Titanium off-centre displacement in isotopically substituted SrTiO3

Bulk SrTiO3 undergoes a ferroelectric phase transition in ambient conditions only when oxygen atoms have been replaced with a heavier isotope. Depending on the experimental technique chosen, there are both indications of the transition being of the displacive type, or order-disorder type. It is possible that the Ti atoms in SrTiO3 are displaced off-centre even in the tetragonal phase, but due to the randomness of the displacement direction the average structure (as seen in diffraction experiments) remains tetragonal. We propose a pair distribution function measurement on two powder samples of SrTiO3, one isotopically normal, and the other containing 60% oxygen-18 isotope. By using Polaris we will gain access to values of Q large enough to detect displacement of the magnitude of 0.04-0.05 Angstrom, which is the range predicted by modelling and implied by gamma ray diffraction experiments. The experiment will verify if titanium atoms are randomly displaced in SrTiO3 already above the ferroelectric transition temperature, and if the magnitude of the displacement is affected by the change in oxygen mass.

体相钛酸锶（SrTiO₃）仅在氧原子被较重同位素取代后，才能在常压环境下发生铁电相变。根据所选实验技术的不同，已有研究表明该相变既可属于位移型，也可属于有序-无序型。即便在四方相（tetragonal phase）中，钛酸锶内的钛原子也有可能偏离中心位置；但由于位移方向具有随机性，其平均结构（如衍射实验所观测到的）仍保持四方相结构。本研究拟对两份钛酸锶粉末样品开展对分布函数（pair distribution function）测试：一份为天然同位素组成的样品，另一份则掺入了60%的氧-18同位素。通过使用Polaris衍射仪，我们可获得足够大的Q值范围，以探测0.04~0.05埃量级的位移——这一位移量级恰为建模预测所得，且与伽马射线衍射（gamma ray diffraction）实验的结果相符。本实验将验证：在铁电相变温度以上的温区中，钛酸锶内的钛原子是否发生随机位移，以及位移量级是否会因氧原子质量的改变而受到影响。