CRYSTAL AND MAGNETIC-STRUCTURE OF PIEZOELECTRIC, FERRIMAGNETIC AND MAGNETOELECTRIC ALUMINUM IRON-OXIDE FEALO3 FROM NEUTRON POWDER DIFFRACTION

The crystal structure of FeAlO3 has been determined at T = 298 K by ne utron diffraction, using polycrystalline samples prepared in a high st ate 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 hexagon al and cubic closed packing of oxygen ions. There are four different c ation sites labelled Fe1, Fe2 (predominantly occupied by iron), Al1 an d Al2 (predominantly occupied by aluminium). The oxygen environment of Al1 forms an almost regular tetrahedron. The other sites have a disto rted 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 dif fraction at T = 30 K reveals a classical Neel ferrimagnetism, the dire ction of easy magnetization being a, with strong '180 degrees cation-a nion-cation' superexchange antiferromagnetic interactions Fe1-O-Fe2 an d Fe1-O-Al2 (Al2 being a site occupied by 0.34 Fe). The Neel sublattic es 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. Piezoelectri city probably originates in the bond arrangement of the four tetrahedr al All sites in the unit cell, each tetrahedron being oriented with an Al1-O bond parallel to the polar c axis.