Dust ion acoustic solitary structures have been investigated in an unmagnetized nonthermal plasma consisting of negatively charged dust grains, adiabatic positive ions and nonthermal electrons. For isothermal electrons, the present plasma system does not support any double layer solution, whereas for nonthermal electrons, negative potential double layer starts to occur whenever the nonthermal parameter exceeds a critical value. However this double layer solution is unable to restrict the

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... nce of all negative potential solitary waves of the present system. As a result, two different types of negative potential solitary waves have been observed, in which occurrence of first type of solitary wave is restricted by M_cM_D, where M_c is the lower bound of Mach number M and M_D(>M_c) is the Mach number corresponding to a negative potential double layer. A finite jump between the amplitudes of negative potential of solitary waves at M=M_D-\epsilon_1 and at M=M_D+\epsilon_2 has been observed, where 0<\epsilon_10. An analytical theory for the existence of double layer has been presented. A numerical scheme has also been provided to find the value of Mach number at which double layer solution exists and also the amplitude of that double layer. Although the occurrence of coexistence of solitary structures of both polarities is restricted by M_cM_max, where M_max is the upper bound of M for the existence of positive potential solitary waves only. Qualitatively different compositional parameter spaces showing the nature of existing solitary structures of the energy integral have been found. These solution spaces are capable of producing new results and physical ideas for the formation of solitary structures.