High-pressure phase transitions in $\mathrm{UP_{1-x} S_x}$

High-pressure X-ray diffraction using synchrotron radiation has been performed on UP1-x -Sx (X=0.1; 0.25; 0.4) up to 53 GPa UP1-x Sx is a solid solution with a B1 (NaCl) structure. For all compositions a second order phase transition is observed around 10 GPa to a distorted B1 structure of rhombohedral symmetry. For UP1-x Sx with x 0.25 a second phase transition is observed, which takes place in the region of 35 GPa This phase transition occurs when the nearest U-U distance reaches the Hill limit of 330–340 pm. The high-pressure phase seems to have orthorhombic or even monoclinic symmetry. It has some similarities to the high pressure phase of UP. UP1-x Sx 4 shows only weak indications for an additional phase at 53 GPa. In conclusion, we observe that the second phase transition and the bulk modulus B, in UP shift to higher pressure, when phosphorus is replaced by sulfur