RESUMO
Polycrystalline samples of Nd(18)Li(8)Fe(5-x)M(x)O(39) (M = Mn, 0 < x < or = 4; M = Co; 0 < x < or = 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) approximately 11.9 A) 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 Nd(18)Li(8)Fe(4)CoO(39), 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 Nd(18)Li(8)Fe(5-x)M(x)O(39) 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) approximately 5 K; the transition temperature decreases with increasing cobalt content but is relatively insensitive to the composition when M = Mn.
