What drives anion order in TiOF?

FeOF and TiOF both adopt the rutile structure, and exhibit anion ordering within 2D {110} planes. This anion order gives rise to diffuse scattering, caused by covalency-driven displacements of cations (M = Fe, Ti) within fac-MO3F3 octahedra. Surprisingly, the anion order is different in each case. Recent theoretical predictions have suggested that the TiOF ordering is caused by Ti-Ti (d1-d1) dimerization, but this would have to persist to extremely high temperatures. The aim of this experiment is therefore to investigate the Ti-Ti distance at high temperatures to (dis)prove this hypothesis.

FeOF与TiOF均采用金红石（rutile）结构，且二者均在二维{110}晶面内呈现阴离子有序化现象。该阴离子有序化会引发漫散射，其成因是阳离子（M=Fe、Ti）在面式fac-MO3F3八面体中受共价键作用发生位移。值得注意的是，二者的阴离子有序化模式并不相同。近期的理论预测提出，TiOF的有序化源于Ti-Ti（d¹-d¹）二聚作用，但该作用需在极端高温条件下才能持续存在。因此本实验旨在通过测定高温下的Ti-Ti键长，对上述假说进行证实或证伪。