Off-centre displacement

A key issue in the realm of impurities experiencing off-centre displacements is the availability of experimental data which prove unambiguously the existence of such a distortion. To achieve this goal a good advance comes out when selective tools (silent for the pure host lattice) can be used a fact which allows one to explore... 

Displacement with reference to a centre an off-centre condition of, say, a cable conductor in relation to its rubber covering, a section of tubing thicker on one side than the other, or a lateral displacement of the two halves of a circular mould. See Off Centre. 

As it was pointed out in the Introduction, the problem of the coexistence of displacive and order-disorder phenomena at the ferroelectric phase transitions of BaTiOs has met growing interest in recent time. Strong support of the order-disorder model comes 30 years ago from EPR measurements performed on Mn" " "-, Cr -, and Fe -doped BaTiOs [218-222] because in the low-temperature rhombohedral phase it was observed that Mn" " ", which substitutes isovalent Ti" " " sites, is displaced off-centre by 0.14 A along <111> directions with a reorientational hopping with correlation times 10 -10 s. 

The fundamental cause of ferroelectricity in oxide perovskites was originally attributed to the idea that small B-site cations could rattle around inside BO, octahedra and off-centre configurations lent stability to the structure. The displaced B-site cation created an electric dipole within each octahedron. The direction of the dipole, or, equivalently, the position of the cation, could be switched from one off-centre position to another under the influence of an applied electric field. 

Tinkler et al [41] showed that when welding a 30 ppm sulphur (LS) plate to a 90 ppm sulphur (HS) plate, the resulting weld was off-centre and displaced towards the LS side. This can be accounted for if it is assumed that Marangoni forces dominate the fluid flow in the weld pool. It can be seen from Figure 21 that the thermocapillary forces in the LS and HS will be from left to right and the diffusocapillary forces will also operate from left to right. Thus these surface flows will cause hot metal to be carried to the LS side and melt back off the steel will result in an asymmetric weld. 

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