Structural Chemistry and Magnetic Properties of Nd₁₈Li₈Fe₅O₃₉ and Nd₁₈Li₈Co₄O₃₉: the Interplay of Cation and Spin Ordering: The interplay of cation and spin ordering

Polycrystalline samples of Nd ₁₈ Li ₈ Fe ₅ O ₃₉ and Nd ₁₈ Li ₈ Co ₄ O ₃₉ have been synthesized using a solid-state method and studied by a combination of neutron diffraction, direct and alternating current magnetometry, and, in the case of Nd ₁₈ Li ₈ Fe ₅ O ₃₉ , Mössbauer spectroscopy. Both compounds adopt a cubic structure (space group Pm3 ^- n, a ₀ ~ 11.9 Å) based on intersecting < 111 > chains made up of alternating octahedral and trigonal-prismatic coordination sites. The Fe ⁴⁺ cations in Nd ₁₈ Li ₈ Fe ₅ O ₃₉ are found on only the smaller of the two distinct octahedral sites in the structure; Fe ³⁺ and Li ⁺ are disordered over the larger octahedral site and the trigonal-prismatic site. The Nd ³⁺ cations occupy sites between the chains. The smaller octahedral site is fully occupied by cobalt in Nd ₁₈ Li ₈ Co ₄ O ₃₉ , with 25% of the larger octahedral sites being vacant; Li ⁺ is only found on the prismatic sites. Nd ₁₈ Li ₈ Fe ₅ O ₃₉ shows spin-glass-like behavior with a spin-freezing temperature of 5.75 K, whereas Nd ₁₈ Li ₈ Co ₄ O ₃₉ appears to order antiferromagnetically at 2.3 K. In both cases, the magnetic coupling involves the Nd ³⁺ sublattice.