In this paper, we develop a nonadiabatic collective model for even–even nuclei with effective triaxial quadrupole and octupole deformations. The model describes three-dimensional quadrupole–octupole rotation coupled with axial quadrupole–octupole vibrations. The moment-of-inertia tensor depends on the angular part of the polar coordinates in the quadrupole–octupole space, the values of which are determined from the ground state of the nucleus. In this model framework, the collective states of the yrast-alternating-parity band in the lanthanides [Formula: see text]Sm, [Formula: see text]Gd, [Formula: see text]Dy and the actinides [Formula: see text]Th, [Formula: see text]U, [Formula: see text]Pu are studied.

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