S. Pechev

Nanoparticles of [Fe(NH2-trz)3]Br2.3H2O (NH2-trz=2-amino-1,2,4-triazole) prepared by the reverse micelle technique: influence of particle and coherent domain sizes on spin-crossover properties.

This paper describes the synthesis of iron(II) spin-crossover nanoparticles prepared by the reverse micelle technique by using the non-ionic surfactant Lauropal (Ifralan D0205) from the polyoxyethylenic family. By changing the surfactant/water ratio, the size of the particles of [Fe(NH2-trz)3]Br2.3H2O (with NH2trz=4-amino-1,2,4-triazole) can be controlled. On the macroscopic scale this complex exhibits cooperative thermal spin crossovers at 305 and 320 K. We find that when the size is reduced down to 50 nm, the spin transition becomes gradual and no hysteresis can be detected. For our data it seems that the critical size, for which the existence of a thermal hysteresis can be detected, is around 50 nm. Interestingly, the change of the particle size induces almost no change in the temperature of the thermal spin transition. A systematic determination of coherent domain size carried out on the nanoparticles by powder X-ray diffraction indicates that at approximately 30 nm individual particles consist of one coherent domain.

Synthesis and characterization of Eu3+, Ti4+ @ ZnO organosols and nanocrystalline c-ZnTiO3 thin films aiming at high transparency and luminescence

Abstract By exploiting colloidal properties, such as transparency, rheology and versatile chemistry, we propose to synthesize new photonic nanomaterials based on colloidal solutions and thin films. This contribution highlights our efforts to elaborate and to characterize nanostructures based on the ZnO–TiO2 system. Using a recently developed sol–gel route to synthesize new Ti4+@ZnO organosols, we were able to prepare, at relatively low temperature (400 °C) and short annealing time (15 min), highly transparent, luminescent, nanocrystalline Eu3+ doped c-ZnTiO3 thin films. The organosols and thin films were characterized with UV-visible-near infrared absorption, ellipsometry, photoluminescence spectroscopy, dynamic light scattering, x-ray diffraction and scanning electron microscopy.

Influence of hydriding on the magnetic properties of Gd3Ni6Al2

Abstract Some details of the synthesis peculiarities of the intermetallic Gd 3 Ni 6 Al 2 compound have been studied. It has been shown that this compound can only be prepared after prolonged annealing of a multiphase product obtained by melting of the constituent metals in a stoichiometric ratio. However, even after this treatment the samples contain some impurities, as proved by X-ray and electron microprobe analysis. As a result of a kinetic study on hydrogen uptake, the formation of a ternary hydride with a maximum content of 8.6 H atoms per formula unit of Gd 3 Ni 6 Al 2 has been confirmed. In this hydride the original cubic Ca 3 Ag 8 -type structure is preserved and only an increase of the unit cell parameter of about 1.45% is observed. Resistivity and low field magnetization measurements have shown the occurrence of ferromagnetic ordering in Gd 3 Ni 6 Al 2 at T c = 118(1) K. The value of M sat = 7.34(4) B = 4.8 T, which is very close to the saturation moment of 7.0 μ B calculated for Gd 3 ion. Hydrogen absorption leads to an important decrease of the Curie temperature ( T c = 69(1) K for Gd 3 Ni 6 Al 2 H 8.6 ). Similarly to the parent compound, a value of 7.0(1) μ B /Gd is obtained for the saturation moment of the hydride at B = 4.8 T. An attempt is made to explain this behaviour by a non-uniform spatial distribution of H atoms in the hydrided sample as well as by a weakening of the GdGd interactions in this sample caused by the increase of the lattice constant.

Synthesis and magnetic behaviour of the ternary germanides UCu2Ge2 and U3Cu4Ge4

Abstract The ternary germanide U 3 Cu 4 Ge 4 crystallizes in the orthorhombic Gd 6 Cu 4 Ge 4 -type structure with a = 1.3932(2) nm, b = 0.6579(1) nm and c = 0.4273(1) nm as unit cell parameters. It orders ferromagnetically below T C = 71 K and is detected as an impurity phase in the as-cast UCu 2 Ge 2 sample. The magnetic properties of this last ternary germanide are strongly dependent on the experimental procedures adopted for its synthesis.

Structural chemistry and magnetic behaviour of the ternary silicides UCuxSi2−x (0.28≤x≤0.96)

The ternary silicides UCuxSi2−x with 0.28≤x≤0.96 have been prepared and investigated by X-ray powder diffraction, electron diffraction, electrical resistivity and magnetization measurements. A structural change occurs with the sequence α-ThSi2 (tetragonal)→AlB2 (hexagonal)→Ni2In (hexagonal) as x increases, respectively, for x≅0.5 and x>0.84. The existence of Ni2In-type, evidenced by transmission electron microscopy, is explained by a crystallographic order between copper and silicon atoms. This system exhibits an interesting magnetic phase diagram: (i) for x<0.50 the compounds show a spin-fluctuation behaviour; (ii) a ferromagnetic ordering appears for 0.50≤x≤0.80 where the Curie temperature goes through a maximum at 91(1) K for x=0.70; (iii) finally the richest compositions in copper 0.92≤x≤0.96 order antiferromagnetically with TN around 43–40 K.

Crystal structure of a new lithium iron vanadate

Iron vanadates and phosphates have been widely explored [1-2] as possible electrode material for Li-ion batteries. In the goal of finding new materials, our approach was to consider existing materials and to investigate the flexibility of their network for possible substitutions. Among the different materials containing iron and vanadium, Cu3Fe4(XO4)6 (X = P, V) are isostructural to Fe7(PO4)6. Lafontaine et al. [3] discussed the structural relationships between β-Cu3Fe4(VO4)6 and several other vanadates, phosphates and molybdates of general formula AxBy(VO4)6. The interesting network flexibility was then demonstrated with the existence of four different crystallographic sites, which can be partially occupied depending on the x+y value : x+y = 7 for β-Cu3Fe4(VO4)6) and x+y = 8 for NaCuFe2(VO4)3. The LixFey(VO4)6 phase was then prepared considering the substitution of Li+ and Fe3+ for Cu2+ ions in βCu3Fe4(VO4)6 and the existence of an extra site to accommodate the charge compensation (7 ≤ x+y ≤ 8). As expected, a new lithium iron vanadate, isotructural to mineral Howardevansite was then obtained. Single crystal diffraction data were collected at room temperature on Enraf-Nonius CAD-4 diffractometer. Structure was refined with JANA-2006 program package. Mössbauer and magnetic measurements were also used to check the oxidation state of iron ions, to support the obtained crystal structure and to consider any possible structural/magnetic transitions. All the results will be presented and discussed in this presentation.

Structural filiations in the new SrLiMTi4O11(M= Cr, Fe) titanates

Titanates occupy an important place among advanced oxide materials. After characterization of MLi2Ti6O14 compounds in the ternary oxide systems MO Li2O TiO2 (M = Sr, Ba, Pb) [1],[2], two new titanates of Sr, Li and a 3d transitionmetal, SrLiMTi4O11 (where M = Cr, Fe) were discovered. Single crystals were obtained by spontaneous nucleation using LiBO2 as flux. The SrLiCrTi4O11 structure was refined using 1491 independent reflections in orthorhombic space group Pmna, with a=13.818A, b=5.755A, c=9.901A, Z=4, R1=0.021 and wR2=0.058. The SrLiFeTi4O11 structure was refined using 5005 independent reflections in orthorhombic space group Pbcn, with a=13.878A, b=11.496A, c=19.895A, Z=16, R1=0.036 and wR2=0.105. Structures can be described by the close-packed arrangement of strontiumand oxygen atoms. The unit cell contains 6 ’’compact planes’’ perpendicular to [100] in the layer sequence ABACBC ((chc)2). Titanium, chromium or iron atoms occupy some of the created interstitial octahedral sites whereas lithium atoms are situated in tetrahedral sites. Depending on the synthesis conditions of SrLiCrTi4O11, the chromium and titanium atom distributions over the four allowed crystallographic sites are not the same. Having a similar compact-planes sequence, SrLiCrTi4O11 and SrLiFeTi4O11 structures differ in the arrangement of strontium and oxygen atoms per layer of close packing which also induces correlated variation in the Li-tetrahedra distribution.

Structural study of UCuGe1.75 by X-ray and electron diffraction

Abstract The new ternary germanide UCuGe 1.75 have been isolated. After annealing treatment (under vacuum at 1073 K for 1 month), microprobe chemical analysis and X-ray powder diffraction revealed the presence of several phases: major phase corresponds to UCuGe 1.75(5) and traces of UGe 2 and UCu 2 Ge 2 were present as impurities phases. The refinement of the crystal structure was performed using a single crystal. It crystallises in orthorhombic system with a =0.4052(3) nm, b =1.697(1) nm and c =0.4058(1) nm. The structure of UCuGe 1.75 corresponds to the TbFeSi 2 structural type. An investigation by transmission electron microscopy (TEM) revealed a substructure 2 a ×2 b ×2 c which has been explained by a three-dimensional order of vacancies corresponding to the germanium deficit. Moreover the electrical resistivity and magnetization measurements performed on UCuGe 1.75 revealed the occurrence of antiferromagnetic ordering below T N =133(1) K.

Structural transition in the TbNi5-xAlx system: Structural and magnetic study

Abstract The crystallographic investigation of the TbNi5-xAlx system has pointed out different structures versus x. In addition to the well known CaCu5 and HoNi2.6Ga2.4 type structures, modulated CaCu5 type structure has been observed for x = 2 as well as ring diffuse scattering for x = 3.