Indexing magnetic structures and crystallographic distortions from powder diffraction data: commensurate and incommensurate propagation vectors

摘要: A considerable challenge is faced by researchers wishing to identify the propagation vector (s) associated with a magnetic structure or a lattice distortion from powder diffraction data, due to the severe destruction of information by powder averaging. Part of this difficulty arises from a decoupling of this procedure from the physical nature of the processes that drive the phase transition: indexing is often carried out by first extracting the peak positions and then calculating predicted peak positions for simple cells that are larger and commensurate with that of the crystal cell before the transition. Other methods use the formalism of a propagation vector, k vector, to enable the exploration of both commensurate and incommensurate trial structures, perhaps following a grid search through the possible k values. Non-linear search procedures have also been introduced that allow the k vector to be refined when in a region of …