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.