Dark sector unifications: dark matter-phantom energy, dark matter --
  constant $w$ dark energy, dark matter-dark energy-dark matter
  The paper brings a novel approach to unification of dark matter and dark
energy in terms of a cosmic fluid. A model is introduced in which the cosmic
fluid speed of sound squared is defined as a function of its equation of state
(EoS) parameter. It is shown how logarithmic part of this function results in
dynamical regimes previously not observed in cosmic fluid models. It is shown
that in a particular dynamical regime the model behaves as a unification of
dark matter and phantom dark energy. Further, it is shown that the model may
describe dark matter - dark energy unification in which dark energy
asymptotically behaves as dark energy with a constant EoS parameter larger than
-1. In a specific parameter regime the unified fluid model also reproduces
global expansion similar to $\Lambda$CDM model with fluid speed of sound
vanishing for small scale factor values and being small, or even vanishing, for
large scale factor values. Finally, it is shown how the model may be
instrumental in describing the cosmic fluid dark matter-dark energy-dark matter
unification. Physical constraints on model parameters yielding such transient
dark energy behavior are obtained.
