Effect of Stepwise Replacement of LiF by Bi 2 O 3 and of Annealing on Optical Properties of LiF · B 2 O 3 Glasses

Bismuth fluoroborate glasses with compositions xBi2O3 · (40− x)LiF · 60 B2O3 (x = 0, 5, 10, 15, and 20) are synthesized by meltquench method. Optical characterization was carried out to examine variation of optical band gap energy (Eg) and Urbach energy (EU) with respect to the concentration. It reflects the effect of stepwise replacement of non-oxide and less polarizable LiF by an oxide and more polarizable (Bi2O3) group on the optical properties of the samples. The value of Eg decreases with increase in concentration of Bi2O3. The samples were subjected to annealing at different temperatures (300◦C, 350◦C, and 400◦C), and the effect of annealing on the optical properties of various samples was again studied. Annealing affects remarkably the values of Eg and EU in the samples with x = 0.


Introduction
Fast cooling of the molten form of a substance makes it difficult to form a crystalline phase.Instead, it forms the amorphous phase of the substance.Glassy nature of the substance, philosophically, is just the lack of time required for the atoms or molecules to form a long range order.The sheet-like structure of boron-oxygen triangles in borate glasses, with their ability to connect themselves to form a network, has popularized B 2 O 3 as one of the best glass former [1,2].The glass formation demands a random arrangement of various atomic and molecular species which easily form in borate glasses.A number of modifications in the properties of the borate glasses, with the addition of alkali halides, have been reported so far [3][4][5][6][7][8][9].The inclusion of LiF in the borate glass network brings out some structural changes [3], which in turn become responsible for the change in various properties.Some oxygen atoms get replaced by fluorine ions in the network to form new units like BO 2 F, BO 2 F 2 , BOF 3 , and BO 3 F [4][5][6].Also, there may be an increase in the number of polyhedral groups of boron and oxygen, which in turn increases the number of nonbridging oxygen atoms [7][8][9].Because of high third-order nonlinear optical susceptibility of Bi 2 O 3 , caused by high density and refractive index [10,11], it has numerous nonlinearity applications such as optical switching [12,13], supercontinuum generation [14], wavelength conversion [15], and so forth.Also, bismuth ion acts as an efficient luminescent activator with applications in lasers as a sensitizer for some rare earth ions [16,17].
Various properties like optical, physical, electrical, and structural, and so forth are always sensitive to the variations in the microstructure of the substance.Due to nonhomogeneous cooling of the melt, the structure and, therefore, optical behavior of the material are affected.B 2 O 3 glasses change their characteristics remarkably, when subjected to different annealing temperatures [18][19][20][21][22].In these glasses, it is never discouraged to expect the formation of B 3 O 6 rings called boroxol rings, by the combination of BO 3 groups.Raman spectroscopy reveals that for annealing temperature greater than the glass transition temperature, the concentration of boroxol rings increases with decrease in temperature [21].There appears a residual stress in such materials.During annealing of a glass, a thermodynamical and mechanical steady state is achieved after a specific time and temperature.α (a.u.)   α (a.u.)   5, 10, 15 and 20) glasses were synthesized through meltquench method using Bi 2 O 3 , LiF, and H 3 BO 3 , reagent grade powders.Various powdered materials were taken in grams equal to their molecular masses and then were mixed uniformly according to their percentage presence in various   samples.The uniform mixture was heated at 1273 K for 30 minutes.The bubble free-melt formed was pressed between two carbon plates at room temperature.The glassy samples were thus obtained in form of thin pallets with an average thickness of 1 mm each.

Optical Characterization.
Optical characterization was carried out using Perkin-Elmer UV-Visible spectrophotometer in the range 200-1000 nm at room temperature.A plot   between the absorption and wavelength of incident radiations is obtained for further analysis.

Annealing.
Samples were annealed at 300 • C, 350 • C, and 400 • C for 2 hrs each.The samples so obtained were again tested for optical and physical properties.The absorption coefficient α(ν) is formulated in terms of transmitted intensity (I t ), incident intensity (I i ), and the thickness of the sample (t) [23] and is given as

Optical Analysis. A series of samples in xBi
From Figures 1, 2, 3, and 4, one can observe that there is an absence of sharp absorption edge in all the plots.This  confirms that the samples are amorphous in nature.Calculation of optical band gap energy explores the optical behavior of a sample in terms of its transparency towards electromagnetic radiations.The optical band gap energy (E g ) is related to the absorption coefficient α(ν) [23] as (2) In this equation, ν is the frequency of incident radiation and B is a constant named as band tailing parameter.Value of the index r suggests the nature of transitions taking place in the sample.For indirect allowed and forbidden transitions r = 2 and 3, respectively, and for direct allowed and forbidden transitions r equals 1/2 and 2/3, respectively.Tauc's plots [(αhν) 1/r versus hν, r = 2] for the reported samples are shown in Figures 5, 6, 7 and 8.The values of optical band gap energy E g are obtained by extrapolating the linear regions of tauc's plots.The variation of E g for as-prepared and annealed samples against composition is listed in Table 1 and plotted in Figure 9.
There is a continuous decrease in the optical band gap energy starting from x = 0 to x = 20.In glassy materials, the decrease in optical band gap energy is often attributed to the increase in polarizability.Also in oxide glasses, it is supported by the increase in disorder due to increase in number of nonbridging oxygen atoms (NBOs).In case of presently reported samples, there is an increase in the polarizability as concentration of LiF, whose polarizability is less, is replaced stepwise by concentration of more polarizable Bi 2 O 3 .Also there is quite a good scope of increase in the number of NBOs, as there is a replacement of a non-oxide group (LiF) taking place by an oxide group (Bi 2 O 3 ) from x = 0 to x = 20.
Interestingly, annealing at different temperatures has remarkably affected E g for the sample with no Bi 2 O 3 added.For samples with x = 5 also, it first decreases for annealing temperature 300 • C and the increases for higher temperatures.E g increases with annealing temperature, in general, for all other concentrations.In samples with x = 0, it is expected that annealing causes BO 4 units to be transformed into BO 3 − units, which have one NBO each.Therefore, E g decreases on account of increasing no. of NBOs.In samples other than x = 0, annealing causes more BO 3 groups to connect themselves to form boroxol rings and to decrease the no. of NBOs.For x = 5, the formation of boroxol rings overshoots the formation of BO 3 − units at 400 • C.Although the trend of variation of E g in annealed samples remains almost same with respect to concentration as it is in the as-prepared samples.
In the low absorption region of tauc's plot, the absorption coefficient is related to photon energy [24] as In this equation E U is named as urbach tail energy which corresponds to the width of the tail states in the mobility gap.E U arises due to presence of the disordered arrangement of atoms, which causes the mobility edges to enter in the mobility gap and give rise to the tail states.Formation of such localized states can thus be attributed to the random potential fluctuations [25].Generally, in such states the phonon-assisted electronic transitions take place [26,27].The formation of such states in the materials makes it the indirect band gap material.Therefore, one can conclude that the reported samples are the indirect band gap materials.The value of E U for the reported samples is calculated from the inverse of the slopes of the linear parts of the Urbach plots (ln α v/s hν), shown in Figures 10,11,12,and 13.The values of E U for as-prepared and annealed samples are listed in Table 1, and the variation of E U is plotted against the composition in Figure 14.
Value of E U remains almost same for all as-prepared samples but increases remarkably for x = 0 when annealed at different temperatures.The reason is the same as given for decrease in band gap energy that is, conversion of BO 4 units to BO 3 − units, thereby increasing the no. of NBOs and hence disorder.For other samples, E U increases due to annealing, keeping the trend of variation similar with respect to the concentration.(2) Annealing at different temperatures decreases the value of E g for x = 0 due to conversion of BO 4 units into BO 3 − units, thereby increasing the no. of NBOs.For samples with x = 5 also, annealing at 300 • C firstly causes E g to decrease but it increases for increase in annealing temperature.This is because with increase in annealing temperature, formation of boroxol rings dominates over formation of BO 3 − units, hence decreasing NBOs.For other samples, E g the increases with annealing temperature, again due to increase in NBOs.

Conclusions
(3) In case of as-prepared samples, Urbach energy varies slightly with concentration with maximum for x = 0.
(4) For same reasons explained above, after annealing, E U is affected appreciably for x = 0 and x = 5.For other samples, it is slightly increased keeping the trend of variation similar.

( 1 )
Optical band gap energy in the series of samples xBi 2 O 3 •(40 − x)LiF•60 B 2 O 3 decreases from x = 0 to x = 20 due to increased polarizability and no. of NBOs in the samples.