High pressure phase stability of U₆X(X = Mn, Fe, Co, Ni) compounds

The dependence of the superconducting transition temperature T_c on pressure P has been measured for samples of U₆Fe and U₆Co with residual resistivity ratios RR = 10 and 4, respectively, for 0≤P≤2.0 GPa. Respective initial increases dT_c dP = 0.64 and 0.15 K{plus 45 degree rule}GPa are found for the two materials, the value for U₆Fe being considerably enhanced with respect to those of less pure samples [L.E. De Long et al., Mater. Res. Soc. Symp. Proc. Vol. 22 (North-Holland, Amsterdam, 1983) p. 47]. A pronounced kink in T_c(P) for U₆Fe is observed at P ≈ 0.4 GPa, indicating a second-order phase transition, above which dT_c dP = -0.24 K{plus 45 degree rule}GPa. The anomalous form of T_c(P) and its marked sensitivity to sample purity for both U₆Fe and U₆Co suggest that these materials have Fermi surface anisotropies and/or undergo transitions to charge- or spin-density wave states.