Nuclear and Magnetic Structures of K 2 NiF 4 -type Iron(III) Oxide Halides
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Nuclear and Magnetic Structures of K2NiF4-type Iron(III) Oxide Halides Andrew L. Hector,* Alexander I. MacDonald, Daniel J. Price and Mark T. Weller Department of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, UK
ABSTRACT K2NiF4-type iron(III) oxides show a very common form of magnetic ordering, XY antiferromagnetic ordering within the layers combined with layer stacking based on alignment of spins in alternate layers. The Ising antiferromagnet Ca2MnO4 has been reported to have a doubled c-axis (ca 24Å) in the magnetic structure and we have found a similar stacking in the XY antiferromagnet Sr2FeO3F. We show here that this unusual c-axis doubling is related to the exposure of the material to air and suggest that in both Sr2FeO3F and Ca2MnO4 it may be related to the occupation of interstitial sites. INTRODUCTION The K2NiF4 structure type consists of Perovskite layers separated by Rocksalt layers and is thus magnetically an example of a pseudo-2D square lattice. However it also often exhibits 3D ordering and the magnetic structures of many materials have been resolved using powder neutron diffraction. Three magnetic exchange interactions exist in this system, as illustrated in Figure 1 for a system A2BX4 in which the layers exhibit XY antiferromagnetic ordering: 1. An intralayer coupling interaction, e.g. within layer (i) in which alternate B-cation spins are opposed. This is the nearest neighbour B-X-B superexchange interaction which is repeated in every layer. Crystallographically this means that to model this form of order we need a √2 increase in the a-parameter and thus two distinct B-cation sites. 2. The nearest interlayer interaction links a B-cation site in layer (ii) to those in layer (i) by a B-X-A-X-B pathway. The interaction with each of the four spins in the layer below is equivalent. Thus an antiferromagnetic exchange interaction will result in geometric spin frustration, which may be manifest as a non-collinear spin alignment. In Fig. 1 this is shown by layer (ii) spins aligned perpendicular to those in layer (i). 3. Layer (iii) is linked to layer (ii) by interaction type 2 (above) so these layers must have perpendicular spin alignments. The question at this point is: what is the effect of the next-nearest interlayer interaction – will the spins be parallel or antiparallel to those in layer (i)? There are B-X-A-X-A-X-B pathways from a B-cation in layer (iii) to five different B-cations in layer (i). The arrangement shown on the left in figure 1 represents one ferro- and four antiferromagnetic interactions. There are no literature examples of the arrangement on the right. Crystallographically this arrangement would result in a doubling of the c-parameter and this is observed in the Ising (spins point along c) antiferromagnet Ca2MnO4.1
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Figure 1. Two magnetic models for the K2NiF4 structure. One with next-nearest layers aligned parallel (left) and the other with next-nearest layers aligned antiparallel (right). The measured low temperature magnetic structures of
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