Jonathan Bulled | Searcharxiv

arXiv2026

X-ray Thermal diffuse scattering from real-space displacement correlations

We introduce a method for calculating X-ray thermal diffuse scattering based on the three-dimensional difference pair distribution function (3D-dPDF). Within the harmonic approximation the method is exact, as it includes all orders of multi-phonon scattering. A single Fourier transform delivers the diffuse intensity over large volumes of reciprocal space. We tested the method against experimental diffuse scattering measured on a silicon single crystal. With nothing refined beyond a scale and a background, the calculation reproduces the measured intensity to an R2 residual below 5%. Because the method uses the same pair-correlation language as the established Yell program, thermal and static correlated disorder enter the analysis on equal footing.

cond-mat.mtrl-sci↗

arXiv2024

Negative thermal expansion in transition-metal dicyanides: the hidden role of the underlying diamondoid framework

The transition-metal dicyanides M(CN)$_2$ (M = Zn, Cd) are amongst the most important negative thermal expansion (NTE) materials known, favoured for the magnitude, isotropy, and thermal persistence of the NTE behaviour they show. The conventional picture of the NTE mechanism in this family is one of correlated rotations and translations of M(C/N)$_4$ polyhedra acting to draw the diamondoid network of M--CN--M linkages in on itself. An implication of this mechanism is increased transverse vibrational motion of C and N atoms relative to the isotropic displacements of M atoms, which act as anchors. Here, we use a combination of neutron total scattering measurements and \emph{ab initio} calculations to reassess the vibrational behaviour of the M(CN)$_2$ family. We find that M, C, and N atoms all exhibit similar degrees of local thermal motion, such that the cyanide linkages behave as pseudo-springs connecting M$\ldots$M pairs. This interpretation leads us to uncover a `hidden' dispersion in the M(CN)$_2$ phonon dispersions, closely related to that of diamond and silicon themselves. By virtue of this mapping, a simple geometric model based on the competing energy scales of network stretching and flexing -- long applied to interpret NTE modes in C and Si -- turns out to capture the key NTE physics of M(CN)$_2$, especially at low temperatures. Our study highlights the potential insight gained by coarse-graining the complex lattice dynamics of framework materials in terms of what we call `framework modes' -- the correlated distortions of the underlying network structure itself.

cond-mat.mtrl-sci↗