Structural and magnetic characterisation of the pyrochlores Bi_2-xFe_x(FeSb)O₇, (x = 0.1, 0.2, 0.3), Nd_1.8Fe_0.2(FeSb)O₇ and Pr₂(FeSb)O₇

Four antimony-containing pyrochlores with compositions Bi2−xFex(FeSb)O7, where x=0.1, 0.2, 0.3, and Nd1.8Fe0.2(FeSb)O7 have been synthesised, their structural properties examined, and compared to that of the pyrochlore phase Pr2(FeSb)O7 which we have structurally characterised for the first time. The Fe3+ ions in Pr2(FeSb)O7 are located only on the B-sites whereas in the bismuth- and neodymium-containing pyrochlore structures the Fe3+ ions are present on both the A- and B-sites giving rise to static displacive disorder in the A2O' chains. The presence of a smaller cation on the A-site in materials of composition Bi2−xFex(FeSb)O7 appears to lower the radius ratio (rA/rB) and leads to a stabilisation of the structure. Magnetic susceptibility measurements reveal no long range order, but Bi1.9Fe0.1(FeSb)O7 and Bi1.8Fe0.2(FeSb)O7 undergo transitions at ca. 12K to a spin-glass state; Nd1.8Fe0.2(FeSb)O7 and Pr2(FeSb)O7 are paramagnetic.