  The potential energy surfaces of even-even $^{146-156}$Sm are investigated in
the constrained reflection-asymmetric relativistic mean-field approach with
parameter set PK1. It is shown that the critical-point candidate nucleus
$^{152}$Sm marks the shape/phase transition not only from U(5) to SU(3)
symmetry, but also from the octupole-deformed ground state in $^{150}$Sm to the
quadrupole-deformed ground state in $^{154}$Sm. By including the octupole
degree of freedom, an energy gap near the Fermi surface for single-particle
levels in $^{152}$Sm with $\beta_2 = 0.14 \sim 0.26$ is found, and the
important role of the octupole deformation driving pair $\nu 2f_{7/2}$ and $\nu
1i_{13/2}$ is demonstrated.
