Growth and piezoelectric properties of Al-substituted langasite-type La3Nb0.5Ga5.5O14 crystals

Materials Research Bulletin 43 (2008) 1731–1736 www.elsevier.com/locate/matresbu

Growth and piezoelectric properties of Al-substituted langasite-type La3Nb0.5Ga5.5O14 crystals Hiroaki Takeda *, Tomoaki Kuze, Takashi Nishida, Kiyoshi Uchiyama, Tadashi Shiosaki Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), Takayama-cho 8916-5, Ikoma, Nara 630-0192, Japan Received 31 May 2007; received in revised form 4 July 2007; accepted 12 July 2007 Available online 22 July 2007

Abstract The influences of aluminum substitution for gallium in the langasite-type La3Nb0.5Ga5.5O14 (LNG) crystals on their growth and electric properties were investigated. Al-substituted LNG (La3Nb0.5Ga5.5xAlxO14; LNGAx) single crystals up to the solubility limit x = 0.2 have been grown by the conventional Czochralski technique. The electric properties of the LNGAx crystals were investigated and compared to those of LNG. With Al substitution, the piezoelectric constants, d11 and d14, were slightly higher. The LNGAx crystals showed a temperature dependence of d11 similar to that of the LNG crystal. # 2007 Elsevier Ltd. All rights reserved. Keywords: B. Crystal growth; D. Piezoelectricity

1. Introduction Recently, La3Ga5SiO14 (langasite, LGS) crystals have attracted the attention as combustion pressure sensor materials used at high temperature, because LGS shows no phase transitions up to its melting temperature. For such applications, piezoelectric materials with a low temperature dependence of the piezoelectric properties at high temperature are required. Although many LGS-type crystals have already been reported [1–6], there is little information about the piezoelectric properties of these LGS-type crystals at high temperature. Therefore, it has not been clarified that LGS is the most appropriate material for all the LGS-type crystals. There is an issue that expensive gallium oxide is needed as the main component of the raw material for the mass production of the sensors made of the LGS crystals. Kumatoriya et al. [7] reported the effective reduction of Ga in the LGS crystal by Al substitution. In Ref. [8], we reported that the Al-substituted LGS crystal shows a higher electromechanical coupling factor when compared to the pure LGS crystal. Under this situation, we have paid attention to other LGS-type single crystals and chosen the La3Nb0.5Ga5.5O14 (LNG) single crystals, because it can easily be grown to a large size using the Czochralski (Cz) technique, as well as exhibiting a high frequency stability and high electromechanical coupling factors [9,10]. In this paper, we describe the synthesis, crystal growth and the electric properties at high temperature of the Al-substituted LNG and a comparison to those of the pure LNG crystal.

* Corresponding author. Tel.: +81 743 72 6063; fax: +81 743 72 6069. E-mail address: [email protected] (H. Takeda). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.07.029

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2. Experimental procedure The determination of the Al solubility limit in LNG was performed by a solid-state reaction. The starting materials were prepared by mixing 99.99% pure La2O3, Nb2O5, Al2O3 and Ga2O3 powders at the compositions corresponding to the chemical formula La3Nb0.5Ga5.5xAlxO14 (LNGAx; x = 0.0–5.0). The powders were fired for 2 h at 1400–1480 8C. The phase identification and lattice constant measurements for each sample were done by the powder X-ray diffraction (XRD) method. The LNGAx crystals with the Al-content(x) between 0.0 and 1.0 were grown by the conventional Czochralski technique using an Ir crucible that was 50 mm in diameter and height. The growth atmosphere was an Ar plus 1 vol% O2 mixture gas. The pulling direction was h0 0 1i. The pulling rate and the rotation rate were 1.5 mm/h and 15 rpm, respectively. Observations of bubbles and inclusions in the crystals were performed using an optical microscope. The density of the grown crystals was measured by the Archimedes method using distilled water at room temperature. The chemical composition was measured by a quantitative X-ray fluorescent analysis (XRF) technique. The grown crystals were pulverized and their phases were identified by the powder XRD analysis. For investigation of the Al substitution effect on acoustic properties of LGS-type crystals, all material constants, composed of the dielectric, piezoelectric and elastic compliance constants (eij, dij and sij), were determined as reported in Refs. [11,12]. The resonance and the antiresonance frequencies for various vibration modes were measured using an impedance/gain phase analyzer (HP-4194A: Agilent). The dielectric constants eTij were determined by measuring the capacitances of the resonators by taking the parasitic capacitance into account. The changes in d11 in the temperature range from room temperature to 500 8C were investigated. 3. Results and discussion We attempted to synthesize the LNGAx polycrystals by changing the composition x using a solid-state reaction at 1470–1525 8C. We observed the LNGAx single phase at x values up to 1.5. As secondary phases, La(Ga,Al)O3 appeared over x > 1.6. When the melting point was visually determined, the temperature increased with the changing in the Al content. This observation suggested that Al3+ ions are easily incorporated into the crystals during the growth process, and the segregation coefficient of Al is greater than unity. This result agreed with the melting behavior of the Al-substituted LGS [7] and La3Ta0.5Ga5.5O14 (LTG) single crystals [13]. The powder samples synthesized at 1450 8C and without any secondary phases were used for the measurement. Both the lattice constants a and c decreased with an increase in x. This indicated that Al3+ with an ionic radius smaller than Ga3+ [14] was smoothly incorporated into the LNG crystal lattice. Based on these observations, we found that the solubility limit of Al in the LNG as a polycrystal state was determined to be 1.5. In Refs. [7,13], the solubility limit value of Al in the Al-substituted LGS and LTG single crystals was lower than those in the polycrystal state. Therefore, in this study, we tried to grow LNGAx crystals at 0.0 (LNG)  x  1.0. Fig. 1(a) shows an as-grown LNGA0.2 crystal. Since no growth instability appeared during the growth, the uniform diameter of the growing crystals was easily controlled. The crystal showed a smooth surface and had a transparent orange color. No cracks and inclusions were found inside the grown crystals. Fig. 1(b) shows an LNGA0.4 as-grown crystal. The crystal was a transparent deep orange color. Dubovskiy et al. [15] grew LGS crystals from the melts with

Fig. 1. As-grown (a) LNGA0.2 and (b) LNGA0.4 crystals.

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Fig. 2. Powder X-ray diffraction profiles of pulverized specimen of (a) LNG, (b) LNGA0.2, (c) the inside of LNGA0.4 and (d) the rim of the crystals.

various compositions, and then suggested that the coloring originates from the existence of (VO, 2e)x – two electrons localized at an oxygen vacancy – in the crystal. We agree with their explanation, because we confirmed that the orange color of pure and Al-substituted LGS, LNG and LTG crystals grown in oxygen-contained atmosphere disappeared after annealing in N2 or Ar gases and also identified that the crystals grown in N2 or Ar gases were transparent and colorless. There were some adhesive substances, as the showing white color parts seen in Fig. 1(b), on the crystal surface. The adhesive substances were found to be a La(Ga,Al)O3 phase by XRD analysis. However, no inclusions were observed on the inside of the crystal. Fig. 2 shows powder XRD profiles of the pulverized specimens of the LNG, LNGA0.2, LNGA0.4 crystals. All peaks in the powder XRD patterns of the LNG, LNGA0.2 and the inside of the LNGA0.4 crystals were identified to be those having the LGS-type structure. On the other hand, the rim part of the LNGA0.4 crystal contained the La(Ga,Al)O3 phase. This was because the rim part included the adhesive La(Ga,Al)O3. Since the La(Ga,Al)O3 phase has higher melting temperature (1720 8C) than LGS-type phase (ca. 1500 8C) [16], it is considered that the excess aluminum enhanced an accidental crystallization of La(Ga,Al)O3 during growth process. Finally, we obtained the LNGAx (0 < x  0.2) single crystals with no secondary phases. These LNGAx crystals were easily grown in the same way as the LNG one. The measured density (Dm = 5.903 g/cm3) of the LNGA0.2 crystals were slightly smaller than that (5.856 g/cm3) of the pure LNG ones. This result is supported by the atomic weight (WAl(26.98) < WGa(69.72)). Using the LNGA0.2 crystals, the chemical composition was measured by XRF. The cationic ratio of the top and tail parts of the grown crystals was determined to be La:Nb:Ga:Al = 3.05:0.47:5.31:0.20 and 3.01:0.47:5.31:0.18, respectively. The Al substitution into the LNG was confirmed. Moreover, the Al content slightly decreased during the growth process. This result also proved that the segregation coefficient of Al into LNG is greater than unity. Fig. 3 shows the change in the lattice constants of LNGAx versus the Al content. The powder samples made of single crystals without any secondary phases were used for the measurement. Both lattice constants, a and c, decreased with an increase in x. This indicated that Al3+ with an ionic radius smaller than Ga3+ [14] was smoothly incorporated into the LNG crystal lattice. Since the inside part of the LNGA0.4 crystals did not contain impurity phases, it was clarified that this inside part had a higher Al content compared to LNGA0.2. The growth speed of 1.5 mm h1 used in this study was too high to obtain pure LNGA0.4 crystals. In Ref. [17], the Sr-substituted LTG single-crystal without any impurities was successfully grown by decreasing the growth speed from 1.5 to 0.4 mm h1. Although a similar observation was expected for the LNGAx crystal, the growth speed was fixed at 1.5 mm h1 based on the assumption that the crystals were being mass-produced. Two crystal samples (LNG and LNGA0.2) without impurity phases were chosen for the investigation of the Al substitution effect on the physical properties. The elecrtromechanical coupling factors and evaluated material constants of the LNG and LNGA0.2 single crystals are shown in Table 1. The material constants of the LNG crystals determined in this study were comparable to those reported in Ref. [5]. All the factors of the LNGA0.2 sample are

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Fig. 3. Variation in lattice constants of LNGAx single crystals.

greater than those of the pure LNG. This result is the same as that reported for the Al-substituted LGS and LTG crystals [7,13]. The LNGA0.2 crystal has also higher piezoelectric constants, d11 and d14, as the absolute values compared to the pure LNG crystal. With an increase in the Al content, the piezoelectric constant d11 and d14 increases by about 1.4 and 5.2%, respectively. We explained the relationship between the crystal structure and the piezoelectric properties in the LGS-type crystals. There are four kinds of cation sites in this structure, and this structure can be represented by the chemical formula, A3BC3D2O14. In this chemical formula, A and B represent the decahedral (twisted Thomson cube) site coordinated by eight oxygen anions, and the octahedral site coordinated by six oxygen anions, respectively. While both C and D represent tetrahedral sites coordinated by four oxygen anions, the size of the D site is slightly smaller than that of the C site. In the case of the LGS crystal, La3+ occupies the A site, Ga3+ occupies B, C and half of the D sites, and Si4+ half of the D site [18]. The piezoelectric modulus d11 of LGS was found to decrease with the increasing atomic number of the rare-earth element occupying the decahedral site, i.e., when La3+ is replaced with Pr3+ and Nd3+ [4]. In these crystals, the corresponding mean interatomic distance A–O(oxygen) decreased with the decreasing ionic radii of the rare-earth element (rLa > rPr > rNd [14]), but the other B–O, C–O and D–O distances did not change [19]. By the way, according to Ref. [8,20], in the Al-substituted LGS and LTG crystals, the single-crystal X-ray structure analysis revealed that the Al atoms are distributed in all the cation sites except for the A-site. Furthermore, it was reported that the A–O distance seems to relatively extend due to the reduction of the other B–O, C–O and D–O distances. Based on these observations, the tendency was obtained for increasing the piezoelectric modulus d11 of the LGS-type crystals if the A–O distance was extended. More data are needed for explaining the change in d14.

Table 1 Properties of LNG and LNGA0.2 crystals

Dm k12 k23 eT11 =e0 eT33 =e0 d11 d14 sE11 sE12 sE13 sE14 sE33 sE44 sE66

LNG

LNGA0.2

5.903(6) 15.8 6.20 21.0 80.0 6.40(5) 3.86(3) 9.00 5.10 1.51 3.53 5.47 21.2 28.2

5.856(3) 16.2 6.57 20.2 75.5 6.49(2) 4.06(4) 9.22 4.87 1.61 3.68 5.25 20.8 28.2

Dm: experimental density (g/cm3), kij: electromechanical coupling factor, eTij =e0 : Relative dielectric constant, dij: piezoelectric constant (pC/N), sE11 : elastic compliance constant (1012 m2/N).

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 Fig. 4. Temperature dependences of the piezoelectric constant d11 of the LNG and LNGA0.2 crystals. Corresponding data of isomorphic La3Ga5SiO14 (LGS), La3Ga4.1Al0.9SiO14 (LGAS0.9) crystals are plotted from Ref. [21].

The thermal behavior of the piezoelectric properties of the LNG and LNGA0.2 single crystals were investigated. Fig. 4 shows the temperature dependence of the piezoelectric constant d11 of the LNG and LNGA0.2 crystals. Those of the LGS and La3Ga4.1Al0.9SiO14 (LGAS0.9) crystals [21] are plotted for comparison. In this figure, the relative  piezoelectric constant d11 represents the ratio of the d11 value at the measurement temperature and that at room  temperature (25 8C). The d11 value increased with temperature for all the crystals. Small differences in the thermal behaviors were observed between the pure (LGS and LNG) and the Al-substituted (LGAS0.9 and LNGA0.2) crystals. Therefore, we found that the Al substitution hardly affected the thermal behavior of the piezoelectric properties of the LGS-type crystals. Moreover, both the LNG and LNGA0.2 crystals show a small change (approximately 2%) compared to the LGS and LGAS0.9 crystals, and this is an advantage for a real device. In addition, we mentioned that the Al substitution enables the use of a lower amount of expensive gallium oxide as the raw material. For the LNGA0.2 crystal, a reduction (approximately 4%) in the Ga2O3 amount versus LNG is expected. 4. Conclusions We grew Al-substituted LNG crystals by the conventional Cz technique and investigated their characteristics at high temperature. By Al-substitution, the LNG crystals acquired higher piezoelectric properties. Furthermore, the Al substitution enables the use of a lower amount of expensive gallium oxide as the raw material. At present, the LNGA0.2 crystal is a candidate for combustion pressure sensor materials. Acknowledgements This work was partly supported by a Grant-in-Aid for Young Scientists Research, No. 17686058, from the Ministry of Education, Culture, Sports, Science and Technology of Japan and a research grant from the KRF Foundation. References [1] [2] [3] [4]

A.A. Kaminskii, B.V. Milll, G.G. Khodzhabagyan, A.F. Konstatinova, A.I. Okorochkov, I.M. Silvestrova, Phys. Stat. Sol. (a) 80 (1983) 607. I.M. Silvestrova, Yu.V. Pisarevskii, B.V. Mill, A.A. Kaminskii, Sov. Phys. Dokl. 30 (1985) 402. I.M. Silvestrova, Yu.V. Pisarevskii, A.A. Kaminskii, B.V. Mill, Sov. Phys. Solid State 29 (1987) 870. J. Sato, H. Takeda, H. Morikoshi, K. Shimamura, P. Rudolph, T. Fukuda, J. Cryst. Growth 191 (1998) 746.

1736 [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

H. Takeda et al. / Materials Research Bulletin 43 (2008) 1731–1736 J. Bohm, E. Chilla, C. Flannery, H.-J. Fro¨hlich, T. Hauke, R.B. Heimann, M. Hengst, U. Straube, J. Cryst. Growth 216 (2000) 293. M.M.C. Chou, S. Jen, B.H.T. Chai, in: Proceedings of the 2001 IEEE Ultrasonics Symposium, 2001, p. 225. M. Kumatoriya, H. Sato, J. Nakanishi, T. Fujii, M. Kadota, Y. Sakabe, J. Cryst. Growth 229 (2001) 289. H. Takeda, M. Kumatoriya, T. Shiosaki, Appl. Phys. Lett. 79 (2001) 4201. J. Bohm, E. Chilla, C. Flannery, H.-J. Fro¨hlich, T. Hauke, R.B. Heimann, M. Hengst, U. Straube, J. Cryst. Growth 204 (1999) 128. H. Takeda, K. Shimamura, T. Kohno, T. Fukuda, J. Cryst. Growth 169 (1996) 503. IEEE Standard on Piezoelectricity 176-1987, 1987. W.P. Mason, Piezoelectric Crystals and their Application to Ultrasonics, D. Van. Nostrand Company, New York, 1950. H. Takeda, S. Tanaka, H. Shimizu, T. Nishida, T. Shiosaki, Key Eng. Mater. 320 (2006) 239. R.D. Shannon, Acta Crsytallogr. A 32 (1976) 751. A. Dubovskiy, E. Domoroshchina, G. Kuz’micheva, G. Semenkovich, in: Proceedings of the 2004 IEEE Ultrasonics Symposium, 2004, p. 642. M. Mizuno, T. Yamada, T. Ohtake, Yoguo-kyokai-shi 93 (1985) 295. H. Takeda, T. Kato, V.I. Chani, H. Morikoshi, K. Shimamura, T. Fukuda, J. Alloys Compd. 290 (1999) 79. B.V. Mill, A.V. Batushin, G.G. Hodzhabagjan, E.L. Belokoneva, N.V. Belov, Dokl. Akad. Nauk. SSSR 264 (1982) 1385. H. Takeda, S. Izukawa, H. Shimizu, T. Nishida, S. Okamura, T. Shiosaki, Trans. Mater. Res. Soc. Jpn. 30 (2005) 63. H. Takeda, unpublished data. H. Takeda, S. Tanaka, S. Izukawa, H. Shimizu, T. Nishida, T. Shiosaki, in: Proceedings of the 2005 IEEE Ultrasonics Symposium, 2005, p. 560.