Epr, optical and other physical studies of Cr3+-doped MgO-BaO-B2O3-TeO2 glasses Текст научной статьи по специальности «Физика»

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ФИЗИКА КОНДЕНСИРОВАННОГО СОСТОЯНИЯ

D0l:10.18721/JPM.10401

UDC 538.95

EPR, OPTICAL AND OTHER PHYSICAL STUDIES OF Cr3+-DOPED MgO-BaO-B2O3-TeO2 GLASSES

M. Samdani1, G. Ramadevudu2, M.N. Chary3, M. Shareefuddin3

'Salalah College of Technology, Salalah, Oman;

2 Vasavi College of Engineering, Hyderabad, India;

3Osmania University, Hyderabad, India

Physical, optical and electron paramagnetic resonance (EPR) studies of 15MgO-15BaO-(59.8 - x)B203-(10.0 + x)TeO2-0.2Cr2O3 (x = 0, 10, 20, 30 mole %) glass samples have been reported. EPR spectra exhibited some resonance signals. The first one (g = 4.80) had a large zero field splitting parameter (D) and E/D < 1/3, it was attributed to isolated Cr3+ centers in strongly distorted octahedral sites; the second one (g = 1.98) was assigned to Cr3+ centers at axially distorted octahedral sites. The resonance signal at g = 4.26 was assigned to Fe3+ ions, which were in the glass matrix as unwanted impurities. The resonance signal at g = 1.91 indicated the Cr3+-Cr3+ exchange coupled pairs. In the optical absorption spectra, the two observed broad optical bands with 16025 cm-1 and 23474 cm-1 were assigned to 4A (F) ^ 4T2g(F) and 4Ag(F) ^ 4T1g(F) transitions, respectively. The site symmetry of Cr3+ is C4v o/C3v. Racah parameters, crystal field (Dq) parameters and energy gap were evaluated from the optical spectra. Various other physical parameters like density, etc., are also reported.

Key words: borotellurite glass; electron paramagnetic resonance (EPR); optical absorption; physical properties

Citation: M. Samdani, G. Ramadevudu, M.N. Chary, M. Shareefuddin, EPR, optical and other physical studies of Cr3+-doped MgO-BaO-B2O3-TeO2 glasses, St. Petersburg Polytechnical State University Journal. Physics and Mathematics. 10 (4) (2017) 7-20. DOI: 10.18721/JPM.10401

ИССЛЕДОВАНИЯ ЭПР, ОПТИЧЕСКИХ И ДРУГИХ ФИЗИЧЕСКИХ СВОЙСТВ СТЕКОЛ СОСТАВА MgO-BaO-B2O3-TeO2 , АКТИВИРОВАННЫХ ИОНАМИ Cr3+

М. Самдани1, Г. Рамадевуду2, М.Н. Чари 3, М. Шарифуддин3

1 Технологический колледж г. Салала, г. Салала, Оман;

2 Инженерный колледж Васави, г. Хайдарабад, Индия;

3Османский университет, г. Хайдарабад, Индия

Представлены исследования электронного парамагнитного резонанса (ЭПР), оптических и других физических свойств стекол системы 15MgO-15BaO-(59,8 - x)B203-(10,0 + x)TeO2-0,2Cr2O3 (x = 0, 10, 20, 30 мол. %). В спектрах ЭПР обнаружено несколько резонансных сигналов. Сигнал с

g = 4,80, высоким значением параметра тонкого расщепления в нулевом магнитном поле (В) и значением отношения Е/В < 1/3 приписан изолированным центрам Сг3+ в сильно искаженных октаэдрических позициях. Сигнал с g = 1,98 отнесен к центрам Сг3+, локализованным в аксиально искаженных октаэдрических позициях. Резонансный сигнал с g-фактором, равным 4,26, идентифицирован как парамагнитное поглощение СВЧ ионами Бе3+, которые присутствуют в матрице стекла как нежелательная примесь. Резонансный сигнал с g = 1,91 указывает на присутствие обменно-связанных пар ионов хрома Сг3+-Сг3+. Спектры оптического поглощения характеризуются двумя широкими полосами на частотах 16025 и 23474 см1; они соотнесены с переходами 4А2 (!) ^ 4Т2(Е) и 4А/—) ^ 4Т1е(!) соответственно. Симметрия позиций ионов Сг3+ — С4у или С3у. По оптическим спектрам оценены параметры Рака, параметр кристаллического поля Вд и величина энергетического зазора. Дополнительно приводятся другие физические параметры стекла, такие как плотность и пр.

Ключевые слова: боротеллуритное стекло; электронный парамагнитный резонанс (ЭПР); оптическое поглощение; физические свойства

Ссылка при цитировании: Самдани М., Рамадевуду Г., Чари М.Н., Шарифуддин М. Исследования ЭПР, оптических и друтих физических свойств стекол состава М§0-Ва0-В203-Те02, активированных ионами Сг3+// Научно-технические ведомости СПбГПУ. Физико-математические науки. 2017. Т. 10. № 4. С. 7-20. БОГ: 10.18721/1РМ.10401

1. Introduction

Currently, a great scientific revolution has been carried out by means of telecommunication, through mobile phones or via the Internet. People are connected to the entire world through the Internet and the world has become a global village. The entire high-speed telecommunication signal transmission basically depends on the optical fiber network. The optical fiber network requires switches and routers. These electronic boxes particularly cannot meet the demands of the communication industry [1] because their linear and nonlinear properties play an important role in determining the features [2, 3]. For example, linear properties like the refractive index of transparent medium change with the intensity of light travelling through it [4]. Thus, the glass industry plays a crucial role in glass fibers, whereas organic materials show very poor mechanical and thermal properties. Finally, an amorphous material like glass is a very good option for high intrinsic transparency, fast response time [5], excellent resistance to atmospheric conditions, mechanical and chemical durability. Hence glass is an important material in the field of optics [6] and optical communication systems.

Boric acid (H3BO3) is one of the important

oxides for the glass formation. It exhibits a variety of structural changes when combined with different alkali and alkaline earth oxides. Linear and nonlinear properties of borate glasses change with the composition of oxides [7]. It is also used as dielectric and insulating medium as a shield against infrared radiation [8]. TeO2-based glasses are used in nonlinear devices due to their property of forming glasses with a higher refractive index (n > 2), good thermal stability and low melting temperature [9]. The desired linear and nonlinear optical susceptibility changes can be obtained by the combination of B2O3 and TeO2 in the glass matrix. Addition of different oxides and transition-metal ions in borotellurite glasses will tend to increase the photoluminescence response.

Transition-metal ion chromium (Cr3+) is used in the fabrication of stainless steel and alloys. This ion has an incomplete 3dn (n < 10) shell, so it can be used as a paramagnetic spin probe. Several researchers [10 — 13] have carried out studies of EPR spectral response and optical studies of Cr3+-doped alkali and alkaline earth oxide borate glasses. To the best of our knowledge, until now no studies in the properties of MgO-BaO-B2O3-TeO2 glasses have been conducted. In this context, we would like to focus on the effect of variation of B2O3 and TeO2 concentrations on various physical, optical and EPR properties of these glasses.

Table 1

Composition of glass samples and their physical parameters

Glass code MgO BaO B2O3 TeO2 C2O3 M Ph VM n

mole % g/mole g/cm3 cm3/mol

MBBTCr-1 15 15 59.8 10 0.2 125.8 3.34 37.67 2.74 0.173

MBBTCr-2 15 15 49.8 20 0.2 129.4 3.70 34.98 2.77 0.152

MBBTCr-3 15 15 39.8 30 0.2 133.0 4.06 32.76 2.78 0.131

MBBTCr-4 15 15 29.8 40 0.2 136.6 4.43 30.84 2.79 0.110

Notations: M (± 0.001) is the average molecular weight; pth (± 0.001) is the theoretical density; VM (± 0.01) is the molar volume; n is the refractive index; Ah is the interaction parameter.

2. Experimental procedure

The melt-quenching technique was used to prepare the glass samples in the composition 15MgO-15BaO-(59.8 - x)B203-(10.0 + x) Te02-0.2Cr203 (x = 0, 10, 20, 30 mole %). Analar grade boric acid (H3B03), tellurium oxide (Te02), magnesium oxide (MgO), barium oxide (Ba0) were used for the preparation of present glasses. These oxides were weighed proportionally in mole % and then transferred to porcelain crucibles and kept in an electrically heated furnace at a temperature of around 1000°C. It took approximately an hour to melt the mixtures congruently. The molten liquid was poured onto a steel plate (maintained at 200 °C) and then pressed with another steel plate. The glasses obtained by this way were transparent and bubble-free. To relieve mechanical stress, the samples were annealed at around 200 °C for 2 hrs. The variations of chemical composition in these glasses are given in Table 1. The amorphous nature of the samples was confirmed by X-ray diffraction. EPR spectra were recorded for dry and perfectly powdered glass samples at room temperature using an EPR spectrometer JE0L FE 1X operating at X-band frequency (9.205 GHz) with a modulation frequency of 100 kHz. The optical absorption spectra were recorded with a UV-VIS-NIR spectrophotome-ter in the wavelength range of 300 - 800 nm at room temperature.

3. Results and discussion

3.1. Density and molar volume. The following empirical equation [14] was used to calculate the theoretical densities of the prepared

glass samples

Ph = 0.53X (M • x,.)/[X (V • x)], (1)

where x. is the mole fraction of different oxides;

i '

M. is the molecular weight of each oxide; V is the packing density parameter of each oxide, calculated using the relation

V= (4/3)n Na(X • J + Y • r3), (2)

where NA is the Avogadro number; rm , ro are (respectively) the ionic radii of metal and oxygen atoms for MX0Y oxide .

The calculated density and molar volume values are presented in Table 1. The density values are reproducible to ±0.02 g/cm3, the molar volume (VM) is calculated using the relation

VM= M / P*

where Mt is the total molecular weight of the multicomponent glass, ph is the density.

Fig. 1 shows changes in the density ph and the molar volume VM with the Te02 content. By substituting Te02 for B203 we observed an increase in density values from 3.34 to 4.43 g/cm3, whereas the molar volume decreased from 37.67 to 30.84 cm3/mol. The changes are attributed to the structural rearrangement of Te02 atoms in the borate glass network. Te02 acts as a network modifier causing the creation of more nonbridging oxygen atoms. Hence packing of molecules becomes denser as the concentration increases, and thereby there is an increase in density [15]. A reduction in the molar volume is due to a decrease in the bond length or interatomic spacing between the atoms.

35 40 XTe02mole%

Fig. 1. Changes in the density and the molar volume of the multicomponent glasses with TeO2 content

3.2. Optical absorption. The spectra of Cr3+ (3d 3) free ion are characterized by two states, the 4P, 4F quartet state (Fig. 2, a) and the 2G, 2H doublet state (Fig. 2, b). The quartet state purely depends on the crystal field, where as the double state does not depend on the crystal field. An octahedral crystal field symmetry (quartet state) exhibits broad spin-allowed absorption bands due to

4A2g(F) - 4T2g(F),

4A7g(F) - 4Tjg(F),

^(F) - 4Tg(P)

transitions.

In a weak field, the ground state represents the t|g orbital (Fig. 3, a). On the other hand, the two spin-forbidden bands are due to

4Alg(F) -2Eg(G),

4A2g(F) - 2 Tjg( G)

transitions (Fig. 3, b).

Fig. 4 shows the optical absorption spectra of glass samples. These spectra exhibited various broad bands. The band at a wave number of 16025 cm1 was assigned to 4A2g(F) - 4T2g(F) spin-allowed transition. Such an assignment argues for the site symmetry of Cr3+ as either C4v or C3v in the glass structure. The other band with the wave number of 23474 cm1 was assigned to the 4A2 (F) - 4T (F) spin-allowed

transition. A low-resolved optical absorption band with a wave number of 28011 cm-1 may be masked by the fundamental absorption edge; it was assigned to the 4A2g(F) ^ 4Tlg(P) transition [16]. The dip in the absorption spectra around 700 nm indicates the presence of the Fano

4 WF) 2 1

___tl e1 - orbital

2 g g

4 a4F) £ - orbital

2g

4 Th (F) , 1 71 ^ ' tl e1 - orbital

- 2 g g

_tl e2 - orbital

- 2g g

WP) b)

2e9(V 2 a4G)

2WG)

Doublet state \

\ X-2Eg(H)

-y 2W">

2W">

Fig. 2. Splitting scheme of energy levels for the 4P, 4F quartet (a) and the 2 G, 2H doublet (b) states in the crystal field of the octahedral symmetry

Cr3+ (3d3)

а)

Ground state ofCr3*

b)

Ground state

of Cr;

4 r^)

•4WF)

4

2 Ыв)

Fig. 3. Spin-allowed (a) and spin-forbidden (b) transitions in the weak crystal field; the former depend on the field, the latter do not

antiresonance [17 — 19].

Optical absorption spectra can be used to obtain additional information such as the structural and bonding nature of Cr3+ ions in the boric oxide from the Racah parameters (B, C and Dq). These parameters can be calculated from the following relations [16]:

Dq = E(4A2g- *T2g) /10; Dq/B = 15(X - 8) / (X2 - 10X),

where X = (E1 - E2) / Dq; (Ag - 4T2) = 10Dq + 4B + 3C;

(3)

(4)

(5)

(6)

h = [(f - B) / BJ/K^

where Dq is the (1/10)-th of the energy of the 4A2g(F) ^ 4 T2g(F) transition, Bfree = 918 cm-1, B is the interelectron repulsion in the J-shell, C is the bonding between Cr3+ ion and its ligand, KCr3+ = 0.21 [20].

The calculated Racah parameter values are in good agreement with Cr3+-containing glasses [21]. The value Dq / B < 2.3 corresponds to the 4A2g(F) ^ 4T2g(F) spin-allowed transition. This transition has had an impact on laser applications and it indicates that Cr3+ centers are mostly localized in the low field sites [22]. The parameter h indicates the nephelauxetic function of ligands. The h parameter indicated an increase with increasing TeO2 content in the glass samples under study. This result points to an increase of ionic-bond nature between Cr3+ and its ligands in the glass matrix.

It is known that the Dq/B value is less than 2.3 in the weak crystal-field sites, greater than 2.3 in the strong ones, and this value is equal to 2.3 in the intermediate fields. In the present work, the Dq /B value was found to be

3

CO

<D О С

си

.Q

О

M

■О <

X= 20 -ч, /-W^V (357 nm) \ I-*=3C|

х=ю\ \ (624 nm) Ч^Д Л ( 700 nm) (426 nm) \ /

200 300 400 500 600 700 800

x=o \

300 400 500 600

Wave length, nm

700

800

Fig. 4. Optical absorption spectra of 15MgO-15BaO--(59.8 - x)B203-(10.0 + x)TeO2 glasses doped with Cr3+

Table 2

Optical absorption spectra identification and some parameters of chromium ions in the glass systems MBBTCr

Transition, parameter Parameter value

MBBTCr-1 MBBTCr-2 MBBTCr-3 MBBTCr-4

4A2g(F) - 2Tlg(P) 1, nm 357 360 357 351

Г1, cm-1 28011 27777 28011 28490

44g(F) - 4 Tig(F) 1, nm 426 430 431 440

1-1, cm-1 23474 23255 23201 22757

4A2g(F) - 4 Tg(F 1, nm 624 630 633 640

1-1, cm-1 16025 15873 15797 15625

h 0.659 0.665 0.665 0.883

Dq/B, cm-1/cm-1 2.03 2.01 2.00 2.09

B, cm1 790 787 789 747

C, cm1 2940 2915 3018 3291

Ip 0.63 0.60 0.58 0.55

Notations: X, X 1 are the wavelength and the wave number for the optical absorption peak positions; Dq/B, B, C are the Racah parameters; h is the nephelauxetic parameter; Ip is the Cr3+ ionic porocity.

Notes: According to Ref. [24], the Racah parameters are the following: Dq/B = 2.65, B = 619 cm1, C = 2233 cm1

around 2.0. This indicated that Cr3+ ions are in the weak ligand field sites. Different calculated Racah parameters are given in Table 2.

By correlating EPR and optical data, the chemical bonding parameter a is evaluated using the following relation [23]:

8а1/Л,

(7)

g0 ge

where ge is the g-factor of a free electron (ge = 2.0023), 1 is the spin-orbit coupling constant (1 = 91 cm-1), A(4A2g(F) ^ 4T2(F)) is the energy gap between the excited and the ground levels. The value g0 = 1.98 was taken.

3.3. Energy gap, refractive index and interaction parameters. In the ultraviolet region, the studies of optical absorption edge from the absorption spectra have revealed various optical transitions such as a direct and an indirect band transitions. It is well-known that Cr3+ ions have three unpaired electrons in the 3 J-shell, hence, in the presence of electromagnetic waves, these unpaired electrons exchange their energies in the valence band and shift to the conduction band. Various anions, involved in the glass composition system as glass-forming oxides, influence the conduction band, even though the significant role of cations should not be

ignored [24].

Fig. 4 shows the optical absorption spectra of 15MgO-15BaO-(59.8 - x)B2O3--(10.0 + x)TeO2-0.2 Cr2O3 glasses. The absorption edges of these glasses are not sharp as seen from the figure. Diffuse absorption edges are characteristic of amorphous nature. The shifting of absorption edges towards the higher wavelength may be due to a decrease in the rigidity of glasses as the TeO2 concentration increases over that of B2O3.

The relationship between the photon energy ha and the optical energy gap Eopt is given by the following relation [25]:

a(a) = [const/(ha)]/(ha - EJr. (8)

The direct and indirect allowed transitions are given by r = 1/2 and r = 2, respectively.

Plotting the graphs for (aha)1/2 versus ha yields direct band gap (Eo ) energies, while plotting the graphs for (aha)2 versus ha yields indirect band gap (E) energies, known as Tauc's plots. Optical band gaps calculated from Tauc's plots are shown in Fig. 5,a. By extrapolating the linear portion of (aha)1/2 and (aha)2 curves at ha = 0 we obtain the direct and indirect band gap values (in eV). The optical

Photon energy, eV

0.09-

0.06-

с<

X {я

J3 <

0.03-

0.00

0.0015

Fig. 5. The Tauc's (a) and the absorption spectrum fitting (ASF) (b) plots of all glass samples

band gap values can also be obtained from the absorption spectrum fitting (ASF) method proposed by Escobar — Alarcon, et al. [26]. Energy band gap values calculated by the ASF

f = 1239.83/ Xg

(9)

method are denoted as and obtained by

extrapolating the linear portion of the (a/X)1/r versus (1/X) curve (Fig. 5, b) at (a/A)1/r = 0.

The value of the band gap E^F in eV can be obtained from the parameter lg using the expression

The direct and indirect band gap values are given in Table 3. The optical band gap values obtained from Tauc's plots are in agreement with those obtained by the ASF method (Fig. 6). The decrease in energy gaps from 1.96 to 1.86 eV for indirect band transitions and from 3.15 to 3.05 eV for direct ones indicates that the structure of the resulting glasses has become less

Table 3

Optical energy gaps and some other parameters obtained from optical and EPR spectra of Cr3+-doped glasses

Glass code E, eV E ,ASF, eV opt 1 R. , a Nx1022, (kg)-1 Xх 10-3, m3(kg)-1

r = 1/2 r = 2 r = 1/2 r = 2

MBBTCr-1 1.96 3.15 1.95 3.13 21.84 0.490 1.35 1.35

MBBTCr-2 1.91 3.10 1.90 3.09 21.31 0.486 5.43 5.45

MBBTCr-3 1.89 3.08 1.87 3.07 20.85 0.480 8.90 8.95

MBBTCr-4 1.86 3.05 1.84 3.03 20.44 0.470 13.22 13.28

Notations: E, E^are the optical energy gaps, obtained using the Tauc's and the absorption spectrum fitting (ASF) plots, relatively; r = 1/2 and r = 2 correspond to the direct and indirect allowed transitions, relatively; R. is the interionic distance; a is the chemical bonding parameter; N is the spin concentration; X is the magnetic susceptibility.

ordered. The increase in TeO2 concentration results in the breaking of regular structure of borotellurite glasses, which leads to a decrease in the energy gap [2]. The decrease may also be due to an increase in disorder in the glass and a further extension of localized states within the gap according to Ref. [27]. It was also observed that the indirect band gap values were larger than the direct ones.

The relation proposed by Dimitrov and Sakka [28] is in the form

(n2 - 1)/(n2 + 2) = 1 - (Eopt / 20)1/2 (10)

and used to calculate the refractive index n.

The molar refraction R (expressed in cm3)

is related to the structure of the glass given by the Lorentz — Lorentz equation

Rm= (n2 — 1)Vm / (n2 + 2), (11) where Vm is the molar volume; the quantity

(n2 — 1)/(n2 + 2) represents the reflection loss.

From relation (10), it is clear that as Eopt decreases, the refractive index n shall obviously increase. As V decreases, n shall increase. However, as the TeO2 content increases in the glass matrix, Te — O — Te bonds break up and nonbridging oxygen is created. Thus, the ionic character of bonding in the glasses increases [7].

1.96-

1.94-

1.92-1

■C

LLTI.90-

1.88-

1.86-

10

T 15

— □ — Egrfrom Tauc's plot —*— Eg from ASF method

20

25

30

T 35

1.96

1.94

-1.92 m

"î—4 >

СГ

1.90 f X

1.88 w*

-1.84

40

X TeO,mole%

Fig. 6. Eg as functions of TeO2 concentration obtained from Tauc's and ASF plots for glasses with 0.2 mol% of Cr3+ ions

The theoretical interaction parameter Ath was calculated using the following relation:

Ath XMgOAMgO + XbaOABaO + + XB2O3 AB2O3+ XTeO2 ATeO2 +

+ X A '

Cr2O3 Cr2O3'

(12)

their values along with R. are presented in Table 3.

Average oxide ion polarizability of the ion is described by Ath, and indicates the interatomic interaction.

3.4. Electron paramagnetic resonance (EPR). In the glass matrix, Cr3+ ions may exist as isolated Cr3+ ions and exchange-coupled Cr3+-Cr3+ ion pairs [24]. The ground state of a Cr3+ free ion is 4F. It belongs to the 3d3 electronic configuration. In the absence of a magnetic field, spin-orbit coupling splits one state level into the Kramer's doublet (|±1/2> and |±3/2>) separated by 2D energy distance, where D is the zero field splitting parameter. In the presence of the external magnetic field, the spin-orbit coupling splits into various transitions like

|—3/2 > ^ |—1/2 >, |—1/2 > ^ |1/2 >, |1/2 > ^ |3/2 >

at gPB — 2D, gPB, gPB + 2D, respectively. In all the transitions the maximum separation is 4D [29].

4Ag is the ground state level of Cr3+ ion in an octahedral crystal field. In this field 4F state splits into the 4A2g singlet, 4T1g and 4T2g orbital triplets [24]. A large separation in spin-orbit transition leads to two resonance signals at g « 2 — 5 [30 — 33]. So far EPR spectra of Cr3+ ion-doped borate glasses [34] and borotellurite glasses [35, 36] were reported. Spin-Hamil-tonian parameters of the 15MgO-15BaO--(59.8 — x)B2O3-(10.0 + x)TeO2-0.2Cr2O3 glass systems under consideration were calculated by the following relation [22]:

H = ^(BgS) + D{S2 - [S(S + 1)/3]} + E(S2 -s2),

(13)

where |B is the Bohr magneton; B is the magnetic field; D, E are the fine structure constants for axial and rhombic fields, respectively.

EPR spectra of the glass samples doped with chromium are shown in Fig. 7.

W -Q CD

<D "O

g=4.80

g= 1.91

x= 10

x= 0

1000 2000 3000 4000 5000

Magnetic field, Gs

6000

7000

Fig. 7. EPR spectra of 15MgO-15BaO--(59.8 — x)B2O3-(10.0 + x)TeO2-0.2 Cr2O3 glasses for various x at room temperature

The resonance signal at g = 4.80 assigned to Cr3+(I) indicated isolated Cr3+ centers in strongly distorted octahedral sites. This resonance signal exhibited a large zero field splitting paramer D; E > g'^BB, and E/D < 1/3 [30, 31]. The resonance signal at g = 1.98 assigned to Cr3+(II) indicated isolated Cr3+ centers in axially-distorted octahedral sites. This resonance signal also has a large value of D and E/D << 1/3. The resonance signal at g = 1.91 indicates Cr3+-Cr3+ exchange-coupled pairs [13]. Apart from these resonance signals, one more resonance signal at g = 4.26 was observed and assigned to Fe3+ ions, which were unwanted impurities and can be also present in the undoped glasses [20, 37]. The values of g obtained for the glass compositions in the present study are in tune with other glass systems reported in the literature [24, 38].

3.5. Number of spins and susceptibility. The variations in the number of spins N and the susceptibility x with TeO2 content are shown in Fig. 8 and the values are given in Table 3. The number N of spins participating in the resonance at g = 1.98 was obtained from the formula given by Weil, et al. [39]:

N = (AJAJ • [(Scan )2/(ScanJ2] x

x (Gd /C) [(Hm)d/ №J [(sjyis)2] x ^

x {[ S(S + 1)] sd/[S(S + 1)] x} x (14)

x [(PJ1/2/(Px)1/2] • St),

where A is the area under the absorption curve, which can be obtained by double integrating the first derivative of the EPR absorption curve; Scan is the magnetic field corresponding to a unit length of the chart; G is the gain; Hm is the width of the modulation field; g is the g-factor; S is the spin of the system in its ground state; P is the power of the microwave source. The subscripts x and std represent the corresponding quantities for the glass and the standart samples (CuSO4 5H2O), respectively.

The susceptibility x of the sample can be calculated using the formula [40]:

X = W/(/ + 1)/(3kB7),

(15)

where N is the number of spins per m3; the other symbols have their usual meaning.

N can be calculated from Eq. (14) while calculating the x values; g = 1.98 was taken. In borate glasses the addition of TeO2 content results in the increase of open network, thus the number of nonbridging oxygens increases in the glass network, and this effect weakens the O2- bonds for every Cr3+ ions. Hence, the interatomic distance between Cr3+ ions decreases and so the number of spins participating in the resonance increases.

4. Conclusions

On the basis of EPR and optical absorption studies on 15MgO-15BaO-(59.8 - x)B2O3--(10.0 + x)TeO2-0.2 Cr2O3 the following conclusions can be made.

i. Optical absorption spectra showed two broad bands (16025 and 23474 cm-1) due to spin-allowed transitions that were assigned to 44g(F) ^ 4 Tg(F) and to 4A2g(F) ^ 4Tlg(F) transitions. The 4A2g(F) ^ 4T2g(F) transition revealed that the site symmetry of Cr3+ was either C or C . The unresolved broad band

4v 3v

corresponding to 28011 cm-1 was assigned to the 4A2g(F) ^ T1g(P) transition.

ii. The value of Dq/B around 2.0 (which is less than 2.3) indicated that Cr3+ ions were

14-

12-

10-

S

<D Q. 8-

Ri

О

X 6-

г

4-

2-

0-

—I— 10

~~i— 15

—I—

20

—I—

25

—I—

30

—I—

35

—I— 40

X TeO mole%

Fig. 8. The concentration of spins N for the resonance signal at g « 1.98 as a function of TeO2 concentration

in the weak-field ligand sites. The increase in nephelauxetic (h) values indicated the increasing ionic nature between Cr3+ and its ligands in the glass.

iii. The decrease of the energy gap from 1.96 to 1.86 eV for indirect band transitions and from 3.15 to 3.05 eV for direct band transitions was attributed to the structure of the resulting glasses becoming less ordered. The increase in TeO2 concentration results in the breaking of the regular structure of borotellurite glasses and, hence, is responsible for the decrease in the energy gap.

iv. The resonance signal at g = 4.80 was attributed to isolated Cr3+ centers in strongly distorted octahedral sites. The resonance signal at g = 1.98 was assigned to Cr3+ centers at axially distorted octahedral sites.

v. In addition to this, two more resonance signals at g = 4.26 and g = 1.91 were also observed. The resonance signal at g = 4.26 was assigned to the presence of unwanted impurity Fe3+ ions, and the resonance signal at g = 1.91 indicated the Cr3+-Cr3+ exchangecoupled pairs.

REFERENCES

[1] G.A. Thomas, D.A. Ackerman, P.R. Prucnal, S.L. Cooper, Physics in the whirlwind of optical communications, Physics Today. 53 (9) (2000) 3036.

[2] D. Cotter, R.J. Manning, K.J. Blow, A.D.

Ellis, A.E. Kelly, D. Nesset, I.D. Phillips, A.J. Poustie, D.C. Rogers, Nonlinear optics for highspeed digital information processing, Science. 286 (5444)(1999) 1523-1528.

[3] M. Yamane, Y. Asahara, Glasses for

photonics. Cambridge University Press, Cambridge, 2000.

[4] R.L. Sutherland, Handbook of nonlinear optics, Dekker, New York, 1996.

[5] M.E. Lines, Oxide glasses for fast photonic switching: A comparative study, Journal of Applied Physics. 69 (10) (1991) 6876-6884.

[6] A.J. Marker, N. Neuroth, The properties of optical glass, Ed. by H. Bach, N. Neuroth. SpringerVerlag, Berlin, 1995.

[7] P. Becker, Borate materials in nonlinear optics, Advanced Materials. 10 (1998) 979.

[8] D.L. Griscom, Borate glasses, in: Mater. Sci. Res. Vol. 12, Ed. by L.D. Pye, V.D. Frechette, N.J. Kreidle, Plenum Press, New York, 1978, 36 p.

[9] R.A.H. El-Mallawany, Tellurite glasses handbook. Physical properties and data. CRC Press, Boca Raton, 2002.

[10] F. Rasheed, K.P. O'Donnell, B. Henderson, D. Hollis, Disorder and the optical spectroscopy of Cr3+-doped glasses. II. Glasses with high and low ligand fields, J. Phys.: Cond. Matter. 3 (21) (1991) 3825.

[11] B. Henderson, M. Yamaga, Y. Gao, K.P. O'Donnell, Disorder and nonradiative decay of Cr3+-doped glasses, Phys. Rev. B. 46 (2) (1992) 652.

[12] S.M. Kaczmarek, Li2B4O7 glasses doped with Cr, Co, Eu and Dy, Opt. Mater. 19 (1) (2002) 189.

[13] R.V.V.S.N. Ravikumar, K. Kayalvizhi, A.V. Chandrasekhar, Y.P. Reddy, J. Yamauchi,

K. Arunakumari, P.S. Rao, Strontium tetraborate glasses doped with transition metal ions: EPR and optical absorption study, Appl. Magn. Reson. 33 (1) (2008) 185.

[14] S. Inaba, S. Fujino, Empirical equation for calculating the density of oxide glasses, J. Am. Ceram. Soc. 93 (1) (2010) 217.

[15] B. Edlen, The refractive index of air, Metrologia. 2 (2) (1966) 71.

[16] B. Henderson, G.F. Imbush, Optical spectroscopy of inorganic solids, Clarendon Press, Oxford, 1989.

[17] U. Fano, Effects of configuration interaction on intensities and phase shifts, Phys. Rev. 124 (1961) 1866.

[18] M.D. Sturge, J. Guggenheim, M.H.L. Pryle, Antiresonance in the optical spectra of transition-metal ions in crystals, Phys. Rev. B. 2 (7) (1970) 2459.

[19] S.A. Payne, L.L. Chase, W.F. Kropke, Optical properties of Cr3+ in fluorite-structure hosts and in MgF2, J. Chem. Phys. 86 (6) (1987) 3455.

[20] V. Ramesh Kumar, J. Lakshmana Rao, N.O. Gopal, EPR and optical absorption studies of Cr3+ ions in alkaline earth alumino-borate glasses, J.

Mater. Sci. 41 (7) (2006) 2045.

[21] R.V.S.S.N. Ravikumar, R. Komatsu, K. Ikeda, A.V. Chandrasekhar, B.J. Reddy, Y.P. Reddy, P.S. Rao, EPR and optical studies on transition metal doped LiRbB4O7 glasses, Journal of Phys. and Chem. of Sol. 64 (2) (2003) 261-264.

[22] B.V. Padlyak, W. Ryba-Romanowski, R. Lisiecki, V.T. Adamiv, Ya.V. Burak, I.M. Teslyuk, Synthesis, EPR and optical spectroscopy of the Cr-doped tetraborate glasses, Optical Materials. 34 (12) (2012) 2112-2119.

[23] M. Haouari, H. Ben Ouada, H. Maaref,

H. Hommel, A.P. Legrand, Optical absorption and electron paramagnetic resonance study of Cr3+-doped phosphate glasses, J. Phys.: Cond. Matter. 9 (31) (1997) 6711.

[24] C.R. Kesavulu, R.P.S. Chakradhar, C.K. Jayasankar, J. Lakshmana Rao, EPR, optical, photoluminescence studies of Cr3+ ions in Li2O-Cs2O-B2O3 glasses - An evidence of mixed alkali effect, Journal of Molecular Structure. 975 (1-3) (2010) 93-99.

[25] E.A. Davis, N.F. Mott, Conduction in non-crystalline systems V. Conductivity, optical absorption and photoconductivity in amorphous semiconductors, Philos. Mag. 22 (179) (1970) 903.

[26] L.E. Alarcon, A. Arrieta, E. Camps, S. Muhl, S. Rudil, E.V. Santiago, An alternative procedure for the determination of the optical band gap and thickness of amorphous carbon nitride thin films, Applied Surface Science. 254 (1) (2007) 412-415.

[27] N.F. Mott, E.A. Davis, Electronic processes in non-crystalline materials, Oxford University Press, London, 1971, p. 293.

[28] V. Dimitrov, S. Sakka, Electronic oxide polarizability and optical basicity of simple oxides.

I, J. Appl. Phys. 79 (3) (1996) 1736-1740.

[29] S. Lakshmi Reddy, T. Endo and G. Siva Reddy, Electronic (absorption) spectra of 3d transition metal complexes, Advanced Aspects of Spectroscopy, ed. by Muhammad Akhyar Farrukh, InTech, 2012.

[30] B.V. Padlyak, A. Gutsze, EPR study of the impurity paramagnetic centres in (CaO-Ga2O3-

GeO2) glasses, Appl. Magn. Reson. 14 (1) (1998) 59. 2

[31] R.J. Landry, G.T. Fournier, C.G. Young,

Electron spin resonance and optical absorption studies of Cr3+ in the phosphate glass, J. Chem. Phys. 46 (4) (1967) 1285.

[32] D.L. Griscom, Characterization of three ¿"-center variants in X- and y-irradiated high purity a-SiO2, J. Non-Cryst. Solids. 40 (2-3) (1980) 21.

[323] G. Little Flower, M. Srinivasa Reddy, G. Sahaya Baskaran, N. Veeraiah, The structural influence of chromium ions in lead gallium

phosphate glasses by means of spectroscopic studies, Opt. Mater. 30 (3) (2007) 357.

[34] C. Leign, J.Y. Buzare, J. Emery, C. Jacoboni, Electron paramagnetic resonance determination of the local field distribution acting on Cr3+ and Fe3+ in transition metal fluoride glasses (TMFG), J. Phys. Condens. Matter. 7 (20) (1995) 3853.

[35] R.P. Sreekanth Chakradhar, J. Lakshmana Rao, G. Sivaramaiah, N.O. Gopal, Chromium ions in alkali lead borotellurite glasses — An EPR and optical study, Phys. Status Solidi (b). 242 (14) (2005) 2919.

[36] I. Ardelean, S. Filip, EPR and magnetic investigations of chromium ions in TeO2 based glasses, J. Opt. Adv. Mater. 7 (2) (2005) 745.

Received 09.04.2016, accepted 01.10.2017.

[37] M. Casalboni, V. Ciafardone, G. Giuli, B. Izzi, E. Paris, P. Prosposito, An optical study of silicate glass containing Cr3+ and Cr6+ ions, J. Phys. Condens. Matter. 8 (1996) 9059.

[38] J. Santhan Kumar, J. Lakshmi Kumar, M. Subba Rao, Sandhya Cole, An optical study of silicate glass containing and ions, Optical Materials. 35 (2013) 1320-1326.

[39] J.A. Weil, J.R. Bolton, J.E. Wertz, Electron paramagnetic resonance: elementary theory and practical applications, John Wiley, New York, 1994.

[40] N.W. Ashcroft, N.D. Mermin, Solid state physics, Harcourt College Publishers, New York, 2001.

THE AUTHORS

SAMDANI Mohammed

Salalah College of Technology

Thumrait Rd, Thumrayt St, Salalah 211, Oman

samdanimohd82@gmail.com

RAMADEVUDU Gokarakonda

Vasavi College of Engineering

9-5-81, Ibrahim Bagh, Hyderabad, Telangana 500031, India dr.ramdev@gmail.com

CHARY M. Narasimha

Osmania University

Osmania University, Main Rd, Amberpet, Hyderabad, Telangana 500007, India mnchary_phy@rediffmail.com

SHAREEFUDDIN Mohammed

Osmania University

Osmania University, Main Rd, Amberpet, Hyderabad, Telangana 500007, India shareef1775@gmail.com

СПИСОК ЛИТЕРАТУРЫ

1. Thomas G.A., Ackerman D.A., Prucnal P.R., Cooper S.L. Physics in the whirlwind of optical communications // Physics Today. 2000. Vol. 53. No. 9. P. 3036.

2. Cotter D., Manning R.J., Blow K.J., Ellis A.D., Kelly A.E., Nesset D., Phillips I.D., Poustie A.J., Rogers D.C. Nonlinear optics for high-speed digital information processing // Science. 1999. Vol. 286. No. 5444. Pp. 1523-1528.

3. Yamane M., Asahara Y. Glasses for photonics. Cambridge: Cambridge University Press, 2000.

4. Sutherland R.L. Handbook of nonlinear optics. New York: Dekker, 1996.

5. Lines M.E. Oxide glasses for fast photonic switching: A comparative study // Journal of Applied

Physics. 1991. Vol. 69. No. 10. Pp. 6876-6884.

6. Marker A.J., Neuroth N. The properties of optical glass. Ed. by Bach H., Neuroth N. Berlin: Springer-Verlag, 1995.

7. Becker P. Borate materials in nonlinear optics // Advanced Materials. 1998. Vol. 10. P. 979.

8. Griscom D.L. Borate glasses, in: Mater. Sci. Res. Vol. 12. Ed. by Pye L.D., Frechette V.D., Kreidle N.J. New York: Plenum Press, 1978. 36 p.

9. El-Mallawany R.A.H. Tellurite glasses handbook. Physical properties and data. Boca Raton: CRC Press, 2002.

10. Rasheed F., O'Donnell K.P., Henderson B., Hollis D. Disorder and the optical spectroscopy of Cr3+-doped glasses. II. Glasses with high and low

ligand fields // J. Phys.: Cond. Matter. 1991. Vol. 3. No. 21. P. 3825.

11. Henderson B., Yamaga M., Gao Y., O'Donnell K.P. Disorder and nonradiative decay of Cr 3+-doped glasses // Phys. Rev. B. 1992. Vol. 46. No. 2. P. 652.

12. Kaczmarek S.M. Li2B4O7 glasses doped with Cr, Co, Eu and Dy// Opt2. M4 a7ter. 2002. Vol. 19. No. 1. P. 189.

13. Ravikumar R.V.V.S.N., Kayalvizhi K., Chandrasekhar A.V., Reddy Y.P., Yamauchi J., Arunakumari K., Rao P.S. Strontium tetraborate glasses doped with transition metal ions: EPR and optical absorption study // Appl. Magn. Reson. 2008. Vol. 33. No. 1. P. 185.

14. Inaba S., Fujino S. Empirical equation for calculating the density of oxide glasses // J. Am. Ceram. Soc. 2010. Vol. 93. No. 1. P. 217.

15. Edlen B. The refractive index of air // Metrologia. 1966. Vol. 2. No. 2. P. 71.

16. Henderson B., Imbush G.F. Optical spectroscopy of inorganic solids. Oxford: Clarendon Press, 1989.

17. Fano U. Effects of configuration interaction on intensities and phase shifts // Phys. Rev. 1961. Vol. 124. P. 1866.

18. Sturge M.D., Guggenheim J., Pryle M.H.L. Antiresonance in the optical spectra of transition-metal ions in crystals // Phys. Rev. B. 1970. Vol. 2. No. 7. P. 2459.

19. Payne S.A., Chase L.L., Kropke W.F. Optical properties of Cr3+ in fluorite-structure hosts and in MgF2 // J. Chem. Phys. 1987. Vol. 86. No. 6. P. 3455.

20. Ramesh Kumar V., Lakshmana Rao J., Gopal N.O. EPR and optical absorption studies of Cr3+ ions in alkaline earth alumino-borate glasses // J. Mater. Sci. 2006. Vol. 41. No. 7. P. 2045.

21. Ravikumar R.V.S.S.N., Komatsu R., Ikeda K., Chandrasekhar A.V., Reddy B.J., Reddy Y.P., Rao P.S. EPR and optical studies on transition metal doped LiRbB4O7 glasses // Journal of Phys. and Chem. of Sol. 2003. Vol. 64. No. 2. Pp. 261-264.

22. Padlyak B.V., Ryba-Romanowski W., Lisiecki R., Adamiv V.T., Burak Ya.V., Teslyuk I.M. Synthesis, EPR and optical spectroscopy of the Cr-doped tetraborate glasses // Optical Materials. 2012. Vol. 34. No. 12. Pp. 2112-2119.

23. Haouari M., Ben Ouada H., Maaref H., Hommel H., Legrand A.P. Optical absorption and electron paramagnetic resonance study of Cr3+-doped phosphate glasses // J. Phys.: Cond. Matter. 1997. Vol. 9. No. 31. P. 6711.

24. Kesavulu C.R., Chakradhar R.P.S., Jayasankar C.K., Lakshmana Rao J. EPR, optical,

photoluminescence studies of Cr3+ ions in Li2O-Cs2O-B2O3 glasses - An evidence of mixed alkali effect // Journal of Molecular Structure. 2010. Vol. 975. No. 1-3. Pp. 93-99.

25. Davis E.A., Mott N.F. Conduction in non-crystalline systems V. Conductivity, optical absorption and photoconductivity in amorphous semiconductors // Philos. Mag. 1970. Vol. 22. No. 179. P. 903.

26. Alarcon L.E., Arrieta A., Camps E., Muhl S., Rudil S., Santiago E.V. An alternative procedure for the determination of the optical band gap and thickness of amorphous carbon nitride thin films // Applied Surface Science. 2007. Vol. 254. No. 1. Pp. 412-415.

27. Mott N.F., Davis E.A. Electronic processes in non-crystalline materials. London: Oxford University Press, 1971. 293 p.

28. Dimitrov V., Sakka S. Electronic oxide polarizability and optical basicity of simple oxides. I. // J. Appl. Phys. 1996. Vol. 79. No. 3. Pp. 1736-1740.

29. Lakshmi Reddy S., Endo T., Siva Reddy G. Electronic (absorption) spectra of 3d transition metal complexes // Advanced Aspects of Spectroscopy. Ed. by M.A. Farrukh. InTech. 2012.

30. Padlyak B.V., Gutsze A. EPR study of the impurity paramagnetic centres in (CaO-Ga2O3-GeO2) glasses // Appl. Magn. Reson. 1998. Vol. 14. No. 1. P. 59.

31. Landry R.J., Fournier G.T., Young C.G. Electron spin resonance and optical absorption studies of Cr3+ in a phosphate glass // J. Chem. Phys. 1967. Vol. 46. No. 4. P. 1285.

32. Griscom D.L. Characterization of three E '-center variants in X- and y-irradiated high purity a-SiO2 // J. Non-Cryst. Solids. 1980. Vol. 40. No. 2-3. P. 21.

33. Little Flower G., Srinivasa Reddy M., Sahaya Baskaran G., Veeraiah N. The structural influence of chromium ions in lead gallium phosphate glasses by means of spectroscopic studies // Opt. Mater. 2007. Vol. 30. No. 3. P. 357.

34. Leign C., Buzare J.Y., Emery J., Jacoboni C. Electron paramagnetic resonance determination of the local field distribution acting on Cr3+ and Fe3+ in transition metal fluoride glasses (TMFG) // J. Phys. Condens. Matter. 1995. Vol. 7. No. 20. P. 3853.

35. Sreekanth Chakradhar R.P., Lakshmana Rao J., Sivaramaiah G., Gopal N.O. Chromium ions in alkali lead borotellurite glasses - An EPR and optical study // Phys. Status Solidi (b). 2005. Vol. 242. No. 14. P. 2919.

36. Ardelean I., Filip S. EPR and magnetic investigations of chromium ions in TeO2 based glasses

// J. Opt. Adv. Mater. 2005. Vol. 7. No. 2. P. 745.

37. Casalboni M., Ciafardone V., Giuli G., Izzi B., Paris E., Prosposito P. An optical study of silicate glass containing Cr3+ and Cr6+ ions // J. Phys. Condens. Matter. 1996. Vol. 8. P. 9059.

38. Santhan Kumar J., Lakshmi Kumar J., Subba Rao M., Cole S. An optical study of silicate glass containing and ions // Optical Materials. 2013.

Vol. 35. Pp. 1320-1326.

39. Weil J.A., Bolton J.R., Wertz J.E. Electron paramagnetic resonance: elementary theory and practical applications. New York: John Wiley, 1994.

40. Ashcroft N.W., Mermin N.D. Solid state physics. New York: Harcourt College Publishers, 2001.

Статья поступила в редакцию 09.04.2017, принята к публикации 01.10.2017.

СВЕДЕНИЯ ОБ АВТОРАХ

САМДАНИ Мохаммед — сотрудник кафедры инженерных наук Технологического колледжа, г. Салала, Оман.

Thumrait Rd, Thumrayt St, Salalah 211, Oman samdanimohd82@gmail.com

РАМАДЕВУДУ Гокараконда — сотрудник кафедры физики Инженерного колледжа Васави, г. Хайдарабад, Индия.

9-5-81, Ibrahim Bagh, Hyderabad, Telangana 500031, India dr.ramdev@gmail.com

ЧАРИ М. Нарасимха — сотрудник кафедры физики Османского университета, г. Хайдарабад, Индия.

Osmania University, Main Rd, Amberpet, Hyderabad, Telangana 500007, India mnchary_phy@rediffmail .com

ШАРИФУДДИН Мохаммед — сотрудник кафедры физики Османского университета, г. Хайдарабад, Индия.

Osmania University, Main Rd, Amberpet, Hyderabad, Telangana 500007, India shareef1775@gmail.com

© Санкт-Петербургский политехнический университет Петра Великого, 2017