Structural Chemistry and Magnetic Properties of Nd₁₈Li₈Fe₅-ₓMₓO₃₉ (M = Mn, Co)


Polycrystalline samples of Nd18Li8Fe5-xMxO39 (M = Mn, 0 < x ≤ 4; M = Co; 0 < x ≤ 3) have been synthesized using a solid-state method and have been studied using a combination of dc and ac magnetometry, neutron diffraction, and Mössbauer spectroscopy. All compositions adopt a cubic structure (space group Pm3n, a 0 ~ 11.9 Å) based on intersecting <.111> chains made up of alternating octahedral and trigonal-prismatic coordination sites. These chains occupy channels within a Nd-O framework. The trigonal prismatic site is occupied by Li + and, in some cases, high-spin Fe 3+. When M = Mn, the smaller of the two distinct octahedral sites in the structure is occupied by a disordered distribution of Mn 4+ and low-spin Fe 4+, but in Nd18Li8Fe4CoO39, this site is occupied exclusively by Fe 4+. The larger of the octahedral sites is occupied by a disordered distribution of M 3+, high-spin Fe 3+, and, in some cases, Li +. Unusually, the Mn 3+ cations in Nd18Li8Fe5-xMxO39 adopt a low-spin state. This is attributed to the presence of an internal chemical pressure generated by the Nd-O framework. Competing magnetic superexchange interactions lead to the formation of a spin-glass-like phase in all compositions below T f ~ 5 K; the transition temperature decreases with increasing cobalt content but is relatively insensitive to the composition when M = Mn.



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© 2009 American Chemical Society (ACS), All rights reserved.

Publication Date

01 Feb 2009