NiO-induced crystallization and optical characteristics of Li2O–CaF2–P2O5 glass system

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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phys. stat. sol. (a) 205, No. 1, 177–187 (2008) / DOI 10.1002/pssa.200723371

NiO-induced crystallization and optical characteristics of Li2O–CaF2–P2O5 glass system

G. Murali Krishna, Y. Gandhi, and N. Veeraiah*

Department of Physics, Acharya Nagarjuna University, Nuzvid Campus, Nuzvid – 521201, India

Received 9 June 2007, revised 1 August 2007, accepted 14 August 2007 Published online 25 October 2007

PACS 61.10.Nz, 75.50.Kj, 78.20.Ci * Corresponding author: e-mail [email protected], Phone: +91 8656 235551/91 9440 015188, Fax: +91 8656 235200

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction There has been an increasing demand for the development of materials suitable for ultra-broadband optical amplifiers to revolutionize telecommu-nication systems. Although rare earth ion-doped glasses and ceramics were considered as suitable candidates for such applications, the optical amplification band width in these materials is narrowed, due to the fact that the emis-sion bands of the 4f–4f transition of the rare earth ions are very narrow. Glass ceramic materials with appropriate transition metal ions as nucleating agents are considered as better candidates for such applications over glass and crys-talline materials. The reasons are obvious. In these materi-als the dominance of non-radiative losses over the relaxa-tion of excited states of luminescence ions is low. Further, the glass ceramic materials possess outstanding physical properties like high mechanical and insulating strengths, high thermal conductivity and high chemical durability when compared with glass materials. Additionally, if care is taken to minimize the size of the microcrystals (far less than the wavelength of interest) in glass ceramics, the light scattering caused by these crystals is negligibly low and thereby a substantial improvement in the quantum effi-ciency of the broadband emission can be achieved. Further, the lasing ions disperse more evenly in crystalline nuclei of bulk glass ceramic samples when compared with as-quenched glass samples.

The characteristics of glass ceramics, however, depend on the kind and quantity of the crystal phase formed as well as on the residual glass composition. Hence, the selec-tion of a suitable nucleating agent in the correct concentra-tion and determination of the crystallization temperature and the time of nucleation and growth are important factors in the formation of a glass ceramic. The crystallizing agents normally used are oxides of titanium, chromium, manganese, vanadium and nickel. Among these, Cr4+ ions have been used as potential candidates for high-gain opti-cal amplifiers with larger bandwidths in glass ceramics; nevertheless, the chromium ions exist in multivalent states, i.e. Cr3+, Cr4+, Cr5+ and Cr6+ [1, 2]. The same is true in the case of other transition metal ions like titanium and man-ganese [3, 4]. Hence, it is too difficult to achieve strict con-trol over the required or suitable valence state of these ions embedded in crystal phases. Unlike these ions, nickel ions mostly exist in the divalent state and are extremely stable and there is no need for special care in experimentation in retaining nickel ions in the divalent state. There have been hardly any reports so far about reduction or oxidation of Ni2+ ions to lower or higher oxidation states respectively in glasses or glass ceramic matrices. Further, Ni2+ is an ion with exceptionally large crystal stabilization energies par-ticularly when it is in an octahedral field [5]. Ni2+ ions ex-hibit several strong absorption bands in the visible and

Li2O–CaF2 –P2O5 glasses mixed with different concentra-tions of NiO (ranging from 0 mol% to 2.0 mol%) were crystal-lized. The samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy disper-sive spectroscopy and differential thermal analysis. Studies were extended to optical absorption and magnetic susceptibil-ity of these glass ceramic samples. The XRD and SEM studies

reveal the presence of lithium phosphate, calcium phosphate and nickel phosphate crystal phases. The optical absorption studies together with magnetic susceptibility measurements indicate a gradual transformation of nickel ions from tetrahe-dral sites to octahedral sites (lasing sites) as the concentration of NiO is increased beyond 0.8 mol%.

178 G. Murali Krishna et al.: Crystallization and optical characteristics of Li2O–CaF2 –P2O5

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Table 1 Physical parameters of Li2O–CaF2 –P2O5 :NiO glass ceramics.

sample density, d (g/cm3)

total nickel ion conc., Ni (1021 ions/cm3)

interionic distance of Ni ions, ri (Å)

dissociation factor (10–6 g/cm2/min)

N0 2.1866 – – 3.31 N2 2.3852 2.82 7.08 0.52 N4 2.4090 5.68 5.60 0.47 N6 2.4171 8.55 4.89 0.36 N8 2.4436 11.51 4.43 0.12 N10 2.4430 14.37 4.11 0.23 N15 2.4425 21.50 3.60 0.33 N20 2.4410 28.59 3.27 0.46

near-infrared (NIR) regions where pumping sources are

easily available. The octahedrally positioned Ni2+ ions in

glass networks are expected to exhibit eye-safe laser emis-

sion of wavelength about 1.5 µm due to the 3T2 → 3A2

transition, even at room temperature, which is of great im-

portance in telecommunications [6]. There have been a

considerable number of recent studies on the lasing action

of nickel ions in various glass and glass ceramic materials

[7, 8].

0.0

2.0

4.0

6.0

0 0.2 0.4 0.6 0.8 1 1.5 26

6.2

6.4

6.6

6.8

a

Conc. NiO (mol %)

pHva

lue

Dis

soci

atio

nfa

ctor

(10-6

g/cm

2 /min

)

bc

Figure 1 (online colour at: www.pss-a.com) (a) Variation of dis-sociation rate of Li2O–CaF2 –P2O5 :NiO glass ceramics in dis-tilled water solution at 90 °C; (b) dissociation rate of Li2O–CaF2 –P2O5 :NiO glass samples; (c) variation of pH of solution with NiO concentration after corrosion testing of the glass ce-ramic samples.

Although crystallization with transition metal ions is mostly confined to SiO2-based glasses, there are a consid-erable number of recent reports where transition metal ions have been used as nucleating agents and observed to in-duce phase separation in other glass systems like P2O5 and GeO2 [9, 10]. Lithium phosphate glasses are well known due to their far-infrared transmission and owing to their other technological applications like solid electrolytes and electrochemical devices such as high energy density batter-ies [11]. Additionally, the Li2O–P2O5 glass network offers a suitable environment for hosting lasing ions like nickel. Further, addition of CaF2 to alkali phosphate glass matrices is predicted to lower the viscosity and to decrease the liq-uidus temperature by a substantial extent and it is also found to act as an effective mineralizer [12]. The current investigation is aimed at understanding the catalyst action of NiO on the crystallization of the Li2O–CaF2–P2O5 glass system by means of X-ray diffraction (XRD), electron mi-croscopy, energy dispersive spectroscopy (EDS), thermal analysis and optical absorption studies and to provide hints on the possible use of these glass ceramic materials for la-ser emission in the NIR region. 2 Experimental For the present study, particular compositions (30 − x)Li2O–10CaF2–60P2O5 :xNiO with eight values of x ranging from 0 to 2.0 were chosen. The details of the compositions are:

N0: 30Li2O–10CaF2–60P2O5 N2: 29.8Li2O–10CaF2–60P2O5 :0.2NiO N4: 29.6Li2O–10CaF2–60P2O5 :0.4 NiO N6: 29.4Li2O–10CaF2–60P2O5 :0.6 NiO N8: 29.2Li2O–10CaF2–60P2O5 :0.8 NiO N10: 29Li2O–10CaF2–60P2O5 :1.0NiO N15: 28.5Li2O–10CaF2–60P2O5 :1.5 NiO N20: 28.0Li2O–10CaF2–60P2O5 :2.0 NiO

Analytical-grade reagents of NH4H2PO4, Li2CO3, CaF2 and NiO powders in appropriate amounts (all in mol%) were thoroughly mixed in an agate mortar and melted us-ing a thick-walled platinum crucible at 1000 ± 10 °C in a PID temperature-controlled furnace for about 2 h. The re-sultant bubble-free melt was then poured in a brass mould

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and subsequently annealed at 250 °C. The samples pre-pared were free from visible inhomogeneities, such as in-clusions, cracks or bubbles. The specimens with various concentrations of NiO were heat treated in an automatic temperature controlling furnace at their crystallization temperatures Tc (520–550 °C, identified from differential thermal analysis) for 4 h. After the heat treatment in the furnace at specified temperature, the samples were quenched in air to room temperature. The crystalline phases in the glass ceramic samples were identified by XRD using an Xpert PRO’panalytical X-ray diffractometer with Cu Kα radiation. The microstructures were observed with scanning electron microscopy (SEM; Hitachi model S-3400N) using polished surfaces. Differential thermal analysis was carried out using a Netzsch STA 409 C/CD instrument with a programmed heating rate of 10 °C/min in the temperature range 30–1000 °C, to determine the glass transition temperature and crystalline peaks. The density d of the glass ceramics was measured to an accuracy of ±0.0001 g by the standard principle of Archimedes using o-xylene (99.99% pure) as the buoyant liquid with an Ohaus balance (model AR 2140). The mass of the samples was measured to an accu-racy of 0.1 mg using this balance. For evaluating the chemical durability, bulk samples were suspended by a weightless strand in about 100 ml of water of pH = 7 at 90 °C for 12, 24 and 48 h. The weight loss (∆W) was measured each time and the average dissociation rate (DR) was evaluated using DR = ∆W/(S × t) (in g/cm2/min), where S is the surface area of the sample and t is the time

of immersion. The pH value of the residual solution at the end of each immersion time was measured at 30 °C. Opti-cal absorption spectra of the glasses were recorded at room temperature in the wavelength range 300–1500 nm up to a resolution of 0.1 nm using a Cary 5E UV–visible–NIR spectrophotometer. The dimensions of the samples used for these measurements were ~1.0 cm × 1.0 cm × 0.2 cm. Magnetic susceptibility measurements were also carried out on these samples by Guoy’s method using fine pow-ders of the samples to an accuracy of 10–6 emu. 3 Results The density of the NiO-free glass ceramic material was measured to be 2.1866 g/cm3. The density d of the glass ceramic samples is found to increase with in-creasing concentration of NiO nucleating agent up to 0.8 mol%, and beyond this concentration a slight decrease in the value of d is observed (Table 1). From the measured values of the density and average molecular weight M of the samples, various other physical parameters such as nickel ion concentration Ni and mean nickel ion separation ri in the Li2O–CaF2–P2O5 :NiO glass ceramic samples were computed and are presented in Table 1. The values of DR measured as per the procedure described in Section 2 are also given in Table 1. DR decreases considerably with gradual increase of NiO up to 0.8 mol% (Fig. 1; Table 1); beyond this concentration DR is observed to increase slightly. However, when compared with the as-quenched glass samples there is a substantial improvement in the degrada-tion of these glass ceramic samples (Fig. 1, curve a). The

Figure 2 SEM images of some of the crystallized Li2O–CaF2 –P2O5 :NiO glass samples.



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pH of the distilled water solution after corrosion testing of the Li2O–CaF2–P2O5 :NiO glass ceramic samples in-creased significantly with increasing concentration of NiO up to 0.8 mol%; beyond this concentration of the nucleat-ing agent a slight decline in the pH value of the solution could be observed. The variation in pH of the liquid in contact with samples is consistent with the dissolution of phosphate species from these glasses and the subsequent formation of phosphoric acid. SEM images of some of the crystallized samples are shown in Fig. 2. The SEM images of the crystallized samples exhibit well-defined and randomly distributed crystals entrenched in glassy matrix. The chemical makeup of the

phases characterized using EDS for some of the samples is shown in Fig. 3a; Fig. 3b shows EDS spectra of sample N8 at three different points. The EDS analysis of the glass ce-ramic materials indicates lithium, phosphorus, calcium and nickel elements in various crystalline phases. We also re-corded the X-ray maps of these glass ceramics for nickel ions. The maps indicated the reasonably uniform distribu-tion of nickel ions in the entire glass ceramic material. The XRD patterns (Fig. 4) of the glass ceramic sam-ples exhibit peaks due to lithium phosphate, calcium phos-phate and nickel ortho- and pyrophosphate crystal phases. With increasing content of crystallizing agent, i.e. NiO, in the glass matrix, the structure of the diffraction pattern be-

Figure 3 (online colour at: www.pss-a.com) a) EDS spectra of some of the Li2O–CaF2 –P2O5 :NiO glass ceramic samples. b) EDS spectra of Li2O–CaF2 –P2O5 :NiO glass ceramic sample N8 recorded at three different points. The spectra indicate the reten-tion of all the elements that are present originally in the glass samples at different points.

a)

b)

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10 20 30 40 50 60 70 80

Ni3(PO4)2

LiPO3

Ni2P2O7

β-Ca2P2O7

Pure un crystallized

Pure crystallized at 400 oc

N8

N6

N4

2θ

Inte

nsity

(a.u

)

Ni(PO3)2

Figure 4 (online colour at: www.pss-a.com) XRD patterns of NiO-doped Li2O–CaF2–P2O5 glasses crystallized at 400 °C. comes more complex and a clear variation in the intensity and the pattern of diffraction peaks can be observed. XRD analysis indicates that crystallization with nickel ions causes not only the appearance of diffraction peaks due to new crystalline phases with nickel ions as constituents but also an increase in the intensity of the diffraction peaks due to lithium and calcium phosphate crystals phases. Thus from the SEM images and XRD patterns it can be con-cluded that NiO enhanced the phase separation tendency of various crystalline phases in the glass ceramic samples. Figure 5 shows the differential thermal analysis traces of the Li2O–CaF2–P2O5 :NiO samples. For the pre-crystallized sample an endothermic change at about 403 °C due to the glass transition temperature Tg is observed. At still higher temperature, an exothermic peak due to the crystal growth followed by another endothermic peak due to re-melting of the glass are also observed. All the crystal-lized samples exhibit an endothermic change due to the glass transition followed by three clear exothermic peaks Tc1, Tc2 and Tc3 due to the crystal growth. An endothermic peak due to re-melting of the samples is observed in the traces of all the samples. The pertinent data related to dif-ferential thermal analysis are presented in Table 2. The optical absorption spectra recorded at room tem-perature of the Li2O–CaF2–P2O5 glasses crystallized with

350 550 750 950

N0

N2

N4

N6

N8

N10

N15

N20

TgTc3

Tc2

TmTc1

Temperature 0C

Exo

Endo

Figure 5 (online colour at: www.pss-a.com) Differential thermal analysis traces of Li2O–CaF2 –P2O5 :NiO glass ceramics. different concentrations of NiO in the wavelength range 250–1500 nm are shown in Fig. 6. The absorption edge observed at 314 nm for sample N0 (not shown in Fig. 6) is observed to shift to 300 nm with the introduction of nucle-ating agent (0.2 mol%). When the NiO content is raised to 0.8 mol%, the edge is observed to shift further towards lower wavelength, and beyond this concentration it is shifted gradually towards higher wavelength. From the ob-served absorption edges, we evaluated the optical band gaps (E0) of these samples by drawing Tauc plots (Fig. 6c) between (αħω)1/2 and ωħ according to

2

0( ) ( )c Eα ω ω ω� �= - . (1)

Table 2 Summary of data for differential thermal analysis studies of Li2O–CaF2 –P2O5 :NiO glass ceramics.

sample Tg (°C)

Tc1 (°C)

Tc2 (°C)

Tc3 (°C)

Tm (°C)

N0 403 – 527 – 885 N2 406 514 532 556 889 N4 411 520 541 562 896 N6 419 529 553 574 904 N8 427 545 566 599 908 N10 422 542 557 587 900 N15 418 538 551 580 895 N20 412 528 544 576 890



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3T2(F)3A2(F)

Oh1Td1

N8

N2

N20

N10

N15

N6

N4

Wavelength (nm)

abso

rban

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rb.U

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300 500 700 900

Oh43T1(P)

3T1(F)1T2(D)

Oh2Oh3

Td2

N20N2N15N4N6N10N8

3T1(P)

1.5

2.0

2.5

3.0

0 0.4 0.8 1.2 1.6 2Conc. NiO (mol

abso

rptio

nco

eff.

(cm

-

3T1(F) 3A2(F)

3A2(F) 3T2(F)

(b)

(a)

2.0

2.5

3.0

2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2

N8

N4

N10N6

N15

N2

N20

hν (eV)

( αh ν

)1/2

(cm

-1eV

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a)

b)

c)

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300 500 700 900 1100 1300 1500Wavelength (nm)

Oh4

3T1(P)1T2(D)

Oh3

Td2

3T1(P)

3T1(F)

Oh23T2(F)

3A2(F)

Oh1

Td1

pre-crystallized sample

crystallized sample

rela

tive

abso

rban

ce(a

rb.

Uni

ts)

Figure 6 (online colour at: www.pss-a.com) Optical absorption spectra of Li2O–CaF2 –P2O5 :NiO glass ceramics in the wavelength region (a) 300–900 nm and (b) 900–1500 nm. The inset shows the variation of the absorption coefficient at the peak position for one of the octahedral and one of the tetrahedral bands. c) Tauc plots for evaluating the Li2O–CaF2 –P2O5 :NiO glass ceramics. d) Compari-son of optical absorption spectra for sample N20 before and after crystallization.

From the extrapolation of the linear portion of the curves of Fig. 6c, the values of optical band gap (E0) were deter-mined and are presented in Table 3. The value of the opti-cal band gap is observed to be the highest for the glass ce-ramic sample N8. Additionally, the spectrum of sample N2 exhibited six clearly resolved intense absorption bands in the visible and NIR regions at 1285 nm (Oh1), 1034 nm (Td1), 793 nm (Oh2), 617 nm (Td2), 481 nm (Oh3) and 432 nm (Oh4). As the con-centration of NiO crystallizing agent is increased up to 0.8 mol%, the intensity of the octahedral bands (Oh bands) is observed to decrease with a shift towards slightly lower wavelength; in this concentration range the intensity of the

two tetrahedral bands (Td bands) is observed to increase with no considerable shift in the band positions. With the increase of NiO content from 0.8 mol% to 2.0 mol%, the positions of the octahedral bands are shifted towards slightly higher wavelength with increasing intensity at the expense of Td1 and Td2 bands (Fig. 6). The variation of peak absorption co-efficients of one of the octahedral bands and one of the tet-rahedral bands are plotted against the concentration of nu-cleating agent in the inset of Fig. 6b. Figure 6d shows a comparison of optical absorption spectra for one of the sam-ples (i.e. N20) before and after crystallization; the compari-son indicates larger absorption for crystalline samples. A summary of the data for the positions of various absorption

Table 3 Summary of the data for optical absorption spectra of Li2O–CaF2 –P2O5 :NiO glass ceramics.

sample N2 N4 N6 N8 N10 N15 N20

absorption edge (nm) 300 296 293 285 289 296 304 optical band gap, E0 (eV) 3.44 3.62 3.74 3.90 3.82 3.52 3.36

Ni2+ band positions, octahedral transitions (nm) 3A2(F) → 3T2(F) (Oh1) 1285 1281 1279 1277 1278 1282 1286 3A2(F) → 3T1(F) (Oh2) 793 789 787 784 786 791 794 3A2(F) → 1T2(D) (Oh3)

481 476 466 464 469 479 483 3A2(F) → 3T1(P) (Oh4)

432 430 427 421 424 431 434

Ni2+ band positions, tetrahedral transitions (nm) 3T1(F) → 3A2(F) (Td1) 1034 1035 1036 1038 1037 1034 1033 3T1(F) → 3T1(P) (Td2)

617 618 619 620 619 618 617 Dq (cm–1) 757 761 766 768 763 757 743 B (cm–1) 841 845 847 850 848 841 835 nephelauxetic ratio (β) 0.817 0.820 0.818 0.825 0.823 0.817 0.810

d)



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Table 4 Magnetic susceptibilities and effective magnetic mo-ments of Li2O–CaF2 –P2O5 :NiO glass ceramics.

sample magnetic susceptibility, χ (10–5 emu)

µeff (µB)

N2 2.374 3.52 N4 5.237 3.68 N6 8.576 3.84 N8 12.848 4.05 N10 13.805 3.76 N15 17.579 3.48 N20 19.245 3.16

bands is given in Table 3 along with other pertinent data on optical absorption spectra of these samples. The magnetic susceptibility of the Li2O–CaF2–P2O5 :NiO glass ceramics measured at room temperature is observed to increase gradually with increasing content of NiO crystallizing agent. From the values of the magnetic susceptibilities, the magnetic moment of Ni2+ ion was evaluated, as presented in Table 4. 4 Discussion P2O5 is a well-known strong glass-forming oxide, participating in the glass network with PO4 structural units. The PO4 tetrahedrons are linked together with covalent bonding in chains or rings by bridging oxy-gens. The glass network is built up of Q3 tetrahedra with three bridging oxygens; the fourth oxygen is doubly bonded to the phosphorus atom with substantial π-bond character to account for the pentavalency of phosphorous. Similar to the fluoroborate glass network, the phosphate glass network containing alkali earth fluorides like CaF2 may consist of P(O,F)4 or PO3F or PO2F2 units. Li2O is a conventional modifier oxide and enters the glass network, either by rupturing or by breaking up the P–O–P bonds, introducing coordinated defects known as dangling bonds along with non-bridging oxygen ions. The Li2O–CaF2–P2O5 :NiO glass ceramic materials contain a fine-grained uniform structure, consisting of small crystals of irregular and distorted form, aggregated into spherulites, with residual glass interlayers acting as in-terconnecting zones making the samples free of voids and cracks. XRD patterns of these ceramic samples confirm the presence of such microstructural grains. The crystalline lithium phosphate (LiPO3), nickel phosphates (Ni3(PO4)2, Ni(PO3)2 and Ni2P2O7) and β-calcium phosphate (Ca2P2O7) phases that are kinetically and thermodynamically feasible, seemed to be the main products in these glass ceramic samples. These metastable crystals are usually in solid-solution phases that can incorporate the major constituents of the glass composition in approximately the same pro-portion as they were present in the original glass matrix. The nickel-rich areas in the samples may enhance the reac-tivity of nickel with the other oxides that precipitate as a high density of fine nickel-rich crystals. These tiny crystals act as heterogeneous nuclei for the crystallization of the

remaining glass. The average diameter dc of the induced crystal phases is evaluated using Scherrer’s formula:

c

0.9

∆ cosd

λ

θ θ= , (2)

where λ is the wavelength of the X-rays and ∆θ is the half-width of the diffraction peak. The average diameters of various crystalline phases evaluated using this formula are found to lie in the range 10–50 nm. This diameter range is sufficiently far away from the wavelengths of the visible and NIR regions. In view of this, the scattering losses due to the difference of the refractive indices between crystal-lites and glass ceramic are expected to be negligibly small. Probably for this reason, these samples retain transparency even after crystallization. The SEM images of NiO-doped glass ceramics show larger crystals than those visible in the NiO-free samples. The SEM images show a reasonably homogeneous distribution of the crystals in glass ceramic samples N8 to N20. The appearance of different crystallization tempera-tures in the differential thermal analysis patterns of the glass ceramic samples obviously suggests the presence of different phases of crystallization in the samples. Crystalli-zation in the samples may take place based on surface and bulk nucleations. The non-isothermal devitrification pro-cess is represented by the equation [13]

1ln exp

1 n

AN nE

RTα β

Ê ˆ Ê ˆ= -Ë ¯ Ë ¯- , (3)

where α is the fraction of the volume crystallized at tem-perature T, E is the activation energy, β is the differential thermal analysis heating rate and A is a constant. The nu-clei centres N are the sum of surface and bulk nuclei. The higher the value of N, the lower is the crystallization tem-perature Tc. In the surface nucleation mechanism, the nu-cleating centres are formed only on the surface and the crystals start developing from the surface to the inside of the glass one-dimensionally (in this case n = 1). In contrast, in the bulk nucleation the crystals grow three-dimensio-nally (in this case n = 3). The general shape of the crystal-lization peaks is strongly dependent on the value of n. The higher the value of n the narrower is the width of the crys-tallization peak [14]. For the surface crystallization we may therefore expect relatively wide peaks when compared with the bulk crystallization peaks. The pattern of the dif-ferential thermal analysis peaks suggests that the crystalli-zation is predominantly due to surface crystallization in samples N2 to N8; in samples N10 to N20, bulk crystalliza-tion seems to prevail even though it is generally believed that volume crystallization can take place in the samples only if the glass system is crystallized at a temperature far above the first crystallization peak [15]. The density of Li2O–CaF2–P2O5 :NiO glass ceramics is observed to depend strongly on the content of NiO in the glass ceramic. In general, the degree of structural com-pactness, modification of the geometrical configuration of

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the glassy network, size of the microcrystals formed, change in the coordination of the glass-forming ions and fluctuations in the dimensions of the interstitial holes are some of the factors that influence the density of the glass ceramic material. The larger values of the density of sam-ples N8–N20 when compared with the other samples (Ta-ble 1) is an indication of growing structural compactness due to the crystallization in these samples. The Ni2+ ions seem to exist in both four- and six-fold coordination in the glass ceramic samples. In general, tetra-gonally positioned Ni2+ ions do not induce the formation of any non-bridging oxygen ions, but octahedrally positioned ions may act as modifiers [16]. The decrease in the average DR of the glasses with increasing concentration of NiO up to 0.8 mol% indicates that the concentration of undisturbed P–O–P, P–O–Ni bonds is higher in sample N8. The pres-ence of such bonds, in higher concentrations, makes the glass ceramic samples more hydration resistant. Addition-ally, the length and the orientation distribution of PO4 chains also play a major role in deciding the chemical du-rability of these samples. Previous empirical studies showed that shortened PO4 chains in the network are re-sponsible to some extent for the high corrosion resistance of the glasses [17]. This conclusion obviously suggests that there is a gradual increase in the concentration of tetrahe-dral sites of nickel ions that facilitate the formation of more and more P–O–Ni bonds in the glass ceramics as the concentration of NiO increases from 0 mol% to 0.8 mol%. Ceramic samples crystallized with NiO > 0.8 mol% showed a marked improvement in the dissolution rate when compared with samples N2 to N8, even though an in-significant increase in the dissolution rate for samples N10 to N20 (compared to that of N8) could be seen. Surfaces of samples N8 to N20 after the corrosion testing are observed to be smooth, with the corners and the edges retaining their original sharpness. The pH of the residual water solution also correlates with a notable improvement of the durabil-ity of these samples. The improvement of chemical dura-bility means the presence of undisturbed P–O–P and P–O–Ni bonds in higher concentrations in the glass ce-ramic network. Using Tanabe–Sugano diagrams for d8 ions, the opti-cal absorption spectra of Ni2+ ions in mixed glass ceramics were analysed and the bands Oh1, Oh2, Oh3 and Oh4 are as-signed to the transitions from the 3A2 ground state of octa-hedrally positioned Ni2+ ions to 3T2(F), 3T1(F), 1T2(D) and 3T1(P) excited states, respectively. The ligand field pa-rameters Dq (crystal field splitting energy) and B (Racah parameter; interelectron repulsion parameter) were evalu-ated using energies of these transitions and the values ob-tained are given in Table 3. The energy level diagram for one of the glass ceramic samples (i.e. N10) is shown in Fig. 7. The bands with barycentres at about 1035 nm (Td1) and 620 nm (Td2) nm are attributed to 3T1(F) → 3A2(F) and 3T1(F) → 3T1(P) transitions, respectively, of Ni2+ ions posi-tioned in tetrahedral sites [18]; there are also reports sug-gesting that in the region of the 3T1(F) → 3T1(P) transition,

Figure 7 (online colour at: www.pss-a.com) Energy level dia-

gram of octahedral Ni2+ ions in Li2O–CaF2–P2O5 glass ceramic

crystallized with 1.0 mol% of NiO.

a weak spin-forbidden transition 3A2(F) → 1E(D) due to distorted octahedral site Ni2+ ions may also occur [5]. The band observed at about 480 nm represents a spin-forbidden octahedral transition 3A2 → 1T2 [19]. Thus, the optical ab-sorption spectra of Li2O–CaF2–P2O5 :NiO glass ceramic samples indicate that Ni2+ ions exist both in octahedral and tetrahedral sites in the network. The tetrahedrally posi-tioned nickel ions participate in the glass network with NiO4 structural units and alternate with PO4 units and strengthen the structure. A schematic illustration of the phosphate network containing Ni2+ in octahedral and tetra-hedral positions is shown in Fig. 8. Further, the way the intensity of these bands varies with the concentration of NiO suggests that at smaller con-centrations (≤0.8 mol%), Ni2+ ions prefer to occupy tetra-hedral positions; at larger concentrations, these ions prefer mostly octahedral sites in the glass ceramic lattice. Using the interelectronic repulsion parameter B, the nephe-lauxetic ratio was also evaluated using the formula β = B (complex)/B′ (free ion), for all the ceramic samples; the value of β l is observed to be a maximum for sample N8 and to decrease gradually for further increase in the con-centration of NiO crystallizing agent (Table 3). This ratio gives information on the delocalization of electrons from the metal ions into molecular orbitals covering both the metals and ligands. The lower the value of β the greater is the delocalization effect. The highest value of β obtained for ceramic sample N8 indicates the minimum delocaliza-tion effect in this sample [20]. The octahedrally positioned Ni2+ ions act as modifiers similar to Li+ and Ca2+ ions and induce non-bridging oxy-



ph

ysic

ap s sstat

us

solid

i a

gens in the glass ceramic network. The lower the concentra-tion of these modifier ions, the lower is the concentration of non-bridging oxygens in the glass matrix. This leads to a de-crease in the degree of localization of electrons thereby de-creasing the donor centres in the glass matrix. The presence of smaller concentrations of these donor centres increases the optical band gap and shifts the absorption edge towards lower wavelength, as observed (Fig. 6c). Further support for the conclusion that there is a grad-ual increasing presence of Ni2+ ions in tetrahedral sites in the networks of ceramic samples N2 to N8 and an increas-ing occupancy of octahedral sites in samples N10 to N20 can also be obtained from the analysis of the results of the magnetic properties. The magnetic properties of Li2O–CaF2–P2O5 :NiO glass ceramic samples arise from the paramagnetic Ni2+ (both tetrahedral and octahedral) ions. Since the ground state 3T1(F) of tetrahedral Ni2+ ions pos-sesses much inherent orbital angular momentum, the mag-netic moment of perfect tetrahedral Ni2+ should be ~4.2µB. Even a slight distortion reduces this value markedly be-cause of the orbital degeneracy. The fairly regular tetrahe-dral complexes of these ions are expected to have a mag-netic moment in the range 3.5–4.1µB [20]. The value of the effective magnetic moment (4.05µB) obtained for the ce-ramic sample N8 indicates that most of the Ni2+ ions occupy tetrahedral positions in the network of the sample. Magneti-cally, the octahedral Ni2+ complexes have relatively simple behaviour and their magnetic moments are expected to lie in the range 2.9–3.4µB [20] depending on the magnitude of the orbital contribution. The gradual decrease of the effec-tive magnetic moment from 4.05µB (for sample N8) to 3.16µB (for sample N20) confirms that there is a gradual transformation of the positions of Ni2+ ions from the tetrahe-dral sites to the octahedral sites as the concentration of NiO crystallizing agent is increased beyond 0.8 mol%. Thus the observations on optical absorption and mag-netic properties reveal that the ratio Ni2+ (oct) /Ni2+ (tet) increases with increasing concentration of NiO crystalliz-ing agent beyond 0.8 mol% in the Li2O–CaF2–P2O5 glass ceramics; further, the d–d transitions of the tetrahedral complexes are electric dipole allowed whereas those of oc-tahedral complexes are electric dipole forbidden and are

mainly due to the static or dynamic distortions from the regular octahedral geometry of the glass network and they can also be magnetic dipole allowed. As was reported by a number of other researchers, it is the octahedrally posi-tioned Ni2+ ion that is responsible for the important lumi-nescence emission transition 3T2(F) → 3A2(F) that peaks around 1350 nm; although we could not record the lumi-nescence emission in the NIR range for lack of facility, the high intensity of octahedral optical absorption bands in-cluding 3A2(F) → 3T2(F) and the results of the magnetic moments clearly indicate the possibility of high lumines-cence emission in glass ceramic samples N10 to N20. A slight degradation observed for these samples (N10 to N20), however, could be minimized by adding a small amount of Fe2O3 to these phosphate glass ceramics, as was reported in our earlier paper [21]. 5 Conclusions Divalent nickel ions could success-fully be used as strong nucleating agents for inducing vari-ous crystal phases in Li2O–CaF2–P2O5 glass samples. XRD and SEM studies clearly reveal the presence of vari-ous crystalline phases in these glass ceramic samples. The EDS analysis indicates the retention of all the elements in these samples even after crystallization. These studies fur-ther indicate the uniform distribution of the nucleating agents, i.e. divalent nickel ions, in the bulk glass ceramic samples. The approximate dimensions of various crystal phases evaluated from XRD data indicate that the size of the microcrystals is such that scattering loss in the visible and NIR regions in these glass ceramic samples is negligi-bly small. The corrosion test experiments indicate that the glass ceramic samples are much more hydration resistant when compared with pre-crystallized glass materials. Dif-ferential thermal analysis clearly indicates the formation of different crystallization phases in these ceramic samples. The optical absorption studies together with magnetic sus-ceptibility measurements indicate a gradual transformation of nickel ions from tetrahedral environment to octahedral environment as the concentration of NiO is increased be-yond 0.8 mol%. The Li2O–CaF2–P2O5 glass samples crys-tallized with NiO beyond 0.8 mol% are of much practical importance as laser materials in the NIR region.

Figure 8 (online colour at: www.pss-

a.com) Schematic of phosphate glass ce-

ramic network containing Ni2+ ions in

octahedral and tetrahedral positions.

phys. stat. sol. (a) 205, No. 1 (2008) 187


Original

Paper

Acknowledgement The authors wish to thank Department

of Science and Technology, Govt. of India for providing financial

assistance (Proj. No. SR/S2/CMP-16/2033) for carrying out this

work.

References

[1] A. M. Malyarevich, Yu. V. Volk, K. V. Yumashev, V. K.

Pavlovskii, S. S. Zapalov, O. S. Dymshits, and A. A. Zhilin,

J. Non-Cryst. Solids 351, 3551 (2005).

[2] M. Srinivasa Reddy, S. V. G. V. A. Prasad, and N. Veeraiah,

phys. stat. sol. (a), 204, 816 (2007).

[3] B. V. Raghavaiah, C. Laxmikanth, and N. Veeraiah, Opt.

Commun. 235, 341 (2004).

[4] N. Krishna Mohan, M. Rami Reddy, and N. Veeraiah, J. Al-

loys Compd. (in press), DOI: 10.1016/j.jallcom.2007.04.143.

[5] H. Keppler and N. Bagdassarov, Chem. Geol. 158, 105

(1999).

[6] E. Zannoni, E. Cavalli, and M. Bettinelli, J. Phys. Chem.

Solids 67, 789 (2006).

[7] G. Feng, S. Zhou, J. Bao, S. Xu, and J. Qiu, J. Alloys

Compd. (in press), DOI: 10.1016/j.jallcom.2007.03.034.

[8] T. Suziki, K. Horibuchi, and Y. Ohishi, J. Non-Cryst. Solids

351, 2304 (2005).

[9] B. Rolling, J. Non-Cryst. Solids 244, 34 (1999).

[10] B. P. Antonyuk, N. N. N. Novikona, N. V. Didenko, and

O. A. Aktsipetrov, Phys. Lett. A 287, 161 (2001).

[11] B. V. R. Chowdari, G. V. Subba Rao, G. Y. H. Lee, and

X. P. Sand, Solid State Ion. 136, 1067 (2000).

[12] G. Venkateswara Rao and N. Veeraiah, Phys. Chem. Glas-

ses 43, 205 (2002).

[13] K. Matusita and S. Sakka, Bull. Inst. Chem. Res., Kyoto

Univ. 59, 159 (1981).

[14] R. M. Abdelouhab, R. Braunstein, and K. Baerner, J. Non-

Cryst. Solids 108, 109 (1989).

[15] S. Ram, K. Ram, and B. S. Shukla, J. Mater. Sci. 27, 511

(1992).

[16] P. Subbalakshmi and N. Veeraiah, J. Non-Cryst. Solids 298,

89 (2002).

[17] Hui-Fen Wu, Chung-Cheng Lin, and Pouyan Shen, J. Non-

Cryst. Solids 209, 76 (1997).

[18] J. L. Rao, G. L. Narendra, and S. V. J. Laksmana, Polyhe-

dron 9, 1475 (1990).

[19] E. Zamoni, G. Cavalli, and M. Bettinelli, J. Phys. Chem.

Solids 60, 449 (1999).

[20] J. D. Lee, Concise Inorganic Chemistry, 5th ed. (Blackwell

Science, 1996).

[21] N. Krishna Mohan, K. Sambasiva Rao, Y. Gandhi, and

N. Veeraiah, Physica B 387, 213 (2007).

Documents

NiO-induced crystallization and optical characteristics of Li2O–CaF2–P2O5 glass system