Optical properties of pure Ytterbium Alluminium perovskites

Optical Materials 33 (2011) 1000–1003

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Optical properties of pure Ytterbium Alluminium perovskites P.C. Ricci a,⇑, A. Casu a, D. Chiriu a, C. Corpino a,b, C.M. Carbonaro a,b, M. Marceddu b, M. Salis a, A. Anedda a,b a b

Dipartimento di Fisica, Università di Cagliari, s.p.n°8, Km 0,700 09042 Monserrato (Ca), Italy Centro d’Ateneo Grandi Strumenti, Università di Cagliari, s.p.n°8, Km 0,700 09042 Monserrato (Ca), Italy

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Article history: Received 3 June 2010 Received in revised form 27 October 2010 Accepted 4 November 2010 Available online 4 December 2010 Keywords: Ytterbium Alluminium perovskites YbAP Fast scintillators Charge-transfer state

a b s t r a c t We present the first report about growth and optical properties of pure Ytterbium Alluminium perovskites single crystals (YbAlO3). The sample crystal structure was studied and assigned by means of Raman spectroscopy while the photoluminescence measurements from Yb3+ charge-transfer state show a broad ultraviolet emission bands with nanosecond lifetimes at room temperature. Yb emissions are also studied as a function of temperature revealing an abrupt quenching in the 180–240 K range. The fast time decay and the high material density suggest Ytterbium Alluminium perovskites crystal as a good candidate for the development of fast scintillators for high energy physics applications. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction In recent years there has been a growing interest in developing and studying new scintillation materials characterized by high efficiency output, fast response and high density [1–3]. The difficulty in finding out a compound that fits all the mentioned characteristics has often lead to choose materials suitable for just a partial set of different applications. As an example, the Yb-based scintillator gives a lower optical response in respect to the Cerium-based scintillator (such as Ce:YAG or Ce:LSO), but with a faster time response (lifetime smaller than 10 ns versus 30 ns of Ce:LSO) [4–6]. In this regards, the charge transfer luminescence of Yb3+ is an attractive subject for the development of new scintillator materials for neutrino detection in high energy physics experiments. Recent results on Yb doped YAlO3 revealed a maximum light output of 3000 eh/ MeV at 105 K and a light yield of 7.5% (in respect to the BGO one), which suggest that Yb:YAP can be an attractive scintillator also for nuclear imaging applications (PET) [2]. Moreover, the integrated radioluminescence spectra in Yb:YAP increase monotonously with Yb concentration reaching an efficiency more than 11% higher than BGO for a Yb/Yttrium ratio in the 45–55% range [7]. Thus far this was the highest known limit for the stability of Yb-doped perovskite structures. In this work we overcame the 45% limit, reaching the total substitution of Yttrium with Ytterbium ions and growing pure Ytterbium Aluminium perovskite crystal (YbAP). YbAP sample presents a larger density (8.2 g/cm3) ⇑ Corresponding author. Tel.: +39 0706754755; fax: +39 070510171. E-mail address: [email protected] (P.C. Ricci). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.11.002

than pure Yttrium perovskite (YAP) (5.38 g/cm3) and Yb doped YAP (5.42 g/cm3 for Yb at 2% and 6.64 g/cm3 for Yb at 45%). Two main arguments outline the importance of the reported study: the development of a brand new crystal with immediate application as scintillator and the opportunity to obtain fundamental insights in the optical properties of Yb ions, that can be extended to other Yb based crystals. Charge transfer (CT) optical transition in a solid-state structure implies the transfer of the involved electron from ligand to central metal ion, with some of the absorption transitions possibly followed by the radiative return of the system to its ground state. In some cases the radiative recombination is subsequent to nonradiative processes to lower levels and only the final step features a luminescent process, as for Eu3+:Y2O3. This phosphor adsorbs in the UV range with a transition to the charge-transfer state of the Eu3+ ion and luminescence occurs from the 5DJ states after nonradiative decay to the lower 4f levels. The Yb3+ ion presents a very efficient charge transfer luminescence, since the only excited 4f state (2F5/2) is located 10,000 cm1 above the ground 2F7/2 state and no other absorption band has been identified. Because of the large energy difference between the charge-transfer state and the highest excited 4f state, nonradiative channels are precluded and charge transfer luminescence can be observed. In this paper, the first report on the optical properties (luminescence spectra and decay kinetics) and Raman spectra of Ytterbium Alluminium perovskite single crystal are presented. Possible energy transfer mechanisms to the Yb3+ 4f states are discussed.

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P.C. Ricci et al. / Optical Materials 33 (2011) 1000–1003

2. Experimental

3. Results and discussions Ytterbium Aluminum perovskite is an orthorhombic crystal of the D16 2h space group, featuring a 20 atoms unit cell. The general structure of perovskite crystals can be expressed as ABX3, where elements A and B occupy, respectively, the vertexes and the central site of the body centred cube, while X anions, situated on the six faces of the cube, form an octahedron around the central site B. Fig. 1 presents the unpolarized Raman spectrum of YbAP sample along the X-axis. All our spectra show well-defined peaks, thus allowing us to clearly assign the crystal structure. The irreducible point representations of orthorhombic perovskite (x axis in Pnma) allows 24 Raman active vibrational modes (upon a total of 60): [8]

U ¼ 7A1g þ 7B1g þ 5B2g þ 5B3g þ 8A1u þ 8B1u þ 10B2u þ 10B3u Among them the Raman active modes are: 7 A1g modes, 7 B1g modes, 5 B2g modes and 5 B3g modes. Each of these vibrational modes can be assigned to different atomic motions, although not everyone of them was shown by our measurements. Moreover, some of the Raman bands are not resolved and/or overlapped and did not allow a clear identification of the different vibrational modes. This problem led to the choice of acquiring differently-polarized Raman spectra along the three main axes of the crystal in order to show the Raman active modes.

Raman Intensity (arb. units)

350 300 250 200 150 100 50 0 0

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Raman Shift (cm ) Fig. 1. Unpolarized Raman spectrum of YbAp along X-axis, kexc = 514.5 nm.

For a crystal in a D16 2h , space group, it is possible to discriminate between all the permitted vibrational modes by choosing different Raman polarization configurations along single crystal samples. Defining a orthogonal axis tern x, y and z with z along the highest symmetry axes and in reference to the Porto’s Notation [9], the Ag vibrational modes should be observed when the components of the polar tensor xx, yy, zz are non-null, while contribution to the B1g, B2g and B3g modes should be given, respectively, from xy, yz and xz components. Polarized Raman spectra (Fig. 2) allows us to assign seven A1g modes (114, 144, 283, 361, 464, 564, 598), four B1g modes (285, 360, 403 and 463 cm1), five B2g modes (115, 160, 319, 422 and 561 cm1) and three B3g modes (138, 469 and 548 cm1). The A1g, B1g, B2g and B3g vibrational modes of YbAP were assigned by confrontation with their vibrational correspondents in similar perovskites, particularly YAP and LuYAP [10]. The excitation and emission spectra of the Yb3+ charge transfer luminescence band are shown in Fig. 3. The edge of the excitation spectrum coincides with the edge of the charge transfer absorption transition observed in Yb-doped perovskites, [11] suggesting that a partial overlap with the fundamental gap of the crystal could no be excluded. The emission spectrum, excited at 250 nm, presents two

z(xx)z: A1G Raman Intensity

High purity powders of Yb2O3 and Al2O3 were mixed and presintered at the pressure of 140 MPa and processed at 1400 °C for 24 h. Sintered tablets were melted at 2050 °C, the crystal structure was obtained seeding the melt with supercooling technique and it was consequently grown by Czochralski method. The samples were cut along the principal axis and optically finished. Raman scattering measurements were carried out in back scattering geometry with the 514.5 nm line of an Argon-ion laser. Measurements were performed in air at room temperature with a triple spectrometer Jobin–Yvon Dilor integrated system with a spectral resolution of about 1 cm1. A first round of photoluminescence measurements (PL) were performed by exciting the samples with pulsed synchrotron radiation (SR). PL signal was dispersed by a 0.5 m Czerny-Turner monochromator and detected in the 1.5–5.0 eV energy range with a charge-coupled device (CCD). Emission spectral bandwidth was 0.1 nm. Excitation of photoluminescence spectra (PLE) were recorded under multibunch operation mode and detected with an integral time window of 192 ns correlated to the SR pulses. These experiments were performed at the SUPERLUMI experimental station of the I beamline at DESY Hasylab in Hamburg. A second round of time resolved photoluminescence measurements (TR-PL) were performed with sample excitation provided by an optical parametric oscillator pumped by the third harmonic of a pulsed Nd:YAG laser (Spectra Physics Quanta Ray Serie). The laser pulse was 10 ns long with 10 Hz repetition rate. PL spectra were recorded in 90° geometry by focusing the light emission into the entrance slit of a triple grating spectrograph (ARC SpectraPro 300i) and detecting light signal with an intensified gatable CCD detector (Princeton Instruments PI MAX). Depending on the PL bands under examination, different delays from laser excitation were used. Spectral resolution was better than 102 eV. The spectra were corrected for the spectral response of the optical systems. Temperature dependent measurements were performed in the 10–290 K range by means of a closed-cycle He cryogenerator in cold finger configuration.

400

z(xy)zB1G x(yz)x B2G y(xz)y B3G 200

400

600

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Raman Shift (cm-1) Fig. 2. YbAP Raman modes revelead through different geometries. Porto’s notation has been used to describe the scattering geometry. The spectra were taken at room temperature on single crystal samples.

P.C. Ricci et al. / Optical Materials 33 (2011) 1000–1003

PL Intensity (arb. units)

λem = 338 nm

the same time kinetic is observed for the whole spectrum. The two bands of the YbCTS ? 2F7/2, YbCTS ? 2F5/2 recombinations present a very weak shift towards high energy and a reduced full width at half maximum with respect to room temperature measurements (peaks at 336 and 480 nm respectively). From the analysis of the decay time curve (see inset in Fig. 4) it is possible to observe at least two decay components. The red line in the inset of Fig. 4 represents the curve fitting obtained with two exponentials with decay time of 6.1 ns and 34.2 ns respectively. The well pronounced non-single-exponential decay and the decrease of decay constants with increasing temperature indicate the thermal activated luminescence quenching of Yb3+ ions. The nature of these effects is attributed to the presence of Yb2+ from the growing process or to more probable thermally activated effects (Yb3+ + e ? Yb2+) that generate non-radiative recombinations [19]. Another effect of the presence of Yb2+ ions is the distortion of lattice due to the discrepancy between the differently charged Yb ions: RYb2+ = 1.13 Å RYb3+ = 0.98 Å. Emission characteristics of Yb3+ CT are strongly dependent on the local symmetry and are also influenced by the symmetry properties of the second coordination sphere. The thermal activated luminescence quenching is well evidenced in Fig. 5, that reports the luminescence integrated intensity as a function of the sample temperature. A significant decrease of the PL signal is observed between 190 and 210 K. The inset clearly shows that the main shortening of the decay times is observed in the same 190–210 K range. It is worth to note that in Yb:YAP the variation of the PL signal with temperature occurs in a wider temperature range (10–250 K) [3]. Several authors observed pronounced quenching of ytterbium charge transfer luminescence with temperature in different matrices [15–18] and it was previously assigned to non-radiative energy transfer from the CT band to the 4f levels of ytterbium [19,20]. According to this attribution, we retain that the shift to higher temperature of the CT quenching, which had been partially observed in previous works on Yb-doped perovskites at different concentrations [14], can be due to the restricted relaxation of the charge transfer excited state due to the decreasing volume of the elementary unit cell, which should increase the energy barrier for thermal quenching. The elementary unit cell in pure YAP crystal is 210.85 Å3 while in YbAP crystal, according to our calculations, should be 199.17 Å3. The decrease in the volume of the unit cell is in agreement with the variations previously observed in the density of these perovskites, showing an increasing trend relative to

100 1

Instrum. responce λ em = 338 nm

10

0.1 0.01

1 1E-3 0.1

1E-4 1E-5

Normalized PL Intensity (arb. units)

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Excitation wavelength (nm) Fig. 3. PL and PLE spectra at room temperature (288 K). The inset reports the decay time recorded for the 338 nm emission and the instrumental time response.

bands, peaked at about 340 nm and 500 nm, assigned to the radiative recombination from the charge transfer level to the 2F7/2 and to 2F5/2, respectively, in agreement with the predicted energetic separation of about 10,000 cm1 [12,13]. No radiative emission from 2F5/2 level to 2F7/2 was observed in the IR spectral region. The fast RT decay time of YbAP sample is reported in the inset of Fig. 3. It was previously reported that a 30%Yb-doped YAP crystal showed two time components at room temperature, with time constants of 0.87 ns and 2.2 ns for the faster and slower components respectively [7,14]. However, due to the laser pulse width of 8 ns (laser pulse time profile is also shown in the inset) we can just give an estimation of the decay time in Ytterbium Aluminium perovskites at room temperature. Actually, we performed time measurements also with synchrotron radiation at DESY with excitation wavelength at 250 nm., Though not reported here those measurements are in good agreement with the above shown decay time analysis: in particular the fast decay time component at room temperature was estimated in 1 ns, but it is worth nothing again that the time response of the acquisition system (about 1 ns) does not permit a more accurate evaluation. The time evolution of the Yb radiative decay can be better observed in the 3D plot obtained at 10 K (Fig. 4). The plot shows that

PL Intensity (arb. units)

T = 10 K

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350

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)

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e(

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Emission wavelength (nm) Fig. 4. 3D plot of the PL at different time delay from the excitation light at 10 K. The inset shows the decay time of the emission at 338 nm.

P.C. Ricci et al. / Optical Materials 33 (2011) 1000–1003

quenching is also observed. The nature of PL quenching is attributed to thermally activated Yb2+ ions that generate non-radiative recombinations. The fast decay time and the high crystal density suggest Ytterbium Alluminium perovskites crystal as a good candidate for the development of fast scintillators in high energy physics applications.

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Acknowledgements

PL Intensity

Normalized Integral PL emission

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Temperature (K) Fig. 5. Integrated PL at 338 nm vs temperature. The inset reports the decay time at different temperatures.

the the presence of Ytterbium over Yttrium (from 5.38 g/cm3 (YAP) and going up to 6.64 g/cm3 (Yb:YAP), to 8.2 g/cm3 for YbAP) [19]. The optical properties of YbAP crystal could cause a growing interest for direct applications in the field of high energy physics if its scintillation efficiency resembles the characteristics previously observed in Yb:YAP [20]. Nickl et al. reported that the total RL intensity of integrated emission spectra is monotonously increasing with the Yb concentration, reaching more than 11% of the BGO intensity for a 45%Yb-doped crystal at room temperature and rising up to 170% at 80 K [7]. Considering the higher efficiency and higher temperature quenching of YbAP crystal with respect to the published data of Yb:YAP crystal, a study focusing on the scintillation properties of YbAP crystal is mandatory. 4. Conclusions Ytterbium Alluminium perovskites single crystal was grown for the first time and its optical properties were studied. Luminescence spectra are dominated by the charge transfer bands peaked in the UV (340 nm) and in the visible (500 nm) range. No other PL bands were observed with excitation at 250 nm in the temperature range between 10 and 290 K in the 300–1000 nm spectral range. The radiative decay time at room temperature is well below 8 ns and can be estimated in 1–2 ns while at 10 K a bi-exponential decay trend was observed, with radiative lifetime of 6.1 ns and 34.2 ns respectively. The decay time shortening occurs mainly in the temperature range between 190 and 210 K, where the luminescence

This work was partially supported by the European Community – Research Infrastructure Action under the FP6 ‘‘Structuring the European Research Area’’ Programme (through the Integrated Infrastructure Initiative ‘‘Integrating Activity on Synchrotron and Free-Electron-Laser Science’’). P.C. Ricci and M. Marceddu are grateful for the economic support from the RAS (Regione Autonoma della Sardegna) and European community through the grants in aid for Young Researcher, L.R. n° 7, 2007, N° CRP 2_539 and CRP 1_455, respectively.’’ References [1] N. Guerassimova, N. Garnier, C. Dujardin, A.G. Petrosyan, C. Pedrini, Chem. Phys. Lett. 339 (2001) 197. [2] S. Belogurov, G. Bressi, G. Carugno, M. Moszynski, W. Czarnacki, M. Kapusta, M. Szawlowski, Nucl. Instrum. Methods Phys. Res. A 496 (2003) 385. [3] M. Nikl, A. Yoshikawa, A. Vedda, T. Fukuda, J. Cryst. Growth 292 (2006) 416– 421. [4] M. Kucˇera, K. Nitsch, M. Nikl, M. Hanuš, S. Daniš, J. Cryst. Growth 312 (2010) 1538. [5] C.M. Pepin, P. Berard, A.L. Perrot, C. Pepin, D. Houde, R. Lecomte, C.L. Melcher, H. Dautet, IEEE Trans. Nucl. Sci. 51 (2004) 789. [6] P. Yang, P. Deng, Z. Yin, J. Lumin. 97 (2002) 51. [7] M. Nikl, N. Solovieva, J. Pejchal, J.B. Shim, A. Yoshikawa, T. Fukuda, A. Vedda, M. Martini, D.H. Yoon, Appl. Phys. Lett. 84 (2004) 882. [8] R.W.G. Wychoff, Crystal Structure, Krieger, Florida, 1986. [9] The first and the last letter outside the bracket represent the incident and the scattered direction respect to the crystal axes, assuming the z component parallel to the C-axis. The two letters inside the bracket represent the polarization direction of the incident and scattered beam, respectively. The ‘‘–’’ symbol indicates no polarization of the backscattered light: [10] A. Casu, P.C. Ricci, A. Anedda, J. Raman Spectrosc. 40 (2009) 1224. [11] Y. Dong, G. Zhou, J. Xu, G. Zhao, F. Su, L. Su, G. Zhang, D. Zhang, H. Li, J. Si, J. Cryst. Growth 289 (2006) 676–680. [12] A. Denoyer, Y. Levesque, S. Jandl, Ph. Goldner, O. Guillot-Noel, B. Viana, F. Thibault, D. Pelenc, J. Lumin. 128 (2008) 1389. [13] R. Lisiecki, W. Ryba-Romanowski, A. Speghini, M. Bettinelli, J. Lumin. 129 (2009) 521. [14] J. Pejchal, M. Nikl, J.B. Shim, A. Yoshikawa, T. Fukuda, A. Voloshinovskii, V. Múcˇka, D.H. Yoon, J. Appl. Phys. 98 (2005) 016104. [15] E. Nakazawa, J. Lumin. 18/19 (1979) 272–276. [16] L. Van PietersonL, M. Heeroma, E. deHeer, A. Meijerink, J. Lumin. 91 (2000) 177. [17] I.A. Kamenskikh, N. Guerassimova, C. Dujardin, N. Garnier, G. Ledoux, C. Pedrini, M. Kirm, A. Petrosyan, D. Spassky, Opt. Mater. 24 (2003) 267. [18] I. Kamenskikh, C. Pedrini, A. Petrosyan, A. Vasil’ev, J. Lumin. 29 (2009) 1509. [19] M. Nikl, A. Yoshikawa, T. Fukuda, Opt. Mater. 26 (2004) 545–549. [20] J.B. Shim, A. Yoshikawa, M. Nikl, J. Pejchal, A. Vedda, T. Fukuda, Radiat. Meas. 38 (2004) 493.