The crystallographic features of Gd-doped ceria were investigated at the operating temperature of solid oxides fuel cells, where these materials are used as solid electrolytes. (Ce(1-x)Gd(x))O(2-x/2) samples (x = 0.1, 0.3, 0.5, 0.7) were prepared by coprecipitation of mixed oxalates, treated at 1473 K in air, and analyzed by synchrotron X-ray diffraction in the temperature range 673 K ≤ T ≤ 1073 K at the Elettra synchrotron radiation facility located in Trieste, Italy. In the whole temperature span a boundary was found at x ∼ 0.2 between a CeO2-based solid solution (for x ≤ 0.2) and a structure where Gd2O3 microdomains grow within the CeO2 matrix, taking advantage of the similarity between Gd(3+) and Ce(4+) sizes; the existence of the boundary at x ∼ 0.2 was confirmed also by measurements of ionic conductivity performed by impedance spectroscopy. Similar to what observed at room temperature, the trend of the cell parameter shows the presence of a maximum; with increasing temperature, the composition corresponding to the maximum moves toward lower Gd content. This evidence can be explained by analyzing the behavior of the coefficient of thermal expansion as a function of composition.

To unveil the structural mechanisms associated with T c variations induced by either mechanical or chemical pressure, samples of the Y(Ba 1Ϫx Sr x ) 2 Cu 3 O w system with xр0,0. 02,0.1,0.25,0.35,0.5,0.625,0.75,1, as well as those with xϭ0.5 and wϭ6.685,6.80,6.96,6.98, have been prepared and characterized. Characterization includes crystal structural refinements based on powder neutron diffraction data taken at room temperature, electron microscopy for detecting a possible Ba/Sr ordering, and resistive and magnetic measurements of the superconducting transition. The effects of Sr substitution on the structural parameters are equivalent to those of a pressure of approximately 10 GPa/x. The main difference is the thickness of the superconducting block CuO 2 -Y-CuO 2 , which increases with increasing x and decreases with increasing pressure. As a consequence of the displacement of O4 from the ͑0, 1 2 ,0͒ to the (x, 1 2 ,0) position, the Ba/Sr-O4 distance decreases with increasing Sr content. At constant w, T c decreases at the rate of 20 K/x. For xϾ0, the maximum of T c occurs at a value of w higher than for xϭ0. For xϭ0.5 the thickness of the superconducting block increases with increasing w and consequently with increasing T c . This indicates that, at least in this system, the thickness of the superconducting block is not the key parameter controlling T c . The stress of the Ba/Sr and the Y sites, estimated from the bond valence sums ͑BVSs͒, decreases with increasing x. For xϭ0.5, the BVS values are almost equal to the formal values of 2ϩ and 3ϩ, respectively. The average BVS of Cu1 and Cu2 increases with increasing x. However, this does not correspond to a real increase because the total charge of the Cu cations, as determined by iodometric titration or neutron diffraction, remains constant with x. Our analysis of the structural data suggests that the relaxation of the Ba/Sr layer hinders the charge transfer from Cu1 to Cu2, which accounts for the decrease of T c with increasing x. ͓S0163-1829͑98͒05645-8͔

Both ac and dc magnetic measurements have been performed on polycrystalline RuSr 2 GdCu 2 O 8 samples, aimed at studying the magnetic ordering of such a phase. Linear ac susceptibility measurements give clear evidence of the antiferromagnetic transition at a temperature T = T N , which has been already widely observed in such system with different techniques. In addition, nonlinear ͑third-order͒ susceptibility shows that another magnetic structure exists at a temperature near but slightly higher than T N . Shape and sign of this small signal are compatible with the presence of ferromagnetic nanoparticles exhibiting superparamagnetic behavior. Such possibility has been confirmed by magnetization decay measurements performed at temperatures lower than the blocking temperature T B of the superparamagnetic system. A fit of magnetization decay measurements with a model describing the slow dynamic of magnetic nanoparticle systems gives the size distribution of the nanoparticles and a mean volume of the order of V = 2.5ϫ 10 −19 cm 3 , in accord with previous estimates. The scenario of RuSr 2 GdCu 2 O 8 as a phase separated material, already proposed to explain some peculiarities of its magnetic behavior, is here confirmed.

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The effects of a tryptophan-free amino acid mixture on tilt aftereffect, movement aftereffect, and the Mueller-Lyer illusion were studied. 12 male subjects ingested either a balanced amino acid mixture or a tryptophan-free mixture which causes a marked depletion of brain tryptophan and serotonin. The tryptophan-free mixture significantly increased the strength of tilt aftereffect but had no effect on movement aftereffect or the Mueller-Lyer illusion. These results were discussed with reference to the pharmacological activity of serotonin and its influence on the strength of lateral inhibition in mammalian brains.