Langasite-type single crystals have been energetically investigated as a piezoelectric material due to the high piezoelectric constant and electromechanical coupling factor at high temperature.  Therefore, the langasite-type crystals have been expected to be applied for various sensor devices at high temperature such as the combustion pressure sensor and temperature sensor.  Recently, rare-earth (RE) free langasite-type crystals such as Ca₃TaGa₃Si₂O₁₄ (CTGS) and Ca₃NbGa₃Si₂O₁₄ (CNGS) have been developed and they are playing the role of an engine for recent researches of langasite-type crystals because their electrical resistivity at high temperature is higher than the langasite-type containing RE such as La₃Ta_0.5Ga_5.5O₁₄ and La₃Ga₅SiO₁₄.  In addition, the manufacturing cost would be decreased due to the change from crystals with RE ion (La ion) to RE free crystal.  However, the RE free langasite-type crystals still contain the Ga ion which is relatively high cost of starting materials and the manufacturing cost of langasite-type crystals are more than ten times higher than quartz element.  If the mount of Ga ion in the crystal would be decreased, the manufacturing cost would be decrease.  In the previous reports about the langasite-type containing RE ion, the 20% Ga site in the crystal could be substituted by the Al ion.  In this study, we grew the Al doped CTGS single crystals with various Al concentrations to decrease the amount of Ga ion in the crystals and their physical properties were investigated.
  Al doped Ca₃TaGa₃Si₂O₁₄ crystals were grown by the μ-PD method using the Pt-Rh crucible as nominal compositions of Ca₃Ta(Ga_1-xAl_x)₃Si₂O₁₄ [CTGAS] x = 0, 0.2, 0.4, 0.6, 0.8 and 1.  The mixed powders were prepared using CaCO₃, Ta₂O₅, Ga₂O₃, Al₂O₃, SiO₂ and the mixed powders were sintered at 1200°C 12 hours in air several times.  The sintered powder was set in the crucible and the crucible was put in the center of the high-frequency induction coil.  The crucible was heated in air up to the melting point of by the CTGAS by the coil and the melt was pulled down by the seed crystal with a-axis along to the growth direction.  During the crystal growth, the liquid-solid interface was observed by the CCD camera.  The structural phases of grown crystals were identified by the powder X-ray diffraction (XRD) measurement.  The plate crystals were cut from the grown crystals and they were polished for the measurements of crystallinity, chemical composition, local observation, piezoelectric constant and electrical resistivity.
   The Al20% doped CTGS crystal was grown by the μ-PD method and the liquid-solid interface during crystal growth is shown in Fig.1(a).  The meniscus was limited by the plate-shaped die of crucible and plate-shaped Al doped CTGS crystal was obtained (Fig.1(b)).  The obtained crystal had high transparency and there is no visible cracks in the crystal. 
  The XRD patterns of undoped and Al20% doped CTGS crystals are indicated in Fig.2.  The results showed that both undoped and Al20% doped CTGS crystals had a single phase of langasite-type structure (trigonal, P321).  The lattice parameters were calculated from the XRD patterns and the a- and c-axes of Al20% doepd crystal were smaller than those of undoped crystal (Fig.2).  The decrease of lattice parameters suggests the Ga sites in CTGS crystal were substituted by the Al ions with smaller ionic radius than Ga ion.  The crystal growth, actual chemical compositions and other physical properties of Al doped CTGS crystals with various Al concentrations will be reported.

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