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Title: The synthesis, structures and properties of multi-anion solids
Author: Smura, Catherine F.
ISNI:       0000 0001 3469 3536
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2008
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This thesis reports a study of oxychalcogenide compounds with general formula A2M02Cu2X2 (A is an alkaline earth metal; M is a first row transition metal; X is a chalcogen). The structure consists of alternating layers of square-planar (M02i- and antifluorite-type (Cu2X2i- with A2+ cations in between. There is huge scope for chemical substitution, both within and between the two layer types. It is consequently possible to tune the inter- and intra-layer distances in a controlled manner, gaining a handle on the properties of these solids. Low temperature neutron powder diffraction on A2Co02CU2X2 (A = Sr, Ba; X = S, Se) has revealed in-plane antiferromagnetic ordering of C02+ in all cases. An orbital contribution to the moment arises for C02+ in a highly square-planar coordination. The strength of spin-orbit coupling is modified by A-site substitution: the ordered moment in Ba2Co02Cu2X2, 4.3(1) !lB, is approximately 1 !lB greater than that in Sr2Co02CU2X2. The larger spin-orbit interaction in the Ba compounds also results in a low temperature orthorhombic distortion when the moments are aligned antiferromagnetically. The conclusions drawn have been further supported by investigation of the Sr(2-x)BaxCo02CU2S2 solid solution. As Co is brought into an increasingly square-planar coordination, the size of the ordered moment and orthorhombic distortion increase accordingly. The alignment of Co moments between 111111 C002 layers in Sr2Co02CU2S(2_x)Sex depends critically on x. At a Se concentration greater than 70 %, the alignment switches from ferromagnetic to antiferromagnetic. The energy difference between the magnetic structures arising from the two types of alignment is clearly very small and at certain compositions of Sr2Co02CU2S(2-x)Sex they are found to coexist. This system therefore provides a rare example of magnetic polytypism in a layered Co compound. The Mn ions in Sr2Mn02Cu1.56Se2 are ferromagnetically coupled in the plane, giving rise to Atype antiferromagnetism. In the oxysulfide analogue the moments in the plane are ordered in a CE-type arrangement which has zig-zag ferromagnetic chains coupled antiferromagnetically. The magnetic ordering in the oxysulfide may be related to Cu-site/vacancy ordering; electron diffraction measurements have shown that any ordering in the oxyselenide occurs only at short range. An absence of long range magnetic ordering in Ba2Mn02CuI.86(4)Se2 arises from the lower Mn oxidation state in this compound which is controlled by the size of the A-cation. The first neutron powder diffraction and property measurements have also been performed on Sr2Ni02Cu2S2, synthesised by a new flux route. The results suggest that a localisation of Ni2 + dx2-1 electrons (delocalised at room temperature) occurs on cooling. Below 140 K, a transfer of this electron density to the half occupied dxy gives low spin cf Ni2+, i.e. a high to low spin state transition arises for Ni2+ in square-planar coordination.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available