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Luminescence of the orthoborate YBO3:Eu3+. Relationship with crystal structure
JOURNAL OF
LUMINESCENCE Journal of Luminescence 72-74 (1997) 564-566
Luminescence of the orthoborate YB03 : Eu3 +. Relationship with crystal structure G. Chadeyron, R. Mahiou *, M. EL-Ghozzi
,
A. Arbus, D. Zambon, J.C. Cousseins
Laboratoire des Mattkiaux Inorganiques U.R.A. 444, Universitk Blaise-Pascal and ENSCCF, 63177 Aubihre Cedex, France
Abstract Pseudo-vaterite structure of YB03 has been determined from X-Ray data taken from a single crystal. This orthoborate crystallizes in the P63/m space group. The structure exhibits two types of [YOs] polyhedra due to the existence of two
environments for the yttrium ions. The Eu 3f luminescence in this compound corroborates fairly well the structural determination by identification of two crystallographic sites both showing a C3 symmetry. Keywords:
Yttrium
orthoborate;
Crystal
structure;
Luminescence
1. Introduction
Yttrium and lanthanide orthoborates are an interesting class of materials having efficient luminescent properties which make them potential candidates for practical applications in the realization of gas discharge panels. The large number of studies concerning the YBOJ orthoborate has not clearly identified its crystallographic structure. Newnham et al. [l] proposed two different descriptions both in hexagonal symmetry, a disordered one with P6,/mmc space group, showing unique Dsd point symmetry for the rare earth and an ordered one with P6,/mcm space group showing two kinds of point symmetry for the rare earth, D3 and D3+ Then Bradley [Z] confirmed the D3 point symmetry for the rare earth but using the P&2 space group. However investigations on the vaterite type of LuBO, : Eu3+ [3] and YB03 : Eu3+ * Corresponding author. Fax: (33)-73-40-71-08. 0022-2313/97/$17.00 0 PII SOO22-23
1997 Elsevier Science B.V. All rights reserved
13(96)00191-2
[4] showed disagreement between crystallographic and luminescence results. In this paper, we report some optical results using the Eu3 + ion as structural probe in order to analyse the origin of this discrepancy. For this purpose, the YB03 structure has been refined [S] to apprehend clearly the correlation between luminescence and structure.
2. Experimental
details
YB03 and Y1 _.Eu,B03 (0
G. Chadeyron et al. 1 Journal of Luminescence 72-74 (1997) 564-566
excite into the 5D, level of Eu3+ ions. The luminescence was recorded using a Jobin-Yvon HR 1000 monochromator (focal length: 1 m, 1200 groove/ mm grating and band pass of 8 A/mm slits). The detector is a Hamamatsu R1104 photomultiplier tube. Data were collected with a digital signal processor. An EG & G boxcar averager model PAR 162/164 is used for time resolved spectroscopy (TRS). The samples were mounted in a Cryomech GB15 cryorefrigerator which allows to work between 10 and 300 K.
3. Description of the structure YB03 crystallizes in the hexagonal system with P63/m as space group [S]. Y atoms are surrounded by eight oxygen atoms in an arrangement which can be described as a trigonal bicapped prism. These oxygens surrounding the Y3 ’ ions occupy two crystallographic sites: six Or atoms in (4f) and two O2 atoms in (6h) with a partial occupancy of 3. Two types of [YO,] polyhedron are observed (Fig. 1) due to delocalization of O2 oxygens owing to a mirror. Thus deviation from the S6 ideal local symmetry appears. In this description boron is fourfold coordinated; two forms of [BO,] tetrahedra are noted, a first one which is very regu@ with B-O distances ranging from 1.37 to 1.57 A and aOsecond one with two similar short bondsoat 1.37 A and two larger ones at 1.89 and 1.92 A. rlB nuclear magnetic resonance (NMR) and infrared spectroscopy (IR) confirm that YB03 contains only tetrahedrally coordinated boron. [YO,] polyhedron shares Or-O1 edges and O2 corners with two other [YO,] polyhedra. The structure can be described as a 3D
Fig. 1. Yttrium
environment
in YBO,.
565
unit by connection of [YO,] polyhedra, in which each [BO,] group is linked with two adjacent [BO,] groups by O2 atoms and with two adjacent [YO,] groups by O1 atoms. More information on the crystallographic structure is given in Ref. [S].
4. Spectroscopic results Luminescence measurements were performed on polycrystalline samples. The emission spectra of the Eu3 + luminescence in YB03 : Eu3 ’ are described in Ref. [4]. From these results it is clear that the magnetic dipole 5D0 --+‘F1 transition is stronger than the electric dipole 5D0 -+ ‘F, emission. Observation of the latter transition indicates the absence of an inversion centre confirming that the Eu3+ ion occupies sites of symmetry lower than Se. The excitation spectra in the ‘FO --) 5D0 wavelength range were recorded at 34 K for the 10 mol% Eu3+-doped sample prepared by both methods, by monitoring different 5D0 --) ‘Fi emission bands. TRS was performed using a delay of 150 us and a gate width of 5 us. ‘F,, + ‘Do excitation bands can be fitted by the sum of three
Fig. 2. ‘F, + 5D, excitation spectra of Eu3 + fluorescence (A,, = 592.6 nm) in Y0,9Eu,,IB03 and their deconvolution as a sum of three Gaussian bands: (a) sample prepared by sol-gel method (SG), (b) sample prepared by solid state reaction (SS).
566
G. Chadeyron et al. /Journal
of Luminescence
i.2
72-74 (1997) 564-566
observation has already been made, for example, for Ca,GdO(B03)3 : Eu3+ [7]. TRS emission presented in Fig. 3 were recorded at 34 K for the 10% Eu3+-doped sample prepared by the SG technique (the same spectra were observed for samples prepared by SS reaction). The observed 5D,, -+ 7F1 emissions can be unambiguously separated and identified. For excitation in the A, B and C peaks, counting of the 5D,, -+ 7Fz transitions is difficult due to the overlapping which occurs between the different emission bands related to the three excitations. The number of recorded bands for the 5D0 + 7F,,1 transitions corresponding to A and B excitation peaks is 1 and 2, respectively. Thus, it can be concluded that the two intrinsic sites have C3 symmetry. The presence of two sites with a small deviation from the ideal local S6 symmetry is due to the delocalization of O2 oxygen atoms. In conclusion, the luminescence studies using the Eu3+ ion as structural probe confirm the crystallographic results.
Fig. 3. TRS emission spectra at 34 K for YBOs: Eu3+ (10%) sample prepared by sol-gel technique.
Acknowledgements Gaussian functions (Fig. 2) instead of two at 300 K. The number of bands confirms that the Eu3+ ion introduced into the matrix lies in more than one crystallographic site. Important variation is observed in the intensity of the C excitation peak depending on synthesis conditions and temperature. The C peak appears only under 77 K and its intensity increases for samples prepared by the sol-gel method while A and B peaks remain unchanged. Thus, it can be clearly stated that two A and B peaks correspond to intrinsic transitions while the third one can be assigned to transitions which originate from a perturbed site. The increase in C peak intensity for the sample prepared by sol-gel technique indicates that this site results from the presence of Eu3 + ions near an oxygen impurity. The presence of these oxygen atoms is a consequence of the preparation method. Such an
The authors acknowledge financial support from Rhone-Poulenc Chimie (France).
References [l] R.E. Newnham, M.J. Redman and R.P. Santoro, J. Amer. Ceram. Sot. 46 (1963) 253. [2] W.F.Bradley, D.L. Graf and R.S. Roth, Acta Crystallogr. 20 (1966) 283. [3] J. H&a, Inorg. Chim. Acta 139 (1987) 257. [4] G. Chadeyron, A. Arbus, M.T. Fournier, D. Zambon and J.C. Cousseins, C. R. Acad. Sci. Paris 320, serie II B (1995) 199. [S] G. Chadeyron, M. El-Ghozzi, A. Arbus, R. Mahiou and J. C. Cousseins to be published. [6] J.P. Laperches and P. Tarte, Spectrochim. Acta 22 (1966) 1201. [7] G.J. Dirksen and G. Blasse, J. Alloys Compounds 191(1993) 121.
LUMINESCENCE Journal of Luminescence 72-74 (1997) 564-566
Luminescence of the orthoborate YB03 : Eu3 +. Relationship with crystal structure G. Chadeyron, R. Mahiou *, M. EL-Ghozzi
,
A. Arbus, D. Zambon, J.C. Cousseins
Laboratoire des Mattkiaux Inorganiques U.R.A. 444, Universitk Blaise-Pascal and ENSCCF, 63177 Aubihre Cedex, France
Abstract Pseudo-vaterite structure of YB03 has been determined from X-Ray data taken from a single crystal. This orthoborate crystallizes in the P63/m space group. The structure exhibits two types of [YOs] polyhedra due to the existence of two
environments for the yttrium ions. The Eu 3f luminescence in this compound corroborates fairly well the structural determination by identification of two crystallographic sites both showing a C3 symmetry. Keywords:
Yttrium
orthoborate;
Crystal
structure;
Luminescence
1. Introduction
Yttrium and lanthanide orthoborates are an interesting class of materials having efficient luminescent properties which make them potential candidates for practical applications in the realization of gas discharge panels. The large number of studies concerning the YBOJ orthoborate has not clearly identified its crystallographic structure. Newnham et al. [l] proposed two different descriptions both in hexagonal symmetry, a disordered one with P6,/mmc space group, showing unique Dsd point symmetry for the rare earth and an ordered one with P6,/mcm space group showing two kinds of point symmetry for the rare earth, D3 and D3+ Then Bradley [Z] confirmed the D3 point symmetry for the rare earth but using the P&2 space group. However investigations on the vaterite type of LuBO, : Eu3+ [3] and YB03 : Eu3+ * Corresponding author. Fax: (33)-73-40-71-08. 0022-2313/97/$17.00 0 PII SOO22-23
1997 Elsevier Science B.V. All rights reserved
13(96)00191-2
[4] showed disagreement between crystallographic and luminescence results. In this paper, we report some optical results using the Eu3 + ion as structural probe in order to analyse the origin of this discrepancy. For this purpose, the YB03 structure has been refined [S] to apprehend clearly the correlation between luminescence and structure.
2. Experimental
details
YB03 and Y1 _.Eu,B03 (0
G. Chadeyron et al. 1 Journal of Luminescence 72-74 (1997) 564-566
excite into the 5D, level of Eu3+ ions. The luminescence was recorded using a Jobin-Yvon HR 1000 monochromator (focal length: 1 m, 1200 groove/ mm grating and band pass of 8 A/mm slits). The detector is a Hamamatsu R1104 photomultiplier tube. Data were collected with a digital signal processor. An EG & G boxcar averager model PAR 162/164 is used for time resolved spectroscopy (TRS). The samples were mounted in a Cryomech GB15 cryorefrigerator which allows to work between 10 and 300 K.
3. Description of the structure YB03 crystallizes in the hexagonal system with P63/m as space group [S]. Y atoms are surrounded by eight oxygen atoms in an arrangement which can be described as a trigonal bicapped prism. These oxygens surrounding the Y3 ’ ions occupy two crystallographic sites: six Or atoms in (4f) and two O2 atoms in (6h) with a partial occupancy of 3. Two types of [YO,] polyhedron are observed (Fig. 1) due to delocalization of O2 oxygens owing to a mirror. Thus deviation from the S6 ideal local symmetry appears. In this description boron is fourfold coordinated; two forms of [BO,] tetrahedra are noted, a first one which is very regu@ with B-O distances ranging from 1.37 to 1.57 A and aOsecond one with two similar short bondsoat 1.37 A and two larger ones at 1.89 and 1.92 A. rlB nuclear magnetic resonance (NMR) and infrared spectroscopy (IR) confirm that YB03 contains only tetrahedrally coordinated boron. [YO,] polyhedron shares Or-O1 edges and O2 corners with two other [YO,] polyhedra. The structure can be described as a 3D
Fig. 1. Yttrium
environment
in YBO,.
565
unit by connection of [YO,] polyhedra, in which each [BO,] group is linked with two adjacent [BO,] groups by O2 atoms and with two adjacent [YO,] groups by O1 atoms. More information on the crystallographic structure is given in Ref. [S].
4. Spectroscopic results Luminescence measurements were performed on polycrystalline samples. The emission spectra of the Eu3 + luminescence in YB03 : Eu3 ’ are described in Ref. [4]. From these results it is clear that the magnetic dipole 5D0 --+‘F1 transition is stronger than the electric dipole 5D0 -+ ‘F, emission. Observation of the latter transition indicates the absence of an inversion centre confirming that the Eu3+ ion occupies sites of symmetry lower than Se. The excitation spectra in the ‘FO --) 5D0 wavelength range were recorded at 34 K for the 10 mol% Eu3+-doped sample prepared by both methods, by monitoring different 5D0 --) ‘Fi emission bands. TRS was performed using a delay of 150 us and a gate width of 5 us. ‘F,, + ‘Do excitation bands can be fitted by the sum of three
Fig. 2. ‘F, + 5D, excitation spectra of Eu3 + fluorescence (A,, = 592.6 nm) in Y0,9Eu,,IB03 and their deconvolution as a sum of three Gaussian bands: (a) sample prepared by sol-gel method (SG), (b) sample prepared by solid state reaction (SS).
566
G. Chadeyron et al. /Journal
of Luminescence
i.2
72-74 (1997) 564-566
observation has already been made, for example, for Ca,GdO(B03)3 : Eu3+ [7]. TRS emission presented in Fig. 3 were recorded at 34 K for the 10% Eu3+-doped sample prepared by the SG technique (the same spectra were observed for samples prepared by SS reaction). The observed 5D,, -+ 7F1 emissions can be unambiguously separated and identified. For excitation in the A, B and C peaks, counting of the 5D,, -+ 7Fz transitions is difficult due to the overlapping which occurs between the different emission bands related to the three excitations. The number of recorded bands for the 5D0 + 7F,,1 transitions corresponding to A and B excitation peaks is 1 and 2, respectively. Thus, it can be concluded that the two intrinsic sites have C3 symmetry. The presence of two sites with a small deviation from the ideal local S6 symmetry is due to the delocalization of O2 oxygen atoms. In conclusion, the luminescence studies using the Eu3+ ion as structural probe confirm the crystallographic results.
Fig. 3. TRS emission spectra at 34 K for YBOs: Eu3+ (10%) sample prepared by sol-gel technique.
Acknowledgements Gaussian functions (Fig. 2) instead of two at 300 K. The number of bands confirms that the Eu3+ ion introduced into the matrix lies in more than one crystallographic site. Important variation is observed in the intensity of the C excitation peak depending on synthesis conditions and temperature. The C peak appears only under 77 K and its intensity increases for samples prepared by the sol-gel method while A and B peaks remain unchanged. Thus, it can be clearly stated that two A and B peaks correspond to intrinsic transitions while the third one can be assigned to transitions which originate from a perturbed site. The increase in C peak intensity for the sample prepared by sol-gel technique indicates that this site results from the presence of Eu3 + ions near an oxygen impurity. The presence of these oxygen atoms is a consequence of the preparation method. Such an
The authors acknowledge financial support from Rhone-Poulenc Chimie (France).
References [l] R.E. Newnham, M.J. Redman and R.P. Santoro, J. Amer. Ceram. Sot. 46 (1963) 253. [2] W.F.Bradley, D.L. Graf and R.S. Roth, Acta Crystallogr. 20 (1966) 283. [3] J. H&a, Inorg. Chim. Acta 139 (1987) 257. [4] G. Chadeyron, A. Arbus, M.T. Fournier, D. Zambon and J.C. Cousseins, C. R. Acad. Sci. Paris 320, serie II B (1995) 199. [S] G. Chadeyron, M. El-Ghozzi, A. Arbus, R. Mahiou and J. C. Cousseins to be published. [6] J.P. Laperches and P. Tarte, Spectrochim. Acta 22 (1966) 1201. [7] G.J. Dirksen and G. Blasse, J. Alloys Compounds 191(1993) 121.