Luminescence quenching by manganese ionsin MO–CaF2–B2O3 glasses

G. Venkateswara Rao a, N. Veeraiah a,*, P. Yadagiri Reddy b

a Department of Physics, Nagarjuna University P.G. Centre, Nuzvid 521201, AP, Indiab Department of Physics, O.U. College of Science, Osmania University, Hyderabad, AP, India

Received 29 January 2001; received in revised form 19 August 2002; accepted 5 September 2002

Abstract

Thermoluminescence (TL) characteristics of X-ray irradiated calcium fluoro borate glasses mixed with three different

alkali oxide modifiers viz., Li2O, Na2O and K2O have been studied in the temperature range 303–600 K; all the glasses

have exhibited single TL peak at 485, 541 and 497 K respectively. The glasses containing Na2O as modifier has ex-

hibited the maximum TL light output. The doping of manganese oxide by a small concentration (0.2 mol%) in all these

glasses has been observed to inhibit TL light output drastically with shifting of peak positions towards lower tem-

peratures. The trap depth parameters associated with the observed TL peaks have been evaluated using Chen�s for-mulae. The probable mechanism responsible for quenching of TL emission by manganese ions in these glasses has been

suggested with the aid of optical absorption, IR spectra and differential scanning calorimetric studies.

� 2002 Elsevier Science B.V. All rights reserved.

1. Introduction

The understanding of the glass structure by

detailed studies on radiation induced defect centres

has been an interesting subject of investigation in

recent years. Extensive studies on the activating or

killing effect of luminescence produced by some

transition metal ions like Fe2þ, Cu2þ, Ti4þ etc., in

amorphous materials are available in literature [1–

5]. Borate glasses are very advantageous materialsfor the radiation dosimetry applications in view of

the fact that their effective atomic number is very

close to that of human tissue. However, pure bo-

rate glasses have certain disadvantages to use inradiation dosimetry since they are highly hygro-

scopic and exhibit weak glow peak at relatively

low temperatures. Alkali oxy borate glasses are

considered as good materials for dosimetry appli-

cations since they are relatively moisture resistant

when compared with the pure borate glasses. Ad-

dition of CaF2 in to the glass matrix lowers the

viscosity and decreases the liquidus temperature toa substantial extent and further it acts as an ef-

fective mineralizer, giving scope for the formation

of large concentration of colour centres when the

glasses are exposed to ionizing radiations [6].

Further F� ions come from CaF2 act as co-acti-

vators and facilitate the substitution of activators

into the lattice.

*Corresponding author. Tel.: +91-8656-32560; fax: +91-

8656-35200.

E-mail address: nvr8@rediffmail.com (N. Veeraiah).

0925-3467/03/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0925-3467(02)00237-9

Optical Materials 22 (2003) 295–302

www.elsevier.com/locate/optmat

Manganese ion, is an interesting one because it

exists in different valence states in different glass

matrices, for example as Mn3þ in borate glass with

octahedral coordination whereas in silicate and

germanate glasses it exists in Mn2þ state with both

octahedral and tetrahedral coordination [7]. Fur-ther, among different manganese ions, Mn2þ and

Mn4þ ions are identified as luminescence activators

[8]. The content of manganese in different forms in

different valence states exist in the glass depends

on the quantitative properties of modifiers and

glass formers, size of the ions in glass structure,

their field strength, mobility of the modifier cation

etc. Hence the connection between the state andthe position of the manganese ion and lumines-

cence properties of the glass is highly interesting.

The objective of the present investigation is to

study the effect of manganese ions on the radiative

electron–hole recombination in the X-ray irradi-

ated calcium flouro borate glasses mixed with

three different modifier oxides viz., Li2O, Na2O

and K2O, to evaluate various trap depth parame-ters of the TL glow curves, to compare the lumi-

nescence efficiencies of these glasses and to suggest

the possible mechanism for TL emission in these

glasses.

2. Experimental

For the present study the following composi-

tions in mol% are chosen:

A : 20 MO� 20 CaF2 � 60 B2O3ðMO ¼ Li2OðA1Þ; Na2O ðA2Þ; K2O ðA3ÞÞ

A0 : 19:8 MO� 20 CaF2 � 60 B2O3 : 0:2 MnOðMO ¼ Li2O ðA0

1Þ; Na2OðA02Þ; K2O ðA0

3ÞÞ

MnO is introduced as carbonate. The methods

of preparation and characterisation of these glas-

ses are similar to those of other glasses reported

earlier [9,10]. The density d of these glasses wasdetermined by the standard principle of Archime-

des� using xylene (99.99% pure) as the buoyantliquid. The glass transition temperatures of these

glasses were determined by differential scanning

calorimetry traces recorded using universal V2.3C

TA differential scanning calorimeter. The IR

transmission spectra of these glasses in KBr ma-

trices were recorded using Perkin-Elmer 283 B

spectrophotometer in the frequency range 400–

4000 cm�1. X-ray irradiation on these glasses wascarried out at room temperature for 1 h with an X-

ray tube operated at 35 kV, 10 mA. The optical

absorption spectra of these glasses were recorded

(before and after X-ray irradiation) on Shimadzu-

3101 pc UV–vis–NIR spectrophotometer in the

wavelength range 200–600 nm. The thermolumi-

nescence (TL) glow curves of these glasses were

recorded on computerized Nucleonix–TL set up(Nucleonix Pvt., Ltd., Hyderabad, India) in the

temperature range 303–600 K; the rate of heating

of the glasses was maintained at 1 �C/s.

3. Results

From the measured values of density and theaverage molecular weight ..

.various other physical

parameters such as manganese ion concentration

Ni, mean manganese ion separation distance are

calculated and presented in the Table 1.

Fig. 1 represents the DSC curves of these glas-

ses; from these traces the glass transition temper-

ature Tg for Li2O–CaF2–B2O3 glass is determinedto be 490 �C. The highest value of Tg is observed

Table 1

Physical parameters of glasses A and A0

Sample Density ...

NIð�1021Þ riðA0Þ Tg

A1 2.553 62.548 – – 490

A2 2.525 68.982 – – 508

A3 2.462 75.428 – – 500

A1 2.53 62.740 4.86 5.90 510

A2 2.511 69.109 4.38 6.11 520

A3 2.48 75.491 3.96 6.32 517

296 G. Venkateswara Rao et al. / Optical Materials 22 (2003) 295–302

for A2 glass (i.e. glass containing Na2O as modi-

fier). The doping of MnO caused an increase ofglass transition temperature for all the glasses

(Table 1). The IR transmission spectra recorded

for these glasses (Fig. 2) exhibit two groups of

bands: (i) in the region 1200–1600 cm�1, due to the

stretching relaxation of the B–O bond of the tri-

gonal BO3 units, (ii) in the region 800–1200 cm�1

due to B–O bond stretching of the tetrahedral BO4units and (iii) a band at about 710 cm�1 due to thebending of B–O linkages in the borate network

[11]. With the introduction of manganese oxide

into the glass network the intensity of the second

group of bands are found to be shifted towards

slightly higher frequencies with decreasing in-

tensities. The optical absorption spectra of pure

glasses recorded before X-ray irradiation are

shown in Fig. 3a. The cut-off wavelength for thethree glasses are observed to be 325 (A1), 315 (A2),

329 (A3) nm. With the doping of manganese, these

edges are observed to shift towards slightly higher

wavelengths (Fig. 3b). All the glasses exhibit threeconventional absorption bands at 501, 421 and

Fig. 1. DSC tracings of MO–CaF2–B2O3 glasses with different modifiers: (A) pure; (B) MnO doped glasses.

Fig. 2. Infrared transmission spectra ofMO–CaF2–B2O3 glasses.

G. Venkateswara Rao et al. / Optical Materials 22 (2003) 295–302 297

402 nm which are attributed to 6A1g(S)4T1g(G),

6A1g(S)4T2g(G) and 6A1g(S)

4A1g(G),4Eg(G)

transitions of Mn2þ ions respectively.

Among these, the first band i.e., the band at 501

nm is observed to be more sharp since it arisesfrom intra-configurational transitions where as the

other two bands are observed to be broaden since

they involve a change of configuration from ðt2gÞ3ðegÞ2 to ðt2gÞ2 ðegÞ1 [12,13]. These Mn2þ ion bandsare observed to be obscured with a presence of

broad absorption band with a maximum at about

490 nm when these glasses were X-ray irradiated

(Fig. 3c). In addition a weak band at about 360 nmis observed in both pure and Mn2þ doped glasses

after they were X-ray irradiated. The band at 490

nm is identified due to 5Eg5T2g transition of Mn

3þ

ions [14] where as the band at 360 nm is due to Caþ

ions [15,16]. However, the manganese free glasses

after X-ray irradiation have not shown any de-

tectable bands except a kink at 360 nm.

Fig. 4a presents the TL glow curves of pureMO–CaF2–B2O3 glasses where as Fig. 4b repre-

sents those of doped with MnO. The three pure

glasses exhibit a TL peak at 485 K (A1), 541 K

(A2), 497 K (A3) respectively. The maximum TL

light output is observed to be exhibited by Na2O–

CaF2–B2O3 glasses (A2 glass) among the three

pure glasses. The doping of manganese ions is

observed to reduce the TL light output of all the

three glasses (Fig. 4b); the comparison of TL

emission for these glasses with respect to pure

glasses is shown in Fig. 5. The quenching of TLemission due to manganese ions is observed to be

the highest for the glasses containing lithium as

modifier (A1). The reduction of TL light output for

these glasses is nearly 93% where as for sodium

and potassium oxide modifier glasses the distruc-

tion is found to be 90% and 73% respectively.

The activation energies for these glow peaks are

computed using Chen�s formulae [17]:

Es ¼ 1:52ðKT 2m=sÞ � 1:58ð2KTmÞ ð1Þ

Ed ¼ 0:976ðKT 2m=dÞ ð2Þfor the first order kinetics. The frequency factor Sis calculated from the relation

S ¼ ðblg=KT2mÞelg=KTm ð3Þ

In the above equation K is Boltzmann constant, bis the rate of heating, s ¼ Tm � T1, d ¼ T2 � Tm,lg ¼ d=ðT2 � T1Þ, where Tm is the glow peak tem-perature, T1 (rising end) and T2 (falling end) are thetemperatures at the half widths of the glow peaks.The A2 glasses are found to have the highest values

Fig. 3. Optical absorption spectra of MO–CaF2–B2O3 glasses: (a) pure; (b) MnO doped; (c) MnO doped and X-ray irradiated.

298 G. Venkateswara Rao et al. / Optical Materials 22 (2003) 295–302

of trap depth parameters among the three pure

glasses; though the values of these parameters

after doping of manganese are high for Li2O

modifier glasses, the TL light output is recorded to

be the lowest for these glasses. The summary of the

data on TL peaks with corresponding trap depth

parameters of the present glasses is furnished in

Table 2.

Fig. 4. TL recordings of X-ray irradiated MO–CaF2–B2O3 glasses: (a) pure; (b) MnO doped.

Fig. 5. A comparison plot of TL light output of MO–CaF2–B2O3: MnO glasses.

Table 2

Data on various trap depth parameters of A and A glasses

Glasses Tm (K) s (K) d (K) lg Es (eV) Ed (eV) S (s�1) 10�4 Area (arb. units)

A1 485 67 63 0.485 0.323 0.3097 0.3087 1740

A2 541 71 74 0.510 0.387 0.3280 0.1353 3135

A3 497 69 71 0.507 0.329 0.2886 0.3945 2050

A01 468 43 45 0.511 0.532 0.4037 1.0491 128

A02 490 67 61 0.476 0.331 0.3265 0.2175 318

A03 475 66 68 0.507 0.314 0.2752 0.7592 561

G. Venkateswara Rao et al. / Optical Materials 22 (2003) 295–302 299

4. Discussion

It is well known that the effect of introduction

of alkali oxides into B2O3 is the conversion of sp2

planar BO3 units into more stable sp3 tetrahedral

BO4 units and may also create non-bridging oxy-

gens. Each BO4 unit is linked to two such other

units and one oxygen from each unit with a metal

ion and the structure leads to the formation of

long chain tetrahedron. The presence of such BO4units in the present glasses is evident from the IR

spectral studies. Earlier NMR investigations by

different people [18,19] on alkali earth fluoro bo-rate glasses indicate that in addition to BO4 units

there exist B(O,F)4 (or BO3F) and BO2F2 units;

the presence of the highest probability was esti-

mated for BO3F units. The probable linkages be-

tween various ions in the present glasses are shown

in Fig. 6; it is to be noted in the figure that different

borate groups are connected by oxygen and that

the alkali ion (M) located near the fluorine serve ascharge compensator.

The alkali ions viz., Liþ, Naþ, Kþ have closed

structure, do not have energy levels within 10 eV of

the ground state hence these ions do not participate

directly in luminescence but may act as activator

ions. The calcium ions occur as Ca2þ; in the acti-

vation process the substitution of Mþ (Liþ, Naþ,

Kþ) by doubly charged calcium ions, the chargebalance would be upset. To maintain the charge

balance an equal number of F� ions in the lattice

have to be introduced. Since the calcium ion is

doubly charged, the energy levels of the surround-

ing oxygen ions will be slightly lifted up in com-

parison with the normal ions and give rise to

occupied energy levels close to the top of the va-

lance band. These levels form the ground state of

the luminescence. The lifting of these energy levels is

in a lesser extent in glasses containing Naþ ions

since the ionic radius of Naþ (0.098 nm) is very close

to that of Ca2þ (0.104 nm) where as the ionic radii

of Liþ and Kþ are 0.078 and 0.133 nm respectively.This is also borne out by the fact that the highest

value of glass transition temperature Tg for the glassA2 indicating the highest value of the coordination

number (Z) as per the equation lnðTgÞ ¼ 1:6Z þ 2:3[20]. The lesser degree of deformation in Naþ ion

present glasses indicate deeper trap depths for the

colour centres in these glasses giving rise to the

highest TL light output with the highest activationenergies (Table 2) as observed. Similarly the co-

activator F� ions give rise to unoccupied energy

levels close to the bottom of the conduction band

and these levels act as traps.

The action of X-ray irradiation on glasses is to

produce secondary electrons from the sites where

they are in a stable state and have excess energy.

Such electrons may traverse in the glass latticedepending upon their energy and the composition

of the glass and are finally be trapped, thus forming

color centres (or alternatively they may form exci-

tons with energy states in the forbidden gap). The

trapping sites may be the metal cations which

constitute the glass structure, ions of admixtures to

the main composition and the structural defects

due to impurities in the glass. Thus this processleads to the formation of (1) boron E centres

(BEC), (2) non-bridging oxygen hole centres, (3)

boron oxygen hole centres (BOHC) and (4) calcium

E centre––evident from the presence of a weak

absorption band at about 360 nm in the X-ray ir-

radiated glasses. TL is a radiative recombination

between the electrons (released by heating from a

BEC) and an anti-bonding molecular orbital of thenearest of the BOHC�s. However the TL emissiondue to such recombination is possible only at low

temperature at about 140 K as reported earlier

[21]. Alternatively, the TL emission in these glasses

may be explained as follows: the glasses with and

without Mn, after the X-ray irradiation, show an

absorption band at 360 nm which is attributed to

Caþ ions. During the heating process for recordingthe TL light output the electrons that were cap-

tured by Ca ions are liberated and later trapped byFig. 6. Schematic illustration of alkali fluoro borate glass.

M stands for alkali ion.

300 G. Venkateswara Rao et al. / Optical Materials 22 (2003) 295–302

holes in the recombination centre giving out TL

light output as shown in the Fig. 7a.

Coming to the glasses with Mn, during the

heating process for recording the TL, a fraction of

the electrons liberated by the calcium ions are

captured by the Mn3þ ions and the remaining

electrons may be trapped by the holes leading to a

decrease in the TL light output (Fig. 7b).The presence of predominant absorption band

in the X-ray irradiated MnO doped calcium flouro

borate glasses around 490 nm and the missing of

402 and 421 nm bands that are observed in the

non-irradiated glasses indicate a major part of the

manganese ions in the X-ray irradiated calcium

fluoro borate glasses with different modifiers ap-

pears to be Mn3þ ions. We observe the highest

amplitude of this 490 nm band for A01 samples

(containing Li2O as modifier) which indicates the

presence of the highest concentration of Mn3þ ions

in these glasses; this conclusion is based upon the

fact that the samples have almost identical thick-

nesses and the full width at half maximum for this

band is nearly the same for all the samples. Higherthe concentration of Mn3þ ions higher is the killing

action of the TL light output. This may explain

why there is more quenching of TL light output in

A01 glasses.

Alternatively the quenching action of lumines-

cence in the present glasses may also be explained

as follows: the electron–hole recombination occurs

Fig. 7. (a) A proposed TL mechanism for Mn free MO–CaF2–B2O3 glasses. (b) A proposed TL mechanism with killing action of Mn3þ

ions in MO–CaF2–B2O3 glasses.

G. Venkateswara Rao et al. / Optical Materials 22 (2003) 295–302 301

by means of the energy transfer of exciton re-

combination to the excited states of d band ofmanganese ion, this is followed by non-radiative

relaxation of the excited ion. with phonon emis-

sion. This is possible because the free electron (or

the hole) do not have sufficient electron–phononcoupling necessary for the self trapping to give rise

radiative recombination; on the other hand the

exciton has a strong coupling with the lattice so

that predominant non-radiative recombination

occurs with the phonon emission [22]. The nearly

equal radii of substitutional Mn3þ (0.07 nm) and

Liþ (0.078 nm) indicate a replacement of more

activator ions in glass A01 by Mn

3þ (luminescencekillers) and thus causing the highest percentage

of luminescence quenching in these glasses; Fur-

ther it may be noted here that the concentration of

manganese ions is the highest in these glasses with

lowest inter ionic distance (Table 1) when com-

pared with that in other two glasses.

5. Conclusions

TL study on X-ray irradiated MO–CaF2–B2O3could satisfactorily be explained with a radiative––

recombination between the electrons released on

heating from calcium ion centers and holes in re-

combination centers. The doping of MnO in small

percentage in these glasses caused a drastic de-crease in the TL light output. This has been ex-

plained due to the killing action of Mn3þ ions.

Acknowledgements

One of the authors G. Venkateswara Rao is

grateful to University Grants Commission of Indiafor granting fellowship under Faculty Improve-

ment Programme scheme. He also wishes to thank

Fr.C. Peter Raj, S.J., Principal, Andhra Loyola

College, Vijayawada for kindly granting the study

leave.

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