Copper-deficiency in Ln2-xCexCuO4 (Ln=Nd, Gd) crystals and oxygen disorder in Gd2CuO4 crystals

The structure of seven Nd2-xCexCuO4 (x=0 and x=0.18) and Gd2CuO4 single crystals is determined by means of X-ray diffraction. All crystals have the Nd2CuO 4 tetragonal T' structure, space group I4/mmm (139). The copper site is found deficient (3 - 6%) in six crystals whereas both oxygen sites (within the CO2 planes O(1) and between rare earth layers 0(2)) remain fully occupied within standard deviation. The vibration ellipsoids of Nd, Cu and O(2) appear to be more ellongated in the c direction in Nd-based crystals reannealed in an inert atmosphere than in as-grown ones. The oxygen of the CuO2 basal plane (0(1)) in Gd2CuO4 is found off-centered in the a direction by 0.18 (2) Å.La structure de sept monocristaux de Nd2-xCexCuO 4 (x=0 et x=0.18) et Gd2CuO4 est déterminée par diffraction des rayons X. Tous les cristaux ont la structure tétragonale T' du Nd2CuO4 (groupe d'espace I4/mmm, n° 139). Un défaut de cuivre de l'ordre de 3 à 6% est trouvé dans six cristaux alors que les deux sites de l'oxygène (dans les plans CuO2 O(1) et entre les plans de terre rare O(2)) sont complètement occupés dans la limite d'erreur. Les ellipsoïdes de vibration des sites Nd, Cu et O(2) sont plus allongés parallèlement à l'axe c dans les cristaux recuit sous atmosphère inerte que dans les cristaux bruts de croissance. L'atome d'oxygène des plans CuO 2 (O(1)) dans Gd2CuO4 est trouvé décentré dans la direction de l'axe a de 0.18 (2) Å

The Ca0.80CuO1.93 phase : New way of synthesis by the sol-gel method

We report on the reaction kinetics for the Ca0.80CuO1.93 phase prepared by the sol-gel method using acrylamide for gelification. The phase was prepared 95% pure after a 2 hour treatment at 800°C. Comparative stability under vacuum and oxygen has been determined. The kinetics parameters for the decomposition have been established : the activation energy is Ea= 626 kJ/mol and the pseudo reaction order n = 0.5

Preparation and characterization of Au/ZrO2 composite obtained by oxidation of thin film ZrxAuy alloy

We present the preliminary results of a study aiming at preparing Au/ZrO2 mixed nanosized particles from thin films. In a first step, pure Zr and Au layers are alternatively deposited by RF sputtering on Si and W substrates under vacuum. Then, the intermetallic compounds are formed at high temperature under an inert atmosphere or in vacuum. The resulting samples are characterized by X-ray diffraction and by scanning electron microscopy. Keywords : Intermetallic compounds, Gold nanoparticles, Thin films, Gas sensor

Formation pathways in the synthesis and properties of (Tl0.5Pb0.5)(Sr0.9Ba0.1)2Ca2Cu3Oz and (Tl0.5Pb0.5)(Sr0.8Ba0.2)2Ca2Cu3Oz-1223 superconductors

The formation pathway of (Tl0.5Pb0.5)(Sr0.9Ba0.1)2Ca2Cu3Oz and (Tl0.5Pb0.5)(Sr0.8Ba0.2)2Ca2Cu3Oz was studied by neutron and x-ray diffraction. The following reaction pathway was proposed: thallium oxide and lead oxide react with Sr-rich (Sr1-xCax )CuO2 and Ca-rich (CaxSr1-x)O to form Sr4Tl2O7 and (CaxSr1-x)PbO3, respectively. The thallate and the plumbate compounds then form the (Tl0.5Pb0.5)-1212 phase starting at a temperature of 600 ◦C. Finally, between 850 and 900 ◦C, the 1223 phase is formed from (Tl0.5Pb0.5)-1212, (CaxSr1-x)2CuO3 and CuO. Parallel experiments to fabricate the (Tl, Pb)-1223 superconductor resulted in specimens with critical temperatures of 117.5 K and 116 K, respectively, and transition widths of 2 K. Differences between the pathways for the formation of Pb-doped, Sr-rich and Pb-free, Ba-rich Tl-1223 superconductors are discussed

Contribution to the study of the Ce-Zr-Au system

The equilibrium phase diagram of the Ce-Zr-Au system was studied by differential thermal analysis, X-ray diffraction and electron probe microanalysis along the isopletic section at 50 at. % Au. Our main result concerns the discovery of a new ternary phase and a ternary invariant. The polymorphic transitions of the ZrAu phase, previously reported, was confirmed and we precised the melting and the polymorphic transition temperatures of the CeAu phase. A schematic representation of the isopletic section is given

Preparation of TI-1223 and TI-2223 superconducting films by spray pyrolysis

Starting from nitrate solutions, precursors with composition Ba:Ca:Cu = 2:2:3 have been sprayed on MgO and LaAlO3 single crystal substrates to form homogeneously distributed multiphase layers with a grain size smaller than 1 µm. Based on the knowledge of the Tl-Ba-Ca-Cu-O system, ex-situ thallination leading to TlBa2Ca2Cu3O2 (Tl-1223) and Tl2Ba2Ca2Cu3O2 (Tl-2223) films, 1 to 2 µm in thickness, has been performed. SEM images reveal plate-like grains, 20 µm or more along the large dimension, and X-Ray diffraction experiments show that the films have both a c-axis and an in-plane texture more pronounced on LaAlO3 than on MgO. The critical temperatures for Tl-1223 and Tl-2223 films are 113 K and 120 K, respectively, with transition width of about 2 K. The critical current densities Jc (77 K, 0 T), deduced from screening method, is above 7x105 A/cm2. The optimisation of the parameters conditioning the film fabrication and the properties are under work

On the ternary compounds of the BaO-CaO-CuO system

The crystal structure and the stability of the two ternary compounds Ba$_4$CaCu$_3$O8+$\delta$ and Ba$_6$CaCu$_3$O$_{\rm 10+vardelta}$ have been investigated by means of X-ray diffraction, differential thermal analysis and thermogravimetry analysis. We find that the Ba$_4$CaCu$_3$O$_{\rm 8+vardelta}$ phase has a cubic structure (Im-3m, a = 8.1515(1) Å for $\delta$ = + 0.8) for an oxygen content in the range 8.7-9.3 as reported in the literature but undergoes a transformation into a tetragonal structure (I4/mmm, a = 8.1888(1) Å, c = 8.0634(1) Å for $\delta$ = - 0.3) when the oxygen content is lowered (7.7-8.7). Both Ba$_4$CaCu$_3$O8+$\delta$ and Ba$_6$CaCu$_3$O$_{\rm 10+vardelta}$ phases are metastable at low temperature in p(O$_2$) = 1 bar. The DTA/TGA curves indicate an equilibrium recovery between 300°C and 400°C corresponding to the decomposition into BaO$_2$, CaO and Ba$_2$Cu$_3$O$_6$.La structure cristallographique et la stabilité des deux composés ternaires Ba$_4$CaCu$_3$O$_{\rm 8+vardelta}$ et Ba$_6$CaCu$_3$O$_{\rm 10+vardelta}$ ont été étudiées par diffraction des rayons X, analyse thermique différentielle et analyse thermogravimétrique. Nous trouvons que la phase Ba$_4$CaCu$_3$O$_{\rm 8+vardelta}$ a une structure cubique (Im-3m, a = 8.1515(1) Å, $\delta$ = + 0.8) pour des contenus en oxygène élevés (8.7-9.3) comme cela est reporté dans la littérature mais subit une transition vers une structure tétragonale (I4/mmm, a = 8.1888(1) Å, c = 8.0634(1) Å, $\delta$ = - 0.3) lorsque ce contenu est diminué (7.7-8.7). Ces deux composés ternaires sont métastables à basse température sous p(O$_2$) = 1 bar. Les courbes d'ATD/TG indiquent en effet un retour à l'équilibre entre 300°C et 400°C qui correspond à une décomposition en BaO$_2$, CaO et Ba$_2$Cu$_3$O$_6$

Critical parameters for the preparation of TI-1223 superconducting films by spray pyrolysis

Thick films of the high temperature TIBa2Ca2Cu3Oz superconductor have been prepared from a precursor Ba2Ca2Cu3Oz obtained by spray pyrolysis. In this paper, we report on the parameters which mainly control the formation of the superconducting phase : the composition of the nitrates solution, the substrate temperature and the subsequent ex-situ thallination annealing. The sample were analyzed by X-ray diffraction and by secondary electron microscopy. Epitaxial film ( 1 to 2 µm thick ) with a critical temperature Tc=107 K and a critical density of current Jc(77 K, OT)=2.103 A/cm2 have been obtained