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Proton-irradiation effect on the proton motion in TlH2PO4
Current Applied Physics 7 (2007) 28–30 www.elsevier.com/locate/cap www.kps.or.kr
Proton-irradiation effect on the proton motion in TlH2PO4 S.H. Kim a, K.W. Lee a, Cheol Eui Lee a
a,*
, K.-S. Lee
b
Department of Physics and Institute for Nano Science, Korea University, 1 Anam-dong, Sungbuk-ku, Seoul 136 713, Republic of Korea b School of Computer-Aided Science, Inje University, Gimhae 621 749, Republic of Korea Received 20 May 2005; accepted 11 June 2005 Available online 14 October 2005
Abstract We have investigated the proton-beam irradiation effect on TlH2PO4 (TDP) undergoing an antiferroelectric phase transition and a ferroelastic one. The polycrystalline sample was irradiated by a 1 MeV hydrogen ion beam to a dose of 1015 ions/cm2. The changes in the 1H nuclear magnetic resonance (NMR) line shapes are discussed in view of the two distinct proton motions, which were identified from the line shape analysis, exhibiting contrasting behaviors after the proton-irradiation. Ó 2005 Elsevier B.V. All rights reserved. PACS: 64.70.Kb; 76.60.k; 77.90.+k; 61.80.Lj; 79.20.Rf Keywords: Proton-beam irradiation; Nuclear magnetic resonance; TlH2PO4
1. Introduction TlH2PO4 (TDP) is closely related to the KH2PO4 (KDP)-type crystals, which are interesting hydrogenbonded materials undergoing structural phase transitions accompanied by ferroelectricity or antiferroelectricity. In these crystals, it is known that protons in the double well potentials on the hydrogen bonds are involved in a phase transition accompanied by displacements in the heavy atom (K,P,O) structure. One outstanding phenomenon is the proton–deuteron ‘‘isotope effect’’ that raises the transition temperature by about 100 K and decreases the pressure dependence of the transition temperature. Traditionally, the proton–deuteron isotope effect has been explained by the tunneling model, in which the marked increase in the transition temperature was attributed to a change in the tunneling frequency by the mass change, and the decrease in the transition temperature with increasing pressure was attributed to an increase in the tunneling integral [1]. Recently, neutron Compton scattering experi*
Corresponding author. Tel.: +82 2 32903098; fax: +82 2 9273292. E-mail address: [email protected] (C.E. Lee).
1567-1739/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2005.06.009
ments have shown that the protons are self-trapped in one or the other of its equivalent positions, and jump from position to position through the phonon-assisted tunneling [2]. However, the tunneling model has mainly been concerned with the proton–deuteron mass difference and does not take into account the effect of the hydrogen bond geometry. In order to vary the hydrogen bond geometry, the isotope substitution [3,4] as well as the high pressure studies have been carried out [5,6]. It is worthwhile to note that the transition temperature did not shift by deuteron substitution when the hydrogen bond lengths were retained [5]. The geometrical aspects of the hydrogen bonds were considered in the proton-lattice coupling model [7,8], and redistribution of the atomic positions by the electric fieldinduced structural changes was suggested [9,10]. Yet, the irradiation methods may provide another way of modifying the hydrogen bond geometry. Most irradiation studies in the hydrogen-bonded ferroelectrics have been concentrated on the transient defects induced by ionizing-radiations such as X-ray and UV-ray (ultraviolet ray), where the defects are closely related to the optical properties [11–14]. On the other hand, light ion beam irradiation
S.H. Kim et al. / Current Applied Physics 7 (2007) 28–30
effects have rarely been studied [15–17]. Nature of the transient defects and their relaxation processes are fairly well studied, for cases of apparently very dilute defects, indicating close relation to the modification of the hydrogen bonds [11–13]. The hydrogen bond lengths were reported to change due to the defects, particularly the hydrogen vacancies [14]. While the pressure is a macroscopic tool, the irradiation method may be used as a microscopic tool to modify the hydrogen bond. Thus, the hydrogen bond length can be locally modified by the hydrogen ion irradiation while retaining the crystalline structure, and hydrogen bonds with various bond lengths may be realized in a sample. TDP undergoes two major phase transitions: an antiferroelectric phase transition at Tc = 230 K and a ferroelastic phase transition at T 0c ¼ 357 K [18–20]. The room temperature phase is known to be paraelectric and ferroelastic while the low-temperature phase is antiferroelectric. The high-temperature phase is known to be paraelectric and paraelastic. TDP has a monoclinic primitive cell at room ˚ , b = 4.518 A ˚ , c = 6.516 A ˚, temperature with a = 14.308 A and b = 91.76° [21,22]. TDP has three different crystallographic hydrogen bonds as determined by X-ray and neutron diffraction, and the crystal structure of TDP illustrating three inequivalent H sites can be found in the literature [18,22]. The two shorter bonds, 0.243 nm and 0.245 nm, respectively, are centrosymmetric and form zigzag chains along the c-axis. Hydrogens of these bonds are at special positions at a center of inversion and undergo an order–disorder phase transition through the phase transition temperature Tc. The longest bond, 0.25 nm, is asymmetric along the b-axis and the protons are at a general position both above and below Tc [18]. In this work, we have studied the hydrogen ion irradiated TDP in comparison to the virgin (unirradiated) TDP. In view of the hydrogen bonds, TDP has very short bond lengths R = 0.243 and 0.245 nm, whereas the transition temperature Tc = 230 K is very high, distinct from other KDP-type ferroelectrics. Proton-irradiation effect in this system was also expected in the structural changes and proton dynamics. One of our main interests was the effect of irradiation on the proton motion in the hydrogen bond network.
29
3. Results and discussion Fig. 1 shows the NMR line shapes in TDP at T = 253 K, above the antiferroelectric phase transition, before and after the irradiation. The line shapes were well fitted by a Gaussian line component and a Lorentzian one. The 1H NMR line shape and the linewidth are dictated by the proton–proton magnetic dipole interaction [23]. In Fig. 1, changes in the line components are apparent due to the proton-irradiation. The Gaussian line represents the ‘‘rigid lattice’’ protons, whereas the Lorentzian line represents the motionally narrowed mobile protons in the time scale probe by the NMR Larmor frequency [24]. The motional narrowing condition is expressed as xds 1, where xd is the the typical frequency of dipolar interaction and s is the correlation time of the motion. The presence of the two distinct line components has been reported in the KDP ferroelectrics and attributed to the O–H–O protons [25]. Fig. 2 shows temperature dependencies of the Gaussian linewidth in TDP, obtained from deconvolution of the line shape into the Gaussian and Lorentzian line components as indicated in Fig. 1, before and after the proton-beam irradiation. After proton- irradiation, an overall increase in the Gaussian linewidth is noticed, indicative of the ‘‘rigid protons’’ undergoing slower motions. In fact, our rotatingframe spin-lattice relaxation measurements gave the activation energies of 0.41 eV and 0.59 eV before and after the proton-irradiation, respectively, just below the ferroelastic phase transition. This indicates that the proton-irradiation modifies the microscopic environments of the rigid proton sites giving rise to their slowed motion, presumably by rendering the proton double well potentials more asymmetric [11,13,14].
Before
2. Experiment A polycrystalline TDP sample was irradiated with 1 MeV hydrogen ions to a dose of 1015 ions/cm2. A BRUKER MSL 200 spectrometer was used for the free induction decay (FID) of broadband 1H NMR (nuclear magnetic resonance) measurements at the Larmor frequency of x0/2p = 200 MHz in the temperature range of 180–360 K. The line shapes were obtained by Fouriertransforming the FID signals, and were fitted into separate line components. The line shape analysis was done by using the integration routines with least-squares method for minimization of the covariances.
After
-40
-20
0
20
40
Frequency (kHz) Fig. 1. Line shape of TDP at 253 K. The open squares correspond to the data points, and the dotted lines and the dashed lines correspond to the Gaussian and the Lorentzian line components, respectively.
30
S.H. Kim et al. / Current Applied Physics 7 (2007) 28–30
crease after the irradiation, indicative of enhanced motional narrowing.
Tc
Gaussian Linewidth (kHz)
18
Acknowledgements 16
14
12
Before After
10 180
210
240
270
300
330
360
Temperature (K) Fig. 2. Temperature dependence of the Gaussian linewidth in TDP before and after the proton-beam irradiation.
Fig. 3 shows the temperature dependence of the Lorentzian linewidth in TDP before and after the protonbeam irradiation. In contrast to the case of the Gaussian linewidth, the Lorentzian linewidth shows a decrease up to room temperature after the proton-beam irradiation. This decreased Lorentzian linewidth represents the shortened correlation times giving rise to a further motional narrowing of the mobile protons undergoing rapid hopping motions after the proton-irradiation [26]. In summary, we have studied the 1H lineshapes in polycrystalline TlH2PO4 before and after the proton-irradiation. The Gaussian line component showed an increase in the linewidth after the irradiation reflecting slowed rigid lattice proton motions. On the other hand, the Lorentzian linewidth representing the mobile protons showed a de-
Tc
3
Before
Lorentzian Linewidth (kHz)
After
2
1
0 210
240
270
300
330
360
Temperature (K) Fig. 3. Temperature dependence of the Lorentzian linewidth in TDP before and after the proton-beam irradiation.
This work was supported by the Korea Science and Engineering foundation (Proton Accelerator User Program No. M202AK010021-04A1101-02110 and RO1-2005-00010798-0) and by the Korea Research Foundation (Grant No. KRF-2004-005-C00060 and Brain Korea 21 Project in 2005). We thank Dr. H.-J. Woo at the KIGAM for the proton-beam irradiation. The measurements at the Korean Basic Science Institute (KBSI) are acknowledged. The authors also gratefully acknowledge Korea UniversityÕs support for the Korea UniversityÕs 100th Anniversary Symposium on the State of the Art and the Prospect of the Interdisciplinary Nano Sciences. References [1] R. Blinc, B. Zeks, Ferroelectrics 72 (1987) 193. [2] G.F. Reiter, J. Mayers, P. Platzman, Phys. Rev. Lett. 89 (2002) 1355051. [3] S. Tanaka, Phys. Rev. B 42 (1990) 10488. [4] S. Koval, J. Kohanoff, R.L. Migoni, E. Tosatti, Phys. Rev. Lett. 89 (2002) 187602. [5] M.I. McMahon, R.J. Nelmes, W.F. Kuhst, R. Dorwarth, R.O. Piltz, Z. Tun, Nature (London) 348 (1990) 317. [6] I.V. Stasyuk, R.R. Levitskii, A.P. Moina, Phys. Rev. B 59 (1999) 8530. [7] A. Bussmann-Holder, K.H. Michel, Phys. Rev. Lett. 80 (1998) 2173. [8] N. Dalal, A. Klymachyov, A. Bussmann-Holder, Phys. Rev. Lett. 81 (1998) 5924. [9] S.J. van Reeuwijk, A. Puig-Molina, H. Grassfsma, Phys. Rev. B 62 (2000) 6192. [10] S.J. van Reeuwijk, A. Puig-Molina, H. Grassfsma, Phys. Rev. B 64 (2001) 134105. [11] S.D. Setzler, K.T. Stevens, L.E. Halliburton, M. Yan, N.P. Zaitseva, J.J. DeYoreo, Phys. Rev. B 57 (1998) 2643. [12] N.Y. Garces, K.T. Stevens, L.E. Halliburton, S.G. Demos, H.B. Radousky, N.P. Zaitseva, J. Appl. Phys. 89 (2001) 47. [13] M.M. Chirila, N.Y. Garces, L.E. Halliburton, S.G. Demos, T.A. Land, H.B. Radousky, J. Appl. Phys. 94 (2003) 6456. [14] C.S. Liu, N. Kioussis, S.G. Demos, H.B. Radousky, Phys. Rev. Lett. 91 (2003) 015505. [15] T. Som, M.S. Navati, V.N. Kulkarni, Nucl. Instr. Meth. B 179 (2001) 551. [16] V.T. Kuanyshev, T.A. Belykh, I.N. Ogorodnikov, B.V. Shulgin, M.K. Satybaldieva, M.M. Kidibaev, Radiat. Meas. 33 (2001) 503. [17] S.O. Kucheyev, T.E. Felter, J. Appl. Phys. 95 (2004) 8475. [18] J. Seliger, V. Zagar, R. Blinc, V.H. Schmidt, J. Chem. Phys. 88 (1988) 3260. [19] K. Hanazawa, M. Komukae, T. Osaka, Y. Makita, M. Arai, T. Yagi, J. Phys. Soc. Jpn. 60 (1991) 188. [20] N. Yasuda, S. Fujimoto, T. Asano, Phys. Lett. A 76 (1980) 174. [21] Y. Oddon, A. Tranquard, G. Pe´pe, Acta Crystallogr., Sect. B 35 (1979) 542. [22] R.J. Nelmes, R.N.P. Choudhary, Solid State Commun. 38 (1981) 321. [23] E. Fukushima, S.B.W. Roeder, Experimental Pulse NMR, AddisonWesley, Reading, 1981. [24] K.W. Lee, D.K. Oh, J.K. Kang, C.H. Lee, J. Kim, J. Chem. Phys. 17 (2002) 8004. [25] M. Karayanni, G. Papavassiliou, M. Fardis, J. Dolinsˇek, Phys. Rev. B 42 (1990) 10488. [26] J.H. Kim, C.E. Lee, K.-S. Lee, Phys. Rev. B 53 (1996) 6104.
Proton-irradiation effect on the proton motion in TlH2PO4 S.H. Kim a, K.W. Lee a, Cheol Eui Lee a
a,*
, K.-S. Lee
b
Department of Physics and Institute for Nano Science, Korea University, 1 Anam-dong, Sungbuk-ku, Seoul 136 713, Republic of Korea b School of Computer-Aided Science, Inje University, Gimhae 621 749, Republic of Korea Received 20 May 2005; accepted 11 June 2005 Available online 14 October 2005
Abstract We have investigated the proton-beam irradiation effect on TlH2PO4 (TDP) undergoing an antiferroelectric phase transition and a ferroelastic one. The polycrystalline sample was irradiated by a 1 MeV hydrogen ion beam to a dose of 1015 ions/cm2. The changes in the 1H nuclear magnetic resonance (NMR) line shapes are discussed in view of the two distinct proton motions, which were identified from the line shape analysis, exhibiting contrasting behaviors after the proton-irradiation. Ó 2005 Elsevier B.V. All rights reserved. PACS: 64.70.Kb; 76.60.k; 77.90.+k; 61.80.Lj; 79.20.Rf Keywords: Proton-beam irradiation; Nuclear magnetic resonance; TlH2PO4
1. Introduction TlH2PO4 (TDP) is closely related to the KH2PO4 (KDP)-type crystals, which are interesting hydrogenbonded materials undergoing structural phase transitions accompanied by ferroelectricity or antiferroelectricity. In these crystals, it is known that protons in the double well potentials on the hydrogen bonds are involved in a phase transition accompanied by displacements in the heavy atom (K,P,O) structure. One outstanding phenomenon is the proton–deuteron ‘‘isotope effect’’ that raises the transition temperature by about 100 K and decreases the pressure dependence of the transition temperature. Traditionally, the proton–deuteron isotope effect has been explained by the tunneling model, in which the marked increase in the transition temperature was attributed to a change in the tunneling frequency by the mass change, and the decrease in the transition temperature with increasing pressure was attributed to an increase in the tunneling integral [1]. Recently, neutron Compton scattering experi*
Corresponding author. Tel.: +82 2 32903098; fax: +82 2 9273292. E-mail address: [email protected] (C.E. Lee).
1567-1739/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2005.06.009
ments have shown that the protons are self-trapped in one or the other of its equivalent positions, and jump from position to position through the phonon-assisted tunneling [2]. However, the tunneling model has mainly been concerned with the proton–deuteron mass difference and does not take into account the effect of the hydrogen bond geometry. In order to vary the hydrogen bond geometry, the isotope substitution [3,4] as well as the high pressure studies have been carried out [5,6]. It is worthwhile to note that the transition temperature did not shift by deuteron substitution when the hydrogen bond lengths were retained [5]. The geometrical aspects of the hydrogen bonds were considered in the proton-lattice coupling model [7,8], and redistribution of the atomic positions by the electric fieldinduced structural changes was suggested [9,10]. Yet, the irradiation methods may provide another way of modifying the hydrogen bond geometry. Most irradiation studies in the hydrogen-bonded ferroelectrics have been concentrated on the transient defects induced by ionizing-radiations such as X-ray and UV-ray (ultraviolet ray), where the defects are closely related to the optical properties [11–14]. On the other hand, light ion beam irradiation
S.H. Kim et al. / Current Applied Physics 7 (2007) 28–30
effects have rarely been studied [15–17]. Nature of the transient defects and their relaxation processes are fairly well studied, for cases of apparently very dilute defects, indicating close relation to the modification of the hydrogen bonds [11–13]. The hydrogen bond lengths were reported to change due to the defects, particularly the hydrogen vacancies [14]. While the pressure is a macroscopic tool, the irradiation method may be used as a microscopic tool to modify the hydrogen bond. Thus, the hydrogen bond length can be locally modified by the hydrogen ion irradiation while retaining the crystalline structure, and hydrogen bonds with various bond lengths may be realized in a sample. TDP undergoes two major phase transitions: an antiferroelectric phase transition at Tc = 230 K and a ferroelastic phase transition at T 0c ¼ 357 K [18–20]. The room temperature phase is known to be paraelectric and ferroelastic while the low-temperature phase is antiferroelectric. The high-temperature phase is known to be paraelectric and paraelastic. TDP has a monoclinic primitive cell at room ˚ , b = 4.518 A ˚ , c = 6.516 A ˚, temperature with a = 14.308 A and b = 91.76° [21,22]. TDP has three different crystallographic hydrogen bonds as determined by X-ray and neutron diffraction, and the crystal structure of TDP illustrating three inequivalent H sites can be found in the literature [18,22]. The two shorter bonds, 0.243 nm and 0.245 nm, respectively, are centrosymmetric and form zigzag chains along the c-axis. Hydrogens of these bonds are at special positions at a center of inversion and undergo an order–disorder phase transition through the phase transition temperature Tc. The longest bond, 0.25 nm, is asymmetric along the b-axis and the protons are at a general position both above and below Tc [18]. In this work, we have studied the hydrogen ion irradiated TDP in comparison to the virgin (unirradiated) TDP. In view of the hydrogen bonds, TDP has very short bond lengths R = 0.243 and 0.245 nm, whereas the transition temperature Tc = 230 K is very high, distinct from other KDP-type ferroelectrics. Proton-irradiation effect in this system was also expected in the structural changes and proton dynamics. One of our main interests was the effect of irradiation on the proton motion in the hydrogen bond network.
29
3. Results and discussion Fig. 1 shows the NMR line shapes in TDP at T = 253 K, above the antiferroelectric phase transition, before and after the irradiation. The line shapes were well fitted by a Gaussian line component and a Lorentzian one. The 1H NMR line shape and the linewidth are dictated by the proton–proton magnetic dipole interaction [23]. In Fig. 1, changes in the line components are apparent due to the proton-irradiation. The Gaussian line represents the ‘‘rigid lattice’’ protons, whereas the Lorentzian line represents the motionally narrowed mobile protons in the time scale probe by the NMR Larmor frequency [24]. The motional narrowing condition is expressed as xds 1, where xd is the the typical frequency of dipolar interaction and s is the correlation time of the motion. The presence of the two distinct line components has been reported in the KDP ferroelectrics and attributed to the O–H–O protons [25]. Fig. 2 shows temperature dependencies of the Gaussian linewidth in TDP, obtained from deconvolution of the line shape into the Gaussian and Lorentzian line components as indicated in Fig. 1, before and after the proton-beam irradiation. After proton- irradiation, an overall increase in the Gaussian linewidth is noticed, indicative of the ‘‘rigid protons’’ undergoing slower motions. In fact, our rotatingframe spin-lattice relaxation measurements gave the activation energies of 0.41 eV and 0.59 eV before and after the proton-irradiation, respectively, just below the ferroelastic phase transition. This indicates that the proton-irradiation modifies the microscopic environments of the rigid proton sites giving rise to their slowed motion, presumably by rendering the proton double well potentials more asymmetric [11,13,14].
Before
2. Experiment A polycrystalline TDP sample was irradiated with 1 MeV hydrogen ions to a dose of 1015 ions/cm2. A BRUKER MSL 200 spectrometer was used for the free induction decay (FID) of broadband 1H NMR (nuclear magnetic resonance) measurements at the Larmor frequency of x0/2p = 200 MHz in the temperature range of 180–360 K. The line shapes were obtained by Fouriertransforming the FID signals, and were fitted into separate line components. The line shape analysis was done by using the integration routines with least-squares method for minimization of the covariances.
After
-40
-20
0
20
40
Frequency (kHz) Fig. 1. Line shape of TDP at 253 K. The open squares correspond to the data points, and the dotted lines and the dashed lines correspond to the Gaussian and the Lorentzian line components, respectively.
30
S.H. Kim et al. / Current Applied Physics 7 (2007) 28–30
crease after the irradiation, indicative of enhanced motional narrowing.
Tc
Gaussian Linewidth (kHz)
18
Acknowledgements 16
14
12
Before After
10 180
210
240
270
300
330
360
Temperature (K) Fig. 2. Temperature dependence of the Gaussian linewidth in TDP before and after the proton-beam irradiation.
Fig. 3 shows the temperature dependence of the Lorentzian linewidth in TDP before and after the protonbeam irradiation. In contrast to the case of the Gaussian linewidth, the Lorentzian linewidth shows a decrease up to room temperature after the proton-beam irradiation. This decreased Lorentzian linewidth represents the shortened correlation times giving rise to a further motional narrowing of the mobile protons undergoing rapid hopping motions after the proton-irradiation [26]. In summary, we have studied the 1H lineshapes in polycrystalline TlH2PO4 before and after the proton-irradiation. The Gaussian line component showed an increase in the linewidth after the irradiation reflecting slowed rigid lattice proton motions. On the other hand, the Lorentzian linewidth representing the mobile protons showed a de-
Tc
3
Before
Lorentzian Linewidth (kHz)
After
2
1
0 210
240
270
300
330
360
Temperature (K) Fig. 3. Temperature dependence of the Lorentzian linewidth in TDP before and after the proton-beam irradiation.
This work was supported by the Korea Science and Engineering foundation (Proton Accelerator User Program No. M202AK010021-04A1101-02110 and RO1-2005-00010798-0) and by the Korea Research Foundation (Grant No. KRF-2004-005-C00060 and Brain Korea 21 Project in 2005). We thank Dr. H.-J. Woo at the KIGAM for the proton-beam irradiation. The measurements at the Korean Basic Science Institute (KBSI) are acknowledged. The authors also gratefully acknowledge Korea UniversityÕs support for the Korea UniversityÕs 100th Anniversary Symposium on the State of the Art and the Prospect of the Interdisciplinary Nano Sciences. References [1] R. Blinc, B. Zeks, Ferroelectrics 72 (1987) 193. [2] G.F. Reiter, J. Mayers, P. Platzman, Phys. Rev. Lett. 89 (2002) 1355051. [3] S. Tanaka, Phys. Rev. B 42 (1990) 10488. [4] S. Koval, J. Kohanoff, R.L. Migoni, E. Tosatti, Phys. Rev. Lett. 89 (2002) 187602. [5] M.I. McMahon, R.J. Nelmes, W.F. Kuhst, R. Dorwarth, R.O. Piltz, Z. Tun, Nature (London) 348 (1990) 317. [6] I.V. Stasyuk, R.R. Levitskii, A.P. Moina, Phys. Rev. B 59 (1999) 8530. [7] A. Bussmann-Holder, K.H. Michel, Phys. Rev. Lett. 80 (1998) 2173. [8] N. Dalal, A. Klymachyov, A. Bussmann-Holder, Phys. Rev. Lett. 81 (1998) 5924. [9] S.J. van Reeuwijk, A. Puig-Molina, H. Grassfsma, Phys. Rev. B 62 (2000) 6192. [10] S.J. van Reeuwijk, A. Puig-Molina, H. Grassfsma, Phys. Rev. B 64 (2001) 134105. [11] S.D. Setzler, K.T. Stevens, L.E. Halliburton, M. Yan, N.P. Zaitseva, J.J. DeYoreo, Phys. Rev. B 57 (1998) 2643. [12] N.Y. Garces, K.T. Stevens, L.E. Halliburton, S.G. Demos, H.B. Radousky, N.P. Zaitseva, J. Appl. Phys. 89 (2001) 47. [13] M.M. Chirila, N.Y. Garces, L.E. Halliburton, S.G. Demos, T.A. Land, H.B. Radousky, J. Appl. Phys. 94 (2003) 6456. [14] C.S. Liu, N. Kioussis, S.G. Demos, H.B. Radousky, Phys. Rev. Lett. 91 (2003) 015505. [15] T. Som, M.S. Navati, V.N. Kulkarni, Nucl. Instr. Meth. B 179 (2001) 551. [16] V.T. Kuanyshev, T.A. Belykh, I.N. Ogorodnikov, B.V. Shulgin, M.K. Satybaldieva, M.M. Kidibaev, Radiat. Meas. 33 (2001) 503. [17] S.O. Kucheyev, T.E. Felter, J. Appl. Phys. 95 (2004) 8475. [18] J. Seliger, V. Zagar, R. Blinc, V.H. Schmidt, J. Chem. Phys. 88 (1988) 3260. [19] K. Hanazawa, M. Komukae, T. Osaka, Y. Makita, M. Arai, T. Yagi, J. Phys. Soc. Jpn. 60 (1991) 188. [20] N. Yasuda, S. Fujimoto, T. Asano, Phys. Lett. A 76 (1980) 174. [21] Y. Oddon, A. Tranquard, G. Pe´pe, Acta Crystallogr., Sect. B 35 (1979) 542. [22] R.J. Nelmes, R.N.P. Choudhary, Solid State Commun. 38 (1981) 321. [23] E. Fukushima, S.B.W. Roeder, Experimental Pulse NMR, AddisonWesley, Reading, 1981. [24] K.W. Lee, D.K. Oh, J.K. Kang, C.H. Lee, J. Kim, J. Chem. Phys. 17 (2002) 8004. [25] M. Karayanni, G. Papavassiliou, M. Fardis, J. Dolinsˇek, Phys. Rev. B 42 (1990) 10488. [26] J.H. Kim, C.E. Lee, K.-S. Lee, Phys. Rev. B 53 (1996) 6104.