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Floating-zone growth and physical properties of CaxBa1-xNb2O6 single crystals
|Yunfeng Ma 1, Yijian Jiang 1, Yue Wang 2, Guoqing Liu 2|
1. Beijing University of Technology, Institute of Laser Engineering, Beijing 100124, China
In this presentation, the growth of single crystals of calcium barium niobate (CaxBa1-xNb2O6, 0.22≤x≤0.37) with using the optical floating zone method was reported. The relationship between crystal quality and growth condition was studied and the optimum technological condition was obtained. At a growth rate of 1-5 mm/h and a rotation rate of 20-40 min-1, the grown crystals with 6-8 mm in diameter and 70-115 mm in length exhibit pale yellow and transparent after annealing in the O2 , showed no macroscopic defects such as second phase precipitates and cellular growth. The compositional profile in the rod-like crystals was measured by electron probe micro-analyzer (EPMA). In the case of sample CBN-35.1, the radial deviation of composition could be neglected. However, the composition x at the upper, middle and end region were 0.337, 0.351, 0.366 respectively. The result may be due to the changing composition in the melting zone and Segregation effect. X-ray powder diffraction experiment indicated that only the c0 lattice parameter offers a clearly marked dependence from the composition while the a0 lattice constant is nearly constant. We can explain it on the basis of the general crystal structure of the TTB. Which also provide for CBN crystals with excellent piezoelectric, dielectric, ferroelectric, electro-optic and photorefractive properties. In addition, X-ray densities calculated from lattice parameter and actual densities measured by buoyancy method agree very well, they all decrease linearly with increasing Ca proportion x from 0.224 to 0.366. The rocking curves on (001) and (100) crystal planes showed that the symmetry of the diffraction peak is very well. The full-width at half-maximum (FWHM) value was measured to be approximately 0.1°. Which indicate that the as-grown single crystals have medium crystalline quality. The flake XRD patterns of the CBN-24.6, 32.5, 35.1, 36.6 showed that the direction of crystal growth is along c-axis. The transmittance spectra of Z-cut plates were recorded on a UV-3600 spectrophotometer at room temperature in the range of 190-2500 nm, the cut-off wavelength is 361-370 nm depending on the actual composition, which was shorter than that of SBN crystal (400 nm). CBN crystals have high optical transmittance above 80% for wavelength above 500 nm and wide transparency range covering visible and near-infrared lights. Micro-Raman spectrum was measured at room temperature for X-cut plates of CBN crystals. We can see that with x increased, two Raman characteristic peaks ν2, ν5 broaden and move to the low wave number direction, the FWHM of Raman peak is related to the NbO6 octahedral distortion and the relative intensity of v5 peak relative to v2 peak increase gradually. That is because the degree of disorder increases in the crystal cell structure. In order to characterize the vibration modes, we measured the Raman spectrum for CBN-22.4 crystal in different polarizations. The result of the single crystal X-ray structure analysis showed no miscellaneous points in the projection map of lattice structure along , ,-orientation, indicate good microcosmic crystalline quality. At room temperature and 1MHz, with increasing crystal composition x from 0.224 to 0.366, ε33 and ε11 increase linearly up to 468.1 and 342.0 respectively. The measured dependence of dielectric constants on temperature showed the Curie temperature of the CBN-22.4 crystal has maximum 347 oC, which is about 270 oC higher than SBN-60, as a main advantage, makes CBN potentially attractive for applications to high temperature devices. There is a clear linear correlation between actual composition x and the values of piezoelectric constants d33, d15, d31. The optimum values d33, d31 of CBN-36.6 and d15 of CBN-22.4 were 90.0, -54.7, 89.2 p C/N respectively. Typical P-E hysteresis loops were measured at room temperature and 1 Hz for (0 0 1) surface of CBN crystals, the remnant polarization of CBN-28 was 31.7 μC/cm2, which was a little smaller than the first-best result of 32.2 μC/cm2, but the coercive field (Ec) was 16.9 kV/cm, better than informational 38.1 kV/cm. The spontaneous polarization, remnant polarization and coercive field of CBN-36.6 crystal were 27.4μC/cm2, 25.5μC/cm2 and 14.4 kV/cm, and Ec decreased with increasing actual composition x. In conclusion, the mechanism of electrical property modulation by actual composition x for CBN crystals can be investigated on the basis of the composition-structure-property relationship.
Presentation: Oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, Topical Session 6, by Yunfeng Ma
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17
Submitted: 2013-03-31 05:29 Revised: 2013-07-18 20:25