Correlation between positive temperature coefficient of resistance, double Schottky barrier, and anionic substituents in BiFeO3

Various types of conduction (leakage current) mechanisms in metal oxides deteriorate the system's electrical response. The existence of impurity phases, the formation of oxygen vacancies, and high porosity are the main source of free charge carriers in the system. Acceptor states are created due to the formation of oxygen vacancies in the system. In BiFeO3, acceptor states are formed within the grains and at the grain boundaries. The formation of double Schottky barriers at the grain boundaries is interpreted to be associated with the formation of such acceptor states formed at the grain boundary. These acceptor states are the consequence of oxygen stoichiometry which is also responsible for the appearance of a positive temperature coefficient of resistance (PTCR). The effect of substituting isovalent and aliovalent dopants in BFO has been comprehensively analyzed in the present study. These substitutions are responsible for forming a double Schottky barrier at the grain boundary. Barrier potential and barrier width are calculated and plotted as a function of the substituent (di, tri, or tetravalent) to highlight the role of the substituent's valence in the formation of a double Schottky barrier at the grain boundary and the appearance of the PTCR effect in the resistivity profile. The present study reveals that the occupation of the acceptor states at the grain boundaries affects the inter-grain conductivity.