Synthesis, crystal growth and characterization of a new organic NLO material: Caffeinium picrate (CAFP)—A charge transfer molecular complex salt

ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 5409–5415

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Synthesis, crystal growth and characterization of a new organic NLO material: Caffeinium picrate (CAFP)—A charge transfer molecular complex salt A. Chandramohan a, R. Bharathikannan b, J. Chandrasekaran b, P. Maadeswaran b, R. Renganathan c, V. Kandavelu c, a

Department of Chemistry, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641 020, Tamil Nadu, India Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641 020, Tamil Nadu, India c School of Chemistry, Bharathidasan University, Palakalai Perur, Tiruchirappalli 620 024, Tamil Nadu, India b

a r t i c l e in fo

abstract

Article history: Received 11 February 2008 Received in revised form 7 August 2008 Accepted 3 September 2008 Communicated by M. Roth Available online 17 September 2008

Good-optical-quality single crystals of caffeinium picrate (CAFP), a new organic charge transfer molecular complex salt, were successfully grown by the slow evaporation solution growth technique at room temperature. Formation of the new crystal has been confirmed by single-crystal X-ray diffraction (XRD) and NMR spectroscopic techniques. CAFP crystallizes under monoclinic system with cell parameters a ¼ 8.2996(10) A˚, b ¼ 9.0014(11) A˚ and c ¼ 23.0210(3) A˚. The suitability of this material for optical application was studied by optical transmission spectroscopy. Fourier transform infrared (FT-IR) spectral analysis was used to confirm the presence of various functional groups in the grown crystals. The thermal stability of the crystal was investigated using thermogravimetric and differential thermal analyses studies. The nonlinear optical (NLO) property of the crystal was confirmed by the Kurtz–Perry powder second harmonic generation (SHG) test. & 2008 Elsevier B.V. All rights reserved.

PACS: 34.70.+e 42.70.Mp 61.10.Nz 65.40.b 77.84.Jd 81.10.Dn Keywords: A1. Charge transfer A1. Thermal A2. Solution growth B1. Caffeinium picrate B2. Nonlinear optic (NLO)

1. Introduction The field of crystal growth has been enriched by organic molecular crystals due to their high optical nonlinearities and quick response in the electro-optic effect compared to the inorganic analogues [1–8]. Molecular flexibility of organic materials is an added advantage to enhance the nonlinear optical (NLO) properties in a desired manner [9]. In addition, they have large structural diversity. By adopting molecular engineering and chemical synthesis one can easily refine the optical properties of organic molecules [10]. Various methodologies akin to the formation of salts and metal complexes, introduction of steric effects

 Corresponding author. Tel.: +91 431 2407053; fax: +91 431 2407045.

E-mail address: [email protected] (V. Kandavelu). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.09.020

and hydrogen bonding interactions and etc., have successfully tapped the best results in synthesizing such crystals and channelizing their properties to useful ends [11]. However, the ease of growth, simplicity of the technique, the less cumbersome procedures involved coupled with the economic friendly facet have won acclaims for the formation of salts over the methodologies mentioned above. The various organic sub-networks induce noncentrosymmetry in the bulk and enhance the thermal and mechanical stabilities through hydrogen bonding interactions [12,13]. Picric acid forms crystalline picrate salts with various organic molecules by virtue of its Lewis acid behavior and serves a better acidic ligand in the formation of salts through specific electrostatic or hydrogen bonding interactions [14]. The strength and nature of the electron donor–acceptor type bonding in the picric acid complexes are dictated by the nature of the partners involved in the bond-formation process. The linkage encompasses

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O H3C O

O

NO2

CH3

H3C

N

N

+ N CH3

N

O 2N

O

NO2 OH

NO2

CH3 N

N N

N

CH3

H

O2 N

NO2

O

Scheme 1.

electrostatic interactions as well as the formation of the molecular complexes [15]. Second harmonic generation (SHG) requires materials with a noncentrosymmetric structure that gives rise to a large secondorder NLO susceptibility. For molecular materials, it is widely assumed that the molecular structure as well as the packing arrangement must also be noncentrosymmetric, unless a magnetic dipole [16–18] or an electric quadruple [19] contributes to the bulk susceptibility. Consequently, most studies have focused on dipolar molecular systems in which the optical nonlinearities arise from intramolecular charge transfer [20–23]. But the criteria for SHG may also be satisfied by centrosymmetric molecules if they aggregate in a noncentrosymmetric manner and there is a contribution to the bulk susceptibility from intermolecular charge transfer. Based on the above discussions, the title material was synthesized and characterized through elemental analysis, powder X-ray diffraction (XRD), UV–vis, and NMR techniques. Singlecrystal XRD data reveal that the title material crystallizes under centrosymmetric space group P21/n. The centrosymmetric feature of the material does not hamper its SHG prospects as a bolster for the same is obtained from the noncentrosymmetric aggregation of the molecules augmented duly by the intermolecular charge transfer contributing to the bulk susceptibility [24]. The existence of such charge transfer interaction in the case of organic molecular charge transfer complexes has been confirmed by the spectroscopic techniques [25]. The molecular aggregation through intermolecular hydrogen bonding has been confirmed by the single-crystal XRD studies [26].

2. Experimental procedure

Fig. 1. Photograph of an as-grown crystal of CAFP.

Table 1 Elemental analysis of CAFP Element

C N H

Composition (%) Experimental

Theoretical

39.41 23.40 3.12

39.41 23.16 3.03

2.1. Material synthesis Analar grade caffeine and picric acid were dissolved in pure chloroform separately in 1:1 molar ratio and the two solutions were mixed together. The mixture was stirred well for about 15 min, when a yellow crystalline precipitate of the charge transfer complex salt caffeinium picrate (CAFP) was obtained. It was filtered off and repeatedly recrystallized using activated animal charcoal to improve the purity of the material since highquality crystals are warranted for evaluating the NLO properties of molecular materials [27]. The reaction involved is illustrated in Scheme 1. 2.2. Growth of single crystal A saturated solution of CAFP in the solvent mixture of pure chloroform and methanol (1:1) was prepared, stirred well for about 30 min and heated slightly to dissolve the undissolved particles. Suspended impurities were removed by using Whatman 41 grade filter paper. The clear filtrate so obtained was kept aside

unperturbed in a dust-free room for the growth of single crystals. Well-defined, bright-yellow crystals of average dimension 8  6  4 mm3 were collected at the end of the 10th day. The photograph of an as-grown crystal of CAFP is shown in Fig. 1.

3. Results and discussion The contribution of elements such as C, N and H in the resulting compound was analyzed by the elemental analysis using Perkin-Elmer 2400 CHN analyzer. The data (Table 1) show good agreement between the experimentally determined and theoretically calculated values. The result indicates that CAFP is free from impurities and devoid of water molecules. Further, it confirms the stoichiometry and hence the molecular formula of the material.

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5411

220

140

100

10

20

30

– (2 1 11)

20

(1 2 5) (1 3 -1)

(0 1 2)

60

– (1 1 4) – (1 2 1)

(0 1 3)

Lin (Counts)

(2 0 4)

180

40 2θ (degree)

50

60

70

80

Fig. 2. Powder XRD pattern of CAFP.

Table 2 Powder XRD data of CAFP 2y

(h k l)

d Values experimental (A˚)

d Values simulated (A˚)

12.390 15.106 20.786 22.767 26.679 29.794 31.898 49.007

(0 1 2) (0 1 3) (11 4¯) (1 2 1¯) (2 0 4) (1 2 5) (1 3 1¯) (2 11¯ 1)

7.138 5.860 4.269 3.902 3.338 2.996 2.803 1.857

7.112 5.866 4.244 3.908 3.334 2.992 2.804 1.859

The powder XRD pattern obtained by using Shimadzu XRD 6000 diffractometer with CuKa radiation (l ¼ 1.5406 nm) for CAFP is shown in Fig. 2, the simulated hkl and d values along with experimental d values are given in Table 2. The single-crystal XRD structure has already been reported by our group [26] and the molecular structure of CAFP is depicted in Fig. 3. This study reveals that CAFP belongs to monoclinic crystal system with space group P21/n. The lattice parameters are a ¼ 8.2996(10) A˚, b ¼ 9.0014(11) A˚, c ¼ 23.0210(3) A˚ and b ¼ 91.474(2)1. The volume of the system is 1733.4(4) A˚3. The electronic absorption spectrum of CAFP was recorded using Shimadzu 1601 UV–vis spectrophotometer in the range 280–500 nm and is shown in Fig. 4. The spectrum exhibits a strong absorption band at 335.5 nm, which is attributed to pp* transition of picrate ion and no different bands due to an assumed charge transfer transition (the promotion of an electron from the highest occupied molecular orbital (HOMO) of the donor to the vacant molecular orbital (LUMO) of the acceptor) appear in the range 460–800 nm. To explain such a conclusion it has been thought that these labile complexes which are chromatographically decomposed could have disassociated from the reaction with pure hydroxylic solvent [28]. The absence of absorption above 400 nm is an advantage, as it is the prime requirement for materials having NLO properties. As a result it can be used as an SHG material around the visible range. The optical transmission spectrum of CAFP was recorded using VARIAN CARRY 5E-UV–vis–NIR spectrometer in the wavelength

range 190–2500 nm. The spectrum is shown in Fig. 5. The attained percentage of transmission is 82% in the visible region. The absence of any transmission before 400 nm is due to the absorbance leading to pp* transition. At longer wavelength side the crystal is transparent up to 2000 nm. This suggests the suitability of the material for the optical applications above 400 nm. The functional groups of CAFP are confirmed by recording the Fourier transform infrared (FT-IR) spectrum in the range of 4000–400 cm1 using Perkin-Elmer FT-IR spectrometer using the KBr pellet technique. The FT-IR spectrum of CAFP is shown in Fig. 6 and the assignment of the well-defined bands in the infrared spectrum is given in Table 3. The formation of the charge transfer complex during the reaction between caffeine and picric acid is strongly evidenced by the presence of the main characteristic infrared bands of the donor and acceptor in the spectrum of the product. However, the bands of the donor are slightly shifted to lower frequency and that of the acceptor are slightly shifted to higher frequency. This shift owes to the changes in the electronic structure upon the formation of charge transfer complex. In general for acid–base reaction, a proton transfer from the acceptor (acid) to the donor is expected to take place. This activity is observed in the case of caffeine interacting with picric acid as well. Such an assumption is strongly supported by the appearance of a new band of medium intensity in the spectrum of CAFP. The band is observed at 3160.25 cm1 and is due to the v (+N–H) stretching vibration of hydrogen against positively charged nitrogen [25]. The absorptions at 3071.16 and 2960.37 cm1 confirm the aromatic C–H stretching vibrations and C–H symmetric stretching vibrations of methyl group, respectively. The band due to C–H asymmetric vibration of methyl group overlaps with aromatic C–H stretching vibration. The CQO stretching vibration has been observed at 1716.52 cm1. Like other aromatic nitro compounds, CAFP also exhibits two sharp absorption bands corresponding to the asymmetric and symmetric stretching vibrations of the NO2 group at 1545 and 1322.42 cm1, respectively. Normally, the asymmetric stretching vibration, vasy (NO2) is sensitive to polar influences and electronic states of the nucleus. Therefore, it has been realized that the shift to lower frequency of vasy (NO2) vibration (1545 cm1) in the spectrum of the complex compound compared with its value in free picric acid

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08A C6 08B O9B

N7

C5 07

C13

N1

O9A N4

C14 C12

01

C1

O3

N6

C2 C4

C9

C11 O6

N2 C10 N3

C3

C7

C8

O4A N5

O5B

O2

O4B O5A

Fig. 3. Single-crystal molecular structure of CAFP.

90

3.318 2.435

75 335.5

60 T (%)

Absorbance (AU)

4.200

1.553 0.671 -0.212 280

45 30

324

368 412 Wavelength (nm)

456

500

Fig. 4. Electronic absorption spectrum of CAFP.

15 0 0

500

1000 1500 Wavelength (nm)

2000

2500

Fig. 5. Optical transmission spectrum of CAFP.

(1607 cm1) is due to the increased electron density on the picrate moiety as a result of charge transfer interaction in the complex. The absorptions at 1671, 1566 and 1497 cm1 are due to the CQC stretching vibrations of the aromatic ring. The asymmetric and symmetric deformation modes of the CH3 group bring forth bands at 1434 and 1363.25 cm1, respectively. The C–O stretching vibration is observed at 1273 cm1. The assignment of the various absorption frequencies for the crystal (CAFP) is found to be in good agreement with those of similar compounds [25]. The 1H NMR spectrum was recorded using AMX 400 spectrometer in DMSO with tetramethyl silane as the internal reference. The 1H NMR spectrum (d in ppm) of CAFP is depicted in Fig. 7. The spectrum exhibits six singlet signals indicating six different proton environments. The intense singlet appearing at 8.585 ppm is assigned to proton attached to tertiary nitrogen at position 9 in xanthenium moiety. The singlet at 7.971 ppm is due to the two protons of the same kind in picrate moiety whereas in free picric acid the same was observed at 8.57 ppm. This up-field shift in frequency has been attributed to the increased electron density as a result of charge transfer from donor (caffeine) to acceptor (picric acid) in the complex. The singlet signal at 7.469 ppm is due to the proton at C8 carbon of xanthenium moiety. The singlet at 3.838 ppm is due to the protons of CH3

group attached to the 1-nitrogen flanked by two electronwithdrawing carbonyl groups (deshielding effect). The singlet at 3.358 ppm has been assigned to the protons of the methyl group on 3-nitrogen atom in xanthenium moiety. The protons of the methyl group on 7-nitrogen in the same moiety appear as singlet at 3.165 ppm. The 13C NMR spectrum of CAFP is depicted in Fig. 8. The appearance of 12 distinct peaks in the spectrum establishes the molecular structure of the title complex species with greater degree of certainty. The weak signal at 160.24 ppm owes to the ipso carbon of picrate moiety. The C2 and C6 carbon atoms of the same kind in picrate moiety appear at 141.67 ppm. The highly intense peak at 125.14 ppm is due to C3 and C5 carbon atoms of the same kind in picrate moiety. The weak signal at 124.17 ppm is assigned to C4 carbon atom of the same moiety in the complex. The peaks appearing at 154.40, 150.91, 147.82 and 142.55 ppm have been assigned, respectively, to C2, C6, C5 and C8 carbon atoms in xanthenium moiety in the complex, taking into account the deshielding effect. The three methyl carbon atoms on 1-, 3-, 7nitrogen atoms in xanthenium moiety appear, respectively, at 33.05, 29.29 and 27.36 ppm in the upfield [29].

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5413

100

% Transmittance

80

60

40

20

0 4000

3500

3000

2500

1500

2000

1000

500

Wavenumber (cm−1)

3.358 3.165 3.838

6

4 (δ) ppm

2

0

0.80 0.81

8

0.82

2750 1716.52 1608 1545 1322.42 1434 1363.25 1671, 1566, 1497 1165 1273 1029 912 839.6 794 745 526 484

7.971

+ N–H stretching C–H asymmetric stretching (aromatic) C–H asymmetric stretching of methyl group/aromatic symmetric C–H stretching Symmetric C–H stretching vibration of methyl group CQO stretching + N–H deformation mode NO2 asymmetric stretching NO2 symmetric stretching CH3 asymmetric bending CH3 symmetric bending CQC stretching (aromatic) C–H bending (in plane) C–O stretching C–H bending (in plane) C–NO2 stretching NO2 scissoring C–H bending (out of plane) NO2 wagging NO2 scissoring CQO bending (out of plane)

7.469

3160.25 3071.16 2960.37

1.52

Assignment

0.26

Observed frequency (in cm1)

1.00

Table 3 FT-IR spectral assignments of CAFP

8.585

Fig. 6. FT-IR spectrum of CAFP.

Fig. 7. 1H NMR spectrum of CAFP.

The thermal stability of CAFP was identified by thermo gravimetric (TG) and differential thermal analyses (DTA) using Perkin-Elmer thermal analyzer. The sample was analyzed between the temperatures 39 and 1056 1C at a heating rate of 10 1C/min in nitrogen atmosphere. The CAFP sample weighing 8.790 mg was taken for the analysis, and the thermogram is shown in Fig. 9. The DTA reveals exactly the same changes shown by the TGA. The sharp endothermic peak in DTA at 142.97 1C indicates the melting point of the material. The sharpness of this endothermic peak shows good degree of crystallinity and purity of the material [30].

From the TG curve it is inferred that the decomposition of the title compound takes place in two stages. The first-stage decomposition commences at 190 1C and ends at 277.33 1C with the loss of 68% of material into gaseous products. The major weight loss noticed at low temperatures occurs in the region from 190 to 277.33 1C. The second-stage weight loss takes place gradually between 310 and 400 1C with the removal of about 22% of the material as gaseous products. The residue left out at the end is about 10% by weight. This may be due to the residual carbon mass at the end of the decomposition reactions.

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The SHG property of CAFP crystals was examined through the modified Kurtz and Perry powder technique [31]. The crystal was ground into a powder of particle size 100–150 mm and packed densely between two transparent glass slides. An Nd:YAG laser was used as the light source. A fundamental laser beam of 1064 nm wavelength, 8 ns pulse width with 10 Hz pulse rate and 2.75 mJ energy per pulse was made to fall normally on the sample cell. The power of the incident beam was measured using a power meter. The transmitted fundamental wave was passed over a monochromator, which separates 532 nm (SHG) signal from 1064 nm and was absorbed by CuSO4 solution that removes the 1064 nm radiation. A BG-38 filter kept in the path also removes the residual 1064 nm radiation. The green radiation was detected by a photomultiplier tube (Hamamatsu R 2059, a visible PMT) and displayed on a storage oscilloscope. This confirms the NLO property of the title material.

4. Conclusions The organic molecular charge transfer complex salt CAFP was synthesized and single crystals of the title compound were grown by the slow evaporation solution growth technique at ambient temperature. Elemental analysis data confirm the crystallinity and purity of the material. The stoichiometry and hence the molecular formula of CAFP stands verified. From the powder XRD pattern the various planes of reflections have been identified. The UV–vis spectrum exhibits the pp* band of picrate ion in the complex salt. FT-IR, NMR spectral techniques establish the molecular structure of CAFP and also bring forth the evidence for the prevalent charge transfer activity in the complex salt. The decomposition temperature and percentage weight loss of the material were obtained from TG/DTA analyses. The degree of crystallinity and purity of the material from a sharp endothermic peak in DTA was also confirmed. The NLO activity in the complex was confirmed by employing Nd:YAG laser as source.

References

33.05 29.29 27.36

106.50

124.83

The authors thank Prof. P.K. Das (Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore) for the measurement of powder SHG efficiency. One of the authors (A. Chandramohan) thanks the University Grants Commission (UGC) for the financial assistance provided under faculty development programme.

140

100

60

[1] R.E. Newnham, Structure–Property Relations, Springer, New York, 1975. [2] J. Zyss, Molecular Nonlinear Optics: Materials, Physics and Devices, Academic Press, New York, 1993. [3] F. Agullo´-Lo´pez, J.M. Cabrera, F. Agullo´-Rueda, Electrooptics: Phenomena, Materials and Applications, Academic Press, New York, 1994. [4] V.A. Russel, C.C. Evans, W. Li, M.D. Ward, Science 276 (1997) 575. [5] R. Hierle, J. Badamn, J. Zyss, J. Crystal Growth 69 (1984) 545. [6] C.A. Discoll, H.J. Hoffmann, R.E. Stone, P.E. Perkins, J. Opt. Soc. Am. B 31 (1986) 683. [7] J. Zyss, J. Phys. D: Appl. Phys. 26 (1993) 8198. [8] G.R. Desiraju, Crystal Engineering: The Design of Organic Solids, Elsevier, New York, 1989.

20

(δ) ppm Fig. 8.

13

C NMR spectrum of CAFP.

71.5

9.1 8

50

30

10

Peak=277.33°C

7 Weight (%) / mg

Heat flow endo down (mW) – –

180

154.40 150.91 147.82 142.55 141.67

160.24

125.10

Acknowledgements

6 Area=6152.999 mg Deta H=-699.9772 J/g

5 4 3

-10

2

Area=982.189 mJ Delta = III.7358 J/g

1

Peak=142.97°C

40

200

400

600 Temperature (°C)

Fig. 9. TG/DTA thermogram of CAFP.

800

1000

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[9] A. Datta, S.K. Pati, J. Chem. Phys. 118 (2003) 8420. [10] G. DeMatos, V. Venkataraman, E. Nogueria, M. Belsley, P.A. Criado, M.J. Dianez, E. Perez Garrido, Synth. Met. 115 (2000) 225. [11] D.S. Chemla, J. Zyss (Eds.), Nonlinear Optical Properties of Organic Molecules and Crystals, vols. 1 and 2, Academic Press, New York, 1987. [12] A. Criado, M.J. Dianez, S.P.L. Garrido, I.M. Fernandes, M.E. Belsley, M. de Gomes, Acta Crystallogr. C 56 (2000) 888. [13] J. Zaccaro, J.P. Salvestrini, A. Ibanez, P. Ney, M.D.J. Fontana, Opt. Soc. Am. 17B (2000) 427. [14] Y. In, H. Nagata, M. Doi, T. Ishida, A. Wakahara, Acta Crystallogr. C 53 (1997) 367. [15] P. Zaderenko, M.S. Gel, P. Lopez, P. Ballesteros, I. Fonseco, A. Albert, Acta Crystallogr. B 53 (1997) 961. [16] B. Koopmans, A.-M. Janner, H.T. Jonkman, G.A. Sawatzky, F. Van der Woude, Phys. Rev. Lett. 71 (1993) 3569. [17] B. Koopmans, A. Anema, H.T. Jonkman, G.A. Sawatzky, F. Van der Woude, Phys. Rev. B 48 (1993) 2759. [18] Z. Shuai, J.-L. Bre´das, Adv. Mater. 6 (1994) 486. [19] P. Guyot-Sionnest, Y.R. Shen, Phys. Rev. B 38 (1988) 7985. [20] J.F. Nicoud, R.J. Twieg, in: D.S. Chemla, J. Zyss (Eds.), Nonlinear Optical Properties of Organic Molecules and Crystals, Academic Press, Orlando, 1987.

5415

[21] P.N. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects in Molecules and Polymers, Wiley, New York, 1991. [22] S. Allen, in: G.J. Ashwell (Ed.), Molecular Electronics, vol. 207, Research Studies, Taunton, 1992. [23] G.J. Ashwell, D. Bloor (Eds.), Organic Materials for Nonlinear Optics III, Royal Society of Chemistry Press, Cambridge, 1993. [24] G.J. Ashwell, G. Jefferies, D.G. Hamilton, D.E. Lynch, M.P.S. Roberts, G.S. Bahra, C.R. Brown, Nature 375 (1995) 385. [25] A. Chandramohan, R. Bharathikannan, M.A. Kandhaswamy, J. Chandrasekaren, R. Renganathan, V. Kandavelu, Cryst. Res. Technol. 43 (2008) 173. [26] A. Chandramohan, D. Gayathri, D. Velmurugan, K. Ravikumar, M.A. Kandhaswamy, Acta Crystallogr. E 63 (2007) o2495. [27] I.V. Kityk, B. Marciniak, A. Mefleh, J. Phys. D: Appl. Phys. 34 (2001) 1. [28] O.L. Tombesi, M.A. Frontera, M.A. Tomas, M.A. Badajoz, Appl. Spectrosc. 46 (1992) 873. [29] R.M. Silverstein, G.C. Bassler, T.C. Morrill, Spectrometric Identification of Organic Compounds, fifth ed., Wiley, New York, 1991. [30] A.S.H. Hameed, G. Ravi, R. Dhanasekaran, P. Ramasamy, J. Crystal Growth 212 (2000) 227. [31] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798.