FLUOROBERYLLATE VITREOUS SYSTEMS

From:

Alfred Margaryan

“Ligands and Modifiers in Vitreous Materials: Spectnoscopy of condensed Systems”

Publisher: World Scientific, Singapore, New Jersey, London, Honk Hong 1999

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Fluoroberyllate crowns are a type of non-oxygen containing glasses, where a glass forming component is beryllium fluorine (BeF2). Fluoroberyllate vitreous materials have higher coefficient of dispersion n=80-105 and lower refractive index nD=1.3-1.4. They occupied a special place on the Abbe diagram.

Fluoroberyllate glasses are characterized by significant transmission in the wide spectral interval (150 to 5000nm) of frequencies. These glasses are promising materials for ultraviolet and infrared optics.

Fluoroberyllate glasses possess high resistance against radioactive irradiation, which allows to create stable ultraviolet filters. High transparency in ultraviolet, visible and infrared range of spectra permit to investigate all of absorption and luminescence bands of dopants (¦n, dn) in fluoroberyllate matrices.

Goldschmidt [128, 129] first showed the possibility of creation of fluoroberyllate glasses, providing analogy between crystalline silicates and fluoroberyllates. Glasses were synthesized in the BeF2 – KF ­– NaF system. Systematic investigations of simple and multicomponente fluoroberyllate glasses was accomplished Heyne [130]. Table 5.8 shows some compositions of fluoroberyllate glasses according to the reference [130]. Density of glasses close to 2.35g/cm3, refractive indices nD=1.33 – 1.34, temperature of softening between interval 170 - 300°C.

Domain of transparency of shown glasses is from 220nm to 5500nm [130]. Sun, Huggins, and Callear [131-138] accomplished wide and fundamental research of different compositions of fluoroberyllate glasses for optics.

Table 5.9 shows some compositions of multicomponent fluoroberyllate glasses, which were developed by Sun, and Huggins [138].

Table 5.9

Compositions of Fluoroberyllate Glasses (from [138])

(A – wt% and B – mol%) . n = 95 – 100.

Above presented compositions contain more than 50mol% of BeF2 and AlF3 taken together.

Table 5.10 summarizes some compositions of fluoroberyllate glasses presented by different researchers.

Table 5.10

Compositions of Fluoroberyllate Glasses According to the Different Authors.

Imaoka and Mizusawa [140, 142] developed fluoroberyllate glasses with refractive indices nD = 1.36 – 1.40 and dispersion n = 90-100. Optical constants of these glasses were very close to LiF (nD = 1.39, n = 98.5) and CaF2 (nD = 1.43, n = 95.4).

Table 5.11 shows optical constants, densities and compositions of alkaline fluoroberyllate glasses, which have a low chemical stability and are soluble in water[142].

Table 5.11

Compositions of Alkaline Fluoroberyllate Glasses (from [142])

Vogel and Gerth [143-146] developed fluoroberyllate glasses in binary and ternary systems BeF2 – RF(R – Li, Na, K, Rb), BeF2 – RF2(R-Mg, Ca, Sr) and BeF2 – KF – RF2 (R – Mg, Ca, Sr, Ba), BeF2 – NaF – RF2(R-Mg, Ca, Sr), BeF2 – LiF – MgF2. They estimated boundaries of glass formation, refractive indices, densities and refraction and considered also problems of structures of fluoroberyllate glasses.

Henrikh and Ignatiev [147] presented domain of glass formation in ternary fluoroberyllate systems. Chalilev et al. [148, 149] investigated influence of gas atmosphere on the ultraviolet transparency at melting time of fluoroberyllate glasses. For this purpose was used aluminum-containing fluoroberyllate compositions (Table 5.12).

Table 5.12

Compositions of Alumo-Fluoroberyllate Glasses (in mol%)(from [148])

Spectroscopic investigations of doped (¦n, dn) fluoroberyllate glasses by Margaryan [150 –156] considered the base non-alkaline glasses (Table 5.13)

Table 5.13

Compositions of Non-Alkaline Fluoroberyllate Glasses (in mol%)(from [150])

Kocik, and Kocikova [157] studied boundaries of glass formation for ternary and multicomponent fluoroberyllate systems. Contents of BeF2 = 25 – 35mol%, AlF3 = 19-28mol%, optical constants are between nD = 1.38 -1.39 and n = 97 – 100. In the system BeF2 – AlF3 – MeF2, where Me – Mg, Ca, Sr, Ba, Pb it was established that the order of crystallization follows Mg ® Ca ® Pb ® Sr ® Ba (increase of crystallization).

Many of the physico-chemical properties of fluoroberyllate glasses, for the first time, were investigated in the referes [148-156, 158-164].

In the USA (Corning Glass Works Co.) [165, 166] developed new composition and technology of creation of fluoroberyllate glasses doped with neodimium, for application to powerful lasers used in the thermonuclear syntheses and technology of laser melting.

Margaryan [167 –169] studied irradiation stability of fluoroberyllate glasses, doped with fluorides of rare earth elements. Glasses containing fluorides of rare earth elements in 0.02 – 0.05mol% possess much higher irradiation stability. High positive effect was discovered for glasses doped with fluorides of samarium, ytterbium and europium. Cerium worse is the irradiation stability of doped fluoroberyllate glasses.

Figure 5.30 illustrates spectra of fluoroberyllate glasses, doped with fluoride of rare earth elements before and after gamma-irradiation.

Figure 5.30: Curves of transparency of fluoroberyllate glasses before and after gamma-

irradiation. (1) initial glass, (2) NdF3, (3) SmF3, (4) YbF3, (5) EuF3, (6) CeF3, in 0.02 mol%. Irradiation dose 105 roentgen (from [167-169]).

Curves 3, 4 and 5 show the transformation of part of the rare earth ions, under gamma-irradiation, from Sm(III) ® Sm(II) (280 – 360nm), Yb(III) ® Yb(II) (333nm), Eu(III) ® Eu(II) (310nm).

Influence of the hydroxyl (-OH) groups on the irradiation stability of fluoroberyllate glasses was studied by Margaryan [118, 170]. The investigation included glasses with and without hydroxyl content. Spectral band of –OH group lies at 2800nm. After gamma-irradiation (Figure 5.31) curves of transmission are distributed in the following order: glass with –OH characterized with insignificant high transmission, than glass without hydroxyl groups.

Figure 5.31: Curves of transparency of fluoroberyllate glasses before and after gamma-

irradiation. Irradiation dose 105 roentgen (from [118, 170]).

Therefore, significant anti-irradiation effect at presence of –OH groups in fluoroberyllate glasses are not displayed. In oxide and quartz type of glasses presence of –OH groups show very high irradiation stability [171,172].

Margaryan et al. [173, 174] first studied spectroscopy of fluoroberyllate glasses doped with Mn(II). Defined meaning of Racha Parameter –B and C makes to use its for calculation scheme of energetic levels.

Diagram of energetic levels of Mn(II), are present for glass, where BeF2 = 35, AlF3 = 20, CaF2 = 20, CaF2 = 20, SrF2 = 15, MgF2 = 10mol%, concentration of dopant MnF2 = 10wt%, with B = 700cm-1 and C = 3600cm-1 (Figure 5.32).

Figure 5.32: Diagram of energetic levels of Mn(II) in fluoroberyllate glass at B=700cm-1 and

C=3600cm-1(from [173, 174]).

Figure 5.32 diagram shows that two terms which connected with transitions 6A1g(S) ® 4Eg(G) and 6A1g(S) ® 4Eg(D) are responsible for two narrow and intense bands. All of the other bands are not uniform broadening according to the slope of their terms.

Free ion of bivalence manganese has a fundamental state 6S and above arranged four quartet terms 4G, 4P, 4D and 4F. Closest term 4G has a 27000cm-1 of distance at fundamental state. Position of energetic levels depends on parameters D, B and C. Magnitude of Racha parameters B and C depend on nature of chemical bonds Mn(II)-ligand and can be determined using absorption spectra of Mn(II) for each type of glass [175-177]. For Mn(II) exist energetic conditions when their positions are not changed with change of strength of ligand field. This situation permits comparison of distance between terms of free ion and ion in structural cell of glass. Above indicated terms are 4Eg(G) and 4Eg(D). Using position of bands of absorption 6A1g(S) ® 4Eg(G) and 6A1g(S) ® 4Eg(D) we may determine parameter of B and C from equation:

6S ® 4G = 10B + 5C

6S ® 4D = 17B + 5C

B and C makes use for calculation of schemes of energetic levels for different type of matrices. Introduction of Mn(II) in glass or other matrix leads to decreasing of Racha B. This is the result of the high state of covalency between dopant-ligands in the glass or other matrix [118-120].

Figure 5.33 presents absorption spectra of fluoroberyllate glass, containing 16wt% MnF2.

Figure 5.33: Absorption spectra of fluoroberyllate glass, doped with bivalence manganese.

Concentration of MnF2=16wt% (from [173, 174]).

According to diagram of energy levels (Figure 5.32) intense and narrow bands of absorption are result of transitions 6A1g(S) ® 4Eg(G) and 6A1g(S) ® 4Eg(D) where position of maximuma are 25200cm-1 and 30050cm-1 respectively. Transition on level 6A1g(S) ® 4T1g(G) is sensitive to change of strength of ligand field [118]. Difference of energy between levels 6A1g(S) and 4T1g(G) is for fluoroberyllate glass – 4050cm-1, phosphate glass – 4700 and silicate glass – 8250cm-1. Strength of ligand field increases in the order fluoroberyllate, phosphate and silicate glass.

Maximuma of absorption bands for transition 6A1g(S) ® 4T1g(G) are composed for fluoroberyllate glass – 21150 (Figure 5.33), phosphate and silicate 19850cm-1 and 15450 respectively [178].

Figure 5.34 shows luminescence spectra with increasing concentration of dopant (at 0.25 to 25wt% MnF2). Colour of emission changes from yellow to red. The luminescence spectra of glasses show wide bands with shifting to the long wave of frequency when the contents of Mn(II) is increased.

Figure 5.34: Luminescence spectra of fluoroberyllate glass doped with bivalence manganese.

(1) 0.25wt%, (2) 8wt%, (3) 25wt% MnF2 (from [173, 174]).

Position of maximuma at concentration of MnF2 0.25wt% is 17300cm-1, when MnF2 is 25wt% it is 15750cm-1.

Tsurikova [179] studied luminescence spectra of Mn(II) in the multicomponent fluoroberyllate glasses composition (mol%): 60BeF2 . 10AlF3 . 10CaF2 . 20MF, where M = Li, Na, K, Rb, Cs and in the glass form BeF2. The results of these developments are presented in Figure 2.7 (See Chapter No. 2). Equimolecular displacement of LiF®NaF®KF®RbF®CsF lead to systematic decreasing of hyperfine splitting of Mn(II) in glasses and minimum value of hfs finded for glass forming BeF2.

Table 5.14 shows values of hyperfine splitting for multicomponent fluoroberyllate and beryllium fluorine glasses.

Table 5.14

Values of Hyperfine Splitting for Fluoroberyllate Glasses (from [179])

Figure 5.35 illustrates EPR spectra of Mn(II) in fluoroberyllate (curve 4), phosphate (curve 5), and silicate (curves 1,2,3) glasses. EPR spectra of Mn(II) in fluoroberyllate glass typically show the presence of a single band with g = 2.00, which is same also for phosphate type of glass. In the silicate type of glass a second band is observed with g = 4.27. Intensity of this band growing according to K2O®Na2O®Li2O (Figure 5.35)

Figure 5.35: EPR spectra of Mn(II) in different type of glasses: (1) Li2O . 2SiO2: 2MnO2,

(2) Na2O . 2SiO2: MnO2, (3) K2O . 2SiO2, (4) 35BeF2 . 20AlF3 . 20CaF2 . 15SrF2 . 10MgF2: 0.05MnF2, (5) ZnO . P2O5: 0.05MnO2 (from[118, 173,174]).

Constant of hyperfine splitting (A) decreases in the line of glasses fluoroberyllate (96 Oe) ® phosphate (95 Oe) ® Silicate (89-85 Oe), which shows systematic growth of degree of covalency Mn(II)-ligand in the resulting glasses [173, 174].

Abdrashitova and Ptrovski [180, 181] investigated EPR of the ions of transition groups of the ferrium in fluoroberyllate glasses. Developed glasses contains Ti, V, Cr, Mn, Fe, Co, Ni, Cu (Figure 5.36). Titanium in fluoroberyllate glass is present in the form of Ti(III). At 293°K on the EPR spectra of titanium (curve 1) were observed narrow antisymmetric line with g = 1.95. Decrease of temperature to 4°K leads to significant widening of lines with increasing asymmetry (curve 2).

Vanadium containing glass are characterized by complicated EPR spectra (curves 3, 4) with fine permission of structure. Vanadium in fluoroberyllate glasses present in the form of ion – (VO)2+.

In the glass with chromium 1wt% were observed wide antisymmetrical lines with complicated form.

Figure 5.36: EPR spectra of transition ions in fluoroberyllate glasses (from [180, 181]).

(1) Ti(III), temperature 293°K; (2) Ti(III), 4°K; (3) (4) (VO)2+, 77°K; (5) (6) (7)

Mn(II), 293°K; (8) Ni(II), 293°K; (9) Co(II), 77°K; (10) Co(II), 4°K.

At 77°K g1=4.7, g2=2 and g3=1.5. When increasing concentration of chromium to 3wt% at 293°K, only one symmetrical line is observed, with g=2.

Figure 5.36 (curves 5,6,7) show EPR of Mn(II) at temperature 293°K [180, 181].

Fluoroberyllate glass with content of Fe»0.01wt% shows bands of absorption on the EPR spectra at Fe(III) with g»4.27. Additional introduction of Fe(III) (to 0.1wt%) causes appearance of wide lines of EPR with g»2.2. At low temperature (77°K) the band of g»2.2 disappears, but the wide asymmetric line with g»4 remains.

Analogous effect is observed for glass containing Ni(II) in 0.5wt%. At temperature 293°K discovered wide line of EPR (curve 8), which disappears at 77°K.

On the Figure 5.36 (curves 9 and 10) show EPR spectra of Co(II) at 77°K and 4°K. EPR spectra at 77°K consist of one wide line with g»4.2 (concentration of cobalt 3wt%). Decrease of temperature to 4°K leads to narrowing of EPR line, but the same g – factor [180, 181].

Fluoroberyllate glass with Cu(II) gives complicated form of EPR spectra with weak hyperfine structure [180,181].

Udin, Tsurikova and Petrovski [182, 183] presented EPR spectra of Co(II) in the two type of fluoroberyllate glasses in Figure 5.37. In both cases was observed wide resonance line with g=4.28. Cobalt in these glasses has an octahedral coordination.

Figure 5.37: EPR spectra of glass composition 60BeF2 . 20KF . 10AlF3 . 10CaF2(mol%):

(1) 0.25wt%, (2) 0.5wt%, (3) 1.0wt% CoF2 . 70BeF2 . 10AlF3 . 20KF(mol%):

(4) 0.5wt% CoF2 (from [182, 183]).

Abdrashitova and Raaben [184] first discovered EPR for Co(II) in the glass forming beryllium fluorine(BeF2).

A large number of publications devoted to investigation of glass forming matrices for purpose of creation of new laser materials [185 – 193]. This created interest for systematic research and development of spectroscopic properties of rare earth ions in different type of glass forming materials. In this case fluoroberyllate glasses have a special interest.

On the absorption spectra of rare earth ions are observed two type of bands: narrow bands which lie in the infrared, visible and near ultraviolet range and wide bands which lie in the far ultraviolet range of spectra.

Absorption bands of the first type connected with transition of electrons between ¦ – levels, bands of second type (wide bands) depend on transition of electrons between levels ¦n – ¦nd [194].

Prosedymium – Pr(III)(4¦2, 3H4). Fluoroberyllate glasses doped PrF3 have a green colour. Basic bands of absorption are placed in the range between 25000 to 4500cm-1. Characteristic bands of absorption are found at wave numbers: 4500, 5100, 6500, 6850, 9800, 11450, 12450, 13400, 17000, 19200, 19600, 21000, 21400, 22600, 22800cm-1 (Figure 5.38 (a)) [184, 154].

Figure 5.38: Absorption spectra of fluoroberyllate glass doped: (a) PrF3, (b) NdF3 – 1mol%

(from [118, 154]).

Neodymium-Nd(III)( 4¦3, 4I9/2). Glasses doped with NdF3 have a special interest for creation of new type of laser.

Figure 5.38(b) shows, for this glass a large number of absorption bands in the transparent interval of spectra. Most intense bands lie at 28900, 28600, 28200, 19600, 19200, 17400, 13400, 12400, 11500cm-1 [118, 154]. Fluoroberyllate glass containing NdF3 has a liliac colour. Excitation at any band of absorption lead to IR luminescence with the maximuma 900, 1060 and 1300nm.

Petrovski, Tolstoy et al. [195, 196] studied spectra of absorption and luminescence of Nd(III) for three type of fluoroberyllate glasses:

1. 60BeF2 . 10AlF3 . 10CaF2 . 15KF . 5MF (M=Li, Na, K, Rb, Cs)

2. 70BeF2 . 10AlF3 . 20MF (M=Li, Na, K, Rb, Cs)

3. 60BeF2 . 10AlF3 . 20KF . 5CaF2 . 5MF2 (M=Mg, Ca, Cd, Sr, Ba, Zn, Pb)

Characteristics of absorption and luminescence spectra of the Nd(III) for all investigated glasses are found to be very close. Absorption bands, by comparison with oxygen – containing glasses, shifts little on the short – wave of spectra. Figure 5.39 shows absorption spectra of Nd(III) in fluoroberyllate, fluorophosphate and silicate type of glass [195].

Figure 5.39: Absorption spectra of neodymium in glasses:

(a) fluoroberyllate, (b) fluorophosphate, (c) silicate (from [195]).

Value of term splitting for fluoroberyllate glass is significantly lower than for oxygen-containing glasses and crystals (Figure 5.39). Therefore, strength of ligand field on the Nd(III) in the fluoroberyllate glasses concerning low [118].

Duration of luminescence of neodymium in fluoroberyllate glasses is 4.10-4sec at 300°K, 5.10-4sec at 77°K [196].

Samarium – Sm(III)(4¦5, 6H5/2). Fluoroberyllate glass doped SmF3 has a weak yellow colour. Ultraviolet radiation cause intense orange colour of luminescence in glass [118]. Figure 5.40 shows absorption spectra of Sm(III) in fluoroberyllate glass. Intense bands of absorption are disposed between interval of wave numbers 29000 to 24000cm-1 and 10000 to 6000cm-1 [118, 154].

Figure 5.40: Absorption spectra of fluoroberyllate glass doped SmF3 – 1mol% (from [118,

154]).

On the spectral curve of absorption between interval 20000 to 11000cm-1 do not observed expressed bands, just at 20060 and 17900cm-1 displayed weak maximuma.

Intense bands of absorption of Sm(III) in the fluoroberyllate glass discovered at: 29200, 27800, 26950, 25000, 21050, 10550, 9220, 8050, 7220, 6700cm-1 (Figure 5.40).

Glass doped with bivalence rare earth fluorides, with SmF2, has a red-orange colour with wide intense band of absorption with maximum 520nm (Figure 5.41a). Spectra of luminescence (Figure 5.41b) of Sm(II) consists from bands with maximuma at 682.5, 696.0, 720.0, 760.0 and 815.0nm [118, 197].

Figure 5.41: Absorption (a) and luminescence (b) spectra of bivalence samarium in

fluoroberyllate glass (from [118, 197]).

Gadolinium – Gd(III) (4¦7, 8S7/2). Glass with GdF3 is colourless. Basic bands of absorption lie in the ultraviolet part of the spectrum. Characteristic absorption bands Gd(III) in the fluoroberyllate glass observed at 36750 and 36250cm-1 (Figure 5.42a). In the interval at 50000 to 40000cm-1 appear a wide band of absorption [118, 154].

Figure 5.42: Absorption spectra of fluoroberyllate glass doped: (a) GdF3, (b) TbF3, (c) DyF3

– 1 mol% (from [118, 154]).

Glass doped with Gd(III) are capable of lasing in the ultraviolet range of spectra (at 312.5nm)[118]. Ion of Gd(III) in fluoroberyllate glass has absorption bands according transitions from basic state on the terms 6P, 6I, 6D and two bands of luminescence at 32150 and 32700cm-1 (transition 6P7/2,5/2 ® 8S7/2).

Terbium – Tb(III)(4¦8, 7F6). Glass with TbF3 also is colourless. Ultraviolet radiation cause green colour of luminescence in fluoroberyllate glass [118]. Basic bands of absorption lie in the short wave range of spectra between the interval of wave numbers at 45000 to 26000cm-1. In this spectral interval observed weak absorption bands. In the infrared part of spectra (at 6000 to 4000cm-1) for Tb(III) appeared sharp and intense bands at 5250, 5050 and 4450cm-1 (Figure 5.42b).

Dysprosium – Dy(III) (4¦9, 6H15/2). Glass doped DyF3 has a weak yellow colour. Here also are absent sharp and intense bands of absorption (Figure 5.42c). Basic bands are disposed in the interval of wavelength at 3500 to 21000cm-1 and in the long wave at 13000 to 5000cm-1 [118]. Characteristic bands of absorption for Dy(III) are observed at wave numbers: 39150, 34000, 30950, 28700, 27550, 25950, 22100, 12350, 11000, 9050, 7800, 5850cm-1 [154].

Holmium – Ho(III) (4¦10, 5I8). Fluoroberyllate glass containing HoF3 has a yellow colour. Figure 5.43a shows absorption spectra of Ho(III). In the wide interval of spectra from 49000 to 4000cm-1 can be observed narrow and intense bands of absorption. In the ultraviolet part of spectra more characteristic bands are disposed at 41500, 35950, 34850, 29950, 28960 and 27700cm-1 [154].

In the visible distance are observed intense bands at wave numbers: 25950, 24000, 22200, 21400, 21200, 20600, 18700, 15600cm-1.

Figure 5.43: Absorption spectra of fluoroberyllate glass doped: (a) HoF3, (b) ErF3, (c) TuF3, (d)

YbF3 – 1mol% (from [118, 154]).

In the long spectral wavelength (to 5000cm-1) discovered bands are found respectively at 8600, 8300 and 5050cm-1 [154]. Excitation in the blue range of spectra generates intense infrared luminescence in fluoroberyllate glass.

Erbium – Er(III) (4¦11, 4I15/2). Glass doped with ErF3 has a lilac colour. Absorption curves show large number of narrow bands placed in the ultraviolet, visible and infrared interval of spectra (Figure 5.43b). On some maximuma are observed tendency of hyperfine structure of spectra. Main absorption bands of Er(III) in the fluoroberyllate glass discovered at wave numbers: 43520, 41150, 39200, 36400, 28100, 27500, 26600, 26400, 24700, 22300, 20600, 19200, 18500, 15350, 12350, 10150, 6600, 6450cm-1 [118, 154]. In the present time glasses doped with Er(III) have a special and important application for creation new type of solid state lasers, fiber lasers and planar waveguide amplifiers [198-208].

Thulium – Tu(III) (4¦12, 3H6). Fluoroberyllate glass with TuF3 is colourless. Basic bands of absorption lie at wave numbers (Figure 5.43c): 5950, 8050, 12420, 12700, 14520, 15100, 21200, 21600, 28000, 35150, 36600, 38400cm-1.

Figure 5.44 shows luminescence spectra of Tu(III) in fluoroberyllate glass at excitation in 28000cm-1 [118].