AMPLIMODES: symmetry-mode analysis on the Bilbao Crystallographic Server

AMPLIMODES is a computer program available on the Bilbao Crystallographic Server that can perform a symmetry‐mode analysis of any distorted structure of displacive type. The analysis consists in decomposing the symmetry‐breaking distortion present in the distorted structure into contributions from different symmetry‐adapted modes. Given the high‐ and the low‐symmetry structures, AMPLIMODES determines the atomic displacements that relate them, defines a basis of symmetry‐adapted modes, and calculates the amplitudes and polarization vectors of the distortion modes of different symmetry frozen in the structure. The program uses a mode parameterization that is as close as possible to the crystallographic conventions, expressing all quantities for the asymmetric unit of the low‐symmetry structure. Distorted structures are often related to their higher‐symmetry counterparts by temperature‐ and/or pressure‐driven phase transitions, ferroic phase transitions being a particular example. The automatic symmetry‐mode analysis performed by AMPLIMODES can be very useful for establishing the driving mechanisms of such structural phase transitions or the fundamental instabilities at the origin of the distorted phases.