Tuneable Correlated Disorder and Disorder-Phonon Coupling in the pseudo-bcc Uranium Molybdenum System γ-(U_1-xMo_x)

Understanding the role of disorder, and the correlations that exist within it, is one of the defining challenges in contemporary materials science. However, there are few material systems, devoid of other complex interactions, that can be used to systematically study the effects of correlated disorder arising from crystallographic conflict. The pseudo-<i>bcc</i> uranium molybdenum system is however an exemplar case study and as such we fabricated thin (∼300 nm) epitaxial films of γ-(U_1-xMo_x) alloys in the range 0.16 &lt; x &lt; 0.31 to be studied at the ID28 beamline (ESRF, France).<br/><br/>We established, via extensive diffuse x-ray scattering studies [1], that the intrinsic symmetry conflict; where uranium, which prefers a locally anisotropic environment, is forced into an isotropic <i>bcc</i> global symmetry, produces a new form of correlated disorder where every atom is displaced to form a short-range superstructure with correlations existing over nanometre sized regions. Furthermore, we showed that the strength of this unusual form of disorder displays both intrinsic and extrinsic tuneability, via alloy composition and heavy ion irradiation, respectively.<br/><br/>Given the correlated disorder serves to modulate the local crystallographic periodicity, this gives rise to the possibility of a form of disorder-phonon coupling. To investigate this possibility one alloy composition was measured with grazing incidence inelastic x-ray scattering (GI-IXS) and the results compared with extensive <i>ab-initio</i> modelling [2]. We discovered strong disorder-phonon coupling that relaxes degeneracy conditions at the <b>P</b> position, hardens the LA-2/3〈111〉mode ubiquitous to monotonic <i>bcc</i> crystals and produces significant phonon linewidth broadening at almost all positions in the 1^st Brillion zone [1]. This broadening is almost entirely due to the presence of correlations, and results in a strong reduction of the phononic contribution to thermal conductivity.<br/><br/>Within the conventional setting of advanced nuclear fuels, a proper understanding of the local structure of γ-(U_1-xMo_x) alloys, and its behaviour under irradiation, is of great interest for accurate modelling efforts, especially those concerning diffusion related properties, and may also help shed light on other currently unexplained phenomena such as the apparently contradictory <i>fcc</i> nanobubble lattice that forms from fission gasses. However, more broadly, as similar effects are expected in any system able to host comparable levels of crystallographic conflict we believe this novel form of correlated disorder also constitutes a compelling tool for designing disorder into future functional materials.<br/><br/><b>References</b><br/>[1] D. Chaney <i>et al</i>., Phys. Rev. Materials <b>5</b>, 035004 (2021)<br/>[2] A. Castellano <i>et al</i>., Phys. Rev. B <b>101</b>, 184111 (2020)