ESR and ligand field theory studies of Nd2O3 doped borochoromate glasses

Journal of Alloys and Compounds 539 (2012) 233–236

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ESR and ligand field theory studies of Nd2O3 doped borochoromate glasses M.A. Hassan a,⇑, M. Farouk a, A.H. Abdullah b, I. Kashef a, M.M. ElOkr a a b

Physics Department, Faculty of Science, Azhar University, Cairo, Egypt Applied Science Department, College of Technological Studies, Kuwait

a r t i c l e

i n f o

Article history: Received 24 April 2012 Received in revised form 7 June 2012 Accepted 13 June 2012 Available online 22 June 2012 Keywords: Borochromate glasses Rare earth ESR Ligand field theory Infrared spectra

a b s t r a c t Glassy system of compositions xCr2O3–(25x)Li2O–yNd2O3–(75y)B2O3 where x = 0, 0.1, 0.2, 0.3, 0.5 at y = 1.0 mol% and x = 1.0, at y = 0 mol% has been prepared by conventional melt quenching technique. The electron spin resonance (ESR) of the prepared glassy system was investigated; the obtained ESR parameters indicate the presence of chromium ions predominantly in Cr3+ with small traces of Cr6+. The former is suggested to occupy modifier positions at lower Cr2O3 content (up to x = 0.3, y = 1 mol%). For higher Cr2O3 content (x > 0.3, y = 1 mol%) the chromium ions participate in the network as a former. The optical results have been analyzed in the light of ligand field theory. The value of the crystal field strength (Dq) and the inter-electronic repulsion Racah parameter (B) has been calculated from the optical transitions energies for all chromium doped glassy. The (Dq/B) ratio is found to be around 2.06; which indicate that the Cr3+ ions are in the weak ligand field sites. The IR absorption and density has been measured, the obtained data reveal that, both N4 ratio and density are almost composition independent. Ó 2012 Published by Elsevier B.V.

1. Introduction Transition metal ions are being greatly used in the present days to probe the glass structure since their outer d-electron orbital function have a broad radial distribution and due to their high sensitive response to the changes in the surrounding cations [1–3]. The basic features of trivalent transition metal and rare earth ions in a large number of crystalline and amorphous host matrices are so well established that these transition metal and rare earth ions are extensively used as a spectroscopic probe for studying the structures [4,5]. Borate glasses are particularly interesting model systems as they exhibit a variety of structural changes with alkali content. It is often used as dielectric and insulating material and it is known as a good shield against infrared radiation, also B2O3 compounds are a promising host for incorporation of Cr3+ ions. However Cr3+-doped borate glasses are also of interest in this context because the emission band is much broader than in ionic crystals [3,6]. Ruby, natural or synthetic, is a-Al2O3 containing occasional Cr3+ ions in place of Al3+ ions. The environment of the Cr3+ in ruby is slightly distorted (D3d) octahedron of oxide ions. The frequencies of the spin-allowed bands of Cr3+ in ruby indicate that the Cr3+ ion is under considerable compassion [7,8]. Extensive investigations on the optical absorption and ESR spectroscopy of Cr3+ ion ⇑ Corresponding author. Tel./fax: +20 222629356. E-mail addresses: [email protected] (M.A. Hassan), mf_egypt22375@ yahoo.com (M. Farouk). 0925-8388/$ - see front matter Ó 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jallcom.2012.06.060

a varity of inorganic glasses have been investigated in the recent years due to their technological importance in the development of tunable solid state laser, an electro optic modulators, electro-optic switches, non-liner optical, parametric converters and new luminescence materials [9–11]. The optical properties of Cr3+ ions are modified by the random disordered structure of glasses, because the environment of each Cr3+ ion in the glass varies as a consequence of differences in bonding to nearest-neighbor ions. This results in site-to-site differences in the energy level structure, and in the radiative and non radiative transition probabilities of the Cr3+ ions in glasses. Since the frequencies and radiative transition probabilities vary from site-to-site, the optical absorption and emission spectra are super positions of contributions from individual crystal-field sites [12]. The analysis of published data of ESR spectra predicts a possible co-existence of chromium ions in Cr5+, Cr6+ state with Cr3+ state in glass system, hence, ESR signals can only be observed in Cr5+ and Cr3+. The Cr5+ ion exhibit ESR signal at 800G, while as Cr3+ at about 3400G [13]. Among various transition metal ions, the chromium is a paramagnetic ion, when dissolved in glass matrices even in very small quantities makes the glasses colored and has a strong influence over the optical properties of the glasses. The energy diagram (Tanabe–Sugano diagram) in octahedral symmetry predicts that the spectrum of Cr3+ exhibit mainly two broad bands, which occur in the UV–Visible range and impart the glasses their characteristic green color. The two main absorption bands arise from the spin-allowed d–d transitions of octahedral Cr3+, 4A2g ? 4T2g and 4 A2g ? 4T1g, and thereafter referred to as m1 and m2 bands in the increasing order of energy respectively [8–18]. Additionally, the

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absorption spectra show a fine structure in the vicinity of the profile of 4A2g ? 4T2g, it is well known that the spectrum of Cr3+ doped glasses exhibits two dips. These dips are due to the Fano anti-resonances resulting from the interaction of the 2T1g and 2E2g with vibrationally broadened 4T2g state [3,15]. Other some chromium doped system exhibit 4A2g ? 2A1g transition at higher energy, suggesting the presence of a part of the chromium ions in the Cr6+ state, Cr3+ (acting as modifier with CrO6 structural units) and Cr6+ (acting as network former with CrO2 structural units) 4 [3,6,11,17]. Based on the above assignments the energy matrices of d3 configuration are solved for different values of crystal field (Dq), and the Coulomb inter-electronic repulsion Racah parameter (B) is used to measure a degree of covalency/ionicity of ligand bonding, which was significantly higher in crystals than in glass systems [11,15,18]. 2. Experimental The glass system xCr2O3–(25x)Li2O–yNd2O3–(75y)B2O3 was prepared by mixing all specified weights, where x = 0, 0.1, 0.2, 0.3, 0.5 at y = 1.0 mol% and x = 1.0, at y = 0 mol%. The required weights of the high purity ingredients is grinded homogeneously in an agate mortar for 15 min, the powders taken in a porcelain crucible and melted in an electric furnace in the temperature range 1000–1050 °C for 1.0 h. The melts were stirred to ensure homogeneity and then quenched in between two well polished plates in order to obtain bubble free glasses and smooth surface. The samples were transferred to an annealing furnace kept at 300 °C for 10 h. The ESR spectra were obtained by ESR spectrometer (EMXBruker) operating in X-band frequency. The optical absorption spectra were recorded in the wave length rang (190–1100 nm) for the bulk glass samples with the two parallel polished faces were measured using (UV–Visible JenWay 6405) spectrophotometer. Infrared absorption spectra were obtained using FTIR (Perkin Elmer) in the range (2000–400 cm1), the sample powder were mixed with KBr of high purity. The density of the prepared glassy samples was determined by simple Archimedes method using carbon tetrachloride as immersing liquid. All measurements curried out at room temperature. 3. Results and discussion 3.1. ESR Fig. 1 shows ESR spectra for the investigated glass system doped with Cr2O3. No signals were obtained for chromium free samples; 7000 6000

hence all the obtained spectra are due to Cr ions. This indicates that the host glass is free from any paramagnetic centers, which means that the observed signals are only due to Cr3+ ions (d3 configuration). The intensity of signals is found to decrease with increasing of Cr2O3 content up to x = 0.3 mol%. For x > 0.3 mol% ESR signal follows opposite behavior as shown in Fig. 2. The decrease allows concluding that some of Cr3+ ions are converted to the Cr6+ ions, which reduces the spin density [10]. From the previous publishers, the concentration of chromium ion increased gradually up to 0.5 mol%, there are two signals, one at g = 1.98 and the second at g 5 should be observed, for chromium in alkali lead borotellurite glasses. While the signals occur at g = 4.50 and 1.98 for chromium in alkaline earth alumino borate, which can be attributed to Cr3+ and Cr5+ ions, respectively [3,19–21]. However, in the present work show only one signal has been observed, which is due to the available range of the magnetic fields. The observed behavior suggested that the Cr ions are of trivalent state (Cr3+) and acted up on by octahedral ligand field. In an octahedral field, the ground level of the Cr3+ ion is 4A2g; under the action of a low symmetric field component and spin orbit coupling, the spin state fourfold degenerate   splits into two Kramers doublets  32 and  52 [13]. In the present work the signal at g  5 was not be observed because it is out of range of the spectrometer i.e. low magnetic field. The separation between these two doublets leads to the resonance at g = 2 to g 5. According to the theory of Landry, the low field line with g 5 is attributed to the isolated Cr3+ ions that have local rhombic sites subjected to strong crystal field effects. This signal arises mainly due to 3/2 M +3/2 that are allowed due to low symmetry of Cr3+ ions. Comparatively larger intensity of low field peak (g  5) indicates higher concentration of isolated Cr3+ ions in this glass network [22]. The intensity and the line width of the resonance signal at g  1.98 arises due to the exchange coupling between tow Cr3+– Cr3+ ion pairs are observed to increase with the increase in the concentration of chromium ions. Such as increase indicates an increasing presence of chromium ions in the trivalent state Cr3+ in the glass network and also the near absence of anti ferromagnetic interactions between Cr3+ ions [19,22,23]. Fig. 2 shows the dependence of number of unpaired electrons (spin density) participating in resonance in glass system as a function of Cr2O3 content up to x = 0.3%. It is observed that the number of spins participating in resonance decreases as result of increasing chromium oxide content up to x = 0.3 then start to increase i.e. it follows the same pattern of the intensity. This is most likely due to the coexistence of both Cr3+ and Cr6+. Moreover, it seems that the number of Cr6+ and Cr3+ ions increases by increasing Cr2O3 content, when Cr3+ ions enter the net work as modifier and Cr6+ ions enter as net work former with CrO2 structural units [17]. It is 4 worth mentioning that the free Nd2O3 sample does not follow

5000

45000

10000

35000

8000

25000

6000

15000

4000

2000 1000 0 x=0.0,y=1

-1000

x=0.3,y=1

x=1.0,y=0

-2000

x=0.2,y=1 x=0.4,y=1

-3000 -4000 3350

Intensity

3000

Spin density

Intensity

4000

x=0.5,y=1 x=0.1,y=1

3400

3450

3500

H

3550

3600

5000

3650

3700

(G)

Fig. 1. ESR spectra of Cr3+ ions in xCr2O3–(25x)Li2O–yNd2O3–(75y)B2O3 glasses.

2000 0

0.1

0.2

0.3

0.4

0.5

Cr2O3 mol % Fig. 2. The spin density and the intensity of ESR as a function of Cr2O3 content.

M.A. Hassan et al. / Journal of Alloys and Compounds 539 (2012) 233–236

the same observed behavior of the other Nd2O3 doped sample. Where it was expected to the intensity and/or the spin density are high due to the high value of Cr2O3 content. This can be explained by assuming that the Nd2O3 plays considerable role in determining the structural units. 3.2. Optical absorption Absorption spectra for borate glasses doped with Cr3+ and Nd3+ ions observed out at room temperature. Fig. 3 shows spectra in which absorption bands due to 3d transitions of Cr3+ and 4f transitions of Nd3+ can be observed. All chromium doped samples exhibit green color which may indicate the presence Cr3+ ions in octahedral sites, as crystal field strength is a sensitive measurement of the surrounding of transition elements. In this respect, the main two visible absorption bands of such ions were observed. In average, the first broad band is m1 centered at 622 nm, corresponding to the 4A2g ? 4T2g transition and the second band is m2 locate at 426 nm corresponding to the 4A2g ? 4T1g transition [5,6]. Another third weak band is m3 at 380 nm exhibit 4A2g ? 2A1g transition; this is most likely due to the presence of traces of the chromium ions in the Cr6+ state [6]. The ligand field parameters; crystal field strength (Dq) and Racah parameter (B,C) are related only to electronic transition among the 3d electrons were evaluated from spectral positions of absorption bands using the following relations [3–7]:

Dq ¼

m1 10

ð1Þ

B¼

C¼

ð2m21 þ m22  3m1 m2 Þ ð15m2  27m1 Þ

m3  10Dq  4B 3

235

ð2Þ

ð3Þ

where m1 , m2 and m3 represent the energies in cm1. The estimated values of ligand field parameters are listed in Table 1. It is clear that the crystal field parameter Dq decreases with increasing of Cr2O3 content up to x = 0.3 mol%, by more addition (for x > 0.3 mol) the behavior follows opposite trend. As well as the values of the Racah parameter B follow opposite behavior. For x = 0.1 to 0.3 mol% region, some of ions Cr3+ are converted to the Cr6+ ions, the data suggests that the environment of the chromium ion is more ionic, and the more the electrons are localized on the d-shell [6,10,11,15], and vice versa, this behavior exhibit good agreement with the ESR data. However as tabulated in Table 1, the results reveals that the ratio (Dq/B) is found to be 2.06, which indicates the Cr3+ ions are occupy weak-field octahedral sites, according to typical range of Dq/B values; in strong crystal-field, the ratio of is greater than 2.3 and in weak crystal-field sites, the ratio is less than 2.3, while for intermediate fields it is equal to 2.3 [6,18]. The inter-electronic repulsion Racah parameter C can be calculated by using Eq. (3) [6], yielding the value of parameter C shows the same pattern of B, as it listed in Table 1. The estimated value of B in the investigation samples is lower than the Bfree ¼ 918 cm1 value of the free (gaseous) Cr3+ ion. The bonding can also be predicted using the following expression [3,15]:

h¼

½ðBfree  BÞ=Bfree  kCr3þ

ð4Þ

where h and k are nephelauxetic functions of the ligand and the central metal ion, respectively, the value of kCr3þ ¼ 0:21. The nephelauxetic function h value is decreases with increasing of Cr2O3 content up to x = 0.3 mol%, which suggests the decrease in covalent bonding nature between Cr3+ and the surrounding ligands, the lower values of h mean an increased localization of the d-electrons. Then for x > 0.3 mol; the function h is increases, the larger values of h mean an increased delocalization of the d-electrons due to d orbitals overlapping with ligand orbitals [3,15]. It is worth noting, the values of the crystal field parameters for the glassy sample (x = 1, y = 0) are almost the same values of (x = 0.5, y = 1); this is most likely reveals that the addition of Nd2O3 reduces the number of Cr6+ ions in comparing with the neodymium doped glasses. However the glassy sample (x = 1, y = 0) exhibit very weak absorption band at 670 nm, this band is not observed in the spectrum of the other investigated glassy samples. This absorption is resulting from the interaction of the 2E2g with vibrationally broadened 4T2g state [5,15,17]. On the other hand, the optical absorption spectrum of all glassy samples doped with Nd3+ in the wavelength range of 400–900 nm are shown in Fig. 3. The spectrum was analyzed, the spectra consists of various absorption levels corresponding to the transitions between the ground state and higher energy states (4F3/2, 4 F5/2 + 2H9/2, 4S3/2 + 4F7/2, 4F9/2, 2H11/2, 4G5/2 + 2G7/2, 2K13/2 + 4G7/2, 4 4 G9/2 I9/2) have been observed inside the 4f3 electronic configuration of the Nd3+ ions. The spectra show no change in the barycenters, and the intensity of these bands. The transitions were assigned by comparing the band positions in the absorption spectra with those reported in literatures [24–27]. 3.3. IR spectra

Fig. 3. Typical optical absorption spectra for xCr2O3–(25x)Li2O–yNd2O3– (75y)B2O3 where (x = 0, y = 1), (x = 0.5, y = 1) and (x = 1, y = 0) glassy samples.

The infrared spectroscopy was used to study the structural change in the samples due to the interactions between the different atoms. It is well known that the effect of introduction of alkali

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Table 1 Crystal field strength (Dq), Racah parameters (B & C), nephelauxetic functions (h) of Cr3+ ions and the experimental density in xCr2O3–(25x)Li2O–yNd2O3–(75y)B2O3 glasses. Concentration

Ligand field parameters

x (Cr2O3)

y (Nd2O3)

Dq (cm

0.0 0.1 0.2 0.3 0.4 0.5 1.0

1.0 1.0 1.0 1.0 1.0 1.0 0.0

– 1610.3 1607.7 1600.0 1605.1 1615.5 1615.5

1

)

Archimedes density

B (cm

1

)

C (cm

– 769.9 783.3 797.7 797.0 769.2 769.2

– 2044.5 2654.5 3072.8 2267.5 2136.4 2647.3

0.6

N4

0.5 0.4 0.3

Absorbance (arb. u.)

0.2 0

0.1

0.2 0.3 0.4 Cr2O3 mol %

0.5

x=1 ,y=0 x=0.5,y=1

)

Dq/B

h

Density (g/cm3)

M. Volume (cm3)

– 2.092 2.053 2.006 2.014 2.100 2.100

– 0.768 0.699 0.624 0.628 0.772 0.772

2.236 2.290 2.274 2.276 2.305 2.292 2.312

27.89 26.49 26.72 26.75 26.47 26.67 26.35

chromium ions from Cr3+ to Cr6+ structural units at lower concentration (up to x = 0.3 mol%), that take part in network forming positions and make the glass network more stable when Cr2O3 is present at higher concentrations (>0.3 mol%) in this system. Moreover, the optical absorption spectra recorded that evidently exhibit bands characteristic of Cr3+ ions in an octahedral symmetry, and the addition of Nd2O3 reduces the number of Cr6+ ions.

x=0.4,y=1 x=0.3,y=1 x=0.2,y=1 x=0.1,y=1 x=0.0,y=1

400

1

600

800

1000

1200

1400

1600

1800

2000

-1

Wavenumber cm

Fig. 4. Infrared spectra of xCr2O3–(25x)Li2O–yNd2O3–(75y)B2O3 glassy samples; insert is the N4 composition dependence.

oxides into B2O3 glass is the conversion of triangle BO3 units into more stable tetrahedral BO4 units and may also create non-bridging oxygen’s [11,16]. Fig. 4 shows the IR absorption spectra, all samples follow the same pattern, the spectra shows three main bands related to BO3 and BO4 units. The first is locates between 1180– 1600 cm1 and is usually attributed to the asymmetric stretching vibration mode of B–O on boron triangles (BO3). While as the second band is the splitting band ranged from 760 to 1180 cm1 and is most probably due to B–O stretching and rocking vibration of tetrahedral (BO4) groups. The third is the weak band locates between 670 and 570 cm1 is usually attributed to bending vibration of BO3 groups. Also there are very weak band below 570 cm1 which is most likely attributed to the vibration of lithium ions [28,29]. The broad bands in Fig. 4 are composed of overlapping individual bands. To get quantitative information about the structural groups in samples, the spectra are deconvoluted into the minimum number of bands using Gaussian line shapes. The N4 factor is usually used to measure the BO4 ratio (N4 = concentration of BO4 groups/concentration of all borate groups) [30]. As shown the insert in Fig. 4, it is clear that the N4 factor almost unchanged due to the small value of Cr2O3 content. Also as listed in Table 1 the obtained density and molar volume composition dependence. Both density and molar volume shows almost unchanged with increasing Cr2O3 content follow the same pattern of N4 factor in IR results. 4. Conclusions The analysis of ESR spectra and ligand field theory of neodymium doped borochromate glasses, reveals that a conversion of

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