Optical and dielectric properties of Sr3NbGa3Si2O14 crystals

Journal of Alloys and Compounds 425 (2006) 264–267

Optical and dielectric properties of Sr3NbGa3Si2O14 crystals Z.M. Wang a,∗ , D.R. Yuan b , Y.Sh. Yin a , X. Wang a a

Institute of Material Science and Engineering, Ocean University of China, Qingdao 266003, PR China b State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China Received 23 December 2005; received in revised form 2 January 2006; accepted 3 January 2006 Available online 21 February 2006

Abstract Refractive indices of Sr3 NbGa3 Si2 O14 (SNGS) crystal were determined in the wavelength region between 0.365 and 0.7065 ␮m. While phasematched optical second harmonic generation is impossible for SNGS crystal. Temperature-dependent examination of the dielectric properties and dielectric loss of Y-cut crystal wafer were studied in the temperature range from 298.15 to 873.15 K. The frequency dependence on the dielectric loss in the frequency range from 10 Hz to 13 MHz was also measured. © 2006 Elsevier B.V. All rights reserved. Keywords: Inorganic crystal materials; Refractive indices; Dielectric response; Dielectric properties; Dielectric loss

1. Introduction Piezoelectric crystals with the Ca3 Ga2 Ge4 O14 structure reported by Sil’vestrova et al. [1,2] attracted great interest, and their applications in mobile communication were reported by Sato et al. [3] and Fedoretz et al. [4]. The growth piezoelectric crystals by the Czochralski technique has yielded langasitetype crystals with high thermal stability of frequency and large electromechanically coupling factor. La3 Ga5 SiO14 (LGS, langasite), La3 Nb0.5 Ga5.5 SiO14 (LNG) and La3 Ta0.5 Ga5.5 O14 (LTG) are the current leading candidates for the requirements because of their unique acoustic characteristics. There are some reports concerning the crystal growth and the precise piezoelectric properties of LGS crystals reported by Detaint et al. [5], Fukuda et al. [6] and Shimamura et al. [7], LNG reported by Sil’vestrova et al. [2], Pisarevskii et al. [8] and Takeda et al. [9] and LTG reported by Mill et al. [10], Takeda et al. [11] and Kavanaka et al. [12]. But most of the known langasite structure compounds are disordered. Disorder causes higher acoustic losses and also non-uniform mechanical properties. In order to seek for new ordered langasite structure compounds, we had synthesized and grown Sr3 NbGa3 Si2 O14 (SNGS) single crystals which had all the cations totally ordered. SNGS crystals had the advantages of low Ga2 O3 content and good properties, such as high acous∗

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tic velocity and low acoustic loss. Chou et al. [13], Mill’ et al. [14,16], Jung et al. [15,17–19], Heimann et al. [20] and Wang et al. [21] reported the growth and some physical properties of SNGS crystals, respectively. However, most researchers’ interest focused on the piezoelectric properties of SNGS crystals, no details of the refractive indices and dielectric properties of SNGS crystal have been published until now in our knowledge. In this paper, we will present results of measurement of refractive indices and dielectric properties of SNGS crystal. 2. Experiment 2.1. Measurement of refractive indices The growth process of SNGS crystal was similar to that of La3 Ga5 SiO14 crystal and Wang et al. [21] had reported crystal growth process in detail. The as-grown crystals had good optical quality, and there were no dispersion observed when a beam laser passed through the crystal. A 15 mm × 10 mm × 5 mm (X × Y × Z) SNGS crystal wafer was cut and polished. The Ellipse Polarization Light Measurement Technique was used to measure the refractive indices. The principle of the method was described as follows. When a beam of linearly polarized monochromatic light was incidence on the planes of the crystal material, the reflected light beam was not linearly polarized light but elliptical polarized light. The changes contained some information about optical

Z.M. Wang et al. / Journal of Alloys and Compounds 425 (2006) 264–267

parameters, such as refractive indices, etc., so we can measure the polarized states changes of reflected light to former incidence light and calculate the optical parameters including refractive indices of the crystal material. This method was called Reflected Ellipse Polarization Light Measurement Technique. In our experiment, photometric automatic ellipse polarization light measurement device was used, and synchronous rotation detected device was used to detect the polarized state of reflected light wave and obtained some energy periodically change signals, then used the Fourier Transform and deduced the polarized state of the reflected light. In general, the incidence angle of the incidence light was near the Brewster’s angle, and Xe-flash lamp was used as the light source, and grating monochromator was used to form monochromatic light, the precision of the wavelength was 0.2 nm. Using the method described above, we measure the refractive indices in the wavelength range of 0.365–0.7065 ␮m.

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Table 1 Refractive indices of SNGS crystal Wavelength (␮m)

no

ne

0.7065 0.68 0.66 0.6438 0.6 0.52 0.4358 0.4078 0.38 0.365

1.725 1.74 1.745 1.755 1.775 1.77 1.78 1.825 1.84 1.845

1.785 1.755 1.76 1.77 1.79 1.74 1.795 1.84 1.855 1.86

2.2. Measurement of the dielectric properties For charactering dielectric properties of SNGS single crystals, Y-cut of SNGS crystal was fabricated in the form of 6.12 mm × 1.00 mm × 10 mm (X × Y × Z) wafer and it was coated with a layer of silver conducting paste. The sample was placed in a RF oven with cooling and heating rates set to about 1 K/min with a Eurotherm 818 temperature. Measurement of the sample capacitance value at frequency of 10, 100 kHz and 1 MHz (from which the dielectric constant ε can be calculated) were made using a HP-4192A LF IMPEDANCE ANALYZER and the temperature varied from 273.15 to 873.15 K. Moreover, dielectric loss was measured as a function of frequency at the temperature of 473.15 and 673.15 K by using HP-4294 IMPEDANCE ANALYZER. 3. Results and discussions 3.1. Refractive indices As a basic quantity for further investigation of optical properties, measurement of the crystals’ refractive indices and their dispersion is essential. It is important to develop a high quality data set as a precursor to electro-optical and non-linear optical investigations. Table 1 shows the two main refractive indices of SNGS and its dispersion measured at 10 discrete wavelengths between 0.365 and 0.7065 ␮m, and the error of refractive indices was 0.01. Fig. 1 shows these indices fitted by a four parametric Sellmeier equation of the form n2 (λ) = A +

B − D × λ2 . λ2 − C

(1)

where n(λ) is the refractive index at the wavelength λ, A–D the Sellmeier coefficients and λ is the wavelength (␮m). The corresponding Sellmeier coefficients are given in Table 2. From Fig. 1, we can see that most measured dots distribute near the fitting curve, however, several data have large deviation, it may be due to the error of the measurement, such as

Fig. 1. Refractive indices of SNGS and its dispersion.

man-made or instrument-made. Furthermore, there is slightly different slope in the fitting curve, this may be caused by the instrument error, and the crystalline imperfection is another reason. There are very weak changes in the optical transmission curve in the range of 0.365–0.7065 ␮m corresponding to some very weak absorption peaks, and this may cause different slope in the fitting curve. To non-centrosymmetric crystals the question arises whether a phase-matched optic second harmonic generation (SHG) is ω possible, i.e. whether one can find direction for which n2ω o = ne , referring to phase-matching type I: ee → o, however, owing to lower birefringence for SNGS crystal, a phase-matched process is impossible. Table 2 Sellmeier equation coefficients of SNGS Coefficients

no

ne

A B C D

3.51257 0.01572 0.05541 0.14612

3.54935 0.01462 0.05898 0.40628

χ2a

1.9 × 10−4

1.9 × 10−4

a

χ2 = sum of squares of the residuals.

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Z.M. Wang et al. / Journal of Alloys and Compounds 425 (2006) 264–267

Fig. 2. Dielectric loss dependence of the frequency of Sr3 NbGa3 Si2 O14 single crystal.

3.2. Dielectric properties Fig. 2 shows the curve of the dielectric loss versus the logarithm of frequency at 473.15 and 673.15 K. As can be seen that the dielectric loss of SNGS crystal is very small, however, there are several peaks in the graph, these weak peaks may be leak electric current caused by the small impurity and crystalline imperfection. And the maximal dielectric loss occurs at 478.289 Hz for SNGS crystal wafer of Y-cut, which is corresponding to the relaxation time (τ), that is to say, 1/τ is equal to 478.289 Hz. Dielectric polarization is mainly caused by the inherent electric dipole moment orientation at 478.289 Hz, when the inherent electric dipole moment orientation changes, it will spend time and energy which are corresponding to the relaxation time and dielectric loss, respectively. When the frequency is less than 1/τ, the polarized relaxation is increased with the frequency increase and the dielectric loss is also increased with the frequency increases. However, when the frequency exceeds 1/τ, dipole moment does not keep up with the change of electric field, so the dielectric loss is decreased with the frequency increases and the dielectric loss becomes maximal when the frequency reaches some middle frequency ωm = 1/τ. We should avoid resonance at 478.289 Hz in the application in order to make the dielectric loss minimum. The relaxation time is easily calculated to be 2.09 × 10−3 s. Using HP 4192A LF IMPEDANCE ANALYZER, we obtained the capacitance at different temperature and frequency, and then calculated relative dielectric constant εr according to the form εr =

d×c s × ε0

Fig. 3. Temperature dependence of relative dielectric constant of SNGS crystal of Y-cut. Table 3 Coefficients of polynomial approximation for SNGS crystal wafer Y-cut relative permittivity components Coefficients (ai )

10 kHz

100 kHz

1 MHz

a4 a3 a2 a1 a0

2.50675 × 10−10 4.49065 × 10−7 −2.71688 × 10−4 0.0764104 6.76146

−2.58452 × 10−10 4.89995 × 10−7 −3.19379 × 10−4 0.090906 5.74409

−1.68311 × 10−10 3.64624 × 10−7 −2.736041 × 10−4 0.089280 4.46218

imation of the behavior of SNGS crystal, we fitted preceding data with fourth-order polynomial laws assuming that εr (T ) ≈ a4 T 4 + a3 T 3 + a2 T 2 + a1 T + a0

(3)

where coefficients ai for frequency of 10, 100 kHz and 1 MHz are given in Table 3. At ambient temperature, the εr shows low temperature coefficients as follows (Fig. 3): 1 ∂εr = 0.025491 K−1 , εr ∂T

f = 10 kHz

1 ∂εr = 2.473068 × 10−4 K−1 , εr ∂T 1 ∂εr = 3.66024 × 10−4 K−1 , εr ∂T

f = 100 kHz f = 1 MHz

(4) (5) (6)

4. Conclusions (2)

where d is the thickness of sample (␮m), c the capacitance (pF), s the area of electrode (mm2 ) and ε0 is the dielectric constant of vacuum and is equal to 8.85. In order to study the temperature dependence of relative dielectric constant, we draw the curve of the relative dielectric constant versus temperature (Fig. 3). To give a useful approx-

Measurements for refractive indices of Sr3 NbGa3 Si2 O14 (SNGS) crystal showed that it could not realize the phasematching due to lower birefringence for SNGS crystal. Low dielectric loss and small dielectric temperature coefficients showed that the SNGS crystals were probably good piezoelectric materials. And at 478.289 Hz, the dielectric loss reaches maximum, the relaxation time is calculated to be 2.09 × 10−3 s.

Z.M. Wang et al. / Journal of Alloys and Compounds 425 (2006) 264–267

Acknowledgements This work was supported by grants from National Natural Science Foundation of China (No. 50242008 and No. 50372034) and Changjiang Scholar Awarding Project. The authors are grateful to State Key Laboratory of Crystal Materials of Shandong University for supporting this work. References [1] I.M. Sil’vestrova, Yu.V. Pisarevskii, B.V. Mill’, A.A. Kaminskii, Dokl. Akod, Nauk SSSR 282 (1985) 575. [2] I.M. Sil’vestrova, Yu.V. Pisarevskii, A.A. Kaminskii, B.V. Mill, Fiz. Tverd Tela 29 (1987) 1520 (Leningrad, 1987). [3] S. Sato, V. Tshigami, K. Moroishi, S.A. Sakkharov, Proceedings of the International IEEE Frequency Control Symposium, Orlando, 1996, p. 379. [4] V.N. Fedoretz, Yu.P. Kondratyev, B.V. Mill, V.A. Pankov, Yu.V. Pisarevsky, V.V. Timashev, Proceedings of the International IEEE Frequency Control Symposium, Orlando, 1997, pp. 816–820. [5] J. Detaint, J. Schwartzel, A. Zarka, B. Capelle, J.P. Denis, E.P. Hilippot, Proceedings of the International IEEE Frequency Control Symposium, 1994, p. 58. [6] T. Fukuda, K. Shimamura, T. Kohno, H. Takeda, M. Sato, J. Jpn. Assoc. Cryst. Growth 22 (5) (1995) 358 (in Japanese). [7] K. Shimamura, H. Takeda, T. Kohno, T. Fukuda, J. Cryst. Growth 163 (1996) 388–392.

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