Comments on Raman Spectra of SrTi¹⁸O₃: Imperfect Softening and Intrinsic Inhomogeneity of the Ferroelectric Phase

A peculiar nature of the ferroelectric phase transition of isotope exchanged SrTi 18 O 3 (STO18) are discussed by comparing three independent Raman studies which have been recently published. The results reported in a paper by the present authors (Phys.Rev. B 74, 174121 (2006)) are quite different from others as follows: (a) Contrary to the report by Takesada et al. (Phys. Rev. Lett. 96 , 227602 (2006)), the perfect softening does not take place. Instead, a quasi-elastic mode appears near T c indicating the order-disorder nature of STO18; (b) Contrary to the report by Taniguchi et al. (Phys.Rev.Lett. 99 , 017602 (2007)), the ferroelectric phase of STO18 is intrinsically inhomogeneous regardless of the isotope concentration. By comparison, it is shown that the conclusions in the latter two papers are not the case. The reason is that in these papers the spectra for only a single phonon propagation direction K p were measured and analyzed without taking into account the consistency to the spectra for other K p 's. Contrary to other ordinary ferroelectric crystals, the transition in STO18 is peculiar because the ferroelectric domains are generated from the polar nano-region (PNR) and its percolation.