The effect of copper ions addition on structural and optical properties of zinc borate glasses

Journal of Non-Crystalline Solids 358 (2012) 839–846

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The effect of copper ions addition on structural and optical properties of zinc borate glasses Razvan Stefan a, Eugen Culea b, Petru Pascuta b,⁎ a b

Biophysics Department, Agricultural Science and Veterinary Medicine University, Cluj-Napoca, Romania Physics Department, Technical University, 400020 Cluj-Napoca, Romania

a r t i c l e

i n f o

Article history: Received 30 October 2011 Received in revised form 16 December 2011 Available online 12 January 2012 Keywords: Zinc borate glasses; XRD; FTIR; EPR; Optical properties

a b s t r a c t Glasses in the ternary system xCuO∙(100 − x)[55B2O3·45ZnO] (0 ≤ x ≤ 20 mol%) have been prepared by melting at 1200 °C and rapidly cooling at room temperature. The effect of copper ions addition in 55B2O3·45ZnO glass matrix together with the matrix effect on paramagentic behavior has been investigated using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, differential thermal analysis (DTA), electron paramagnetic resonance (EPR), ultraviolet–visible (UV–VIS) spectroscopy and density measurements. The increase of the number of non-bridging oxygen (NBO) atoms as a function of CuO content in these glasses leads to the decrease of glass polymerization which reduces the stability of the glasses and favors the association of copper ions in clusters. This leads to the major changes of structural and optical properties of the studied glasses as can be seen from the data obtained by FTIR and EPR spectroscopies. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Zinc borate glasses are of technological interest owing to their applications in different fields of electronic products since they are known to have low melting temperatures [1-3]. This is due to the fact that the higher the polarizability of an ion, the lower the melting temperature of the substance. Low melting glasses have been widely used for lowering the sintering temperature and optimizing the thermal expansion coefficient in the field of electric devices such as multi layer ceramic capacitors, low temperature co-fired ceramics (LTCC), plasma display panels, cathode ray tubes, electric modules, etc. [1-3]. The structure and properties of such glasses are certainly related to the nature of the constituents. B2O3 is one of the best glass formers known and is present in almost all commercially important glasses. The ability of boron existing in three and four oxygen co-ordinated environments and the high strengths of covalent B―O bonds enable borates to form stable glasses. In general, ZnO is a glass modifier and enters the glass network by breaking up the B―O―B bonds (normally the oxygens of ZnO break the local symmetry while Zn2 + ions occupy interstitial positions) and introduces co-ordinated defects known as dangling bonds along with non-bridging oxygen (NBO) atoms; in this case Zn2 + is octahedrally coordinated. However, ZnO may also participate in the glass network with ZnO4 structural units when zinc is linked to four oxygen ions in a covalent bond configuration. In such a case the network structure is considered to be built up of ZnO4 and BO4 pyramidal units, which are linked together by B―O―Zn bonds [4,5].

⁎ Corresponding author. Tel.: + 40 264 401 262; fax: + 40 264 595 355. E-mail address: [email protected] (P. Pascuta). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.12.079

On the other hand, glasses doped with transitional metal ions (TMI) attract much attention because of their interesting electrical [6-8], optical [9-11] and magnetic properties [12-16]. These properties are determined by the presence of TMI in multivalent states, by the environment of TMI as well as by the TMI content and its distribution in the glass matrix. Because of these properties glasses containing TMI are useful for large practical and potential applications in many fields, such as electronics [6,7,10,12], magnetic information storage [11,12,15], tunable solid state lasers and fiber optic communication systems [9]. The properties of glasses are mainly determined by the degree of local order/disorder, so that investigations giving information about the local structure are important in order to understand the correlation between properties and structure. EPR, UV–VIS and FTIR spectroscopies are very useful experimental techniques because they may provide valuable information related to the local symmetry, nature of the chemical bonds and other structural properties of glasses [17,18]. Copper ions have been frequently used as paramagnetic probes for exploring the structure and properties of glasses. A large number of interesting studies are available on the environment of copper ion in glasses [19-23]. Thus, in glasses, copper ions exist in two stable ionic states, the divalent Cu2 + and monovalent Cu+ ions, and may also exist as metallic copper [19]. Cu2 + ions are well-known paramagnetic ions and it is also quite likely for these ions to have links with zinc borate groups [1]. Such links may strengthen the glass structure and raise the chemical resistance of the glass [1,23]. It may also be useful to note that there is a possibility for the Cu 2 + ions to occupy the octahedral zinc ion sites since their ionic radii are very close to each other (the radius of Cu2 + ion is 0.72 Å and that of Zn2 + is 0.74 Å) [1]. This work aims to present our results obtained by means of XRD, FTIR DTA, EPR and density measurements performed on some zinc

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borate glasses doped with CuO in order to establish the structural changes induced by copper oxide addition and to obtain information concerning the structural and optical properties of these glasses. 2. Experimental Samples from the xCuO∙(100 − x)[55B2O3·45ZnO] system (0 ≤ x ≤ 50 mol%) were prepared using pure reagent-grade compounds, i.e. H3BO3, ZnO and CuO, in appropriate ratios. The mixtures corresponding to the desired compositions were melted in air, in sintered corundum crucibles, in an electric furnace at 1200 °C for t = 15 minutes. The melts were quickly cooled to room temperature by pouring onto stainless-steel plates. The XRD measurements of the studied samples were performed with an XRD-6000 Shimadzu diffractometer, with a monochromator of graphite for Cu-Kα radiation (λ = 1.54060 Å) at room temperature. The glass density, ρ, was measured using Archimedes' method with distilled water as immersion liquid. Mass was measured using a digital balance sensitive to 0.00001 g. The glass transition temperature, Tg, of the studied samples was obtained from DTA curves recorded at the heating rate of 20 °C/min using a Perkin Elmer TG/DTA 6300 thermal analyzer. About 50 mg of bulk glass was heated in Pt-holder with another Pt-holder containing αalumina as a reference material. The FTIR absorption spectra were registered at room temperatures using a JASCO FTIR 6200 spectrometer. The IR absorption measurements were done using the KBr pellet technique. In order to obtain good quality spectra, the samples were crushed in an agate mortar to obtain particles of micrometer size. This procedure was applied every time to fragments of bulk glass to avoid structural modifications due to ambient moisture. The samples were mixed with KBr powder in the proportion of 1:100 (3 mg sample: 300 mg KBr) for 20 minutes. The mixture was subjected to a load of 8 tons/cm 2 in an evocable die for 5 minutes to produce clear homogenous disks. The IR absorption spectra were measured immediately after preparing the disks. The FTIR spectra were recorded in the wavenumber range of 400– 1650 cm − 1 and normalized to eliminate the concentration effect of the powder sample in the KBr disk. The EPR measurements were performed on a PS 8400 spectrometer in the X-band (9.1–9.6 GHz) with 100 kHz field modulations. The measurements were made at room temperature. To avoid the alteration of the glass structure due to the ambient conditions, especially humidity, samples were powdered immediately after preparation and enclosed in tubular holders of the same caliber. Equal quantities of samples were studied. The UV–VIS absorption spectra were recorded at room temperature in the 200–1000 nm range using a PerkinElmer Lambda 45 UV–VIS spectrometer equipped with an integrating sphere. UV–VIS transparent samples have been obtained for all CuO investigated concentration. Thus the bulk samples have been crushed and sieved in order to obtain similar glass particles with biggest dimension smaller than 0.045 mm. As obtained samples were mixed together with glycerin and homogeneous mixtures were prepared. The samples measured through UV–VIS spectrometry contained the same volume of powder diffused in the same glycerin volume and exhibit the same linear density of particles as big as attenuation by scattering is negligible. The overall powders volume has been established with respect of the transparency of the most CuO concentrate sample. Only relative calculations have been done in order to avoid errors.

[55B2O3·45ZnO] samples with various contents of copper oxide (0 ≤ x ≤ 50 mol%) are presented in Fig. 1. These patterns show that crystalline phase is present only in the samples with x ≥ 30 mol%. Up to this concentration, glasses were obtained. For x ≥ 30 mol% the vitreous phase coexists with a crystalline phase and the pattern shows large maxima overlapped with the peaks characteristic to the crystalline phase. All detectable peaks can be indexed as belonging to the CuO crystal in the standard data (PDF#050667). The results indicated that the CuO crystal structure is cubic with the Pn3m space group. Considering these data, we propose to ourselves to study through FTIR, EPR and UV–VIS spectroscopies only the glass samples (x≤20 mol%) from the considered system.

3.2. Density and glass transition temperature data The composition dependence of the density, ρ, of the present glass samples is shown in Fig. 2. It may be observed that density increases gradually with the increasing of copper oxide content in the glass compositions. The relationship between density and composition of an oxide glass system can be expressed in terms of an apparent volume Vm occupied by 1 g atom of oxygen. The molar volume, Vm, of all the prepared glasses, is defined as the mean molecular weight (M) of its constituents divided by its experimental density as given by the equation, Vm = M/ρ and the sum of the molar volumes, Vc, of the components of the glasses in their crystalline states has been calculated and is also shown in Fig. 2. Glass transition temperature are useful in suggesting structural changes that achieve by composition change because Tg is very sensitive to any change of the coordination number of the network forming atoms and to the formation of NBO. The effect of copper content on the glass transition temperature for the studied glasses is shown in Fig. 3.

3. Results 3.1. XRD data The vitreous or/and crystalline nature of the studied samples was tested by XRD. The obtained XRD patterns of the xCuO∙(100 − x)

Fig. 1. The XRD patterns of the xCuO∙(100 − x)[55B2O3·45ZnO] samples.

R. Stefan et al. / Journal of Non-Crystalline Solids 358 (2012) 839–846

841

Fig. 2. Composition dependence of density and molar volume for the xCuO∙(100 − x) [55B2O3·45ZnO] glasses.

3.3. FTIR data The experimental normalized FTIR spectra of xCuO∙(100 − x) [55B2O3·45ZnO] samples with various contents of copper oxide (0≤ x ≤ 20 mol%) were presented in Fig. 4. To obtain quantitative information about the structural groups in the studied glasses, the FTIR spectra have been deconvoluted using the Spectra Manager program and a Gaussian type function. Each individual band has its characteristic parameters such as its center (C), which is related to some type of vibration of a specific structural group, and its relative area (A), which is proportional to the concentration of this structural group. Fig. 5 shows the deconvolution, in Gaussian bands, of the spectrum for glasses containing 10 mol% CuO. The deconvolution parameters, namely the band centers C and the area A, as well as the bands assignment are given in Tables 1 and 2 for the studied glasses. The result of peak deconvolution, indicates a number of 7 peaks in the spectral region from 400 to 1650 cm− 1.

Fig. 4. FTIR spectra of the xCuO∙(100 − x)[55B2O3·45ZnO] glasses.

(0.5 ≤ x ≤ 1 mol%) the EPR spectrum shows the hyperfine structure (hfs) resolved in both parallel and perpendicular bands (Fig. 7). For 3 ≤ x ≤ 7 mol%, three parallel components are observed in the lower field region while the fourth parallel component is overlapped with

3.4. EPR data No EPR signal was observed in the spectra of undoped glasses indicating that no paramagnetic impurities were present in the starting materials. EPR signals were observed for all the glasses containing copper ions and are shown in Fig. 6. As can be seen from this figure, there is a strong dependence of the absorption spectra structure and parameters on the CuO content of the samples. In the low concentration range

Fig. 3. Composition dependence of the glass transition temperature for the xCuO∙(100−x) [55B2O3·45ZnO] glasses.

Fig. 5. Deconvoluted FTIR spectra of the (CuO)10∙(55B2O3·45ZnO)90 glass using a Gaussian-type function.

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Table 1 Deconvolution parameters of the FTIR spectra of the xCuO∙(100 − x)[55B2O3·45ZnO] glasses. C is the component band center (cm− 1) and A is the area of the component band. x=0

x = 0.5

x=1

x=3

x=7

x = 10

x = 15

x = 20

C

A

C

A

C

A

C

A

C

A

C

A

C

A

C

A

608 685 878 1051 1226 1360 1482

59 47 37 141 68 192 49

620 693 885 1033 1231 1379 1500

36 50 20 89 103 186 38

615 693 884 1031 1229 1379 1503

30 55 18 86 102 195 34

619 694 881 1032 1230 1378 1499

32 51 19 86 106 193 39

618 693 882 1027 1228 1380 1502

37 54 18 83 110 196 40

619 695 884 1025 1222 1376 1499

31 54 16 71 112 192 39

602 696 882 1026 1220 1371 1494

15 59 16 67 114 198 34

611 697 882 1023 1219 1372 1494

18 61 14 64 118 190 40

the perpendicular components. For these samples the perpendicular components in the high field region are not resolved, indicating a width of the individual components exceeding the |A⊥| separation. For glasses containing less than 1 mol% CuO, we could calculate values of the EPR spectrum parameters: g|| ≈2.29, g⊥ ≈2.06, A|| ≈126.1⋅10− 4 cm− 1 and A⊥ ≈ 70.8⋅10− 4 cm− 1. The estimated values of the g|| and g⊥ satisfy the g|| > g⊥ >ge =2.0023 relation, evidencing the presence of Cu2 + ions in the predominant axially distorted octahedral symmetric sites. The ge spectroscopic factor corresponds to the free electron. 3.5. Optical absorption data The UV–VIS absorption spectra of xCuO∙(100 − x)[55B2O3·45ZnO] glasses, with 0 ≤ x ≤ 20 mol%, recorded at room temperature are shown in Fig. 8. For glasses containing CuO the optical absorption spectra exhibit a single broad peak in the 760–780 nm range. Optical absorption offers useful information for optical band gap of glasses. The principle of the technique is that a photon with energy greater than the band gap energy will be absorbed. There are two kinds of optical transitions at the fundamental absorption edge: direct and indirect transitions. In both cases, electromagnetic waves interact with the electrons in the valance band, which are raised across the fundamental gap to the conduction band. For glasses, the expression for the absorption coefficient, α(ν), as a function of photon energy, hν, for direct and indirect optical transitions was given by Davis and Mott [24] as: αðνÞ ¼

p α 0 ðhν−Eopt g Þ hν

studied glasses. The values of indirect optical band gap energy Egopt have been estimated from the linear regions of the curves by extrapolating them to meet the hν axis at (αhν) 1/2 → 0 as shown in Fig. 9. The values obtained for Egopt are shown in Table 3 for all the studied glasses. 4. Discussion It is well known that pure B2O3 glass is composed essentially of BO3 triangles forming three-membred (boroxol) rings. By adding the modifier oxide to the B2O3 glass, some BO3 triangles change to BO4 tetrahedra, breaking bridging oxygen bonds to form NBOs and residing in interstitial sites of the tetrahedral network in the vicinity of the negatively charged NBOs. In order to understand the effect of addition of CuO in the (B2O3)55·(ZnO)45 glass matrix, we analyzed the mid infrared region (400–1650 cm − 1) where the vibration modes of zinc borate glasses are active. The absorption band from 602– 620 cm –1 (Table 2) can be due to the Zn―O stretching vibrations in ZnO4 tetrahedral units [27,28]. For samples containing copper ions this band may be due also to the Cu―O bonds [29,30]. However, the relative area of this band decreases with increasing of CuO content (Table 1). Under such circumstances, the mentioned absorption band from 602–620 cm − 1 cannot be considered as an indicator for the presence of Cu―O bonds but can be considered as an indicator

ð1Þ

where Egopt is the optical band gap energy in eV (the optical band gap in glasses is closely related to the energy gap between the valence band and conduction band), α0 is an energy independent constant and p is an index which can be assumed to have values of 1/2, 3/2, 2 and 3, depending on the nature of the electronic transition responsible for absorption; p is equal to 1/2 for allowed direct transitions, 3/ 2 for direct forbidden transitions, 2 for allowed indirect transitions and 3 for forbidden indirect transitions [25]. For vitreous materials indirect transitions (p = 2) are valid according to the Tauc's relations [26]. The variation of (αhν) 1/2versus hν is shown in Fig. 9 for the

Table 2 Assignment of the FTIR absorption bands of the xCuO∙(100 − x)[55B2O3·45ZnO] glasses. Wavenumber Assignment [cm− 1] 602–620 685–697 878–885 1023–1051 1219–1231 1360–1380 1482–1503

Zn―O stretch in ZO4 units Cu―O bonds B―O―B bend B―O stretch in BO4 units from tri-, tetra- and penta-borate groups B―Osymm stretch in BO3 units from pyro- and ortho-borate groups B―O stretch in BO3 units from varied types of borate groups B―O− stretch in BO2O− units from varied types of borate groups

Fig. 6. EPR spectra of Cu2 + ions in the xCuO∙(100 − x)[55B2O3·45ZnO] glasses.

R. Stefan et al. / Journal of Non-Crystalline Solids 358 (2012) 839–846

843

Fig. 8. UV–VIS absorption spectra of xCuO∙(100 − x)[55B2O3·45ZnO] glasses.

Fig. 7. Evolution of the hfs in (a) the parallel and (b) perpendicular band for the xCuO∙(100 − x)[55B2O3·45ZnO] glasses for 0.5 ≤ x ≤ 7 mol%.

of the presence of the Zn―O tetrahedral bonds. The decrease of relative area of this band suggests a gradual displacement of zinc ions from tetrahedral positions to octahedral positions. The increase in concentration of octahedrally coordinated zinc ions denotes an

increase in the depolymerization of the glass network. The peak from 685–697 cm− 1 is assigned to bending vibration of B―O―B linkages in the borate network [31,32]. The relative area of this band increases with the increase of copper ions content and shifts to higher wavenumbers. This shift may be produced by the electrostatic field of the Cu2 + ions [33]. The increasing of CuO content results in an increase of the electron cloud density around the oxygen in the BO3 unit, leading to an increase in the roll-torque of the B―O―B band and consequently contributing to the shift towards higher wavenumbers. This process forms new B―O―Cu bridging bonds due to the aforementioned induced electrostatic field causing a weakening of the borate network [33]. The peaks from 878–885 cm− 1 and 1023–1051 cm− 1 are due to the stretching vibrations of B―O bonds in BO4 units from tri-, tetraand penta-borate groups [31,32]. The absorption band from 1219– 1231 cm− 1 is attributed to the asymmetric stretching vibrations of B―O bonds in BO3 units from pyro- and orto-borate groups, these groups containing a large number of NBOs. With the increase of the copper ions content the relative area of this band increases indicating the increase of the number of NBOs in very good agreement with the appearance of octahedral coordinated copper. The band from 1360– 1380 cm− 1 can be due to the stretching vibrations of B―O in BO3 units from different borate groups while the band from 1482– 1503 cm− 1 is attributed to the stretching vibrations of B―O − in BO2O− units from different borate groups [31,32]. The structural changes involved by the addition of CuO were analyzed based on the changes produced by the copper ions in the relative population of triangular and tetrahedral borate units in glasses. To follow the evolution of the triangular and tetrahedral borate units in the studied glasses we used the fraction of four-coordination boron atoms, N4, as was defined previously [31,34]:

N4 ¼

A4 A3 þ A4

ð2Þ

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suggest that the copper ions play a network modifier role in the studied glasses. Density responds to variations in glass composition sensitively in technological practice. Density of glass, in general, is explained in terms of a competition between the masses and size of the various structural groups present in glass. Accordingly, the density is related to how tightly the ions and ionic groups are packed together in the structure. From Fig. 2 it can be observed that the molar volume of the glasses is always greater than their hypothetical mixed crystalline phase, indicating the presence of excess structural volume in the glasses. On the other hand, the increasing of copper oxide content leads to the molar volume decreasing. This result may be due to the fact that CuO plays the role of network modifier and introduces excess structural free volumes. The decrease of Tg is probably because N4 decreases and can be explained by the significant increase in NBO, which depolymerizes and weakens the glass network. The addition of copper ions in zinc borate glass matrix offers the possibility to investigate the local order by EPR spectroscopy. In glasses, copper ions exist in two stable ionic states as Cu+ and Cu2 +, but only Cu2 + shows EPR absorptions at room temperature. The copper ions in Cu2 + 2 state belong to the 3d9 configuration with D as ground state with the 63 electron spin S =1/2 and the nuclear spin I = 3/2 for both Cu (natural 65 abundance 69%) and Cu (natural abundance 31%). The dipole–dipole interaction between the electronic spin and the nuclear spin leads to four parallel and four perpendiculars hyperfine lines. The EPR spectra of Cu2 + ions in glasses can be described using an axial spin Hamiltonian which includes the Zeeman and hyperfine interactions [20,35]:     H ¼ g jj βSz Bz þ g ⊥ β Sx Bx þ Sy By þ Ajj Sz Iz þ A⊥ Sx Ix þ Sy Iy

Fig. 9. The variation of (αhν)1/2versus hν for the xCuO∙(100 − x)[55B2O3·45ZnO] glasses with 0 ≤ x ≤ 1 mol% (a) and 3 ≤ x ≤ 7 mol% (b).

where A4 and A3 denote the areas of the BO4 and BO3 units. The fraction of four-coordination boron atoms, N4, decreases with increasing the content of CuO in the whole concentration range (Table 3). This is due to the structural changes involving the conversions of the BO4 into BO3 structural units as the content of the CuO glass modifier increases. Therefore for the studied glasses the BO4 units will be destroyed and converted into BO3 and BO2O− units with NBOs, which reduces the stability of the glasses. The threefold boron atoms are favored in the investigated glass system as compared with the fourfold ones in the whole studied concentration range (Table 3). Thus, the presence of copper ions within these glasses seems to influence the surrounding of the B3 + and Zn2 + cations favoring the formation of the BO3 and ZnO4 structural units as well. The structural changes observed by increasing the CuO content in (B2O3)55·(ZnO)45 glass matrix evidenced by the FTIR investigation Table 3 Fraction of four-coordination boron atoms (N4) and optical band gap (Egopt) for the xCuO∙(100 − x)[55B2O3·45ZnO] glasses. x [mol%]

N4

Egopt [eV]

0 0.5 1 3 7 10 15 20

0.365 0.252 0.239 0.237 0.226 0.202 0.193 0.183

4.49 2.63 2.53 2.47 2.24 1.75 1.37 1.11

ð3Þ

where z is the tetragonal symmetry axis; g|| and g⊥ are the parallel and perpendicular components of the axial g tensor; A|| and A⊥ are the parallel and perpendicular components of the axial hyperfine coupling; Bx, By and Bz are the component of the static magnetic field; Sx, Sy and Sz, Ix, Iy and Iz are the components of the spin operators of the electron and the nucleus, respectively; β is the Bohr magneton. The first two terms concern the interaction between the electronic spin and the magnetic field; the third term concerns the coupling between the electronic and nuclear spins. The solution of the spin Hamiltonian gives the expressions for the peak positions of the principal g and A tensors as [35]:   A2 2 ⊥ hv ¼ g jj βB þ mAjj þ 15=4−m 2g jj βB 2

ð4Þ 2

  A þ A⊥ 2 jj hv ¼ g ⊥ βB þ mA⊥ þ 15=4−m 4g ⊥ βB

ð5Þ

for the parallel and perpendicular hyperfine peaks, respectively. Here m is the nuclear magnetic quantum number of the copper nucleus with values 3/2, 1/2, −1/2 and −3/2 and ν is the microwave frequency at resonance. The relatively broad range in which the EPR spectrum of our glasses shows a resolved hfs denotes an appreciable degree of short range ordering in the vitreous matrix built by approximately identical structural units, involving Cu 2 + ions. The values of the g factors and hyperfine constant attest the predominantly ionic character of bonding between Cu 2 + ions and the ligand atoms [36], but there is also a covalency effect which has to be taken into account. Because Cu 2 + is a network modifier there is competition between Cu 2 + and the B 3 + network forming cations in attracting the oxygen pairs available in their vicinity. The covalency of the Cu 2 +―O bonds increases when the B―O bonds become weaker in the structural aggregates involving them. The shape of EPR spectra is modified with increasing of copper ions content. This consists in the disappearance of the hfs and the

R. Stefan et al. / Journal of Non-Crystalline Solids 358 (2012) 839–846

appearance of a broad line centered at g ≈ 2.1, value characteristic for clustered ions. Thus for samples with x ≥ 3 mol% the obtained spectra may be considered as the result of the superposition of two signals, one with resolved hfs typical for isolated Cu 2 + ions and the other one consisting in a broad line typical for clustered ions. The evolution of the g ≈ 2.1 absorption lines as a function of the copper content can be followed in the concentration dependence of the EPR parameters: the peak-to-peak linewidth ΔB and the line intensity J, estimated as the line integral. These variations are plotted in Fig. 10. The composition dependence of the g ≈ 2.1 absorption line intensity shows an increase up to 10 mol% CuO followed by a decrease for higher content of copper ions. Generally, the signal intensity is proportional to the number of EPR active species involved in the resonance absorption. Thus, the decrease of the line intensity of the g ≈ 2.1 resonance shows that the Cu 2 + ions content diminishes suggesting the presence of diamagnetic Cu + ions, which co-exist with Cu 2 + species in the glass matrix when copper ions are added. The Cu + ions do not manifest in the EPR absorption, but they can interact with Cu 2 + ions. Mixed valence states of Cu 2 + and Cu + ions were also detected in other glasses [37]. These new ions balance the paramagnetic Cu 2 + species so that the addition of CuO does not imply more proportional changes of the EPR line intensity with the x value. The Cu + ionic species simultaneously present with the Cu 2 + ones occur when sample preparation conditions favored an oxygen rich glass melt. The linewidth increases with the copper ions accumulation in the whole concentration range, more pronounced for x > 10 mol%. This broadening can be due to the increased disordering of the glasses structure, to the dipole–dipole interactions and to the interactions between ions in multivalent states. Thus, the presence of Cu 2 +–Cu + pairs and the dipole–dipole interactions can explain the evolution of the EPR absorption line, with the CuO content detected for studied glasses. Optical absorption study in glasses has proved to be very useful for elucidation of optical transitions and electronic band structure of these materials. TMI possess characteristic optical absorption spectra which are mostly extended in the UV–VIS range depending on the valence state and coordination number of the TMI and consequently the state of d level. It is well known that copper exists in glasses the forms of Cu2 + and Cu+ ions. While Cu+ ions have no absorption in the UV–VIS region, the Cu2 + ions have a strong absorption [21]. The 2 free ion term for Cu 2 + ion (3d 9) is D. In a ligand field of octahedral 2 symmetry the D ground state splits into the eg and t2g states. However, as the ground state for Cu2 + ions in an octahedral ligand field is eg, tetragonal splitting due to Jahn–Teller distortion will occur and the eg 2 2 2 2 level splits to B1g and A1g and the t2g level to B2g and Eg levels [38,39]. Thus, in general for copper ions three bands corresponding to

2

2

845 2

2

2

2

the transitions B1g → A1g, B1g → B2g and B1g → E2 are expected. But in the present case, we have observed a single broad absorption band 2 2 (Fig. 8) which was assigned to the B1g → B2g transition of Cu2 + ions [19,38]. The broadening of this band can be due to the overlap of all the three transitions. The values of the optical band gap energy decrease from 2.63 eV to 1.11 eV when the content of CuO increases from 0.5 to 20 mol%. T. Srikumar et al. [40] showed that with the increase in the CuO content in the glasses, a large number of donor centers are created; the excited states of localized electrons originally trapped on Cu + sites begin to overlap with the empty 3d states on the neighboring Cu 2 + sites, and as a result, the polaron band becomes more extended into the main band gap. This new polaronic development can lead to a significant shrinkage, in the band gap as the concentration of CuO is increased up to 20 mol%. Thus we can conclude that the decreases of the optical band gap energy with increasing of CuO content may be due to the fact that the optical absorption in the studied glasses is dominated by polaronic transfer between the Cu + and Cu 2 + ions. 5. Conclusions Homogeneous glasses of the xCuO∙(100 − x)[55B2O3·45ZnO] system were obtained within the 0 ≤ x ≤ 20 mol% composition range. For x ≥ 30 mol% the XRD patterns show a crystalline phase, identified as the CuO one. FTIR spectra of these glasses have been analyzed in order to identify the spectral contribution of each component on the structure and to point out the role of the copper ions. The CuO plays the network modifier role in the studied glasses determining the formation of NBOs. The fraction of four-coordination boron atoms, N4, decreases with increasing the CuO content for all the studied compositional range. This is due to the structural conversion of BO4 into BO3 units as the content of the CuO glass modifier oxide increases. The EPR absorption spectra reveal the presence of isolated Cu 2 + ions in axially distorted octahedral sites (octahedron elongated along one axis) up to about 7 mol% CuO in the investigated vitreous system. The EPR spectra are modified with increasing the CuO content, leading to the disappearance of the copper hfs and the appearance of a broad line centered at g ≈ 2.1, characteristic of clustered ions. Simultaneously with Cu 2 + ions, the Cu + ions were also found in the studied glasses. The EPR absorption line broadening can be due to the increased disordering of the glasses structure, to the presence of Cu 2 +–Cu + pairs and to the dipole–dipole interactions. The UV–VIS absorption spectra of these glasses show a single broad band due to 2 2 B1g → B2g transitions of the Cu 2 + ions. This proves that the Cu 2 + ions are placed in distorted octahedral sites. From optical absorption spectra the Egopt has been calculated. It is interesting to observe that the Egopt for the studied glasses decrease with the addition of copper ions. Acknowledgement This work was supported by CNCSIS—UEFISCDI, projects number 1117/2009, PNII–IDEI code 2528/2008. References [1] [2] [3] [4] [5] [6] [7]

Fig. 10. Composition dependence of intensity and line-width for g ≈ 2.1 absorption line of xCuO∙(100 − x)[55B2O3·45ZnO] glasses. The lines are drawn as a guide for the eyes.

[8]

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