Optical transitions in ZnS type crystals containing cobalt

1. Phys.

Chem. Solids

OPTICAL

Pergamon

Press 1966. Voi. 27, pp. 187-195

in Great Britain.

TRANSITIONS IN ZnS TYPE CONTAINING COBALT H.-E.

Institut

Printed

fur Elektronenmikroskopie (Received

GUMLICH

and H.-J.

am Fritz-Haber-Institut

CRYSTALS

SCHULZ der Max-Planck-Gesellschaft,

14 Janrrnr_v 1965; in revised form 20 May

Berlin-Dahlem

1965)

Zusammenfassung-Die optischen Eigenschaften von ZnS-Kristallen und einiger ZnS- und (Zn.Cd)S-Leuchtstoffe, die mit Co aktiviert sind, wurden im Temperaturbereich zwischen 4,2”K und 3&K untersucht. Die Absorptionsbanden bei Photonen-Energien von 1,75 eV und 0,85 eV soalten bei Abkiihluna auf: eine Absorntionsbande bei 0.4 eV tritt bei 76°K deutlicher als bei 412°K in Erscheinungr Die-dem Kobalt-zugeschriebene infrarote Emission bei etwa 0,4 eV wird bei Abkiihlung starker und zeigt bei tiefen Temperaturen Struktur. Diese Emission kann in den vom Co++ herriihrenden Absorptionsbanden bei 1,75 und 0,85 eV angeregt werden. Zusltzliche Bestrahlung mit kurzwelligem Licht (25 eV 5 E 5 3,2 eV verandert bei tiefen Temperaturen das Absorptions- und das Emissions-Spektrum: (1) Die charakteristischen Co++-Anregungsmaxima werden verkleinert. Eine zusatzliche Absorption mit einem Maximum bei etwa 1,3 eV erscheint. (2) Die dem Co++ zugeschriebene Infrarot-Emission bei 0,4 eV wird schwlcher, dafiir tritt eine neue Emission im Energiebereich 0,6 eV 6 E 5 0,9 eV auf, die dem Cu++ zugeschrieben aird. Die Anderung der optischen Eigenschaften durch zusatzliche Bestrahlung und durch Temperaturanderung wird durch Ionen-Umladungsprozesse Co ++ e Co+ und Cu++ Cu++ gedeutet. Abstract-The optical properties of ZnS crystals, ZnS and (Zn,Cd)S phosphors activated by cobalt were studied in the temperature range from 4.2”K to 300°K. The absorption bands with an energy of 1.75 and 0.85 eV split when the crystals are cooled. However, the absorption at 0.4 eV is more pronounced at 76°K than at the temperature of liquid helium. The infrared emission due to cobalt at @4 eV increases by cooling the crystal and shows some structure at low temperature. This emission can be excited by irradiation into the absorption bands at 1.75 and 0.85 eV. Additional irradiation by short wavelength light (2.5 eV $ E s 3.2 eV) at low temperatures changes the absorption spectra and the emission spectra: (1) The characteristic excitation peaks of Co+ are reduced, whereas an additional absorption with a maximum at about 1.3 eV occurs. (2) The infrared emission due to Co ++ at 0.4 eV decreases, but a new emission within an energy range of @6 eV s E s 0.9 eV appears, generated in Cut-f centres. The changes of optical properties by additional radiation and by cooling the crystals are explained by charge transfer processes Co++ t’ Co+ and Cu+ ?~r.Cu++.

INTRODUCI’ION

THE ACTIVATION of ZnS by cobalt gives rise to a number of very specific optical phenomena and, therefore, to information concerning the optical transitions due to cobalt centres. It is well known that cobalt in phosphors of the ZnS type causes a decrease of visible luminescence without showing by itself luminescence within the visible range.(r) Therefore, cobalt is generally classified as a ‘killer’. On the other hand, Garlick and Dumbleton found the presence of cobalt in the phosphors to 187

be the reason for an infrared emission with a peak near 3 p.(s) Furthermore, in ZnS and (Zn,Cd)S cobalt creates an electron trap about 0.52 eV below the conduction band, as has been investigated estensively by HOOGENSTRAATEN.(~) Recently, the transfer of energy from luminescence centres to the Co has been studied by means of electroand electrophotoluminescence.(d) luminescence From these experiments with electric fields follo\vs that the energy is transported by free carriers. From the theoretical point of view considerable

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progress has been made by explaining the optical and chemical properties of materials containing transition metals. As for Co++ ions in II-VI compounds, PAPPALARDO and DIETZ(~) successfully applied the crystal field theory to the absorption spectra of CdS(Co++) whereas WEAKLIEM(@ succeeded in classifying the absorption levels of ZnS(Co++) on the basis of the crystal field approximation. ALLEN(~) tried to fit the infrared cobalt emission into the energy level scheme gi, cn by Weakliem. The ideas of Pappalardo, Dietz, Weakliem, and Allen are based on the assumption that the absorption and the emission are due to internal transitions within the Co++ ion disturbed by the crystal field of the lattice, so a 3d7 configuration is the starting point of all theoretical considerations. The role which the Co impurities play in electroluminescence, electrophotoluminescence, and trapping processes, as well as the fact that the Co is able to create levels in the forbidden gap of II-VI compounds lead to the question: are optical transitions in Co ions in ZnS influenced by the charge-transfer? In order to get information on that point we measured absorption and emission spectra of ZnS and (Zn,Cd)S containing Co in the temperature range of 4.2”K to 300°K without and with additional radiation. The results of these measurements show the influence of the stimulating radiation on both emission and absorption. EXPERIMENTAL The measurements were made on a ZnS(Co) single crystal containing cobalt. The amount of cobalt within the crystal has been measured by spectrochemical methods*. The value found in this way is 7 x 10-s g Co/g ZnS. Copper has not been introduced intentionally into the crystal. However, the spectrochemical analysis showed that copper in the order of magnitude of 10-s g Cu/g ZnS is present within the crystal. The ZnS and (Zn,Cd)S powder phosphors contain different amounts of cobalt. The values are indicated in Fig. 3. In emission and excitation measurements the samples were irradiated by a xenon lamp, in the absorption measurements by a tungsten lamp or * Bundesanstalt Berlin, Germany.

fiir Materialpriifung,

Dr. Miinchow,

and

H.-J.

SCHULZ

by a globar. The chopped radiation was recorded by thermocouples, PbS or cooled InSb cells. To study the influence of additional irradiation on the crystal unchopped light of a d.c. operated mercury discharge lamp was applied. The spectra were obtained by means of a double monochromator having CaFa or flint glass prisms. A cryostat with sapphire windows allowed measurements in a steady state procedure between 4~2°K and room temperature. ABSORPTION As has already been discussed by WEAKLIEM@) it is reasonable to assume that each Co ion is surrounded by four sulfur-atoms, so the Co ion lies in the tetrahedral electric field of four negative charges, which lift the degeneracy of the 4F, 4P, sG levels. We found at room temperature broad absorption bands with maxima at about 0.85 and 1.75 eV (Fig. 1) in good accordance with Weakliem. When the crystals are cooled down both maxima become more pronounced and split into several peaks. At the temperature of liquid helium the 1.75 eV group shows at least four peaks at 1.73 ; l-80; l-86; and l-90 eV and one shoulder at 1.75 eV. The assignments of these peaks are 4A2(F) +4T$?),4A2(F) -+2A1(G),4A2(F) -f2T2(G), respectively. Some of them show spin-orbit splitting. The O-85 eV group splits into a number of components (e.g. at O-78; O-84; and O-89 eV), which have been classified as transitions from the 4As(F) level to the spin-orbit split levels of “Tr(F). However, the absorption band at 0.40 eV behaves differently, when the crystal is cooled. Weakliem mentioned that this band was quite difficult to detect and was only observed at room temperature. We found that this group, being due to transitions from 4As(F) levels to the spin-orbit levels of 4Ts(F), is very weak at room temperature, but showed a more pronounced structure when the crystal was cooled down to liquid nitrogen temperature. When the temperature decreases below the temperature of liquid nitrogen, the structure obscures and is finally much weaker at liquid helium than at liquid nitrogen temperature. EMISSION At room temperature we observe a rather broad emission band with a peak at 0.42 eV (Fig. 2). When the crystal is cooled, the main emission peak

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IN ZnS

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CRYSTALS ----Waveiengf

energy,

COBALT

189

h,

I.5

I.0

Photon

CONTAINING

eV -

FIG. 1. Transmission spectrum of a ZnS : Co crystal. The origin of the logarithmic transmission scale is arbitrary and the curves have been shifted relative to one another.

appears at O-433 eV and several other maxima and shoulders occur. Besides the O-433 eV peak the 0,395 eV peak is a very pronounced one. The assignment of this is not possible in a straightforward manner. ALLEN(~)concluded from the peak positions of the absorption, excitation, and emission that the emission should be due to a 4Tl(F) -+ eTs(F) transition. Allen’s conclusion is based on the fact that the Wavelength.

p

4Ts(F) -+4As(F) transition is a first order forbidden one. However, the comparison with Weakliem’s calculation shows that at least the energy of the main peak is closer to the calculated value of the aTa -+ JAs(F) transition than to the value of 4Tl(F) -+ 4Ts(F). Therefore there are at present several possibilities to be discussed in order to explain the i.r. emission: (1) The i.r. emission is entirely due to a 4Tl(F) --+aTs(F) transition as Allen suggested. The splitting into several components might be due either to transitions between different spin-orbit components of the involved levels or to phonon coupling. We do not have any direct proof against this proposal, but the fact that the peak at 0.40 eV appears in emission and in absorption as well makes other assignments more likely.

Phofon energy, eV

FIG. 2. Normalized

emission spectra of a ZnS : Co crystal. Excitation: l-4 eV $ E 5 2-Z eV.

(2) The 0.40 eV emission is due to the *Tl(F) --+eTs(F), the O-43 eV emission is due to the *Ta(F) -+ 4As(F) transition. In this case we meet a similar

problem:

Since

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the 4T#) + 4Ts(F) transition should be the low energy transition according to Weakliem, it is somewhat surprising to find the same value (O-40 eV) in absorption as we did in emission. Of course the coincidence might be purely accidental and due to phonon interaction. (3) The emission is due to transitions from the zero phonon state of JTQ(F) to the zero phonon state of 4AQ(F) and from the zero phonon state of 4TQ(F) to the first vibrational state of ‘JAs(F). The opposite transitions give rise to absorption bands. This particular model which explains best the experimental facts, implies that the 0.40 eV emission and absorption belong to the transitions between the first vibrational state of JAQ(F) and the zero phonon state of 4TQ(F). In this model the 0.43 eV emission and absorption belongs to the transitions between the zero-phonon-states of 4Ts(F) and 4AQ(F). As for the transitions from the first vibrational state of 4AQ(F) to 4TQ(F) the selection rule due to symmetry considerations is overcome by electronphonon-coupling. Therefore the absorption and emission are allowed and the bands in the 0.40 range may be attributed to these transitions. The disappearance of this absorption band on going from 77°K to 4°K may be understood in this way as well as the fact that the emission does not disappear. The absence or weakness of the non-phonon absorption is reasonable as a consequence of the symmetry selection rule already mentioned. The emission peak separation is just that of a transversal optical phonon,@) a fact which fits well in this model. The temperature dependence of the ratio of emission intensities in both peaks is not yet explained in this model. The low energy peak corresponding to the transition from the zero vibrational state of “Tz(F) to the first vibrational state of 4As(F) is not influenced very much by change of temperature between 4.2”K and lSO”K, whereas the high energy peak increases markedly by cooling the crystal in this range. It is not clear how the change of the thermal distribution of electrons on different spin-orbit components of the levels by cooling the crystal may influence the spectral distribution of emission.

and

H.-J.

SCHULZ

Such an effect could be taken into account as a reason for the different behavior of different peaks in emission as soon as one can classify the spinorbit components of the emission. This is not yet the case. Wavelength, 30

35

Photon

,AL 25

energy.

e’.’

Wavelength. 35

30

Photon

/A 25

energ& eV

FIG. 3. Normalized emission spectra of powders at 300°K. (a) Different lattices; (b) Different amounts of cobalt. Excitation: 1.2 eV 6 E 6 2.2 eV.

The intensity of i.r. emission is of course a function of the amount of available Co++ centres. This amount can be changed by a charge transfer process either Co++ --+ Co+ or Co++ --f Co+++. Since the quasifermi levels can be changed by changing the temperature, one has to take into account the possibility of such charge transfer processes. Similar to the model explaining the i.r. emission of copper(QJQ) the excited (Co++)* centres should be able to capture electrons and be transformed into Co+. The process of capturing electrons from the valence band requires thermal energy, therefore the loss of emitting centres by the transformation (Co++)* -+ Co+ decreases with decreasing temperature. If this happens we can calculate from the initial slope of the curve shown in Fig. 4, which represents the temperature dependence of the i.r. emission, a quenching energy of 0.24 eV. A comparison of the main maxima at 0.40 eV and 0.43 eV shows that in both spectral ranges the emission intensity decreases by warming up the crystal from 150-300”K, whereas in the region below 150°K only the 0.43 eV peak depends on temperature as mentioned earlier. So it is consistent with our model to assume that the

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TYPE

decrease of i.r. emission by warming up the crystal to temperatures higher than 150°K is mostly due to losses of Co++ centres by charge transfer processes like Co++ -+ Co+. This assumption is supported by the results which we obtained with additional radiation. This will be discussed later on.

CRYSTALS

CONTAINING

confirms the assumption

COBALT

191

that the transitions

*b(F) +-*-h(F), *b(F) -, *‘G(P), *Az(F) -+ 2A1(G), *b(F) --f2T2(G)

Temperature.

OK

give rise to the infrared

emission

25

4T2(F) +*A2(F). Wovelenglh, p 14 12 :/If

120

.c z

vo I

o-9 I

O-8 I

07 ii

I

0.6 I

*o

2 15

20

25

I03 -7’

OK4

30

35

40

.E

6 60 'Z

FIG. 4. Temperature dependence of the Co++ emission intensity of the main peak. Escitation: 1.2 eV 5 E 5 2.2 eV.

.s aI 40 &

Incidentally, the position of the emission is shifted towards smaller energies by substituting zinc by cadmium in the lattice. As an example Fig. 3(a) shows the emission of a ZnS phosphor without cadmium having a peak at O-41 eV and with 50% cadmium having a peak at 0.37 eV. The amount of cobalt, however, does not influence the spectral distribution, but the intensity of the infrared emission (Fig. 3 b).

O

EXCITATION

SPECTRA

The excitation spectra of the infrared cobalt emission show the same bands as the absorption spectrum does. Since the resolution of the apparatus when used for measuring excitation spectra was less than in the case of absorption spectra, because the emission intensity is weak, we did not obtain so much structure within the maxima (Fig. 5). We observed an excitation band in the 1.75 eV range with some structure at the temperature of liquid helium and another excitation band, which can be resolved into two peaks at 0.85 eV and 0.59 eV at liquid helium temperature. This excitation spectrum does not give more information on the fine structure of the level svstem. , I but -~ it

08

t0

12 14 I.6 Photon energy,eV

20

22

FIG. 5. Normalized excitation spectra of the infrared emission of a ZnS : Co crystal. Emission of photons with energies less than 0.79 eV measured.

Incidentally the i.r. emission is only very weak when the radiation is absorbed in the lattice (E 2 3 -8 eV). This fits reasonably in our model because the excited electrons have the possibility to recombine without energy exchange with the inner shells of the cobalt, for instance they may recombine radiationless via lattice defects, surface states or other irregularities. Therefore only a small fraction of the absorbed energy finds its way to the Co++ centres. On the other hand the amount of available Co++ centres itself is reduced by the absorbed radiation because some of the Co++ centres may catch electrons and therefore be lost by the charge transfer process Co++ -+ Co+ until holes recombine with the Co+ centres and establish the Co++ centres again. THE

ACTION OF ADDITIONAL RADIATION

The explanation of the temperature dependence __ i.r. emission and the weakness of the i.r.

of the

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emission in case of lattice excitation lead us to the assumption that the amount of Co++ ions can be changed by free carriers. This assumption implies the possibility to reduce or enhance the amount of Co++ centres by additional radiation as well. In order to check this, we excited the crystal by chopped radiation in the range of the 1.75 eV absorption band. In addition, the sample was irradiated by unchopped light of higher energy (2.5 eV 6 E 5 4.0 eV). Under these conditions the additional light reduces indeed the response of the crystal to the 1.75 eV excitation. As an example, Fig. 6 shows an emission spectrum at 76°K of a crystal irradiated in the spectral region between 1.0 eV and 2.2 eV. The luminescence shows the well known maximum at 0.43 eV. An interesting feature of this measurement is that the emission bands were quenched equally. In the curve shown in Fig. 6 the quenching rate was

and

H.-J.

SCHULZ

luminescence by diminishing the number of Co++ centres. If the additional irradiation causes a loss of Co++ centres by charge transfer, it might be possible to detect corresponding changes of emission in other ranges of the spectra. In other words, the carriers involved in the change of ionization state of Co should be missed on other places and energetic levels of the crystal. It turns out that this is indeed the case: A new emission appears in the range 0.6 eV to 0.9 eV, when the additional radiation acts on the crystal (Fig. 6). A comparison of the spectral distribution of this additional emission and of that of the Cu++ emission(sJs) shows the striking similarity of both. A slight deviation is shown by the 0.71 eV peak which is somewhat more pronounced here than in crystals containing copper only. In view of the good agreement of both spectra we assume that the additional radiation creates Cu++ centres. Since it is very likely that copper in ZnS appears in the ionization states Cu+ and Cu++, we conclude that the additional Wavelenath. IL radiation creates Cu++ centres by the transition Cu+ + Cu++. At high irradiation levels one can observe some weak traces of Cu++ emission even ZnS:Co without additional irradiation. But also in this case the Cu++ emission is considerably enhanced by additional radiation. It should be emphasized that a change of temperature by the additional radiation as a reason for the decrease of the O-4 eV Co++ emission is excluded. The quenching of Co++ emission by additional radiation occurs whenever the crystal is excited in the main absorption bands at 0.85 eV However, when -0-25 and 1.75 eV, as Fig. 7 shows. 03 0.4 0.5 0.7 08 09 0.6 the crystal is excited in an intermediate range Photon energy, eV (l-06 eV 2 E 5 1.67 eV) the infrared emission FIG. 6. Emission spectrum of a ZnS : Co crystal at 76”K, increases in case of the simultaneous irradiation. (a) without and (b) with additional irradiation (2.5 In this experiment the emission was measured eV s E 5 4.0 eV). Excitation: 1.0 eV 5 E s 2.2 eV. Note that the intensity scale in the high energy part through a germanium filter, so infrared light of is expanded a hundred times. energies lower than O-79 eV was recorded, which could have been the well known Co++ or Cu++ about 20% regardless of the wavelength of emisemission and/or a new emission band. In view of sion. From this observation we may conclude that the bad resolution, Fig. 7 gives only quite a rough the range of spectral emission between 0.37 eV idea what happens. However, since neither Cu++ and O-46 eV belongs to the same kind of emission nor Co++ show excitation bands in this intermecentre. Thus the possibility that the two maxima diate range, one might tentatively assume that the belong to two different ionization states of Co additional radiation creates a third kind of i.r. may be ruled out. emitting centre, the nature of which is to be The answer to our initial question is that addiexplained. tional irradiation as well can reduce the i.r. Considering the shape of the excitation spectrum

OPTICAL

TRANSITIONS

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ZnS TYPE

of Fig. 7, one should expect some influence of the additional irradiation on the absorption of the crystal. Measurements of the absorption coefficient as function of photon energy without and with additional radiation show indeed an enhanced absorption with a maximum at about 1.3 eV (Fig. 8). This is just the spectral range where the i.r. emission can be enhanced by additional radiation (Fig. 7). So the enhancement of i.r. emission and the increased absorption in this range seem to be connected with each other. too Y a

tz

II

Excitation waveiength, p 0% 10 09

07

CRYSTALS

CONTAINING

COBALT

193

In order to compare the absorption of copper with our stimulated absorption we plotted schematically the range of Cu absorption taken from Ref. 10 on Fig. 8. The comparison of both Cu absorption and the new stimulated absorption supports the idea that other absorbing centres apart from the copper are created. However, the high energy range of the absorption is influenced by the increased Cu+f absorption.(s) Incidentally one can understand that the increase in absorption due to additional light shows minima in the l-75 eV range (Fig. 8) because of the absorption due to Co++ ions superposed there, which should decrease under the action of the additional light as the number of Co++ ions does. The wavelength dependence of the action of additional light fits well in the charge transfer model. Figure 9 shows that the action of quenching exists in the violet, blue, and green range, but that Wovelength.

p

Photon energy, eV FIG. 7. Excitation spectrum of a ZnS : Co crystal at 76°K without and with additional irradiation (3.2 eV 5 E $ 4.0 eV). Emission of photons with energies less than 0.79 eV measured.

Wovelenglh,

p

24

a% 27

2%

29

32

33

Photon energy, eV

FIG. 9. Quenching of infrared emission at 76°K by additional irradiation. Excitation: 1.55 eV $ E 5 2-O eV.

Photon

energy,

eV

FIG. 8. Increase of absorption coefficient of a ZnS : Co crystal 7

at

76°K under the influence of additional irradiation (2-S eV s E s 3.6 eV).

there is a limit at about 2.5 eV, where the quenching disappears. The half height corresponds roughly to an energy of 2.7 eV. Provided that the valence or conduction bands are involved, the transition should be connected with levels of this energetic distance from one of the bands. A comparison of the spectrum of sensitization(W of copper activated crystals led us to the conclusion that the absorption of additional radiation may

194

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and

H.-J.

SCHULZ

cal infrared emission of Cu++ appears. The increase of infrared emission under excitation at about 1.3 eV (Fig. 7) by additional irradiation could at least partially be due to Cu++ ions as well. 2. The free electrons in the conduction band are partially captured by Co++ ions, which are transformed in that way into Co+ ions. So the number of Co-H remains reduced until the Co+ ions capture free holes from the valence band. This is consistent with earlier measurements of ANTONOVROMANOVSKIand SHCHUKIN.(~~) DISCUSSION AND SUMMARY The problem if the additional absorption and We have seen that the absorption spectrum of the the additional emission caused by simultaneous crystal containing cobalt fits well into the level irradiation are connected with the appearance of diagramme of Co++ calculated by WEAKLIEM,@) Co+ ions cannot yet be solved for the following and that the excitation spectrum corresponds to reasons: the Co+ having 383 configuration like the absorption spectrum. Moreover, the infrared Ni++ has, the optical data of both should be emission finds its place in the system, even if there similar in some way. As for any emission of Ni++ is no quantitative explanation for the relative there is no clear proof until now, whereas the probabilities of radiative and non-radiative transiabsorption has been studied by WEAKLIEM,(@who tions at present, or more specific: why most transigave also the assignments according to his calculations are non-radiative ones, although relatively tion. In view of the fact that Ni* and Co* are large energy intervals are involved, while the isoelectronic one might be tempted to give the transition 4Ts(F) -+4As(F) with a low energy of assignments sTl(F) + sAs(F); sTl(F) -+ lTz(P) O-4 eV gives rise to the characteristic infrared and sTl(F) -+ lE(D) to the additional absorption emission ? Summarizing, it is reasonable to in the range of 1.3 eV. If this were true, there assume that the optical phenomena in absorption, should also appear a strongly increased absorption excitation, and emission take place essentially in the visible due to the sTl(F) --f 3Tr(P) transition, within the Co++ centre. On the other hand it is which for Ni++ m ZnS is markedly stronger than likely that some of the transition metals in II-VI the absorption sTl(F) --+ aAs( Since we did not compounds are present in different ionization find this absorption we have no evidence for states. By ESR measurements MATOS~I and cotransitions within Co+. workers,(ll) and R%JBER and SCHNEIDER~~)proved The increase of infrared emission with decreasthe change of the ionization states of Cr and Fe in ing temperature may also be understood by the ZnS crystals caused by ultraviolet irradiation and change of equilibrium Co+ -+ Co++. Obviously by additional doping of the crystal. The increase of the thermal energy is not sufficient to empty optical absorption in ZnS(Cu) due to appropriate cobalt traps in the range of lOO-300”K, and to irradiation has already been shown in an earlier transform Co+ into Co++. But the opposite process work.(ls) Following that line it is likely that the Co++ --f Co+ may occur on thermal excitation of results we obtained on irradiating the crystals valence electrons and will decrease with decreasing additionally are due to a charge transfer process in temperature, and therefore the rate of available the sense of a reduction of the number of available Co++ should increase. In this process other traps, Co++ centres, and are therefore connected with a for instance chlorine, may play an important role. decrease of the absorption and emission at these So it turns out that the action of an additional centres. A comparison with the work done on radiation on cobalt activated phosphors and the ZnS(Cu) shows that in case of the absorption of temperature dependence of the luminescence additional radiation we are dealing with transitions whenever copper is present may be understood by from Cu levels in the band gap to the conduction assuming a charge transfer process and by a band. This absorption has two consequences: change in the equilibrium Co++ e Co+ connected 1. The Cuf becomes Cu++, therefore, the typiwith the change Cu+ e Cu++. take place at Cu+ centres with a level 1.1 eV above the top of the valence band. This gives rise to a change of ionization state Cu+ -+ Cu++. The free electrons are captured by the cobalt centre Co++ -+ Co+. The charge transfer from Cu to Co is in good agreement with the results one obtains in electroluminescence and electrophotoluminescence of phosphors activated by copper and cobalt.@)

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TYPE

Acknowledgments-The authors gratefully acknowledge the discussions with Professor I. BROSER and the helpful comments of the referee. In addition we wish to acknowledge the assistance of Mr. H. MAIER in the low temperature technique and of Mr. R. MOSER in preparing phosphors containing cobalt. We thank Miss M. KINDLER and Miss H. DOBBENV~ANN, who assisted in performing the experimental work. This work was supported by a grant of the Deutsche Forschungsgemeinschaft.

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5. 6. 7. 8.

9.

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of the Electrochemical Society, Los Angeles 1962. Abstract 49. An~esu. Ckem. 74. 355 11962). PAPPALARDOR. and DI~TZ R. E., Phjls. Re;. lZ& 1188 (1961). WEAKLIEM H. A., J. Chem. Phys. 36,2117 (1962). ALLEN J. W., Proc. Phys. Sot. SO, 1385 (1962). MARSHALL R. and MITRA S. S., Phys. Rev. 13&L, 1019 (1964); BALKANSKIM., NUSIMOVICIM. and LE TOULLEC R., J. Phys. 25, 305 (1964). BROSER I. and FRANCE K.-H. J. Phys. Chem. Solids 26,1013 (1965). BROSER I. and SCHULZ H.-J., J. Electrochem. Sac. 108, 545 (1961); SCHULZ H.-J., Phys. Stat. Sol. 3,485 (1963). MATOSSI F., R~UBER A, and SUPPER F. W., 2. Naturforschg. 18A, 818 (1963). RXUBER A. and SCHNEIDERJ., Z. Nasurforschg. 17a,

REFERENCES 1. ARPIARIANN. and CURIE D., C.R. Acad. Sci. Paris 234,75 (1952). 2. GARLICK G. F. J. and DUMBLETON M. J., PYOC. Phys. Sot. B67,442 (1954). 3. HOOGENSTRAATENW., Philips Res. Repts. 13, 515

12.

4. BROSER I., GUMLXCH H.-E. and MOSER R., Pkysik. Verkandl. 3,227 (1963); GUMLICH H.-E., Meeting

13. A~~Fuw-R~~N~~~Iu V. V. and SHWUKIN I. P., Dokl. Akad. Nauk. SSSR 71,445 (1950).

(1958).

10.

Il.

266 (1962).