YCrO3 (YCO) is known to be a multiferroic with orthorhombic, Pnma, structure with center of inversion. However, the local structural inhomogeneity in this compound is believed to give rise to ferroelectric behavior. In this study we explore high temperature Raman investigations of YCO and observed that one of its Raman mode B3g (3) (CrO6 Octahedral tilt mode) softens around the structural phase transition which could be the origin of ferroelectric nature in YCO. In addition, we substitute bismuth (Bi) in YCO to understand the structural distortions that lead to local structural inhomogeneity. Besides B3g (3) mode softening with composition and high temperature studies in Y1-xBixCrO3 the study reveals the structural distortions and the structural tunability Bi offers in such systems.

Abstract

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The intrinsic distortions present in rare-earth orthochromites (RCrO 3) observed from lanthanum to lutetium (in R-site) can influence the magnetic properties like Neel transition and weak ferromagnetic coupling. A nonmagnetic cation with similar ionic radius would be a suitable candidate to engineer the inherent distortions of particular orthochromite. In this study, bismuth (Bi 3+) with a 6s 2 lone pair was chosen to substitute in neodymium (Nd 3+) site of NdCrO 3 (NCO) to tailor the intrinsic structural distortions. The variation of optical absorption edge evidently suggests that Bi (6s 2) substituted in the magnetic rare-earth Nd 3+ influences the Cr-O overlap integral. The interaction of Bi cation with oxygen bonds influences the structural distortions through Cr-O polyhedral, which are evident from Raman scattering studies. The observed structural and magnetic properties of similar ionic radius of Bi 3+ in Nd 3+ reveal that intrinsic structural distortions are interrelated to enhanced weak ferromagnetic component and change in Neel and spin reorientation temperatures in our compounds. In addition, a reduction in the optical bandgap of NCO from 3.1 to 2.6 eV was observed.

YCrO 3 and the substitution of Bi +3 (6s 2) in the Y-site were synthesized by the sol gel process. The conductivity measurements using dielectric spectroscopy reveals that conductivity increases with increase of composition. Dielectric loss were used to understand the nature of conducting species and it reveals that chromates exhibits the usual reciprocal behaviour of charge carriers to motion of oxygen vacancies as we increase the Bi composition and temperature. Impedance and microstructural studies revealed that conductivity of the chromates was dominated by the grain contribution.