Giorgio Flor

Raman spectroscopy of AMn2O4(A = Mn, Mg and Zn) spinels

First order Raman spectra in the region 200–800 cm−1 have been collected for AMn2O4 (A = Mn, Mg and Zn) tetragonal spinels. A possible correlation between Raman phonons and AO4 and MnO6 unit vibrations is proposed. Structural changes have been analyzed following the evolution of the Raman spectra with x for the Mg1−xMnxMn2O4 solid solution; the effect of the spinel inversion has been also studied on MgMn2O4 quenched samples from high temperatures. The results, taking into account also the X-ray diffraction and electron paramagnetic resonance data, show the influence of the Jahn–Teller effect on the Raman scattering for this class of materials.

Role of oxygen content on the transport and magnetic properties of La1−xCaxMnO3+δ manganites

Abstract In this paper we report about the synthesis and X-ray powder diffraction characterization of La 1− x Ca x MnO 3+ δ samples with x =0.1, 0.3 and −0.025≤ δ ≤0.054. Transport and magnetization measurements on these samples were performed as a function of the oxygen content both in over and under-stoichiometric regimes. We point out the role of the cation and anion vacancies and the obtained results suggest taking in relevant account the role of oxygen content when dealing with this kind of materials.

Effect of alkaline-doping on the properties of La2Mo2O9 fast oxygen ion conductor

Synthesis by means of a modified Pechini method of pure and alkaline-doped (Na, K and Rb) La2Mo2O9 (LAMOX) is presented. The electrical conductivity of LAMOX prepared by this method is completely analogous to that of the solid state prepared material. Doping with K and Rb hinders the α → β phase transition found in the pure and Na-doped material around 580 °C. Electrical measurements as a function of pO2 shows that LAMOX has a mainly ionic conductivity down to 10−18 atm. The electrical conductivity of the doped samples is close to that of the pure molybdate for temperatures higher than about 600 °C, while for K- and Rb-doped samples the conductivity is higher for lower temperatures, as a consequence of the absence of the phase transition. Finally, a non-Arrhenius behaviour is observed for all the investigated samples and correlated, at a first approximation, to the structural peculiarities of the LAMOX family.

Reactivity of β-aluminas with water

Abstract The reactivity with air moisture of finely divided crystals of Na β and Li + -containing β-aluminas is analyzed with thermogravimetric and X-ray techniques. For Na β-alumina two distinct water intake processes are evident: 1) a fast one, controlled by the elastic interactions among water molecules near the crystal boundaries; 2) a slow one, driven by water diffusion away from the boundaries. For finely divided crystals of Li β-alumina the two processes cannot be distinguished but the slow one is observable in crystals of macroscopic dimensions. The water intake is generally accompanied by an expansion of the c-axis. However, the relationship between water content and c-parameter is highly non-linear and the c-axis length at room temperature depends upon the thermal history of the sample. Our results confirm previous reports suggesting that β-aluminas absorb about one water molecule per cation of the conducting plane.

Magnetic and X-ray diffraction investigation on Mg1−xMn2+xO4 spinels

X-ray powder diffraction, electron paramagnetic resonance and static magnetisation measurements of slowly cooled Mg1−xMn2+xO4 solid solutions for 0≤x≤1 have been performed in order to investigate the effects of the Mg/Mn ratio on the spinel inversion. Our results prove that a small inversion degree exists at room temperature for the samples with x<0.5 and suggest that the inversion decreases with increasing x, that is by reducing the Mg2+ content. The multi-technique approach for the definition of the correct synthetic and cooling procedures enables the formation of single-phase high purity materials.

Electrons and holes in undoped Nd2CuO4

Abstract Nd 2 CuO 4± δ is the non-superconducting prototype of the Re 2− x M x CuO 4 t − y family (Re=Pr, Nd, Sm and M=Ceor Th) of n-type oxide superconductors. Four-probe DC conductivity, EMF in P (O 2 ) gradient, and thermopower measurements have been used to characterise its electric transport and defect structure between 300 and 900°C and between 5×10 −4 and 1 atm oxygen partial pressure. The results show that Nd 2 CuO 4±δ can be oxygen under-stoichiometric (with n-type conductivity), near-stoichiometric, and over-stoichiometric (with p-type conductivity) in different T, P (O 2 ) ranges.

KINETICS AND MECHANISMS OF THE FORMATION OF YBA2CU3O7-X

Abstract The paper reports diffusion experiments on the systems: BaCO 3 +CuO, BaCO 3 +Y 2 O 3 , Y 2 O 3 +CuO, BaCO 3 +Y 2 Cu 2 O 5 , Y 2 O 3 +BaCuO 2 , and Y 2 Cu 2 O 5 +BaCuO 2 . The results are discussed in connection with X-ray diffraction measurements on the corresponding powder mixtures and on ternary BaCO 3 +CuO+Y 2 O 3 mixtures. For YBa 2 Cu 3 O 6.5 formation, a three-step mechanism is proposed on the basis of transport and thermodynamic properties according to the sequence: (i) BaCO 3 +CuO→BaCuO 2 +CO 2 ; (ii) Y 2 O 3 +2CuO→Y 2 Cu 2 O 5 ; (iii) 4BaCuO 2 +Y 2 Cu 2 O 5 →2YBa 2 Cu 3 O 6.5 .

NdBa2Cu3O6+δ (Nd-123): low- and high-temperature conductivity

Abstract Conductivity measurements are presented for the NdBa 2 Cu 3 O 6+ δ superconductor. The measurements were carried out both in the high temperature range (350 T −4 P (O 2 ) −1 atm, and in the low temperature range (10 T δ δ δ δ >0.64–0.69; (c) in the tetragonal phase an Anderson localisation of the charge carriers driven by band fluctuation at the Fermi level is likely responsible for the semiconducting trend of resistivity vs. T below 300 K; (d) the T c vs. δ curve does not present the typical 60 K plateau, and (e) the room temperature ageing has no effect on T c .

Kinetics and mechanisms of formation of the Bi2Sr2CuOx superconductor

Abstract The solid state reactivity in the Bi, Cu, Sr/O pseudoternary system has been investigated using monodimensional diffusion experiments on selected reacting couples and differential thermal analysis, thermogravimetry, X-ray powder diffraction, optical and scanning electron microscopy and electron microprobe analysis on binary and ternary powder mixtures during isothermal or polythermal reactions. The results show that only three binary compounds are relevant for 221 formation kinetics. They are Bi 2 CuO 4 , Bi 2 Sr 2 O 5 and the wide range solid solution with rhombohedral symmetry and composition close to Bi 4 SrO 7 . The last compound shows the fastest formation kinetics from almost any couple of Bi- and Sr-containing phases, but does not eventually lead to the superconducting material, and therefore acts as a spurious phase or as a source of spurious phases in many ternary mixture reactions. Bi 2 Sr 2 O 5 is formed at a reasonable rate from Bi 4 SrO 7 but does not significantly react with CuO. Attention should be given also to some particular features of the ternary system, such as: (i) Bi 2 O 3 sublimation (which makes more complex some solid state reaction by providing alternate mechanisms); (ii) the very slow reactivity of SrO in comparison to its precursors (so that conditions favouring precursor decomposition should be avoided); and (iii) the equilibrium between the superconducting phase and its insulating polymorph. A promising way towards the 221 superconductor involves: (i) Bi 2 CuO 4 formation from the parent oxides and (ii) its reaction with a Sr-containing phase. SrO 2 precursor, low temperature and P (O 2 ) = 10–20 atm are required for the latter step.

Phase formation and physical properties in the Bi2O3/SrO/CuO system near the 2:2:1 (Bi:Sr:Cu) stoichiometry

Abstract The phases of the Bi 2 O 3 /SrO/CuO system near the 2:2:1 (Bi:Sr:Cu) stoichiometry have been prepared by solid state reaction using different precursors and characterized by X-ray powder diffractometry, thermogravimetry, optical and scanning electron microscopy, electron microprobe analysis and electrical resistivity measurements. The solid state reaction between Bi 2 O 3 , CuO and SrO 2 at 800°C and under P(O 2 ) > 10 5 Pa (1 atm) is effective in producing a single phase material having almost stoichiometric oxygen content, and belonging to the well-known crystal structure of the 221 superconductor (SU), with a pseudo-tetragonal subcell [ a = 0.5384(3), c = 2.4664(13) nm ; a ≅ b ] and almost commensurate modulation. An heterogeneous material is formed using the SrCO 3 precursor, the SU phase being mixed with variable amounts of its insulating polymorph , which shows a stoichiometry close to 17:16:7 (Bi:Sr:Cu), a temperature-dependent oxygen content, and a monoclinic subcell [ a = 2.4515(31), b = 0.5417(7), c = 2.1941(24) nm , β = 105.55(5)°] with fourfold superstructure along, c and is formed by decomposition of the SU phase. The two phases show room temperature resistivities (ρ) differing by almost two orders of magnitude, different ρ vs T behaviours, and different reactivities with environmental atmosphere.