3.1 Structural properties
Under normal conditions, double perovskites RE2SnFeO6 (RE=Ca,Ba) crystalizes in a cubic geometry possessing space group notation Fm-3m (225) having stoichiometry formula unit A2(BB’)O6 with distinct four elements (A, B, B and X) in the ratio of 2: 1: 1: 6. The constituents (atoms) occupy their positions at their corresponding different cites has been quoted in Fig. 1 . The position of A larger cation with 12-fold coordination resides at body centered cubic site. The remaining B and B’ atoms are located in the centre of O atoms having crystallographic sites at corner positions forming alternate BO6 and B’O6 octahedral patterns. Further the accuracy of relaxed lattice constants is acquired from well-known Birch Murnaghan’s equation of state [28-29]. From the literature survey and also from the graphical plots as shown in Fig. 2 , it is visualized that the least ground state energy in the ferromagnetic phase (magnetic configuration) is thermodynamically stable. Hence, the stability curve for both the oxide materials displays an essential parameters including lattice constant a0 (Ǻ), volume in (a.u.3), bulk modulus (B), derivative of bulk modulus (B’) and crystal free minimum energy (E0) are presented in Table 1 . Bond lengths exposed in Table 2 interplays a cructial role in understanding the stability as well as tolerance factor of the alloys. Tolerance factor [19] varies from crystal structures and henceforth, it is mentioned, that if the value of t lies in the interval range of 0.9-1.0, means all the atoms are ideally fit at individual positions resulting ideal cubic structure. When t = 0.71-0.9, 12-folded cation is relatively so small that it cannot fits its position resulting orthorhombic or rhombohedral structure. However if t >1, A site cations are oversized to fit their positions resulting hexagonal structures. For t < 0.71, diferent structures are formed.