Synthesis and characterisation of chemically modified schafarzikite and pyrochlore minerals

Whitaker, Mariana J. (2013). Synthesis and characterisation of chemically modified schafarzikite and pyrochlore minerals. University of Birmingham. Ph.D.

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Abstract

Schafarzikite, FeSb $_2$ O $_4$ , is a tetragonal mineral (space group P4 $_2$ /mbc) belonging to a family of compounds with the general formula MX $_2$ O $_4$ . Schafarzikite has successfully been synthesised and chemically modified in various different ways. Lead has been substituted onto the pyramidal X sites to form compounds of formulation FeSb $_{2-x}$ Pb $_x$ O $_4$ (x = 0.2-0.75), excess oxygen ions have been incorporated into the structure to form FeSb $_{2-x}$ Pb $_x$ O $_{4+y}$ and compounds with mixed octahedral M ions, Fe $_{1-x}$ Mg $_x$ Sb $_2$ O $_4$ , have been prepared. The modifications give rise to changes in the magnetic ordering and magnetic structures and the conductivity of FeSb $_{2-x}$ Pb $_x$ O $_4$ is increased with respect to that of schafarzikite.

Pyrochlores (space group Fd-3m) have the general formula A $_2$ B $_2$ O $_7$ . The structures of Bi $_{2-x}$ Fe $_x$ (FeSb)O $_7$ (x = 0.1, 0.2, 0.3) and Nd $_{1.8}$ Fe $_{0.2}$ (FeSb)O $_7$ have been studied and found to undergo static displacive disorder in the A $_2$ O’ chains. Pyrochlore materials are magnetically frustrated and the samples Bi $_{2-x}$ Fex(FeSb)O $_7$ (x = 0.1, 0.2, 0.3) display spin-glass behaviour. The magnetic properties of RE $_2$ CrSbO $_7$ pyrochlores have also been investigated because, despite the inherent magnetic frustration in the pyrochlore structure, Ho $_2$ CrSbO $_7$ is known to order magnetically at 10 K. Neutron diffraction showed that the Ho $^{3+}$ moments form an ordered spin-ice arrangement while the Cr $^{3+}$ moments order ferromagnetically.