Magnetic Susceptibility of (U1−cThc)O2

The magnetic susceptibility χ of ${\mathrm{UO}}_2$ diluted by its diamagnetic isomorph, ${\mathrm{ThO}}_2$ , is reported for $\text{1.7°\hspace{0.17em}≤\hspace{0.17em} T\hspace{0.17em}≤\hspace{0.17em}60°}\mathrm{K}$ . In dilute samples, 5%–10% ${\mathrm{UO}}_2$ in ${\mathrm{ThO}}_2$ , χ⁻¹ decreases monotonically with decreasing temperature. Blume's singlet ground‐state mechanism for the first‐order magnetic phase transition in ${\mathrm{UO}}_2$ requires a ${\Gamma }_1$ singlet ground state 50°K below a ${\Gamma }_5$ triplet. The crystal‐field structure of U⁴⁺ in ${\mathrm{ThO}}_2$ should be similar and should lead to a rise in χ⁻¹ at low temperature as the triplet depopulates, in striking contrast with observation. Concentrated samples with 0%–40% ${\mathrm{ThO}}_2$ in ${\mathrm{UO}}_2$ show a linear dependence of $T_{\mathrm{N}}$ on concentration, with a critical concentration of 42% ${\mathrm{ThO}}_2$ . The singlet ground‐state mechanism in the molecular field approximation predicts that $T_{\mathrm{N}}$ is sharply nonlinear with dilution of the molecular field and falls steeply to zero when diluted by 14%. It is apparent from these two lines of evidence that the static ground state in ${\mathrm{UO}}_2$ is not a singlet. The first‐order transition in ${\mathrm{UO}}_2$ is probably related to a dynamic lattice interaction which opposes ordering down to 30°K where a cooperative lattice and magnetic transition takes place.