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Optical spectroscopy and crystal field analysis of Eu3+ in calcium tartrate tetrahydrate
Volume 161, number 2
CHEMICAL PHYSICS LETTERS
8 September 1989
OPTICAL SPECTROSCOPY AND CRYSTAL FIELD ANALYSIS OF Eu3+ IN CALCIUM TARTRATE TETRAHYDRATE
J.A. CAPOBIANCO, P.P. PROULX and N. RASPA Department
of Chemistry,Concordia University, 1455 de Maisonneuve Boulevard, West, Montreal, Quebec, Canada H3G lM8
Received 14 March 1989; in final form 3 July 1989
The fluorescence of Eu’+-dopedsingle crystals of calcium tartrate tetrahydrate was investigated using laser-excited site-selective spectroscopy. The fluorescence spectrum at 514.5 and 488 nm revealed the presence of three sites for Eu3+. A phenomenological crystal field analysis was conducted on the basis of the observed ‘FO_4energy levels, using D, symmetry. The covalent character of the europium oxygen bond in the three sites is found to vary.
1. Introduction The advent of fluorescence Iine narrowing (FLN) by laser-induced site-selective spectroscopy has provided the spectroscopist with the means of probing the local environment of an activator ion in amorphous systems (glasses) and of characterizing different crystal field sites in crystals such as Y,O, [ 11, Y3A15012[2], and Gd203 [3]. The Eu3+ ion is particularly useful in this regard in that it possesses nondegenerate ground ( ‘FO) and emitting ( 5D,,) states, so that neither the ground nor emissive level can be split by a crystal field. Thus, in principle, a one-toone correspondence exists between the number of peaks ( ‘DO+‘FO) in the emission spectrum and the number of distinct Eu3+ ion environments. Laser-induced site-selective spectroscopy has proven valuable in providing a wide range of information, including local site symmetry and properties of the chemical bonding. The fluorescence of Eu3+ in calcium tartrate tetrahydrate, CaC4H406.4H20, has been investigated by Sperka and Bettinelli [ 41. They identified at least eight components in the ‘DO-’‘FOregion within a total spread of 35 cm- ’ . The authors concluded that the Eu3+ ion occupies a set of energetically similar sites of D, symmetry. However, they did not assign the spectral lines to any particular ion site. The aim of the present research is a detailed study of the optical emission properties exhibited by tri-
valent europium ions in calcium tartrate tetrahydrate crystals using laser-induced site-selective spectroscopy. The subject is covered in three parts. Firstly, the identification and the number of distinct fluorescence spectra originating from the 5D0 levels of Eu3+ in calcium tartrate tetrahydrate; secondly, the assignment of each spectrum to a particular ion site and thirdly, a phenomenological crystal field analysis of the spectral data to obtain information regarding the local site symmetry and properties of the chemical bonding.
2. Experimental Crystals of calcium tartrate tetrahydrate doped with Eu3+ were grown from silica gel using a method first described by Henisch, Dennis and Hanoka [ 5 1. A Spectra Physics 375 dye laser operating with rhodamine 6G ( 10e3 mol/dm3 in ethylene glycol) pumped by a Coherent Innova 70 argon ion laser was used for excitation of the site-selective spectra. The laser had a typical linewidth of 2 cm-’ full width at half maximum ( fwhm). Emission spectra with 5 14.5 and 488 nm excitation were excited directly with the green and blue line of the argon ion laser. The fluorescence was monitored with an RCA-C3 1034-02 photomultiplier operating in the photon counting mode and recorded under computer control using the Stanford SR 465 software data acquisition/analysis
0 009-26 14/89/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
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emission peaks are only slightly broader than the holinewidths of Eu3+ in crystalline solids such as Gd203:Eu3+ (2 cm-‘) [3]. The four lines were identified as 5D0+7F0 transitions and the laser was tuned successively in exact resonance with each of them. Four distinct fluorescence spectra (figs. 2a and 2b) were obtained for the 5D0+7F0 transition excited at 578.00, 578.65 (A), 579.21 (B) and 579.71 nm (C). In figs. 2a and 2b the top spectrum is under 514.5 nm excitation. The results of these observations are summarized as follows: (i ) The spectrum obtained by exciting resonantly into the maximum of each ofthe 5D,,+7F0 levels does not correspond to the fluorescence of Eu3+ ions from a single site. (ii) For each excitation wavelength there exists some overlap in lines especially in the ‘F2, 7F3 and ‘F4 region but with different relative intensities. We were able to separate the 5D0-+7D,_4emission into three groups of lines originating from Eu3+ in different sites (table 1,) . For the purpose of this discussion it is convenient to designate the emission spectra obtained at 578.65, 579.2 1 and 579.71 nm to sites I, II and III, respectively. Excitation at 578 nm
systefi. Data were acquired at liquid-nitrogen temperature (77 K) using an Oxford Instruments continuous flow cryostat (CF 204). Energy levels were determined by deconvoluting the measured spectra with a Lorentzian bands leastsquares minimization routine [ 6 ] employing a Marquardt algorithm [ 71. Crystal-field fits with J-mixing were performed by diagonalizing parametrized crystal field Hamiltonians ( DZdand Dz symmetries) using 1JM) states of the ‘F, multiplet as basis functions. The sum of squared differences between observed and calculated energies was minimized by successive linear minimization of individual crystal field parameters using Powell’s algorithm [ 8 1.
mogeneous
3. Results Figs. 1a and lb show the 77 K $D+‘F,, region of the Eu3+ emission spectra excited at 514.5 and 488 nm, respectively. The salient feature of these spectra is that four emission peaks are observed with maxima at 578.00, 578.65, 579.21 and.579.71 nm indicating emission from more than one site. The linewidths (5 cm-’ fwhm) of each of the ‘DO+‘FO
1
e
o
8 576
8 September 1989
I
I
578
Wavelength
I WI
(nm)
I
576
I
I
570
Wavelength
/
5Kl
(nm)
Fig. 1.Emission spectra (77 K) of the 5Da+7F0 region of Eu‘+ in calcium tartrate tetrahydrate excited at (a) 514.5 nm and (b) 488 nm.
152
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8 September 1989
CHEMICAL PHYSICS LETTERS
(b)
(i)
(ii)
(iii)
(iv)
r
I
I
I
590 Wavelength
I
610
I
I
650
d---I 650
I
650
(nm)
I
I
I
670
I
69D
I
I
i
710
Wavelength
Fig. 2. Emission spectra of the (a) ‘D,,+‘FI,~ and (b) ‘D o+7F3,4transitions in calcium tartrate tetrahydrate at 77 K. (i) Under 514.5 nm excitation; (ii) under 578 nm excitation; (iii) under 578.65 nm excitation (spectrum A); (iv) under 579.21 nm excitation (spectrum B); (v) under 579.71 nm excitation (spectrum C).
will not be discussed further since it corresponds to a mixture of all the sites present. The spectra obtained for the different excitation wavelengths all show electric dipole transitions to the 7Fz and 7F, levels. The magnetic dipole transition ‘DO+‘FI is, however, clearly weaker than the magnetic dipole transition, 5D,,-‘7F2.The spectra all show the “forbidden” transition ‘Do+‘F3 to be very weak indicating that the free ion selection rule for the electric dipole transition, J= 2,4 or 6 (for an initial state of J= Cl)is obeyed quite rigorously. We also observe that for emission spectra obtained by excitation at 514.5 and 488 nm the transition ‘D,+‘F, has significant intensity. This is an electric-dipole-allowed transition for C,,, or lower site symmetries made possible by the presence of the linear odd-rank crystal field term.
4. Discussion 4.1. Crystallographic background Crystallographic studies by Ambady [9] and Hawthorne and Ferguson [ 10 ] have shown that calcium tartrate tetrahydrate belongs to the space group P2,2,2,. The unit cell is orthorhombic with a = 9.24 A, b= 10.63 A, c=9.66 8, and 2=4. The calcium ion was found to be eightfold coordinated with the coordination polyhedron of oxygen atoms forming a distorted dodecahedron. Fig. 3 shows schematically such a dodecahedron, drawn using the crystallographic data of Hawthorne and Ferguson [ 10 1. In the figure 01 and 02 belong to one molecule of tartrate ion with 01 being a hydroxyl and 02 a neighbouring carboxyl oxygen. The same relation holds for 05 and 04; 03 and 06 are both carboxyl oxygen atoms belonging ‘to two different tartrate ions. 07 153
Table 1 Observed and calculated energy levels of Eu3+ in CaC4H406.4H20 Exditation
Level
Energy (cm-‘)
(nm) observed 578.65
‘F0
0.0
barycentre
rms
-0.02
68.46
9.59
7FI
257 318 609
257 316 612
440
%
869 950 1160 1207
871 1129 1162 1207
1098
‘h
1823 1826 1850 I907 1940 2020 2030
1819 1830 1852 1925 1941 2012 2018
1938
‘F4
2734 2799 2857 2865
2129 2798 2860 2875 2877 2910 2972 3039 3097
2919
2919 2971 3036 3097 579.21
calculated
%I
0.0
-0.17
53.32
‘F,
243 390 512
243 397 504
427
‘F*
936 951 1030 1162 1205
940 963 1022 1167 1197
1094
1837 1865 1887 1949 1958 1998 2035
1842 1857 1877 1945 1964 200 1 2039
1945
2700
2707 2731 2795 2815 2859 2926 2942 3038 3085
2885
‘F4
2795 2818 2860 2932 2940 3038 3084
8.5
Volume I6 I, number 2
CHEMICALPHYSICSLETTERS
8 September 1989
Table 1 Continued Excitation (nm)
Level
Energy (cm ’ ) observed
579.71
‘F0
0.0
26.80
7.27
292 317 436
372
%
1837 1846
I845
1941
1861 1884
1858 1877
1937 1960 1988
1929 1953 1995
2690 2732
2692 2729
2757 2819 2862 2958
2765 2812 2857 2959
2964 3010 3054
2971 3009 3055
I858
of Ca*+ in calcium
tar-
and 08 are oxygen atoms of two water molecules. has an approximate DZdsymmetry.
The polyhedron
4.2. Crystaljeld calculations The spectroscopy
rms
295 318 432
33
dodecahedron
0.16
barycentre
‘F,
‘F.S
Fig. 3. The coordination trate tetrahydrate.
calculated
results show that the symmetry
2894
of the sites occupied by the Eu3+ ion in calcium tartrate tetrahydrate is lower than D,,. This is borne out by two observations; the full Stark splitting of the J-manifolds and the presence of the ‘Do-‘F,, transition (excitation 5 14.5 and 488 nm, fig. 1) an electric dipole transition allowed for C,, or lower site symmetries. From the experimentally determined energy level values (24 Stark levels) it is unreasonable to expect a meaningful simulation of the crystal field for symmetry lower than D2, since for Cz symmetry there are 14 independent crystal parameters B,,. We treat the europium coordination as having an effective site symmetry of DZ. This is reasonable since the emission from the Eu3+ ion is much more sensitive to the immediate surroundings of the ion than the more distant neighbours. In view of this we do not feel that the small refinements in energy that would arise by calculation at symmetries lower than D1 would justify the effort such a consideration would require. The crystal field Hamiltonian for D2 symmetry is written as:
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Table 2 Crystal field parameters (cm-‘) for Eu‘+ in CaC,H,O,+4H,O assuming DI symmetry Excitation
B20
B22
ho
B 42
B44
B 60
Ba
B64
B 66
578.65 579.21 579.71
985 710 -132
123 -393 -255
917 -185 161
370 885 349
-538 -446 119
-1101 -357 39
-171 -346 -897
-288 -535 - 1057
28 47 310
+&dG-fJ+C66).
(1)
The simulation of the energy level scheme involves nine experimental crystal field parameters. In order to find the starting parameters we use the technique of successive reduction of symmetry. The procedure considers that the “real” site symmetry D, is not far from Dzd where the number of parameters is only five, Bzo, BJo, B44, Be0 and Be4. For Eu3+ the first-order splitting of the ‘F, state depends only on BZoand the splitting of ‘F2 on Bzo and Bqo. This allows us to estimate the values of B2,, and B40, neglecting J-mixing. Proceeding on this basis we refine the crystal field parameters for D,, symmetry and then incorporate additional parameters required by the lowering of symmetry to Dz. The observed and calculated energy level positions of the 7FJ (J= 1 to 4) manifolds are reported in table 1, where the root mean-squared (rms) deviations are also reported. The resultant crystal field parameters are given in table 2. 4.3. Analysis of the observed spectra Table 1 reports the observed and calculated emission lines associated with the different excitation wavelengths. The laser linewidth is sufficiently narrow to select each site except in the case of accidental coincidences. The measurements were performed
under continuous selective excitation and weak emission from Eu3+ in other sites not directly excited appears as a result of site-to-site energy transfer. However, this does not preclude the identification of groups of fluorescence lines arising from Eu3 + 156
ions whose 5D, level is in resonance with the laser wavelength, since these lines are enhanced in intensity with respect to the others. The ‘F, splittings for spectra A, B and C are different and hence they exhibit different Stark level distributions for the three sites. However, ‘F2, ‘F3 and ‘F, exhibit substantial overlap in several lines originating from the three sites giving credence to an energy transfer process between sites. The Eu3+ ions substitute for Ca2+ ions in calcium tartrate tetrahydrate. The ionic radius of Eu3+ is 1.03 A which is somewhat larger than the ionic radius of Ca’+, 0.99 A. The Eu3+ impurity ions probably create local distortion in the neighbouring tartrate complexes and these local distortions may be different in the three calcium sites making them nonequivalent. Also the substitution of Eu3+ for Ca2+ into calcium tartrate tetrahydrate requires charge compensation. One would expect one Na+ (available in the gel) ion to be incorporated (in place of Ca’+ ) for every Eu’+ ion. Therefore, one way in which three distinct sites could arise is from having the closest Ca2+ site occupied by either Ca’+ (with remote charge cpmpensation ) or Na+ (local charge compensation). Thus, if a chemical or structural imperfection is located near the Eu3+ ion, different sites would result. The energy of the $DOlevel for the three sites lies at 17282, 17265 and 17250 cm-‘. This suggests different degrees of covalency between the Eu3+ and the oxygen ions. The energy of the 5Do ( 172>0 cm- ’ ) level for site III lies in the “covalent” region of the nephelauxetic scale [ 111 whereas the energy of the 5D0 ( 17282 cm-‘) level for site I lies higher suggesting a more ionic bond. This is confirmed by the values of the second-order crystal field parameters (table 2) which provide an estimate of the electrostatic contribution to the crystal field effect. The absolute crystal field strength defined as [ 121
Volume 161, number 2
CHEMICAL PHYSICS LETTERS
applied to the data and a set of optical parameters for each site was obtained.
Table 3 Crystal field strength (cm-‘) for Eu’+ in CaC4H406.4H20
x
Crystal field strength (S) (cm-‘)
Excitation wavelength (cm-’ )
408 390 353
578.65 579.21 579.71
(
B&+2
8 September 1989
Acknowledgement The authors gratefully acknowledge the Natural Science and Engineering Research Council of Canada and the Faculty of Arts and Science of Concordia University for financial support. We are grateful to Dr. T.F. Belliveau for allowing us to use the crystal field programmes.
)I
l/2
c (RBj,, +I&) 1,1> 0
(2)
can render the preceding arguments more quantitative. The crystal field strengths (table 3) of the three sites are seen to increase with increasing excitation energy substantiating the conclusion that the degree of covalent character decreases from site III to site I.
5. Conclusions In summary, site-selective spectroscopy has been shown to give a reasonably complete and consistent picture of the behaviour of Eu3+-doped calcium tartrate tetrahydrate. The Eu3+ ions are shown to occupy at least three different types of site. Despite the low symmetry, crystal field theory was
References [ 11J. Heber, K.H. Hellwege, U. Kobler and H. Murmann, Z. Pbysik 237 (1970) 189. [2] M. Asano and J.A. Koningstein, Chem. Phys. 42 (1979) 369. [ 31 J. Dexpert-Ghys, M. Faucher and P. Caro, Phys. Rev. B 23 (1981) 607. [4] G. Sperka and M. Bettinelli, Inorg. Chim. Acta 149 (1988) 147. [ 51H.K. Hen&h, J. Dennis and J.J. Hanoka, J. Phys. Chem. Solids 26 (1965) 493. [ 61 T.F. Belliveau, Ph.D. Thesis, McGill University, unpublished (1988). [ 71 D.W. Marquardt, J. Sac. Ind. Appl. Math. 1 I ( 1963) 43 I. IS] M.J.D. Powell, Computer J. 1 ( 1964) 303. [ 91 G.K. Ambady, Acta Cryst. B 24 ( 1968) 1548. [lo] F.C. Hawthorne and R.B. Ferguson, Acta Cryst. B 38 ( 1982) 2461. [J 11P. Care, 0. Bcaury and E. Antic, J. Phys. (Paris) 37 (1976) 671. [ 121 N.C. Chang, J.B. Gruber, R.P. Leavitt and CA. Morrison, J. Chem. Phys. 78 (1982) 3877.
157
CHEMICAL PHYSICS LETTERS
8 September 1989
OPTICAL SPECTROSCOPY AND CRYSTAL FIELD ANALYSIS OF Eu3+ IN CALCIUM TARTRATE TETRAHYDRATE
J.A. CAPOBIANCO, P.P. PROULX and N. RASPA Department
of Chemistry,Concordia University, 1455 de Maisonneuve Boulevard, West, Montreal, Quebec, Canada H3G lM8
Received 14 March 1989; in final form 3 July 1989
The fluorescence of Eu’+-dopedsingle crystals of calcium tartrate tetrahydrate was investigated using laser-excited site-selective spectroscopy. The fluorescence spectrum at 514.5 and 488 nm revealed the presence of three sites for Eu3+. A phenomenological crystal field analysis was conducted on the basis of the observed ‘FO_4energy levels, using D, symmetry. The covalent character of the europium oxygen bond in the three sites is found to vary.
1. Introduction The advent of fluorescence Iine narrowing (FLN) by laser-induced site-selective spectroscopy has provided the spectroscopist with the means of probing the local environment of an activator ion in amorphous systems (glasses) and of characterizing different crystal field sites in crystals such as Y,O, [ 11, Y3A15012[2], and Gd203 [3]. The Eu3+ ion is particularly useful in this regard in that it possesses nondegenerate ground ( ‘FO) and emitting ( 5D,,) states, so that neither the ground nor emissive level can be split by a crystal field. Thus, in principle, a one-toone correspondence exists between the number of peaks ( ‘DO+‘FO) in the emission spectrum and the number of distinct Eu3+ ion environments. Laser-induced site-selective spectroscopy has proven valuable in providing a wide range of information, including local site symmetry and properties of the chemical bonding. The fluorescence of Eu3+ in calcium tartrate tetrahydrate, CaC4H406.4H20, has been investigated by Sperka and Bettinelli [ 41. They identified at least eight components in the ‘DO-’‘FOregion within a total spread of 35 cm- ’ . The authors concluded that the Eu3+ ion occupies a set of energetically similar sites of D, symmetry. However, they did not assign the spectral lines to any particular ion site. The aim of the present research is a detailed study of the optical emission properties exhibited by tri-
valent europium ions in calcium tartrate tetrahydrate crystals using laser-induced site-selective spectroscopy. The subject is covered in three parts. Firstly, the identification and the number of distinct fluorescence spectra originating from the 5D0 levels of Eu3+ in calcium tartrate tetrahydrate; secondly, the assignment of each spectrum to a particular ion site and thirdly, a phenomenological crystal field analysis of the spectral data to obtain information regarding the local site symmetry and properties of the chemical bonding.
2. Experimental Crystals of calcium tartrate tetrahydrate doped with Eu3+ were grown from silica gel using a method first described by Henisch, Dennis and Hanoka [ 5 1. A Spectra Physics 375 dye laser operating with rhodamine 6G ( 10e3 mol/dm3 in ethylene glycol) pumped by a Coherent Innova 70 argon ion laser was used for excitation of the site-selective spectra. The laser had a typical linewidth of 2 cm-’ full width at half maximum ( fwhm). Emission spectra with 5 14.5 and 488 nm excitation were excited directly with the green and blue line of the argon ion laser. The fluorescence was monitored with an RCA-C3 1034-02 photomultiplier operating in the photon counting mode and recorded under computer control using the Stanford SR 465 software data acquisition/analysis
0 009-26 14/89/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
151
Volume I6 I, number 2
CHEMICAL PHYSICS LETTERS
emission peaks are only slightly broader than the holinewidths of Eu3+ in crystalline solids such as Gd203:Eu3+ (2 cm-‘) [3]. The four lines were identified as 5D0+7F0 transitions and the laser was tuned successively in exact resonance with each of them. Four distinct fluorescence spectra (figs. 2a and 2b) were obtained for the 5D0+7F0 transition excited at 578.00, 578.65 (A), 579.21 (B) and 579.71 nm (C). In figs. 2a and 2b the top spectrum is under 514.5 nm excitation. The results of these observations are summarized as follows: (i ) The spectrum obtained by exciting resonantly into the maximum of each ofthe 5D,,+7F0 levels does not correspond to the fluorescence of Eu3+ ions from a single site. (ii) For each excitation wavelength there exists some overlap in lines especially in the ‘F2, 7F3 and ‘F4 region but with different relative intensities. We were able to separate the 5D0-+7D,_4emission into three groups of lines originating from Eu3+ in different sites (table 1,) . For the purpose of this discussion it is convenient to designate the emission spectra obtained at 578.65, 579.2 1 and 579.71 nm to sites I, II and III, respectively. Excitation at 578 nm
systefi. Data were acquired at liquid-nitrogen temperature (77 K) using an Oxford Instruments continuous flow cryostat (CF 204). Energy levels were determined by deconvoluting the measured spectra with a Lorentzian bands leastsquares minimization routine [ 6 ] employing a Marquardt algorithm [ 71. Crystal-field fits with J-mixing were performed by diagonalizing parametrized crystal field Hamiltonians ( DZdand Dz symmetries) using 1JM) states of the ‘F, multiplet as basis functions. The sum of squared differences between observed and calculated energies was minimized by successive linear minimization of individual crystal field parameters using Powell’s algorithm [ 8 1.
mogeneous
3. Results Figs. 1a and lb show the 77 K $D+‘F,, region of the Eu3+ emission spectra excited at 514.5 and 488 nm, respectively. The salient feature of these spectra is that four emission peaks are observed with maxima at 578.00, 578.65, 579.21 and.579.71 nm indicating emission from more than one site. The linewidths (5 cm-’ fwhm) of each of the ‘DO+‘FO
1
e
o
8 576
8 September 1989
I
I
578
Wavelength
I WI
(nm)
I
576
I
I
570
Wavelength
/
5Kl
(nm)
Fig. 1.Emission spectra (77 K) of the 5Da+7F0 region of Eu‘+ in calcium tartrate tetrahydrate excited at (a) 514.5 nm and (b) 488 nm.
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CHEMICAL PHYSICS LETTERS
(b)
(i)
(ii)
(iii)
(iv)
r
I
I
I
590 Wavelength
I
610
I
I
650
d---I 650
I
650
(nm)
I
I
I
670
I
69D
I
I
i
710
Wavelength
Fig. 2. Emission spectra of the (a) ‘D,,+‘FI,~ and (b) ‘D o+7F3,4transitions in calcium tartrate tetrahydrate at 77 K. (i) Under 514.5 nm excitation; (ii) under 578 nm excitation; (iii) under 578.65 nm excitation (spectrum A); (iv) under 579.21 nm excitation (spectrum B); (v) under 579.71 nm excitation (spectrum C).
will not be discussed further since it corresponds to a mixture of all the sites present. The spectra obtained for the different excitation wavelengths all show electric dipole transitions to the 7Fz and 7F, levels. The magnetic dipole transition ‘DO+‘FI is, however, clearly weaker than the magnetic dipole transition, 5D,,-‘7F2.The spectra all show the “forbidden” transition ‘Do+‘F3 to be very weak indicating that the free ion selection rule for the electric dipole transition, J= 2,4 or 6 (for an initial state of J= Cl)is obeyed quite rigorously. We also observe that for emission spectra obtained by excitation at 514.5 and 488 nm the transition ‘D,+‘F, has significant intensity. This is an electric-dipole-allowed transition for C,,, or lower site symmetries made possible by the presence of the linear odd-rank crystal field term.
4. Discussion 4.1. Crystallographic background Crystallographic studies by Ambady [9] and Hawthorne and Ferguson [ 10 ] have shown that calcium tartrate tetrahydrate belongs to the space group P2,2,2,. The unit cell is orthorhombic with a = 9.24 A, b= 10.63 A, c=9.66 8, and 2=4. The calcium ion was found to be eightfold coordinated with the coordination polyhedron of oxygen atoms forming a distorted dodecahedron. Fig. 3 shows schematically such a dodecahedron, drawn using the crystallographic data of Hawthorne and Ferguson [ 10 1. In the figure 01 and 02 belong to one molecule of tartrate ion with 01 being a hydroxyl and 02 a neighbouring carboxyl oxygen. The same relation holds for 05 and 04; 03 and 06 are both carboxyl oxygen atoms belonging ‘to two different tartrate ions. 07 153
Table 1 Observed and calculated energy levels of Eu3+ in CaC4H406.4H20 Exditation
Level
Energy (cm-‘)
(nm) observed 578.65
‘F0
0.0
barycentre
rms
-0.02
68.46
9.59
7FI
257 318 609
257 316 612
440
%
869 950 1160 1207
871 1129 1162 1207
1098
‘h
1823 1826 1850 I907 1940 2020 2030
1819 1830 1852 1925 1941 2012 2018
1938
‘F4
2734 2799 2857 2865
2129 2798 2860 2875 2877 2910 2972 3039 3097
2919
2919 2971 3036 3097 579.21
calculated
%I
0.0
-0.17
53.32
‘F,
243 390 512
243 397 504
427
‘F*
936 951 1030 1162 1205
940 963 1022 1167 1197
1094
1837 1865 1887 1949 1958 1998 2035
1842 1857 1877 1945 1964 200 1 2039
1945
2700
2707 2731 2795 2815 2859 2926 2942 3038 3085
2885
‘F4
2795 2818 2860 2932 2940 3038 3084
8.5
Volume I6 I, number 2
CHEMICALPHYSICSLETTERS
8 September 1989
Table 1 Continued Excitation (nm)
Level
Energy (cm ’ ) observed
579.71
‘F0
0.0
26.80
7.27
292 317 436
372
%
1837 1846
I845
1941
1861 1884
1858 1877
1937 1960 1988
1929 1953 1995
2690 2732
2692 2729
2757 2819 2862 2958
2765 2812 2857 2959
2964 3010 3054
2971 3009 3055
I858
of Ca*+ in calcium
tar-
and 08 are oxygen atoms of two water molecules. has an approximate DZdsymmetry.
The polyhedron
4.2. Crystaljeld calculations The spectroscopy
rms
295 318 432
33
dodecahedron
0.16
barycentre
‘F,
‘F.S
Fig. 3. The coordination trate tetrahydrate.
calculated
results show that the symmetry
2894
of the sites occupied by the Eu3+ ion in calcium tartrate tetrahydrate is lower than D,,. This is borne out by two observations; the full Stark splitting of the J-manifolds and the presence of the ‘Do-‘F,, transition (excitation 5 14.5 and 488 nm, fig. 1) an electric dipole transition allowed for C,, or lower site symmetries. From the experimentally determined energy level values (24 Stark levels) it is unreasonable to expect a meaningful simulation of the crystal field for symmetry lower than D2, since for Cz symmetry there are 14 independent crystal parameters B,,. We treat the europium coordination as having an effective site symmetry of DZ. This is reasonable since the emission from the Eu3+ ion is much more sensitive to the immediate surroundings of the ion than the more distant neighbours. In view of this we do not feel that the small refinements in energy that would arise by calculation at symmetries lower than D1 would justify the effort such a consideration would require. The crystal field Hamiltonian for D2 symmetry is written as:
155
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Table 2 Crystal field parameters (cm-‘) for Eu‘+ in CaC,H,O,+4H,O assuming DI symmetry Excitation
B20
B22
ho
B 42
B44
B 60
Ba
B64
B 66
578.65 579.21 579.71
985 710 -132
123 -393 -255
917 -185 161
370 885 349
-538 -446 119
-1101 -357 39
-171 -346 -897
-288 -535 - 1057
28 47 310
+&dG-fJ+C66).
(1)
The simulation of the energy level scheme involves nine experimental crystal field parameters. In order to find the starting parameters we use the technique of successive reduction of symmetry. The procedure considers that the “real” site symmetry D, is not far from Dzd where the number of parameters is only five, Bzo, BJo, B44, Be0 and Be4. For Eu3+ the first-order splitting of the ‘F, state depends only on BZoand the splitting of ‘F2 on Bzo and Bqo. This allows us to estimate the values of B2,, and B40, neglecting J-mixing. Proceeding on this basis we refine the crystal field parameters for D,, symmetry and then incorporate additional parameters required by the lowering of symmetry to Dz. The observed and calculated energy level positions of the 7FJ (J= 1 to 4) manifolds are reported in table 1, where the root mean-squared (rms) deviations are also reported. The resultant crystal field parameters are given in table 2. 4.3. Analysis of the observed spectra Table 1 reports the observed and calculated emission lines associated with the different excitation wavelengths. The laser linewidth is sufficiently narrow to select each site except in the case of accidental coincidences. The measurements were performed
under continuous selective excitation and weak emission from Eu3+ in other sites not directly excited appears as a result of site-to-site energy transfer. However, this does not preclude the identification of groups of fluorescence lines arising from Eu3 + 156
ions whose 5D, level is in resonance with the laser wavelength, since these lines are enhanced in intensity with respect to the others. The ‘F, splittings for spectra A, B and C are different and hence they exhibit different Stark level distributions for the three sites. However, ‘F2, ‘F3 and ‘F, exhibit substantial overlap in several lines originating from the three sites giving credence to an energy transfer process between sites. The Eu3+ ions substitute for Ca2+ ions in calcium tartrate tetrahydrate. The ionic radius of Eu3+ is 1.03 A which is somewhat larger than the ionic radius of Ca’+, 0.99 A. The Eu3+ impurity ions probably create local distortion in the neighbouring tartrate complexes and these local distortions may be different in the three calcium sites making them nonequivalent. Also the substitution of Eu3+ for Ca2+ into calcium tartrate tetrahydrate requires charge compensation. One would expect one Na+ (available in the gel) ion to be incorporated (in place of Ca’+ ) for every Eu’+ ion. Therefore, one way in which three distinct sites could arise is from having the closest Ca2+ site occupied by either Ca’+ (with remote charge cpmpensation ) or Na+ (local charge compensation). Thus, if a chemical or structural imperfection is located near the Eu3+ ion, different sites would result. The energy of the $DOlevel for the three sites lies at 17282, 17265 and 17250 cm-‘. This suggests different degrees of covalency between the Eu3+ and the oxygen ions. The energy of the 5Do ( 172>0 cm- ’ ) level for site III lies in the “covalent” region of the nephelauxetic scale [ 111 whereas the energy of the 5D0 ( 17282 cm-‘) level for site I lies higher suggesting a more ionic bond. This is confirmed by the values of the second-order crystal field parameters (table 2) which provide an estimate of the electrostatic contribution to the crystal field effect. The absolute crystal field strength defined as [ 121
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applied to the data and a set of optical parameters for each site was obtained.
Table 3 Crystal field strength (cm-‘) for Eu’+ in CaC4H406.4H20
x
Crystal field strength (S) (cm-‘)
Excitation wavelength (cm-’ )
408 390 353
578.65 579.21 579.71
(
B&+2
8 September 1989
Acknowledgement The authors gratefully acknowledge the Natural Science and Engineering Research Council of Canada and the Faculty of Arts and Science of Concordia University for financial support. We are grateful to Dr. T.F. Belliveau for allowing us to use the crystal field programmes.
)I
l/2
c (RBj,, +I&) 1,1> 0
(2)
can render the preceding arguments more quantitative. The crystal field strengths (table 3) of the three sites are seen to increase with increasing excitation energy substantiating the conclusion that the degree of covalent character decreases from site III to site I.
5. Conclusions In summary, site-selective spectroscopy has been shown to give a reasonably complete and consistent picture of the behaviour of Eu3+-doped calcium tartrate tetrahydrate. The Eu3+ ions are shown to occupy at least three different types of site. Despite the low symmetry, crystal field theory was
References [ 11J. Heber, K.H. Hellwege, U. Kobler and H. Murmann, Z. Pbysik 237 (1970) 189. [2] M. Asano and J.A. Koningstein, Chem. Phys. 42 (1979) 369. [ 31 J. Dexpert-Ghys, M. Faucher and P. Caro, Phys. Rev. B 23 (1981) 607. [4] G. Sperka and M. Bettinelli, Inorg. Chim. Acta 149 (1988) 147. [ 51H.K. Hen&h, J. Dennis and J.J. Hanoka, J. Phys. Chem. Solids 26 (1965) 493. [ 61 T.F. Belliveau, Ph.D. Thesis, McGill University, unpublished (1988). [ 71 D.W. Marquardt, J. Sac. Ind. Appl. Math. 1 I ( 1963) 43 I. IS] M.J.D. Powell, Computer J. 1 ( 1964) 303. [ 91 G.K. Ambady, Acta Cryst. B 24 ( 1968) 1548. [lo] F.C. Hawthorne and R.B. Ferguson, Acta Cryst. B 38 ( 1982) 2461. [J 11P. Care, 0. Bcaury and E. Antic, J. Phys. (Paris) 37 (1976) 671. [ 121 N.C. Chang, J.B. Gruber, R.P. Leavitt and CA. Morrison, J. Chem. Phys. 78 (1982) 3877.
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