ARTICLE IN PRESS

0921-4526/$ - see

doi:10.1016/j.ph

�Correspondifax: +918656 2

E-mail addre

Physica B 403 (2008) 702–710

www.elsevier.com/locate/physb

Features of the local structural disorder in Li2O–CaF2–P2O5

glass–ceramics with Cr2O3 as nucleating agent

G. Murali Krishnaa, Y. Gandhia, N. Venkatramaiahb, R. Venkatesanb, N. Veeraiaha,�

aDepartment of Physics, Acharya Nagarjuna University, Nuzvid Campus, Nuzvid 521 201, IndiabDepartment of Chemistry, Pondicherry University, Pondicherry, India

Received 30 August 2007; received in revised form 14 September 2007; accepted 18 September 2007

Abstract

Li2O–CaF2–P2O5 glasses mixed with different concentrations of Cr2O3 (ranging from 0 to 1.0mol%) were crystallized. The samples

were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy (EDS), differential thermal

analysis and conventional spectroscopic techniques. The X-ray diffraction and scanning electron microscopic studies reveal the presence

of lithium phosphate, calcium phosphate and chromium phosphate (complexes of Cr3+, Cr5+ and Cr6+ ions) crystal phases. The study

on DTA suggests that the crystallization is predominantly due to the surface crystallization when the concentration of nucleating agent

Cr2O3 is around 0.8mol%. The IR and Raman spectral studies of these samples indicate that the sample crystallized with 0.8mol%

Cr2O3 is more compact and possesses high rigidity due to the presence of chromium ions largely in tetrahedral positions.

r 2007 Elsevier B.V. All rights reserved.

Keywords: Glass–ceramics; X-ray diffraction; SEM; Spectroscopic studies

1. Introduction

The crystalline glass materials contain a fine-graineduniform structure, consisting of small crystals of irregularand distorted form, often aggregated into spherulites, withresidual glass inter-layers, cementing the crystalline glass–ceramic concretion. These materials possess very small andstrain-free intertwined crystals that hinder the crackgrowth. As a result, glass–ceramics are mechanically vigor,possess large electrical and thermal insulating strengths,low coefficient of thermal expansion and are chemicallydurable. In view of such features, these materials areconsidered to be potential candidates as gain media forultra broadband optical fiber amplifiers, tunable lasers andultra short pulse lasers in telecommunication wavelengthregions. Glass–ceramic materials are superior materialsover the glass materials for such application since thedominance of non-radiative losses over the relaxations ofexcited states of transition metal ions is low in these

front matter r 2007 Elsevier B.V. All rights reserved.

ysb.2007.09.085

ng author. Tel.: +918656 235551/9440 015188;

35200.

ss: nvr8@rediffmail.com (N. Veeraiah).

materials. Additionally, these glass–ceramics are also beingwidely used in the field of vacuum sealing, electronics,biotechnology, etc. [1–4].The characteristics of glass–ceramic, however, depend on

the kind and quantity of the crystal phase formed as well ason the residual glass composition. Hence, the selection of asuitable nucleating agent in the correct concentration anddetermination of the temperature and the time of nuclea-tion and growth are important factors, in the formation ofa glass–ceramic. The nucleating agents that are generallyused for controlled crystallization processes, giving rise toenormous numbers of nucleation centers in the originalglass, are gold, silver, platinum or the oxides of Ti, Cr, Mn,Ce, V, Ni and Zr or certain sulfides or fluorides.Among various transition metal ions, the chromium ions

are considered as effective and useful nucleating agentsowing to the fact that these ions may exist in differentvalence states simultaneously in the final glass–ceramic.Materials containing mixed valence ions like chromium areof recent interest, as cathode materials in rechargeablebatteries due to their very high energy density and highcapacitance. Lithium phosphate glasses are particularlysuitable as solid electrolytes in the above-mentioned

ARTICLE IN PRESSG. Murali Krishna et al. / Physica B 403 (2008) 702–710 703

electrochemical devices [5–8]. However, it is well knownthat the poor chemical durability, high hygroscopic andvolatile nature of alkali phosphate glasses have preventedthem from replacing the conventional glasses in a widerange of technological applications. The crystallization oflithium phosphate glasses with Cr2O3 as stimulatormay improve the physical properties and widen the rangeof the applications. Addition of CaF2 further in toalkali phosphate glass matrices is predicted to lower theviscosity and to decrease the liquidus temperature to asubstantial extent and is also found to be an effectivemineralizer [9–11].

In addition to the conventional X-ray diffraction andelectron microscopic methods, it is also quite possible tounderstand the structure of the glass–ceramics to asubstantial extent by investigating various physical proper-ties viz., optical, magnetic and electrical properties. Thoughsome studies are available on different glass–ceramicsincluding phosphate with Cr2O3 as nucleating agent[12–14], still there is a lot of scope to throw some new lighton the above-mentioned physical properties of lithiumcalcium fluoro phosphate glass–ceramics. Recently, we havereported quantitative experimental results on optical,magnetic and electrical properties of Li2O–CaF2–P2O5:Cr2O3 glass–ceramic materials [15]; these studies haveyielded valuable information regarding various valencestates of chromium ions and their environment in thematerial. The current investigation is aimed at under-standing the catalyst action of the Cr2O3 on the crystal-lization of Li2O–CaF2–P2O5 glass system and the structuralanalysis of the material by means of different characteriza-tion (XRD, SEM and DTA) and spectroscopic (IR andRaman) techniques.

2. Experimental procedure

For the present study, a particular composition(30�x)Li2O–10CaF2–60P2O5:xCr2O3 with six values of x

ranging from 0 to 1 is chosen. The details of thecompositions are as follows:

C0 : 30Li2O210CaF2260P2O5

C2 : 29:8Li2O210CaF2260P2O5 : 0:2Cr2O3

C4 : 29:6Li2O210CaF2260P2O5 : 0:4Cr2O3

C6 : 29:4Li2O210CaF2260P2O5 : 0:6Cr2O3

C8 : 29:2Li2O210CaF2260P2O5 : 0:8Cr2O3

C10 : 29Li2O210CaF2260P2O5 : 1:0Cr2O3

Analytical grade reagents of NH4H2PO4, Li2CO3, CaF2

and Cr2O3 powders in appropriate amounts (all in mol%)were thoroughly mixed in an agate mortar and meltedusing a thick walled platinum crucible at 1000710 1Cin a PID temperature controlled furnace for about 2 h.

The resultant bubble-free melt was then poured in a brassmold and subsequently annealed at 250 1C. The synthesizedglass samples were free from visible inhomogeneities, suchas inclusions, cracks or bubbles. The glass samplesobtained were heat treated in an automatic temperaturecontrolled furnace at crystallization temperature Tc (iden-tified from differential thermal analysis) for 4 h. After theheat treatment in the furnace at specified temperature, thesamples were quenched in air to room temperature.The crystalline phases in the glass–ceramic samples wereidentified by X-ray diffraction using Xpert PRO’panaly-tical X-ray diffractometer with Cu Ka radiation. Themicrostructures are observed with a scanning electronmicroscope (HITACHI Model S-3400N) using polishedsurfaces.Differential thermal analysis was carried out using

NETZSCH STA 409 C/CD instrument with a programmedheating rate of 10 1C/min, in the temperature range30–1000 1C, to determine the glass transition temperatureand crystalline peaks. The density d of the glasses isdetermined to an accuracy of (70.0001 g) by the standardprinciple of Archimedes’ using o-xylene (99.99% pure) asthe buoyant liquid with OHAUS balance (model AR2140). The mass of the samples was measured to anaccuracy of 0.1mg using this balance. For evaluating thechemical durability, the bulk samples were suspended by aweightless strand in 100ml of water of pH 7 and heated at90 1C for 12, 24 and 48 h. The weight loss (DW) for eachinterval is measured and the average dissociation rate (DR)is evaluated using, DR ¼ DW/(S� t) g/cm2/min, where S isthe surface area of the sample and t is the time ofimmersion. The pH value of the residual solution at the endof each immersion time is also measured at 30 1C.Infrared transmission spectra are recorded on a JASCO-

FT/IR�5300 spectrophotometer with a resolution of0.1 cm�1 in the range of wavenumber 400–2000 cm�1 usingpotassium bromide pellets (300mg) containing pulverizedsample (1.5mg). These pellets were pressed in a vacuum dieat �680MPa. The Raman spectra were recorded to anaccuracy of 0.1 cm�1 on Raman Spectrometer (BrukerFRA 106/RFS) using 514 nm exciting light of argon laser.

3. Results

The density of the Cr2O3-free glass–ceramic material ismeasured to be 2.1866 g/cm3. Significant variations havebeen observed in the density of the ceramic material withthe variation in the concentration of the nucleating agentCr2O3 in the glass matrix (Table 1); sample C8 is found tohave the highest density. From the measured values of thedensity and average molecular weight M of the samples,various other physical parameters such as chromium ionconcentration Ni, mean chromium ion separation ri,polaran radius rp in Li2O–CaF2–P2O5:Cr2O3 ceramicsamples are computed and presented in Table 1. The DRof the samples measured as per the procedure described inexperimental part is also presented in the same table;

ARTICLE IN PRESS

Table 1

Physical parameters of Li2O–CaF2–P2O5:Cr2O3 glass ceramics

Glass Density (g/cm3) Cr ion concentration Ni

(1021 ions/cm3)

Mean Cr ion

separation, ri (A)

Polaron radius, rp (A) Dissociation factor

(10�6 g/cm2/min)

C0 2.1866 – – – 3.31

C2 2.3932 2.820 7.077 2.85 2.46

C4 2.2593 5.310 5.732 2.31 1.74

C6 2.4288 8.541 4.892 1.97 0.85

C8 2.5006 11.690 4.406 1.78 0.32

C10 2.1787 12.700 4.286 1.73 1.46

0.0

1.0

2.0

3.0

0.2 0.4 0.6 0.8 1

0.2 0.4 0.6 0.8 16.4

6.5

6.6

6.7

6.8

Dis

socia

tion facto

r (1

0-6

g / c

m2 / m

in)

pH

valu

e

conc. of Cr2O3 (mol %)

Fig. 1. (a) Variation of dissociation rate of Li2O–CaF2–P2O5:Cr2O3

glass–ceramics in distilled water solution at 90 1C. (b) Variation of pH of

the solution with concentration of Cr2O3 after corrosion testing.

G. Murali Krishna et al. / Physica B 403 (2008) 702–710704

the DR is observed to be minimum for the sample C8

(Fig. 1, Table 1). The change in the pH of the distilledwater solution after corrosion testing with respect to that ofthe pure water is observed to be minimal for the sample C8

(Fig. 1). The change in pH of the liquid in contact withsamples is consistent with the dissolution of phosphatespecies from these glasses and the subsequent formation ofphosphoric acid.The scanning electron microscopic (SEM) pictures of

some of the crystallized samples are shown in Fig. 2. TheSEM pictures of the crystallized samples exhibit welldefined and randomly distributed crystals entrenched inglassy matrix. The chemical makeup of the phasescharacterized by energy dispersive spectroscopy (EDS)for some of the samples is shown in Fig. 3. The EDSanalysis of the glass–ceramic materials exhibits Li, P, Caand Cr elements in various crystalline phases. Thus, fromthese pictures it can be concluded that Cr2O3 enhanced thephase separation tendency of various crystalline phases;this fact reflects specific overlap integral between the d Crlevels and the delocalized anti-bonding p-states of phos-phate anion complexes.The X-ray diffraction pattern (Fig. 4) of the glass–

ceramic samples exhibits diffraction peaks due to lithiumphosphate, lithium chromium phosphate and chromiumphosphate crystal phases. With an increase in the contentof crystallizing agent, i.e., Cr2O3, in the glass matrix, thestructure of the diffraction pattern becomes more complexand a clear variation in the intensity and the pattern ofdiffraction peaks can be visualized. The appearance ofLi2CrO4 and Li3CrO4 crystalline phases could also bedetected in the spectra of ceramic samples C6 and C8,indicating that the chromium ions also exist in Cr6+ andCr5+ states along with Cr3+ state especially in the samplesC6 and C8.Fig. 5 shows the differential thermal analysis traces of

Li2O–CaF2–P2O5:Cr2O3 samples. For the pre-crystallizedsample an endothermic change due to the glass transitiontemperature Tg is observed. At still high temperature, Tc,an exothermic peak due to the crystal growth followed byanother endothermic peak due to re-melting of the glass arealso observed. The appearance of single crystallizationpeak in the pre-crystallized samples indicates that the

ARTICLE IN PRESS

Fig. 2. SEM pictures of crystallized Li2O–CaF2–P2O5:Cr2O3 samples.

G. Murali Krishna et al. / Physica B 403 (2008) 702–710 705

dopant, i.e., Cr2O3, is homogeneously distributed in thematerial. All the crystallized samples exhibit endothermicchange due to the glass transition followed by three clearexothermic peaks Tc1, Tc2 and Tc3 due to the crystalgrowth. At the end, an endothermic peak due to re-meltingof the samples has been observed in the traces of all thesamples. The pertinent data related to DTA study havebeen presented in Table 2.The infrared transmission spectra studies on these

glasses indicated that there are several signs of thecrystallinity or even nanocrystallinity. The IR spectrumof Cr2O3-free Li2O–CaF2–P2O5 glass–ceramics (Fig. 6)exhibits vibrational bands at 1300 cm�1 (identified due toanti-symmetrical vibrations of PO2

�, this region may alsoconsist of bands due to PQO stretching vibrationsgroups), 1050 cm�1 (a normal vibrational mode in PO4

3�

group arising out of n3-symmetric stretching), 907 cm�1

(due to P–O–P asymmetric bending vibrations) andanother band at 793 cm�1 due to P–O–P symmetricstretching vibrations [16–19]. With the introduction ofCr2O3 (from 0.4 to 0.8mol%) and heat treatment, theintensity of the bands due to symmetrical vibrations ofphosphate groups is observed to increase with a shift in theband position toward slightly lower wavenumber; theintensity and the position of P–O–P asymmetric bendingvibrational band exhibited a reversal trend with in thisconcentration range; this fact may be of interest to usethese material-like optically operated fibers, as was shownby Makowska-Janusik et al. [20] in other phosphate glasssystems. However, for the samples C0–C4, the intensityassociated with the symmetrical bands is observed todecrease gradually while that of the asymmetrical band isobserved to increase. It may be noted here that the bandpositions of various phosphate structural groups observedfor the present glass–ceramic samples are found to be wellwithin the ranges reported in the literature [16–19].Additionally, a feeble band, the intensity of which isobserved to increase gradually with the increase inconcentration of Cr2O3 from 0.4 to 0.8mol%, is locatedat about 860 cm�1; this band is identified due to n3vibrations of CrO4

2� structural units [21]. The intensity ofthis band, however, seems to be trivial in the spectra of theglasses C4 and C10. The summary of the band positions ofvibrations due to various structural groups is presented inTable 3.Fig. 7 shows the Raman spectra of Li2O–CaF2–P2O5:

Cr2O3 glass–ceramics. The spectra of all the glasses haveexhibited three conventional bands due to phosphategroups: (i) in the region 1040–1070 cm�1 due to (ns PO2

mode), (ii) 700–730 cm�1 due to (ns POP mode) and (iii) atabout 400 cm�1 due to bending and torsional vibrations ofPO4 structural units [22]. With the increase in theconcentration of Cr2O3 from 0.4 to 0.8mol%, the intensityof the symmetrical vibrational groups is observed to growat the expense of asymmetrical groups. The spectra of thesesamples have also exhibited an additional band at about860 cm�1 (with the maximum intensity in the spectrum of

ARTICLE IN PRESS

Fig. 3. EDS spectra of Li2O–CaF2–P2O5:Cr2O3 ceramic samples.

G. Murali Krishna et al. / Physica B 403 (2008) 702–710706

sample C8); this band is predicted due to Cr–O vibrationsof CrO4

2� units [23]. The positions of Raman bands due tovarious structural units are summarized in Table 4.

4. Discussion

P2O5 is a well-known strong glass forming oxide,participates in the glass network with PO4 structural units.The PO4 tetrahedrons are linked together with covalentbonding in chains or rings by bridging oxygens. Normally,the structure of the simple phosphate glasses is dependenton O/P ratios and the fraction of Q phosphate tetrahedra.For single P2O5 glass O/P ¼ 2.5 and the glass networkis build up of Q3 tetrahedra with the bridging oxygens;and with the fourth oxygen, is doubly bonded to the

phosphorus atom with the substantial p-bond character toaccount for pentavalency of phosphorous. With theaddition of Cr2O3 an ultraphosphate network consistingof Q2 and Q3 tetrahedra may form with O/Po3.0 [24–27].Similar to fluoro borate 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 aconventional modifier oxide and enters the glass network,either by rupturing or by breaking up the P–O–P bonds,and introduces coordinated defects known as danglingbonds along with non-bridging oxygen ions.Chromium ions seem to exist primarily in Cr3+ state in

the present glass–ceramic. Though the starting oxidationstate of chromium ion in the composition of the glasssystem is Cr3+, the oxidation of Cr3+ to Cr5+ and Cr6+

ARTICLE IN PRESS

10 20 30 40 50 60 70 80

CrPO4

LiCrP2O7

LiCr(PO3)4

Li2CrO4

β - Li3CrO4

β - Ca2P2O7

Pure un-crystallized

Pure crystallized

C8

C6

C4

Inte

nsity (

a.u

)

2θ (degrees)

LiPO3

Fig. 4. XRD patterns of Cr2O3-doped Li2O–CaF2–P2O5 glass–ceramics.

Exo

Endo

Tg

Tc2

Tc1

Tc3Tm

C2

C10

C8

C6

C4

300 400 500 600 700 800 900

Temparature °C

Fig. 5. DTA traces of Li2O–CaF2–P2O5:Cr2O3 glass–ceramics.

Table 2

Summary of data on differential thermal analysis studies of Li2O–CaF2–

P2O5:Cr2O3 glass ceramics

Sample Tg (1C) Tc1 (1C) Tc2 (1C) Tc3 (1C) Tm (1C)

C2 368 484 495 526 861

C4 360 476 485 521 870

C6 386 493 517 538 864

C8 395 512 528 554 858

C10 390 506 518 547 867

G. Murali Krishna et al. / Physica B 403 (2008) 702–710 707

appears to be viable during melting, crystallization andannealing processes. The presence of such oxidation statesof chromium ions in these glass–ceramic systems wasreported in our earlier paper by studying the ESR andoptical absorption spectra [15]. Cr (VI) ion may enter theglass network with CrO4

2� structural units and alternatewith PO4 units. Cr(III) ions, however, act as modifierssimilar to Li2O and CaF2 [28,29].

Density of Li2O–CaF2–P2O5:Cr2O3 glass–ceramics hasbeen observed to depend strongly on the content of Cr2O3

in the glass ceramic. In general, the degree of structuralcompactness, the modification of the geometrical config-uration of the glassy network, the size of the microcrystalsformed, change in the coordination of the glass formingions and the fluctuations in the dimensions of theinterstitial holes are the some of the factors that influencethe density of the glass–ceramic material. The increasingconcentration of the nucleating agent Cr2O3 (from 0.4 to0.8mol%) caused a considerable increase in the density;this is an indicative of growing structural compactness ofthe material probably due to rising presence of chromiumions in Cr6+ state; the field strength of these Cr6+ ions isvery high when compared with that of Cr3+ state andmakes the glass–ceramic samples more compact.

The observed lowest average DR of the sample crystal-lized with 0.8mol% of Cr2O3 indicates that the concentra-tion of undisturbed P–O–P, P–O–Cr bonds is the highest inthis sample. The presence of such bonds, in higherconcentrations, makes the glasses more hydration resistant.Additionally, the length and the orientation distribution ofPO4 chains also play a major role in deciding the chemicaldurability of these samples. Previous empirical studiesshowed that the shortened PO4 chains in the network areresponsible to some extent for the high corrosion resistanceof the glasses [30]. This conclusion obviously suggests thatup to 0.8mol% concentration, the chromium ions prefer togo in to tetrahedral positions, facilitate to form more andmore P–O–Cr bonds rather than acting as modifiers thatcreate more dangling bonds in the glass network.Ceramic sample crystallized with 0.8mol% Cr2O3

(sample C8) showed a remarkable improvement of chemi-cal durability; surfaces of these samples after the corrosion

ARTICLE IN PRESS

.

Non bridging of

PO2 symmetric

Symmetric

stretching of P - O - PBending mode of

phosphate PO4 polyhedraCrO4

2-

C10

G. Murali Krishna et al. / Physica B 403 (2008) 702–710708

testing are observed to be smooth, with the corners and theedges retaining original sharpness. The pH of the residualwater solution also correlates notable improvement of thedurability of glass C8. The high corrosion resistance of thissample is obviously due to the presence of large concentra-tion of unruffled P–O–P and P–O–Cr bonds in thenetwork.

X-ray diffraction pattern exhibits microstructuralchanges. The crystalline lithium phosphate (orthophosphate,metaphosphate and pyrophosphate), chromium phosphatesthat are kinetically and thermodynamically feasible seemedto be the main products in the present glass–ceramicsamples. In these samples, in addition to lithium phosphate,b-calcium phosphate, lithium chromium phosphate andchromium phosphate crystal phases (viz., LiCrP2O7 andLiCr(PO3)4 in which chromium exists in Cr3+ state) are also

C0

C2

C4

C6

C8

C10

Tra

nsm

itta

nce %

.

PO2-

asym. groups PO3- & PO4

3-

sym : stretching

P - O - P asy :bending

CrO2-4

P- O - P sym.bending

Wavenumber (cm-1)

1250 1050 850 650 450

Fig. 6. IR spectra of Li2O–CaF2–P2O5:Cr2O3 glass–ceramics.

Table 3

Summary of the data on band positions (in cm�1) of various absorption band

Structural unit Glass C0 Glass C2

PO2� asymmetric stretching 1300 1290

PO43� groups 1052 1060

P–O–P asymmetric bending 905 900

P–O–P symmetric stretching 801 805

CrO42� – 873

detected. Further, crystalline phases of Cr6+ and Cr5+ ionsviz., LiCrO4 and Li3CrO4 are also identified. These metast-able crystals are usually in solid-solution phases that canincorporate the major constituents of the glass composition

s in the IR spectra of Li2O–CaF2–P2O5:Cr2O3 glass ceramics

Glass C4 Glass C6 Glass C8 Glass C10

1285 1311 1313 1305

1071 1039 1030 1041

895 912 925 918

809 788 783 789

– 865 861 871

10030050070090011001300

Wavenumber (cm-1)

Ram

an u

nits

C0

C2

C4

C6

C8

Fig. 7. Raman spectra of Li2O–CaF2–P2O5:Cr2O3 glass–ceramics.

Table 4

Band positions (in cm�1) in the Raman spectra of Li2O–CaF2–P2O5:Cr2O3

glass ceramics

Glass PO2 mode POP mode PO4 units CrO42� groups

C0 1068 718 400 –

C2 1056 714 403 871

C4 1053 710 408 867

C6 1050 707 411 864

C8 1044 703 416 861

C10 1064 709 406 863

ARTICLE IN PRESSG. Murali Krishna et al. / Physica B 403 (2008) 702–710 709

in the approximately same proportion as they were presentin the original glass matrix. The Cr rich areas in the samplesmay enhance the reactivity of Cr with the other oxidesthat precipitate as a high density of fine Cr rich crystals.These tiny crystals act as heterogeneous nuclei for thecrystallization of the remaining glass. The SEM pictures ofCr2O3-doped glass–ceramics exhibit larger crystals thanthose visible in the Cr2O3-free samples. The SEM picture ofsample C8 shows a reasonably homogeneous distribution ofthe crystals. The residual glass phase may act as inter-connecting zones among the crystallized areas making thesamples free of voids and cracks.

The appearance of different crystallization temperaturesin the DTA pattern in the glass–ceramic samples obviouslysuggests the presence of different phases of crystallizationin the samples. The crystallization in the samples may takeplace based on the surface and bulk nucleations. The non-isothermal devitrification process is represented by theequation [31]:

ln1

1� a

� �¼

AN

bn e�ðnE=RTÞ, (1)

where a is the fraction of the volume crystallized attemperature T, E the activation energy and b is the DTAheating rate and A is the constant. The nuclei centers N arethe sum of surface and bulk nuclei. The higher the value ofN, the lower is the crystallization temperature Tc. In thesurface nucleation mechanism, the nucleating centers areformed only on the surface and the crystals startdeveloping from the surface to the inside of the glass onedimensionally (in this case n ¼ 1); on the other hand, in thebulk nucleation, the crystals grow three dimensionally(in this case n ¼ 3). The general shape of the crystallizationpeaks is strongly dependent on the value of n. The higherthe value of n the narrower is the width of the crystal-lization peak [32]. For the surface crystallization, we may,therefore, expect relatively wider peaks when comparedwith those of bulk crystallization peaks. The pattern of theDTA peaks suggests that the crystallization is predomi-nantly due to the surface crystallization in the sample C8;however, in the sample C4, the bulk crystallization seems toprevail even though it is generally believed that volumecrystallization can take place in the samples only if theglass system crystallized well above the first crystallizationpeak [13].

With a gradual increase in the concentration of thenucleating agent Cr2O3 in the Li2O–CaF2–P2O5 glass–ceramic (from 0.4 to 0.8mol%), a progressive increase inthe intensity of the bands due to symmetrical phosphatestructural units is observed at the expense of bands due toasymmetrical groups. These observations clearly suggest adecreasing disorder or an increasing rigidity of theglass–ceramics with the increase of nucleating agent in thisconcentration range. The observed gradual hike in theintensity of the band due to CrO4

2� structural units thatparticipate in the network forming (within this concentra-tion range of Cr2O3) also supports this viewpoint.

In the IR spectrum of glass C6, the n3 vibrational band ofCrO4

2� units appeared at about 865 cm�1, exhibits degen-eracy with two closed distinct bands at 860 and 870 cm�1.Such splitting may be explained in terms of site symmetryand local lattice interactions on chromium ion. When theseions occupy tetrahedral positions of Td symmetry in theglass network, they are subjected to perturbations (likebonding, compression and chemical interactions) due tochange in the environment and the incompatibility in ionsize. As a result, CrO4

2� ions undergo structural distortionsinvolving changes in bond lengths and angles of Cr–Obonds. Such changes seem to be responsible for theobserved splitting. In this case, the distortion leads tolower the symmetry element of Td point group from C3 toD2d [33]. The elimination of such degeneracy in thespectrum of ceramics C8 clearly indicates a low degree ofstructural fluctuations in the network. The decay of theband due to these vibrational groups in the spectrum of thesample C10 indicates presence of a low concentration ofCr(VI) ions or high concentration of Cr(III) ions thatoccupy octahedral positions. Similarly, in the Ramanspectra, the strength of the bands due to conventionalsymmetric vibrations viz., PO2 and POP modes, is observedto increase, while that of the torsional bending vibrationalmode is observed to decrease with the variation in theconcentration of Cr2O3 from 0.4 to 0.8mol%. Thus, theanalysis of the results of IR and Raman spectral studiespoints out that there is a low degree of disorder in thenetwork or high rigidity of glass ceramic when theconcentration of Cr2O3 is �0.8mol%. This argument isconsistent with the conclusion drawn from the study ofother physical properties of these samples [15].

5. Conclusions

The X-ray diffraction and SEM studies on Li2O–CaF2–P2O5:Cr2O3 glass–ceramics indicate that these ceramicsamples contain lithium phosphate (ortho, meta and pyro),calcium phosphate and chromium phosphate (complexes ofCr3+, Cr5+ and Cr6+ ions) crystal phases. The EDS ana-lysis of these samples confirms the presence of Li, P, Caand Cr elements in these samples. The study on DTAsuggests that the crystallization is predominantly due to thesurface crystallization when the concentration of nucleat-ing agent Cr2O3 is around 0.8mol%. The IR and Ramanspectral studies indicate that the chromium ions occupytetrahedral positions in larger concentrations in the sampleC8 and increase the rigidity of the sample.

References

[1] M. Goswami, P. Sengupta, K. Sharma, R. Kumar, V.K. Shrikhande,

J.M.F. Ferreira, G.P. Kothiyal, Ceram. Int. 33 (2007) 863.

[2] T. Suzuki, Y. Arai, Y. Ohishi, J. Non-Cryst. Solids 353 (2007) 36.

[3] M.P. Shepilov, Opt. Mater. (2007).

[4] G. Murali Krishna, N. Veeraiah, N. Venkatramaiah, R. Venkatesan,

J. Alloys Compd. (2006).

ARTICLE IN PRESSG. Murali Krishna et al. / Physica B 403 (2008) 702–710710

[5] B.M. Casari, E. Wingstr, V. Langer, J. Solid State Chem. 179 (2006)

296.

[6] T. Fuss, A. Mogus-Milankovic, C.S. Ray, C.E. Lesher, R. Young-

man, D.E. Day, J. Non-Cryst. Solids 352 (2006) 4101.

[7] K. Sambasiva Rao, M. Srinivasa Reddy, N. Veeraiah, Physica B 396

(2007) 29.

[8] K. Sambasiva Rao, N. Krishna Mohan, N. Veeraiah, Turk J. Phys.

21 (2007) 11.

[9] I.A. Bondar, N.A. Toropov, in: E.A. Poraikoshits (Ed.), The

Structure of Glass, Consultants Bureau, New York, 1964, p. 35.

[10] G. Venkateswara Rao, N. Veeraiah, Phys. Chem. Glasses 43 (2002)

205.

[11] G. Venkateswara Rao, N. Veeraiah, Mater. Lett. 57 (2002) 403.

[12] A. Karamanov, P. Pisciella, M. Pelino, J. Eur. Ceram. Soc. 19 (1999)

2641.

[13] V.K. Marghussian, A. Beitollahi, M. Haghi, Ceram. Int. 29 (2003)

455.

[14] A. Santic, A. Mogus-Milankovic, K. Furic, V. Bermanec, C.W. Kim,

D.E. Day, J. Non-Cryst. Solids 353 (2007) 1070.

[15] G. Murali Krishna, B. Anila Kumari, M. Srinivas Reddy,

N. Veeraiah, J. Solid State Chem. (2007).

[16] D. Ilieva, B. Jivov, D. Kovacheva, Ts. Tsacheva, Y. Dimitriev,

G. Bogachev, Ch. Petkov, J. Non-Cryst. Solids 293 (2001) 562.

[17] J. Schwarz, H. Ticha, L. Tichy, R. Mertens, J. Optoelectron. Adv.

Mater. 6 (2004) 737.

[18] K. Nakamoto, IR and Raman Spectra of Inorganic and Coordina-

tion Compounds, Wiely-Interscience, New York, 1978.

[19] N. Krishna Mohan, K. Sambasiva Rao, Y. Gandhi, N. Veeraiah,

Physica B 387 (2007) 213;

G. Little Flower, M. Srinivasa Reddy, M.V. Ramana Reddy,

N. Veeraiah, Z. Naturforsch. 62a (2007) 315.

[20] M. Makowska-Janusik, I.V. Kityk, J. Berdowski, J. Matejec,

I. Kasik, A. Mefleh, J. Opt. Pure Appl. Opt. 2 (2000) 43.

[21] M. Srinivasa Reddy, S.V.G.V.A. Prasad, N. Veeraiah, Phys. Status

Solidi (a) 204 (2007) 816.

[22] J. Schwarz, H. Ticha, L. Tichy, R. Mertens, J. Optoelectron. Adv.

Mater. 6 (2004) 737.

[23] S. Ram, K. Ram, J. Mater. Sci. 23 (1988) 4541.

[24] S.V.G.V.A. Prasad, M. Srinivasa Reddy, N. Veeraiah, J. Phys. Chem.

Solids 67 (2006) 2478.

[25] A. Mogus-Milankovic, A. Santic, V. Licina, D.E. Day, J. Non-Cryst.

Solids 351 (2005) 3235.

[26] P. Subbalakshmi, N. Veeraiah, J. Non-Cryst. Solids 298 (2002) 89.

[27] I. Ardelean, Gh. Ilonca, M. Peteanu, E. Barbos, J. Mater. Sci. 17

(1982) 1988.

[28] D.K. Durga, N. Veeraiah, Physica B 324 (2002) 127.

[29] G. Srinivasa Rao, N. Veeraiah, Eur. Phys. J. Appl. Phys. 16 (2001) 11.

[30] C.K. Loong, K. Suzuya, D.L. Price, B.C. Sales, L.A. Boatner,

Physica B 890 (1998) 241.

[31] K. Matusita, S. Sakka, Bull. Inst. Chem. Res., Kyoto Univ. 59 (1981)

159.

[32] A. Marotta, A. Buri, F. Branda, J. Mater. Sci. 16 (1981) 341.

[33] J.M. Yanez-Limon, J.F. Perez-Roblezand, D. Ramirez-Rosales, Thin

Solid Films 373 (2000) 184.