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

TL;DR

This study uses Rietveld analysis of X-ray diffraction data to precisely identify the monoclinic phase presence and phase boundaries in a multiferroic solid solution, clarifying previous phase coexistence reports.

Contribution

It provides a detailed structural phase diagram of (1-x)Bi(Ni1/2Ti1/2)O3-xPbTiO3, confirming the monoclinic phase at the MPB and refuting earlier rhombohedral-tetragonal coexistence claims.

Findings

Monoclinic (Pm) phase exists in the MPB region.

Phase coexistence occurs between tetragonal and monoclinic structures.

B-O bonds are ionic in cubic phases and partially covalent in others.

Abstract

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.