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Journal of Crystal Growth 275 (2005) e279–e282 www.elsevier.com/locate/jcrysgro
Growth of piezoelectric single crystals by the ﬂux method M. Beaurain, P. Armand, P. Papet LPMC, UMR5617, Universite´ Montpellier II (UMII), CC003, Place Euge`ne Bataillon, 34095 Montpellier Cedex 5, France Available online 7 December 2004
Abstract Transparent single crystals of the piezoelectric materials a-GaPO4 have been grown by the spontaneous nucleation method from Li2Mo3O10 ﬂux. The solubility of a-GaPO4 in Li2Mo3O10 ﬂux has been determined between 650 and 950 1C. Lattice parameters of crystals have been measured by X-ray diffraction. Result of infrared spectroscopy indicates the possibility of growing a-GaPO4 crystal without hydroxyl groups by the ﬂux method. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.Dn Keywords: A1. Solubility; A2. Growth from solutions; A2. Single-crystal growth; B1. Gallium compounds; B1. Quartz; B2. Piezoelectric materials
1. Introduction The a-modiﬁcation of gallium orthophosphate, GaPO4, is a piezoelectric material isostructural with a-quartz, SiO2. This material exhibits better piezoelectric characteristics: higher coupling coefﬁcient for its temperature compensated cut and wider thermal stability than quartz single crystals . The high temperature of the allotropic transition a-quartz - b-cristobalite at 933 1C does not
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permit the growth of a-GaPO4 single crystals by conventional melt techniques. Up to now a-GaPO4 single crystals are commonly grown by the hydrothermal technique from an aqueous solution of phosphoric acid [1,2]. However, this method has the disadvantage of incorporation of hydroxyl (OH) groups which deteriorates the piezoelectric properties and limits their device application at high temperature. For these reasons, it will be interesting to grow aGaPO4 single crystals with low amount of OH groups. The high-temperature solution or ﬂux growth technique seems to be a good alternative for this purpose. After several ﬂux tests, clear aGaPO4 single crystals have been grown by the
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M. Beaurain et al. / Journal of Crystal Growth 275 (2005) e279–e282
spontaneous nucleation method with Li2Mo3O10 as the ﬂux.
2. Experimental procedure The Li2O–MoO3 system was used as a ﬂux, because it is soluble in water and vaporizes little at high temperatures . The Li2Mo3O10 composition was chosen due to its low melting temperature obtained by DSC (SETARAM DSC-121). The sample was run in sealed silica tube at a heating rate of 5 1C/min from the room temperature to 800 1C. Fig. 1 shows that the Li2Mo3O10 ﬂux melts at 55971 1C. Li2Mo3O10 ﬂux was synthesized at 545 1C for 4 days in a nickel crucible by the solid-state reaction (1) Li2 CO3 þ 3MoO3 ! Li2 Mo3 O10 þ CO2 "
using Li2CO3 and MoO3 in the respective purities of 99% and 99.5%. The solubility was determined by introducing excess a-GaPO4 crystals in the ﬂux (solvent) at different controlled temperatures (from 650 to 950 1C). After dissolution has proceeded for 3 days, the Pt crucible was removed from the furnace and quenched into water. Then, the undissolved material was separated from the saturated solution and weighted. The weight loss at each temperature gives the corresponding solubility, Fig. 2.
Fig. 1. Thermal analysis DSC curve for Li2Mo3O10 ﬂux.
Fig. 2. Solubility of a-GaPO4 in Li2Mo3O10 ﬂux.
The crystal growth experiments reported were carried out in air in a single zone, SiC resistance heater furnace with an Eurotherm temperature controller. Different amounts (wt%) of the Li2Mo3O10 ﬂux were mixed with the a-GaPO4 powder and homogenized in an agate mortar. The a-GaPO4-ﬂux mixture was put in a 36 ml Pt crucible covered with a lid, heated from room temperature to 950 1C at a ramp rate of 100 1C h1 and held at this temperature for 5 h for homogenization. The melt (a-GaPO4-ﬂux) was then slowly cooled at a rate of 2 1C h1 from 950 to 600 1C and held here for 5 h. After this, the charge was cooled to room temperature at 200 1C h1. The growth period was about eight days including the heating up and homogenization time. The asgrown single crystals were removed from the ﬂux with water (a-GaPO4 is soluble in acids and not in water). From different solute–ﬂux ratios experiments, it was found that 85 wt% of the ﬂux for 15 wt% of a-GaPO4 gave crystals of better size. The phase identiﬁcation of the as-grown crystals was done by a Seifert X-ray powder diffractometer (XRD) using the CuKa radiation and the lattice parameters were determined by an enraf-nonius XRD using the MoKa radiation. The infrared transmission measurements using the KBr pellet method, were carried out using a Perkin-Elmer Spectrum One FT-IR spectrometer with a resolution of 4 cm1. Chemical analyses were performed on a CAMECA SX-100 electron probe instrument
ARTICLE IN PRESS M. Beaurain et al. / Journal of Crystal Growth 275 (2005) e279–e282
equipped with ﬁve wavelength dispersive X-ray spectrometers (WDS) and a tungsten electron gun .
3. Results and discussion The transparent and colorless as-grown crystals appeared to nucleate only throughout the ﬂux volume. The dimensions of the single crystals obtained with the solute/ﬂux ratio of 15/85 wt% in a 36 ml Pt crucible were up to 5 3 1 mm3 as
Table 1 Lattice parameters of a-GaPO4 a-GaPO4
Lattice parameters (A˚)
a ¼ 4.905(3) c ¼ 11.04(1) a ¼ 4.8990 c ¼ 11.0340 a ¼ 4.8740 c ¼ 11.0330 a ¼ 4.9352 c ¼ 11.0790
JCPDS card ﬁle 85-0977 JCPDS card ﬁle 85-0978 JCPDS card ﬁle 85-2061
Fig. 3. Optical photograph of a-GaPO4 as-grown single crystals.
Fig. 5. IR spectrum of a-GaPO4 crystals grown in Li2Mo3O10 ﬂux.
Fig. 4. X-ray powder diffraction pattern of a-GaPO4 crystals.
shown on the optical photograph (Fig. 3). A room temperature X-ray powder diffraction pattern of a-GaPO4 grown in Li2Mo3O10 is shown in Fig. 4. All peaks could be indexed in agreement with the JCPDS Card File 85-0977. The lattice parameters of the hexagonal phase were determined by XRD using 20 reﬂections and are given in Table 1. In this table, lattice parameters from other JCPDS Card Files are also listed for comparison. The infrared transmission spectrum recorded in the range 450–4000 cm1 of a-GaPO4 crystals grown in the Li2Mo3O10 ﬂux is shown in Fig. 5. The absence of vibration bands in the experimental mid-IR spectrum indicates that as-grown aGaPO4 crystals are free of hydroxyl groups. The chemical analysis of the as-grown a-GaPO4 crystals was determined with an electron probe
ARTICLE IN PRESS M. Beaurain et al. / Journal of Crystal Growth 275 (2005) e279–e282
Table 2 Chemical composition of a a-GaPO4 single crystal grown in Li2Mo3O10 ﬂux Element
Ga P Mo
16.66 16.67 0
16.31 16.80 0
analyzer. The experimental values compared with the ideal values are presented in Table 2. The results show no detectable Mo incorporation from the Li2Mo3O10 ﬂux into the a-GaPO4 crystals. Both the IR and the chemical results encourage us to follow the development of the a-GaPO4 single crystals growth by the high-temperature ﬂux method.
single crystals without hydroxyls by the ﬂux method was established. So, it should be likely possible to grow larger a-GaPO4 crystals using the top seeded solution growth method.
Acknowledgements The authors are very grateful to Drs. E. Philippot and D. Balitski of the LPMC of UMII for their encouragement and valuable discussion. They also thank Dr. N. Fretty (Institut Gerhardt, UMII), Dr. R. Astier (Institut Gerhardt, UMII), Mr. J.-M. Peiris (Service Microsonde Sud, UMII) and Mr. D. Maurin (Service Infrarouge, UMII) for their kind help with SEM analysis, X-ray diffraction measurements, electron probe data collection and infrared measurements.
4. Conclusion References Transparent a-GaPO4 single crystals were grown by the spontaneous crystallization method (without the use of seed crystals) from hightemperature Li2Mo3O10 ﬂux. The size (5 3 1 mm3) was limited by the small volume of our Pt crucible. But the feasibility of growing a-GaPO4
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