Modeling Thermal Diffuse Scattering Using Joint Atomic Displacement Parameters in YELL

Phonons play a role in various phenomena, from superconductivity through phonon–electron coupling and spintronics via phonon–spin interactions, to the dynamical stability of solids, and are fundamental to elastic properties. They can be probed by measuring Thermal Diffuse Scattering (TDS) from single crystals. However, modelling of the TDS is a computational challenge due to the number of intensities in the experiment. With existing software such as AB2TDS, the full experiment can be calculated only in one- or two-phonon approximations. Approximations of higher order are possible, but they are computationally expensive and can be performed only on small portions of reciprocal space.

In this work, we propose a new method for modelling and fitting TDS signals using joint atomic displacement parameters in YELL. This approach uses the crystal’s dynamical matrix as input, which can be derived by various methods including universal potentials, DFT, or approximations from elastic constants. By using a Fast Fourier Transform, our method can efficiently calculate large volumes of TDS in the infinite-phonon approximation.

This development will enhance the 3D-DeltaPDF suite, enabling extraction of elastic constants from various materials and extending to the analysis of high-amplitude soft phonons, which are relevant in negative thermal expansion materials like ScF3. The software’s capability to handle higher-order phonon contributions makes it particularly valuable for systems where these effects are significant, addressing a current gap in available tools.