The oxygen ionic conductivity of apatite-type La 9.83 Si 4.5 Al 1.5Ϫy Fe y O 26Ϯ␦ (y ϭ 0-1.5), La 10Ϫx Si 6Ϫy Fe y O 26Ϯ␦ (x ϭ 0-0.77; y ϭ 1-2), and La 7Ϫx Sr 3 Si 6 O 26Ϫ␦ (x ϭ 0-1) increases with increasing oxygen content. The ion transference numbers, determined by faradaic efficiency measurements at 973-1223 K in air, are close to unity for La 9.83 Si 4.5 Al 1.5Ϫy Fe y O 26ϩ␦ and La 10 Si 5 FeO 26.5 , and vary in the range 0.96-0.99 for other compositions. Doping of La 9.83 (Si, Al) 6 O 26 with iron results in an increasing Fe 4ϩ fraction, which was evaluated by Mössbauer spectroscopy and correlates with partial ionic and p-type electronic conductivities, whereas La-stoichiometric La 10 (Si, Fe͒O 26ϩ␦ apatites stabilize the Fe 3ϩ state. Among the studied materials, the highest ionic and electronic transport is observed for La 10 Si 5 FeO 26.5 , where oxygen interstitials are close neighbors of Si-site cations. Data on transference numbers, total conductivity, and Seebeck coefficient as a function of the oxygen partial pressure confirm that the ionic conduction in Fe-substituted apatites remains dominant under solid oxide fuel cell operation conditions. However, reducing p (O 2 ) leads to a drastic decrease in the ionic transport, presumably due to a transition from the prevailing interstitial to a vacancy diffusion mechanism, which is similar to the effect of acceptor doping. Iron additions improve the sinterability of silicate ceramics, increase the n-type electronic conductivity at low p(O 2 ), and probably partly suppress the ionic conductivity drop. The thermal expansion coefficients of apatite solid electrolytes in air are (8.8-9.9) ϫ 10 Ϫ6 K Ϫ1 at 300-1250 K.

Abstract. The study of the magnetic properties depending upon mechanical milling of the ferromagnetic polycrystalline TbAl 2 material is reported. The Rietveld analysis of the Xray diffraction data reveals a decrease of the grain size down to 14 nm and -0.

The structural state and static and dynamic magnetic properties of TbCu2 nanoparticles are reported. The nanoparticles were produced by mechanical milling under inert atmosphere with low milling times ≤ 15 hours. The randomly dispersed nanoparticles as detected by TEM retain the bulk symmetry with an orthorhombic Imma lattice and Tb and Cu in 4c positions. Rietveld refinements confirm that the milling produces a controlled reduction of particle sizes reaching down to ≃ 6 nm and an increase of the microstrain ≃ 0.6 %. The DC-susceptibility shows a reduction of the Néel transition (from 49 K to 43 K) and a progressive increase of the spin glass peak (from 9 to 15 K) in the zero field cooled magnetization with size reduction. The exchange anisotropy is very weak (bias field of ≃ 30 Oe) and is due to the presence of a disordered (thin) shell coupled to the antiferromagnetic core. The dynamic susceptibility evidences a critical slowing down in the spin disordered state with a tendency to increase of zv and β exponents when the size becomes smaller (zv ≃ 6.6 and β ≃ 0.85). A Rietveld analysis of neutron diffraction patterns 1.8 ≤ T ≤ 60 K including the magnetic structure determination reveals that there is a reduction of the expected moment (≃ 80 %) which must be connected to the presence of the disordered particle shell. The core magnetic structure retains the bulk antiferromagnetic arrangement. The overall interpretation is based on a superantiferromagnetic behavior which at low temperatures coexists with a canting of surface moments. We propose a novel magnetic phase diagram as a function of the particle size.

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An unusual 4f-superantiferromagnetic state characterized by simultaneous antiferromagnetic and spin-glass behaviors induced by particle-size reduction is revealed in metallic nanoparticles (≈9 nm) of TbCu 2. The Néel temperature is 46 K and the glassy freezing is below ≈9 K and governed by a critical slowing down process. Neutron diffraction at 1.8 K establishes the superantiferromagnetism. The latter is settled by the nanoparticle moments and the freezing mechanism is provided by the surface spins.