Project supported by the National Natural Science Foundation of China (Grant Nos. 11179030 and 90714011);the Knowledge Innovation Project of the Chinese Academy of Sciences (Grant No. KJCX2-SW-N20)
Nearly all displacive transitions have been considered to be continuous or second order, and the rigid unit mode (RUM) provides a natural candidate for the soft mode. However, in-situ X-ray diffraction and Raman mea...