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Single crystals of the ferroelectric BaTi 2 O 5 and BaTiO 3 were prepared from a solution of 33-mol% BaO and 67-mol% TiO 2 by a rapid cooling method. The dielectric constant (ε ) and dielectric loss tangent (tanδ) were measured in a wide temperature range of 10-860 K and in a frequency range of 0.1-3,000 kHz. The ε along the b-axis of the BaTi 2 O 5 crystal, prepared in air, shows a sharp dielectric anomaly reaching 30,000 at the ferroelectric Curie temperature of T C = 752 K. By contrast, the crystal prepared in a reducing atmosphere shows a diffuse phase transition near T C = 703 K. The values of ε and tanδ are compared between these three crystals consisting of two kinds of BaTi 2 O 5 and one BaTiO 3 .

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Ferroelectric single crystals of BaTi 2 O 5 have been studied in detail by Raman and high-resolution micro-Brillouin scattering with a focus on the central component of the relaxation mode over a wide temperature range ͑20-600°C͒. A relaxation mode has been clearly observed in these single crystals. The temperature dependences of the relaxation time and integrated intensity of the relaxation mode are well reproduced by the extended semiclassical tunneling model. The ferroelectric phase transition shows, at least near the vicinity of T c , an order-disorder nature. Our results indicate that the spontaneous polarization mainly resulted from the dynamic disorder of the Ti ions in the octahedra along the polar b axis of BaTi 2 O 5 .