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Ce co-doped lithium alumino-phosphate glasses
Optical Materials xxx (2016) 1e8
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The influence of different alkaline earth oxides on the structural and optical properties of undoped, Ce-doped, Sm-doped, and Sm/Ce codoped lithium alumino-phosphate glasses € ncke c, * H.A. Othman a, G.M. Arzumanyan b, D. Mo a b c
Physics Department, Faculty of Science, Menoufia University, Shebin El-Koom, Egypt Joint Institute of Nuclear Research, Dubna, 141980, Russia Kazuo Inamori School of Engineering, Alfred University, NY, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 September 2016 Received in revised form 11 October 2016 Accepted 28 October 2016 Available online xxx
Undoped, singly Sm doped, Ce doped, and Sm/Ce co-doped lithium alumino-phosphate glasses with different alkaline earth modifiers were prepared by melt quenching. The structure of the prepared glasses was investigated by FT-IR and Raman, as well as by optical spectroscopy. The effect of the optical basicity of the host glass matrix on the added active dopants was studied, as was the effect doping had on the phosphate structural units. The optical edge shifts toward higher wavelengths with an increase in the optical basicity due to the increased polarizability of the glass matrix, but also with increasing CeO2 concentration as a result of Ce3þ/Ce4þ inter valence charge transfer (IV-CT) absorption. The optical band gap for direct and indirect allowed transitions was calculated for the undoped glasses. The glass sample containing Mg2þ modifier ions is found to have the highest value (4.16 eV) for the optical band gap while Ba2þ has the lowest value (3.61 eV). The change in the optical band gap arises from the structural changes and the overall polarizability (optical basicity). Refractive index, molar refractivity Rm and molar polarizability am values increase with increasing optical basicity of the glasses. The characteristic absorption peaks of Sm3þ were also investigated. For Sm/Ce co-doped glasses, especially at high concentration of CeO2, the absorption of Ce3þ hinders the high energy absorption of Sm3þ and this effect becomes more obvious with increasing optical basicity. © 2016 Elsevier B.V. All rights reserved.
Keywords: Ce, Sm ion co-doped glasses UVeVis spectroscopy FT-IR and Raman spectroscopy Oxygen polarizability Optical basicity
1. Introduction Phosphate glasses are easy to fabricate and can be considered efficient rare-earth ions host matrices as they provide a high solubility for rare-earth ions without showing strong clustering effects. Depending on the composition, phosphate glasses possess good chemical durability, ion exchangeability and excellent optical properties [1e3]. In the studied multicomponent glass series, lithium oxide, as any alkali oxide, acts as network modifier. When added to vitreous P2O5, P-O-P linkages break and non-bridging oxygen ions (nbO) are created [4]. Network intermediates such as Al2O3 are added to glasses to improve their chemical durability and to decrease their solubility in water [5]. Introducing Al2O3 into phosphate glasses stabilized the glass through strong cross-linking
* Corresponding author. €ncke). E-mail address: [email protected] (D. Mo
between the PO4-tetrahedra and therefore improves also the physical properties such as increased Tg, lower coefficient of thermal expansion, or the already mentioned increased chemical stability [6e8]. The low coefficient of thermal expansion for example makes alumino-phosphate glasses ideal candidates for ion exchange planar waveguide devices [9]. For Al3þ, the high field strength (Z/r2), that is, the ratio of the charge (Z) to the square ionic radius (r), is behind its strong cross linking capability. This effect is also seen to a lesser degree for small Li-ions. Lithium aluminophosphate glasses are of great interest due to their technological significance in a lot of applications such as high energy capacitors, re-cycling of nuclear waste, and other electro chromic devices [10]. Introducing alkaline earth oxides as modifier oxides to phosphate glass, such as magnesium, calcium, strontium, and barium oxide, impacts the physical properties as this can strengthen or weaken the chemical durability of the glasses, or prevents devitrification of the glass host [5]. Furthermore, the different alkaline earth oxides affect the optical basicity, which in turn may lead to a change in the
http://dx.doi.org/10.1016/j.optmat.2016.10.051 0925-3467/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: H.A. Othman, et al., The influence of different alkaline earth oxides on the structural and optical properties of undoped, Ce-doped, Sm-doped, and Sm/Ce co-doped lithium alumino-phosphate glasses, Optical Materials (2016), http://dx.doi.org/10.1016/ j.optmat.2016.10.051
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energy states, for example of Ce-ions, and can influence the energy transfer to other rare earth ions [11]. Phosphate glasses doped with rare earth ions have a wide range of applications in photonics, highpower lasers, or in optical communications [12e14]. Cerium ions act as sensitizers in luminescent glasses, and are used in scintillators, or in laser glasses. They are also widely used as sensitizers in photo-thermo-refractive glasses [15]. Depending on the redox state, cerium ions might also be added to decrease photo-darkening in optical fibers and are used as decolorizing agent for container glasses [11]. As the spectroscopic properties of rare earth ions are highly influenced by the host in which they are embedded, the aim of this work is to investigate the effect of alkaline earth oxides (MgO, CaO, SrO and BaO) on the structure, as well as on physical and optical properties of undoped, Sm doped, Ce-doped, and Sm/Ce co-doped lithium alumino-phosphate glasses. 2. Experimental methods
was used for detection of spectra collected at different localizations of the samples with an integration time of 10 s for Raman measurements. The signals are probed in the 200e1400 cm1 fingerprint region of the vibrational frequencies with a spectral resolution of ~2.0 cm1. Rayleigh scattering was blocked using a Semrock long-pass edge filter. Baseline correction was employed during data processing using the rollingball method. All measurements were performed at room temperature. 2.3.3. Ultraviolet and visible absorption spectroscopy The optical absorption spectra were recorded at room temperature in the ultra violet to visible and near infrared region, using a Spectrophotometer, Jasco Model (V-570), in the wavelength range from 190 to 1800 nm. 2.3.4. Refractive index measurements The refractive index was measured using a PTR refractometer 46X at room temperature at the sodium wavelength 589 nm with an accuracy of about ±0.00005 RI.
2.1. Glass preparation 3. Results and discussion The glass samples were prepared according to the following formula: 50 P2O5 - 10 Al2O3 - 10 Li2O - 30 MO where M ¼ Mg, Ca, Sr and Ba. Doped glasses were also prepared with the addition of 3 1020 ions/cm3 of Sm2O3, or CeO2 (1 1020 ions/cm3), and codoped with Sm2O3 (3 1020 ions/cm3) and cerium (1 1020 ions/cm3). Another series was co-doped with 3 1020 ions/cm3 of Sm2O3 and 3 1020 ions/cm3 of CeO2. All these glass samples were prepared by the melt quenching method. They were synthesized from P2O5 (99.5%), Al2O3 (99.9%), Li2O (99.9%) and MgO, BaO, SrO, or CaO (all of purity 99%), from Aldrich. Sm2O3 (purity 99.999%), and CeO2 (99.99%), were used for rare earth doping, both from Aldrich. The mixed raw materials were placed in an alumina crucible and preheated in an electrical oven at 400 C for 3 h, and subsequently melted at 1200 C for 30 min. The glasses were cast into pre-heated stainless steel molds and were immediately transferred to an annealing furnace where they were kept at 450 C for about 1 h before cooling them slowly down to room temperature to remove internal stress. 2.2. Density measurement Glass densities of the prepared glasses were measured by the Archimedean method using toluene as an immersion liquid at room temperature. The accuracy of the measurements was approximately ±0.01 g cm3. 2.3. Optical measurements 2.3.1. FT-IR spectroscopy The infrared spectra of the prepared glasses were recorded on an FT-IR spectrophotometer type Tensor 27, Brulear over the 4000e200 cm1 wavenumber range at room temperature. Powdered samples with a grain size of about 60 mm were mixed with KBr and pressed into pellets of about 0.2 mm thickness. 2.3.2. Raman spectroscopy Raman measurements were conducted on a confocal microspectroscopy set-up. The system comprises a “Confotec CARS” scanning laser spectrometer coupled to the NIKON TE2000-E inverted microscope. Excitation at 633 was provided by a He-Ne laser (MellesGriot 05-LHP-991). All Raman spectra were collected in the backscattering geometry and dispersed by 600 grooves/mm diffraction grating mounted in the MS520 monochromatorspectrograph. A Peltier-cooled CCD camera (ProScan HS101 H)
3.1. Structural investigation 3.1.1. Density and molar volume The composition of the prepared glasses, their density and molar volume are reported in Table 1. The density decreases with the following sequence (BaO > SrO > CaO > MgO) as the molecular weight of the modifier oxide decreases, BaO > SrO > CaO > MgO. The molar volume was then calculated according to the following formula: Vm ¼ Mrw , where (Mw) is the glass molecular weight and (r) is its density. As the increase in the molecular weight is higher than the increase in density of the prepared samples, an increase in the molar volume is observed from Mg to Ba containing glasses. 3.1.2. FT-IR spectroscopy Examples of the transmittance spectra of the prepared samples are shown in Fig. 1. The following relation A ¼ [2 - Log(T%)], was used to convert these spectra to absorption spectra for more detailed analysis. One example with band assignments is shown in Fig. 2. All the IR absorption spectra are characterized by similar absorption peaks which can mostly be assigned to metaphosphate structural units, as desribed in detail in the following [16]. Vibrations of Al-polyhedra coupled with bending modes of the phosphate network d(P-O) extend up toz 520 cm1 [16]. The band envelop at 650 to 800 cm1 can be assigned to symmetric stretching modes ns(P-O-P) of bridges between phosphate tetrahedra. In the following we use the Qn nomenclature, where n denotes the number of briging oxygen atoms per phosphate tetrahedra. Metaphosphate units have 2 bridging and 4-n ¼ 2 terminal oxygen atoms per network forming tetrahedra. For bridging Q2 units this band is split into a low and high frequency component around 710 and 790 cm1, while P-O-P bridges between Q1 units show only one band around 700 cm1 [7,16]. The low energy feature at 710 cm1 might also be related to tetrahedral AlO4-units [17]. The asymmetric P-O-P bonds of metaphosphate chains and rings contribute strongly to the high frequency band envelop between 900 and 1400 cm1. These bands absorb at almost 900 cm1 for Q2 chains and around 980 cm1 for metaphosphate rings. Around 910 cm1 absorbs also the asymmetric P-O-P stretching mode of Q1 groups, which are also found at chain endings [7,16]. The asymmetric stretching modes nas(P-O) are found around 1110 cm1 for Q1 units and near 1245 cm1 for Q2 groups [7,16]. The effect of the charge balancing cations on the P-O bond strength is reflected in the shift of this band toward lower energies with
Please cite this article in press as: H.A. Othman, et al., The influence of different alkaline earth oxides on the structural and optical properties of undoped, Ce-doped, Sm-doped, and Sm/Ce co-doped lithium alumino-phosphate glasses, Optical Materials (2016), http://dx.doi.org/10.1016/ j.optmat.2016.10.051
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3
Table 1 Selected properties of the undoped base glasses 50 P2O5 - 10 Al2O3 - 10 Li2O - 30 MO (with M ¼ Mg, Ca, Sr, Ba), density (r) and molar volume (Vm), refractive index nd (measured at 589 nm), and the optical energy gap for direct (dir.) and indirect (ind.) transitions respectively. Sample PAL-Mg PAL-Ca PAL-Sr PAL-Ba
MO (mol%)
r
(g/cm3) ±0.01
Vm (cm3mol1)
nd ± 0.00005
Eopt.dir. (eV)
Eopt.ind. (eV)
30 30 30 30
2.55 2.63 2.95 3.27
37.76 38.44 39.02 39.80
1.50572 1.53398 1.54388 1.56281
4.11 3.72 3.65 3.58
4.16 3.74 3.68 3.61
MgO CaO SrO BaO
polymerized Q3 groups. These are not expected in the glasses of this study, which have a nominal O:P ratio of 3.2, halfway between the metaphosphate (Q2) and pyrophosphate (Q1) composition. Since the KBr pellet technique is prone to hydrolysis it is unclear if the observed smeared bands near 1600 cm1 in Fig. 1, which can be assigned to P-OH, is purely intrinsic in nature [18,19]. It should also be noted, that the KBr pellet technique suffers from other effects like ion exchange which might change the IR spectra [18,20,21]. But even though, from the obtained IR data it was observed that the center at 710 moves toward higher wavenumber with increasing concentration of cerium, see Fig. 3. Such an observation reflects the influence of the Ce-ions with extreme low electronegativity, and the additional depolymerization of the network as an increasing amount of CeO2 is added. Within a glass series, the IR spectra confirm earlier studies which showed a higher fraction of metaphosphate chains for Mg-metaphosphate glasses through the n(P-O) stretching mode near 900 cm1, and a progressively stronger contribution of metaphosphate rings, n(P-O) stretching mode near 980 cm1, as Mg2þ is replaced by the larger alkaline earth ions Ca2þ, Sr2þ, and finally Ba2þ [16].
Fig. 1. FT-IR spectra of 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO with M ¼ Mg, Ca, Sr, Ba glasses co-doped with Sm2O3 3 1020 and CeO2 3 1020 ion/cm3.
decreasing electronegativity of the added alkali earth modifier: Mg2þ > Ca2þ > Sr2þ > Ba2þ. Another band at about 1060 cm1 can be assigned to asymmetric stretching vibrations of chain terminating Q1 groups [16]. The highest known absorption band of phosphate groups is the P]O double bond which, because of the delocalization of the electrons of non-bridging oxygen ions over all terminal oxygen atoms, can only be discerned for the fully
Fig. 2. FT-IR spectrum after conversion into absorbance on the example of PAL-Ba (50 P2O5 - 10 Al2O3 - 10 Li2O - 30 BaO) co-doped with Sm2O3 3 1020 and CeO2 3 1020 ion/cm3.
3.1.3. Raman spectroscopy Raman scattering provides complementary information to the infrared studies on the network structure of the glass samples. Examples of Raman spectra are represented in Fig. 4 in the 200 cm1 -1400 cm1 wavenumber range. All the spectra are characterized by two strong features at 710 cm1 and 1175 cm1 along with other low intensity bands. The observed low frequency band at about 340 cm1 can be assigned to network deformation modes. Another envelope of weak intensity is observed near 500 cm1 and can be assigned to bending modes of differently
Fig. 3. Variation of the position of the peak center (z710 cm1) with increasing CeO2 content for each of the 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO glass series (M ¼ Mg, Ca, Sr, Ba). The Mg-series shows a significant higher frequency, which is related to the high field strength of the small Mg2þ ion. The relative high value of the undoped Ba-glass might be a sign of increased depolymerization due to higher water content (see Fig. 1).
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(700 cm1) over the symmetric and asymmetric stretching modes of Q2 groups near 1100 cm1 can be used as a measure of polymerization, since it gives the ratio of P-O-P linkages over P-O bonds. Fig. 5 shows how the depolymerization of the glasses increases as CeO2 is added. Even though we did not include the stretching modes of Q1 and Q3 groups, the error will be low, since both bands are of very low intensity in the spectra of the studied glass series. As expected, the degree of polymerization decreases as more modifier oxide is added. 3.2. Optical measurements
Fig. 4. Raman spectra of the Sm/Ce co-doped 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO (M ¼ Mg, Ca, Sr, Ba) glasses series co-doped with Sm2O3 3 1020 and CeO2 3 1020 ion/cm3.
connected aluminate and phosphate groups. Coupling of aluminate with phosphate bonds contribute to the overall band width [7,16,22]. The band at 680 cm1 to 710 cm1 can be assigned to the symmetric stretching of P-O-P bridges. The band shifts to higher frequencies as the glass contains more Q1 than Q2 units, that is, with increasing degree of depolymerization. Conversely, this band shifts to lower frequencies with decreasing electron negativity of the charge balancing cation, that is, with the increase in the optical basicity [7,16,23]. The peak of high intensity at 1170 cm1 is 2 attributed to the symmetric stretching vibrations ns(PO 2 ) of Q groups. It can be observed from Fig. 4 that the relative intensity of this peak increases with increasing mass of the charge balancing cations, that is, with increasing optical basicity. The weaker band at 1273 cm1 can be assigned to the asymmetric stretching of the same Q2 units. Interestingly, this asymmetric band increases relative to the intensity of the symmetric band, with increasing mass and subsequently radius and coordination number of the charge balancing modifier cations from Mg < Ca < Sr < Ba. The ratio of the integrated area of the stretching modes of P-O-P bridges
Fig. 5. Ratio of the integrated area of the Raman band at 700 cm1 (stretching mode of P-O-P bridges) to the bands centered at 1100 and 1200 cm1 (symmetric and asymmetric stretching mode of Q2 groups) for 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO glasses (M ¼ Mg, Ca, Sr, Ba) containing 3 1020 Sm2O3 ions/cm3, and co-doped with none, 1, or 3 1020 ions/cm3 CeO2. Decreasing values reflect an increase in depolymerization.
3.2.1. Absorption spectra of undoped glasses The absorption spectra of undoped glasses are shown in Fig. 6. It can be observed that the absorption edge shifts toward higher wavelengths for each added alkaline earth modifier due to the change in the optical basicity, that is, with a higher overall polarizability of the glasses. The optical band gap can be related to the absorption coefficient a(n). At this point it is important to note that even though all modern spectrometer use the decadic logarithm when converting transmission into absorbance, in physical terms, the natural logarithm should be used. Therefore, the optical absorption coefficient a(hn) was calculated using the following formula [24]:
1 d
I0 I
a ¼ ln
(1)
where, d is the thickness of the sample and ln(I0/I) corresponds to the absorbance. An expression for the absorption coefficient, a(n), as function of photon energy (hn) for direct and indirect optical transitions was given by Davis and Mott [25] as follow:
aðnÞ ¼
n A hn Eopt hn
(2)
where, A is an energy-independent constant, Eopt is the optical band gap and n is a constant which determines the type of the optical transition as follow: Eopt values can be obtained by extrapolation to (ahn)1/2 ¼ 0 for direct allowed transitions and (ahn)2 ¼ 0 for indirect allowed transitions. The obtained Eopt values are reported in Table 1. These values are similar to reported values for phosphate glasses [4,26]. The observed change in the values of the Eopt can be attributed to
Fig. 6. Absorption spectra of the undoped glasses: 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO with M ¼ Mg, Ca, Sr, Ba.
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n ¼ 1/2
Direct allowed transition
n¼2 n ¼ 1/3 n¼3
Indirect allowed transition Direct forbidden transition Indirect forbidden transition
the structural changes and the overall polarizability (optical basicity). The concept of optical basicity was established by Duffy and Ingram who provided micro basicity data for many oxides, so that the theoretical optical basicity can be calculated for any composition using the following equations [27e31]:
Lth ¼ ¼
XA
gA
þ
XB
gB
þ/
yNA qNB L Ax Oy þ L Bp Oq þ / yNA þqNB þ / yNA þ qNB þL (3)
The basicity moderating parameters gA, gB, …determine the polarization ability of each oxide with 1/gA ¼ L(AxOy). The oxygen equivalent fraction Х A, Х B, …considers the number of oxygen ions per mole metal oxide. This is important since the optical basicity of an oxidic medium NA(AxOy)NB(BpOq) … depends on the cations ability to polarize the oxide anions and on the oxygen contribution of each oxide (AxOy; BpOq; …). These calculated theoretical or average basicity values are given in Table 2 for all undoped glasses. The optical basicity can also be determined from the oxygen polarizability, which in turn can be deduced from the refractive index of the glasses [30e32].
Lth ¼ 0:7 aO2 0:547
(4)
In order to determine the oxygen polarizability form the optical properties the following calculations were applied. To begin with, the indirect optical band gap energy can be used to calculate the molar refractive index Rm of phosphate glasses using LorentzLorentz relation [31,33]:
rffiffiffiffiffiffi! Eg 1 Vm 20
Rm ¼
(5)
The molar polarizability am is proportional to Rm by the Clausius-Mosotti relation as follow [25]:
am ¼
3 Rm 4pNA
5
The oxygen polarizabilities were determined from am values by subtracting the cation polarizabilities of each molar fraction and dividing the obtained value by the number of oxygen ions per mol, aM(P5þ) ¼ 0.021 Å3, aM(Al3þ) ¼ 0.052 Å3, aM(Liþ) ¼ 0.029 Å3, aM(Mg2þ) ¼ 0.094 Å3, aM(Ca2þ) ¼ 0.47 Å3, aM(Sr2þ) ¼ 0.86 Å3, aM(Ba2þ) ¼ 1.55 Å3 [29,32]. However, as can be seen from Table 2, the obtained values are off by almost a factor of two compared to the analogous calculations from the refractive index data (see later). This fact might highlight a general problem when calculating band gap data from the optical absorption edge of glasses, where many impurities contribute to the intrinsic band gap by s-p or CT transitions. These contributions determine actually the Urbach tail [34]. Iron impurities have their charge transfer transitions in the UV for Fe2þ, which shifts to longer wavelegths for Fe3þ [35,36]. Of course, a higher optical basicity of the base glass shifts the Fe2þ/Fe3þ equilibrium to the higher redox state and the absorption edge will be shifted further into the visible. Thus, for glasses, intrinsic and extrinsic contributions to the absorption edge need to be considered at all times and large deviations from optical basicity values determined from the optical band gap from those determined from the refractive index have been observed before [35,37]. The refractive index of the studied samples was measured at the d-line, that is, at 589 nm, and is therefore not impacted by impurities. The refractive index values follow the same behavior of density. It was found that the glass sample containing barium has the highest density and the highest value of refractive index while that containing magnesium has the lowest value for both density and refractive index, see Table 1. Using another version of the Lorentz-Lorenz equation, the average polarizability can be calculated from the refractive index and molar volume as follow [16,38]:
am ¼
3 Vm n2 1 4pN n2 þ 2
(8)
with p, the mathematical constant, N the Avogadro number and n the refractive index. Just as described earlier for the band gap calculations, the oxygen polarizability can be deduced from the average polarizability of the glass by subtracting the cation polarizabilities of each oxide fraction and dividing the thus obtained value by the overall number of oxygen atoms, see Table 2. Fig. 7 shows the behavior of optical basicity, oxygen polarizability and refractive index.
(6)
After rearranging Equation (5) and replacing the constants in numbers, a simpler expression is obtained:
Rm ¼ 2:52am
(7)
3.2.2. Absorption spectra of RE doped glasses For the prepared glasses that were singly doped with 1 1020 ion/cm3 of CeO2, further increase in the shift of the UV cut-off was observed for all the prepared samples. This can be attributed to Ce3þ absorption [11]. The position and intensity of the Ce3þ absorption depends on the optical basicity of the host [11], see Fig. 8. If some of the cerium ions are oxidized to Ce4þ, inter valence charge
Table 2 Theoretical optical basicity (Lth), in comparison with optical basicity values (La(n)) calculated from the oxygen polarizability a(O2-) which was determined from the measured refractive index values (see Table 1), and optical basicity values (La(E)) from the measured direct band gap (see Table 1). The corresponding oxygen polarizabilities a(O2-) are listed next to the molar refractive index (Rm) values respectively. Sample PAL-Mg PAL-Ca PAL-Sr PAL-Ba a b
Ltha 0.433 0.469 0.477 0.500
La(n) 0.412 0.451 0.459 0.466
La(E)
a(O2-)nb
Rm(n) (cm3mol1)
a(O2-)Eb
(Å3)
(Å3)
Rm(Ed) (cm3 mol1)
1.161 1.189 1.165 1.142
1.370 1.425 1.438 1.446
11.21 11.93 12.33 12.92
2.427 2.475 2.438 2.405
21.77 21.99 22.22 21.76
Calculated using L(P2O5) ¼ 0.4, L(Al2O3) ¼ 0.4, L(Li2O) ¼ 0.81, L(MgO) ¼ 0.61, L(CaO) ¼ 1, L(SrO) ¼ 1.08, and L(BaO) ¼ 1.33. Calculated with a(Mnþ): a(P5þ) ¼ 0.021 Å3, a(Al3þ) ¼ 0.052 Å3, a(Liþ) ¼ 0.029 Å3, a(Mg2þ) ¼ 0.094 Å3, a(Ca2þ) ¼ 0.047 Å3, a(Sr3þ) ¼ 0.86 Å3, a(Ba2þ) ¼ 1.55 Å3.
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Fig. 7. Comparison of the optical basicity Lth, as calculated from the glass composition, and LaO2 , as determined from the oxygen polarizability (aO2 ), that is, from refractive index data. All values are from undoped glass samples 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO with M ¼ Mg, Ca, Sr, Ba.
Fig. 10. Sm3þ related absorption bands in the near infrared spectral range for Sm3þ (3 1020 ion/cm3) doped glasses in the 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO (M ¼ Mg, Ca, Sr, Ba) series.
Fig. 8. The effect of CeO2 doping (1 1020 ion/cm3) on the UV cut-off is shown on the example of the PAL-Sr glass (50 P2O5 - 10 Li2O - 10 Al2O3 - 30 SrO).
Fig. 9. Sm3þ related absorption bands in the UVeVis spectral range for each of the Sm3þ (3 1020 ion/cm3) doped glasses in the 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO (M ¼ Mg, Ca, Sr, Ba) series.
transfer (IV-CT) transitions between Ce4þ and Ce3þ absorb strongly near 300 nm [39,40]. The absorption spectra of Sm3þ singly doped glasses are shown in Figs. 9 and 10, respectively. The related Sm3þ absorption bands in the UVeVis spectral range are found at 360, 372, 400, 414, 439, 460, 468 and 475 nm, see Fig. 9. The obtained absorption bands for doped samples are similar to Sm3þ absorption bands reported for other Sm3þ doped glasses and are due to transition from the ground state (6H5/2) to different excited states 4D3/2, 4D1/2, 6P3/2, 6P5/2, 6P3/2 4G9/2, 4 I9/2, 4M15/2 and 4I13/2 [41]. The near infrared absorption peaks at about 940, 1070, 1225, 1370, 1470, 1520 and 1585 nm are due to transitions from 6H5/2 to 6F11/2, 6F9/2,6F7/2, 6F5/2, 6F3/2, 6F1/2 and 6H15/2, see Fig.10. These transitions are sharp and of high intensity as a result of the effective shielding of the electrons in the 4f shell by 5s and 5p shells [42]. The intensity of the absorption bands increases from Mg to Ba in the same direction of the increase in the optical basicity. It is known that the intensity and the position of the hypersensitive
Fig. 11. An example of the effect of increasing CeO2 concentration on the high energy absorption peaks of Sm3þ in barium containing PAL-Ba (50 P2O5 - 10 Al2O3 - 10 Li2O 30 BaO) glasses.
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H.A. Othman et al. / Optical Materials xxx (2016) 1e8 Table 3 Measured refractive indeces for the doped glasses, the rare earth ion concentration is given in ions/cm3. Sample
Sm3þ (3 1020)
Sm3þ (3 1020) & Cenþ (1 1020)
Sm3þ (3 1020) & Cenþ (3 1020)
PAL-Mg PAL-Ca PAL-Sr PAL-Ba
1.51562 1.54298 1.55379 1.57291
1.51144 1.53449 1.54871 1.56754
1.54118 1.54439 1.55005 1.55822
transition (6H5/2 / 6F1/2) are sensitive to the matrix in which the rare earth ion is embedded [42]. It can be observed that the position of this transition shifts toward lower energies from Mg2þ to Ba2þ and this reflects on the effect the matrix has on the covalence of the Sm-O bond [43]. For Sm/Ce co-doped glasses, especially at a high concentration of CeO2, the absorption of Ce3þ hinders the high energy absorption of Sm3þ which are assigned to the transitions from 6H5/2 to 4D3/2 and 4D1/2. This effect becomes more obvious with increasing optical basicity. Consequently, this effect is more pronounced for the barium containing sample that has the highest value of optical basicity, see Fig. 11. Table 3 shows the the refrective index values for increased levels of rare earth ion doping in the studied glass series. For samples with high optical basicity, such as the barium containing glass, the strong overlapping of Ce3þ and Sm3þ bands indicate toward a higher probability of energy transfer between Ce and Sm. This assumption shall be studied in a future project. 4. Conclusion The prepared glasses were studied using FT-IR, Raman, UVeVis spectroscopy. Several optical parameter were calculated from the composition and from measured properties, such as the optical basicity or the optical band gap. The FT-IR and Raman spectra show the characteristics vibrations of phosphate groups which were found to be strongly influenced by the optical basicity of the host, that is, by changing the alkaline earth modifier oxide. The structure was also changed through addition of rare earth oxides. The absorption edge shifts toward higher wavelengths with the increase in the optical basicity for each glass sample. With increasing CeO2 content, this shift increases due to the absorption of Ce3þ. The characteristic absorption peaks of Sm3þ ions in samarium doped glasses could be assigned to the characteristic f-f transitions, The optical edge of Sm/Ce co-doped samples shifts from the UV into the visible wavelength region and this shift increases with increasing concentration of CeO2, as observed for singly doped Ce glasses, but contrary to the singly doped Sm2O3 containing glasses. For the sample with high optical basicity, such as barium containing glass, the strong overlapping of Ce3þ and Sm3þ bands indicate higher probability of energy transfer between Ce and Sm ions. All these studied glasses are good hosts for Sm3þ ions and can be recommended as a solid state laser host material, while the Sm/Ce co-doped glasses are suitable for WLEDs applications. References [1] S. Jiang, T. Luo, B.-C. Hwang, F. Smekatala, K. Seneschal, J. Lucas, N. Peyghambarian, Er3þ-doped phosphate glasses for fiber amplifiers with high gain per unit length, J. Non-Cryst. Solids 263e264 (2000) 364e368, http://dx.doi.org/10.1016/S0022-3093(99)00646-8. [2] P. Laporta, S. Taccheo, S. Longhi, O. Svelto, C. Svelto, Erbiumeytterbium microlasers: optical properties and lasing characteristics, Opt. Mater. 11 (1999) 269e288, http://dx.doi.org/10.1016/S0925-3467(98)00049-4. [3] D.L. Veasey, D.S. Funk, P.M. Peters, N.A. Sanford, G.E. Obarski, N. Fontaine, M. Young, A.P. Peskin, W.-C. Liu, S.N. Houde-Walter, J.S. Hayden, Yb/Ercodoped and Yb-doped waveguide lasers in phosphate glass, J. Non-Cryst. Solids 263e264 (2000) 369e381, http://dx.doi.org/10.1016/S0022-3093(99)
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The influence of different alkaline earth oxides on the structural and optical properties of undoped, Ce-doped, Sm-doped, and Sm/Ce codoped lithium alumino-phosphate glasses € ncke c, * H.A. Othman a, G.M. Arzumanyan b, D. Mo a b c
Physics Department, Faculty of Science, Menoufia University, Shebin El-Koom, Egypt Joint Institute of Nuclear Research, Dubna, 141980, Russia Kazuo Inamori School of Engineering, Alfred University, NY, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 September 2016 Received in revised form 11 October 2016 Accepted 28 October 2016 Available online xxx
Undoped, singly Sm doped, Ce doped, and Sm/Ce co-doped lithium alumino-phosphate glasses with different alkaline earth modifiers were prepared by melt quenching. The structure of the prepared glasses was investigated by FT-IR and Raman, as well as by optical spectroscopy. The effect of the optical basicity of the host glass matrix on the added active dopants was studied, as was the effect doping had on the phosphate structural units. The optical edge shifts toward higher wavelengths with an increase in the optical basicity due to the increased polarizability of the glass matrix, but also with increasing CeO2 concentration as a result of Ce3þ/Ce4þ inter valence charge transfer (IV-CT) absorption. The optical band gap for direct and indirect allowed transitions was calculated for the undoped glasses. The glass sample containing Mg2þ modifier ions is found to have the highest value (4.16 eV) for the optical band gap while Ba2þ has the lowest value (3.61 eV). The change in the optical band gap arises from the structural changes and the overall polarizability (optical basicity). Refractive index, molar refractivity Rm and molar polarizability am values increase with increasing optical basicity of the glasses. The characteristic absorption peaks of Sm3þ were also investigated. For Sm/Ce co-doped glasses, especially at high concentration of CeO2, the absorption of Ce3þ hinders the high energy absorption of Sm3þ and this effect becomes more obvious with increasing optical basicity. © 2016 Elsevier B.V. All rights reserved.
Keywords: Ce, Sm ion co-doped glasses UVeVis spectroscopy FT-IR and Raman spectroscopy Oxygen polarizability Optical basicity
1. Introduction Phosphate glasses are easy to fabricate and can be considered efficient rare-earth ions host matrices as they provide a high solubility for rare-earth ions without showing strong clustering effects. Depending on the composition, phosphate glasses possess good chemical durability, ion exchangeability and excellent optical properties [1e3]. In the studied multicomponent glass series, lithium oxide, as any alkali oxide, acts as network modifier. When added to vitreous P2O5, P-O-P linkages break and non-bridging oxygen ions (nbO) are created [4]. Network intermediates such as Al2O3 are added to glasses to improve their chemical durability and to decrease their solubility in water [5]. Introducing Al2O3 into phosphate glasses stabilized the glass through strong cross-linking
* Corresponding author. €ncke). E-mail address: [email protected] (D. Mo
between the PO4-tetrahedra and therefore improves also the physical properties such as increased Tg, lower coefficient of thermal expansion, or the already mentioned increased chemical stability [6e8]. The low coefficient of thermal expansion for example makes alumino-phosphate glasses ideal candidates for ion exchange planar waveguide devices [9]. For Al3þ, the high field strength (Z/r2), that is, the ratio of the charge (Z) to the square ionic radius (r), is behind its strong cross linking capability. This effect is also seen to a lesser degree for small Li-ions. Lithium aluminophosphate glasses are of great interest due to their technological significance in a lot of applications such as high energy capacitors, re-cycling of nuclear waste, and other electro chromic devices [10]. Introducing alkaline earth oxides as modifier oxides to phosphate glass, such as magnesium, calcium, strontium, and barium oxide, impacts the physical properties as this can strengthen or weaken the chemical durability of the glasses, or prevents devitrification of the glass host [5]. Furthermore, the different alkaline earth oxides affect the optical basicity, which in turn may lead to a change in the
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Please cite this article in press as: H.A. Othman, et al., The influence of different alkaline earth oxides on the structural and optical properties of undoped, Ce-doped, Sm-doped, and Sm/Ce co-doped lithium alumino-phosphate glasses, Optical Materials (2016), http://dx.doi.org/10.1016/ j.optmat.2016.10.051
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energy states, for example of Ce-ions, and can influence the energy transfer to other rare earth ions [11]. Phosphate glasses doped with rare earth ions have a wide range of applications in photonics, highpower lasers, or in optical communications [12e14]. Cerium ions act as sensitizers in luminescent glasses, and are used in scintillators, or in laser glasses. They are also widely used as sensitizers in photo-thermo-refractive glasses [15]. Depending on the redox state, cerium ions might also be added to decrease photo-darkening in optical fibers and are used as decolorizing agent for container glasses [11]. As the spectroscopic properties of rare earth ions are highly influenced by the host in which they are embedded, the aim of this work is to investigate the effect of alkaline earth oxides (MgO, CaO, SrO and BaO) on the structure, as well as on physical and optical properties of undoped, Sm doped, Ce-doped, and Sm/Ce co-doped lithium alumino-phosphate glasses. 2. Experimental methods
was used for detection of spectra collected at different localizations of the samples with an integration time of 10 s for Raman measurements. The signals are probed in the 200e1400 cm1 fingerprint region of the vibrational frequencies with a spectral resolution of ~2.0 cm1. Rayleigh scattering was blocked using a Semrock long-pass edge filter. Baseline correction was employed during data processing using the rollingball method. All measurements were performed at room temperature. 2.3.3. Ultraviolet and visible absorption spectroscopy The optical absorption spectra were recorded at room temperature in the ultra violet to visible and near infrared region, using a Spectrophotometer, Jasco Model (V-570), in the wavelength range from 190 to 1800 nm. 2.3.4. Refractive index measurements The refractive index was measured using a PTR refractometer 46X at room temperature at the sodium wavelength 589 nm with an accuracy of about ±0.00005 RI.
2.1. Glass preparation 3. Results and discussion The glass samples were prepared according to the following formula: 50 P2O5 - 10 Al2O3 - 10 Li2O - 30 MO where M ¼ Mg, Ca, Sr and Ba. Doped glasses were also prepared with the addition of 3 1020 ions/cm3 of Sm2O3, or CeO2 (1 1020 ions/cm3), and codoped with Sm2O3 (3 1020 ions/cm3) and cerium (1 1020 ions/cm3). Another series was co-doped with 3 1020 ions/cm3 of Sm2O3 and 3 1020 ions/cm3 of CeO2. All these glass samples were prepared by the melt quenching method. They were synthesized from P2O5 (99.5%), Al2O3 (99.9%), Li2O (99.9%) and MgO, BaO, SrO, or CaO (all of purity 99%), from Aldrich. Sm2O3 (purity 99.999%), and CeO2 (99.99%), were used for rare earth doping, both from Aldrich. The mixed raw materials were placed in an alumina crucible and preheated in an electrical oven at 400 C for 3 h, and subsequently melted at 1200 C for 30 min. The glasses were cast into pre-heated stainless steel molds and were immediately transferred to an annealing furnace where they were kept at 450 C for about 1 h before cooling them slowly down to room temperature to remove internal stress. 2.2. Density measurement Glass densities of the prepared glasses were measured by the Archimedean method using toluene as an immersion liquid at room temperature. The accuracy of the measurements was approximately ±0.01 g cm3. 2.3. Optical measurements 2.3.1. FT-IR spectroscopy The infrared spectra of the prepared glasses were recorded on an FT-IR spectrophotometer type Tensor 27, Brulear over the 4000e200 cm1 wavenumber range at room temperature. Powdered samples with a grain size of about 60 mm were mixed with KBr and pressed into pellets of about 0.2 mm thickness. 2.3.2. Raman spectroscopy Raman measurements were conducted on a confocal microspectroscopy set-up. The system comprises a “Confotec CARS” scanning laser spectrometer coupled to the NIKON TE2000-E inverted microscope. Excitation at 633 was provided by a He-Ne laser (MellesGriot 05-LHP-991). All Raman spectra were collected in the backscattering geometry and dispersed by 600 grooves/mm diffraction grating mounted in the MS520 monochromatorspectrograph. A Peltier-cooled CCD camera (ProScan HS101 H)
3.1. Structural investigation 3.1.1. Density and molar volume The composition of the prepared glasses, their density and molar volume are reported in Table 1. The density decreases with the following sequence (BaO > SrO > CaO > MgO) as the molecular weight of the modifier oxide decreases, BaO > SrO > CaO > MgO. The molar volume was then calculated according to the following formula: Vm ¼ Mrw , where (Mw) is the glass molecular weight and (r) is its density. As the increase in the molecular weight is higher than the increase in density of the prepared samples, an increase in the molar volume is observed from Mg to Ba containing glasses. 3.1.2. FT-IR spectroscopy Examples of the transmittance spectra of the prepared samples are shown in Fig. 1. The following relation A ¼ [2 - Log(T%)], was used to convert these spectra to absorption spectra for more detailed analysis. One example with band assignments is shown in Fig. 2. All the IR absorption spectra are characterized by similar absorption peaks which can mostly be assigned to metaphosphate structural units, as desribed in detail in the following [16]. Vibrations of Al-polyhedra coupled with bending modes of the phosphate network d(P-O) extend up toz 520 cm1 [16]. The band envelop at 650 to 800 cm1 can be assigned to symmetric stretching modes ns(P-O-P) of bridges between phosphate tetrahedra. In the following we use the Qn nomenclature, where n denotes the number of briging oxygen atoms per phosphate tetrahedra. Metaphosphate units have 2 bridging and 4-n ¼ 2 terminal oxygen atoms per network forming tetrahedra. For bridging Q2 units this band is split into a low and high frequency component around 710 and 790 cm1, while P-O-P bridges between Q1 units show only one band around 700 cm1 [7,16]. The low energy feature at 710 cm1 might also be related to tetrahedral AlO4-units [17]. The asymmetric P-O-P bonds of metaphosphate chains and rings contribute strongly to the high frequency band envelop between 900 and 1400 cm1. These bands absorb at almost 900 cm1 for Q2 chains and around 980 cm1 for metaphosphate rings. Around 910 cm1 absorbs also the asymmetric P-O-P stretching mode of Q1 groups, which are also found at chain endings [7,16]. The asymmetric stretching modes nas(P-O) are found around 1110 cm1 for Q1 units and near 1245 cm1 for Q2 groups [7,16]. The effect of the charge balancing cations on the P-O bond strength is reflected in the shift of this band toward lower energies with
Please cite this article in press as: H.A. Othman, et al., The influence of different alkaline earth oxides on the structural and optical properties of undoped, Ce-doped, Sm-doped, and Sm/Ce co-doped lithium alumino-phosphate glasses, Optical Materials (2016), http://dx.doi.org/10.1016/ j.optmat.2016.10.051
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3
Table 1 Selected properties of the undoped base glasses 50 P2O5 - 10 Al2O3 - 10 Li2O - 30 MO (with M ¼ Mg, Ca, Sr, Ba), density (r) and molar volume (Vm), refractive index nd (measured at 589 nm), and the optical energy gap for direct (dir.) and indirect (ind.) transitions respectively. Sample PAL-Mg PAL-Ca PAL-Sr PAL-Ba
MO (mol%)
r
(g/cm3) ±0.01
Vm (cm3mol1)
nd ± 0.00005
Eopt.dir. (eV)
Eopt.ind. (eV)
30 30 30 30
2.55 2.63 2.95 3.27
37.76 38.44 39.02 39.80
1.50572 1.53398 1.54388 1.56281
4.11 3.72 3.65 3.58
4.16 3.74 3.68 3.61
MgO CaO SrO BaO
polymerized Q3 groups. These are not expected in the glasses of this study, which have a nominal O:P ratio of 3.2, halfway between the metaphosphate (Q2) and pyrophosphate (Q1) composition. Since the KBr pellet technique is prone to hydrolysis it is unclear if the observed smeared bands near 1600 cm1 in Fig. 1, which can be assigned to P-OH, is purely intrinsic in nature [18,19]. It should also be noted, that the KBr pellet technique suffers from other effects like ion exchange which might change the IR spectra [18,20,21]. But even though, from the obtained IR data it was observed that the center at 710 moves toward higher wavenumber with increasing concentration of cerium, see Fig. 3. Such an observation reflects the influence of the Ce-ions with extreme low electronegativity, and the additional depolymerization of the network as an increasing amount of CeO2 is added. Within a glass series, the IR spectra confirm earlier studies which showed a higher fraction of metaphosphate chains for Mg-metaphosphate glasses through the n(P-O) stretching mode near 900 cm1, and a progressively stronger contribution of metaphosphate rings, n(P-O) stretching mode near 980 cm1, as Mg2þ is replaced by the larger alkaline earth ions Ca2þ, Sr2þ, and finally Ba2þ [16].
Fig. 1. FT-IR spectra of 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO with M ¼ Mg, Ca, Sr, Ba glasses co-doped with Sm2O3 3 1020 and CeO2 3 1020 ion/cm3.
decreasing electronegativity of the added alkali earth modifier: Mg2þ > Ca2þ > Sr2þ > Ba2þ. Another band at about 1060 cm1 can be assigned to asymmetric stretching vibrations of chain terminating Q1 groups [16]. The highest known absorption band of phosphate groups is the P]O double bond which, because of the delocalization of the electrons of non-bridging oxygen ions over all terminal oxygen atoms, can only be discerned for the fully
Fig. 2. FT-IR spectrum after conversion into absorbance on the example of PAL-Ba (50 P2O5 - 10 Al2O3 - 10 Li2O - 30 BaO) co-doped with Sm2O3 3 1020 and CeO2 3 1020 ion/cm3.
3.1.3. Raman spectroscopy Raman scattering provides complementary information to the infrared studies on the network structure of the glass samples. Examples of Raman spectra are represented in Fig. 4 in the 200 cm1 -1400 cm1 wavenumber range. All the spectra are characterized by two strong features at 710 cm1 and 1175 cm1 along with other low intensity bands. The observed low frequency band at about 340 cm1 can be assigned to network deformation modes. Another envelope of weak intensity is observed near 500 cm1 and can be assigned to bending modes of differently
Fig. 3. Variation of the position of the peak center (z710 cm1) with increasing CeO2 content for each of the 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO glass series (M ¼ Mg, Ca, Sr, Ba). The Mg-series shows a significant higher frequency, which is related to the high field strength of the small Mg2þ ion. The relative high value of the undoped Ba-glass might be a sign of increased depolymerization due to higher water content (see Fig. 1).
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(700 cm1) over the symmetric and asymmetric stretching modes of Q2 groups near 1100 cm1 can be used as a measure of polymerization, since it gives the ratio of P-O-P linkages over P-O bonds. Fig. 5 shows how the depolymerization of the glasses increases as CeO2 is added. Even though we did not include the stretching modes of Q1 and Q3 groups, the error will be low, since both bands are of very low intensity in the spectra of the studied glass series. As expected, the degree of polymerization decreases as more modifier oxide is added. 3.2. Optical measurements
Fig. 4. Raman spectra of the Sm/Ce co-doped 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO (M ¼ Mg, Ca, Sr, Ba) glasses series co-doped with Sm2O3 3 1020 and CeO2 3 1020 ion/cm3.
connected aluminate and phosphate groups. Coupling of aluminate with phosphate bonds contribute to the overall band width [7,16,22]. The band at 680 cm1 to 710 cm1 can be assigned to the symmetric stretching of P-O-P bridges. The band shifts to higher frequencies as the glass contains more Q1 than Q2 units, that is, with increasing degree of depolymerization. Conversely, this band shifts to lower frequencies with decreasing electron negativity of the charge balancing cation, that is, with the increase in the optical basicity [7,16,23]. The peak of high intensity at 1170 cm1 is 2 attributed to the symmetric stretching vibrations ns(PO 2 ) of Q groups. It can be observed from Fig. 4 that the relative intensity of this peak increases with increasing mass of the charge balancing cations, that is, with increasing optical basicity. The weaker band at 1273 cm1 can be assigned to the asymmetric stretching of the same Q2 units. Interestingly, this asymmetric band increases relative to the intensity of the symmetric band, with increasing mass and subsequently radius and coordination number of the charge balancing modifier cations from Mg < Ca < Sr < Ba. The ratio of the integrated area of the stretching modes of P-O-P bridges
Fig. 5. Ratio of the integrated area of the Raman band at 700 cm1 (stretching mode of P-O-P bridges) to the bands centered at 1100 and 1200 cm1 (symmetric and asymmetric stretching mode of Q2 groups) for 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO glasses (M ¼ Mg, Ca, Sr, Ba) containing 3 1020 Sm2O3 ions/cm3, and co-doped with none, 1, or 3 1020 ions/cm3 CeO2. Decreasing values reflect an increase in depolymerization.
3.2.1. Absorption spectra of undoped glasses The absorption spectra of undoped glasses are shown in Fig. 6. It can be observed that the absorption edge shifts toward higher wavelengths for each added alkaline earth modifier due to the change in the optical basicity, that is, with a higher overall polarizability of the glasses. The optical band gap can be related to the absorption coefficient a(n). At this point it is important to note that even though all modern spectrometer use the decadic logarithm when converting transmission into absorbance, in physical terms, the natural logarithm should be used. Therefore, the optical absorption coefficient a(hn) was calculated using the following formula [24]:
1 d
I0 I
a ¼ ln
(1)
where, d is the thickness of the sample and ln(I0/I) corresponds to the absorbance. An expression for the absorption coefficient, a(n), as function of photon energy (hn) for direct and indirect optical transitions was given by Davis and Mott [25] as follow:
aðnÞ ¼
n A hn Eopt hn
(2)
where, A is an energy-independent constant, Eopt is the optical band gap and n is a constant which determines the type of the optical transition as follow: Eopt values can be obtained by extrapolation to (ahn)1/2 ¼ 0 for direct allowed transitions and (ahn)2 ¼ 0 for indirect allowed transitions. The obtained Eopt values are reported in Table 1. These values are similar to reported values for phosphate glasses [4,26]. The observed change in the values of the Eopt can be attributed to
Fig. 6. Absorption spectra of the undoped glasses: 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO with M ¼ Mg, Ca, Sr, Ba.
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H.A. Othman et al. / Optical Materials xxx (2016) 1e8
n ¼ 1/2
Direct allowed transition
n¼2 n ¼ 1/3 n¼3
Indirect allowed transition Direct forbidden transition Indirect forbidden transition
the structural changes and the overall polarizability (optical basicity). The concept of optical basicity was established by Duffy and Ingram who provided micro basicity data for many oxides, so that the theoretical optical basicity can be calculated for any composition using the following equations [27e31]:
Lth ¼ ¼
XA
gA
þ
XB
gB
þ/
yNA qNB L Ax Oy þ L Bp Oq þ / yNA þqNB þ / yNA þ qNB þL (3)
The basicity moderating parameters gA, gB, …determine the polarization ability of each oxide with 1/gA ¼ L(AxOy). The oxygen equivalent fraction Х A, Х B, …considers the number of oxygen ions per mole metal oxide. This is important since the optical basicity of an oxidic medium NA(AxOy)NB(BpOq) … depends on the cations ability to polarize the oxide anions and on the oxygen contribution of each oxide (AxOy; BpOq; …). These calculated theoretical or average basicity values are given in Table 2 for all undoped glasses. The optical basicity can also be determined from the oxygen polarizability, which in turn can be deduced from the refractive index of the glasses [30e32].
Lth ¼ 0:7 aO2 0:547
(4)
In order to determine the oxygen polarizability form the optical properties the following calculations were applied. To begin with, the indirect optical band gap energy can be used to calculate the molar refractive index Rm of phosphate glasses using LorentzLorentz relation [31,33]:
rffiffiffiffiffiffi! Eg 1 Vm 20
Rm ¼
(5)
The molar polarizability am is proportional to Rm by the Clausius-Mosotti relation as follow [25]:
am ¼
3 Rm 4pNA
5
The oxygen polarizabilities were determined from am values by subtracting the cation polarizabilities of each molar fraction and dividing the obtained value by the number of oxygen ions per mol, aM(P5þ) ¼ 0.021 Å3, aM(Al3þ) ¼ 0.052 Å3, aM(Liþ) ¼ 0.029 Å3, aM(Mg2þ) ¼ 0.094 Å3, aM(Ca2þ) ¼ 0.47 Å3, aM(Sr2þ) ¼ 0.86 Å3, aM(Ba2þ) ¼ 1.55 Å3 [29,32]. However, as can be seen from Table 2, the obtained values are off by almost a factor of two compared to the analogous calculations from the refractive index data (see later). This fact might highlight a general problem when calculating band gap data from the optical absorption edge of glasses, where many impurities contribute to the intrinsic band gap by s-p or CT transitions. These contributions determine actually the Urbach tail [34]. Iron impurities have their charge transfer transitions in the UV for Fe2þ, which shifts to longer wavelegths for Fe3þ [35,36]. Of course, a higher optical basicity of the base glass shifts the Fe2þ/Fe3þ equilibrium to the higher redox state and the absorption edge will be shifted further into the visible. Thus, for glasses, intrinsic and extrinsic contributions to the absorption edge need to be considered at all times and large deviations from optical basicity values determined from the optical band gap from those determined from the refractive index have been observed before [35,37]. The refractive index of the studied samples was measured at the d-line, that is, at 589 nm, and is therefore not impacted by impurities. The refractive index values follow the same behavior of density. It was found that the glass sample containing barium has the highest density and the highest value of refractive index while that containing magnesium has the lowest value for both density and refractive index, see Table 1. Using another version of the Lorentz-Lorenz equation, the average polarizability can be calculated from the refractive index and molar volume as follow [16,38]:
am ¼
3 Vm n2 1 4pN n2 þ 2
(8)
with p, the mathematical constant, N the Avogadro number and n the refractive index. Just as described earlier for the band gap calculations, the oxygen polarizability can be deduced from the average polarizability of the glass by subtracting the cation polarizabilities of each oxide fraction and dividing the thus obtained value by the overall number of oxygen atoms, see Table 2. Fig. 7 shows the behavior of optical basicity, oxygen polarizability and refractive index.
(6)
After rearranging Equation (5) and replacing the constants in numbers, a simpler expression is obtained:
Rm ¼ 2:52am
(7)
3.2.2. Absorption spectra of RE doped glasses For the prepared glasses that were singly doped with 1 1020 ion/cm3 of CeO2, further increase in the shift of the UV cut-off was observed for all the prepared samples. This can be attributed to Ce3þ absorption [11]. The position and intensity of the Ce3þ absorption depends on the optical basicity of the host [11], see Fig. 8. If some of the cerium ions are oxidized to Ce4þ, inter valence charge
Table 2 Theoretical optical basicity (Lth), in comparison with optical basicity values (La(n)) calculated from the oxygen polarizability a(O2-) which was determined from the measured refractive index values (see Table 1), and optical basicity values (La(E)) from the measured direct band gap (see Table 1). The corresponding oxygen polarizabilities a(O2-) are listed next to the molar refractive index (Rm) values respectively. Sample PAL-Mg PAL-Ca PAL-Sr PAL-Ba a b
Ltha 0.433 0.469 0.477 0.500
La(n) 0.412 0.451 0.459 0.466
La(E)
a(O2-)nb
Rm(n) (cm3mol1)
a(O2-)Eb
(Å3)
(Å3)
Rm(Ed) (cm3 mol1)
1.161 1.189 1.165 1.142
1.370 1.425 1.438 1.446
11.21 11.93 12.33 12.92
2.427 2.475 2.438 2.405
21.77 21.99 22.22 21.76
Calculated using L(P2O5) ¼ 0.4, L(Al2O3) ¼ 0.4, L(Li2O) ¼ 0.81, L(MgO) ¼ 0.61, L(CaO) ¼ 1, L(SrO) ¼ 1.08, and L(BaO) ¼ 1.33. Calculated with a(Mnþ): a(P5þ) ¼ 0.021 Å3, a(Al3þ) ¼ 0.052 Å3, a(Liþ) ¼ 0.029 Å3, a(Mg2þ) ¼ 0.094 Å3, a(Ca2þ) ¼ 0.047 Å3, a(Sr3þ) ¼ 0.86 Å3, a(Ba2þ) ¼ 1.55 Å3.
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H.A. Othman et al. / Optical Materials xxx (2016) 1e8
Fig. 7. Comparison of the optical basicity Lth, as calculated from the glass composition, and LaO2 , as determined from the oxygen polarizability (aO2 ), that is, from refractive index data. All values are from undoped glass samples 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO with M ¼ Mg, Ca, Sr, Ba.
Fig. 10. Sm3þ related absorption bands in the near infrared spectral range for Sm3þ (3 1020 ion/cm3) doped glasses in the 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO (M ¼ Mg, Ca, Sr, Ba) series.
Fig. 8. The effect of CeO2 doping (1 1020 ion/cm3) on the UV cut-off is shown on the example of the PAL-Sr glass (50 P2O5 - 10 Li2O - 10 Al2O3 - 30 SrO).
Fig. 9. Sm3þ related absorption bands in the UVeVis spectral range for each of the Sm3þ (3 1020 ion/cm3) doped glasses in the 50 P2O5 - 10 Li2O - 10 Al2O3 - 30 MO (M ¼ Mg, Ca, Sr, Ba) series.
transfer (IV-CT) transitions between Ce4þ and Ce3þ absorb strongly near 300 nm [39,40]. The absorption spectra of Sm3þ singly doped glasses are shown in Figs. 9 and 10, respectively. The related Sm3þ absorption bands in the UVeVis spectral range are found at 360, 372, 400, 414, 439, 460, 468 and 475 nm, see Fig. 9. The obtained absorption bands for doped samples are similar to Sm3þ absorption bands reported for other Sm3þ doped glasses and are due to transition from the ground state (6H5/2) to different excited states 4D3/2, 4D1/2, 6P3/2, 6P5/2, 6P3/2 4G9/2, 4 I9/2, 4M15/2 and 4I13/2 [41]. The near infrared absorption peaks at about 940, 1070, 1225, 1370, 1470, 1520 and 1585 nm are due to transitions from 6H5/2 to 6F11/2, 6F9/2,6F7/2, 6F5/2, 6F3/2, 6F1/2 and 6H15/2, see Fig.10. These transitions are sharp and of high intensity as a result of the effective shielding of the electrons in the 4f shell by 5s and 5p shells [42]. The intensity of the absorption bands increases from Mg to Ba in the same direction of the increase in the optical basicity. It is known that the intensity and the position of the hypersensitive
Fig. 11. An example of the effect of increasing CeO2 concentration on the high energy absorption peaks of Sm3þ in barium containing PAL-Ba (50 P2O5 - 10 Al2O3 - 10 Li2O 30 BaO) glasses.
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H.A. Othman et al. / Optical Materials xxx (2016) 1e8 Table 3 Measured refractive indeces for the doped glasses, the rare earth ion concentration is given in ions/cm3. Sample
Sm3þ (3 1020)
Sm3þ (3 1020) & Cenþ (1 1020)
Sm3þ (3 1020) & Cenþ (3 1020)
PAL-Mg PAL-Ca PAL-Sr PAL-Ba
1.51562 1.54298 1.55379 1.57291
1.51144 1.53449 1.54871 1.56754
1.54118 1.54439 1.55005 1.55822
transition (6H5/2 / 6F1/2) are sensitive to the matrix in which the rare earth ion is embedded [42]. It can be observed that the position of this transition shifts toward lower energies from Mg2þ to Ba2þ and this reflects on the effect the matrix has on the covalence of the Sm-O bond [43]. For Sm/Ce co-doped glasses, especially at a high concentration of CeO2, the absorption of Ce3þ hinders the high energy absorption of Sm3þ which are assigned to the transitions from 6H5/2 to 4D3/2 and 4D1/2. This effect becomes more obvious with increasing optical basicity. Consequently, this effect is more pronounced for the barium containing sample that has the highest value of optical basicity, see Fig. 11. Table 3 shows the the refrective index values for increased levels of rare earth ion doping in the studied glass series. For samples with high optical basicity, such as the barium containing glass, the strong overlapping of Ce3þ and Sm3þ bands indicate toward a higher probability of energy transfer between Ce and Sm. This assumption shall be studied in a future project. 4. Conclusion The prepared glasses were studied using FT-IR, Raman, UVeVis spectroscopy. Several optical parameter were calculated from the composition and from measured properties, such as the optical basicity or the optical band gap. The FT-IR and Raman spectra show the characteristics vibrations of phosphate groups which were found to be strongly influenced by the optical basicity of the host, that is, by changing the alkaline earth modifier oxide. The structure was also changed through addition of rare earth oxides. The absorption edge shifts toward higher wavelengths with the increase in the optical basicity for each glass sample. With increasing CeO2 content, this shift increases due to the absorption of Ce3þ. The characteristic absorption peaks of Sm3þ ions in samarium doped glasses could be assigned to the characteristic f-f transitions, The optical edge of Sm/Ce co-doped samples shifts from the UV into the visible wavelength region and this shift increases with increasing concentration of CeO2, as observed for singly doped Ce glasses, but contrary to the singly doped Sm2O3 containing glasses. For the sample with high optical basicity, such as barium containing glass, the strong overlapping of Ce3þ and Sm3þ bands indicate higher probability of energy transfer between Ce and Sm ions. All these studied glasses are good hosts for Sm3þ ions and can be recommended as a solid state laser host material, while the Sm/Ce co-doped glasses are suitable for WLEDs applications. References [1] S. Jiang, T. Luo, B.-C. Hwang, F. Smekatala, K. Seneschal, J. Lucas, N. Peyghambarian, Er3þ-doped phosphate glasses for fiber amplifiers with high gain per unit length, J. Non-Cryst. Solids 263e264 (2000) 364e368, http://dx.doi.org/10.1016/S0022-3093(99)00646-8. [2] P. Laporta, S. Taccheo, S. Longhi, O. Svelto, C. Svelto, Erbiumeytterbium microlasers: optical properties and lasing characteristics, Opt. Mater. 11 (1999) 269e288, http://dx.doi.org/10.1016/S0925-3467(98)00049-4. [3] D.L. Veasey, D.S. Funk, P.M. Peters, N.A. Sanford, G.E. Obarski, N. Fontaine, M. Young, A.P. Peskin, W.-C. Liu, S.N. Houde-Walter, J.S. Hayden, Yb/Ercodoped and Yb-doped waveguide lasers in phosphate glass, J. Non-Cryst. Solids 263e264 (2000) 369e381, http://dx.doi.org/10.1016/S0022-3093(99)
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Please cite this article in press as: H.A. Othman, et al., The influence of different alkaline earth oxides on the structural and optical properties of undoped, Ce-doped, Sm-doped, and Sm/Ce co-doped lithium alumino-phosphate glasses, Optical Materials (2016), http://dx.doi.org/10.1016/ j.optmat.2016.10.051