Growth and Characterization of Lanthanum Calcium Oxoborate LaCa 4 O(BO 3 ) 3 Single Crystals Hiroaki TAKEDA  , Hideki SAKO, Hiroyuki SHIMIZU, Kaoru KODAMA, Masahiro NISHIDA 1 , Hiroshi NAKAO 1 , Takashi NISHIDA, Soichiro OKAMURA, Takashi SHIKIDA 1 and Tadashi SHIOSAKI Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan 1 Research Development Division, Sakai Chemical Industry Co., Ltd., 5-1 Ebisujima-cho, Sakai, Osaka 590-0985, Japan (Received June 16, 2003; accepted for publication July 9, 2003) Lanthanum calcium oxoborate LaCa 4 O(BO 3 ) 3 (LaCOB) bulk single crystals were grown by the Czochralski technique. The crystals have almost constant transmittances between 320 and 2500 nm. The relationship between the growth habit and crystallographic abc - rectangular XYZ axes was clarified. Some dielectric and piezoelectric constants as well as elastic compliance constants of the LaCOB crystals were evaluated. The surface acoustic wave (SAW) and pseudo-SAW (PSAW) parameters of the LaCOB crystals were also measured. For the SAW, the largest electromechanical coupling coefficient of approximately 0.26% and a velocity of 3550 m/s were obtained for the YX orientation. For the PSAW, these parameters were approximately 0.6% and 4000 m/s for the ZY orientation. [DOI: 10.1143/JJAP.42.6081] KEYWORDS: borate, crystal growth, transmittance, piezoelectricity, SAW property 1. Introduction Recent progress of electronic technology requires new piezoelectric crystals with the high thermal stability of frequency and large electromechanical coupling coefficients. For designing devices such as filters with a wide pass band while maintaining high stability and small insertion attenu- ation, the necessity has resulted in discovering new piezo- electric crystals with intermediate properties between those of quartz and lithium tantalate (LiTaO 3 ). Lithium tetraborate (Li 2 B 4 O 7 ; abbreviated as LBO), -AlPO 4 , and La 3 Ga 5 SiO 14 (LGS) have been candidates for satisfying those require- ments. 1–3) Actually, LBO and LGS wafers have been mass- produced for use in intermediate frequency-surface acoustic wave (IF-SAW) devices. However, these crystals have some disadvantages in mass production. In LBO, a very low growth rate (<1 mm/h) is needed to produce inclusion-free crystals. 4) In LGS, a large amount of expensive gallium oxide is used as the raw material. Moreover, it was pointed out that LGS melts incongruently and is a peritectic compound. 5) This suggests that more detailed investigations are indispensable for the growth of a bulk single crystal with high quality. Due to above-mentioned situation, we have paid attention to rare-earth calcium oxoborate RCa 4 O(BO 3 ) 3 (R = La- Lu,Y; abbreviated as RCOB). RCOB crystals have been developed, which exhibit excellent nonlinear optical proper- ties and can be grown by the Czochralski technique at a low cost. 6–9) The effective nonlinear coupling coefficient, d eff ¼ 1:1 pm/V, of YCa 4 O(BO 3 ) 3 (YCOB) is larger than those of KH 2 PO 4 and LBO (d eff ¼ 0:38 and 0.68 pm/V, respectively) in which bulk crystals can be easily grown. 10) This report has led to the study of RCOB crystals as piezoelectric materials. To date, the surface acoustic wave (SAW) properties of YCOB and GdCa 4 O(BO 3 ) 3 (GdCOB) crystals have been reported. 11,12) The dielectric, piezoelec- tric, and elastic compliance constants of GdCOB crystals have been determined. 13) Although there is a report concerning the optical properties of LaCa 4 O(BO 3 ) 3 (La- COB) single crystals, 10) to the best of our knowledge, none has described their piezoelectric properties. In this study, we demonstrate the successful growth of large LaCOB single crystals by the Czochralski (Cz) technique, present their optical and piezoelectric properties, and determine the relationship between the growth habit and the crystallographic abc - rectangular XYZ orientation of the crystals. The SAW properties of the LaCOB crystals are also described. 2. Experimental 2.1 Crystal growth and characterization of grown crystals Single crystals were grown by a conventional RF-heating Cz technique using an iridium crucible (50 mm in diameter and height). Starting materials were prepared by mixing of 99.99% pure La 2 O 3 , CaCO 3 and B 2 O 3 powders in a stoichiometric ratio. The powders were mixed under air atmosphere, and then calcined at 1000  C for 2 h. The calcined powders were heated at 1200  C for 2 h. The LaCOB powders were pressed uniaxially in a desk form of 48 mm diameter and 30 mm thickness, and were charged into the crucible. The growth atmosphere was an Ar gas flow rate of 10 3 m 3 /min. The pulling rate and the rotation rate were 2.0–3.0 mm/h and 20 rpm, respectively. The observa- tion of bubbles and inclusions in crystals was performed using an optical microscope. The phase identification of the as-grown crystals was carried out by powder X-ray diffraction (XRD) method. The density of the grown crystals was measured by the Archimedes method using distilled water at room temperature. The chemical composition was determined by inductively coupled plasma emission spec- trometry (ICP-ES) analysis. The melting point of LaCOB was determined by thermogravity differential thermal analysis (TG-DTA). To characterize the optical properties, the grown crystals were cut perpendicularly to the h010i growth axis. The sample was formed into a disk with a thickness of 3.7 mm, and was polished carefully. No antireflection coating was applied onto the sample surfaces. The transmission spectra were measured in a wavelength range of 190–2500 nm using a spectrophotometer. 2.2 Electric and elastic measurement LaCOB belongs to the monoclinic space group Cm. Its dielectric, piezoelectric, and elastic compliance constants  E-mail address: hiro-t@ms.aist-nara.ac.jp Jpn. J. Appl. Phys. Vol. 42 (2003) pp. 6081–6085 Part 1, No. 9B, September 2003 #2003 The Japan Society of Applied Physics 6081