Barium titanate single crystals doped with KF (Ba_1-xK_xTiO_3-xF_x with x≤0.1) have been successfully grown; the ferroelectric Curie temperature T_C decreases with increasing x. Moreover, the crystal with x=0.1 has a large dielectric constant of ε'=1200 and has a large piezoelectric constant of d₃₃=300pC N^-1 at room temperature. On the other hand, in barium dititanate (BaTi₂O₅) single crystals, a spontaneous polarization of 7μC cm^-2 has been observed at room temperature. The ε' measured at 1MHz reaches very large value of approximately 30000 at the ferroelectric Curie temperature of T_C=750K, while the dielectric loss tanδ remains small at values below 0.1 at around T_C. dielectric, thermal and structural properties of the single crystals are described in detail.