Luminescence of Pr3+-Activated fluorides

Journal of Luminescence 9 (1974) 288

296.0 North-J-lolland Publishing Company

LUMINESCENCE OF Pr3~-ACTIVATEDFLUORIDES J.L. SOMMERDIJK, A. BRIL and A.W. de JAGER Philips Research Laboratories Eindhoven, The J’Jetherlands Received 13 May 1974

The strong dependence of the emission spectrum of YF 3 Pr~on excitation source (228.8 nm, 213.9 nm or cathode rays) is ascribed to two different types of Pr~sites: one with a relatively strong crystal field and the other with a relatively weak crystal field. The presence of the latter is connected with the conversion of one short-wave 3+ was alsoUV found ( 215 for a-NaYF nm) photon into two visible photons. Two-photon luminescence of Pr 4 and LaF3, but not for CaF2 and BaF2 due to the too strong crystal field in these lattices. The occurrence of two-pho3+ . . ton Pr lummescence 3~,Er3+~activatedlattices. is compared with the The intensity intensity of the of IR-excited the Pr3~luminescence green emission at shortof the corresponding wave UV excitation Yb (213.9 nm) is rather weak. Luminescence of reasonable efficiency is, however, observed on excitation with cathode rays.

1. Introduction The 4f energy level scheme of Pr3+ [1] may be used for the conversion of one short-wave UV( ~ 215 nm) photon into two visible photons (fig. 1). Such a twophoton luminescence is demonstrated for a number of Pr3~-activatedfluorides [2, 3] whereas no such effect is found for Pr3~-activatedoxides. This can be ascribed to the stronger crystal field in the oxide lattices [4]. This stronger field shifts the lowest excited 5d level of Pr3~below the 4f level 1S~.Two-photon luminescence, which should occur from the i~0 level, cannot take place in oxides. In the present paper we will first consider the emission spectrum of YF 3+ 3 Pr as a function of the excitation source. In ref. [2] it was noticed that the emission spectrum of this phosphor on 213.9 nm excitation from a Zn lamp differs significantly from that obtained with 228.8 nm excitation from a Cd lamp. We have found that the emission spectrum with cathode-ray excitation differs even more from that obtained with 228.8 nm excitation and that this spectrum agrees with that obtained with 185 nm excitation [3]. Secondly, we will consider the occurrence of two-photon luminescence in other lattices and, finally, give the radiant efficiencies on cathode-ray excitation of a number of Pr3+~activatedfluorides.

288

J.L. Sommerdijk et al., Luminescence of Pr~+.activatedfluorides

289

50 ISO

40

~ 30

p

____

2

~20 ~D3 10

‘G4 _________

_____

}

~F

Hi

0

3~[1~.

Fig. 1. 4f energy level scheme of Pr

2. Experimental method Powder samples were prepared by firing intimate mixtures of the required amounts of dry fluorides at the following temperatures: 600—650°Cfor a-NaYF 4: 3~and NaLaF 3~900—1000°Cfor CaF 3~and BaF 3~and pr 4 YF : Pr 2 : Pr 2 : fired Pr at least twice 100—1100°Cfor 3~and LaF 3t The mixtures were 3 : hours, Pr the firings 3 : Pr over a period of several being made in Pt crucibles in an N 2/HF atmosphere. The fluorides PrF3, YF3 and LaF3 were obtained from the corresponding oxides: (a) by heating the oxides with NH4F up to 500 °Cor with NH4HF2 up to 300 °C~ or (b) by dissolving the oxides in concentrated HC1, followed by precipitation with HF. The spectral power distribution with UV excitation was measured with a 0.75 m Spex grating monochromator model 1700 II (grating blazed at 500 nm), and that with cathode-ray excitation with a 0.5 m Jarrell-Ash grating monochromator (grating blazed at 750 nm). A photomultiplier EMI 9558Q was used as a detector. Sources of UV radiation were the Zn and Cd quartz spectral lamps, Philips nos. 93106 and 93107, giving excitation with mainly 213.9 nm and 228.8 tim respectively, together with some radiation of longer wavelengths. For cathode-ray excitation the voltage of the demountable cathode-ray tube was kV, the current density being about 2[5]. All optical experiments were20performed at room temperature. 1 ~zA/cm

290

.J.L. Sommerdijk et a!., Luminescence of Pr~-activatedfluorides

3. Results and discussion 3.1.

Emission

spectra of YF3:

J\1N

3~on 228.8 nm excitation from a Cd disThe lamp emission YF3 : 0. 1%from Pr that obtained on 213.9 nm excitation charge (fig. spectrum 2) differsof significantly from a Zn discharge lamp (fig. 3). On 213.9 nm excitation, the spectrum has an emission peak at 407 nm (fig. 3); this peak is not observed at 228.8 nm excitation (fig. 2) ICC

400

450

500

550

600

~ A~nm)

3~onexcitation with mainly 228.8 nm of a Cd lamp. IFig. denotes 2. Emission the spectral spectrum radiant of YF3 power :0.1% in arbitrary Pr units (also in figs. 3 6).

~:1

400

500

450

550 ~-

Fig. 3. Eiiiissjon spectrum

of

YF

600 A(nm)

3~onexcitation with mainly 213.9 nm of a Zn lamp. 3 :0.1% Pr

5’+.activated fluorides

fL. Sommerdi/k et a!., Luminescence ofPr

291

since the excitation wavelength is too long and the ‘Sn level cannot be reached (cf. fig. 1). As has been argued in ref. [2] this peak must be ascribed to a 4f 4f transition of Pr3~,namely 1S~ 3P 3~also shows luminescence from 2 (fig. 1). Since Pr the 3P 1D 0 and 2 levels, the presence of this peak is therefore an indication of the occurrence of two-photon luminescence. In addition to the 407 nm peak, however, the spectra differ also for wavelengths longer than 470 nm, which has been noticed in ref. [21.The spectrum at 213.9 nm excitation (fig. 3) has some additional sharp lines, the strongest ones peaking at 483, 484 and 605 nm. These lines are not observed on 228.8 nm excitation (fig. 2). At cathode-ray excitation, these lines are even stronger than at 213.9 nm excitation (fig. 4).1SThe spectrum on cathode-ray excitation also contains the 407 nm peak due 3P to the 0 2 transition (see above). For wavelengths longer than 470 nm, the spectrum c~onsistsmainly of narrow emission lines and agrees with that obtained with 185 nm excitation [31. The question arises of why the emission spectrum changes so drastically with variation of the excitation source. In our 3+ opinion, change that is connected with theatocsite. It this is assumed the crystal field currence thansay onePr(I), type isofrelatively Pr one type of of more Pr3~site, strong, whereas that at the other type (Pr(II)) is relatively weak. The splitting of the Sd level of Pr(I) is strong due to the stronger crystal field and as a result the lowest excited Sd level of Pr(I) is situated below the 1S 0 level (fig. 1). For Pr(II), on the other hand, the splitting of the Sd level is smaller, and the lowest excited Sd level is situated above the 1~ level. The position of the ~ level is expected to be the same for both sites in view of the screened character of 4f electrons [1]. We may now consider how the experimental results 1S(figs. 2 4) can be fitted into the above assumptions. On 228.8 nm excitation, the 0 level of1(fig. Pr(H)1), cannot be and the excitedexcited at all. The ‘S~level lies too high,On namely 47 Xhand 10~cm lowest 5d level lies even higher. the other Pr(I) is excited, not in the 150 level but in the lowest excited Sd level. From this level Pr3~is de-excited to the 3P 1D 0 and 2 levels, giving rise to the emission spectrum as given in fig. 2. The emission peaks are rather broad, in agreement with the above assumption of the relatively strong crystal field at Pr(fl. 3~sites are excited. For Pr(I) excitation On 213.9 excitation, both types of since Pr this transition is parity-allowed, while occurs mainlynm through 4f Sd absorption, the 4f -4f transition from the ground level to ~ is parity-forbidden [1]. For Pr(II), however, excitation is only possible through this forbiddentransition, since the lowest excited Sd level lies too high for 4f—5d excitation with 213.9 nm. In the spectrum (fig. 3), the emission of Pr(II) is superimposed upon that of Pr(I). This superposition gives rise to the 407 nm emission peak due to the 1S~ 3P 2 transition and to narrow emission lines for wavelengths longer than 470 nm, e.g. at 483, 484 and 605 nm, due to the relatively weak crystal field at Pr(II). On excitation with 185 nm [3] or cathode rays (fig. 4), both Pr(I) and Pr(II) are excited quite effectively to the lowest excited 5d level, either directly (with 185 nm)

292

J.L. Sommerdijk eta!., Luminescence of Pr~”tactivatedfluorides

100

I

II~i

400

450

500

550 600 —.-A (rim)

Fig. 4. Emission spectrum of YF

3 :0.1% Pr~onexcitation with cathode rays (20 kV).

or via excitation 3P of the 1D host lattice (with cathode rays). For Pr(I) de-excitation occurs to the 0 and 2 levels resulting in relatively broad emission lines (cf. figs. 2 and 3). For Pr(II) de-excitation occurs to the 150 level, followed by 407 nm emission and narrow emission lines for wavelengths longer than 470 nm (cf. fig. 3). As can be seen in fig. 4, the emission spectrum with cathode-ray excitation consists mainly of the 407 nm peak and of narrow emission lines for wavelengths longer than 470 nm. Apparently the concentration of excited Pr(lI) is much higher than that of excited Pr(I). Since the efficiency 4f conclude 5d excitation will not differ muchisfor 3~sites,ofwethe must that process the concentration of Pr(II) the twohigher typesthan of Pr much that of Pr(I). In spite of the lower concentration of Pr(I), its spectrum dominates at 213.9 nm and 228.8 nm excitation (figs. 2, 3), since the more abundant Pr(II) is only excited weakly through a forbidden 4f 4f transition (213.9 nm excitation) or cannot be excited at all (228.8 nm excitation). Summarizing, we feel that the drastic change in the emission spectrum of YF 3+ with variation of excitation source can be explained by assuming two types 3: of Pr Pr3~sites: one with a low concentration and strong crystal field (Pr(I)), and the other with a higher concentration and weak crystal field (Pr(II)). Concerning the origin of the two types of Pr3+ sites, we can only speculate. Probably Pr(II) corresponds to Pr3~ions occupying the Y3~sites in YF 3 and the much 3~ions with a locally distorted coordination. Perhaps the less abundant Pr(I) to Pr latter is connected with the difference between the cation coordination in PrF 3 [6] and that in YF3 [7].

J.L. Sommerdijk eta!., Luminescence of Pry-activated fluprides

293

3.2. Two-photon luminescence ofPr3’ in other lattices As we have argued in ref. [2] and in the previous section, the occurrence of twophoton luminescence is indicated by the presence of an emission peaking at about 400 nm. This emission requires excitation to the 150 level and can therefore occur only at excitation with sufficiently short wavelengths (~215 nm). Such an excitation is provided by a Zn discharge lamp (213.9 nm) or by cathode rays. Fig. S gives the emission spectrum of a-NaYF 3~with cathode-ray excitation. In this spec4 : Prpresent. Emission at about 400 nm was also trum an emission at 407 nm is clearly 1CC

400

450

500

550

600

~ A(nm)

Fig. 5. Emission spectrum of n-NaYF

3~onexcitation with cathode rays (20 kV). 4: 1% Pr

400

500

450

550

600

—0.-A (rim) Fig. 6. Emission spectrum of CaF

3~onexcitation with cathode rays (20 kV). 2 : 0.5% Pr

294

J.L. Sommerdifk et a!., Luminescence of Pr-~-activatedfluorides

found for LaF3 : Pr3~(see also ref. [8]). We conclude, therefore, that in a-NaYF4: 3~and LaF 3~,just as in YF 3~,two-photon luminescence from the S Pr 3 : Pr 3 : Pr 0 level takes place. 3~and BaF 3~do not show this effect. the contrary, fluorides CaF2the : Prusual 3P 1D 2 : Pr TheOn spectra of these the fluorides contain 0, 2 emissions at wavelengths longer than 470 tim, but no emission is observed at about 400 isnm. As in anfig. example 3~with cathode-ray excitation given 6. Ap-the emission spectrum of CaF2 : Pr parently even within the class of fluorides, the occurrence of two-photon luminescence is strongly dependent on the choice of the host lattice. We can correlate the absence of two-photon luminescence in Pr3tactivated CaF 2 and BaF2 with absorption data which indicate that in these lattices the lowest excited Sd level is situated at about 46 X ~ cm 1 [9, 10]. The lowest excited Sd level lies therefore below the 150 level at about 47 X 10~cm 1 (fig. 1). 1S In agreement with this energy level scheme, practically no emission occurs from3tactivated the 0 level (e.g.where fig. 6).the lowest excited Sd oxides A also similar for Pr level lies situation below theexists i5~level. Emission of two visible photons cannot take place from the 5d level due to the large Stokes shift of the Sd 4f luminescence. For example the Sd 4fluminescence of Y 3~,corresponding to transitions terminating 3A15O12 : Pr300 to 450 nm [11]. Oxysulphides are also on the H and F manifolds, extends from unsuitable as the host lattice. A broad excitation band has been found for Y 202S: 3~at about 300 nm [12]. Pr It is interesting to compare the observed dependence on the host lattice with results of our earlier work concerning the host-lattice dependence of the IR-excited green emission of Yb3~,Er3~-activatedphosphors [13, 141. Table 1 compares the occurence of two-photon luminescence of Pr3+ with the relative intensity of the IRexcited green emission of the corresponding Yb3+, Er3tactivated up-conversion phosphors. It appears that two-photon luminescence is suitable in lattices which show a strong IR-excited green emission when activated with Yb3~,Er3~[13, 14]. In our opinion this is not surprising. The IR-excited green luminescence of Yb3+, Er3+.acti~ vated phosphors is suitable in lattices where the crystal-field splitting at the Yb3~, Table 1 Comparison between the occurrence of two-photon Pr3’ luminescence and the relative intensity (Ig) of the infrared excited green emission of the corresponding Yb3’, Er3tactivated lattices. Lattice

Two-photon pr3+ luminescence

a-NaYF 4 YF3 LaF3 CaF2 BaF2 flxides

+ + +

—

100 60 30 10 < 11)

3 ‘~-activatedfluorides

J.L. Sommerdqk et a!., Luminescence ofPr

295

Er3~ions is weak [13, 14]. In lattices with a strong crystal field at the Yb3t Er3~ ions there are more radiative and non-radiative losses from the intermediate and upper levels in the two-step excitation process of the green emission, resulting in a weaker intensity of this emission [13]. The two-photon luminescence of Pr3~is also suitable in lattices with a weak crystal field at the Pr3~ions, since then the lowest excited Sd level remains above the 150 level, so that emission from the latter can take place. 3.3. Cathode-ray efficiencies The luminescence of the Pr3~-activatedfluorides on 213.9 nm excitation with a Zn discharge lamp is rather weak. At low IJr3+ concentrations, the absorption of the UV radiation is too weak, whereas at higher concentrations (~ 1%) the luminescence is effectively quenched by Pr3~ Pr3~interactions [12, 15]. On excitation with cathode rays there is no problem of weak absorption at low Pr3+ concentrations since then Pr3~excitation occurs mainly via excitation of the host lattice. We measured Table 2 Radiant efficiency r~on cathode-ray excitation (20 kV) of some Pr3’-activated fluorides. Lattice

Pr3’~concentration

(%)

~ (%)

YF 3

LaF3

0.1

3.0

0.3

3.0

0.5

2.3

1.0

1.6

0.1

0.6

0.3

0.8

0.5

1.0

1.0

0.9

0.1 0.3 0.5

0.6 0.9 0.8

1.0

1.0

NaLaF4

0.1 0.3 0.5 1.0

0.5 0.6 0.7 0.6

CaF2

0.1 0.5

0.5 0.5

BaF2

0.1 0.5

2.0 1.5

~-NaYF4

296

J.L. Sommerdijk eta!., Luminescence of Pr~-actiyatedfluorides

emission efficiencies with cathode-ray excitation of a number of Pr3~-activated fluorides, the Pr3~concentration being varied between 0.1 and 1%: the phosphors luminesced pink or purple with reasonable efficiency. Table 2 gives the radiant efficiencies (~)for the various phosphors. The highest efficiency was obtained for YF 3 3+ (i~= 3.0%). For comparison, the same value of~has been reported for Y Pr 3~[16], whereas a higher value has been reported for Y 2O3: 3t namely~7 = Pr [l7].The efficiencies of the Pr3~-activatedfluorides 2O2S Pr same order of 8.0% are of: the magnitude as those obtained for the corresponding fluorides activated with Eu3+, Tb3~,Sm3~or Dy3~.

Acknowledgements The authors are indebted to Miss F. Strik for preparation of some of the samples and to Mr. G. de Vries for performing some of the optical experiments.

References 11 12] 13] [4] [5] [6] [7] [8] [9] [10] [11] [121 [13] [14] [15] [16] [17]

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