Frustration of Magnetic and Ferroelectric Long-Range Order in Bi₂Mn_4/3Ni_2/3O₆

Abstract

The slight incommensurate modulation of the structure of Bi(2)Mn(4/3)Ni(2/3)O(6) is sufficient to suppress the electrical polarization which arises in commensurate treatments of the structure, due to antiferroelectric coupling of local polar units of over 900 A(3). The incommensurate structure is produced by the competition between ferroelectric Bi lone pair-driven A site displacement, chemical order of Mn and Ni on the B site, and both charge and orbital order at these transition metals. The interplay between the frustrated polar Bi displacements and the frustrated spin order at the B site, induced by positional disorder, produces magnetodielectric coupling between the incommensurately modulated lattice and the spin-glass-like ground state with an unusual relationship between the magnetocapacitance and the applied field.

Superspace crystallography: a key to the chemistry and properties

“…An example of a locally polar structure, but antiferroelectric on the long scale owing to incommensurate modulation, is Bi 2 Mn 4/3 Ni 2/3 O 6 , where lone pair-driven Bi displacement, chemical order of Mn and Ni and both charge and orbital order at the transition metal sites compete with each other (Claridge et al, 2009). The average oxidation state of Mn in this compound is +3.5, which is known to demonstrate a very strong tendency to charge separation.…”

Magnetodielectric CuCr0.5V0.5O2: an example of a magnetic and dielectric multiglass

“…In the search for new multiferroic materials, the symmetry of the structure is of prime importance and the possibility offered by magnetic structures to break the inversion symmetry has been used to discover numerous spin induced ferroelectrics and strong magnetoelectric (ME) coupling [1]. In contrast, coupling between a spin glass and ferroelectricity has not been studied so extensively [2,3]. Cationic disorder induced by substitution on one crystallographic site may generate magnetic and/or electric glassiness depending on the atoms involved.…”

Local Crystal Structure of Antiferroelectric Bi2Mn4/3Ni2/3O6 in Commensurate and Incommensurate Phases Described by Pair Distribution Function (PDF) and Reverse Monte Carlo (RMC) Modeling

“…The starting model for the RT structure was based on the commensurate 2√2 a p × 4 a p × √2 a p subcell in the polar orthorhombic space group P 2 1 mn , previously identified as the best commensurate approximation to the incommensurate structure. 16 The GdFeO 3 -related structure was used as the starting model for the HT analysis. At the start of the RMC refinement, atom types were assigned randomly to B sites consistent with the fractional occupancies of the parent models, i.e., 2:1 Mn/Ni for the RT and HT models.…”

“…The incommensurate structure of BMN has been determined in the centrosymmetric space group ( Ibmm (α00, 0-β0) mm.ss ). 16 The incommensurate subcell consists of a √2 a p × 2 a p × √2 a p expansion of an orthorhombic perovskite structure ( a p denotes cubic perovskite subcell lattice parameter, ∼3.8 Å) where Bi displacements along the ⟨100⟩ direction cancel to give a nonpolar subcell structure. The subcell is modulated by positional and occupational modulations of the atoms in higher dimensional space along the ⟨100⟩ and ⟨010⟩ directions, which correspond to ideal cubic perovskite directions ⟨11̅0⟩ p and ⟨001⟩ p , respectively (Figure 1 ).…”

“…Displacive modulations of the A site give rise to a wide range of Bi environments and induce locally polar regions along ⟨11̅0⟩ p from the nonpolar subcell which cancel over longer length scales to produce long-range antiferroelectric displacements; the result is that the approximant incommensurate supercell (71√2 a p × 38 a p × √2 a p ) has almost no polarization (2 μC cm –2 ). 16 The short-range structural behavior along the locally polar ⟨11̅0⟩ p direction is therefore of great interest, as this controls the key physical property. A phase transition is observed upon heating at ∼200 °C where the long-range incommensurate modulations are lost.…”