Double perovskite oxides, A₂BB'O₆ or AA'BB'O₆, where A and A' are alkaline―earth and/or rare―earth metals and B and B' are transition metals, have been investigated [1] . These materials are especially interesting in view of the possibility that B and B' cations could be ordered at the octahedral sites, giving rise to novel electrical and magnetic properties due to B - O - B' interaction [2] . The study of these oxides was initiated in the 1950s and since then several hundred of these compounds have been produced and studied because they exhibit quite interesting structural, electronic as well as magnetic properties [2] . One can anticipate many more of these types of compounds to be realized given the diversity of the ions that can occupy both the A- and B-sites. The last few years ordered double perovskite oxides have been received an enormous amount of attention because certain members of this family of oxides exhibit spectacular physical phenomena such as colossal magnetoresistance and half metallicity in the ferrimagnetic Sr₂FeMoO₆ [3] and La₂VRuO₆ [4] compounds.

The physical properties of double perovskite DP are generally determined by the cations occupying the B, B' sites. Thus, special attention has been given to the factors that impact the B-sites arrangement, namely the charge, size and electronic configuration of B cations as well as the A/B size ratio. Anderson et al. [2] have concluded that an ordered structure is most likely to be formed if the charge difference between B and B' is two or higher. An exception to this condition is the case of LaCaMnCoO₆ [5] ; this compound was ordered even though the B - B' charge difference is one. The ideal perovskite has a primitive cubic structure with the formula ABO₃, where the B-ions are at the corners of the cubic cell and the A-ions at the cubic octahedral (12 coordinated) site. There is a variety of ordered structures known in perovskite oxides; among them the double perovskite (A₂BB'O₆ or AA'BB'O₆) and the triple perovskite (A₃BB'₂O₉) structures are the most popular.

These ordered structures can be obtained mainly by substituting other cations at their A or B sites of the ABO₃ structure and occur due to the charge difference. Many of these ordered perovskite cations Ti, Fe and Mn as the B-cations, are interesting due to their optimum dielectric characteristics for applications in electronic industry [6] - [8] .

The double perovskite SrAFeTiO₆ samples where A is (La, Pr, Nd) respectively, were prepared.

Most of the chemicals used in the preparation were Alfa Acer of purity 99.9%. The method used for preparation of these double perovskite is in accordance with the well known procedures [9] [10] . The X-ray diffraction and FTIR measurements were carried out for the samples under investigation.

SrLaFeTiO₆, SrPrFeTiO₆ and SrNdFeTiO₆ were synthesized by both conventional and precursor solid state reaction from the compounds SrCO₃, La₂O₃, Fe₂O₃, TiO₂, Pr₇O₁₁ and Nd₂O₃ raw materials with purity more than 99%. The grouped compounds were mixed together and then ground in an agate mortar for 45 minutes at least. After that the compounds were heated into a Carbolite air furnace to 900 degrees of centigrade for 12 hours. Then it was pressed into pellets under 7 MPa pressure for 30 seconds following by preheating in air at 900˚C for 24 hours with intermediate grinding and finally it was calcinated at 1200˚C for 12 hours. Phase analysis and characterization were carried out by X-ray diffraction (XRD) using Cu K_α radiation on both X’Prot Bruker D8 and Schimadzu 6000.

Determination of the crystal structure, the lattice parameters and the space group symmetry become of central importance in the study of structural, electrical and magnetic properties of the double perovskite materials [11] .

The results of the XRD for the samples obtained after conventional solid state method is shown in Figures 1(a)-(c) for the samples SrLaFeTiO₆, SrPrFeTiO₆ and SrNdFeTiO₆. Clearly, the quality of the samples is poor whatever grinding times and/or heating temperatures are committed.

By following the precursor method, good quality samples are obtained as shown in Figure 2.

A selection range is given in Figure 3 in order to show the difference in the peak position for each sample. The shift in the reflection positions indicates differences in the lattice parameters and/or the type of the crystal structure.

X-ray diffraction patterns measured at room temperature for the samples SrAFeTiO₆ (A = La, Pr or Nd) using two different methods and the precursor method gives better pattern than that the conventional method which gives a multi phase (Figures 1(a)-(c)). The best fitting of these compounds could be const as their structure, and the closed ionic radius for all samples illustrated that, they have an orthorhombic structure with a Pnma space group. There are also some differences in the lattice parameter where differ from SrLaFeTiO₆ up to SrNdFeTiO₆ as follows: a = 5.557654 A˚, b = 7.824075 A˚, c = 5.537659 A˚ for La, and a = 5.52728 A˚, b = 7.79671 A˚, c = 5.51395 A˚ for Pr, and a = 5.51584 A˚, b = 7.79255 A˚, c = 5.51391 A˚ for the Nd. The patterns were subjected to careful analysis by means of the standard Reitveld methods using the FullProf suite [17] . Three structural models were selected instead of the refinement processes for accurate determination of the best crystal structure matching the data. The anti-site effect of the Fe and Ti atoms is considered by their shared occupation between the B and B' sites. The XRD characterization showed that all the samples are of single-phase. The best fitting results are achieved when the anti-site disorder is considered in all the samples. The obtained lattice parameters, atoms positions as well as the reliability factors are shown in Table 1.

The tolerance factor is a measure of the internal pressure for perovskite structures. It is given by [18]

where t represents the tolerance factor, r(A), r(B) and r(X) are the ionic radius for A-site, B-site and oxygen respectively. The values of the tolerance factor for SrLaFeTiO₆, SrPrFeTiO₆, SrNdFeTiO₆, are calculated using the SPuDS program [19] and found to be 0.971, 0.965 and 0.959, respectively. The reduction of symmetry in the compounds is related to the small values of the A-site radius (r_A) in order to accommodate empty spaces, which by its role leads to tilting the octahedral of the Fe and Ti atoms. A guide for this behavior is a plot of the tolerance factor and the ionic radii of the La, Pr and Nd ions against the tolerance factor. Tilting of the octahedral in SrLaFeTiO₆, SrPrFeTiO₆ and SrNdFeTiO₆ leads to different electronic environment around the Fe ions and impose magnetic moments miss alignment. Therefore, the magnetic properties of these compounds might be influenced by the internal pressure [20] .

Successful preparation of double perovskite oxides of chemical formula SrAFeTiO₆ (A = La, Pr or Nd) has been achieved by following the precursor method. The structural characterization analysis proved that it has similar crystal structure (orthorhombic) with a Pnma space group, which found to be consistent with its tolerance factor. The substitution in the A-site has been seen to alter the crystal structure from higher symmetry to lower symmetry as the size of the A-site decreases.

Distortion in the TiO₆ octahedra may result from Jahn-Teller splitting due to the ion existence of Ti⁴⁺ or Fe³⁺ or internal chemical pressure. That belongs to the cation size mismatch (σ²) which depends on the average radius of the cations in A-site of the perovskite structure. The FTIR spectra results of the samples under study are given in the Figure 8. This FTIR spectrum resembles the general appearance of the double perovskite structure. A shift in the peak positions in going from La up to Nd sample in the group has been noticed.