Crystal and magnetic structure of piezoelectric, ferrimagnetic and magnetoelectric aluminium iron oxide FeAlO3 from neutron powder diffraction

The crystal structure of FeAlO3 has been determined at T = 298 K by neutron diffraction, using polycrystalline samples prepared in a high state of purity. The space group is Pna2(1), Z = 8; a = 4.9839(1), b = 8.5544(2), c = 9.2413(2) Angstrom. The structure, which is isomorphous to that of FeGaO3, can be described as a double combination of hexagonal and cubic closed packing of oxygen ions. There are four different cation sites labelled Fe1, Fe2 (predominantly occupied by iron), Al1 and Al2 (predominantly occupied by aluminium). The oxygen environment of Al1 forms an almost regular tetrahedron. The other sites have a distorted octahedral environment, especially irregular for Fe1 and Fe2. The fractions f(i) of iron ions over the four cation sites are: f(1) = 0.78 (1), f(2) = 0.76(1), f(3) = 0.10(1) and f(4) = 0.34(1). Neutron diffraction at T = 30 K reveals a classical Neel ferrimagnetism, the direction of easy magnetization being a, with strong '180 degrees cation-anion-cation' superexchange antiferromagnetic interactions Fe1-O-Fe2 and Fe1-O-Al2 (Al2 being a site occupied by 0.34 Fe). The Neel sublattices are A = Fe1 + Al1 and B Fe2 + Al2. The average magnetic moment per atom is weak (3.4 +/- 0.3 mu(B)) and the spontaneous magnetization at T = 30 K is extremely weak: 0.38 +/- 0.17 mu(B) per atom. Piezoelectricity probably originates in the bond arrangement of the four tetrahedral All sites in the unit cell, each tetrahedron being oriented with an Al1-O bond parallel to the polar c axis.