When Chemical Pressure Fails: Non-ideal Mixing Controls Pressure-induced Transitions in Solid Solutions

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 mixing behavior becomes non-ideal. Using uranothorite (U _x Th _1-x SiO ₄ ), a zircon-structured solid solution series between coffinite (USiO ₄ ) and thorite (ThSiO ₄ ), we establish a limit 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 chemical pressure is inadequate in systems with strongly non-ideal energetics, underscoring the need for full thermodynamic treatment when predicting phase stability in chemically complex materials.