Structural and magnetic study of Yb³⁺ in the perovskites Sr₂YbMO₆ (M=Nb, Ta, Sb)

The compounds Sr₂YbNbO₆, Sr₂YbTaO₆ and Sr₂YbSbO₆ have been prepared using solid state methods by heating pelleted reagents in air at temperatures up to 1400°C. Rietveld refinement against room temperature neutron powder diffraction data show that all three compounds crystallise with a cation-ordered variant of the perovskite structure in the P2₁/n space group. Complete cation ordering occurs between M⁵⁺ and Yb³⁺ over two octahedrally-coordinated sites in the structure and all compounds are stoichiometric in oxygen. The Sb-O bond lengths are similar to related perovskite compounds but differ slightly from those indicated by bond valence sums. Magnetic susceptibility data resemble Curie-Weiss paramagnetic behaviour, but can be better understood as arising from the effect of the octahedral crystal field on the ²F_5/2 ground state of Yb³⁺ leading to a temperature dependent magnetic moment on this ion below 100 K.