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 Pna2₁, Z = 8; a = 4.9839 (1), b = 8.5544 (2), c = 9.2413 (2) Å. 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: f₁ = 0.78 (1), f₂ = 0.76 (1), f₃ = 0.10 (1) and f₄ = 0.34 (1). Neutron diffraction at T = 30 K reveals a classical Néel ferrimagnetism, the direction of easy magnetization being a, with strong `180° cation-anion-cation' super-exchange antiferromagnetic interactions Fe1—O—Fe2 and Fe1—O—Al2 (Al2 being a site occupied by 0.34 Fe). The Néel sublattices are A = Fe1 + Al1 and B = Fe2 + Al2. The average magnetic moment per atom is weak (3.4 ± 0.3 μ_B) and the spontaneous magnetization at T = 30 K is extremely weak: 0.38 ± 0.17 μ_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.

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