Electron spin resonance and optical absorption spectroscopic studies of Cu2+ ions in aluminium lead borate glasses

Journal of Alloys and Compounds 551 (2013) 399–404

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Electron spin resonance and optical absorption spectroscopic studies of Cu2+ ions in aluminium lead borate glasses G. SivaRamaiah a,1, J. LakshmanaRao b,⇑ a b

Department of Physics, Government College for Men, Kadapa 516004, India Department of Physics, Sri Venkateswara University, Tirupati 517502, India

a r t i c l e

i n f o

Article history: Received 2 September 2012 Accepted 6 October 2012 Available online 13 October 2012 Keywords: Amorphous materials Optical materials Optical properties Thermodynamic properties Electron paramagnetic resonance Optical spectroscopy

a b s t r a c t Electron Spin Resonance and optical absorption spectral studies of Cu2+ ions in 5 Al2O3 + 75 B2O3 + (20-z) PbO + z CuO (where z = 0.1–1.5 mol.% of CuO) glasses have been reported. The EPR spectra of all the glasses show resonance signals characteristic of Cu2+ ions at both room and low temperatures. The number of spins and Gibbs energy were calculated at different concentrations and temperatures. From the plot of the ratio of logarithmic number of spins and absolute temperature and the reciprocal of absolute temperature, the entropy and enthalpy have been evaluated. The optical absorption spectra of all the glasses exhibit three bands and these bands have been assigned to 2B1g ? 2Eg, 2B1g ? 2B2g, and 2 B1g ? 2A1g transitions in the decreasing order of energy. It is for the first time to observe three optical absorption bands for Cu2+ ions in oxide glasses. Such type of results is due to excellent sample preparation. From the EPR and optical absorption spectroscopies data, the molecular orbital coefficients have been evaluated. Ó 2012 Published by Elsevier B.V.

1. Introduction In recent years glasses doped with transition metal ions have attracted a great deal of attention because of their significant applications in the development of new tunable solid-state lasers, solar energy converters and fiber-optic communication devices [1]. B2O3 is often used as dielectric and insulating material and it is known that it is a good shield for infrared radiation. The oxide glasses were studied by various authors using EPR and different analytical techniques [2–5]. Glasses prepared with heavy metal oxides such as PbO are interesting because their properties are exploited in many applications like wave-guides in non-linear optics. These are used as radiation shielding windows, scintillation counters, optical transmission devices. The glasses with PbO have stability and give good optical results. The lead aluminate glasses have been little studied [6]. Therefore the authors selected aluminium lead boate glasses as host materials for Cu2+ ions dopants. It has been shown that the addition of Al2O3 improves the glass forming tendency and aqueous durability. The Al2O3 increases the coefficient of thermal expansion and reduces the glass transition temperature (Tg) and the molar volume. Brow and Tallant [7] showed that tetrahedral aluminium sites substitute for the tetrahedral boron species to form modified (AlB2O6)1 metaborate ⇑ Corresponding author. Tel.: +91 877 2249666x272 E-mail addresses: [email protected] (G. SivaRamaiah), [email protected] (J. LakshmanaRao). 1 Tel./fax: +91 8562 244422 0925-8388/$ - see front matter Ó 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jallcom.2012.10.023

chains. However, the cations with large electrostatic field strength, such as Cu2+, stabilize the formation of more highly coordinated (five or six) aluminium. It was previously discussed that the addition of alumina in the glass composition improves the optical response of the material as a results of structural changes involving the TM based clusters [8]. The addition of an extra cation to the glass network exerts an influence on the glass structure because it directly influences the cross-linking between polyhedral constituting the three dimensional network. In particular, the ratio between oxygen linked to one or two network former cation, defined respectively as non bridging oxygen (NBO) and bridging oxygen (BO), changes as a function of the species introduced in the glass matrix. The aluminium acting as a network former promotes the NBO conversion into BO species, the different electron donating abilities of oxygen ions between Pb–NBO and Pb–O–Pb or Pb–O–Al species is the main reason of the change in optical parameters [8]. In this study, the structural evolution induced by the alumina addition has been studied and the modifications occurring in the aluminium and the Cu2+ site have been analyzed in order to understand the structural related optical properties [8]. The purpose of this study is to evaluate the thermodynamic properties such as the Gibbs energy, activation energy, entropy and enthalpy of Cu2+ ions in aluminium lead borate glasses. It is proposed to study the magnetic properties of Cu2+ ions in glasses. Recently SivaRamaiah and Rao [9] studied the Electron Spin Resonance and optical absorption spectroscopic study of manganese

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centers in aluminium lead boarate glasses. To the best of our knowledge, there are no previous reports on thermodynamic and magnetic properties of Cu2+ ions in this glass system. Therefore the authors systematically studied the EPR and optical absorption studies of Cu2+ ions in aluminium lead borate glasses. We are also interested in studying the spin-Hamiltonian parameters and also the site symmetry around Cu2+ ions in these glasses. The temperature dependence and composition on EPR signals have also been undertaken in order to know how the intensity of resonance signals vary with temperature and concentration. 2. Experimental procedure 2.1. Glass preparation The glasses were prepared using melt quenching procedure. The starting materials used for the preparation of glasses were good quality analar Al2O3, B2O3, PbO and CuO with greater than 99% purity. The chemicals were weighed accurately according to their molecular weights (mol.%, as shown in equation in the abstract) in an electrical balance. These were mixed thoroughly and ground to fine powder with an agate mortar for nearly 30 min. The EPR and optical absorption signals should be good if more grinding could be performed on the samples. If grinding is more then paramagnetic interactions are more. The batches were taken in porcelain crucibles and melted in an electric furnace at 1174 K for 3 h. If the batches are placed in electric furnace for longer time, the EPR and optical absorption signals could be excellent. No impurities could be added to the batches, if porcelain crucibles were used. These are cheaper when compared with platinum crucibles. The melts were then poured on a brass plate and pressed quickly with another brass plate. The glasses thus obtained were transparent and bluish in color. The blue color was due to the entering of Cu2+ ions in the glasses. Care should be taken to obtain glasses of uniform thickness (approximately 1 mm) for recording optical absorption spectra. The glasses were annealed at 330 K for several hours to remove the thermal strains. Good quality glasses obtained after polishing were used for optical measurements.

2.2. ESR measurements The ESR spectra were recorded at room temperature for all glasses and at different low temperatures for one glass (APB 0.1 Cu) on a JEOL FE1X EPR spectrometer. The ESR spectrometer was operated at the X-band frequency with a modulation frequency of 100 kHz. The microwave frequency was kept at 9.205 GHz. The magnetic field was scanned from 220 to 420mT with a scan speed of 25mT/min. Powdered glass specimen of 100 mg was taken in a quartz tube for ESR measurements. The ESR spectrum of the CuSO45H2O powdered material was also recorded at 9.205 GHz as a reference to calculate the absolute number of spins. The EPR spectrum of aluminium lead borate glass doped with 0.1 mol.% of CuO was recorded at different temperatures (123–295 K) using a variable temperature controller (JES UCT 2AX). A temperature stability of ±1 K was easily obtained by waiting approximately half an hour at the set temperature before recording the spectrum, at each temperature.

For APB 0.1 Cu glass, the EPR spectrum is characterized by four perpendicular components. For APB 0.5 Cu glass, the EPR spectrum is characterized by two parallel components and two perpendicular components. For APB 1 Cu glass, the EPR spectrum is characterized by three parallel components and one perpendicular component. The third parallel component is merged with the perpendicular component. For APB 1.5 Cu glass, the EPR spectrum is characterized by three parallel components and one perpendicular component. The third parallel component is merged with the perpendicular component. The perpendicular components are not well resolved in all the EPR spectra except for APB 0.1 Cu glass. All the observed ESR spectra are similar to the ESR spectra reported in the literature [11–19]. A regular octahedral site may not exist for Cu2+ center because the cubic symmetry is disturbed by an electronic hole in the degenerate dx2 y2 orbital and therefore this produces a tetragonal distortion.

3.2. ESR studies at low temperatures Fig. 2 shows the EPR spectra recorded at low temperatures (123–273 K) and at room temperature 295 K for APB 0.1 Cu glass. The spectrum is characterized by two parallel components at the low magnetic field region and four perpendicular components at the high magnetic field region. This ESR spectrum was recorded at different experimental conditions when compared with the experimental conditions for the spectra recorded at different concentrations. From the above spectra, it is observed that the intensity of the spectrum decreases with the increase of temperature from 123 to 295 K. This is due to the decrease of population difference between Zeeman levels with the increase of temperature from 123 to 295 K. This is the phenomenon that can be expected from the Boltzmann law. The spin-Hamiltonian parameters such as the g and A values are invariant with the temperature variation from 123 to 295 K. This indicates that the structural environment between the transition metal ion and the glass is invariant with the temperature variation from 123 to 295 K. The linewidth is constant for the parallel and

2.3. Optical absorption measurements The optical absorption spectra of all the glass samples doped with Cu2+ ionswere recorded at room temperature on a JASCO UV–VIS–NIR spectrophotometer (model V-570) in the wavelength region from 200 to 2000 nm. The band position is measured digitally and the accuracy of the measurements is ±1 nm.

3. Results and discussion 3.1. ESR studies at room temperature The ESR signals are not observed in the spectra of undoped glasses. This clearly indicates that there are no paramagnetic impurities in the starting materials. When Cu2+ ions are added to the glasses as dopants, all the samples under investigation exhibit resonance signals characteristic of Cu2+ ions in aluminium lead borate glasses. Fig. 1 shows the EPR spectra recorded at room temperature for APB 0.1 Cu, APB 0.5 Cu, APB 1 Cu and APB 1.5 Cu glasses (APB is aluminium lead borate and Cu is mol.% of Cu2+ ions). The EPR spectra exhibit hyperfine lines which are characteristic of Cu2+ ions in axially distorted symmetric sites [10].

Fig. 1. The room temperature electron paramagnetic resonance spectra of 0.1, 0.5, 1, and 1.5 mol.% of Cu2+ ions doped in aluminium lead borate glasses.

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the spin of the ions in the ground state. The subscripts ‘x’ and ‘std’ denote the sample and the standard respectively. Nstd is number of spins in 100 mg of copper sulphate standard. The known number of spins is called as number of spins for the standard. The population difference between the Zeeman levels (N) for the Cu2+ center at g = 2.14 in APB 0.1 Cu glass at room temperature has been calculated to be 50  1017 spin/g (Table 1). This value is of the same order as calculated for Cu2+ center in alkali lead tetraborate glasses [13]. It is observed that the number of spins increases from 50  1017 to 106  1017 spin/g with the decrease of temperature from 295 to 123 K. The number of spins were calculated for APB 0.1 Cu, APB 0.5 Cu, APB 1 Cu, APB 1.5 Cu glasses and are presented in Table 1. It is observed from Table 1 that the number of spins increases with the increase of concentration of Cu2+ ions. The increase in number of spins with the concentration of Cu2+ ions is due to the increase of unpaired electrons. 3.5. Calculation of Gibbs energy The Gibbs energy can be calculated using the expression given by SivaRamaiah et al. [21],

DG ¼ 2:303RT log10 Fig. 2. The electron paramagnetic resonance spectrum of 0.1 mol.% of Cu2+ ions doped in aluminium lead borate glass at different absolute temperatures (123– 295 K).

perpendicular components with the temperature variation from 123 to 295 K. 3.3. The spin-Hamiltonian (g and A) parameters From the observed ESR spectrum, the spin-Hamiltonian parameters are evaluated and they are found to be g|| = 2.36, g\ = 2.036, A|| = 163  104 cm1 and A\ = 33  104 cm1. The calculated values of g and A for the parallel and perpendicular components are of the same order as reported in the literature [12,13]. The calculated A values for the parallel and perpendicular components are invariant with the temperature and concentration of Cu2+ ions. The calculated values of g and A for the parallel components satisfy the inequalities g|| > g\ > ge (ge = 2.0023) and A|| > A\. This suggests that the Cu2+ ions are coordinated with six ligand atoms in a distorted octahedron elongated along the z axis. The ground state for the Cu2+ ion in APB z Cu (where z = 0.1 to 1.5 mol.%) glasses is 2B1g state i.e., dx2  y2 orbital [13]. The spin-Hamiltonian parameters (g and A) suggests that the bonding between Cu2+ center and the APB glass is ionic in character [10], but the covalence effects should also be taken into consideration. 3.4. Calculation of absolute number of spins The population difference between Zeeman levels (N) can be calculated from the area under the absorption curve with that of a standard of known concentration. Weil et al. [20] gave the following formula including the experimental parameters of both the sample and the standard 1

Nx ¼

Ax ðScanx Þ2 Gstd ðBmÞstd ðgstd Þ2 ½Sðs þ 1Þstd ðPstd Þ2 X Nstd Astd ðscanstd Þ2 Gx ðBm Þx ðgx Þ2 ½SðS þ 1Þx ðPx Þ1=2

ð1Þ

where A is the area under the absorption curve, scan is the magnetic fieldcorresponding to unit length of the spectrum, G is the receiver gain (amplitude), Bm is the modulation amplitude, g is the g factor, S

ðkb TÞ h

ð2Þ

where DG is the Gibbs energy, R is the Universal gas constant (8.31 J/K/mol), kB is Boltzmann constant (1.38  1023 J/K), T is the absolute temperature, k is the rate constant and is equal to the number of spins in 100 mg, h is Planck’s constant (6.63  1034 Js). The Gibbs energy of the Cu2+ center at g = 2.14 has been calculated to be 28 (kJ/mol). This calculated value is consistent with the values reported for glasses and minerals in the literature [21–23]. The Gibbs energy variation with temperature is shown in Fig. 3. From the above figure, it is evident that the Gibbs energy increases with the increase of temperature. This is due to increase of temperature as it is a temperature dependant quantity. This Gibbs energy is useful for coupling the thermodynamic parameters with the phase equilibrium calculations [21–24]. The Gibbs energy is useful for knowing the stability of the thermochemical processes such as the isobaric and isothermal processes [22]. Table 1 presents the calculated Gibbs energies for APB 0.1 Cu, APB 0.5 Cu, APB 1 Cu and APB 1.5 Cu glasses. The Gibbs energy increases with the increase of concentration of Cu2+ ions. This is due to the increase in population difference between the Zeeman levels with the concentration of Cu2+ ions. The large Gibbs energy in APB 1.5 Cu glass indicates that the stability is smaller in this glass. 3.6. Calculation of entropy and enthalpy Fig. 4 shows a graph between log10 (N/T) and reciprocal of absolute temperature. The data of the graph is fit to a straight line. The intercept on Y axis of this linear fit yields the entropy DS at 0.7 meV (11  1023 J). The slope of this linear fit gives the enthalpy DH at 25 meV (400  1023 J). The calculated values of entropy and enthalpy are of the same order as reported in the literature

Table 1 Number of spins (N), magnetic susceptibility (v), and Gibbs energy (DG), for APB z Cu (where z = 0.1 to 1.5 mol.% of CuO) glasses. Name of the glass

N (spin/g)  1017

v (m3/kg)  104

DG (kJ/mol)

APB APB APB APB

6.22 32.74 50.74 69.73

17.60 92.65 143.59 197.33

27.72 32.33 33.40 34.18

0.1 Cu 0.5 Cu 1 Cu 1.5 Cu

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Lower activation energy indicates less sensitive to temperature variation and high activation energy indicates high sensitive to temperature variation. From the intercept on Y axis of Fig. 5, the pre-exponential coefficient (PEC) is calculated to be 42 per second. This value indicates the collision frequency between the Cu2+ center and the APB glass lattice. The PEC may be useful for determining the spin orientation, spin density and spin concentration. This value is of the same order as reported in the literature [22].

27

Δ G (kJ/mol)

24

21

3.8. Calculation of magnetic susceptibility

18

The magnetic susceptibility vCurie for the g = (g|| + 2 g\)/3 = 2.14 resonance signal has been calculated using the expression [9]

15

12 100

vCurie ¼ 120

140

160

180

200

220

240

260

280

300

T (K) Fig. 3. The Gibbs energy of 0.1 mol.% of Cu2+ ions doped in aluminium lead borate glass as a function of different absolute temperatures (123–295 K).

[21,22]. The entropy and enthalpy indicate total heat content in glasses. They show a path for measuring thermal conductivity in glasses. They also indicate strain and exchange interaction at the field boundaries of the domains in the glass lattices. The domains are small regions in the glass lattices which accommodate spins of the order of 10 17 spin/g.

3.7. Calculation of activation energy and pre-exponential coefficient Fig. 5 shows a graph between the logarithmic number of spins and the reciprocal of absolute temperature. The data of the graph is fit to a straight line. The slope of the graph gives the activation energy Ea at 0.010 eV (160  1023 J). This value is similar to the value obtained in literature [13,14]. The activation energy represents the minimum energy required to liberate an unpaired electron from the Cu2+ center in APB glass. The activation energy indicates the molecular force of attraction between the Cu2+ center and the glass lattice.

N g2 b2 JðJ þ 1Þ 3KB T

ð3Þ

where N is number of spins per m3, g is g factor and is equal to 2.14, b is the Bohr magneton (9.27  1024 J/T), J is the total angular momentum quantum number (J = 5/2), kB is Boltzmann constant (1.38  1023 J/K) and T is absolute temperature. The magnetic susceptibilities were calculated from 123 to 295 K. The magnetic susceptibility for APB 0.1 Cu glass at 295 K is calculated as 14  103 m3/kg. The magnetic susceptibilities decrease from 58  103 to 14  103 m3/kg when the temperature increases from 123 to 295 K. Fig. 6 shows a graph between the reciprocal of magnetic susceptibility and the absolute temperature. The data of the graph is fit to a straight line. The intercept of this graph on X axis yields the Curie temperature of +123 K, while the reciprocal of the slope of the graph gives the Curie constant of 0.43 emu/mol. The calculated values of Curie temperature and Curie constant are in good agreement with the values reported in literature [13,14]. The ratio of Curie temperature hc to the Curie constant Cc gives the molecular field constant JC of 286 K/emu/mol. The large molecular field constant demonstrates that a large number of molecular clusters are observed in the glasses. A monotonic decrease of the product vcT with decreasing temperature is observed. This can be attributed to a paramagnetic phase [25]. The Curie temperature is related to the strength and sign of the dominant interactions. The positive value of Curie temperature indicates that ferromagnetic interactions are dominant in APB 0.1 Cu glass. The Curie constant demonstrates the nature of

16.2

16.0

log 10 N (spin in100 mg)

log 10 (N/T)(spin/100mg/K)

18.10

15.8

15.6

18.05

18.00

17.95

17.90

17.85 -3

4.0x10

-3

5.0x10

-3

6.0x10

-3

7.0x10

-3

8.0x10

1/T (1/K) Fig. 4. The graph between the logarithm of (N/T) and reciprocal of absolute temperature for 0.1 mol.% of Cu2+ ions in aluminium lead borate glass.

-3

4.0x10

-3

5.0x10

-3

6.0x10

-3

7.0x10

-3

8.0x10

1/T (1/K) Fig. 5. The graph between the logarithm of number of spins and reciprocal of absolute temperature for 0.1 mol.% of Cu2+ ions doped in aluminium lead borate glass.

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5

350

4

Absorbance (arb.units)

400

1/χ (kg/m3)

300 250 200

2

2

B1g

3

B1g

2

B2g

2

B1g 2A1g

Eg 1.5 Cu 1 Cu

2

1

150 0 100 100

120

140

160

180

200

220

240

260

2

4

6

T (K)

magnetic interactions. The low value of this constant is attributed to high antiferromagnetic interactions. It may also be attributed to the formation of Cu2+–Cu2+ ions pairs, if the exchange coupling between these ions is antiferromagnetic in nature, which is expected to result in a reduction of the Curie constant. The magnetic susceptibilities were calculated for APB 0.1 Cu, APB 0.5 Cu, APB 1 Cu, APB 1.5 Cu glasses and are presented in Table 1. It is observed that the Curie magnetic susceptibilities increase with the increase of concentration of Cu2+ ions. 4. Optical absorption studies We have observed the optical absorption bands for the APB z Cu (where z = 1, 1.5 mol.% of CuO) glasses. Fig. 7 shows the optical absorption spectrum of APB 1 Cu and APB 1.5 Cu glasses. For APB 1 Cu glass, three optical absorption bands have been observed at 975, 785, 365 nm (10256, 12739 and 27397 cm1) respectively. The band at 10256 cm1 has been attributed to 2B1g ? 2A1g transition. The band at 12739 cm1 has been attributed to 2B1g ? 2B2g transition. The band at 27397 cm1 has been attributed to 2 B1g ? 2Eg transition. For APB 1.5 Cu glass, three bands have been observed at 1009, 765, 371 nm (9911, 13072, 26954 cm1) respectively. Some authors obtained single optical absorption band in optical absorption spectrum of Cu2+ ions doped glasses [26]. Our optical absorption results are quite interesting when compared with the previously published results [26,27]. Such type of optical absorption results is due to excellent sample preparation. The transitions for APB 1.5 Cu and APB 1 Cu glasses are the same. It is for the first time to observe three transition bands in optical absorption spectra for Cu2+ centers in oxide glasses. The variation in band energies with the composition has been attributed to the variation in structural environment between Cu2+ centers and the glasses. By correlating the EPR and optical data, the molecular orbital coefficients such as a2, b2, b21 , Cr (%), Cp (%) are calculated using the formulae [26].

7 4

b2 ¼

2

B1g ! 2 Eg ½1  g ? =ge  ka2

10

12

14

16

18

20

100 λ (nm)

Fig. 6. The graph between the reciprocal of magnetic susceptibility and absolute temperature for 0.1 mol.% of Cu2+ ions doped in aluminium lead borate glass.

a2 ¼ ½Ak =P  A=P þ 2=3g k  5=21g ? 6=7

8

ð4Þ

ð5Þ

Fig. 7. The room temperature optical absorption spectrum of 0.5 and 1 mol.% of Cu2+ ions doped in aluminium lead borate glasses.

b21 ¼

2

B1g ! 2 Eg ½1  gk =ge  4ka2

ð6Þ

where A = 1/3 (A|| + 2A\), P = 0.036 cm1, is a constant, k is the spin–orbit coupling constant and is equal to 828 cm1, ge is the g factor of free electron 2.0023). The coefficient a2 can be taken as a measure of the in-plane r bonding between the d orbital of central metal ion and p orbital of the ligand. The coefficient b2 can be taken as a measure of the out of-plane p bonding between the d orbital of central metal ion and p orbital of the ligand. The coefficient b21 can be taken as a measure of the in-plane p bonding between the d orbital of central metal ion and p orbital of the ligand [13]. The Cr, Cp indicate the normalized covalency of the Cu2+O inplane bondings of r and p symmetries. The Cr, Cp are calculated usingthe formulae [13]

ur ¼

200ð1  SÞð1  a2 Þ % 1  2S

up ¼ 200ð1  b21 Þ%

ð7Þ ð8Þ

where S is the overlapping integral between the d orbital of the Cu2+ center and the p orbital of the oxygen ligand and is equal to 0.076 cm1. Using Eqs. 7 and 8, the coefficients such as a2, b2, b21 , Cr, Cp are calculated for all glasses and are presented in Table 2. The crystal field splitting energies such as D|| and D\ are observed for APB 1 Cu and APB 1.5 Cu glasses and are presented in Table 2. The values of a2, b2, b21 lie between 0.5 and 1, the limits of pure covalent and pure ionic bonding. In the case of many oxide glasses b2 is equal to 1. The transitions 2 B1g ? 2B2g, 2B1g ? 2Eg indicate the crystal field splitting energies D|| and D\. The value of a2 for all the APB z Cu (z = 0.5– 1.5 mol.%) glasses is 0.82. This indicates that the in-plane r bonding between the d orbital of centralmetal ion and p orbital of the ligand is significantly ionic in nature. The a2 value is invariant with the increase of concentration of Cu2+ ions. This indicates that the ionic bonding between the d orbital of the central metal ion and p orbital of the ligand of in-plane r bonding is invariant with the concentration of Cu2+ ions. The value of b2 changes between 0.67–0.68 for APB z Cu (z = 1–1.5 mol.%) glasses. This indicates that the out of-plane p bonding between the d orbital of central metal ion and p orbital of the ligand is significantly covalent in nature. The value of b21 lies between

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Table 2 The observed energies and the molecular orbital coefficients for APB z Cu (where z = 0.5 to 1.5 mol.% CuO) glasses. Name of the glass

D|| (cm1)

D\ (cm1)

a2

b2

b21

Cr (%)

Cp (%)

APB 0.5 Cu APB 1 Cu APB 1.5 Cu

13141 12739 13072

– 27397 26954

0.82 0.82 0.82

– 0.68 0.67

0.86 0.84 0.86

39 39 39

28 32 28

0.84–0.86 for APB z Cu (z = 1–1.5 mol.%) glasses. This indicates that the in-plane p bonding between the d orbital of central metal ion and p orbital of the ligand is significantly ionic in nature [13]. The value of Cr is invariant with the concentration of Cu2+ centers. This indicates that the in-plane bonding of r-symmetry is invariant with the concentration of Cu2+ centers. The value of Cp increases with the increase of concentration of Cu2+ centers from 0.5 to 1 mol.% of Cu2+ centers and decreases from 1 to 1.5 mol.% of Cu2+ centers. This indicates that the in-plane bonding of p-symmetry increases from 28% to 32% when the concentration increases from 0.5 to 1 mol.% of Cu2+ centers and decreases from 32% to 28% when the concentration increases from 1% to 1.5 mol.% of Cu2+ centers. This type of behavior is due to variation of structural changes within the glasses.

5. Conclusions The ESR and optical absorption spectra of Cu2+ centers in aluminium lead borate glasses confirm that the Cu2+ centers have occupied octahedral symmetric sites with an axial symmetry. The Cu2+ ions are surrounded by the oxygen ligands and coordinated to aluminium ions in the glasses. The linear relationship between the Gibbs energy and the absolute temperature is observed. It is observed that the number of spins increases with the decrease of temperature. This is the phenomenon that is expected from the Boltzmann law. The linear relationship is observed between the temperature and the reciprocal of magnetic susceptibility in accordance with the Curie’s law. The molecular orbital coefficients obtained from the correlation of the EPR and optical data indicate that the in-plane r and p bondings are significantly ionic in nature where as the out of plane p bonding is significantly covalent in nature.

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