Birefringence study of ferroelectric ordering in oxygen isotope exchanged systems SrTi(16O1-x18Ox)3

An optical birefringence study was performed on the quantum ferroelectric phase transition in the oxygen isotope exchanged system SrTi (O-16(1-x) O-18(x))(3). The ferroelectric ordering in (110)(c) plates with the substitutional ratio x > 0.33 brought about a marked enhancement in the temperature dependence of birefringence. The temperature derivative of the birefringence had a singularity at a temperature which can be regarded as the ordering temperature T-c. The antiferrodistortive phase transition temperature T-a of a (110)(c) sample with x = 0.86 reached 107.7 K. The (001)-plane birefringence of a (110)(c) sample with x = 0.83 exhibited a slight, but clear change at T-c.