A search for disorder in the spin glass double perovskites Sr ₂CaReO₆ and Sr₂MgReO₆ using neutron diffraction and neutron pair distribution function analysis

The geometrically frustrated, B-site ordered, S = 1/2, double perovskites Sr₂CaReO₆ and Sr₂MgReO₆, which show spin frozen magnetic ground states, have been investigated using neutron powder diffraction (ND) and neutron pair distribution function (NPDF) analysis in a search for evidence for atomic positional disorder. For both materials, data were taken above and below the spin freezing temperatures of ∼ 14K and ∼ 45K for the CaRe and MgRe phases, respectively. In both cases the fully B-site ordered model was in excellent agreement with the data, both ND and NPDF, at all temperatures studied. Thus, the structure of these materials, from the average and the local perspectives, is very well described by the fully B-site ordered model, which raises questions concerning the origin of the spin glass ground state. These results are compared with those for the spin glass pyrochlore Y₂Mo₂O₇ and other B-site ordered double perovskites.