Chemical pressure exerted by rare earth substitution in BiFeO3: Effect on crystal symmetry, band structure and magnetism

Abstract

Single phase BiFeO3 (BFO) is difficult to synthesize due to the volatile nature of Bi and the formation of impurity phases which restrict its use in commercial applications. Presence of free charge carriers in the system due to the formation of oxygen vacancies leads to an increase in the leakage current and suppresses intrinsic ferroelectric response of the system. In this paper, we present a detailed study of the structural phase transformation triggered by the substitution of rare-earth (RE) ions at A-site of BFO. We report that the chemical pressure induced by the substitutional ions stabilizes lower symmetry polymorphs. A detailed phase diagram showing connection between chemical pressure, ionic radii mismatch, dopant concentration and rare ions is also extracted on the basis of the structural analysis. Change in peak position and intensity of Raman vibrational modes and transformation of structure from rhombohedral to orthorhombic phase are identified to be a function of the concentration of RE ions substituted at the A-site. Five electronic transitions, two d-d and three charge transfer (CT) transitions, are observed in optical measurement within the spectral range of 1–5 eV. Shifts in d-d and CT transitions as a function of substitution are also observed. Dependence of the direct band gap on the concentration of substituent ion is also studied. Presence of spin spiral structure results in net cancellation of magnetic moments in BFO and, as a result, antiferromagnetic hysteresis loop is observed. Substitution of RE ions at A-sites induces chemical pressure which affects Fe-O-Fe bond angle and results in the destruction of spiral spin structure. Consequently, weak ferromagnetism is observed beyond certain concentration of RE in BFO.