Journal of Non-Crystalline Solids 331 (2003) 122–127

www.elsevier.com/locate/jnoncrysol

HYSCORE spectroscopy in the borate glasses

George Kordas *

Sol–Gel Laboratory of Glass and Ceramics, Institute of Materials Science, National Center for Scientific Research Demokritos,

Aghia Paraskevi Attikis, Athens 15310, Greece

Received 8 August 2002; received in revised form 12 May 2003

Abstract

Glasses of the composition xB2O3–Li2O, with x ¼ 1, 2, 3, 4 and 5 were exposed to 60Co c-irradiation and measured

with hyperfine sublevel correlation (HYSCORE) spectroscopy. The HYSCORE spectra were explained with the as-

sistance of a simulation procedure developed in-house in order to extract important parameters describing the Ham-

iltonian of the second boron neighbor of the paramagnetic state. HYSCORE spectroscopy in the xB2O3–Li2O glasses

revealed Aiso couplings, the strength of which varies with x. In few cases, structural models could be proposed which

may account for the HYSCORE spectra. The HYSCORE spectrum of the 4B2O3–Li2O glass exhibits cross peaks in

both (+, +) and (+, )) quadrants indicating the presence of a complex structure.

� 2003 Elsevier B.V. All rights reserved.

1. Introduction

Borate glasses present an intriguing set of ma-terial for structural investigation using EPR spec-

troscopy. In the past, continuous wave electron

paramagnetic resonance spectroscopy (CW-EPR)

was used for the evaluation of the paramagnetic

states occurring in borate glasses. Two centers

were isolated named as Center I (BOHC1) and

Center II (BOHC2) occurring below 25-mol% and

above 25-mol%, respectively [1–6]. Table 1 pre-sents the spectroscopic parameters of these defects.

Recently, FT-EPR spectroscopy was used to

determine the structure of the two defects [7–16].

The two defects involve unpaired electrons trap-

* Tel.: +30-210 650 3301; fax: +30-210 654 7690.

E-mail address: gkordas@ims.demokritos.gr (G. Kordas).

0022-3093/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/j.jnoncrysol.2003.08.058

ped by non-bridging oxygen bonded to three-fold

coordinated boron. The environment of the two

centers is completely different. For the BOHC1,three variants were isolated involving boroxol-

rings [7]. The BOHC2 is composed of an ortho-

borate group in the proximity of a four-fold

coordinated boron [17]. In a recent study [18], the

hyperfine structure splittings were calculated for

various ring structures including pentaborate, tet-

raborate, diborate, coupled triborate borate net-

work with non-bridging atoms (NBO). It wasestablished that Aiso occur in a narrow range be-

tween )10 and )35 MHz [18]. These structure were

identified by NMR, NQR, Raman and IR meth-

ods to occur in the borate glasses the concentra-

tion of which was determined as a function of the

alkali metal concentration, x [19–24]. It is reason-

able to believe that these structures are also pre-

sent in glasses used for generation of paramagnetic

ed.

Table 1

Spectroscopic parameters of the BOHC1 and BOHC2 occurring in borate glasses [1]

Defect g1 g2 g3 A1 (G/MHz) A2 (G/MHz) A3 (G/MHz) Aiso (G/MHz)

Center I (<25-mol%) 2.0020 2.0103 2.0350 12.1 14.2 10.0 12.1

(BOHC1) 33.9 40.0 28.5 34.1

Center II (>25-mol%) 2.0049 2.0092 2.0250 11.2 12.9 8.0 10.7

(BOHC2) 31.4 36.3 22.7 30.1

G. Kordas / Journal of Non-Crystalline Solids 331 (2003) 122–127 123

states and thus they should also contribute to theCW-EPR spectra [18]. This conjecture was sup-

ported by the fact that the BOHC1 and BOHC2

centers were incapable to replicate the signals of

intermediate compositions (0 mol% Li2O< x<50

mol% Li2O) [18]. Thus, one needs to use advanced

FT-EPR techniques that are more sensitive to

spatial resolution than the conventional CW-EPR

spectroscopy.In the present study, the hyperfine sublevel

correlation spectroscopy (HYSCORE) [25–27] was

employed to determine the weak couplings in-

duced by the near neighbors of the unpaired state.

This way the different ring structures might be

identified because they exhibit characteristic Aiso.

Furthermore, it could be possible to figure out

which of the various ring structure are activecontributing to the CW-EPR spectra in the borate

glasses.

2. Experimental

Borate glasses were melted by using H3BO3 and

Li2CO3 in a platinum crucible at temperaturesbetween 800 and 1200 �C. The FT-EPR spectra

were recorded by a 300 E Bruker ESP 380 X-Band

instrument equipped with a Bruker ESP380-1078

IN echo integrator. The instrument dead time was

about 100 ns. The temperature was set below 25 K

using a cryostat. The microwave frequency was

measured using a frequency counter.

The HYSCORE spectra were recorded usingthe sequence:

p2ð16 nsÞ � sð104; 168; 240 nsÞ � p

2ð16 nsÞ � t1

�ð56þ dtð¼ 16 nsÞÞ � pð32 nsÞ � t2

�ð56þ dtð¼ 16 nsÞÞ � p2ð16 nsÞ � echo:

Phase cycling was employed to remove the un-

wanted echoes in the Bruker Pulse Spel library.

3. Results

3.1. HYSCORE spectra

Fig. 1(A) and (B) shows the HYSCORE spectra

of the 4B2O3–Li2O glass recorded with s ¼ 184

and 240 ns, respectively. The spectrum with

s ¼ 184 ns exhibits a cross peak with a separation

�4 MHz. When the spectra were recorded at

s ¼ 240 ns, two cross peaks were observed the one

in the (+, +) quadrant and the other in the (+, ))quadrant. The peak in the (+, +) quadrant has aseparation of �4 MHz. Among all glasses mea-

sured in the present study, the 4B2O3–Li2O glass

composition is the only sample exhibiting cross

peak in the (+, )) quadrant.HYSCORE spectra were recorded also for the

samples with x ¼ 1, 2, 3, 5 and pure B2O3. Table 2

summarizes the cross peaks and the conditions

they were observed. This table indicates that thecross peaks occur in the positive quadrant due to

weak interactions of the paramagnetic state with

the nuclear spin.

3.2. Simulation of the HYSCORE spectra

The interpretation of the HYSCORE spectra

requires simulation in order to understand howthey are created. The generation of the HY-

SCORE signals involves ENDOR transition be-

tween the states shown in Fig. 2. The simulation

program has the option to calculate a complete

spectrum taking into consideration all transitions

together or selectively. For example, it can provide

the spectrum created by single quantum transitions

F2:[MHz]0.0 2.5 5.0 7.5 10.0 12.5

-15

-10

-5

0

5

10

F1:[MHz]

(6,9MHz, 2,8MHz)

(2,7MHz, 6,8MHz)

(-12,4MHz, 3,8MHz)

(-3,7MHz, 12,7MHz)

ω(B (11))=4,7MHz

4MHz

~8 MHz

F2:[MHz]0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

-10

-8

-6

-4

-2

0

2

4

6

8

F1:[MHz]

(3,29MHz, 6,47MHz)

(6,35MHz, 3,17MHz)

3,15 MHz

ω(B(11))=4,7MHz

(A)

(B)

Fig. 1. HYSCORE spectrum of the 411B2O3Li2O glass re-

corded with s ¼ 184 ns (A) and s ¼ 240 ns (B).

-1/2

1/2

m S m I

3/ 2

1/ 2-1/2-3/2

-3/2

-1/2

1/2

3/2

4

3

2

1

4

23

1

Single quantumtransition ∆mI=1

Double quantumtransition ∆mI=2

ω43

ω31

Fig. 2. Energy level diagram for a S ¼ 1=2 and I ¼ 3=2 system

suited for 11B.

Table 2

Locations of cross peaks of the HYSCORE spectra recorded

in the borate glasses

x s (ns) A1 (MHz) A2 (MHz)

5 168 5

5 240 2 4

3 240 3

2 240 3

1 240 Smeared <1.3

B2O3 glass 248 2 6

B2O3 glass 168 6

124 G. Kordas / Journal of Non-Crystalline Solids 331 (2003) 122–127

between the 2$ 3 of the upper and lower levels of

the energy diagram. Changing the commands, itcan also calculate the transitions that are mixed

with the 2–3–4 energy levels with the 1–2–3 levels,

allowing single and double quantum transitions.

Another case would be to permit calculation of the

spectrum due to triple quantum transitions within

the levels 1$ 4 and 4$ 1. This way, it is possibleto assign the HYSCORE spectra to the energy

levels (Fig. 2).

In order to save space, the article will show two

examples ðAiso < 2�xIð11BÞ and Aiso > 2�xIð11BÞÞinvolving the nuclear quadrupole constant (NQC)

2.7 MHz, corresponding to three-fold coordinated

boron [19]. Fig. 3 shows the HYSCORE spectrum

with Aiso ¼ 4 MHz and Taniso ¼ 1:5 MHz. A crosspeak was observed around the Larmor frequency

of boron due to combination of single quantum

transitions (Fig. 2). Fig. 4(A) shows a theoretical

spectrum with Aiso ¼ 16 MHz and Taniso ¼ 0:9MHz. The question is how these cross peaks are

created. When the program was set to calculate

the spectrum with only single or double quantum

transitions, the intensity of the peaks was zero.

0 2 4 6 8 10 12 14 16 18 20-20

-15

-10

-5

0

5

10

15

20

[MHz]

[MH

z]

ω

ω

Fig. 3. Theoretical HYSCORE spectrum obtained using the

parameters: s ¼ 240 ns, gx ¼ 2:0025, gy ¼ 2:0118, gz ¼ 2:0370,

xIð11BÞ¼ 4:782043 MHz, vzz ¼ 2:7 MHz, g¼ 0:37, vrðg�QÞ¼70, Tan ¼ 1:5 MHz, Aiso ¼ 4:0 MHz.

G. Kordas / Journal of Non-Crystalline Solids 331 (2003) 122–127 125

Fig. 4(B) shows the HYSCORE spectrum when

the transitions 1$ 3 and 2$ 4 (lower level) were

mixed with the transitions 2$ 4 and 1$ 3 (upperlevel). Fig. 4(C) occurs when the transitions 2$ 4

and 1$ 3 (lower level) were mixed with the

transitions 1$ 3 and 2$ 4 (upper level).

4. Discussion

The HYSCORE spectra of the B2O3 andB2O3Li2O glass were explained in previous papers

[7–14,17]. The HYSCORE spectra of the B2O3

glass consists of two cross-peaks with Aiso � 2 and

�6 MHz attributed to the BOHC1a and BOHC1b,

respectively. The BOHC1a and BOHC1b centers

can be described by a hole trapped by non-bridg-

ing oxygen bonded to a three fold coordinated

boron. In case of the BOHC1a center, the borontriangle is part of the boroxol-ring [7–14]. The

boron triangle of the BOHC1b center connects two

boroxol rings [7–14]. The HYSCORE spectrum of

the B2O3Li2O glass was attributed to the interac-

tion of the unpaired spin located at an orthoborate

unit with a four-fold coordinated boron in the

neighborhood [17].

As the concentration of the alkali metals in-creases, the boroxol groups decrease and other

units are created. In the 5B2O3Li2O glass, three

weak couplings were observed with Aiso � 2, 4, and

5 MHz (Table 2). The couplings Aiso � 2 and 5

MHz may be due to the BOHC1a and BOHC1b

centers. The Aiso � 4 MHz cannot be explained at

present. It is evident that some more work is nee-

ded to figure out the origin of this HYSCOREpeak.

The HYSCORE spectra of the 4B2O3Li2O glass

are complex and are composed of weak couplings

Aiso � 3 and 4 MHz and a stronger coupling of

Aiso � 16 MHz. The theory gave that the Aiso � 16

MHz HYSCORE spectrum is due to combination

of single and double quantum transition. The

theoretical spectrum of Fig. 4 suggests that thereare not transitions with some significant intensity

in the (+, +) quadrant when Aiso ¼ 16. The cross

peaks with Aiso � 3 and 4 MHz are generated by

an interaction of the unpaired spin with other

boron sites not associated with the boron site

generating the Aiso � 16 MHz coupling. These

couplings can be due to the interaction of the

paramagnetic state in the same complex with threedifferent 11B units.

It is important to mention here that at this

composition the �boron anomaly� becomes maxi-

mum [19]. The term �boron anomaly� was intro-

duced since an assortment of physical properties

including the density, coefficient of expansion, etc.

initiate to alter significantly at about 15–20 mol%

alkali oxide. In the early days, the boron anomalywas attributed to creation and destruction of four-

fold coordinated boron units [28–30]. Though

NMR spectroscopy demonstrated that the fraction

of four-fold coordinated boron ions, N4, shows no

ruptures at �20-mol% modifier but at 40-mol%

with a steady decline to zero at 70-mol%. Since the

�boron anomaly� does not follow the N3 !N4 !N3 conversion with the modifier concentra-tion, a structural reason should account for this

effect. Bray [19] suggested that the large awkward

space filling tetraborate groups are responsible for

�the boron anomaly�. The tetraborate group arise

around �20-mol% alkali content. The HYSCORE

measurements fully support the �occurrence of

unusual structure� occurring in this region. This

0 2 4 6 8 10 12 14 16 18 20-20

-15

-10

-5

0

5

10

15

20

[MHz]

[MH

z]

ω

ω

ω

ω

ω

ω

0 2 4 6 8 10 12 14 16 18 20-20

-15

-10

-5

0

5

10

15

20

[MHz]

[MH

z]

0 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0-20

-15

-10

-5

0

5

10

15

20

[MHz]

[MH

z]

Fig. 4. (A) Theoretical HYSCORE spectrum obtained using the parameters: s ¼ 240 ns, gx ¼ 2:0025, gy ¼ 2:0118, gz ¼ 2:0370,

xI ð11BÞ ¼ 4:782043 MHz, vzz ¼ 2:7 MHz, g ¼ 0:37, vrðg � QÞ ¼ 70, Tan ¼ 0:9 MHz, Aiso ¼ 16:0 MHz. (B) Theoretical spectrum caused

by the transitions 1$ 3 and 2$ 4 (lower level) mixed with the transitions 2$ 4 and 1$ 3 (upper level). (C) Theoretical spectrum

generated though the transitions 2$ 4 and 1$ 3 (lower level) mixed with the transitions 1$ 3 and 2$ 4 (upper level).

126 G. Kordas / Journal of Non-Crystalline Solids 331 (2003) 122–127

complex could be the tetraborate group, complex

of structures, or another structure. The HY-SCORE spectra hold information of the sur-

roundings of the paramagnetic state up to its third

neighbor offering a comprehensive description of

the environment [7–14]. At the moment, one can

capture the information that the HYSCORE

spectrum signifies a significant �structural anomaly�at this composition that might be associated with

the �boron anomaly�. The decoding of the HY-SCORE spectrum of the 4B2O3Li2O glass would

be extremely important.

The addition of alkali metal ions yields to the

destruction of the structure causing the complex

HYSCORE peaks of the 4B2O3Li2O glass and one

different coupling at Aiso � 3–4 MHz occur in the

3B2O3Li2O and 2B2O3Li2O glasses (Table 2). TheseHYSCORE peaks may be generated by the same

structure that might be a metaborate or a pyrob-

orate unit. This unit is destroyed as more alkalis are

added to the glass to observe orthoborate unit in

the 1B2O3Li2O composition (Table 2) [17].

5. Conclusions

The HYSCORE spectroscopy in the borate

glasses revealed changing composite surround-

ings with composition producing dissimilar un-

paired states generating the CW-EPR spectra. The

G. Kordas / Journal of Non-Crystalline Solids 331 (2003) 122–127 127

rationalization of the HYSCORE spectra is diffi-cult for glasses and was done for two B2O3 and

B2O3Li2O glasses, so far. The main result of this

paper was to show the occurrence of a complex

HYSCORE spectrum in the 4B2O3Li2O glass, in

contrast to the other compositions investigated in

the present study. This suggests that something

significant happens to the structure in this range

of composition. This provides the incentives forspectroscopists to acquire data with advanced

EPR methods with the hope to throw some light

on this �boron anomaly�.

References

[1] D.L. Griscom, P.C. Taylor, D.A. Ware, P.J. Bray,

J. Chem. Phys. 48 (11) (1968) 5158.

[2] D.L. Griscom, G.H. Sigel Jr., R.J. Ginther, J. Appl. Phys.

47 (3) (1976) 960.

[3] P.C. Taylor, D.L. Griscom, J. Chem. Phys. 55 (7) (1971)

3610.

[4] M.C.R. Symons, J. Chem. Phys. 53 (1970) 468.

[5] D.L. Griscom, P.C. Taylor, P.J. Bray, J. Chem. Phys. 53

(1970) 469.

[6] P.C. Taylor, D.L. Griscom, P.J. Bray, J. Chem. Phys. 54

(2) (1971) 748.

[7] Y. Deligiannakis, L. Astrakas, G. Kordas, Phys. Rev. B. 58

(17) (1998) 11420.

[8] G. Kordas, Phys. Chem. Glasses 41 (5) (2000) 325.

[9] G. Kordas, in: C. Claeys (Eds.), First ENDEASD Work-

shop, Santorini, 21–22 April 1999, p. 82.

[10] G. Kordas, Phys. Chem. Glasses 41 (5) (2000) 1.

[11] G. Kordas, Phys. Chem. Glasses 41 (6) (2000) 358.

[12] G. Kordas, J. Non-Cryst. Solids 260 (1–2) (1999) 75.

[13] G. Kordas, in: A. Wright, S.A. Feller, A.C. Hannon (Eds.),

Proc. Second Int. Conf. Borate Glasses, Crystals, and

Melts, 1997, p. 148.

[14] G. Kordas, Phys. Chem. Glasses 39 (38) (1996) 21.

[15] I.A. Shkrob, V.F. Tarasov, J. Chem. Phys. 113 (23) (2000)

10723.

[16] I.A. Shkrob, B.M. Tadjikov, A.D. Trifunac, J. Non-Cryst.

Sol. 262 (2000) 6.

[17] G. Kordas, Phys. Rev. B 68 (2003) 024202.

[18] G. Kordas, Phys. Chem. Glasses 42 (3) (2001) 226.

[19] P.J. Bray, in: A.C. Wright, S.A. Feller, A.C. Hannon

(Eds.), Proc. Second Int. Conf. Borate Glasses, Crystals

and Melts, The Soc. Glass Techn., Sheffield, 1997, p. 1.

[20] H.M. Kriz, P.J. Bray, J. Magn. Res. 4 (1971) 69.

[21] D. Kline, P.J. Bray, H.M. Kriz, J. Chem. Phys. 48 (1968)

5277.

[22] Y.H. Yun, P.J. Bray, J. Non-Cryst. Solids 44 (1981) 227.

[23] H.M. Kriz, P.J. Bray, J. Magn. Res. 4 (1971) 76.

[24] E.I. Kamitsos, M.A. Karakassides, Phys. Chem. Glasses 30

(1) (1989) 19.

[25] P. Hoefer, A. Grupp, G. Nebenfuer, M. Mehring, Chem.

Phys. Lett. 132 (1986) 279.

[26] J.J. Shane, P. Hoefer, E.J. Reijerse, E. de Boer, J. Magn.

Res. 99 (1992) 596.

[27] A. Schweiger, G. Jeschke, Principles of Pulse EPR, Oxford

UP, New York, 2001, p. 284.

[28] B.E. Warren, H. Krutter, O. Morningstar, J. Am. Ceram.

Soc. 19 (1936) 202.

[29] T. Abe, J. Am. Ceram. Soc. 40 (1957) 287.

[30] E.C. Evereijn, J.M. Stevels, W.I. Watermann, Phys. Chem.

Glasses 1 (1960) 123.