Accordingly, these mean the increase of the grain boundaries defect and intergranular pores concentration with the increasing of the ionic radius of substituted ions. The slight increase in I2 for V sample relative to Nb ones could attribute to the smaller ionic radius of V5+ than that for Fe3+, r(V5+) = 0.59 Å and r(Fe3+) = 0.63 Å. Furthermore, V sample has smaller (G.S.) than that for Nb one. Dong et al. [20] reported that when the grain size is decreased the thickness of the grain boundaries is increased. The relative intensity I1 might decrease for V-sample relative to Nb-one. Meanwhile, an opposite (reverse) behavior appears for τ2 values. This behavior of τ2 indicates that the small ionic radii of substituted ions (same concentration) permit the positron to reside at defect sites with longer life time. The exception for Sm-sample might be due to the formation of SmO2 during the preparation condition. The dissociation of SmO2 leads to the formation of vacancies inside the grains [21], this causes the concentration of Sm ions to be less than 0.02 [19] and [22]. These accounts on the decrease of I2 for Sm sample relative to the Gd one. According to this conclusion, I1 value for Sm sample might be affected by the formation of these defects inside the grains. To account on this point, one might take into consideration the experimental variation of total porosity Pt and I2 with the ionic radii of the substituted ions ( Fig. 2). From the experimental results, there is a correlation between the total percentage of porosity Pt and the relative intensity I2% as a function of the ionic radii of the substituted ions (magnetic or non magnetic ions). There is an increase of porosity with the increase of the ionic radii, but decreases for Sm sample due to the decrease in the concentration of Sm ions inside the bulk sample. This could be one factor for the decrease of I2 for Sm sample ( Fig. 1), and consequently the decrease of intergranular pores ( Fig. 2). Since, I1 mostly decreases with increasing the ionic radii of the substituted ions except for Sm sample ( Fig. 3). Then one can conclude that I1 mostly results from the intragranular pores and dominates the crystallographic defects inside the bulk sample. However, I1 mostly decreases with increasing the ionic radii of the substituted ions except for Sm sample ( Fig. 3). Then one can conclude that I1 mostly results from the intragranular pores and dominates the crystallographic defects inside the bulk sample. Meanwhile, as the concentration of I1 decreases the lifetime τ1 of positron to reside at intragranular pores is decreased ( Fig. 3). This behavior mainly attributes to the increase in the hopping of the electrons with decreasing the intragranular pores in case of ferrites. This decreases the lifetime of the positron inside the grains. Furthermore, Gd and Sm ions have large ionic radii and prefer to reside at octahedral sites, B-sites for Gd and Sm samples [15] and [23]. This leads to decrease the hopping of the electrons at B-sites of the ferrite samples which causes to increase τ1. This permits the positron to reside at defects inside the bulk with longer lifetime for these samples relative to the other samples. Accordingly, there is a correlation between the lifetime of the positrons and the hopping of the electrons for substituted Li-ferrite.