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Site-selective fluorescence excitation spectroscopy of a KMgF3:Sm2+ crystal
Journal of Alloys and Compounds 374 (2004) 32–35
Site-selective fluorescence excitation spectroscopy of a KMgF3:Sm2+ crystal Wansong Zhang a , Hyo Jin Seo a,∗ , Byung Kee Moon a , Soung-Soo Yi b , Kiwan Jang c b
a Department of Physics, Pukyong National University, Pusan 608-737, Republic of Korea Department of Photonics, Nano Applied Technology Research Center, Silla University, Pusan 617-736, Republic of Korea c Department of Physics, Changwon National University, Changwon 641-773, Republic of Korea
Abstract We investigate the defect structure of KMgF3 :Sm2+ by site-selective laser spectroscopy. The transitions between 5 D0 and 7 FJ (J = 0, 1 and 2) states of Sm2+ in KMgF3 strictly obey induced electric dipole (ED) and magnetic dipole (MD) selection rules and symmetry selection rules from which three dominant sites with Oh , C4v and C2v symmetries and two minor C3v sites are identified at 11 K. The sites with C4v and C2v symmetries are assigned to Sm2+ with the vacancies at the nearest-neighbor K+ site along the [1 0 0] direction and at the second-nearest-neighbor K+ site along the [1 1 0] direction, respectively. It is assumed that the two C3v sites do not come from Sm2+ with the third-nearest-neighbor K+ vacancy along the [1 1 1] direction but from Sm2+ associated with other defects. © 2003 Elsevier B.V. All rights reserved. Keywords: KMgF3 ; Sm2+ ; Crystal field symmetry; Site selection; Laser spectroscopy
1. Introduction When Sm2+ ions are introduced into KMgF3 lattice, they replace K+ ions and accordingly their charges are compensated by K+ vacancies. The different positions of charge-compensating K+ vacancy relative to Sm2+ give rise to luminescing sites with different crystal field symmetries. The presence of cubic-site Eu2+ and noncubic-site Eu2+ were reported in KMgF3 :Eu2+ by investigation of intraconfigurational 4f 7 → 4f 7 transitions [1,2]. Similar behaviors are expected in KMgF3 :Sm2+ since Sm2+ has nearly identical ionic radius and chemical properties with Eu2+ . Luminescence properties of KMgF3 :Sm2+ were investigated by Gacon et al. [3] and Valyashko et al. [4]. However, to our knowledge, there is as yet no detailed report on locations of K+ vacancies in KMgF3 :Sm2+ and KMgF3 :Eu2+ . Sm2+ is isoelectronic with Eu3+ (4f6 configuration). Sm2+ as well as Eu3+ is suitable as a probe for determination of local structure in the host matrix because of their simple multiplet pattern and straightforward symmetry behavior of the energy levels [5,6]. In this paper we investigate the defect structure of KMgF3 :Sm2+ by site-selective laser ∗
Corresponding author. Fax: +82-51-611-6357. E-mail address: [email protected] (H.J. Seo).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.11.054
spectroscopy and show that there are five different sites with Oh , C4v , C2v and C3v symmetries in KMgF3 :Sm2+ . 2. Experimental procedures A single crystal of KMgF3 doped with Sm2+ (0.5 mol.% in melt) was grown from the melt in Ar-gas atmosphere by the Czochralski method. The excitation source was a dye laser pumped by the second harmonic of a pulsed Nd:YAG laser. The dye used was DCM. The pulse energy of the dye laser output was about 5 mJ with 10 Hz repetition rate and 5 ns duration. The fluorescence was dispersed by a 75 cm monochromator and observed with a photomultiplier tube (PMT). The excitation spectra were measured by monitoring total fluorescence with the monochromator in zero order of diffraction. Suitable filters were used to eliminate noise due to the scattered laser radiation. The signal from the PMT was fed into a digital oscilloscope and then the data were stored in a personal computer.
3. Results Excitation spectrum for the 7 F0 → 5 D0 transition of Sm2+ obtained by monitoring total fluorescence from the
W. Zhang et al. / Journal of Alloys and Compounds 374 (2004) 32–35
Fluorescence Intensity ( arb. units )
1.5 A 1.0
B 0.5
D
C 0.0 680
681
682
683
Excitation Wavelength ( nm )
Fig. 1. Excitation spectrum of the 7 F0 → 5 D0 transition obtained by monitoring total fluorescence at 11 K.
5D
0 level is shown in Fig. 1. The spectrum consists of four peaks at 680.76, 680.85, 681.25 and 682.55 nm (labeled by A, B, C and D, respectively, as indicated in the spectrum). The former two peaks (A and B) are stronger than the latter two peaks (C and D). Each line appearing in the excitation spectrum corresponds to a unique crystallographic center because the 7 F0 → 5 D0 transition can have only one line per site. Emission spectra from individual sites were obtained by tuning the dye laser to resonance with each line in the excitation spectrum. Fig. 2 shows the selectively excited fluorescence spectra in which different 5 D0 → 7 FJ (J = 1, 2) peaks are observed in each spectrum. We note that the 7 F1 and 7 F2 levels of site D shift to higher and lower energies, respectively, with respect to those levels of sites A, B and C. The line numbers of the 5 D0 → 7 F1 and 5 D0 → 7 F2 transitions of Sm2+ are clearly observed to be 2 and 2 for site A, 3 and 4 for site B and 2 and 3 for sites C and D, respectively. Theoretically, the 7 F1 and 7 F2 levels split by crystal-field perturbation into 2 and 4 sublevels for tetragonal site, 2 and
33
3 sublevels for trigonal site and 3 and 5 sublevels for orthorhombic site, while only 1 and 2 sublevels for cubic site, respectively [6]. Induced electric dipole (ED) and magnetic dipole (MD) selection rules predict the permitted transitions between excited and ground states of Sm2+ . The transitions between J-sublevels are further restricted by symmetry selection rules [5]. We assign sites C and D to the C3v symmetry, site A to C4v and site B to C2v symmetries by applying the selection rules. The presence of a cubic-site Sm2+ can be expected since this was also the case for Eu2+ in KMgF3 . However, the induced ED transition cannot occur when the point group of the site contains a center of symmetry (e.g., cubic symmetry) and the 7 F0 → 5 D0 transition for a cubic site is further forbidden by symmetry selection rules. The MD transition is allowed only from the 7 F1 level to the 5 D0 level in cubic symmetry. The 7 F1 levels of Sm2+ ions can be populated thermally by the 7 F0 levels at high temperature due to the small energy difference of about 280 cm−1 between these levels. Excitation spectrum of the MD allowed 7 F → 5 D transition at 100 K is shown in Fig. 3. The dom1 0 inant line appears at 693.98 nm (labeled by E). The peak for site E was observed even at 40 K. The emission spectrum (inset of Fig. 3) under 693.98 nm (site E) excitation reveals only one 5 D0 → 7 F1 peak, whereas no emission line is observed in the 5 D0 → 7 F1 and 5 D0 → 7 F2 transitions. This result indicates that site E originates from cubic-site Sm2+ in KMgF3 . Lifetimes of the 5 D0 emission were measured for sites E, A, B, C and D at 11 K. The temporal evolution of all the fluorescence exhibits a single-exponential decay curve. The lifetimes of sites E, A and B are estimated to be 27.4, 16.3 and 17.9 ms, and for sites C and D are 2.26 and 2.04 ms, respectively. We note that the 5 D0 lifetimes for the latter two sites (C and D) are considerably shorter than the former three sites (E, A and B).
7
Fluorescence Intensity ( arb. units )
F1
7
F2
A B C
D
Fluorescence Intensity ( arb. units )
0.15
E Oh
0.10
0.05
C2v
693
C4v , C2v
680
C3v
C4v
694
700
720
nm
C2v
695
Excitation Wavelength ( nm ) 685
690
695
715
720
725
730
Emission Wavelength ( nm )
Fig. 2. Emission spectra of the 5 D0 → 7 FJ (J = 1 and 2) transitions for sites A, B, C and D exciting at 680.76, 680.85, 681.25 and 682.55 nm, respectively, at 11 K.
Fig. 3. 1 → 0 excitation spectrum monitoring total fluorescence at 100 K. Crystal field symmetries corresponding to the transitions for individual sites are indicated in the spectrum. The inset shows emission spectrum of 5 D0 → 7 FJ transition for site E (Oh symmetry) exciting at 693.98 nm at 100 K. 7F
5D
34
W. Zhang et al. / Journal of Alloys and Compounds 374 (2004) 32–35
C3v (3)
C2v
(1) C4v
(2)
Sm2+
(a)
:K
+
:F
-
: Mg
2+
(b)
Fig. 4. Crystal structure of KMgF3 . (a) Cubic cell with MgF6 octahedron. (b) Twelve-fold K+ cuboctahedron. The lattice site (1), (2) or (3) with its site symmetry indicates a possible position of the charge-compensating K+ vacancy.
4. Discussion The K+ ion has twelve F− nearest neighbors with cuboctahedral Oh symmetry and is surrounded by eight MgF6 octahedrons in KMgF3 (Fig. 4). The Sm2+ ion (1.51 Å) substitutes for the K+ ion (1.78 Å) in KMgF3 giving rise to the formation of a charge-compensating positive ion vacancy located at one of the K+ sites. If such a vacancy is located close to Sm2+ , the site symmetry is reduced from cubic to a lower symmetry. The vacancy at one of the nearest-neighbor K+ site along the [1 0 0] direction (site (1) in Fig. 4) produces tetragonal (C4v ) symmetry. The orthorhombic (C2v ) and trigonal (C3v ) symmetries are created by the K+ vacancies at one of the second-nearest-neighbor along the [1 1 0] direction (site (2)) and third-nearest-neighbor along the [1 1 1] direction (site (3)), respectively. When a K+ vacancy is located away from Sm2+ the site symmetry can be assumed to be cubic (Oh ) although it is not exactly so. We suggest that sites (1) and (2) in Fig. 4 correspond to sites A and B observed in the excitation and emission spectra. However, sites C and D assigned to C3v symmetries seem unlikely to come from Sm2+ bound to the K+ vacancy at the third-nearest-neighbor (site (3) in Fig. 4) because the lifetimes of these sites are considerably shorter than those of sites E, A and B. The substitution of Sm2+ for the large size of twelve coordinated K+ site in KMgF3 gives the large distance between Sm2+ and F− ligands. The small Mg2+ ion (0.86 Å) in the MgF6 octahedron strongly attracts the F− ligand ions. These facts lower the degree of covalency between Sm2+ and F− ligands and weaken the strength of the Sm2+ crystal field causing weak coupling with odd parity vibrations and small admixture of opposite parity states of Sm2+ in KMgF3 . The presence of cubic-site Sm2+ is due to the K+ vacancy loosely bound to Sm2+ in KMgF3 . Sm2+ and Eu2+ in alkali halide crystals (e.g., KCl:Sm2+ [7] and KCl:Eu2+ [8]) substitute not for a cubic site but for a site with C2v or C4v symmetry because of the stronger coupling between
Sm2+ and the K+ vacancy [9]. The 5 D0 levels of sites E, A and B in KMgF3 have relatively longer lifetimes in comparison with Sm2+ in other host lattices (e.g., KCl: 11.5 ms [10], LiBaF3 : 17 ms [11], KY3 F10 : 11.9 ms [12]). Especially the longest lifetime (27 ms), reported up to now, was found for the cubic-site Sm2+ in KMgF3 . The long lifetimes are also attributed to the low covalency and the weak crystal field strength. We assume that when a K+ vacancy is present at the third-nearest-neighbor along the [1 1 1] direction, the crystal field acting on the Sm2+ ion in KMgF3 is cubic (Oh ) rather than trigonal (C3v ) because the larger distance and the presence of MgF6 octahedron between Sm2+ and the K+ vacancy give a weak trigonal perturbation of the crystal field. The vacancies at fourth- and fifth-nearest-neighbors along the [1 0 0] and [2 1 0] directions also have little influence on cubic symmetry of Sm2+ . Sites C and D with C3v symmetries are presumably attributed to the Sm2+ ions associated with other defects, such as interstitial fluorine atoms [13]. The fluorescence properties of sites C and D will be discussed in detail elsewhere.
Acknowledgements This work was supported by Korea Research Foundation Grant (KRF-2002-070-C00042).
References [1] H.J. Seo, B.K. Moon, T. Tsuboi, Phys. Rev. B 62 (2000) 12688. [2] R. Francini, U.M. Grassano, M. Tomini, S. Boiko, G.G. Tarasov, A. Scacco, Phys. Rev. B 55 (1997) 7579. [3] J.C. Gacon, A. Gros, H. Bill, J.P. Wicky, J. Phys. Chem. Solids 42 (1981) 587. [4] E.G. Valyashko, S.N. Bodrug, V.N. Mednikova, V.A. Smirnov, Opt. Spectrosc. 42 (1977) 174. [5] C. Gorller-Walrand, K. Binnemans, Rationalization of Crystal-Field Parametrization, in: K.A. Gschneidner Jr., L. Eyring (Eds.),
W. Zhang et al. / Journal of Alloys and Compounds 374 (2004) 32–35 Handbook on the Physics and Chemistry of Rare Earths, vol. 23, North-Holland, Amsterdam, 1996 (Chapter 155). [6] J.C. Bunzli, Luminescent Probes, in: J.C. Bunzli, G.R. Choppin (Eds.), Lanthanide Probes in Life, Chemical and Earth Sciences, Elsevier, Amsterdam, 1989. [7] A.J. Ramponi, J.C. Wright, Phys. Rev. B 31 (1985) 3965. [8] L.A.O. Nunes, F.M. Matinaga, J.C. Castro, Phys. Rev. B 32 (1985) 8356.
[9] [10] [11] [12]
35
T. Tsuboi, A. Scacco, J. Phys.: Condens. Matter 10 (1998) 7259. M. Guzzi, B. Baldini, J. Lumin. 6 (1973) 270. A. Meijerink, G.J. Dirksen, J. Lumin. 63 (1995) 189. J.-P.R. Wells, A. Sugiyama, T.P.J. Han, H.G. Gallagher, J. Lumin. 85 (1999) 91. [13] J.E. Rhoads, B.H. Rose, L.E. Halliburton, Phys. Rev. B 11 (1975) 5115.
Site-selective fluorescence excitation spectroscopy of a KMgF3:Sm2+ crystal Wansong Zhang a , Hyo Jin Seo a,∗ , Byung Kee Moon a , Soung-Soo Yi b , Kiwan Jang c b
a Department of Physics, Pukyong National University, Pusan 608-737, Republic of Korea Department of Photonics, Nano Applied Technology Research Center, Silla University, Pusan 617-736, Republic of Korea c Department of Physics, Changwon National University, Changwon 641-773, Republic of Korea
Abstract We investigate the defect structure of KMgF3 :Sm2+ by site-selective laser spectroscopy. The transitions between 5 D0 and 7 FJ (J = 0, 1 and 2) states of Sm2+ in KMgF3 strictly obey induced electric dipole (ED) and magnetic dipole (MD) selection rules and symmetry selection rules from which three dominant sites with Oh , C4v and C2v symmetries and two minor C3v sites are identified at 11 K. The sites with C4v and C2v symmetries are assigned to Sm2+ with the vacancies at the nearest-neighbor K+ site along the [1 0 0] direction and at the second-nearest-neighbor K+ site along the [1 1 0] direction, respectively. It is assumed that the two C3v sites do not come from Sm2+ with the third-nearest-neighbor K+ vacancy along the [1 1 1] direction but from Sm2+ associated with other defects. © 2003 Elsevier B.V. All rights reserved. Keywords: KMgF3 ; Sm2+ ; Crystal field symmetry; Site selection; Laser spectroscopy
1. Introduction When Sm2+ ions are introduced into KMgF3 lattice, they replace K+ ions and accordingly their charges are compensated by K+ vacancies. The different positions of charge-compensating K+ vacancy relative to Sm2+ give rise to luminescing sites with different crystal field symmetries. The presence of cubic-site Eu2+ and noncubic-site Eu2+ were reported in KMgF3 :Eu2+ by investigation of intraconfigurational 4f 7 → 4f 7 transitions [1,2]. Similar behaviors are expected in KMgF3 :Sm2+ since Sm2+ has nearly identical ionic radius and chemical properties with Eu2+ . Luminescence properties of KMgF3 :Sm2+ were investigated by Gacon et al. [3] and Valyashko et al. [4]. However, to our knowledge, there is as yet no detailed report on locations of K+ vacancies in KMgF3 :Sm2+ and KMgF3 :Eu2+ . Sm2+ is isoelectronic with Eu3+ (4f6 configuration). Sm2+ as well as Eu3+ is suitable as a probe for determination of local structure in the host matrix because of their simple multiplet pattern and straightforward symmetry behavior of the energy levels [5,6]. In this paper we investigate the defect structure of KMgF3 :Sm2+ by site-selective laser ∗
Corresponding author. Fax: +82-51-611-6357. E-mail address: [email protected] (H.J. Seo).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.11.054
spectroscopy and show that there are five different sites with Oh , C4v , C2v and C3v symmetries in KMgF3 :Sm2+ . 2. Experimental procedures A single crystal of KMgF3 doped with Sm2+ (0.5 mol.% in melt) was grown from the melt in Ar-gas atmosphere by the Czochralski method. The excitation source was a dye laser pumped by the second harmonic of a pulsed Nd:YAG laser. The dye used was DCM. The pulse energy of the dye laser output was about 5 mJ with 10 Hz repetition rate and 5 ns duration. The fluorescence was dispersed by a 75 cm monochromator and observed with a photomultiplier tube (PMT). The excitation spectra were measured by monitoring total fluorescence with the monochromator in zero order of diffraction. Suitable filters were used to eliminate noise due to the scattered laser radiation. The signal from the PMT was fed into a digital oscilloscope and then the data were stored in a personal computer.
3. Results Excitation spectrum for the 7 F0 → 5 D0 transition of Sm2+ obtained by monitoring total fluorescence from the
W. Zhang et al. / Journal of Alloys and Compounds 374 (2004) 32–35
Fluorescence Intensity ( arb. units )
1.5 A 1.0
B 0.5
D
C 0.0 680
681
682
683
Excitation Wavelength ( nm )
Fig. 1. Excitation spectrum of the 7 F0 → 5 D0 transition obtained by monitoring total fluorescence at 11 K.
5D
0 level is shown in Fig. 1. The spectrum consists of four peaks at 680.76, 680.85, 681.25 and 682.55 nm (labeled by A, B, C and D, respectively, as indicated in the spectrum). The former two peaks (A and B) are stronger than the latter two peaks (C and D). Each line appearing in the excitation spectrum corresponds to a unique crystallographic center because the 7 F0 → 5 D0 transition can have only one line per site. Emission spectra from individual sites were obtained by tuning the dye laser to resonance with each line in the excitation spectrum. Fig. 2 shows the selectively excited fluorescence spectra in which different 5 D0 → 7 FJ (J = 1, 2) peaks are observed in each spectrum. We note that the 7 F1 and 7 F2 levels of site D shift to higher and lower energies, respectively, with respect to those levels of sites A, B and C. The line numbers of the 5 D0 → 7 F1 and 5 D0 → 7 F2 transitions of Sm2+ are clearly observed to be 2 and 2 for site A, 3 and 4 for site B and 2 and 3 for sites C and D, respectively. Theoretically, the 7 F1 and 7 F2 levels split by crystal-field perturbation into 2 and 4 sublevels for tetragonal site, 2 and
33
3 sublevels for trigonal site and 3 and 5 sublevels for orthorhombic site, while only 1 and 2 sublevels for cubic site, respectively [6]. Induced electric dipole (ED) and magnetic dipole (MD) selection rules predict the permitted transitions between excited and ground states of Sm2+ . The transitions between J-sublevels are further restricted by symmetry selection rules [5]. We assign sites C and D to the C3v symmetry, site A to C4v and site B to C2v symmetries by applying the selection rules. The presence of a cubic-site Sm2+ can be expected since this was also the case for Eu2+ in KMgF3 . However, the induced ED transition cannot occur when the point group of the site contains a center of symmetry (e.g., cubic symmetry) and the 7 F0 → 5 D0 transition for a cubic site is further forbidden by symmetry selection rules. The MD transition is allowed only from the 7 F1 level to the 5 D0 level in cubic symmetry. The 7 F1 levels of Sm2+ ions can be populated thermally by the 7 F0 levels at high temperature due to the small energy difference of about 280 cm−1 between these levels. Excitation spectrum of the MD allowed 7 F → 5 D transition at 100 K is shown in Fig. 3. The dom1 0 inant line appears at 693.98 nm (labeled by E). The peak for site E was observed even at 40 K. The emission spectrum (inset of Fig. 3) under 693.98 nm (site E) excitation reveals only one 5 D0 → 7 F1 peak, whereas no emission line is observed in the 5 D0 → 7 F1 and 5 D0 → 7 F2 transitions. This result indicates that site E originates from cubic-site Sm2+ in KMgF3 . Lifetimes of the 5 D0 emission were measured for sites E, A, B, C and D at 11 K. The temporal evolution of all the fluorescence exhibits a single-exponential decay curve. The lifetimes of sites E, A and B are estimated to be 27.4, 16.3 and 17.9 ms, and for sites C and D are 2.26 and 2.04 ms, respectively. We note that the 5 D0 lifetimes for the latter two sites (C and D) are considerably shorter than the former three sites (E, A and B).
7
Fluorescence Intensity ( arb. units )
F1
7
F2
A B C
D
Fluorescence Intensity ( arb. units )
0.15
E Oh
0.10
0.05
C2v
693
C4v , C2v
680
C3v
C4v
694
700
720
nm
C2v
695
Excitation Wavelength ( nm ) 685
690
695
715
720
725
730
Emission Wavelength ( nm )
Fig. 2. Emission spectra of the 5 D0 → 7 FJ (J = 1 and 2) transitions for sites A, B, C and D exciting at 680.76, 680.85, 681.25 and 682.55 nm, respectively, at 11 K.
Fig. 3. 1 → 0 excitation spectrum monitoring total fluorescence at 100 K. Crystal field symmetries corresponding to the transitions for individual sites are indicated in the spectrum. The inset shows emission spectrum of 5 D0 → 7 FJ transition for site E (Oh symmetry) exciting at 693.98 nm at 100 K. 7F
5D
34
W. Zhang et al. / Journal of Alloys and Compounds 374 (2004) 32–35
C3v (3)
C2v
(1) C4v
(2)
Sm2+
(a)
:K
+
:F
-
: Mg
2+
(b)
Fig. 4. Crystal structure of KMgF3 . (a) Cubic cell with MgF6 octahedron. (b) Twelve-fold K+ cuboctahedron. The lattice site (1), (2) or (3) with its site symmetry indicates a possible position of the charge-compensating K+ vacancy.
4. Discussion The K+ ion has twelve F− nearest neighbors with cuboctahedral Oh symmetry and is surrounded by eight MgF6 octahedrons in KMgF3 (Fig. 4). The Sm2+ ion (1.51 Å) substitutes for the K+ ion (1.78 Å) in KMgF3 giving rise to the formation of a charge-compensating positive ion vacancy located at one of the K+ sites. If such a vacancy is located close to Sm2+ , the site symmetry is reduced from cubic to a lower symmetry. The vacancy at one of the nearest-neighbor K+ site along the [1 0 0] direction (site (1) in Fig. 4) produces tetragonal (C4v ) symmetry. The orthorhombic (C2v ) and trigonal (C3v ) symmetries are created by the K+ vacancies at one of the second-nearest-neighbor along the [1 1 0] direction (site (2)) and third-nearest-neighbor along the [1 1 1] direction (site (3)), respectively. When a K+ vacancy is located away from Sm2+ the site symmetry can be assumed to be cubic (Oh ) although it is not exactly so. We suggest that sites (1) and (2) in Fig. 4 correspond to sites A and B observed in the excitation and emission spectra. However, sites C and D assigned to C3v symmetries seem unlikely to come from Sm2+ bound to the K+ vacancy at the third-nearest-neighbor (site (3) in Fig. 4) because the lifetimes of these sites are considerably shorter than those of sites E, A and B. The substitution of Sm2+ for the large size of twelve coordinated K+ site in KMgF3 gives the large distance between Sm2+ and F− ligands. The small Mg2+ ion (0.86 Å) in the MgF6 octahedron strongly attracts the F− ligand ions. These facts lower the degree of covalency between Sm2+ and F− ligands and weaken the strength of the Sm2+ crystal field causing weak coupling with odd parity vibrations and small admixture of opposite parity states of Sm2+ in KMgF3 . The presence of cubic-site Sm2+ is due to the K+ vacancy loosely bound to Sm2+ in KMgF3 . Sm2+ and Eu2+ in alkali halide crystals (e.g., KCl:Sm2+ [7] and KCl:Eu2+ [8]) substitute not for a cubic site but for a site with C2v or C4v symmetry because of the stronger coupling between
Sm2+ and the K+ vacancy [9]. The 5 D0 levels of sites E, A and B in KMgF3 have relatively longer lifetimes in comparison with Sm2+ in other host lattices (e.g., KCl: 11.5 ms [10], LiBaF3 : 17 ms [11], KY3 F10 : 11.9 ms [12]). Especially the longest lifetime (27 ms), reported up to now, was found for the cubic-site Sm2+ in KMgF3 . The long lifetimes are also attributed to the low covalency and the weak crystal field strength. We assume that when a K+ vacancy is present at the third-nearest-neighbor along the [1 1 1] direction, the crystal field acting on the Sm2+ ion in KMgF3 is cubic (Oh ) rather than trigonal (C3v ) because the larger distance and the presence of MgF6 octahedron between Sm2+ and the K+ vacancy give a weak trigonal perturbation of the crystal field. The vacancies at fourth- and fifth-nearest-neighbors along the [1 0 0] and [2 1 0] directions also have little influence on cubic symmetry of Sm2+ . Sites C and D with C3v symmetries are presumably attributed to the Sm2+ ions associated with other defects, such as interstitial fluorine atoms [13]. The fluorescence properties of sites C and D will be discussed in detail elsewhere.
Acknowledgements This work was supported by Korea Research Foundation Grant (KRF-2002-070-C00042).
References [1] H.J. Seo, B.K. Moon, T. Tsuboi, Phys. Rev. B 62 (2000) 12688. [2] R. Francini, U.M. Grassano, M. Tomini, S. Boiko, G.G. Tarasov, A. Scacco, Phys. Rev. B 55 (1997) 7579. [3] J.C. Gacon, A. Gros, H. Bill, J.P. Wicky, J. Phys. Chem. Solids 42 (1981) 587. [4] E.G. Valyashko, S.N. Bodrug, V.N. Mednikova, V.A. Smirnov, Opt. Spectrosc. 42 (1977) 174. [5] C. Gorller-Walrand, K. Binnemans, Rationalization of Crystal-Field Parametrization, in: K.A. Gschneidner Jr., L. Eyring (Eds.),
W. Zhang et al. / Journal of Alloys and Compounds 374 (2004) 32–35 Handbook on the Physics and Chemistry of Rare Earths, vol. 23, North-Holland, Amsterdam, 1996 (Chapter 155). [6] J.C. Bunzli, Luminescent Probes, in: J.C. Bunzli, G.R. Choppin (Eds.), Lanthanide Probes in Life, Chemical and Earth Sciences, Elsevier, Amsterdam, 1989. [7] A.J. Ramponi, J.C. Wright, Phys. Rev. B 31 (1985) 3965. [8] L.A.O. Nunes, F.M. Matinaga, J.C. Castro, Phys. Rev. B 32 (1985) 8356.
[9] [10] [11] [12]
35
T. Tsuboi, A. Scacco, J. Phys.: Condens. Matter 10 (1998) 7259. M. Guzzi, B. Baldini, J. Lumin. 6 (1973) 270. A. Meijerink, G.J. Dirksen, J. Lumin. 63 (1995) 189. J.-P.R. Wells, A. Sugiyama, T.P.J. Han, H.G. Gallagher, J. Lumin. 85 (1999) 91. [13] J.E. Rhoads, B.H. Rose, L.E. Halliburton, Phys. Rev. B 11 (1975) 5115.