  The formation and propagation of small amplitude Heavy-ion-acoustic (HIA)
solitary waves and double layers in an unmagnetized collisionless
multi-component plasma system consisting of superthermal electrons, Boltzmann
distributed light ions, and adiabatic positively charged inertial heavy ions
are theoretically investigated. The reductive perturbation technique is
employed to derive the modified Korteweg-de Vries (mK-dV) and standard Gardner
(SG) equations. The solitary wave (SW) solution of mK-dV and SG equations as
well as Double Layers (DLs) solution of SG equation is studied for analysis of
higher-order nonlinearity. It is found that the plasma system under
consideration supports positive and negative potential Gardner solitons but
only positive potential mK-dV solitons. In addition, it is shown that, the
basic properties of HIA mK-dV and Gardner solitons and DLs (viz. polarity,
amplitude, width, and phase speed) are incomparably influenced by the
adiabaticity effect of heavy ions and the superthermality effect of electrons.
The relevance of the present findings to the system of space plasmas as well as
to the system of researchers interest is specified.
