Magnetically induced lattice distortions in actinide compounds

The lattice parameters of a number of actinide compounds have been measured at low temperature by x-ray diffraction. The ferromagnets NpN and NpC exhibit rhombohedral distortions at $T_C$. The antiferromagnets NpP and NpAs exhibit tetragonal distortions, which are compatible with the magnetic symmetry. In NpP the lattice distorts when the magnetic structure becomes commensurate with the atomic lattice. In NpAs the tetragonal distortion occurs at $Y_N$, at which temperature the magnetic structure is a commensurate 4+, 4- arrangement. However, the magnetic structure of NpAs transforms to the simple type-I (+ -) arrangement at 142°K, and the x-ray experiments indicate that the lattice becomes cubic again at this temperature. X-ray experiments on the antiferromagnetic compounds UAs, NpSb, NpS, U${\mathrm{O}}_2$, and Np${\mathrm{O}}_2$ have failed to detect any lattice distortions, $\left|\frac{\mathrm{}(c-a)\mathrm{}}{a}\right|\le 3\times {10}^{-4\mathrm{}}$, in the magnetically ordered state. By listing all compounds examined, the presence of cubic or lower symmetry below the ordering temperature may be correlated with exchange interactions in the magnetic structures.