  Symmetry-adapted distortion modes provide a natural way to describe distorted
structures derived from higher-symmetry parent phases. Structural refinements
using symmetry-mode amplitudes as fit variables have been used for at least 10
years in Rietveld refinements of the average crystal structure from diffraction
data; more recently, this approach has also been used for investigations of the
local structure using real-space pair distribution function (PDF) data. Here,
we further demonstrate the value of performing symmetry-mode fits to PDF data
through the successful application of this method to two topical materials:
TiSe$_2$, where we detect the subtle but long-range structural distortion
driven by the formation of a charge density wave, and MnTe, where we
characterize a large but highly localized structural distortion in terms of
symmetry-lowering displacements of the Te atoms. The analysis is performed
using fully open-source code within the DiffPy framework using two packages we
developed for this work: isopydistort, which provides a scriptable interface to
the ISODISTORT web application for group theoretical calculations, and
isopytools, which converts the ISODISTORT output into a DiffPy-compatible
format for subsequent fitting and analysis. These developments expand the
potential impact of symmetry-adapted PDF analysis by enabling high throughput
analysis and removing the need for any commercial software.
