Neutron diffraction study of the influence of structural disorderon the magnetic properties of Sr2FeMO6 (M=Ta,Sb)

The crystal structure of the perovskite Sr 2 FeTaO 6 has been refined by simultaneous analysis of X-ray and neutron powder diffraction data collected at 280 K; space group Pbnm, a=5.6204(3), b=5.6161(3), c=7.9266(3) A. The structure is of the GdFeO 3 type, with a disordered distribution of Fe and Ta over the six-coordinate cation sites. The structure of Sr 2 FeSbO 6 has been refined in a similar manner; space group P2 1 /n, a=5.6132(5), b=5.5973(5), c=7.9036(7) A, β=90.01(1)°. The two crystallographically distinct six-coordinate sites in Sr 2 FeSbO 6 are occupied in a partially ordered manner [0.795(6):0.205(6)] by Fe and Sb atoms. Neutron diffraction data collected from Sr 2 FeTaO 6 at 1.5 K show no evidence of long-range magnetic ordering and, in the light of previous susceptibility and Mossbauer measurements, it is concluded that Sr 2 FeTaO 6 is a spin glass below 23 K. Neutron diffraction data collected from Sr 2 FeSbO 6 at 1.5 K include magnetic Bragg peaks characteristic of a type I magnetic structure with an average ordered moment of 3.06(9) µ B per Fe atom on the Fe-dominated octahedral site, and no significant ordered moment on the second site. The magnetic Bragg scattering decreases to zero in the temperature interval 1.5≤T/K≤37(2). It is concluded that the partial cation ordering leads to the coexistence of a magnetically ordered spin system and a spin-glass system.