- Email: [email protected]
FT-EPR spectroscopy in the borate system
Journal of Non-Crystalline Solids 345&346 (2004) 45–49 www.elsevier.com/locate/jnoncrysol
FT-EPR spectroscopy in the borate system George Kordas
*
Sol–Gel Laboratory for Glass and Ceramics, Institute of Materials Science, National Center for Scientific Research ÔDEMOKRITOSÕ, Aghia Paraskevi Attikis, 15310 Athens, Greece Available online 15 September 2004
Abstract 80-mol% B2O3 20-mol% Li2O glass has been subjected to c-irradiation at room temperature and subsequently studied by CW EPR (continuous wave electron paramagnetic resonance), ECHO-fs (field sweep), 4p-ESEEM (electron spin echo envelop modulation spectroscopy), 2D-HYSCORE (hyperfine sublevel correlation) spectroscopy and pulse ENDOR (electron nuclear double resonance) spectroscopies. The variation of the EPR signal with the conditions of measurements suggests that several centers contribute to the signal. The Aiso-couplings of the next neighbor have been determined by these methods. One may suggest on this ground the existence of the tetraborate, diborate and boron network non-bridging oxygen (BN-nbo) structures, though this preliminary effort needs supplementary investigational work to be convinced about it. Ó 2004 Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 71.55.Jv; 76.30.V
1. Introduction Recently, pulsed EPR [1–3] and pulsed ENDOR [4] spectroscopies were employed to elucidate the structure of the defect (BOHC1) obtained in a-B2O3. Together with self-consistent field Hartree–Fock (SCF-HF) calculations, ESEEM spectroscopy [1–3], HYSCORE spectroscopy [1–3] and pulsed ENDOR [4] spectroscopy revealed strong and weak hyperfine couplings attributed to boroxol groups. These series of studies revealed the existence of three different BOHC1a,b,c. Another article concerned with the structure of the defect (BOHC2) occurring at 50-mol% B2O3 50-mol% Li2O glass [5]. An orthoborate group in the proximity of fourfold coordinated boron cluster reproduces the experimental parameters most satisfactorily of the BOHC2-center. In the present study, cw-EPR, 4p-1D-ESEEM, HYSCORE and pulsed ENDOR spectroscopies were employed to elucidate the structure of 80-mol% B2O3 20*
Tel.: +30 1 6503301; fax: +30 16547690. E-mail address: [email protected]
0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.07.041
mol% Li2O glass. SCF-HF was used to determine the hfs coupling constants for various ring structures using the B3LYP method with the 6-311G-basis set [6]. These calculations were partially repeated using the EPR II basis set [7]. The ADF program was used to calculate the g-factors using a density function theory (DFT) code [8–10]. GaussView and Chem3D programs were used for the visualization of the results.
2. Experimental Borate glasses were prepared by melting H311BO3 (Aldrich Chemical Company) and Li2CO3 in a platinum crucible at 1200 °C. The B2O3 crystal was provided by US BORAX. The cw-EPR spectra were recorded at 20 K using a 300 E BRUKER instrument. The pulsed EPR work was carried out with a BRUKER ESP 380 spectrometer and with a BRUKER ESP380-1078 IN echo-integrator. The dead time of the instrument was about 100 ns. The pulsed ENDOR measurements were made with a BRUKER ESP 360 DICE ENDOR system
46
G. Kordas / Journal of Non-Crystalline Solids 345&346 (2004) 45–49
and an ENI RF 500 amplifier. A HP 5350B counter was used for microwave frequency measurements. The HYSCORE spectra were recorded using the phase cycling method to remove unwanted echoes in the BRUKER Pulse Spel library. The pulsed-ENDOR spectra were recorded using the Davies and the Mims sequences [11,12]. The conditions of measurements were reported previously [1–5].
3. Results 3.1. CW and field swept spectra Fig. 1 shows the cw-EPR spectrum of the 80-mol% B2O3 20-mol% Li2O glass recorded at 10 and 60 dB. The same figure also gives the ECHO-fs spectra in absorption and first derivative mode. The line shape of the spectra depends significantly on the conditions of
measurements indicating the presence of various defects occurring in this glass. 3.2. Four pulse one dimensional ESEEM Fig. 2 shows the 4p 1D ESEEM spectrum of the 80mol% B2O3 20-mol% Li2O glass recorded using different s-values (=88 + dt (=8 ns)). The peaks between 9 and 10 MHz correspond to the ones of the double frequency of 11B. One can perceive from this figure that double peaks were detected at 88 and 96 ns while a single peak was observed for the other s-values. It has been shown; the splitting of the double frequency peak is due to the quadrupole moment of the trigonal boron. The quadrupole moment of the tetragonal boron is too small to split the double frequency peak of 11B. 3.3. HYSCORE spectroscopy Fig. 3 shows the HYSCORE spectra recorded with s = 248 ns (a) and s = 184 ns (b).
Fig. 1. Echo field sweep spectrum in absorption (A) and first derivative (B) mode. Cw EPR spectrum recorded at 10 (C) and 60 dB (D).
Fig. 2. 4P 1DESEEM spectra recorded at different dt (=8 ns) values starting with 88 ns.
Fig. 3. HYSCORE spectrum of the 80-mol% B2O3 20-mol% Li2O glass recorded at 184 (b) and 248 ns (a).
G. Kordas / Journal of Non-Crystalline Solids 345&346 (2004) 45–49
47
Fig. 4. Spin level system used for the calculations (a). HYSCORE spectra calculated using Aiso = 4 MHz (<2xI(11B)) (b) and 16 MHz (>2xI(11B)) (c).
A number of cross-peaks were detected in this glass. The peaks in the (+, +) quadrant are due to weak interaction (x < 2xI(11B) = 9.564086 MHz for 3.5 kg) while the peaks in the (+, ) quadrant are generated by strong interactions (x > 2xI(11B)). Fig. 4(a) shows the sublevel system of 11B. This system was used to generate the theoretical HYSCORE spectra of Fig. 4(b) and (c). The spectra of the (+, +) quadrant are due to a weak interaction of Aiso = 4 MHz. The cross-peaks of the (+, ) quadrant are due the strong interaction with Aiso = 16 MHz. The peaks in the positive quadrant are due to single quantum transitions while the peaks in the negative quadrant due to double quantum transitions. 3.4. Pulse ENDOR spectra Fig. 5 shows the pulse ENDOR spectra of the B2O3 glass (C), B2O3 crystal (D) in the Davies sequence. Mims (A) and Davies (B) pulse ENDOR spectra of the 80mol% B2O3 20-mol% Li2O glass. The double peaks correspond to strong interactions of the unpaired spin with the nucleus of 11B. The center of the double peak for B2O3 glass (C) and, B2O3 crystal (D) is at Aiso/2 = 18.4 MHz. The center of the double peak is at Aiso/2 = 12.5 MHz for the x = 20-glass. Another weak couple of
peaks was observed for the x = 20-glass with Aiso/2 = 5.2 MHz. 3.5. Theoretical calculations Fig. 6 shows the equilibrium structures for the boron network-non-bridging oxygen, tetraborate and diborate clusters with their respective Aiso-values. Table 1 shows the g-values of various units determined by a DFT method. 3.6. Discussion The paramagnetic states occurring in the B2O3 and 50-mol% B2O3 50-mol% Li2O glasses were attributed to the BOHC1 and BOHC2 centers with g-values listed in Table 1. These parameters can be reproduced by the OBOHOH (boroxol group) and OBOLiOLi (orthoborate group) defects for the BOHC1 and BOHC2, respectively [1–5]. The cw-EPR signal of the 80-mol% B2O3 20-mol% Li2O glass depends on the conditions of measurements. This indicates that several paramagnetic states might contribute the cw-EPR signal. The 4p-1D ESEEM exhibits a double frequency peak between 9 and 10 MHz that depends on s. In some cases a double peak was observed and in another a single peak was obtained. When an element exhibits a quadrupole moment Q, one expects a splitting of the double frequency 4p 1D ESEEM peak with a separation d [13,14]: d¼
Fig. 5. Mims (A) and Davies (B) pulse ENDOR spectra of the 80mol% B2O3 20-mol% Li2O glass Pulse ENDOR spectra of the B2O3 glass (C), B2O3 crystal (D) in the Davies sequence.
3 e2 qQ ð3cos2 h 1Þ; 2 h
ð1Þ
which is affected by the size and orientation of the nuclear quadrupole constant (NQC) tensor with respect to the complex coordinate system [13,14]. Taking d 0.45 MHz and h = 70° (Fig. 2) [13,14], vzz becomes 2.62 MHz for the second neighbor for s = 88 ns. For s = 168 ns, a splitting was not observed in the 4p 1D ESEEM spectrum of the 20-mol% Li2O 80-mol% B2O3 glass, the d-splitting would be less than 0.2 MHz implying that the value for vzz is less than 1.1 MHz.
48
G. Kordas / Journal of Non-Crystalline Solids 345&346 (2004) 45–49
Fig. 6. Equilibrium structures for the BN-nbo, tetraborate and diborate clusters together with the expected Aiso-values. Red (dark) spheres correspond to the oxygen ions. The clouds are the electron spin densities calculated by DFT. Elongated bond of Li ion in the tetraborate group corresponds to the new equilibrium location after irradiation of the cluster.
Table 1 DFT calculations of the g-factors of different potential paramagnetic states in the borate glasses (The uncertainties of the g-factors were given in the respected references) Defect–composition–structure
g1
g2
g3
g-isotropic
Origin
BOHC1 BOHC2 c-B2O3 a-B2O3 OBOH -center% OH OBOLi -center (orthoborate)% OLi Boroxol% OH OH –O–BOH% OH OH B Triborate%
2.0020 2.0049 2.0000 2.0025 2.0023 2.0121 2.0025 2.0023 2.0031
2.0103 2.0092 2.0107 2.0118 2.0117 2.0191 2.0111 2.0027 2.0061
2.0350 2.0250 2.0450 2.0370 2.0334 2.0508 2.0269 2.0032 2.0080
2.0158 2.0130 2.0186 2.0171 2.0158 2.0273 2.0135 2.0027 2.0057
[1–5] [1–5] [1–5] [1–5] DFT DFT DFT DFT DFT
%
Wave function: TZ2P, XC: LDA VWN, GGA Becke Perdew.
Table 2 gives two ranges for NQC in crystalline borates determined by the NMR and NQR spectroscopies [15–19]. Thus, the first neighbor of the paramagnetic state in the first case is fourfold coordinated and in the second threefold coordinated. The structures of Fig. 6 traps the paramagnetic state on bridging oxygen between fourfold and threefold coordinated boron. Such structures will behave in the 4p 1D ESEEM this way. The HYSCORE spectra exhibit peaks with couplings of Aiso = 16, 4 and 3 MHz (Fig. 3). The pulse ENDOR spectra indicate Aiso = 25 and 10.4 MHz (Fig. 5). It has been shown that when an unpaired electron is localized at a non-bridging oxygen attached to a threefold coordi-
nated boron Aiso is greater that j30j MHz [1–3]. The A < j27j MHz are reserved for structures involving combination between fourfold and threefold coordinating boron. These structures are shown in Fig. 6. Most likely, the tetraborate (experiment: Aiso = 3, 4, 10.4) and diborate (experiment: Aiso = 16 MHz), and NBNBO (experiment: Aiso = 3, 4, 25 MHz) structures might bepresent. Though, this preliminary work needs further theoretical and experimental support using more recent developed pulsed EPR methods having better resolution.
4. Conclusion Table 2 Ranges of values in the amorphous materials for threefold and fourfold coordinated boron determined by NMR and NQR [15–19] B coordination
vzz (MHz)
g
Tetragonal Trigonal
0–0.855 2.45–2.81
0–0.23 0.47–0.75
In recent years, the existing models for the BOHC1 and BOHC2 have been reevaluated [1–5,19]. The BOHC1 was attributed to an oxygen-dangling bond attached to threefold coordinated boron in the boroxolring. The BOHC2 can be described with an orthoborate group in the proximity of four-fold coordinated boron.
G. Kordas / Journal of Non-Crystalline Solids 345&346 (2004) 45–49
The g-value calculations fully support these models. The OH original model for the BOHC1 ðHO –O –BOH HO B OH Þ yield gvalues that differ significantly from the experimental data. Having that foundation, one can extend the gvalue calculations to more complex units occurring in the 80-mol% B2O3 20-mol% Li2O glass. These units may include the BN-NBO, tetraborate and diborate ring structures. The HYSCORE and pulse ENDOR spectra (for the selected B) support the presence of these units in the glass. Now, one has good theoretical parameters for these structures to use in a future work for the evaluation of the spectra both HYSCORE and pulse ENDOR. Acknowledgments The author thanks the Greek General Secretariat for Research and Technology and the European Community for funding. I also thank Dr Malgorzata Makowska, Dr Heribert Reis and Dr Erik van Lenthe for their assistance to set up the ADF program and script file in order to calculate the boron g-factors properly. References [1] Y. Deligiannakis, L. Astrakas, G. Kordas, R.A. Smith, Phys. Rev. B 58 (17) (1998) 11420.
49
[2] G. Kordas, Phys. Chem. Glasses 38 (1997) 21. [3] G. Kordas, in: A.C. Wright, S.A. Feller, A. Hannon (Eds.), Borate Glasses, Crystals, and Melts, The Society of Glass Technology, Sheffield, 1997, p. 48. [4] G. Kordas, J. Non-Cryst. Solids 260 (1–2) (1999) 75. [5] G. Kordas, Phys. Rev. B 68 (2003) 024202-1. [6] Gaussian 98W, Revision D. 2, Gaussian, Inc., Pittsburgh, PA, 1995. [7] V. Barone, in: D.P. Chong (Ed.), Recent Advances in DFT Methods, Part I, World Scientific Publ., Singapore, 1996. [8] G. te Velde, F.M. Bickelhaupt, S.J.A. van Gisbergen, C. Fonsesca Guerra, E.J. Baerends, J.G. Snijders, T. Ziegler, Chemistry with ADF, J. Comput. Chem. 22 (2001) 931. [9] C. Fonseca Guerra, J.G. Snijders, G. te Velde, E.J. Baerends, Theor. Chem. Acta 99 (1998) 391. [10] E. van Lenthe, A. van der Avoird, P.E.S. Wormer, J. Chem. Phys. 107 (1997) 2488. [11] E.R. Davies, Phys. Lett. 47A (1974) 1. [12] W.B. Mims, Proc. Roy. Soc. Lond. 283 (1965) 452. [13] A.M. Tyryshkin, S.A. Dikanov, R.G. Evelo, A.J. Hoff, J. Chem. Phys. 97 (1) (1992) 42. [14] A.M. Tyryshkin, S.A. Dikanov, D. Goldfarb, J. Magn. Res. Ser. A 105 (1993) 271. [15] P.J. Bray, Inorg. Chim. Acta 289 (1999) 158. [16] G.E. Jellison Jr., S.A. Feller, P.J. Bray, Phys. Chem. Glasses 19 (3) (1973) 52. [17] G.L. Turner, K.A. Smith, R.J. Kirkpatrick, E. Oldfield, J. Magn. Res. 67 (1986) 544. [18] P.J. Bray, in: A.C. Wright, S.A. Feller, A.C. Hannon (Eds.), Proc. Second Int. Conf. on Borate Glasses Crystals and Melts, The Soc. Glass Techn., Sheffield, 1997, p. 1. [19] I.A. Shkrob, B.M. Tadjikov, A.D. Trifunac, J. Non-Cryst. Solids 262 (2000) 6.
FT-EPR spectroscopy in the borate system George Kordas
*
Sol–Gel Laboratory for Glass and Ceramics, Institute of Materials Science, National Center for Scientific Research ÔDEMOKRITOSÕ, Aghia Paraskevi Attikis, 15310 Athens, Greece Available online 15 September 2004
Abstract 80-mol% B2O3 20-mol% Li2O glass has been subjected to c-irradiation at room temperature and subsequently studied by CW EPR (continuous wave electron paramagnetic resonance), ECHO-fs (field sweep), 4p-ESEEM (electron spin echo envelop modulation spectroscopy), 2D-HYSCORE (hyperfine sublevel correlation) spectroscopy and pulse ENDOR (electron nuclear double resonance) spectroscopies. The variation of the EPR signal with the conditions of measurements suggests that several centers contribute to the signal. The Aiso-couplings of the next neighbor have been determined by these methods. One may suggest on this ground the existence of the tetraborate, diborate and boron network non-bridging oxygen (BN-nbo) structures, though this preliminary effort needs supplementary investigational work to be convinced about it. Ó 2004 Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 71.55.Jv; 76.30.V
1. Introduction Recently, pulsed EPR [1–3] and pulsed ENDOR [4] spectroscopies were employed to elucidate the structure of the defect (BOHC1) obtained in a-B2O3. Together with self-consistent field Hartree–Fock (SCF-HF) calculations, ESEEM spectroscopy [1–3], HYSCORE spectroscopy [1–3] and pulsed ENDOR [4] spectroscopy revealed strong and weak hyperfine couplings attributed to boroxol groups. These series of studies revealed the existence of three different BOHC1a,b,c. Another article concerned with the structure of the defect (BOHC2) occurring at 50-mol% B2O3 50-mol% Li2O glass [5]. An orthoborate group in the proximity of fourfold coordinated boron cluster reproduces the experimental parameters most satisfactorily of the BOHC2-center. In the present study, cw-EPR, 4p-1D-ESEEM, HYSCORE and pulsed ENDOR spectroscopies were employed to elucidate the structure of 80-mol% B2O3 20*
Tel.: +30 1 6503301; fax: +30 16547690. E-mail address: [email protected]
0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.07.041
mol% Li2O glass. SCF-HF was used to determine the hfs coupling constants for various ring structures using the B3LYP method with the 6-311G-basis set [6]. These calculations were partially repeated using the EPR II basis set [7]. The ADF program was used to calculate the g-factors using a density function theory (DFT) code [8–10]. GaussView and Chem3D programs were used for the visualization of the results.
2. Experimental Borate glasses were prepared by melting H311BO3 (Aldrich Chemical Company) and Li2CO3 in a platinum crucible at 1200 °C. The B2O3 crystal was provided by US BORAX. The cw-EPR spectra were recorded at 20 K using a 300 E BRUKER instrument. The pulsed EPR work was carried out with a BRUKER ESP 380 spectrometer and with a BRUKER ESP380-1078 IN echo-integrator. The dead time of the instrument was about 100 ns. The pulsed ENDOR measurements were made with a BRUKER ESP 360 DICE ENDOR system
46
G. Kordas / Journal of Non-Crystalline Solids 345&346 (2004) 45–49
and an ENI RF 500 amplifier. A HP 5350B counter was used for microwave frequency measurements. The HYSCORE spectra were recorded using the phase cycling method to remove unwanted echoes in the BRUKER Pulse Spel library. The pulsed-ENDOR spectra were recorded using the Davies and the Mims sequences [11,12]. The conditions of measurements were reported previously [1–5].
3. Results 3.1. CW and field swept spectra Fig. 1 shows the cw-EPR spectrum of the 80-mol% B2O3 20-mol% Li2O glass recorded at 10 and 60 dB. The same figure also gives the ECHO-fs spectra in absorption and first derivative mode. The line shape of the spectra depends significantly on the conditions of
measurements indicating the presence of various defects occurring in this glass. 3.2. Four pulse one dimensional ESEEM Fig. 2 shows the 4p 1D ESEEM spectrum of the 80mol% B2O3 20-mol% Li2O glass recorded using different s-values (=88 + dt (=8 ns)). The peaks between 9 and 10 MHz correspond to the ones of the double frequency of 11B. One can perceive from this figure that double peaks were detected at 88 and 96 ns while a single peak was observed for the other s-values. It has been shown; the splitting of the double frequency peak is due to the quadrupole moment of the trigonal boron. The quadrupole moment of the tetragonal boron is too small to split the double frequency peak of 11B. 3.3. HYSCORE spectroscopy Fig. 3 shows the HYSCORE spectra recorded with s = 248 ns (a) and s = 184 ns (b).
Fig. 1. Echo field sweep spectrum in absorption (A) and first derivative (B) mode. Cw EPR spectrum recorded at 10 (C) and 60 dB (D).
Fig. 2. 4P 1DESEEM spectra recorded at different dt (=8 ns) values starting with 88 ns.
Fig. 3. HYSCORE spectrum of the 80-mol% B2O3 20-mol% Li2O glass recorded at 184 (b) and 248 ns (a).
G. Kordas / Journal of Non-Crystalline Solids 345&346 (2004) 45–49
47
Fig. 4. Spin level system used for the calculations (a). HYSCORE spectra calculated using Aiso = 4 MHz (<2xI(11B)) (b) and 16 MHz (>2xI(11B)) (c).
A number of cross-peaks were detected in this glass. The peaks in the (+, +) quadrant are due to weak interaction (x < 2xI(11B) = 9.564086 MHz for 3.5 kg) while the peaks in the (+, ) quadrant are generated by strong interactions (x > 2xI(11B)). Fig. 4(a) shows the sublevel system of 11B. This system was used to generate the theoretical HYSCORE spectra of Fig. 4(b) and (c). The spectra of the (+, +) quadrant are due to a weak interaction of Aiso = 4 MHz. The cross-peaks of the (+, ) quadrant are due the strong interaction with Aiso = 16 MHz. The peaks in the positive quadrant are due to single quantum transitions while the peaks in the negative quadrant due to double quantum transitions. 3.4. Pulse ENDOR spectra Fig. 5 shows the pulse ENDOR spectra of the B2O3 glass (C), B2O3 crystal (D) in the Davies sequence. Mims (A) and Davies (B) pulse ENDOR spectra of the 80mol% B2O3 20-mol% Li2O glass. The double peaks correspond to strong interactions of the unpaired spin with the nucleus of 11B. The center of the double peak for B2O3 glass (C) and, B2O3 crystal (D) is at Aiso/2 = 18.4 MHz. The center of the double peak is at Aiso/2 = 12.5 MHz for the x = 20-glass. Another weak couple of
peaks was observed for the x = 20-glass with Aiso/2 = 5.2 MHz. 3.5. Theoretical calculations Fig. 6 shows the equilibrium structures for the boron network-non-bridging oxygen, tetraborate and diborate clusters with their respective Aiso-values. Table 1 shows the g-values of various units determined by a DFT method. 3.6. Discussion The paramagnetic states occurring in the B2O3 and 50-mol% B2O3 50-mol% Li2O glasses were attributed to the BOHC1 and BOHC2 centers with g-values listed in Table 1. These parameters can be reproduced by the OBOHOH (boroxol group) and OBOLiOLi (orthoborate group) defects for the BOHC1 and BOHC2, respectively [1–5]. The cw-EPR signal of the 80-mol% B2O3 20-mol% Li2O glass depends on the conditions of measurements. This indicates that several paramagnetic states might contribute the cw-EPR signal. The 4p-1D ESEEM exhibits a double frequency peak between 9 and 10 MHz that depends on s. In some cases a double peak was observed and in another a single peak was obtained. When an element exhibits a quadrupole moment Q, one expects a splitting of the double frequency 4p 1D ESEEM peak with a separation d [13,14]: d¼
Fig. 5. Mims (A) and Davies (B) pulse ENDOR spectra of the 80mol% B2O3 20-mol% Li2O glass Pulse ENDOR spectra of the B2O3 glass (C), B2O3 crystal (D) in the Davies sequence.
3 e2 qQ ð3cos2 h 1Þ; 2 h
ð1Þ
which is affected by the size and orientation of the nuclear quadrupole constant (NQC) tensor with respect to the complex coordinate system [13,14]. Taking d 0.45 MHz and h = 70° (Fig. 2) [13,14], vzz becomes 2.62 MHz for the second neighbor for s = 88 ns. For s = 168 ns, a splitting was not observed in the 4p 1D ESEEM spectrum of the 20-mol% Li2O 80-mol% B2O3 glass, the d-splitting would be less than 0.2 MHz implying that the value for vzz is less than 1.1 MHz.
48
G. Kordas / Journal of Non-Crystalline Solids 345&346 (2004) 45–49
Fig. 6. Equilibrium structures for the BN-nbo, tetraborate and diborate clusters together with the expected Aiso-values. Red (dark) spheres correspond to the oxygen ions. The clouds are the electron spin densities calculated by DFT. Elongated bond of Li ion in the tetraborate group corresponds to the new equilibrium location after irradiation of the cluster.
Table 1 DFT calculations of the g-factors of different potential paramagnetic states in the borate glasses (The uncertainties of the g-factors were given in the respected references) Defect–composition–structure
g1
g2
g3
g-isotropic
Origin
BOHC1 BOHC2 c-B2O3 a-B2O3 OBOH -center% OH OBOLi -center (orthoborate)% OLi Boroxol% OH OH –O–BOH% OH OH B Triborate%
2.0020 2.0049 2.0000 2.0025 2.0023 2.0121 2.0025 2.0023 2.0031
2.0103 2.0092 2.0107 2.0118 2.0117 2.0191 2.0111 2.0027 2.0061
2.0350 2.0250 2.0450 2.0370 2.0334 2.0508 2.0269 2.0032 2.0080
2.0158 2.0130 2.0186 2.0171 2.0158 2.0273 2.0135 2.0027 2.0057
[1–5] [1–5] [1–5] [1–5] DFT DFT DFT DFT DFT
%
Wave function: TZ2P, XC: LDA VWN, GGA Becke Perdew.
Table 2 gives two ranges for NQC in crystalline borates determined by the NMR and NQR spectroscopies [15–19]. Thus, the first neighbor of the paramagnetic state in the first case is fourfold coordinated and in the second threefold coordinated. The structures of Fig. 6 traps the paramagnetic state on bridging oxygen between fourfold and threefold coordinated boron. Such structures will behave in the 4p 1D ESEEM this way. The HYSCORE spectra exhibit peaks with couplings of Aiso = 16, 4 and 3 MHz (Fig. 3). The pulse ENDOR spectra indicate Aiso = 25 and 10.4 MHz (Fig. 5). It has been shown that when an unpaired electron is localized at a non-bridging oxygen attached to a threefold coordi-
nated boron Aiso is greater that j30j MHz [1–3]. The A < j27j MHz are reserved for structures involving combination between fourfold and threefold coordinating boron. These structures are shown in Fig. 6. Most likely, the tetraborate (experiment: Aiso = 3, 4, 10.4) and diborate (experiment: Aiso = 16 MHz), and NBNBO (experiment: Aiso = 3, 4, 25 MHz) structures might bepresent. Though, this preliminary work needs further theoretical and experimental support using more recent developed pulsed EPR methods having better resolution.
4. Conclusion Table 2 Ranges of values in the amorphous materials for threefold and fourfold coordinated boron determined by NMR and NQR [15–19] B coordination
vzz (MHz)
g
Tetragonal Trigonal
0–0.855 2.45–2.81
0–0.23 0.47–0.75
In recent years, the existing models for the BOHC1 and BOHC2 have been reevaluated [1–5,19]. The BOHC1 was attributed to an oxygen-dangling bond attached to threefold coordinated boron in the boroxolring. The BOHC2 can be described with an orthoborate group in the proximity of four-fold coordinated boron.
G. Kordas / Journal of Non-Crystalline Solids 345&346 (2004) 45–49
The g-value calculations fully support these models. The OH original model for the BOHC1 ðHO –O –BOH HO B OH Þ yield gvalues that differ significantly from the experimental data. Having that foundation, one can extend the gvalue calculations to more complex units occurring in the 80-mol% B2O3 20-mol% Li2O glass. These units may include the BN-NBO, tetraborate and diborate ring structures. The HYSCORE and pulse ENDOR spectra (for the selected B) support the presence of these units in the glass. Now, one has good theoretical parameters for these structures to use in a future work for the evaluation of the spectra both HYSCORE and pulse ENDOR. Acknowledgments The author thanks the Greek General Secretariat for Research and Technology and the European Community for funding. I also thank Dr Malgorzata Makowska, Dr Heribert Reis and Dr Erik van Lenthe for their assistance to set up the ADF program and script file in order to calculate the boron g-factors properly. References [1] Y. Deligiannakis, L. Astrakas, G. Kordas, R.A. Smith, Phys. Rev. B 58 (17) (1998) 11420.
49
[2] G. Kordas, Phys. Chem. Glasses 38 (1997) 21. [3] G. Kordas, in: A.C. Wright, S.A. Feller, A. Hannon (Eds.), Borate Glasses, Crystals, and Melts, The Society of Glass Technology, Sheffield, 1997, p. 48. [4] G. Kordas, J. Non-Cryst. Solids 260 (1–2) (1999) 75. [5] G. Kordas, Phys. Rev. B 68 (2003) 024202-1. [6] Gaussian 98W, Revision D. 2, Gaussian, Inc., Pittsburgh, PA, 1995. [7] V. Barone, in: D.P. Chong (Ed.), Recent Advances in DFT Methods, Part I, World Scientific Publ., Singapore, 1996. [8] G. te Velde, F.M. Bickelhaupt, S.J.A. van Gisbergen, C. Fonsesca Guerra, E.J. Baerends, J.G. Snijders, T. Ziegler, Chemistry with ADF, J. Comput. Chem. 22 (2001) 931. [9] C. Fonseca Guerra, J.G. Snijders, G. te Velde, E.J. Baerends, Theor. Chem. Acta 99 (1998) 391. [10] E. van Lenthe, A. van der Avoird, P.E.S. Wormer, J. Chem. Phys. 107 (1997) 2488. [11] E.R. Davies, Phys. Lett. 47A (1974) 1. [12] W.B. Mims, Proc. Roy. Soc. Lond. 283 (1965) 452. [13] A.M. Tyryshkin, S.A. Dikanov, R.G. Evelo, A.J. Hoff, J. Chem. Phys. 97 (1) (1992) 42. [14] A.M. Tyryshkin, S.A. Dikanov, D. Goldfarb, J. Magn. Res. Ser. A 105 (1993) 271. [15] P.J. Bray, Inorg. Chim. Acta 289 (1999) 158. [16] G.E. Jellison Jr., S.A. Feller, P.J. Bray, Phys. Chem. Glasses 19 (3) (1973) 52. [17] G.L. Turner, K.A. Smith, R.J. Kirkpatrick, E. Oldfield, J. Magn. Res. 67 (1986) 544. [18] P.J. Bray, in: A.C. Wright, S.A. Feller, A.C. Hannon (Eds.), Proc. Second Int. Conf. on Borate Glasses Crystals and Melts, The Soc. Glass Techn., Sheffield, 1997, p. 1. [19] I.A. Shkrob, B.M. Tadjikov, A.D. Trifunac, J. Non-Cryst. Solids 262 (2000) 6.