  The evolution of shapes and low-energy shape coexistence is analyzed in
neutron-deficient Nd and Sm nuclei, using a five-dimensional quadrupole
collective Hamiltonian (5DCH). Deformation energy surfaces, calculated with the
relativistic energy density functional PC-PK1 and a separable finite-range
pairing interaction, exhibit a transition from spherical shapes near $N=80$, to
$\gamma-$soft shapes, and to prolate deformed minima in lighter isotopes. The
corresponding 5DCH model calculation, based on the self-consistent mean-field
potentials, reproduces the empirical isotopic trend of characteristic
collective observables, and predicts significantly different deformations for
the first two $0^+$ states in the $N=74$ isotones $^{134}$Nd and $^{136}$Sm. In
addition to bands based on the triaxial $\gamma-$soft ground state, in
excellent agreement with data, the occurrence of a low-energy rotational band
is predicted, built on the prolate deformed ($\beta\sim0.4$) excited state
$0^+_2$.
