Two-dimensional Generator Coordinate Method calculations for the axial quadrupole ${\beta }_2$ and hexadecapole ${\beta }_4$ collective deformations are carried out with the Gogny force in a series of Xe, Ba, Ce, and Nd isotopes with neutron numbers covering both magic neutron shell closures N = 82 and N = 126. The underlying mean-field configurations are used to characterize the expected dynamic behavior of the system. Two regions of strong coupling between the quadrupole and hexadecapole degrees of freedom are found and characterized. Quantum fluctuations soften the mean-field ground state values of ${\beta }_2$ and ${\beta }_4$ in transitional regions. The ground state correlation energy coming from ${\beta }_4$ is comparable in size to the one coming from ${\beta }_2$, and both together amount to a sizable 1.5 MeV. Shape coexistence in some isotopes and its impact on the excitation energy of the first excited state are analyzed. Finally, the role of a second intruder orbital in the explanation of the large deformation parameters of some nuclei in the region is discussed.

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• Nuclear shapes and moments
• Nuclear structure & decays
• 150 ≤ A ≤ 189
• Nuclear many-body theory