Electronic absorption spectroscopy of neodymium acetate single crystals

Article abstract of DOI:10.1016/S0925-8388(98)00449-6

Electronic absorption spectra of neodymium acetate single crystals were measured at 293 and 4 K. The intensities of 4f-4f transitions were calculated and the Judd-Ofelt Ω_λ parameters evaluated. These results differ in comparison with other lant

Full text of DOI:10.1016/S0925-8388(98)00449-6

Journal of Alloys and Compounds 275–277 (1998) 818–821
L
Electronic absorption spectroscopy of neodymium acetate single crystals
*
´
Anna Mondry , Krystyna Bukietynska
Faculty of Chemistry, University of Wrocław, Joliot-Curie 14, 50-383 Wrocław, Poland
Abstract
Electronic absorption spectra of neodymium acetate single crystals were measured at 293 and 4 K. The intensities of 4f–4f transitions
were calculated and the Judd–Ofelt Vl parameters evaluated. These results differ in comparison with other lanthanide carboxylate
compounds by relatively high ‘hypersensitive’ transition intensity and the V2 parameter value. The vibronic mechanism of the 4f–4f
transitions is discussed and the role of low-energy phonons is shown.
1998 Published by Elsevier Science S.A.
Keywords: Absorption; Intensity analysis; Lanthanides; Acetate
1. Introduction
2. Experimental
Nd2O3 (99.9% Koch and Light Laboratory Ltd) was
dissolved in glacial acetic acid and alkalized with NaOH to
pH54. The well shaped crystals of Nd(CH3COO)3?H2O
were formed during slow evaporation. The X-ray check
showed them to be isomorphic with those reported in
literature [1]. The concentration of Nd(III) ion was de-
termined complexometrically (6.132 M). The crystal den-
sity was measured by the flotation method in a chloroform/
bromoform mixture (d2052.184 g cm23). The refractive
index (nD) of the investigated crystals was assumed to be
1.50.
The IR spectra of the compound were recorded in the
range of 50–4000 cm21 with a Brucker IFS 113V spec-
trophotometer.
Electronic absorption spectra of good optical quality
crystals were recorded on a Cary 2300 and Cary 5
spectrophotometers at room and liquid helium tempera-
tures. No changes of the crystal quality were noticed on
repeated cooling and heating measurements. The investi-
gated spectral range was 250–2500 nm.
It is well known that crystal structures of the light
(Ce–Sm) [1,2] and heavy lanthanide (Eu–Lu) acetates
[2–4] are different. In the structures of both types, acetate
ligands act as bridges between two Ln(III) ions. The light
lanthanides form polynuclear chains, whereas dinuclear
structures are typical for heavy lanthanide acetates.
The influence of the crystal structure of heavy Ln(III)
acetate single crystals on their spectral properties has been
previously discussed [5–7]. It was found that spectral
intensities of the Ln2(CH3COO)6?4H2O single crystals
were different as compared to other Ln(III) carboxylate
compounds with acetic acid derivatives [8–12]. The pos-
sible explanation of this fact was assumption of more
significant contribution of the polarization mechanism [13]
to the spectral intensities of the f–f transitions. It can be
related to the fact that contrary to other heavy lanthanide
carboxylate compounds, the Ln–OH2 bonds are distinctly
shorter than the Ln–OOC– bonds in the Ln(III) acetate
crystal.
So far, absorption and luminescence spectra of chain
lanthanide acetates have been analyzed for the
Pr(CH3COO)3?H2O crystal [14]. The aim of this paper is
to report the spectroscopic properties of another light
lanthanide acetate, Nd(CH3COO)3?H2O.
The intensities of the 4f–4f transition (P) and values of
the Vl parameters were calculated from the following
Judd–Ofelt [15,16] relation:
8p2mcs
3h(2J 1 1)l52,4,6
P 5 x
O
Vl( f ncJiU (l)if nc9J9)2
(1)
where: P denotes oscillator strength, x5(n212)2 /9n, n is
the refractive index, J is the total quantum number of the
*
Corresponding author, Fax:
anm@wchuwr.chem.uni.wroc.pl
148 71 222348; e-mail:
0925-8388/98/$19.00
1998 Published by Elsevier Science S.A. All rights reserved.
PII: S0925-8388(98)00449-6

´
A. Mondry, K. Bukietynska / Journal of Alloys and Compounds 275–277 (1998) 818–821
819
ground state, ( f nCJiU (l)if nC9J9) is the reduced matrix
element of the respective unit tensor operator U (l), tabu-
lated by Carnall et al. [17] and Vl are empirical least-
squares-fitted parameters.
isomorphous Pr(III) acetate they are 4.216 A [2]), in no
f–f transition an additional splitting due to an ion-pair
effect [19] was observed. Some examples of the Stark
splitting of f–f transitions at 4 K are presented in Fig. 1.
˚
The appearance of a single, sharp peak of the I9 / 2 →2P1 / 2
4
transition at 23312 cm21 with a half-width of 6 cm21, as
4
well as a splitting of the F3 / 2 term into two optical lines:
11 510 and 11 544 cm21 is indicative of a single Nd(III)
site, consistent with the X-ray analysis. Since the excited
3. Results
2
state P1 / 2 is a Kramer’s doublet, we could deduce the
The spectral intensities of the Nd(CH3COO)3?H2O
crystal at 293 and 4 K together with the Vl parameter
values for the room temperature are presented in Table 1.
For comparison purposes the spectral data for Nd(III)
acetate solution (1:20) are also included. It may be noticed
that oscillator strength values of all 4f–4f transitions for
the crystal are slightly lower in comparison to those found
for the solution. As it can be seen from Table 1, an
intensity decrease in almost all transitions in the crystal
with the change of temperature from 293 to 4 K is
observed. Similarly, as for Ho(III) [5], Dy(III), Er(III) [6]
4
splitting of the I9 / 2 ground term, which has been found
as: 0, 71, 178, 348, 412 cm21. Some of these energies are
perfectly accordant with the frequencies obtained from the
far IR spectrum for Nd(III) acetate crystal (73, 77, 111,
]
126, 140, 158, 180, 203, 226, 236, 243, 253, 335, 469
cm21). They are also consistent with frequencies from a
laser Raman spectrum of the Pr(CH3COO)3?H2O [14].
It can be noticed from Fig. 1 that for transitions which
obey the selection rule DJ50, 62 additional very weak
side bands are observed (asterisks in Fig. 1b and c). They
are vibronic in origin and are 30–1542 cm21 apart from
and Eu(III) acetates [7] also for Nd(III) acetate, a rela-
4
the main electronic lines. For the I9 / 2 →4G5 / 2
,
2G7 / 2
4
tively high intensity of the ‘hypersensitive’ I9 / 2 →4G5 / 2
,
transition even the vibronics of 3342 cm21 (nOH) in the
]
2G7 / 2 transition and V2 parameter value is observed. They
are distinctly higher than for other known compounds of
Nd(III) ion with acetic acid derivatives i.e. glycinates
[8,9], trichloroacetates [12] and cyanoacetates [18].
The number of Stark components of electronic bands in
spectral region of 485–495 nm are observed. These
4
vibronics can also belong to the I9 / 2 →4G9 / 2 transition
4
(¯330 cm21 from the Stark components of the G9 / 2
term).
The majority of vibronics of about 200 cm21 from Stark
the 4 K spectra of Nd(III) acetate is in a good agreement
2
4
components of the H9 / 2 term lie inside the I9 / 2 →4F5 / 2
transition. This may be a reason of electronic lines
broadening which is observed only for this transition.
Apart from that, some additional peaks are observed for
1
]
with the J 1 manifold predicted by the group theory for
2
a single Nd(III) ion site with the C1 symmetry. In spite of
the fact that the Nd–Nd distances are very short (for the
4
the I9 / 2 →2H9 / 2 transition at 4 K (the insert in Fig. 1c).
These side bands are shifted 21–80 cm21 towards lower
energies from the lowest sublevel (12 527 cm21) of the
2H9 / 2 term. Also very similar strong peaks located on the
Table 1
4
red wavelength side of the I9 / 2 →2H9 / 2 transition were
Oscillator strength values (Pexp) and Vl parameters for the neodymium
acetate complex in solution (cNd 50.01997 M, cAc 50.4001 M) and for the
single crystal of Nd(CH3COO)3?H2O (cNd 56.132 M).
4
found by us in the Nd(III)-diethylenetriaminepentaacetate
(dtpa) crystal [20]. In both these cases there is only one
water molecule in the inner sphere of Nd(III) ion with a
rather short Nd–OH2 bonding. The reason for appearing
these bands is not completely clear for us, however one
may notice that positions of the additional vibronics
Transition(s) I9 / 2
→
Solution
Single crystal
P
exp 3108
293 K
P
4 K
P
exp 3108
exp 3108
4F3 / 2
220.56
827.17
890.03
65.15
183.80
668.94
677.92
44.35
59.60
362.90
464.06
14.33
8.12
893.97
385.38
110.43
4.76
4
4F5 / 2, 2H9 / 2
4F7 / 2, 4S3 / 2
4F9 / 2
correspond to the sum of the energies of the I15 / 2 Stark
components and the first nOH overtone. One of sublevels
]
4
which is undoubtedly a Stark component of the I15 / 2 term
2H11 / 2
19.17
has the energy 5835 cm21. The highest energy of vibronics
4G5 / 2, 2G7 / 2
1971.35
769.27
174.06
29.18
1802.00
552.06
129.77
17.86
7.30
1.92
757.47
5.3760.19
2.8260.17
5.3960.26
4.01
4
lying before the I9 / 2 →2H9 / 2 transition is 12 506 cm21
2K13 / 2, 4G7 / 2, 4G9 / 2
2K15 / 2, 2G9 / 2, (2D, F)3 / 2, 4G11 / 2
2
(5835 cm211233342 cm21512 519 cm21).
2P1 / 2
2D5 / 2
29.18
2.88
(2P, 2D)3 / 2
4D3 / 2, 4D5 / 2, 2I11 / 2, 4D1 / 2, 2L15 / 2
V2 31020 [cm2]
V4 31020 [cm2]
V6 31020 [cm2]
rms3107
886.98
761.75
4. Conclusions
6.2760.51
3.7360.47
7.9560.67
9.39
It is well known that polarizability of the ligand has a
considerable contribution to the ‘hypersensitive’ transitions

´
820
A. Mondry, K. Bukietynska / Journal of Alloys and Compounds 275–277 (1998) 818–821
4
4
Fig. 1. Absorption spectra of the Nd(CH3COO)3?H2O crystal at 293 and 4 K for transitions: (a) I9 / 2 →2D5 / 2
,
2P1 / 2, (b) I9 / 2 →4G5 / 2
,
2G7 / 2, (c)
4I9 / 2 →4F5 / 2, 2H9 / 2, (d) I9 / 2 →4F3 / 2; vibronics are indicated by asterisks. (cNd 56.132 M, crystal thickness l50.071 cm for a and 0.0145 cm for b, c, d)
4

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A. Mondry, K. Bukietynska / Journal of Alloys and Compounds 275–277 (1998) 818–821
821
[2] S. Ganapathy, V.P. Chacko, R.G. Bryant, M.C. Etter, J. Am. Chem.
Soc. 108 (1986) 3159.
[3] Ł.A. Asłanov, J.K. Abdulminev, M.A. Porai-Koshits, V.I. Ivanov,
Dokł. Akad. Nauk S.S.S.R. 205 (1972) 343.
[4] M.C. Favas, D.L. Kepert, B.W. Skelton, A.H. White, J. Chem. Soc.,
Dalton Trans. (1980) 454.
[13]. In the crystals of lanthanide acetates tightly bonded
water molecules exert more polarization effect on the
metal centre than the carboxylic groups. This might
explain qualitatively the high intensity of the ‘hyper-
sensitive’ transitions at room temperature. This does not
explain, however a strong decrease of some transitions
intensity with lowering the temperature to 4 K, what was
especially well observed for Eu(III) acetate crystal [7]. A
possible explanation of this phenomenon can be given if
we assume that low energy phonons play an important role
in the enhancement of intensities of Ln(III) acetates at 293
K. These low energy phonons are particularly strongly
coupled with Stark components of the Ln(III) ground state.
´
[5] A. Mondry, K. Bukietynska, Inorg. Chim. Acta 186 (1991) 135.
´
[6] A. Mondry, K. Bukietynska, Acta Phys. Pol. A. 90 (1996) 233.
´
[7] A. Mondry, K. Bukietynska, in preparation.
´
[8] J. Legendziewicz, E. Huskowska, A. Waskowska, Inorg. Chim. Acta
92 (1984) 151.
[9] J. Legendziewicz, E. Huskowska, Inorg. Chim. Acta 95 (1984) 57.
[10] J. Legendziewicz, E. Huskowska, J. Less-Common Met. 146 (1989)
33.
´
~
[11] J. Legendziewicz, C. Szafranski, G. Oczko, B. Jezowska-Trzebia-
towska, Bull. Acad. Pol. Sci. Chim. 36 (1988) 190.
[12] J. Legendziewicz, G. Oczko, Polyhedron 10 (1991) 1921.
[13] S.F. Mason, R.D. Peacock, B. Stewart, Mol. Phys. 30 (1975) 1829.
Acknowledgements
¨
[14] M.P. Hehlen, H. Riesen, H.U. Gudel, Inorg. Chem. 30 (1991) 2273.
[15] B.R. Judd, Phys. Rev. 127 (1962) 750.
[16] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511.
[17] W.T. Carnall, P.R. Fields, K. Rajnak, J. Phys. Chem. 49 (1968)
4424.
[18] P. Starynowicz, Polish J. Chem. 68 (1994) 621.
[19] P. Caro, O.K. Moune, E. Antic-Fidancev, M. Lemaitre-Blaise, J.
Less-Common Met. 112 (1985) 153.
The authors would like to thank Dr. P. Starynowicz for
X-ray identification of the crystals. This work was sup-
´
ported by Komitet Badan Naukowych (Committee of
Scientific Research), grant 3 T09A 02010.
[20] A. Mondry and P. Starynowicz, in preparation.
References
[1] G.G. Sadikov, G.A. Kukina, M.A. Porai-Koshits, Zh. Strukt. Khim.
8 (1967) 551.