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

Artikel i vetenskaplig tidskrift, 2014

The crystal and magnetic structures of multiferroic Bi0.9La0.1FeO3 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 similar to 1 GPa, transforms the polar R3c structure to an antipolar PbZrO3-like root 2a(p) x 2 root 2a(p) x 2a(p) perovskite superstructure; the second, at similar to 5 GPa, results in a smaller, root 2a(p) x root 2a(p) x 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, lambda approximate to 770 angstrom, 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 BiFeO3 the transition from a rhombohedral to an orthorhombic structure is suppressed by similar to 3 GPa, reflecting the dilution of the stereochemically active bismuth lone pair by lanthanum. A correlation between the cell contraction of Bi1-xLaxFeO3 (0.0 <= x <= 0.3) induced by chemical pressure and hydrostatic pressure on BiFeO3 is determined, with substitution of 1 mol % of La approximately equivalent to application of 0.05 GPa. Bi0.9La0.1FeO3 is found to have a higher bulk modulus than BiFeO3.