Luminescence of Pr3+ in SrAl12O19: Observation of two photon luminescence in oxide lattice

JOURNAL OF

LUMINESCENCE ELSEVIER

Journal of Luminescence

71 (1997) 285-290

Luminescence of Pr3+ in SrAl12019: Observation of two photon luminescence in oxide lattice A.M. Srivastavaa* *, W.W. Beersb a GE Corporate Research and Development Center, 1 Research Circle, Niskayuna, NY 12309, USA b GE Lighting, Nela Park, Cleveland, OH 44112, USA Received

10 May 1996; revised

18 November

1996; accepted

22 November

1996

Abstract The observation of photon-cascade emission (PCE) in Pr3+ activated SrAl12019 under vacuum ultraviolet excitation (VUV) is reported and discussed. In this host lattice, the lowest-energy component of the Pr 3+ 4f5d excited configuration is located above the ‘SO state. The excitation of the Pr3+ 4f5d state by VUV photons results in radiative emission originating from the ‘SO level state. The subsequent radiative emission from the 3P~ state confirms the photon-cascade emission of Pr3’ activated SrAl12019. To the best of our knowledge this is the first observation of PCE in an oxide matrix. Keywords: Oxide; Two photon luminescence;

Pr’+;

SrA112019

1. Introduction The ability of the Pr3+ ion to generate two visible photons per ultraviolet photon absorbed was first described by Piper, DeLuca and Ham in 1976 [l]. The greater than unity quantum yield was achieved at room temperature in Pr3+ activated YF3. The authors reported a visible quantum efficiency of 140% for 185 nm excitation which was subsequently confirmed by other workers [2, 31. The photon cascade emission process (PCE), responsible for the greater than unity quantum yield can be understood on the basis of the energy level scheme of the Pr3’ ion, exhibited in Fig. I. If the lowest energy component of the Pr3+ 4f5d configuration is located above the ‘So state, then nonradiative transitions may populate the ‘SO

* Corresponding (@rd.ge.com.

author. Fax: 001 518 387 5299; e-mail: srivast

0022-23 13/97/$17.00 Published PII SOO22-2313(96)00221-9

by Elsevier Science B.V.

state following 4f2 (3H4) + 4f5d excitation. This transition being allowed the incident photon is strongly absorbed by the activator Pr3’ ions. The large energy gap between the ‘SO state and the next lower-energy state results in predominantly radiative decay of the ‘SO state to the intermediate states of the Pr3+ 4f2 configuration. The first photon emitted corresponds to the ‘SO + ‘16 transition. The next step in the PCE process is the population of the 3Po state through ‘16 +3P~ nonradiative decay (see Fig. 1). The subsequent radiative decay of the 3P~ level results in the emission of the second photon. The generation of PCE requires the isolation of the ‘SO state from the 4f5d configuration. Photoncascade emission is not observed in solids where the 4f5d configuration is situated below the ‘SO state (-46 500 cm-’ ). This is the case, for example, in Pr3+ activated LiYF4 [l] and GdB03 [4]. In these materials, excitation into the 4f5d state results in broad-band ultraviolet luminescence due to the

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50_

1

5dBand

]

-T+ 40-

30-

magnetoplumbite-type structure (hexagonal, space group P6s/mmc) in which the Sg+ ions are coordinated to 12 oxygen ions (six oxygen ions at 2.75 A with an additional six oxygen ions at 2.785 A) [6]. The A13+ ions are present in tetrahedral, octahedral and pentagonal bipyramidal coordination. The Pr3+ ions substitute for the Sff ions and charge compensation is achieved by substituting Mg2+ for A13+ in SrA1120ia. It was hoped that the high coordination number of S?+ sites would result in a low crystal field strength on the Pr3+ 4f5d configuration. We will show that the Pr3+4f5d level is indeed located above the ‘So state in SrAli2Oig. We will further show that under 185 nm excitation, emission from the ‘So state occurs and that the two photon luminescence is confirmed by the subsequent emission from the 3Pc state. 2. Experimental

Pr”

Fig. 1. The energy level diagram of photon cascade emission.

of the Pr3+ ion and the process

4f5d -+ 4f2 transitions. In general, high-energy position of the lowest-energy component of the Pr3+ 4f5d configuration can be induced by doping in a host lattice with predominantly ionic character. The fluorides are ideal candidates. The high electronegativity of the F- ion generally induces weak crystal field splitting of the excited 4f5d configuration resulting in a high-energy position of the 4f5d state of the activator ion. Therefore, it is not surprising that PCE has only been observed in Pr3+ activated fluorides. Indeed, luminescence from the Pr3+ ‘SO state was first observed by Elias et al. in Pr3+-doped LaF3 [5 1. For fluorescent lighting applications, however, the instability of fluorides in mercury discharge does not permit the utilization of YFs-Pr3+ (and other PCE phosphors) in current commercial fluorescent lamp products that are based on low-pressure mercury discharge. In this paper the luminescence of Pr3+ activated SrAli2Oig under vacuum ultraviolet excitation is reported and discussed. The host lattice has the

Samples with the Sri_,Pr,Ali2--xMgx0i9 general composition were synthesized at 1400°C by the classical solid state reaction technique under an atmosphere of 2°hH2-980/N2 forming gas. All starting materials were of the highest purity available commercially. X-ray diffraction indicated single-phase materials. Optical properties of a sample doped with 1 mol% Pr3+ was evaluated with a McPherson 225 VUV spectrometer and an Oriel 77250 visible spectrometer. Data collection was done on a PC using SpectraCalc and custom software written by McPherson. The McPherson spectrometer was equipped with a McPherson Hinteregger type H2 lamp to provide excitation in the VUV region. A quartz window was used to separate the evacuated excitation-sample chamber from the monochromator used for detecting the emitted light. Excitation intensity was checked before and after emission scan to confirm minimal lamp drifting. Correction for gratings and detector system was not done. The present system permits the acquisition of the emission spectrum in the 200500 nm wavelength region. The excitation spectra were corrected for the wavelength-dependent variation in lamp intensity. This was accomplished by directing the excitation beam onto a sodium salicylate window that was monitored by a PMT. The resulting intensity versus wavelength file was applied to the collected sample file to provide the corrected excitation spectra.

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I

I

I

I

I

I

I

200

250

300

350

400

450

500

WAVELENGTH (nm) Fig. 2. Emission

spectrum

of Sro.99Pro.ol Al 11.9Mgo,o,019 at room temperature

3. Results and discussions The

room

temperature

emission

spectra

of

Sro.99Pro.o’Al’ ‘.&Igo.o’0’9 and Y0.999Pr0.00’F3 under 185 nm excitation are exhibited in Figs. 2 and 3, respectively. The assignment of the observed optical transitions in the emission spectrum is given in Table 1. Clearly, the luminescence of Pr3+ in SrA1120’0 is due to the radiative decay of the ‘SO state in the 200-450 nm wavelength region. The room temperature excitation spectrum for Pr3+ emission at 405 nm (‘SO -+ ‘16 transition) is shown in Fig. 4. A broad band with maximum at N 195 nm is observed. This band is assigned to the allowed Pr3+ 4f2 (3H4) ---f4f5d transition. The onset of the excitation spectrum at -2 10 nm (-47 600 cm-‘) confirms the isolation of the ’ SO state (N 46 500 cm-’ ) from the components of the 4f5d configuration. Thus, emission from the ‘SO state is observed following 4f2 --f 4f5d excitation (Fig. 2). The line at -485 nm in the emission spectrum is attributed to the 3P~ -+ 3& transition on the Pr3+ ion. The emission from the 3P~ state under 4f5d excita-

(A,, = 185 nm).

tion is due to the feeding of the 3Pa state through the ‘I6 + 3P~ nonradiative decay. Note that the ‘I6 state in turn has been populated through the ’ SO-+ ’ I6 emission transition. The simultaneous occurrence of the ‘SO+lI6 (at -405nm) and the 3P~ -+3Hq (at -v485nm) in the emission spectrum confirms photon-cascade-type emission in Pr3+ activated SrA1’20’9. The observation of PCE in a Pr3+ activated material does not necessarily imply greater than unity quantum efficiency. Note that the ’ SO--t ’ I6 transition is the most important step in the PCE process. The first photon is emitted in this transition and the transition is also responsible for the population of the 3P~ state under VUV excitation. Thus, the condition for generating visible quantum efficiency in excess of unity requires maximizing the ’ SO-+ ’ I6 transition probability. In the Judd-Oflet theory the strength of this transition is controlled by the !& phenomenological intensity parameter. Thus, the condition for greater than unity quantum efficiency is &/!& = &I526 = 0 and !& # 0. Hence, 526 must dominate over both Qz and s24 [l].

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WAVELENGTH (nm) Fig. 3. Emission

spectrum

of YO.999PrO.OOIF3at room temperature

The requirement of s26 dominating the s22 and s24 intensity parameters for quantum efficiency in excess of unity is best illustrated by comparing the quantum yields in the two PCE materials, LaFs (QE=0.833) and YFs (QE = 1.46) [7]. The s2, intensity parameters in YFs (LaFs) are: s22 = 0.13 (0.12); s2, = 0.70 (1.77) and as = 10.0 (4.78) in 10p2’ cm* units. Note that the ratio 52&&j in LaFs (0.37) is much higher than that in YFs (0.07). This suggests that in LaFs transitions of the type ‘SO -+ ‘Gq, the strength of Table 1 Assignment of optical SrA112019-Pr3+ Wavelength

220 251 276.6 342 404 484.5

(mn)

transitions

in the emission

spectrum

Transition

‘SO +%I4 1So+3F4 ‘SO+‘Gq ‘So -+‘Dz ‘so+‘16 3Po + 3H4

of

(A,, = 185 nm).

which is controlled by 04 will be favored over the IS.0 + ‘16 transition. In agreement with the 52k values, the emission from the ’ SOstate in LaF3-Pr3+ is dominated by the ‘SO + ‘G4 and ‘SO -+ 3F4 transitions [S]. Since these transitions result in the emission of ultraviolet photons (see Table 1), the overall visible quantum yield is expected to be lower. In Pr3+ activated SrAli2019, the ‘SO + ‘G4 transition at 276nm dominates the radiative decay of the ‘SO state. In YFs-Pr 3+, on the other hand, the dominating emission from the ‘SO state is the radiative decay to the ‘16 level (Cf. Figs. 2 and 3). Hence, the ratio &4/526 in SrA1i2019 is expected to be unfavorable for obtaining quantum yield in excess of unity. The overall visible quantum efficiency is thus expected to be lower than that in Pr3+ activated YF3. We should mention that in the estimation of the overall quantum yield due to the PCE process the near infra-red (NIR) emission from the ‘G4 should not be ignored. All previous authors have ignored the possibility of this emission when estimating quantum yield due to the PCE process. In recent years, the radia-

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WAVELENGTH (nm) Fig. 4. Excitation

spectrum

(I,,

= 405 nm) of the Pr3+ emission

tive NIR emission of the ‘Gd state (1.3 urn) has been studied extensively for application in optical communication systems [9]. The probability of this emission depends on the host phonon frequency since the energy gap between the ‘Gq manifold and the ‘F4 manifold (- 3000 cm-’ ) can be easily bridged in lattices where high phonon frequencies are available for coupling with the Pr3+ ion. In fluorides where the maximum phonon frequency is in the range of 550 cm-‘, the ‘G4 + 3F4 nonradiative decay rate due to multiphonon relaxation is reduced. Thus, in Pr3’ activated fluorides the estimation of the overall quantum yield should take into account the NIR emission from the ‘Gq state.

in Sro.99Pro.olAl 11.99Mgo,o, 019.

The ’ Gq --+ 3F4 multiphonon relaxation rate ( W,,) in SrA112019 can be estimated from the modified energy gap law of van Dijik and Schuurmans [lo]: w,, = &I[-(AE

-

-=wnax)~l,

(1)

where ,!&I and CIare constants, AE is the energy gap and tLmrnax is the maximum phonon frequency. Although the position of the ‘Gd state in SrA112019 is not established, a AE value of -3000 cm-’ seems appropriate [9]. With Be, = 4 x 10’ s-l, c(= 5 x lop3 cm, and ha,,,, -700 cm-’ [ 1 l] we estimate W,, w 5 x 1O3 s- ’ Hence, the multiphonon relaxation process can compete with the radiative relaxation (W, w lo3 SC’) of the ‘Gd state in SrA112019.

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In a similar way, we can explain the radiative decay of the 3Po state in this host lattice. The 3Po state can relax nonradiatively into the ‘D2 state through multiphonon transitions. The energy gap between the 3Po and the ‘D2 state is approximately 3300 cm-‘. Hence, W,, N 3 x lo3 s-i from Eq. (1). Assuming a radiant lifetime of 3 ps [12], the radiative decay rate of the 3Ps state is q = 3 x lo5 s-l. Hence, the 1 i honon relaxation rate cannot com3Po+1D2 mutp pete with the radiative decay rate of the 3Pe state. In LaMgB,Oic [13] and LaBsO [14] where emission 3+ ‘SO level is also observed, the absence from the Pr of emission from the 3P~ state was attributed to efficient 3P0 -+ ‘D2 multiphonon relaxation due to the very high phonon frequency of the borate host lattice @Gl,X N 1400 cm-’ ). The concentration dependence of the integrated intensity originating from the ‘So level indicates that the critical concentration for luminescence quenching occurs at approximately 10 mol% Pr. We assume that concentration quenching is due to energy migration over the ‘SO state to quenching sites in the lattice. 4. Conclusion The discovery of PCE of the Pr’+ ion in a oxide host lattice has been achieved for the first time. For practical applications, this opens the possibility of utilizing these materials in commercially available mercury based fluorescent lamps (for efficient conversion of the Hg 185 nm radiation, for example). The highenergy position of the Pr 3+ 4f5d band in SrAli2019 is a result of low crystal field strength due to the high coordination number and the presence of small highly charged ions (A13+) around the Pr3+ ion in the aluminate lattice. This permits the luminescence from

of Luminescence

71 (1997) 285-290

the ‘SO state of this ion. The quantum efficiency of the aluminate is expected to be lower as compared to YFs-Pr3+ for reasons discussed in the paper.

Acknowledgements This work received partial financial support from the NIST ATP program (NIST 70 NANB 3H1372).

References [l] W.W. Piper, J.A. DeLuca and F.S. Ham, J. Lumin. 8 (1974) 344. [2] J.L. Sommerdijik, A. Bril and A.W. de Jager, J. Lumin. 8 (1974) 341. [3] J.L. Sommerdijik, A. Bril and A.W. de Jager, J. Lumin. 9 (1974) 288. [4] A.M. Srivastava, M.T. Sobieraj, SK. Ruan and E. Banks, Mat. Res. Bull. 21 (1986) 1455. [S] L.R. Elias, Wm.S. Heaps and W.M. Yen, Phys. Rev. B 8 (1973) 4989; C.G. Levey, T.J. Glynn and W.W. Yen, J. Lumin. 31/32 (1984) 245. [6] A.J. Lindop, C. Matthews and D.W. Goodwin, Acta Crystallogr. B 31 (1975) 2940. [7] R. Pappalardo, J. Lumin. 14 (1976) 159. [8] W.J. Schipper and G. Blasse, J. Lumin. 59 (1994) 377. [9] K. Wei, D.P. Machewirth, J. Wenzel, E. Snitzer and G.H. Sigel Jr., J. Non-Cryst. Solids 182 (1995) 257. [lo] J.M.F. van Dijk and M.F.H. schuurmans, J. Chem. Phys. 78 (1983) 5317; J.M.F. van Dijk, J. Lumin. 24/25 (1981) 705; M.F.H. Schuurmans and J.M.F. van Dijk, Physica B 123 (1984) 131. [l 1] J.P.M. Van Vliet and G. Blasse, Chem. Phys. Lett. 143 (1988) 221. [12] C. De Mello Donega, A. Meijerink and G. Blasse, J. Phys. Chem. Solids 56 (1995) 673. [13] A.M. Srivastava, D.A. Doughty and W.W. Beers, J. Electrochem. Sot. 143 (1996) 4113. [14] A.M. Srivastava, D.A. Doughty and W.W. Beers, to be published.