High Pressure Crystal and Magnetic Phase Transitions in Multiferroic Bi0.9La0.1FeO3

The crystal and magnetic structures of multiferroic Bi 0.9 La 0.1 FeO 3 have been studied using high resolution neutron powder diffraction in the pressure range 0–8 GPa. Two structural phase transitions are observed. The first, at ∼1 GPa, transforms the polar R 3 c structure to an antipolar PbZrO 3 -like √2a p × 2√2a p × 2a p perovskite superstructure; the second, at ∼5 GPa, results in a smaller, √2a p × √2a p × 2a p unit cell and a structure described with Ibmm (nonstandard setting of Imma ) symmetry, in which the a – a – b 0 octahedral tilt system is retained and the antipolar cation displacements lost. Accompanying the changes in the nuclear structure, the antiferromagnetic spin structure evolves from a cycloid, with a modulation length, λ ≈ 770 Å, to collinear arrangements with the moments aligned along the b -axis ( Pbam ) and the a -axis ( Ibmm ) of the orthorhombic unit cells. In comparison with BiFeO 3 the transition from a rhombohedral to an orthorhombic structure is suppressed by ∼3 GPa, reflecting the dilution of the stereochemically active bismuth lone pair by lanthanum. A correlation between the cell contraction of Bi 1– x La x FeO 3 (0.0 ≤ x ≤ 0.3) induced by chemical pressure and hydrostatic pressure on BiFeO 3 is determined, with substitution of 1 mol % of La approximately equivalent to application of 0.05 GPa. Bi 0.9 La 0.1 FeO 3 is found to have a higher bulk modulus than BiFeO 3 .