Structural chemistry and magnetic properties of Ln_(18)Li _8Rh_(5? x)Fe_xO_(39) (Ln = La, Nd)

摘要

Polycrystalline samples of Ln_(18)Li_8Rh _(5?x)Fe_xO_(39) (Ln = La, Nd; 0.5 ≤ x ≤ 5) have been synthesized by a solid-state method and studied by a combination of dc and ac magnetometry, neutron diffraction, and M?ssbauer spectroscopy. All compositions adopt a cubic structure (space group Pm3n, a_0 ～ 12 ?) based on intersecting 〈111〉 chains made up of alternating octahedral and trigonal-prismatic coordination sites. These chains occupy channels within a Ln?O framework. At low values of x, iron preferentially occupies the smaller (2a) of the two distinct octahedral sites as low-spin Fe(IV). The Rh(III) on the larger (8e) octahedral site is replaced by high-spin Fe(III). Nd-containing compositions having x > 1 show spin-glass-like behavior below ～5 K. La-containing compositions having x > 1 show evidence of a magnetic transition at ～8 K, but the nature of the transition is unclear. This contrasting behavior demonstrates that, although the structural chemistry of the two systems is essentially the same, the magnetic character of the Ln cations plays an important role in determining the properties of these compounds.