J. Choisnet

Sol–gel synthesis and structural characterisation of the perovskite type pseudo solid solution LaNi0.5Cu0.5O3

The composition LaNi0.5Cu0.5O3−δ has been prepared by heat treating a polyacrylamide xerogel in flowing O2. A perovskite type structure starts to crystallise at 700 °C and is achieved at 1000 °C. TGA data point to the formation of a rather strongly oxidised compound (δ n≤ 0.09). As the solid state reaction of a mixture of La2O3/NiO/CuO powder oxides never yields a perovskite, the part of the sol–gel route in this synthesis proves to be crucial. On the basis of X-ray and neutron diffraction results, the existence of a cupronickelate having a rhombohedral perovskite type structure is proved. Rather unexpectedly, an electron diffraction study of the as-prepared composition at 1000 °C reveals the presence of two sets of perovskite type cupronickelate microcrystals: a copper rich one with a fixed composition 67% Cu and a nickel rich one nwith a small range of compositions 59 ≤ Ni% n≤ 67. The latter microcrystals feature the rhombohedral LaNiO3 perovskite whereas the former one, due to the partial presence of Cu(II), frequently shows an orthorhombic [2ap√2 × 2ap√2 × nap] supercell. Thus, the composition LaNi0.5Cu0.5O3−δ. has to be understood in terms of a nanoscale segregated pseudo solid solution formed of two lanthanum cupronickelates, enriched in copper and nickel, respectively.

T*-type substituted neodymium cuprates Nd1.2Sr0.8-xY(Ho, Er, Yb)xCuO4-δ crystal chemistry and electrical conductivity

Abstract The four T*-type solid solutions Nd 1.2 Sr 0.8− x RE x CuO 4− δ (RE=Y, Ho, Er, Yb) were synthesized and studied in terms of homogeneity range, structural study (neutron powder diffraction) and electrical conductivity. The homogeneity ranges have a small extent around the value x =0.4. The main crystal chemical properties are as follows: a full ordering of Sr 2+ and Y 3+ (Ho, Er, Yb) in the Rocksalt (RS) and Calcium Fluoride (CF) slabs, respectively, Nd 3+ distributed on both slabs; an overall oxygen deficiency δ in the range 0.15–0.18; oxygen vacancies distributed in the CF slabs (12%) and the equatorial “CuO 2 ” planes; interstitial oxygen in the RS slabs (# Nd 0.6 Sr 0.4 O 1.04 ). Electrical resistivity measurements (RT — 4 K) of samples annealed under p O 2 up to 1800 bars point to a “dirty metal” behavior up to 25 K (Y, Ho) and 70 K (Er), followed by a semiconductive regime at lower temperature. The big difficulty to get a superconductive transition in these phases and more generally in the T*-type cuprates is strongly dependent on the nature and the amount of small lanthanoidic cation in the CF slabs.

Cationic ordering of small RE3+ cations (RE=Y, Ho, Er, Yb) in the cuprates La1.9−xRExSr(Ca)1.1Cu2O6−δ: similarities between the oxygen-deficient P2/RS and T* structures

Abstract The partial substitution of small rare-earth RE 3+ cations (RE=Y, Ho, Er, Yb) for La 3+ in the cuprates La 1.9 Sr(Ca) 1.1 Cu 2 O 6 results in ordering phenomena of the A cations in the oxygen-deficient double perovskite–rocksalt P 2 /RS intergrowth structure. The preference of these RE 3+ cations for the cubic-like 8-fold coordination in the middle of the P 2 blocks (A 1 sites) is the driving mechanism of formation of the solid solutions La 1.9− x RE x Sr(Ca) 1.1 Cu 2 O 6− δ . The homogeneity range is rather wide in the case of Sr: x max =0.5 (Yb)→0.8 (Ho), as its extent is much more limited in the case of Ca: x max =0.15 (Yb)→0.3 (Ho), owing to a competition between RE 3+ and Ca 2+ for the occupancy of the A 1 sites. In the Sr–Ho solid solution, Ho 3+ is found both in the A 1 and A 2 sites (9-fold coordination): this can be understood in terms of a not so large destabilization of the 4f 10 electronic structure of Ho 3+ in the crystal field of the A 2 sites. The thermal decomposition of La 1.2 Sr 1.1 Er 0.7 Cu 2 O 5.95 at 1050°C gives rise to the formation of a T*-type cuprate whose likely composition is La 1 Sr 0.3 Er 0.7 CuO 3.85 , accompanied by a T-type cuprate and CuO. Such a result, never obtained up to now, exemplifies the strong geometrical similarities between the A–O polyhedral networks of the P 2 /RS and T* structures, namely: A[P 2 /RS]≡A[T*]+ 1 2 A[T].

Intermediate steps in YBa2Cu3O7?x synthesis by sol-gel method: YBaCuO oxycarbonate: Code: DP23

High Tc-superconducting powders of the Y-Ba-Cu-O system are prepared by a solution-polyacrylamide gel using citric acid as a complexing agent. This method provides an easy way to prepare reactive YBaCuO powders by sol-gel synthesis. However this synthesis involves intermediate phases formation which impedes the obtention of the pure phase at low temperature. An intermediate oxycarbonate phase forms between 800° and 850°C in flowing oxygen. From powder X-ray diffraction, thermal analysis and IR spectroscopy, it is concluded that the intermediate oxycarbonate has an average tetragonal structure—SG P4/mmm—similar to that of the parent oxide with a stoichiometry close to YBa2Cu2.95(CO3)0.35O6.6. The carbonate group is located in the center of the basal CuO square. This compound has superconducting properties. A pure 123 phase is obtained when the xerogel precursor is annealed at 925°C in O2 or at 800°C in Ar, then in O2. The grain growth and microstructure development of YBaCuO have also been investigated and compared using two different powders, i.e. sol-gel route and commercial powder from Hoechst.

Chemical reduction of LaNi0.5Cu0.5O3: the oxygen deficient perovskite like cupronickelite LaNi0.5Cu0.5O2.25

By heating the perovskite type cupronickelate LaNi0.5Cu0.5O3 under flowing H2 at 250 °C, the reduced cupronickelite LaNi0.5Cu0.5O3−δ has been prepared. TGA data point to an oxygen content equal to 2.25 which is understood in terms of the simultaneous presence of Ni(I) and Cu(II). Electron diffraction gives evidence for a 2ap√2 × 2ap√2 × 2ap perovskite supercell. Assuming that the crystal chemical properties of the isoelectronic 3d9 cations Cu2+ and Ni+ are rather similar, an average structure derived from the ordered perovskite type of the oxygen deficient cuprate La8−xSrxCu8O20 is retained in Rietveld calculations, from neutron powder diffraction data. A two phase model allows the calculation of two sets of Ni/Cu ratios i.e. the compositions LaNi0.6Cu0.4O2.2 and LaNi0.4Cu0.6O2.3 which originate in the parent components of the oxidized pseudo solid solution. The basic structural trends of the Ni(Cu) rich phase are connected to a strong preference of Ni+ for square and octahedral coordination and Cu2+ for pentagonal pyramidal coordination.