Structural and magnetic characterisation of the pyrochlores Bi{sub 2-x}Fe{sub x}(FeSb)O{sub 7}, (x=0.1, 0.2, 0.3), Nd{sub 1.8}Fe{sub 0.2}(FeSb)O{sub 7} and Pr{sub 2}(FeSb)O{sub 7}

Four antimony-containing pyrochlores with compositions Bi{sub 2-x}Fe{sub x}(FeSb)O{sub 7}, where x=0.1, 0.2, 0.3, and Nd{sub 1.8}Fe{sub 0.2}(FeSb)O{sub 7} have been synthesised, their structural properties examined, and compared to that of the pyrochlore phase Pr{sub 2}(FeSb)O{sub 7} which we have structurally characterised for the first time. The Fe{sup 3+} ions in Pr{sub 2}(FeSb)O{sub 7} are located only on the B-sites whereas in the bismuth- and neodymium-containing pyrochlore structures the Fe{sup 3+} ions are present on both the A- and B-sites giving rise to static displacive disorder in the A{sub 2}O' chains. The presence of a smaller cation on the A-site in materials of composition Bi{sub 2-x}Fe{sub x}(FeSb)O{sub 7} appears to lower the radius ratio (r{sub A}/r{sub B}) and leads to a stabilisation of the structure. Magnetic susceptibility measurements reveal no long range order, but Bi{sub 1.9}Fe{sub 0.1}(FeSb)O{sub 7} and Bi{sub 1.8}Fe{sub 0.2}(FeSb)O{sub 7} undergo transitions at ca. 12K to a spin-glass state; Nd{sub 1.8}Fe{sub 0.2}(FeSb)O{sub 7} and Pr{sub 2}(FeSb)O{sub 7} are paramagnetic. - graphical abstract: Intersite cation mixing in A{sub 2}(FeSb)O{sub 7} (A=Bi, Pr, Nd) pyrochlores. Highlights: Black-Right-Pointing-Pointer The instability of the pyrochlore composition Bi{sub 2}FeSbO{sub 7} is reported for the first time. Black-Right-Pointing-Pointer Structure analysis confirms the presence of Fe on the Bi-site. Black-Right-Pointing-Pointer The presence of Fe{sup 3+} on two distinct sites is supported by Moessbauer spectroscopy. Black-Right-Pointing-Pointer For Nd{sub 2}FeSbO{sub 7} and Pr{sub 2}FeSbO{sub 7,} Fe can also enter the Nd/Prsite. Black-Right-Pointing-Pointer No magnetic order occurs in these materials.