Optical and electronic
properties
of the aluminophosphate glasses doped with 3D-transition...
Rev.Adv.Mater.Sci.
10 (2005)
367-374
367
OPTICAL AND ELECTRONIC PROPERTIES OF
THE ALUMINOPHOSPHATE GLASSES DOPED WITH
3D-TRANSITION METAL IONS
M.Elisa1, Cristiana E.A.Grigorescu1, Cristina Vasiliu1, M.Bulinski2, V.Kuncser3,
Daniela Predoi3, G. Filoti3, Aurelia Meghea4, Nicoleta Iftimie4 and Maria Giurginca4
1
Department for Advanced Materials, National Institute of Optoelectronics-INOE 2000 1, Atomistilor Str., P.O.Box
MG - 5, RO-77125, Bucuresti-Magurele, Romania
2
Department of Optics, Faculty of Physics, University of Bucharest, P.O.Box MG 11, RO-77125, BucharestMagurele, Romania
3
Magnetic Spectroscopy Laboratory, National Institute for Materials Physics, P.O. Box MG-7, RO-77125,
Bucuresti-Magurele, Romania
4
Applied Spectroscopy Laboratory, Faculty of Industrial Chemistry, University POLITEHNICA of Bucharest,
Applied Spectroscopy Laboratory, 1, Polizu Str., 1 Sector, Bucharest, Romania
Received: April 28, 2005
Abstract. Aluminophosphate glasses doped with Fe, Mn, and Cr have been obtained by a wet
non-conventional method. Structural information was provided by IR absorption spectra in the
range 2000-500 cm-1. The optical behaviour (transmission and refractive index) of the samples
has been studied by UV-VIS-NIR spectroscopy. The Fe valence state and the local coordination
were also analysed via 57Fe Mossbauer spectroscopy, whose data revealed the redox equilibrium in the Fe-doped glasses according to the redox potentials of the transition ions.
1. INTRODUCTION
Recently, the aluminophosphate glasses gained a
special scientific interest due to their applications
in optics, optoelectronics and medicine [1]. It is
worth to be mentioned optical filters, laser active
media based on rare-earth-doped glasses [2], implant materials as prosthesis based on invert phosphate glasses [3], protection glass against nuclear
radiation [4], optical switches, wave-guide, screens
for luminescent lamps, composite materials made
of phosphate glass and organic polymers and
fullerens [5], etc.
The investigation of the redox processes in the
phosphate glasses doped with transition ions has
an important role in the obtaining of the coloured
and homogeneous optical glasses [6].
The oxidation state of the redox ions depends
on the chemical composition of the vitreous melt,
temperature and oxygen partial pressure [7].
Iron oxides used as dopants (FeO and Fe2O3)
provide interesting phenomena over the structure and
the properties of the phosphate glasses [8]. Small
amounts of Fe2O3 (2-5 wt.%) induce an increase of
104-fold of the chemical strength against water. Thus,
the iron-doped phosphate glasses are used to embed and to make inert the nuclear wastes (Cs, Sr,
Fe, Co, Ni, Mn, U, Po, rare-earth, sulphides, chlorides, etc.) from the atomo-electrical plants [9,10].
The nuclear wastes, on their turn, increase the glass
chemical stability and diminish the crystallization
tendency of the iron-doped phosphate glass. These
doped-vitreous materials together with policaprolac-
Corresponding author: M. Elisa, e-mail: [email protected]
© 2005 Advanced Study Center Co. Ltd.
368
M.Elisa, Cristiana E.A.Grigorescu, Cristina Vasiliu, M.Bulinski, V.Kuncser, et al.
tone and pollylactic acid are used for bone implants
[11]. On the other hand, the iron-doped glasses
exhibit interesting electrical and magnetic properties that depend on the iron redox species and their
coordination symmetry [12]. It is known that the
undoped phosphate glass has diamagnetic properties; the iron-doped phosphate glass is a paramagnetic material in contrast with the crystallized phosphate glass, which is ferromagnetic. Based on the
IR absorption of the Fe2+ ions (1050 nm), these
doped phosphate glasses are used to retain the heat
radiation in different devices [13].
The manganese-doped phosphate glasses are
interesting due to their magnetic properties. The
dependence of the magnetic susceptibility on temperature and the correlation between the Curie temperature and the manganese ion amount were studied in [14,15].
In this paper we present the optical and electronic properties of the aluminophosphate glasses
doped with iron as well as with additional manganese and chromium ions by a wet method [16]. The
optical, structural and electronic properties of the
doped aluminophosphate glasses were investigated
by ultra-violet-visible-near-infra-red (UV-VIS-NIR),
infrared (IR) and Mössbauer spectroscopy (MS). The
Mössbauer data provide also information about the
oxidation states and the coordination symmetry of
the transition doping ions. The dependence of the
refractive index vs. wavelength and the redox equilibrium of the transition metal ions with multiple oxidation states (iron, manganese and chromium) were
also investigated.
2.EXPERIMENTAL
In the present paper, by the wet method, we have
obtained undoped and 3d transition ion-doped
aluminophosphate glasses, belonging to the following oxide systems:
- S1: 58.6LiPO3 29.3Al(PO3)3 10Ba(PO3)3 2La2O3
- S2: 58LiPO3 29Al(PO3)3 10Ba(PO3)3 2La2O3 1FeO
- S3: 57,33LiPO3 28.66Al(PO 3) 3 10Ba(PO 3) 2
2La2O31FeO 1MnO2;
- S4: 57.33LiPO3 28.66Al(PO3)3 10Ba(PO3)2 2La2O3
1FeO 1CrO3.
The chemical reagents used to obtain the
undoped and doped aluminophosphate glasses are
shown in the Table 1.
The wet-method stages for obtaining the
undoped/doped aluminophosphate glasses were: (a)
the homogenisation and the evaporation of the reagents up to 100-120 °C, (b) the drying process at
180-200 °C, (c) the preliminary heat treatment at
Table 1. The chemical reagents used for the glass
preparation.
Li2CO3 a.g. or LiOH a.g.
Al2O3 a.g.
BaCO3 a.g.
H3PO4 a.g., sol.
conc.85%
La2O3 a.g.
Doping reagents
CrO3 a.g
MnO2 a.g. (Merck)
FeSO4 7H2O a.g.
200-800 °C, (d) the glass melting and the refining
processes at 1000-1200 °C, (e) the casting process
and (f) the annealing stage.
The sample glass S1, S2, S3, S4 (Table 2) were
melted at 1200 °C for 4 hours and subsequently
annealed at 400 °C for 2 hours.
The UV-VIS-NIR transmission spectra were obtained with an UV-VIS-NIR-JASCO 570 V - 1999
spectrophotometer, in the range 250-2300 nm, band
width 2 nm, band width (NIR) 8 nm, data pitch 2 nm,
scanning speed 400 nm/min.
The IR absorption spectra were taken with a FTIR–620-JASCO spectrophotometer, in the range
2500-400 cm-1, accumulation 16, resolution 4 cm-1,
gain 2, aperture 7.1 mm, scanning speed 2 mm/
sec.
The refractive indexes of the analysed samples
were measured with a PR2 Pulfrich refractometer
(Carl-Zeiss Jena) and the data for the whole spectral range were fitted using Cauchy type series.
The 57Fe Mössbauer spectra were acquired at
room temperature (RT) by using a constant acceleration spectrometer and a 57Co(Rh) source. The
isomer shifts were reported to RT α-Fe.
3. RESULTS AND DISCUSSION
The transmission spectra of the analysed samples
in the UV-VIS-NIR range are presented in Fig.1. As
observed, the undoped aluminophosphate glass (S1)
exhibits a relative high optical transmission from
about 250 nm to more than 1500 nm.
The UV-VIS-NIR spectra of S2 and S3 samples,
doped with Fe and with Fe+Mn, respectively, show
mainly the same shape, but they are characterized
by a higher absorption in comparison with the
undoped sample, especially above 500 nm. However, a new small absorption peak appears in these
samples at around 1050 nm, suggesting the presence of Fe2+ ions [13,14]. On the contrary, the absorption spectrum of the S4 sample (doped with Fe
and Cr) shows a very different behaviour, with two
Optical and electronic properties of the aluminophosphate glasses doped with 3D-transition...
369
Table 2. The chemical reagents (a.g.) and the oxide composition (wt.%.) of the glass samples S1, S2, S3,
and S4.
Glass sample
The chemical
reagents
The reagent amount
(g)/(cm3)
The oxide brought
by the reagent
Oxide
composition (wt.%)
The undoped
glass (S1)
Li2CO3
Al2O3
BaCO3
La2O3
H3PO4
Li2CO3
Al2O3
BaCO3
La2O3
H3PO4
FeSO4 7H2O
Li2CO3
Al2O3
BaCO3
La2O3
H3PO4
FeSO4 7H2O
MnO2
Li2CO3
Al2O3
BaCO3
La2O3
H3PO4
FeSO4 7H2O
CrO3
9,97
5,97
7,88
2,6
56,12 (cm3)
8,58
6
7,88
2,6
55
1,112
8,48
5,81
7,88
2,6
54,86
1,112
0,348
8,48
5,81
7,88
2,6
54,86
1,112
0,4
Li2O
Al2O3
BaO
La2O3
P2O5
Li2O
Al2O3
BaO
La2O3
P2O5
FeO
Li2O
Al2O3
BaO
La2O3
P2O5
FeO
MnO2
Li2O
Al2O3
BaO
La2O3
P2O5
FeO
CrO3
5,36
9,12
9,34
3,98
72,2
4,5
7,85
8
3,4
75,88
0,37
4,37
7,4
7,8
3,31
76,31
0,36
0,44
4,49
7,6
8
3,4
75,63
0,37
0,52
Fe-doped
glass (S2)
Fe+Mndoped glass
(S3)
Fe+Crdoped glass
(S4)
absorption bands, specific to the Cr3+ ions at about
460 nm and 660 nm, respectively [14]. The absorption above 1000 nm is lower for S4 sample than that
of the S2 and S3 samples and slowly higher than
that of the undoped glass.
Based on the above observations, we may assess that the transition metal ions are the ones responsible for the peculiar behaviour of the absorption spectra in the visible range of the spectrum.
For the S3 sample, taking into account the high
UV absorption due to Fe3+ ions and the high optical
transmission in the VIS range due to Mn2+ and Fe2+
ions, we may assume the following redox equilibrium:
Mn4+ + Fe2+ ↔ Mn2+ + Fe3+.
Thus, manganese has a positive reduction potential and iron has a positive oxidation potential.
The S4 glass sample has UV absorption peaks
specific to Fe3+ (430-440 cm-1) and Cr6+ (UV absorp-
tion) ions and absorption features characteristic to
Cr3+ ions, mentioned above. Thus, we may suppose
the following redox equilibrium:
Fe2+ + Cr6+ ↔ Fe3+ + Cr3+.
Accordingly, chromium has a positive reduction
potential and iron has a positive oxidation potential.
Information on the local structure of the undoped
(S1) and respectively of the Fe (S2), Fe+Mn (S3),
and Fe+Cr (S4) -doped aluminophosphate glasses
is provided in a first step by the absorption spectra
in the IR range. The IR absorption spectra of the
doped samples are shown in Fig. 2.
It is worth mentioning that the IR spectrum of
the undoped sample resembles quite well with the
IR spectrum of pure P2O5 glasses. The frequencies
of the optical phonons observed in the doped
samples, as compared with those of P2O5 glasses
[13,15] are given in Table 3.
370
M.Elisa, Cristiana E.A.Grigorescu, Cristina Vasiliu, M.Bulinski, V.Kuncser, et al.
Fig. 1. The transmission spectrum of the undoped aluminophosphate glass (S1) as compared with Fe (S2),
Fe+Mn (S3), Fe+Cr (S4)-doped aluminophosphate glass in the UV-VIS-NIR range.
Fig. 2. The IR absorption spectra of the undoped (S1), Fe (S2), Fe+Mn (S3) and Fe+Cr (S4)-doped
aluminophosphate glass.
Typical peaks for optical phonons specific for
aluminophosphate units are evidenced in all the
samples proving that phosphorous atoms are suitable vitreous network formers.
The S2, S3, S4 samples exhibit absorption
bands in the IR range (2500-400 cm-1), that are also
specific to the P2O5-based glass (Table 3). We assigned the 1266 cm-1 peak to PO2 asymmetrical
Optical and electronic properties of the aluminophosphate glasses doped with 3D-transition...
371
Table 3. The IR absorption peaks (cm-1) for the typical P2O5 glasses and for the analyzed glasses (undoped
S1, and transition ion-doped glasses: S2, S3, S4, respectively).
Phosphate units
P2O5 glass
[13,15]
Undoped
glass (S1)
Fe-doped
glass (S2)
Fe+Mndoped glass
(S3)
Fe+Crdoped glass
(S4)
P-O-H
P=O stretch
P-O-P
asymmetrical stretch
P-O-P
symmetrical stretch
P-O-P bend
[P2O7]4pyrophosphate unit
PO43- symmetrical
PO2 symmetrical
stretch
PO2 asymmetrical
stretch
PO32- symmetrical
PO32- asymmetrical
1380
1240-1270
840-950
–
1317
894
–
1266
896
–
1266
896
–
1266
896
670-800
740
790
480
–
742
792
488
–
742
792
510
–
742
792
476
–
–
1092
–
1089
–
1089
–
1089
1200-1300
1317
1266
1266
1266
980-1050
1110-1190
950
1092
946
1089
946
1089
946
1089
420-620
1027
1179
1015
1100-1170
stretch and/or to P=O stretch and the 1089 cm-1
peak to PO32- asymmetrical stretch and/or to PO2
symmetrical stretch. The 480, 488, 510, 476 cm-1
peaks were assigned to P-O-P bend. The 792 cm-1
and 742 cm-1 peaks attributed to P-O-P symmetrical stretch were obtained by the decomposing of a
large IR absorption band. The 1089 cm-1, 946 cm-1
(assigned to PO32- symmetrical stretch) and 896
cm-1 (P-O-P asymmetrical stretch) peaks are also
obtained by IR spectra decomposing.
The dependencies of the refractive index (n) versus the wavelength are presented in the Fig. 3. The
dispersion, measured by the standard parameter
Abbe’s number, ν, shows crown type glasses, with
a low dispersion and a visible refractive index in the
range 1.54 ÷ 1.57. The refractive index decreases
vs. the wavelength via a Cauchy type dependence.
At low wavelengths, where the variation of the refractive index is the sharpest, the behaviour of Fe+Mn
(S3)-doped glass is more similar to the variation of
the refractive index in the only Fe (S2) doped glass,
whereas the variation of the Fe+Cr (S4)-doped glass
is much similar to the Fe (S2)-doped glass, at longer
wavelengths. Both manganese and chromium ions
seem to reduce the decrement of the refractive index, but they have different effects on the absolute
values of the refractive index.
The dispersion curves of booth S3 and S4
samples are reduced as compared with the glass
doped only with iron (sample S2). At the same time,
it is worth mentioning that at any wavelength, the
refractive index of Fe+Mn (S3)-doped glass is higher
than the refractive index of the Fe (S2) doped glass,
which in turn is higher than for the Fe+Cr (S4)-doped
glass.
At this point, we should stress again that the
overall optical behaviour of the analysed glasses is
very sensitive to the electronic configuration of the
3d transition metal ions. In this respect, the 57Fe
Mössbauer spectroscopy can give a direct information about the oxidation state of iron as well as on
the local symmetry and coordination around the iron
sites. Indirectly, through such information can be
evidenced the redox mechanisms involving doping
transition elements in the S2-S4 samples. The 57Fe
Mössbauer spectra of these samples collected at
room temperature (RT) are shown in Fig. 4.
All spectra evidence the presence of the iron
paramagnetic ions with different valence and coordination states. The best convergence test required
a fit with three paramagnetic doublets in the S2 and
S3 samples and only two central doublets in the S4
sample. The fitted Mössbauer parameters (isomer
shift, IS, quadrupolle splitting, QS, and the popula-
372
M.Elisa, Cristiana E.A.Grigorescu, Cristina Vasiliu, M.Bulinski, V.Kuncser, et al.
Fig. 3. Refractive index obtained at different wavelengths corresponding to the undoped (S1), Fe (S2),
Fe+Mn (S3) Fe+Cr (S4)-doped aluminophosphate
glass. The dispersion curves respect Cauchy type
dependence.
tion of each iron position, obtained via the relative
area of the corresponding Mossbauer component,
R.A.) are presented in the Table 4.
The oxidation state and the oxygen coordination around the central iron, as suggested by the
values of the hyperfine parameters, are shown in
the same table. The Mössbauer spectra of the S2
and S3 samples evidence the presence of two significantly split doublets with high quadrupolle splitting (3.17 mm/s and 2.24 mm/s in sample S2 and
2.66 mm/s and 1.98 mm/s in sample S3) and isomer shift (1.17 mm/s and 1.01 mm/s in sample S2
and 1.18 mm/s and 1.14 mm/s in sample S3). The
most shifted doublets, with isomer shifts ranging
between 1.14 mm/s and 1.18 mm/s were assigned
to Fe2+ ions with octahedral oxygen coordination
whereas the ones with isomer shift of about 1.00
mm/s were assigned to Fe2+ ions in tetrahedral coordination [17,18,19]. Finally, the third iron component with much lower IS (0.21 mm/s in sample S2
and 0.34 mm/s in sample S3) and QS (0.54 mm/s
in sample S2 and almost negligible in sample S3)
were assigned to Fe3+ ions with tetrahedral coordination in the S2 sample and octahedral coordination in S3 sample [17,18]. The Mössbauer data involve for both S2 and S3 samples, a mixture of Fe2+
(42%) and Fe3+ (58%) states. The S4 sample contains only Fe3+ ions. Tacking into account the
behaviour of the absorption spectra presented in
Fig.1, one can conclude that the absorption coefficient has to be dependent mainly on the Fe oxida-
Fig. 4. Mössbauer spectra of the Fe (S1), Fe+Mn
(S3) and Fe+Cr (S4) containing aluminophosphate
glasses, respectively, obtained at room temperature.
tion state, which in turn has to influence the oxidation state of the pair-doping ion. In the present case,
Mn and Fe ions should accommodate a bivalent
oxidation state and Cr a hexavalent one. The fact
that Fe3+ ions (in a proportion of 58%) are evidenced
in both S2 and S3 samples stands for a relatively
higher oxidation potential of the aluminophosphate
matrix, inducing electron transfer process from Fe
(or Mn) ions to the matrix. Nevertheless, the matrix
presents the same oxidation potential also in the
S4 sample. The presence of only Fe3+ ions (100%)
in S4 sample, can be explained only via an electron
transfer from the Fe2+ ions (still 42% in sample S2
and S3) to the unstable hexavalent Cr ions, resulting in an additional valence states of Cr (e.g. Cr3+)
in the S4 sample, with direct influence of the UV-
Optical and electronic properties of the aluminophosphate glasses doped with 3D-transition...
373
Table 4. Mossbauer components of the analyzed samples and their corresponding hyperfine parameters.
The oxidation state and the coordination of the different iron sites are also presented.
Sample
Mossbauer
component
IS
(mm/s)
QS
(mm/s)
R. A.
(%)
Oxidation
state
Oxygen
coordination
S2 1%Fe
Doublet 1
Doublet 2
Doublet 3
Doublet 1
Doublet 2
Doublet 3
Doublet 1
Doublet 2
1.17(1)
1.01(2)
0.21(2)
1.18(1)
1.14(2)
0.34(2)
0.16(2)
0.03(1)
3.17(1)
2.24(3)
0.54(2)
2.66(1)
1.98(4)
0.01(3)
0.96(5)
0.40(2)
19
23
58
32
10
58
34
66
Fe2+
Fe2+
Fe3+
Fe2+
Fe2+
Fe3+
Fe3+
Fe3+
6
4
4
6
6
6
4
Lower
than 4
S3 1%Fe
+1%Mn
S4 1%Fe
+1%Cr
VIS-NIR. Therefore, the different absorption spectrum of the S4 sample (in comparison with the spectra of the S2 and S3 samples) has to be due to the
Cr ions in this sample.
On the other site, starting from the Mössbauer
data related to the iron coordination for different iron
sites and the relative population of those sites, could
be estimated an average Fe coordination for each
sample. Average coordination of 4.84, 4.38 and
lower than 4.00 were obtained for the glasses doped
with Fe+Mn (S3), Fe (S2) and Fe+Cr (S4). At a
glance of Fig. 3, can be observed that whatever the
wavelength is, the refractive index are the highest in
glasses doped with Fe+Mn (S3), intermediate in
glasses doped with Fe (S2) and the lowest in Fe+Cr
(S4) doping glasses, suggesting so a correlation
between the absolute values of the refractive index
and the Fe coordination.
4. CONCLUSIONS
Undoped and transition metal-doped aluminophosphate glasses prepared by wet non-conventional
method were investigated by optical methods and
Mössbauer spectroscopy.
The UV-VIS-NIR absorption spectra are strongly
dependent on the doping ions. The Fe and Fe+Mn
– doped samples evidence a 1050 nm peak typical
for the Fe2+ ions, in agreement with Mössbauer results proving a 58% relative amount of Fe2+ ions in
the both samples. The Fe+Cr –doped glass exhibit
absorption peaks at 460 and 660 nm, specific for
Cr3+ ions. The Mössbauer results suggest indirectly
a valence state in the above sample, through the
complete oxidation of the Fe2+ ions. The IR spectra
prove the role of vitreous network former for phosphorous whereas the tetrahedral/octahedral configurations of the Fe doped glasses, evidenced by
Mössbauer spectroscopy, suggests the former/
modifier role of the doping ions. A correlation between the values of the refractive index and the Fe
mean coordination might be suggested via the analysis of the dispersion curves and the Mössbauer data.
REFERENCES
[1] P P.Vast de la Lille // Verre (3) (1996) 3.
[2] M. Bigerelle, A. Iost, F. Gomez and P. Vast //
Journal of Materials Science Letters (2001)
1037.
[3] G.Giabotinsche, Lazernie Fosfatnie Stekla
(Nauca, Moskva, 1980), in Russian.
[4] J.Vogel and P.Wange // Glastech.Ber. %
(1997) 220.
[5] S.V.Stefanovski and I.A.Ivanov // Glass Physics and Chemistry (1994) 103.
[6] Y.B.Peng and J.B.Koen, In: Proc. of XVIIth
International Congress on Glass, China, 2,
(1995) p.301.
[7] L.Ortmann, D.Hohne and G.Nolle //
Glastech.Ber.-Glass $' (1996) 235.
[8] H.Schreiber, L.Peters, J.Beekman and
C.Schreiber // Glastech. Ber. $'(1996) 269.
[9] R.K.Brow and C.M.Arens // Physics and
Chemistry of Glasses !# (1994) 132.
[10] B.C.Sales and L.A.Boatner // J.NonCryst.Solids %' (1986) 83.
[11] B.Kumar and S.Lin // J.Am.Ceram.Soc. %"
(1991) 226.
374
M.Elisa, Cristiana E.A.Grigorescu, Cristina Vasiliu, M.Bulinski, V.Kuncser, et al.
[12] T.Tsuchiya and N.Yoshimura //
J.Mater.Science " (1989) 493.
[13] W.Guomei, L.Jiaheng and Y.Huaishun //
SPIE #' (1991)229.
[14] C.Stalhandske // Glasteknisk Tidskrift ##
(2000) 65.
[15] Xiaoyan Yu, D.E.Day and G.Long // J.NonCryst.Solids # (1997) 21.
[16] Rodica Rogojan, M.Elisa and Marieta
Nicolae, Romanian Patent no.101354/1989.
[17] N.N.Greenwood and T.G.Gibb, Mossbauer
Spectroscopy (Chapmann and Hall Ltd.,
London,1971).
[18] U.Russo, S.Carbonin and A.Della Giusta,
Proceedings of XXX Zakopane School of
Physics (Zakopane, Poland, 1995) p.254.
[19] Tapan Pal, Taraknath Pal and S.Mitra //
Transactions of the Indian Ceramic Society
"& (1989) 115.