NEUTRON-DIFFRACTION STUDY OF THE INFLUENCE OF STRUCTURAL DISORDER ON THE MAGNETIC-PROPERTIES OF SR(2)FEMO(6) (M=TA, SB)

The crystal structure of the perovskite Sr2FeTaO6 has been refined by simultaneous analysis of X-ray and neutron powder diffraction data col lected at 280 K; space group Pbnm, a = 5.6204(3), b = 5.6161(3), c = 7 .9266(3) Angstrom. The structure is of the GdFeO3 type, with a disorde red distribution of Fe and Ta over the six-coordinate cation sites. Th e structure of Sr2FeSbO6 has been refined in a similar manner; space g roup P2(1)/n, a = 5.6132(5), b = 5.5973(5), c = 7.9036(7) Angstrom,bet a = 90.01(1)degrees. The two crystallographically distinct six-coordin ate sites in Sr2FeSbO6 are occupied in a partially ordered manner [0.7 95(6): 0.205(6)] by Fe and Sb atoms. Neutron diffraction data collecte d from Sr2FeTaO6 at 1.5 K show no evidence of long-range magnetic orde ring and, in the light of previous susceptibility and Mossbauer measur ements, it is concluded that Sr2FeTaO6 is a spin glass below 23 K. Neu tron diffraction data collected from Sr2FeSbO6 at 1.5 K include magnet ic Bragg peaks characteristic of a type I magnetic structure with an a verage ordered moment of 3.06(9) mu(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 i nterval 1.5 less than or equal to T/K less than or equal to 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.