Classical behavior of the index of refraction in two ferroelectrics in the high-temperature phase

We report very accurate measurements of the optic index of refraction as a function of temperature n(T) of two ferroelectric materials PbTiO3 and Pb5Ge3O11. In sharp contrast to results in BaTiO3, n(T) is linear in the high-temperature centrosymmetric phases. The implications are discussed. © 1982 The American Physical Society.