHYSIQUE Colloque Cl, supplément au n° 12, Tome 37, Décembre 1976
PROPERTIES OF THE F
ACENTRE
IN MAGNESIUM-DOPED CALCIUM OXIDE
L. S. WELCH, A. E. HUGHES and G. P. PELLS
Materials Development Division, Atomic Energy Research Establishment, Harwell, Didcot, Oxon, UK
A. SCHOENBERG f
Department of Physics, University of Keele, Keele, Staffs, UK
Résumé. — Nous avons étudié des centres liés aux lacunes d'oxygène dans CaO dopé à 1 %
de Mg. Les cristaux obtenus contiennent plusieurs zones distinctes. Immédiatement au-dessous d'une couche fortement colorée à la surface du bain, il existe une zone où de nouveaux centres F+, F et F t peuvent être créés sous irradiation ionisante jusqu'à des concentrations de l'ordre de 1017 cm-3. On obtient par ce processus des rapports FÎ/F+ anormalement élevés. On en conclut que
dans cette zone les centres colorés sont produits par le piégeage d'électrons sur des lacunes d'oxygène préexistantes et qu'une fraction importante de ces lacunes se condense à proximité des impuretés lors du refroidissement à partir du point de fusion plutôt que de former des cavités comme dans les cristaux purs. Une nouvelle bande d'émission à 689 nm avec une raie sans phonon à 656 nm a été trouvée dans ces échantillons dopés. Le facteur de Huang-Rhys est S = 4,4 ± 0,2. La durée de vie est de 12 ms et des mesures de polarisation de la luminescence indiquent un défaut à symétrie tétragonale ce qui est en accord avec le centre FA. On suppose un processus de transfert d'énergie à partir du centre F excité pour expliquer la dépendance en température de l'intensité de la luminescence à 689 nm. La principale bande d'absorption des centres F A se situe probablement à 450 nm.
Abstract. — Oxygen vacancy centres have been studied in crystals of calcium oxide doped with 1 % Mg. The as-grown melt contains several distinct regions. Just below a highly coloured layer at the surface of the melt is a region in which new F+, F and F l centres with a total concentration
up to about 1017 cm- 3 can be produced by ionizing radiation. Unusually high ratios of FA to F+
centres are produced by this process. It is concluded that in this region colour centres are produced by trapping electrons at pre-existing oxygen vacancies, and that a large fraction of these vacancies condense at impurities during cooling from the melting point rather than forming voids as in pure crystals. A new luminescence band peaking at 689 nm with a zero-phonon line at 656 nm has been found in these doped crystals. The Huang-Rhys factor is S = 4.4 ± 0.2. The excited state lifetime is 12 ms and polarized luminescence measurements indicate a defect with tetragonal symmetry, both features being consistent with assignment to the FA centre. An energy transfer process from the excited F centre is invoked to explain the temperature dependence of the 689 nm luminescence intensity. The main absorption band of the FA centre has been tentatively identified at 450 nm.
1. Introduction. — In the last four years the
proper-ties of the FA centre in magnesium doped calcium oxide have been reported [1-3]. This centre is a single electron trapped at an oxygen vacancy with one of the six nearest neighbour calcium ions replaced by a magnesium ion. In undoped CaO crystals the F centre (i. e. two electrons trapped in an anion vacancy) has been studied [4-6] and found to possess interesting bound excited states [7-10]. In previous work on CaO : Mg there has been no evidence to substantiate the presence of the FA centre (i. e.
equi-valent to an F centre but with a magnesium ion in a nearest neighbour site) although, presumably, it must exist. In this paper luminescence and absorption
f Senior Science Research Council Fellow. Present address : Dept. of Physics, Tel Aviv University, Israel.
spectra, and excited state lifetime measurements, are reported which may lead to an identification of this centre. We also report, in section 2.2, some unusual properties of the CaO : Mg crystals which have aided studies of the Mg-associated centres. These results clarify some of the observations described by Boas
etal.[3].
2. Experimental details. — 2.1 SPECTROSCOPY. — Emission and excitation spectra were recorded by exciting luminescence by means of a UV 100 mercury lamp or quartz-halogen lamp in combination with a Hilger D292 monochromator. The luminescence, taken off at right angles to the excitation direction, was detected by a Hilger Monospek 1000 grating monochromator with a cooled EMI 9659 QB extended
PROPERTIES OF THE FA CENTRE IN MAGNESIUM-DOPED CALCIUM OXIDE C7-199
S20 photomultiplier used with a photon counting system. Polarization of radiation was achieved using Polaroid HNP'B filters. Lifetime measurements were made by exciting luminescence with a Wotan super pressure mercury lamp in combina.tion with a 404.7 nm interference filter, or with the 325 nm line of a Liconix He-Cd laser. The luminescence was again taken off at 900 to the excitation direction and recorded using a McPherson
5
metre monochromator withEM1 9659 QB extended S20 photomultiplier in
conjunction with a time resolved spectroscopy photon counting system. The sample was held in a helium gas-flow cryostat. Absorption measurements were made using a Cary 14 spectrophotometer.
2.2 CRYSTALS. -Crystals of CaO, doped with about 1
%
by weight of MgO and grown by the arofusion technique, were obtained from W and C Spicer Ltd. Bulk analysis figures for Mg content are given in Hughes and Pells [I]. A vertical slice through the crystal melt shows four regions of varying properties. The dark brown additively coloured region at the top surface of the melt is a few millimetres in depth. Below this is a very clear region followed by one slightly less clear ; the remainder of the crystal has a cloudy appearance. On irradiating with ionizing radiation (e. g. low energy electrons or X-rays) the very clear region turns yellow due mainly to the production of F andFA+
absorption bands. An elec- tron-probe micro-analysis of the first three regions is shown in figure 1. The magnesium content rises through a maximum in the brown/yellow bands, falling to a steady level (consistent with--
1 %doping) in the clear region.I I I I I J
0 1 2 3 4 5
DISTANCE FROM TOP SURFACE OF CRYSTAL MELT ( m m )
FIG. 1.
-
Electron probe microanalysis indicating the dis- tribution of magnesium in an electron irradiated Mg-doped CaO crystal. The inset shows the extent of the different regionsof the crystal.
During crystal growth the dark brown region 1 is considered to have been additively coloured by the reducing conditions present during arc fusion growth [ l l ] and contains large (- loi8 ~ m - ~ ) con- centrations of F+ and F centres.
FA+
centres are also present, but only at a few percent of the F+ con-centration, as expected from the statistical formation of 0'- vacancies next to Mg2+ impurities [12]. The very clear region 2 contains no observable centres before irradiation, but after electron, y or X-irradiation colour centres are produced. The
FA+
: F+ centre ratio in this region has been found as high as 2 : 1 whereas statistically it is expected to be about 1 : 20. This unexpectedly high ratio ofFA+
to Ff centres has been helpful in studying the properties of theFA+
centre [3]. The origin of these high ratios deserves some comment, since it is quite clear that the rise in Mg concentration in region 2 indicated by the microprobe measurements is quite insufficient to explain the observations. Figure 2 shows the totalFIG. 2. - Growth of the number of F+ and :F centres in Mg-doped CaO using 450 keV electron irradiation at room
temperature. The beam current was 1 MA cm-2.
C7-200 L. S. WELCH, A. E. HUGHES, G. P. PELLS AND A. SCHOENBERG
pre-existing oxygen vacancies and that the abnormally two luminescence bands is shown in figure 4. At high
FA^
to F + ratio is due to these vacancies being 60 K there occurs a cross over of intensity, the zeroth preferentially sited next to Mg2+ impurities. In pure moment of the F centre decreasing more rapidly MgO and CaO crystals it is known that vacancies with increasing temperature than that of the F centre tend to condense into voids and hydrogen-filled in an undoped crystal [5].bubbles during cooling after arc-fusion crystal growth, l o giving the crystals a cloudy appearance [ l l , 131.
g
0,9 I n the very clear region 2 it is possible that some ofthese vacancies condense at and near the dispersed
Mg impurities instead of into voids, thus explaining
i!
the behaviour observed.None of these properties is exhibited by region 3 n 0.3 (the quite clear zone), and the remainder of the
F
0.2- crystal is cloudy (region 4) due to the vacancies2
0 1 -N I I I
0 20 40 60
I " \
condensing in voids. In both regions 3 and 4 colour 80 loo
centres are produced only by collision processes TEMPERATURE ( K J
(e. g. fast neutron or high energy electron irradiation). FIG. 4.
-
The zeroth moments of the 600 nm band of the F The F: : F+ centre ratio has the statistically expected centre and the 689 nm band as a function of temperature. value [I], unlike region 2. A : 600 nm band ; : 689 nm band.3. The FA Centre. - 3.1 LUMINESCENCE. - The CaO : Mg crystal used for the luminescence measu- rements described in this section had been irradiated with 30 MeV electrons at room temperature on the Harwell LINAC to a dose of about 10'' electrons cm-'. On exciting the crystals with 404 nm radiation (the F absorption band peaks at 400.6 nm), a new luminescence band at 689 nm was recorded in addition to the 3T,, -+ 'Alg emission of the F centre at 600 nm
(see Fig. 3). The zero-phonon line of this band is at WAVENUMBER 1103 cm-'1
18 17 16 15 1L 13
WAVELENGTH ( n m l
FIG. 3 . - The 600 nm luminescence band of the F centre and the 689 nm luminescence band, both measured at 3.5 K.
656 nm and the phonon frequency is approximately 200 cm-l. It is possible that this band is the same as the one reported at 677 nm by Henderson et al. [4]
in pure crystals, but which Bates and Wood [6] have shown must be associated with impurities. Using the zero-phonon line intensity, the centroid position and the band second moment the Huang-Rhys factor S is calculated to be 4.4 _+ 0.2. The temperature depen- dence of the intensities (zeroth moments) of the
The polarization properties of the emission spectra of the new band have been investigated using con- ventional right angle geometry as described in [2]. Preliminary polarized excitation spectra for the new emission have also been recorded. As yet, no inde- pendent excitation peaks have been found for the new centre, the strongest excitation coinciding with the absorption of the F centre. However, if the emis- sion is triplet -+ singlet the polarization results
show that the centre involved has the tetragonal symmetry which the FA centre must possess.
3.2 LIFETIMES AND TEMPERATURE DEPENDENCE.
-
The excited state lifetime of the F centre and the centre associated with the new emission band have been measured. The decay time of the F centre is approxi- mately 3.4 ms at 4 K and appears to decrease sharply at about 60 K, whilst that associated with the 689 nm band is 12 ms. These times are similar enough in value to suggest that the same processes are involved in both cases (i. e. forbidden transitions). In con- junction with the polarized excitation results these results are therefore indicative of the FA centre being responsible for the 689 nm band.
A simple energy transfer model explaining the behaviour of figure 4 has been considered. The trans- fer of energy from the F centre to the FA centre is
shown schematically in figure 5a, along with the radiative decay processes of both centres. With this model, the rate equations governing the population of the four energy levels are
= an, - k, n, - Kn,
=
U , - (k,+
K) n, (1)where :
PROPERTIES OF THE FA CENTRE IN MAGNESIUM-DOPED CALCIUM OXIDE C7-201 F CENTRE FA CENTRE LEVELS LEVELS "
1..
L K S
"0 (a)(K+O) TEMPERATURE --, (K-KO) (b)
FIG. 5. - a ) Schematic diagram of energy transfer from the
F to the FA centre. b) Description of the temperature depen- dence of the intensities of the luminescence bands using equa- tions (4) and (5). The full curve refers to the FA emission and the
dashed line to the F emission.
no, n,, n, and n3 are the numbers of electrons in each of the four levels.
U, and UA are constant pumping rates for each
centre.
k , and kA are radiative decay rates.
K refers to the process controlling energy transfer between the two centres and is assumed to possess
a temperature dependence described by equation (3)
K = KO exp(- AlkT) (3) KO = value of K when k T % A
A = activation energy.
Since the results in figure 4 were cbtained when the populations in the levels were in the steady state,
i1
andA,
must be zero. This assumption leads to the following expressions for the intensities (zeroth moments) of the emission bands :I,, = kA n3 = UA
+
U , K O exp(- AlkT)k,
+
K O exp(- AlkT) (5) Equations (4) and (5) predict the temperature depen- dence of the intensities of the F and FA emission bands as shown schematically in figure 5b. The simi- larity to the experimental results in figure 4 is obvious. The activation energy of the energy transfer process is calculated from the data to be approximately 0.05 eV.3 . 3 ABSORPTION BANDS. - The absorption spectra of region 2 of a CaO : Mg crystal before and after X-irradiation at 77 K and 300 K are shown in figure 6 .
WAVENUMBER 110~crn-'l 50 LO 30 25 20 I I I I
i
I I I I I 1 200 300 LOO 500 600 700 WAVELENGTH (nmlFIG. 6. - Absorption spectra of X-irradiated Mg-doped
CaO. Curve A : background before irradiation. Curve B : after irradiation at 300 K. Curve C : after irradiation at 77 K.
All data were taken at 77 K.
The main bands at 340 nm and 374 nm are the F+ and
FA+
bands respectively. A small F band at 400.6 nm is also present and, in addition, new bands at 450 nm and 500 nm have been recorded. Production of the 500 nm band is possible by X-irradiation at 77 K. This band is very unstable with respect to thermal or optical bleaching. On warming the crystal to 300 K or irradiation with white light or light of wavelength 510 nm at 77 K the band decays markedly. The 450 nm band is much more stable at 300 K and does not decay to any great extent when left for some hours. It is also reasonably stable up to 410 K but decays noticably by 500 K. At 750 K this band is no longer present, which is also true of the other colour centre bands. Neither the 450 nm nor 500 nm band has been observed in the cloudy region of the Mg- doped crystal or in an undoped CaO crystal.Growth curves for the F f , F:~, F and 450 nm bands have been measured during X-irradiation at 300 K. The overlapping bands make accurate quan- titative data difficult to obtain, but the results show that all these bands grow in a roughly parallel fashion. Combined ~ i t h the similar thermal stability described above this argues in favour of the 450 nm band being associated with an oxygen vacancy centre and therefore a prime candidate for +he FA absorption band.
C7-202 L. S. WELCH, A. E. HUGHES, G. P. PELLS AND A. SCHOENBERG
absorption results, the 450 nm position is not an and obtain luminescence excitation spectra on well- unacceptable region in which to find the main FA characterized samples in order to correlate absorption
centre absorption band. Although one cannot comple- and excitation bands. tely ignore the 500 nm band in this context, its total
lack of stability at room temperature argues against Acknowledgments. -We should like to thank it being due to the FA centre. Further work on the Professor S. C . Jain for his help with some aspects absorption spectra should help to refine the results of the work on the absorption spectra.
References
[I] HUGHES, A. E. and PELLS, G. P., J. Phys. C : Solid State Physics 5 (1972) 2543.
[2] HUGHES, A. E. and PELLS, G. P., J. Phys. C : Solid State Physics 8 (1975) 3703.
[3] BOAS, J. F., HALL, T. P. P. and E~UGHES, A. E., J. Phys. C :
Solid State Physics 6 (1973) 1639.
[4] HENDERSON, B., STOKOWSKI, S. E. and ENSIGN, T. C.,
Phys. Rev. 183 (1969) 826.
[5] HENDERSON, B., CHEN, Y. and SIBLEY, W. A,, Phys. Rev.
B 6 (1972) 4060.
[6] BATES, J. B. and WOOD, R. F., Phys. Lett. 49A (1974) 389 ;Solid State Commun. 17 (1975) 201.
[7] EDEL, P., HENNIES, C., MERLE D ' A U B I G ~ , Y., ROMES-
TAIN, R. and T~AROWSKI, Y . , Phys. Rev. Lett. 28
(1972) 1268.
[8] EDEL, P., MERLE D'AUBIGN~?, Y . and LOUAT, R., J. Phys.
& Chem. Solids 35 (1 974) 67.
[9] GELINEAU, A., BUCH, T., GEOFFROY, A. and NAUD, C.,
J . Phys. C : Solid State Physics 6 (1973) 774.
[lo] HARRIS, C. B., GLASBEEIC, M. and HENSLEY, E. B., Phys. Rev. Lett. 33 (1974) 537.
[ l l ] BRIGGS, A., J. Mat. Sci. 10 (1975) 729.
[12] WEIGHTMAN, P. and HALI, T. P. P., J. Phys. C : Solid State Physics 6 (1973) 1292.
1131 ABRAHAM, M. M., BUTLER, C. T. and CHEN, Y., J. Chem.
Phys. 55 (1971) 3752.
DISCUSSION
J. DURAN. - Have you tried to get more detailed
information about the vibronic coupling in this sys- tem ? It would probably be very interesting to do for these FA-centers the same work as you have done for the (F+, I?:)-centers.
