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Luminescence of Ni doped CaF2
Journal of Luminescence 22(1981)139—145 North-Holland Publishing Company
139
LUMINESCENCE OF Ni DOPED CaF2 J. CASAS GONZALEZ and P.J. ALONSO Departamento de Fz’sica Fundamental, Cátedra de Optica, Universidad de Zaragoza, Zaragoza, Spain
H.W. DEN HARTOG Solid State Physics Laboratory, 1 Melkweg, Groningen, The Netherlands
R. ALCALA Departamento de Fisica Fundamental, Cdtedra de Optica, Universidadde Zaragoza, Zaragoza, Spain Received 29 April 1980
Luminescence measurements of CaF2 Ni are reported. Before X-irradiation an emission band at 680 nm is observed with the corresponding excitation band at 255 nm. After RT X-irradiation the emission spectrum consists of three peaks at 290, 375 and 680 nm, all of them with an excitation band at 255 nm. The same emission spectrum together with the electron—hole recombination band is obtained by X-ray excitation. A comparison with previous EPR and optical absorption 2+ centers. measurements The emissionindicates processesthat are similar to those found CaF the emission bands are in due to different kinds of Ni 2 Co and CaF2 Mn.
1. Introduction Complex centers consisting of an intrinsic defect near to an impurity ion as well as changes in the charge state of the impurities can be produced by X-irradiation of impurity doped ionic crystals. Defects ofboth kinds have been detected in alkali halides and in MgF2, KMgF3 and RbMgF3 doped with 3d ions [1—8]. In CaF2 doped with rare earth impurities (RE) reductions from the trivalent states2~to the divalent ones that have can been byseveral X-irradiation. The reoxidationby RE3~) processes, beproduced induced in ways, are accompanied (RE the emission of the RE3~ions [9—13]. Similar results have been observed in CaF 2 doped with cobalt and manganese 2+ and Mn2~ respectively in a cation [14—16].These impurity enter CaF2reduces as Co some of them to the monovalent substitutional position andions X-irradiation state. In both cases the reoxidation processes are responsible for some luminescent emissions observed in X-irradiated crystals [14—16]. —~
0 022-2313/81/0000—0000/$02.50 © North-Holland
140
J. Casas Gonzdlez et al.
I Luminescence of Ni doped CaF2
In two previous papers [17,18] we have studied the formation of Ni* centres by X-irradiation of CaF2 : Ni samples. Following our study on the spectroscopy of 3d impurity ions in CaF2 we report in the present paper the emission properties of CaF2 : Ni before and after X-irradiation. We have detected some emission bands The that 2~centers. have beenmechanisms associated with severaltotransitions of different of Ni emission are similar those reported for Cokinds and Mn doped CaF 2.
2. Experimental The CaF2 : Ni samples used in our experiments were from two different sources. Some of them were purchased from Optovac Inc. The others were grown in the Solid State Physics Laboratory (Groningen, Holland) by means of the Bridgman technique employing a 25 kW high frequency generator. The crucible material chosen was high purity carbon. The nickel impurities were added to the starting material as NiF2 the nominal concentrations were about 0.4 mol %. In some of the samples small concentrations of rare earth impurities were present. Photoluminescence measurements were taken using a conventional set-up with two monochromators (0.25 m Bausch—Lomb and 0.50 m Jarrell—Ash). The exciting source was a XBO 150 W Xe lamp. Detection was performed with a RCA/C3 1034 photomultiplier and associated optics and electronic. X-ray luminescence was excited using a Cu-cathode X-ray tube working at 40 kV and 20 mA. The same X-ray source was used for the irradiation of the samples.
3. Experimental results No emission has been found in pure CaF2 : Ni crystals before X-irradiation. In some “as grown” CaF2 : Ni samples also containing RE impurities an emission band centered at 680 nm has been observed in RT measurements. This emission band is shown in fig. I. The corresponding excitation spectrum (also given in fig. 1) shows a main band at about 255 nm. An absorption band that coincides with the excitation one has also been observed in these samples. It has been assigned to position off-center[18]). Ni~ 2~substitutional ionsDuring (displaced in the [100]-the direction from the Cagiven in fig. 2 has been observed. RT X-irradiation emission spectrum It shows, besides the band at 680 nm, two other main bands at 290 and 375 nm. There is also a broad band under the 290 nm one that is due to electron—hole recombination [13]. On the other hand the three bands appear as soon as we start the irradiation of the samples in contrast with the results obtained in Co doped CaF 2 [14]. The relative intensities of the 680 nm band with respect to those of the 290 and 375 nm are different from sample to sample. The relative intensities of the 290 and 375 nm bands are not always the same. This can be partially due to the presence of the electron—hole recombination band and also due to self-absorption
J. Casas Gonzalez etal.
I Luminescence of Ni doped CaF2
141
0
ii
d
I I
C
II
~
,i!
~ I 300
400
_______
500
500
700
)~(nm) Fig. 1. Emission ( ) and excitation spectrum tals. Both have been measured at RT.
(——-—)
of some “as grown” CaF2 Ni crys-
problems because during X-irradiation a strong absorption band is created at 255 nm [18]. The tail of this band and another band that appears in some of the samples in the 290 nm region produce self-absorption of the 290 nm emission. On the other hand the 290 and 375 nm bands always appear together and, as we will see later,
200
400
600
)5(nm)
800
Fig. 2. RT emission spectrum of CaF2 : Ni under X-ray excitation.
142
J. Casas Gonzalez et al.
I Luminescence of Ni doped CaF2
they show a similar behaviour during bleaching with 255 nm light. These results indicate that the two bands at 290 and 375 nm could be due to the same kind of fluorescent centers. We will assume that this is true although we cannot be completely sure about this point. After RT X-irradiation the CaF2 Ni crystals excited with UV light give the same emission spectrum as the one we have observed during X-irradiation except the electron—hole recombination band. The emission intensity of the 680 nm band is much bigger in X-irradiated samples than in untreated samples. The excitation spectra corresponding to all the emission bands show a main peak at 255 nm that coincides with the one given in fig. 1. This excitation band is also the same as the absorption band of X-irradiated samples [18] which has been associated with off-center Ni~ ions. The emission bands at 290, 375 and 680 nm can be bleached with 255 nm light. We give in fig. 3 the emission spectra recorded after several bleaching times with 255 nm light. It can be seen that during the bleaching the intensity of the 680 nm band decreases more slowly than those of the other two emission bands. The 290 and 375 nm bands show a parallel bleaching behaviour. From these results we conclude that the 255 nm light bleaches the centers responsible for the emissions at 290 and 375 nm more strongly than those associated with the 680 nm band. TL experiments corresponding to CaF2 : Ni crystals X-irradiated at 80 K have been reported in ref. [16]. The glow curve coincides with those of CaF2 doped with other 3d [16] and 4f ions [10]. The spectral distribution of each of the glow peaks coincides with that given in fig. 2 except the electron—hole recombination band which only appears in the 100 K glow peak. A decrease of the absorption band at
?~(nm)
—~
Fig. 3. Emission spectra of X-irradiated CaF2 : Ni excited by 255 nm light, recorded after several bleaching times with 255 nm light.
J. Casas Gonzalez et al.
I Luminescence of Ni doped CaF2
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255 nm, which has been created during X-irradiation, is associated with the glow peaks indicating that the TL emission is in some way related to the destruction of the centers responsible for the absorption band at 255 nm.
4. Discussion The emission bands at 290, 375 and 680 nm have been found in CaF2 : Ni crystals from different sources. They have not been observed either in pure CaF2 or in CaF2 doped with other 3d and 4f impurity ions. So we conclude that these three bands are due to Ni emissions. The 680 nm band has also been observed in CaF2 : Ni by Zoroarskaya et al. [19] under X-ray and UV excitation. Unfortunately these authors do not give the complete emission spectrum and consequently we do not know if they have also observed the other two emission bands under X-ray excitation. As we have already mentioned the 290, 375 and 680 nm bands have been observed in all the glow peaks of CaF2 Ni samples X-irradiated at 80 K and warmed up to RT. In a previous paper [16] we have proposed that the TL peaks in CaF2 : Me (Me Mn, Co, Ni) are due to the recombination of holes released from some hole traps (that are also independent of the impurity ions) with Me~centers formed during low temperature X-irradiation. The decrease of the Ni~absorption band associated with the glow peaks seems to be in favour of this mechanism in the case of CaF2 : Ni samples. Consequently, the emission bands weBut haveweobserved CaF2 :that Ni 2~ions. have alsoinfound duringisthe experiments should due to Ni there no TL correlation between thebeintensity of the 680 nm band and those of the other two emission bands. For example the 290 and 375 nm bands have not been observed in any unirradiated sample under excitation with 255 nm light while the 680 nm band was observed in some of the samples. Consequently, we must have at least two different kinds of Ni2~centers in order to account for the observed emissions. It has been shown earlier by EPR measurements that at least two different types of Ni~centers are formed in CaF 2: Ni crystals by X-irradiation [17]. Both of them are very similar. As a matter of fact both give an absorption band at about 255 nm and it has not been possible to separate these bands. Also their EPR signals are very close to one another. In these 2~ two centers the Ni+ ions aretoward displaced from the substitutional position), the center of center one of a cube of fluoride ions (Ca of the faces of that cube except in one of them (which we will call the perturbed Ni~center), there is also a small perturbation that destroys the tetragonal symmetry of the center. We can account for the observed TL results if the released holes are trapped by both kinds of N1’ centers giving two different types of excited Ni2~ions. One of them would give the emission at 680 nm and the other one the bands at 290 and 375 nm.
144
J. Casas Gonzalez et al.
I Luminescence ofNi doped CaF2
4 centers, both of Taking account the existence N1 the other experithem with into an absorption band at 255 of nm,two wedifferent can also kinds explainofall mental results in the following way. It has been found (both by EPR [17] and optical absorption [18]) that, even before X-irradiation, there are some Ni~centers (preferentially perturbed ones) in CaF 2 : Ni containing also rare earth impurities. During X-irradiation the concentration of both kinds of Ni+ ions increases but many more unperturbed than perturbed centers are formed. Bleaching with 255 nm light preferentially destroys the unperturbed Ni~centers [171.Taking into account these results the emission experiments can be understood if the 680 nmproduced band, observed 2~perturbed ions by ioniwhen we with 255Ni+ nmions light, due290 to Ni zation of excite the perturbed andis the and 375 nm bands are due to unperturbed Ni2~centers created in an excited state by ionization of the unperturbed Ni~ ions. Before irradiation we observe the emission of the perturbed Ni2~ions. The concentration of unperturbed Ni~centers in “as grown” crystals is usually smaller than that of the perturbed ones but if the emission intensities were proportional to the concentrations of each type of centers we should see small emission bands at 290 and 375 nm. Since we do not see these bands at all we must conclude that the emission process produced by excitation of the unperturbed Ni~ions is less efficient than the one corresponding to the excitation of perturbed Ni~centers. During X-irradiation both kinds of Ni2~ions are excited and all the emission bands are observed. After X-irradiation we also see both emissions because there are many more unperturbed than perturbed Ni~centers (usually more than 102 times). As the bleaching with 255 nm light goes on the relative intensities of the 290 and 375 nm bands decrease with respect to that of the 680 nm band which is in agreement with the faster destruction of the unperturbed Ni~centers. As a final conclusion we can say that, according to our experimental results, the emission band observed at 680 nm in CaF Ni crystals to per-Ni2t 2~ions and that the bands at 2902 and 375 nmcan are be dueassociated to unperturbed turbed Ni Acknowledgements We are indebted to Mr. P. Wesseling for growing some of the samples. We also thank the JEN and CSIC (Spain) for financial support.
References [1] M. Ikeya and N. Itoh, J. Phys. Soc. Japan 29(1970)1295. [2] M. lkeya, Phys. Stat. Solidi (b) 51(1972) 407. [3] F.J. Lopez, F. Jaque, A.J. Fort and F. Agulló-López, 1. Phys. Chem. Solids 38 (1977) 1101.
J. Casas Gonzalez eta!.
[4] [5] [6] [7] [8] [9] [101 [11] [121 [13] [14] [15] [16] [17] [18]
I Luminescence ofNi doped CaF2
145
S.C. Jam and K. Lal, Cryst. Lattice Defects 1 (1970) 165. W. Hayes and J. Wilkens, Proc. Roy. Soc. A281 (1964) 340. W.E. Vehse and W.A. Sibley, Phys. Rev. B6 (1972) 2443. K.M. Lee and W.A. Sibley, Phys. Rev. B12 (1975) 3392. W.A. Sibley and N. Koumvakalis, Phys. Rev. B14 (1976) 35. D.S. McClure and Z. Kiss, J. Chem. Phys. 39(1963) 3251. J.L. Merz and P.S. Pershan, Phys. Rev. 162 (1967) 217. J.L. Merz and P.S. Pershan, Phys. Rev. 162 (1967) 235. R.W. Ward and P.W. Whippey, Can. J. Phys. 51(1973) 382. J.H. Beaumont, W. Hayes, D.L. Kirk and G.P. Summers, Proc. Roy. Soc. A315 (1970) 69. R. Alcalá and P.J. Alonso, J. Lumin. 20 (1979) 1. R. Alcalá, P.J. Alonso, G. Lalinde and A. Carretero, Phys. Stat. Solidi 98 (1980) 315. P.J. Alonso and R. Alcalá, J. Lumin. 21(1980)147. J. Casas Gonzalez, H.W. den Hartog and R. Alcala, Phys. Rev. B21 (1980) 3826, P.J. Alonso, J. Casas Gonzalez, H.W. den Hartog and R. Alcalá, Phys. Stat. Solidi, (b) 100 (1980) 721. [19] E.I. Zoroarskaya, B.Z. Malkin, A.L. Stalov and Zh.I. Yakovleva, Soy. Phys. Sol. Stat. 10 (1968) 255.
139
LUMINESCENCE OF Ni DOPED CaF2 J. CASAS GONZALEZ and P.J. ALONSO Departamento de Fz’sica Fundamental, Cátedra de Optica, Universidad de Zaragoza, Zaragoza, Spain
H.W. DEN HARTOG Solid State Physics Laboratory, 1 Melkweg, Groningen, The Netherlands
R. ALCALA Departamento de Fisica Fundamental, Cdtedra de Optica, Universidadde Zaragoza, Zaragoza, Spain Received 29 April 1980
Luminescence measurements of CaF2 Ni are reported. Before X-irradiation an emission band at 680 nm is observed with the corresponding excitation band at 255 nm. After RT X-irradiation the emission spectrum consists of three peaks at 290, 375 and 680 nm, all of them with an excitation band at 255 nm. The same emission spectrum together with the electron—hole recombination band is obtained by X-ray excitation. A comparison with previous EPR and optical absorption 2+ centers. measurements The emissionindicates processesthat are similar to those found CaF the emission bands are in due to different kinds of Ni 2 Co and CaF2 Mn.
1. Introduction Complex centers consisting of an intrinsic defect near to an impurity ion as well as changes in the charge state of the impurities can be produced by X-irradiation of impurity doped ionic crystals. Defects ofboth kinds have been detected in alkali halides and in MgF2, KMgF3 and RbMgF3 doped with 3d ions [1—8]. In CaF2 doped with rare earth impurities (RE) reductions from the trivalent states2~to the divalent ones that have can been byseveral X-irradiation. The reoxidationby RE3~) processes, beproduced induced in ways, are accompanied (RE the emission of the RE3~ions [9—13]. Similar results have been observed in CaF 2 doped with cobalt and manganese 2+ and Mn2~ respectively in a cation [14—16].These impurity enter CaF2reduces as Co some of them to the monovalent substitutional position andions X-irradiation state. In both cases the reoxidation processes are responsible for some luminescent emissions observed in X-irradiated crystals [14—16]. —~
0 022-2313/81/0000—0000/$02.50 © North-Holland
140
J. Casas Gonzdlez et al.
I Luminescence of Ni doped CaF2
In two previous papers [17,18] we have studied the formation of Ni* centres by X-irradiation of CaF2 : Ni samples. Following our study on the spectroscopy of 3d impurity ions in CaF2 we report in the present paper the emission properties of CaF2 : Ni before and after X-irradiation. We have detected some emission bands The that 2~centers. have beenmechanisms associated with severaltotransitions of different of Ni emission are similar those reported for Cokinds and Mn doped CaF 2.
2. Experimental The CaF2 : Ni samples used in our experiments were from two different sources. Some of them were purchased from Optovac Inc. The others were grown in the Solid State Physics Laboratory (Groningen, Holland) by means of the Bridgman technique employing a 25 kW high frequency generator. The crucible material chosen was high purity carbon. The nickel impurities were added to the starting material as NiF2 the nominal concentrations were about 0.4 mol %. In some of the samples small concentrations of rare earth impurities were present. Photoluminescence measurements were taken using a conventional set-up with two monochromators (0.25 m Bausch—Lomb and 0.50 m Jarrell—Ash). The exciting source was a XBO 150 W Xe lamp. Detection was performed with a RCA/C3 1034 photomultiplier and associated optics and electronic. X-ray luminescence was excited using a Cu-cathode X-ray tube working at 40 kV and 20 mA. The same X-ray source was used for the irradiation of the samples.
3. Experimental results No emission has been found in pure CaF2 : Ni crystals before X-irradiation. In some “as grown” CaF2 : Ni samples also containing RE impurities an emission band centered at 680 nm has been observed in RT measurements. This emission band is shown in fig. I. The corresponding excitation spectrum (also given in fig. 1) shows a main band at about 255 nm. An absorption band that coincides with the excitation one has also been observed in these samples. It has been assigned to position off-center[18]). Ni~ 2~substitutional ionsDuring (displaced in the [100]-the direction from the Cagiven in fig. 2 has been observed. RT X-irradiation emission spectrum It shows, besides the band at 680 nm, two other main bands at 290 and 375 nm. There is also a broad band under the 290 nm one that is due to electron—hole recombination [13]. On the other hand the three bands appear as soon as we start the irradiation of the samples in contrast with the results obtained in Co doped CaF 2 [14]. The relative intensities of the 680 nm band with respect to those of the 290 and 375 nm are different from sample to sample. The relative intensities of the 290 and 375 nm bands are not always the same. This can be partially due to the presence of the electron—hole recombination band and also due to self-absorption
J. Casas Gonzalez etal.
I Luminescence of Ni doped CaF2
141
0
ii
d
I I
C
II
~
,i!
~ I 300
400
_______
500
500
700
)~(nm) Fig. 1. Emission ( ) and excitation spectrum tals. Both have been measured at RT.
(——-—)
of some “as grown” CaF2 Ni crys-
problems because during X-irradiation a strong absorption band is created at 255 nm [18]. The tail of this band and another band that appears in some of the samples in the 290 nm region produce self-absorption of the 290 nm emission. On the other hand the 290 and 375 nm bands always appear together and, as we will see later,
200
400
600
)5(nm)
800
Fig. 2. RT emission spectrum of CaF2 : Ni under X-ray excitation.
142
J. Casas Gonzalez et al.
I Luminescence of Ni doped CaF2
they show a similar behaviour during bleaching with 255 nm light. These results indicate that the two bands at 290 and 375 nm could be due to the same kind of fluorescent centers. We will assume that this is true although we cannot be completely sure about this point. After RT X-irradiation the CaF2 Ni crystals excited with UV light give the same emission spectrum as the one we have observed during X-irradiation except the electron—hole recombination band. The emission intensity of the 680 nm band is much bigger in X-irradiated samples than in untreated samples. The excitation spectra corresponding to all the emission bands show a main peak at 255 nm that coincides with the one given in fig. 1. This excitation band is also the same as the absorption band of X-irradiated samples [18] which has been associated with off-center Ni~ ions. The emission bands at 290, 375 and 680 nm can be bleached with 255 nm light. We give in fig. 3 the emission spectra recorded after several bleaching times with 255 nm light. It can be seen that during the bleaching the intensity of the 680 nm band decreases more slowly than those of the other two emission bands. The 290 and 375 nm bands show a parallel bleaching behaviour. From these results we conclude that the 255 nm light bleaches the centers responsible for the emissions at 290 and 375 nm more strongly than those associated with the 680 nm band. TL experiments corresponding to CaF2 : Ni crystals X-irradiated at 80 K have been reported in ref. [16]. The glow curve coincides with those of CaF2 doped with other 3d [16] and 4f ions [10]. The spectral distribution of each of the glow peaks coincides with that given in fig. 2 except the electron—hole recombination band which only appears in the 100 K glow peak. A decrease of the absorption band at
?~(nm)
—~
Fig. 3. Emission spectra of X-irradiated CaF2 : Ni excited by 255 nm light, recorded after several bleaching times with 255 nm light.
J. Casas Gonzalez et al.
I Luminescence of Ni doped CaF2
143
255 nm, which has been created during X-irradiation, is associated with the glow peaks indicating that the TL emission is in some way related to the destruction of the centers responsible for the absorption band at 255 nm.
4. Discussion The emission bands at 290, 375 and 680 nm have been found in CaF2 : Ni crystals from different sources. They have not been observed either in pure CaF2 or in CaF2 doped with other 3d and 4f impurity ions. So we conclude that these three bands are due to Ni emissions. The 680 nm band has also been observed in CaF2 : Ni by Zoroarskaya et al. [19] under X-ray and UV excitation. Unfortunately these authors do not give the complete emission spectrum and consequently we do not know if they have also observed the other two emission bands under X-ray excitation. As we have already mentioned the 290, 375 and 680 nm bands have been observed in all the glow peaks of CaF2 Ni samples X-irradiated at 80 K and warmed up to RT. In a previous paper [16] we have proposed that the TL peaks in CaF2 : Me (Me Mn, Co, Ni) are due to the recombination of holes released from some hole traps (that are also independent of the impurity ions) with Me~centers formed during low temperature X-irradiation. The decrease of the Ni~absorption band associated with the glow peaks seems to be in favour of this mechanism in the case of CaF2 : Ni samples. Consequently, the emission bands weBut haveweobserved CaF2 :that Ni 2~ions. have alsoinfound duringisthe experiments should due to Ni there no TL correlation between thebeintensity of the 680 nm band and those of the other two emission bands. For example the 290 and 375 nm bands have not been observed in any unirradiated sample under excitation with 255 nm light while the 680 nm band was observed in some of the samples. Consequently, we must have at least two different kinds of Ni2~centers in order to account for the observed emissions. It has been shown earlier by EPR measurements that at least two different types of Ni~centers are formed in CaF 2: Ni crystals by X-irradiation [17]. Both of them are very similar. As a matter of fact both give an absorption band at about 255 nm and it has not been possible to separate these bands. Also their EPR signals are very close to one another. In these 2~ two centers the Ni+ ions aretoward displaced from the substitutional position), the center of center one of a cube of fluoride ions (Ca of the faces of that cube except in one of them (which we will call the perturbed Ni~center), there is also a small perturbation that destroys the tetragonal symmetry of the center. We can account for the observed TL results if the released holes are trapped by both kinds of N1’ centers giving two different types of excited Ni2~ions. One of them would give the emission at 680 nm and the other one the bands at 290 and 375 nm.
144
J. Casas Gonzalez et al.
I Luminescence ofNi doped CaF2
4 centers, both of Taking account the existence N1 the other experithem with into an absorption band at 255 of nm,two wedifferent can also kinds explainofall mental results in the following way. It has been found (both by EPR [17] and optical absorption [18]) that, even before X-irradiation, there are some Ni~centers (preferentially perturbed ones) in CaF 2 : Ni containing also rare earth impurities. During X-irradiation the concentration of both kinds of Ni+ ions increases but many more unperturbed than perturbed centers are formed. Bleaching with 255 nm light preferentially destroys the unperturbed Ni~centers [171.Taking into account these results the emission experiments can be understood if the 680 nmproduced band, observed 2~perturbed ions by ioniwhen we with 255Ni+ nmions light, due290 to Ni zation of excite the perturbed andis the and 375 nm bands are due to unperturbed Ni2~centers created in an excited state by ionization of the unperturbed Ni~ ions. Before irradiation we observe the emission of the perturbed Ni2~ions. The concentration of unperturbed Ni~centers in “as grown” crystals is usually smaller than that of the perturbed ones but if the emission intensities were proportional to the concentrations of each type of centers we should see small emission bands at 290 and 375 nm. Since we do not see these bands at all we must conclude that the emission process produced by excitation of the unperturbed Ni~ions is less efficient than the one corresponding to the excitation of perturbed Ni~centers. During X-irradiation both kinds of Ni2~ions are excited and all the emission bands are observed. After X-irradiation we also see both emissions because there are many more unperturbed than perturbed Ni~centers (usually more than 102 times). As the bleaching with 255 nm light goes on the relative intensities of the 290 and 375 nm bands decrease with respect to that of the 680 nm band which is in agreement with the faster destruction of the unperturbed Ni~centers. As a final conclusion we can say that, according to our experimental results, the emission band observed at 680 nm in CaF Ni crystals to per-Ni2t 2~ions and that the bands at 2902 and 375 nmcan are be dueassociated to unperturbed turbed Ni Acknowledgements We are indebted to Mr. P. Wesseling for growing some of the samples. We also thank the JEN and CSIC (Spain) for financial support.
References [1] M. Ikeya and N. Itoh, J. Phys. Soc. Japan 29(1970)1295. [2] M. lkeya, Phys. Stat. Solidi (b) 51(1972) 407. [3] F.J. Lopez, F. Jaque, A.J. Fort and F. Agulló-López, 1. Phys. Chem. Solids 38 (1977) 1101.
J. Casas Gonzalez eta!.
[4] [5] [6] [7] [8] [9] [101 [11] [121 [13] [14] [15] [16] [17] [18]
I Luminescence ofNi doped CaF2
145
S.C. Jam and K. Lal, Cryst. Lattice Defects 1 (1970) 165. W. Hayes and J. Wilkens, Proc. Roy. Soc. A281 (1964) 340. W.E. Vehse and W.A. Sibley, Phys. Rev. B6 (1972) 2443. K.M. Lee and W.A. Sibley, Phys. Rev. B12 (1975) 3392. W.A. Sibley and N. Koumvakalis, Phys. Rev. B14 (1976) 35. D.S. McClure and Z. Kiss, J. Chem. Phys. 39(1963) 3251. J.L. Merz and P.S. Pershan, Phys. Rev. 162 (1967) 217. J.L. Merz and P.S. Pershan, Phys. Rev. 162 (1967) 235. R.W. Ward and P.W. Whippey, Can. J. Phys. 51(1973) 382. J.H. Beaumont, W. Hayes, D.L. Kirk and G.P. Summers, Proc. Roy. Soc. A315 (1970) 69. R. Alcalá and P.J. Alonso, J. Lumin. 20 (1979) 1. R. Alcalá, P.J. Alonso, G. Lalinde and A. Carretero, Phys. Stat. Solidi 98 (1980) 315. P.J. Alonso and R. Alcalá, J. Lumin. 21(1980)147. J. Casas Gonzalez, H.W. den Hartog and R. Alcala, Phys. Rev. B21 (1980) 3826, P.J. Alonso, J. Casas Gonzalez, H.W. den Hartog and R. Alcalá, Phys. Stat. Solidi, (b) 100 (1980) 721. [19] E.I. Zoroarskaya, B.Z. Malkin, A.L. Stalov and Zh.I. Yakovleva, Soy. Phys. Sol. Stat. 10 (1968) 255.