Ab initio structural approach on polycrystalline Y2-xCaxBaNiO5 (0 <= x <= 0.33) compounds

The procedure of ab initio structure solution from conventional X-ray powde r diffraction data is successfully applied to a series of Y2-xCaxBaNiO5 com pounds (0 less than or equal to x less than or equal to 0.33). The study ai med at understanding if the presence of dopants in polycrystalline Y2BaNiO5 compound can influence the success of the structure determination process and at quantifying: the substitutional defect. The new code EXPO has been u sed to obtain the most accurate set of structure factor moduli and to perfo rm the direct methods procedure. It is demonstrated that the substitution o n a crystallographic site, as for the above mentioned compound, does not co mpromise the performance of direct methods. The structural parameters obtai ned for samples with different x values are compared with those reported in the literature. It is pointed out that the knowledge of the dependence of lattice parameter and cell volume values on the cation site occupancy facto rs allows the direct determination of the degree of substitution of calcium on yttrium site of the sample.