Non-ideal mixing controls pressure-induced transitions in solid solutions

The conventional view of chemical pressure, defined as structural distortion induced by chemical substitution, serves as a useful analogue to physical pressure, but its predictive power breaks down when the cation mixing behavior becomes non-ideal. Using uranothorite (UxTh1-xSiO4), a zircon-structured solid solution series between coffinite (USiO4) and thorite (ThSiO4), we establish a limit of the lattice strain-derived view of chemical pressure. High-pressure X-ray diffraction reveals an inversion in the thorite-to-huttonite transition pressure, contrary to trends suggested by their endmembers. A Gibbs energy model built from experimental equations of state shows that this inversion arises from non-ideal mixing, which differentially stabilizes competing polymorphs and disrupts the expected linear relationship between composition and transition pressure. These results demonstrate that evaluating lattice distortions alone is inadequate for predicting phase behavior in systems with strongly non-ideal mixing energetics. This underscores the need for a comprehensive thermodynamic framework integrating in situ high-pressure measurements when predicting phase stability in chemically complex materials. The conventional view of chemical pressure, which is defined as a structural distortion induced by chemical substitution, serves as an analogue to physical pressure, but its predictive power breaks down when the cation mixing behavior becomes non-ideal. Here, the authors use uranothorite (UxTh1-xSiO4), a zircon-structured solid solution series between coffinite (USiO4) and thorite (ThSiO4), to assess the limits of lattice strain-derived chemical pressure, showing that evaluating lattice distortions alone is inadequate for predicting phase behavior in systems with strongly non-ideal mixing energetics.