Structural Phase Transition in CeMnSi under Pressure and Comparative Structural Properties of RMnSi (R = La, Ce, Pr, Nd)

Powder X-ray diffraction experiments under pressure up to ~10 GPa were performed on tetragonal CeFeSi-type RMnSi (R = La, Ce, Pr, Nd). A structural phase transition was observed in CeMnSi at a critical pressure of Pₛ ~ 5.7 GPa. In contrast, LaMnSi, PrMnSi, and NdMnSi do not exhibit any structural transitions within the same pressure range. The lattice parameter ratio $c/a$ of CeMnSi decreases rapidly as pressure approaches Pₛ, whereas the $c/a$ ratios of the other RMnSi increase monotonically with pressure. CeMnSi also shows a relatively small bulk modulus: B₀ ~ 41.4(4) GPa in the 0--2 GPa range and B₀ ~ 32.8(2) GPa in the 4--5 GPa range, suggesting valence instability under pressure. The structural transition in CeMnSi is attributed to the pressure-induced decrease in $c/a$ and its low bulk modulus. Above Pₛ, the X-ray diffraction pattern indicates a transition to a monoclinic structure with space group No. 11, P2₁/m. These findings highlight the unique pressure response of CeMnSi and provide insight into the coupling between lattice and electronic degrees of freedom in Ce-based intermetallic systems.