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Presence of a monoclinic (Pm) phase in the morphotropic phase boundary region of multiferroic (1-x)Bi(Ni1/2Ti1/2)O3-xPbTiO3 solid solution: A Rietveld study

We present here the results of structural studies on multiferroic (1-x)Bi(Ni1/2Ti1/2)O3-xPbTiO3 solid solution using Rietveld analysis on powder x-ray diffraction data in the composition range 0.35 to 0.55. We have determined precisely the stability region of various crystallographic phases at room temperature for (1-x)Bi(Ni1/2Ti1/2)O3-xPbTiO3 . Structural transformation from pseudo-cubic to tetragonal phase is observed via phase coexistence region demarcating the morphotropic phase boundary region. The morphotropic phase boundary region consists of coexisting tetragonal and monoclinic structures with space group P4mm and Pm respectively, stable in composition ranges 0.41 to 0.49 as confirmed by Rietveld analysis. The results of Rietveld analysis completely rules out the coexistence of rhombohedral and tetragonal phases in the morphotropic phase boundary region reported by earlier workers. A comparison between the bond lengths for B-site cations- oxygen anions obtained after Rietveld refinement, with the bond length calculated using Shannon-Prewitt ionic radii, reveals the ionic nature of B-O bonds for the cubic phases and partial covalent character for the other crystallographic phases.