Growth of a New Ordered Langasite Structure Crystal Ca3TaAl3Si2O14 Jun Xin, Yanqing Zheng,* Haikuan Kong, Hui Chen, Xiaoniu Tu, and Erwei Shi Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, P. R. China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2617–2619
ReceiVed April 7, 2008; ReVised Manuscript ReceiVed May 14, 2008
ABSTRACT: A new ordered piezoelectric single-crystal Ca3TaAl3Si2O14 (CTAS) with langasite structure has been successfully grown with the dimensions of 14-18 mm in diameter and 45 mm in length using the Czochralski technique. The X-ray powder diffraction (XRPD) of the single crystal was performed at room temperature. The lattice parameters were calculated. They are a ) 8.060 Å, c ) 4.909 Å, V ) 276.19 Å3. X-ray fluorescence analysis was used to determine the molar ratio of the ions of the crystal. That is 3.049:0.945:2.975:2: 13.875 Ca2+:Ta5+:Al3+:Si4+:O2-. High-resolution X-ray diffraction (HRXRD) results indicate that CTAS crystals are free of low-angle boundaries. The piezoelectric strain constant d11 measured by quasi-static d33 meter is 5.0 pC/N. The transmittance spectrum shows that the as-grown crystal is transparent in the wavelength range of 200-800 nm and the highest transmittance exceeds 86%. Piezoelectric materials play a very important role in the modern industry. They are widely used as the raw materials of filters, sensors, resonators, and so on. Until now, quartz has still been the dominant material in the market of surface acoustic waves (SAW) devices. With the development of modern wireless communication technology, the disadvantage of quartz has become a hindrance in the process of improving the performance of SAW devices. It becomes urgent to find a substitution for quartz to make wideband filters. In the last two decades, the langasite family including La3Ga5SiO14 (LGS), Sr3Ga2Ge4O14 (SGG), La3Nd0.5Ga5.5O14 (LGN), and La3Ta0.5Ga5.5O14 (LGT) belonging to point group 32, space group P321 with A3BC3D2O14 structure has attracted increasing attention because of its excellent properties in applications of surface acoustic wave, bulk acoustic wave, and sensors fields.1–8 But with further study on LGS, LGN, LGT, and SGG, several disadvantages of these materials have been found.9 The “disordered” structure of these compounds will cause the randomly distributed distortion of crystal structure leading to the lower achievable acoustic Q and electromechanical coupling. On the basis of the understanding of “disordered” structure, four completely “ordered” compounds of langasite structure including Sr3NbGa3Si2O14 (SNGS), Sr3TaGa3Si2O14 (STGS), Ca3TaGa3Si2O14 (CTGS), and Ca3NbGa3Si2O14 (CNGS) have been found and studied for several years.9–19 These crystals show better SAW performance than those of LGS, but just like LGS, the high price of Ga2O3 strongly limited its application. In 1998, Mill et al. reported the successful preparation of the CTAS ceramic sample for the first time.20 The Al3+ is used to replace the Ga3+ on C site. As the price of Al2O3 is much cheaper than that of Ga2O3, the cost of CTAS is much cheaper correspondingly. The CTAS has a similar cost of the raw materials as quartz while its piezoelectric properties predicted by first-principle calculation are much better than those of quartz.21 This means CTAS is a very competitive material used as the substitution for quartz in the application of SAW. Figure 1 shows the crystal structure of CTAS. In this communication, we report the successful Czochralski growth of CTAS single crystal and evaluate the crystal quality and its optical properties. To the best of our knowledge, this is the first time that a langasite structure single crystal without high cost constituents of Ga or Ge was grown. The polycrystalline CTAS compounds were prepared by solidstate reaction by mixing stoichiometric amounts of CaCO3, Ta2O5, Al2O3, and SiO2 with 4N purity. The compounds were pressed into * Corresponding author. Tel.: 86-21-69987762. Fax: 86-21-59927184. E-mail:
[email protected].
Figure 1. Crystal structure of CTAS viewed along [001].
blocks and then the blocks were put into a Pt crucible. They were heated at 1250 °C for 20 h. After the temperature decreased to ambient temperature, the blocks were put into the crucible for crystal growth. The single crystal of CTAS was grown by a conventional induction-heating CZ furnace with an iridium crucible (60 mm in diameter and 45 mm in height). An after-heater system made of Pt was used. The growth atmosphere was a mixture of N2 and 1 vol % O2 in order to protect the iridium crucible and avoid the oxygen defect in crystal. The CTAS single crystal growth was performed along 〈100〉 directions with STGS seeds. Before crystal growth, the melt of stating materials were maintained for several hours to homogenize the melt. The pulling rate was 1-3 mm/h and the crystal rotation rate was 15-30 rpm. After the growth, the crystal was cooled to room temperature at a rate of 80 °C/h. After the growth, the structure of the CTAS crystal was analyzed by XRPD at room temperature on a Rigaku D/max 2550V diffractometer with Cu KR radiation of wavelength λ ) 1.5406 Å. The lattice parameters of the crystal were calculated from the data of XRPD. Archimedes method was employed to determine the crystal’s density. A small piece of the crystal was cut from the middle of the crystal to determine the molar ratio of the ions of the as-grown CTAS single crystal using X-ray fluorescence analysis. It was carried out on a PW2404 X-ray fluorescence analysis system made by Philips. HRXRD was used to further evaluate the quality of as-grown CTAS single crystal. The rocking curve of the crystal was performed on a y-cut wafer with the size of 10 × 10 × 1 mm3
10.1021/cg800354q CCC: $40.75 2008 American Chemical Society Published on Web 06/18/2008
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Figure 2. CTAS single crystal pulled along 〈100〉.
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Figure 3. XRPD pattern of CTAS single crystal.
Table 1. Experimental Conditions of Crystal Growth and Characteristics of CTAS Crystal atmosphere seed pulling rate (mm/h) rotation rate (rpm) cryst size (mm) color imperfection
N2 + 1 vol % O2 STGS 1-3 15-30 Φ18 × 45 slight yellow opaque core region
Table 2. Molar Ratio of Ca2+, Ta5+, Al3+, Si4+, and O2- Ions of CTAS Single Crystal ion
molar ratio
Ca Ta Al Si O
3.049 0.945 2.975 2 13.875
using the HRXRD D8-discover at room temperature. The angular resolution ratio is 0.0001°. The step time and step size were 1 s and 0.0001°, respectively. A x-cut wafer with the size of 10 × 10 × 2 mm3 were used to measure the piezoelectric strain constant d11 of CTAS by using the ZJ-3A quasi-static d33 meter made by Chinese Institute of Acoustics The transmittance spectrum was measured with the wavelength from 200 to 800 nm using a Shimadzu UV-2501PC spectrophotometer at room temperature. The CTAS crystal was first grown along 〈001〉 direction using a STGS seed. But it was found that no single crystal could be obtained. After that, we found the CTAS crystals had much stronger growth tendency along 〈100〉 direction and CTAS crystal was successfully grown along this direction. Figure 2 shows the as-grown CTAS crystals pulled along the direction of 〈100〉 and Table 1 gives the experimental conditions of crystal growth and the characteristics of crystals. The grown crystals are transparent and slight yellow in color. An opaque core region can be seen in the center of the crystal. The facets of the as grown CTAS crystals are (100) and (001). The XRPD pattern for CTAS crystals is given in Figure 3. The analysis of the XRPD data confirms that the grown crystals are single phase of langasite-family structure. All the diffractive peaks are identified to be those for CTAS and no peaks of second phase are found. By the help of the Jade5.0 program, the lattice parameters were calculated from the XRPD data. They are a ) 8.060 Å, c ) 4.909 Å, V ) 276.19 Å3. The theoretical density is 3.982 g/cm3. The density of CTAS measured by the Archimedes method is 4.046 g/cm3. Table 2 gives the molar ratio of Ca2+, Ta5+, Al3+, Si4+, and 2O ions measured by X-ray fluorescence analysis. The result is
Figure 4. Rocking curves of y-cut CTAS. Table 3. Piezoelectric Strain Constant d11 of CTAS, Quartz, And Some Other Langasite Structure Crystals crystal
d11 (pC/N)
CTAS CTAS (calculated)21 quartz22 LGS23 CTGS24 CNGS18
5.0 -5.78 -2.31 6.15 -4.58 4.40
3.049:0.945:2.975:2:13.875 Ca2+:Ta5+:Al3+:Si4+:O2-, which is very close to the stoichiometric ratio of A3BC3D2O14. Figure 4 shows the rocking curve of a y-cut diffraction wafer. The diffraction angle is 12.9394° and the full width at halfmaximum (fwhm) value is 32 seconds. The result suggests that the CTAS crystal is free of low-angle boundaries. Table 3 shows the value of the piezoelectric strain constant d11 measured by quasi-static d33 meter for CTAS, compared with those of quartz and some other langasite crystals. For CTAS single crystal, d11 is 5.0 pC/N, which is 2.2 times that of the quartz. A y-cut plate (15 × 10 × 0.8 mm3) was cut from the grown crystals and polished for transmittance investigation. As shown in Figure 5, the crystal is transparent and the highest transmittance exceeds 86%. In addition, the minimum transmittance of CTAS crystal is not zero. It rises again when the wavelength is below 238nm. The reason for the increase of the transmittance in deepultraviolet of CTAS may be attributed to the special band structure. Further investigation is needed to explain this phenomenon. The Ca3TaAl3Si2O14 single crystal has been grown successfully for the first time using the Czochralski method. The structure of
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Crystal Growth & Design, Vol. 8, No. 8, 2008 2619
(4) (5) (6) (7) (8) (9)
(10)
(11)
Figure 5. Transmittance spectra of CTAS single crystal.
CTAS was identified by XRPD and the lattice parameters were calculated by JADE5.0 program. They were a ) 8.060 Å, c ) 4.909 Å, V ) 276.19 Å3. The density measured for CTAS crystal is 4.046 g/cm3. The molar ratio of the as-grown single crystal CTAS is 3.049:0.945:2.975:2:13.875 Ca2+:Ta5+:Al3+:Si4+:O2-, which is close to the stoichiometric ratio of A3BC3D2O14. The result of HRXRD indicates that the CTAS crystal is free of low-angle boundaries. The piezoelectric strain constant d11 is 5.0 pC/N, which is 2.2 times that of the quartz. The transmittance spectrum shows that the as-grown crystal is transparent and the highest transmittance exceeds 86%. The piezoelectric properties of CTAS crystal will be reported elsewhere.
Acknowledgment. This work is supported by National Nature Science Foundation of China under Grant 50772121 and the Knowledge Innovation Program of Chinese Academy of Science under Grant KGCX-2-YW-206.
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