Bulk crystal growth and characterization of non-centrosymmetric single crystal CaTa4O11†

Despite that the reported CaO–Ta₂O₅ phase diagram indicates its incongruent melting behavior, for the first time, we found that bulk single crystals of non-centrosymmetric CaTa₄O₁₁ can be grown successfully by the floating-zone method. The as-grown crystal rods (Φ 5–6 mm) have a black and colorless transparent appearance in Ar and O₂ growth atmospheres, respectively. Consequently, the O₂ atmosphere was selected to optimize its bulk crystal growth. The CaTa₄O₁₁ single crystals show good crystallinity with a symmetric X-ray rocking curve for the 004 diffraction peak with an FWHM of 72′′. For this new hexagonal bulk single crystal, its anisotropic optical, dielectric, and thermal properties were characterized. The crystal has a UV cut-off edge at 272 nm and a visible transmittance of ∼75% till 800 nm. When excited by both 254 nm UV and X-ray, it shows a broad-band emission with a peak wavelength of around 427 nm. The dielectric constants ε₁₁ and ε₃₃ were found to be 30 and 52.3, respectively at 1 kHz. Meanwhile, the thermal conductivity along the c-axis is 5.69 W m⁻¹ K⁻¹, which is 1.8 times higher than that along the a-axis, which measures 3.19 W m⁻¹ K⁻¹ at room temperature.