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FLUOROPHOSPHATE VITREOUS SYSTEMS
“Ligands and Modifiers in Vitreous Materials: Spectnoscopy of condensed Systems”
Publisher: World Scientific, Singapore, New Jersey, London, Honk Hong 1999
The use of glass hosts for active elements in lasers and fiber optics has stimulated interest in new vitreous materials, including fluorophosphates.
Sun [65-69] developed first a composition of vitreous fluorophosphate systems on the base metaphosphate aluminum and fluorides of metal from the first and second group of the Periodic system. Optical constant of glasses is nD=1.45-1.59, n=53-73.
Table 5.6 shows some compositions of fluorophosphate glasses and optical constants from the data of Sun [65-69].
Yahn [70-72] offered compositions of fluorophosphate glasses with nD=1.45-1.58 and n=67-88 on the base metaphosphate and pyrophosphate alkaline and earth alkaline metals. Some of those compositions are presented in Table 5.7.
Compositions of Fluorophosphate Glasses (in mol%) (from [65-69] )
Compositions of Fluorophosphate Glasses (in mol %) (from [70-72] ).
Several authors [73-76] studied domains of glass forming and some physicochemical and optical properties in the Al(PO3)3-BaF2-RF2(R-Mg,Ca,Sr) systems. Glass forming range decreases with increasing ion radii of bivalence cations. Barium fluoride is an essential component in these glass systems. IR spectroscopic investigation of the glasses shows that structural lattices of those glasses have phosphate, fluoride and fluorophosphate groups .
According to Murthy [77, 78] structure of glass system NaPO3-NaF has a chain texture with ortho and pyrophosphate groups, and also the presence of monofluorophosphate group – (PO3F)2-.
Laidtorp et al. [79-81] investigated the ability of glass formation and physicochemical and spectroscopic properties of fluorophosphate glasses on barium monofluorophosphate (BaPO3F) base. Optical constants of those glasses was
Birdina et al.  studied the domain of glass forming in BaPO3F-AlF3-RO (R-Be, Mg, Ca, Sr) systems. Density of glasses changes within 3.97-4.28g/cm3, refractive index and dispersion within nD=1.54-1.60, n=65-77.
Tetsuro and Seiichi  created stable fuorophosphate optical glass in the system:
B2O3-P2O5-MF (M-Li, Na, K) and B2O3-P2O5-RF2 (R-Mg, Ba, Ca).
Vogel and Gerth  recommended a method of creating fluorophosphate optical glasses with nD=1.45-1.53, n=55-80. These glasses were synthesized in systems where RF=16-34, MeSiF6=5-54, Al(PO3)3=30-63(wt%), R-Li, Na, K; Me-Mg, Ca, Sr, Ba, Cd, Zn, Pb.
Auzel and Morin  fabricated a composition fluorophosphate glass for laser use, doped with erbium and ytterbium, where BaF2=33.41, AlF3=21.90, CaF2=14.87,
MgF2=12.50, NaPO4=17.32 (in wt%).
Vrtanessian et al.[86, 87] studied the domain of glass formation and physicochemical properties of boron containing fluorophosphate glasses in BaPO3F-B2O3-RxOy, where R-Mg, Ca, Sr, Ba, Al. Ability of glass formation increases with increasing strength of the modifiers field.
Wassilac et al.[88-90] investigated properties, structure and spectroscopy of fluorophosphate glasses in Ba(PO3)2-MgF2-0.4AlF3 . 0.6CaF2. Glasses was doped with Nd(III), Eu(III), Ho(III), Er(III) and Yb(III). They established correlation between shift of spectral lines and concentration of fluorides in the glass. When increasing contents of fluorides in fluorophosphate glass accordingly increases degree of ionicity between dopant-ligand bond.
Pogosyan et al.[91, 92] studied glass formation, properties and structure of alkaline-fluorophosphate glasses in the NaPO3-LiF-(0.4AlF3 . 0.6CaF2) and (0.3LiF . 0.7NaPO3)-AlF3-MeF2 (Me-Mg, Ca) systems.
Domain of glass formation in pseudobinary systems Ba(PO3)2-RF2 increases in the order BaF2(25)®SrF2(35)®CaF2(50)®MgF2(70mol%) . Introduction of RF2 in
Ba(PO3)2 decreases the intensity of characteristic bands of metaphosphates (1260-1240cm-1) in the IR spectra, but increases spectral bands from pyrophosphate groups (1145-1110cm-1 and 930-905cm-1). Increasing the concentration of RF2 assist formation of tetrahedra [PO3F] .
Urusovskaya et al. have done EPR investigation of Al(PO3)3-RF (R-Li, Na, K) glasses, doped with Mn(II) and Co(II). Established data of nature of chemical bonds realizing in alumofluorophosphate glasses. Large lasing effects were obtained from the base neodimium fluorophosphate glasses .
Petrovski  showed the possibility of creating fluorophosphate glasses with monofluorophosphate barium (BaPO3F) only 8mol%, other components are fluorides alkaline earth and rare earth elements. Refractive index of this glass is nD=1.43658, coefficient dispersion n=95.8.
Margaryan et al.[97-105] studied glasses in ternary fluorophosphate systems: P2O5-LaF3-RF2 (R-Mg, Ba), Ba(PO3)2-LaF3-RF2, Ba(PO3)2-YF3-RF2, Ba(PO3)2-Al(PO3)3-RF2 (R-Mg, Ca, Sr, Ba). The domain of glass formation for glass system P2O5-LaF3-MgF2 is P2O5=40-100, LaF3=0-25, MgF2=0-60mol%, for P2O5-LaF3-BaF2 is P2O5=50-100, LaF3=0-25, BaF2=0-50mol%.
The IR spectra of the glasses contain characteristic bands related to alkaline metaphosphates , which are observed in the spectral range 1325-1250cm-1, according to P-O bond in (PO3)n1- anion [99, 100]. Presence of the bands in 760-730cm-1 will be carry to P-O-P vibration of circular phosphates or P-F bonds in monofluorophosphate anion [PO3F]2-. 1080 and 500cm-1 bands according to fundamental vibration of [PO4]3- molecules . Fluorophosphate type of glasses in P2O5-LaF3-RF2 consist from (PO3)n1-, [PO3F]2-, [PO4]3- structural groups [99, 100]. Figure 5.22 presents the domain of glass formation for Ba(PO3)2-LaF3-RF2 and Ba(PO3)2-YF3-RF2 systems .
Figure 5.22: Boundaries of glass formation (from ).
Limits of glass formation increase with decreasing of ion radii of alkaline earth cations.
Yttrium containing systems have a wide limits of glass formation. Glasses in Ba(PO3)2-YF3-BaF2 are formed in Ba(PO3)2=35-100, YF3=0-60, BaF2=0-25mol%. Glasses in Ba(PO3)2-YF3-MgF2 are formed in Ba(PO3)2=25-100, YF3=0-60, MgF2=0-70mol%.
Figure 5.23: IR spectra of glasses of the system Ba(PO2)3 – LaF3 – RF2 (from ).
(1) glass forming Ba(PO3)2, (2) 80Ba(PO3)2 . 20LaF3, (3) 80Ba(PO3)2 . 20BaF2, (4) 80Ba(PO3)2 . 10LaF3 . 10BaF2, (5) 80Ba(PO3)2 . 20SrF2, (6) 80Ba(PO3)2 . 20CaF2, (7) 60Ba(PO3)2 . 40CaF2, (8) 80Ba(PO3)2 . 10LaF3 . 10CaF2,
(9) 60Ba(PO3)2 . 10LaF3 . 30CaF2, (10) 50Ba(PO3)2 . 10LaF3 . 40CaF2,
(11) 80Ba(PO3)2 . 20MgF2, (12) 60Ba(PO3)2 . 40MgF2, (13) 40Ba(PO3)2 . 60MgF2, (14) 80Ba(PO3)2 . 10LaF3 . 10MgF2, (15) 60Ba(PO3)2 . 10LaF3 . 30MgF2,
Figure 5.23 and 5.24 illustrate the IR spectra of Ba(PO3)2-LaF3-RF2 and Ba(PO3)2-YF3-RF2 glasses . IR spectra of glass forming metaphosphate barium-Ba(PO3)2 are characterized by the appearance of the spectral bands on the 1265, 1150, 1090, 1000, 885, 775, 520 and 475cm-1 .
Born  and Thilo  indicated, that the structure of metaphosphate anions depend on the ion sizes of respective cations. Small and big cations form polyphosphates with anions chain. Cations of average size form metaphosphates with circular anions.
Figure 5.24: IR spectra of glasses of the system Ba(PO2)3 – YF3 – RF2 (from ).
(1) glass forming Ba(PO3)2, (2) 80Ba(PO3)2 . 20BaF3, (3) 80Ba(PO3)2 . 20YF3, (4) 60Ba(PO3)2 . 40YF3, (5) 60Ba(PO3)2 . 20YF3 . 20BaF2,
(6) 60Ba(PO3)2 . 20YF2 . 20SrF2, (7) 50Ba(PO3)2 . 20YF3 . 30SrF2,
(8) 60Ba(PO3)2 . 40CaF2, (9) 60Ba(PO3)2 . 20YF3 . 20CaF2,
(10) 50Ba(PO3)2 . 20YF3 . 30CaF2, (11) 60Ba(PO3)2 . 40MgF2,
(12) 40Ba(PO3)2 . 60MgF2, (13) 60Ba(PO3)2 . 20YF3 . 20MgF2,
(14) 50Ba(PO3)2 . 20YF3 . 30MgF2, (15) 30Ba(PO3)2 . 20YF3 . 50MgF2.
Petrovski et al.[110, 111] established that in the crystalline form of barium metaphosphate predominate tetrametaphosphate circular groups type of (P4O12)4-. The crystalline form transforms in the glass forming state to Ba(PO3)2 accompanied by breaking of tetrametaphosphate circles with formation of chains of (PO3)n1- . Introducing fluorides of alkaline earth elements in glass forming Ba(PO3)2 preserves the tetraphosphate radical (P4O12)4- .
Figure 5.23, and 5.24 illustrate, that introduction of any of the alkaline earth fluorides, LaF3 and YF3 in glass forming Ba(PO3)2 leads to decreasing of the bands intensity in IR spectra of metaphosphate anions. In the presence of RF2 and RF3 in 30-50mol% and above observed that IR bands disappear, characteristic of glass forming Ba(PO3)2. When concentration of fluorides increasing in glass forming Ba(PO3)2 on IR spectra displayed intense bands of absorption which is described by the presence of pyrophosphate groups in vibration range of 1145-1110cm-1 (nas PO3) and 930-905cm-1 ( nas P-O-P) (Figure 5.23 and Figure 5.24). Glass 60Ba(PO3)2 . 40CaF2 (curve 7, Figure 5.23), basically formed pyrophosphate groups of structure in the range of frequency 560, 690, 910cm-1 (d-Ba2P2O7).
When the content of fluorides is 40-50mol% (curves 9, 10, Figure 5.23) intensity of corresponding bands increases respectively for pyrophosphate groups with 560, 930-910, 1140-1110cm-1 and tetrametaphosphate (740-730cm-1). Curves 11-15 (Figure 5.23) presents vibration frequency of glasses Ba(PO3)2-LaF3-MgF2. When the concentration of fluorides is above 30mol% (curves 12, 13, 15) basic structural groups formed tetrametaphosphates and pyrophosphates. When MgF2 is 40 and 60mol% (curves 12 and 13) spectral band in 755-735cm-1 interval can be ascribed bonds P-F in the monofluorophosphate anion (PO3F)2-. Probably anions of (PO3F)2- are formed in fluorophosphate glasses in the presence of the high concentration of fluorides.
IR spectra of the glass system Ba(PO3)2-YF3-RF2 (Figure 5.24) present analogous spectra to glasses of Ba(PO3)2-LaF3-RF2 (Figure5.23). Glasses in the system Ba(PO3)2-YF3-MgF2, can have concentration of fluorides up to 70mol%. High content of fluorides (more than 40mol%) assist in the formation of the bonds between nuclei of fluorine and phosphorus in minofluorophosphate tetrahedra (PO3F)2-.
Curves 11-15 (Figure 5.24) show IR spectra of yttriumcontaining fluorophosphate glasses with MgF2 20-60mol%, where this common content of fluoride is 40-70mol%. IR spectra of glasses change according to the content of fluorides. MgF2 has a special role in formation of structure of fluorophosphate and fluoride glasses. Between interval of frequencies 760-755cm-1 on the curves 11, 12 and 15 (Figure 5.24) (MgF2 to 40-60mol%) providing intense bands of the bonds P-F in (PO3F)2- anion. These type of glasses form on the base of the pyrophosphate (930-900cm-1) and monofluorophosphate groups of structure. Glass formation in the Ba(PO3)2-LaF3-RF2 and Ba(PO3)2-YF3-RF2 systems are realized by coexistence of (PO3)n1-, (P2O7)4-, (P4O12)4-, (PO3F )2- and fluoride groups, specially [MgF4]2-.
Figure 5.25 shows boundaries of glass formation of fluorophosphate glasses with two glass former metaphosphates (barium and aluminum) and fluorides of alkaline earth metals . Wide domain of glass forming fluorophosphates form in the line Ba®Sr®Ca®Mg.
Figure 5.25: Boundaries of glass formation (from ).
An exception is Ba(PO3)2-Al(PO3)3-BaF2, where there are two separate ranges of glass formation. Glasses in range I formed at Ba(PO3)2=10-80, Al(PO3)3=10-80, BaF2=10-20mol%, glasses in range II at Ba(PO3)2=10-40, Al(PO3)3=10-20, BaF2=50-70mol%.
The domain of glass formation in the Ba(PO3)2-Al(PO3)3-RF2 systems is according to the follow limits: Ba(PO3)2=0-100, Al(PO3)3=0-100, MgF2=0-70, CaF2=0-50, SrF2=0-35, BaF2=0-25mol%.
Figure 5.26 presents IR spectra of glasses Ba(PO3)2-Al(PO3)3-RF2 systems . IR spectra of glasses present a superposition of bands at glass forming Ba(PO3)2 and Al(PO3)3. Intensity of characteristic bands of IR spectra depend on the concentration of Ba(PO3)2 and Al(PO3)3 in developed glasses.
Figure 5.26: IR spectra of glass of the system Ba(PO3)2 – Al(PO3)3 – RF2 (from ).
(1) glass forming Ba(PO3)2, (2) glass forming Al(PO3)3, (3) 20Al(PO3)3 . 80Ba(PO3)2, (4) 50Ba(PO3)2 . 50Al(PO3)3, (5) 80Al(PO3)3 . 20Ba(PO3)2,
(6) 80Al(PO3)3 . 20BaF2, (7) 80Ba(PO3)2 . 20BaF2, (8) 40Al(PO3)3 . 40Ba(PO3)2 . 20BaF2, (9) 50Al(PO3)3 . 30Ba(PO3)2 . 20BaF2, (10) 60Al(PO3)3 . 20Ba(PO3)2 . 20BaF2, (11) 10Al(PO3)3 . 40Ba(PO3)2 . 50BaF2, (12) 10Al(PO3)3 . 30Ba(PO3)2 . . 60BaF2, (13) 10Al(PO3)3 . 20Ba(PO3)2 . 70BaF2, (14) 20Al(PO3)3 . 10Ba(PO3)2 . 70BaF2, (15) 20Al(PO3)3 . 60Ba(PO3)2 . 20BaF2, (16) 30Al(PO3)3 . 50Ba(PO3)2 . 20BaF2, (17) 80Al(PO3)3 . 20SrF2, (18) 80Ba(PO3)2 . 20SrF2, (19) 10Al(PO3)3 . . 70Ba(PO3)2 . 20SrF2, (20) 70Al(PO3)3 . 10Ba(PO3)2 . 20SrF2, (21) 80Al(PO3)3 . 20SrF2, (22) 80Ba(PO3)2 . 20CaF2, (23) 10Al(PO3)3 . 70Ba(PO3)2 . 20CaF2,
(24) 70Al(PO3)3 . 10Ba(PO3)2 . 20CaF2, (25) 80Al(PO3)3 . 20MgF2,
(26) 80Ba(PO3)2 . 20MgF2, (27) 10Al(PO3)3 . 70Ba(PO3)2 . 20MgF2,
(28) 70Al(PO3)3 . 10Ba(PO3)2 . 20MgF2,
With increasing contents of metaphosphates, IR spectra are displace to the side of the frequencies of vibrations of Ba(PO3)2 and Al(PO3)3 respectively: curve 3-20Al(PO3)3 . 80Ba(PO3)2, curve 4 – 50Al(PO3)3 . 50Ba(PO3)2 and curve 5 – 80Al(PO3)3 . 20Ba(PO3)2 (Figure 5.26).
Introduction in glass fluorides of alkaline earth element causes decreasing of intensity of basic bands of glass forming Al(PO3)3 and Ba(PO3)2 and new bands of absorption appear, which belong to pyrophosphate, monofluorophosphate and tetrametaphosphate groups.
Special interest are glasses of Al(PO3)3-Ba(PO3)2-BaF2. Glasses with BaF2 at 50 to 70mol% (curves 11-14, Figure 5.26) at 760-745cm-1 show formation of tetrahedral anions of (PO3F)2-.
Interval vibration 940-870cm-1 (nas P-O-P in d Ba2P2O7) describes formation of pyrophosphate groups (P2O7)4- (curves 8-16, Figure 5.26) in structure of glass. Thus fluorophosphate glasses Al(PO3)3-Ba(PO3)2-BaF2 are formed from anion type of elements of the structure (PO3F)2-, (P2O7)4-, (P4O12)4 and (PO3)n1-. Glasses Al(PO3)3-Ba(PO3)2-(Sr, Ca, Mg)F2 (curves 17-28, Figure 5.26) which contain fluorides of alkaline earth elements up to 20mol% glass formation is realized by (PO3)n1- and (P2O7)4- structural groups.
Margaryan et al. [105, 112, 113] provided investigations of luminescence, electron absorption and EPR spectra of fluorophosphate glasses doped with Mn(II).
Spectral characteristics of Mn(II) for different types of fluorophosphate systems very close to each other (luminescence spectra Figure 2.8 and Figure 2.10, absorption spectra Figures 3.9; 3.10 and 3.11).
Definite interest presents spectroscopic investigations of Mn(II) in Ba(PO3)2-PbF2(BiF3) and Ba(PO3)2-YF3-PbF2(BiF3) glasses . The glass composition investigated (in mol%) were: 45Ba(PO3)2 . 55PbF2(BiF3), 50Ba(PO3)2 . 20YF3 . 30PbF2(BiF3). The luminescence spectra of glasses, doped with Mn(II), are shown in Figure 5.27. Increasing the concentration of dopant, shifts the broad band spectrum toward a longer wavelength region.
Figure 5.27: Luminescence spectra of Mn(II) in glass with composition(mol%):
50Ba(PO3)2 . 20YF3 . 30PbF2 (curves 1, 2 and 3), and
50Ba(PO3)2 . 20YF3 . 30BiF3 (curves 4, 5 and 6) (from )
The colour of the luminescence changes from yellow to dark red. The peak position is approximately 610nm when the MnF2 concentration is 0.5wt% and 720nm when its concentration is 15wt%.
Changes in the chemical bond between dopant and ligands is the main factor affecting the luminescence [115, 116]. Phosphorus in fluorophosphate and phosphate glasses bonds more strongly to oxygen or fluorine than the silicon in silicate glasses. Phosphorus has a larger nuclear charge and forms five covalent bonds as compared to four for silicon:
In fluorophosphate glasses, in which F-Mn-F, or F-Mn-O-bonds occur, the degree of covalence lies between that of phosphate and fluoroberyllate glasses . Absorption spectra of the glasses are shown in Figure 5.28. The absorption band corresponding to the 6A1g(S)®4Eg(G) transition is the most intense. The position of the maximum for investigated glasses is 24320cm-1. The second most intense band to the 6A1g(S) ® 4Eg(D) transition at 28700 cm-1.
Figure 5.28: Absorption spectra of Mn(II) in glass with composition(mol%):
50Ba(PO3)2 . 20YF3 . 30BiF3 (curves 1 and 2), and
50Ba(PO3)2 . 20YF3 . 30PbF2 (curves 3 and 4) (from ).
Fluorophosphate glasses are close to the phosphate glasses in degree of covalence of the dopant-ligand bond. This is also confirmed by comparison of the Racha coefficient, B, for these glasses. The magnitude of B decreases with decrease in size of the effective nuclear charge for free ions. A decrease in the B value can be used to infer an increase in the degree of covalency of Mn(II) with the surrounding ligands . The coefficient B can be calculated from the Tanabe-Sugano equation . In bismuth and lead containing fluorophosphate glasses B=632 and 634 cm-1 respectively.
For fluoroberyllate, phosphate and silicate glasses, the parameter B is approximately 700, 620 and 600, respectively .
The bands 4Eg(G) and 4Eg(D) (Figure 5.28) do not depend on the strength of the field. In the spectrum, there is a 4T1g(G) band, whose position depends on the strength of the ligand field. This makes it possible to compare the strength of the ligand fields in glasses with various compositions for dopant ions with dn electrons.
According to Figure 5.28, the energy difference between the terms 4Eg(G) and 4T1g(G) for the fluorophosphate glasses is 4320cm-1. For fluoroberyllate, phosphate and silicate glasses the value are 4070, 4700 and 8250cm-1, respectively . The strength of the ligand field increases in the order fluoroberyllate, fluorophosphate, phosphate and silicate of the glass series.
From EPR spectra was found the hyperfine splitting(hfs) for Mn(II) in glasses: 45Ba(PO3)2 . 55BiF3 and 45Ba(PO3)2 . 55PbF2, where A=94.33 and 92.12 Oe respectively. As the concentration of Mn(II) increases, the width of the hfs lines increases, due to spin-spin coupling between adjacent manganese ions (Figure 5.29).
Figure 5.29: EPR spectra of Mn(II) in the glass(mol%) 45P2O5 . 55PbF2 at Mn(II): (1) 0.05, (2) 0.1, (3) 0.5, (4) 1.0, (5) 2.0wt% (from ).
The character of the bond, which exists between the ligand-glass forming agent and ligand-dopant, plays an important role. The hfs for Mn(II) is directly proportional to the number of ionic bonds in the ligand-dopant series [20, 117, 118].
In the present time one of the hot and actual problem is immobilization of radioactive waste obtained from reprocessing irradiated nuclear fuel. Vitrification has been identified as one of the most viable waste treatment alternative for nuclear waste disposal. Currently, the most popular glass compositions being selected for vitrification are the borosilicate family of glasses and soda-lime-silicate variety [121-127].
The United States and former Soviet nations are struggling with how to safely and economically safeguard and dispose of more than 100 tons of weapons grade plutonium from nuclear weapons dismantled under the Strategics Reduction Treaty.
One option being studied is to immobilize plutonium in a solid waste form, such as glass, and place it in a deep, underground repository.
Now one of important problem of vitrification of radioactive waste is a development and investigation of the new effective composition of glasses, which is able to absorb in the maximum level of nuclear irradiation and keep for a long time safely the nuclear waste. For this purpose, fluorophosphate type of glasses have some advantage, containing fluorine and oxides dn and ¦n elements, having a high radioactive resistance, owing to high electronegativity of fluorine and reverse change of valency of dn and ¦n elements. On the other hand high absorption of nuclear irradiation provided by existence of lead fluorine in the vitreous fluorophosphate systems.
The Environmental Protection Agency (EPA) has declared vitrification the best demonstrated available technology for high-level radioactive waste.
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