L. Mestres

Highly textured Sr, Nb co-doped BiFeO3 thin films grown on SrRuO3/Si substrates by rf- sputtering

In this study, (011)-highly oriented Sr, Nb co-doped BiFeO3 (BFO) thin films were successfully grown on SrRuO3/Si substrates by rf-magnetron sputtering. The presence of parasite magnetic phases was ruled out based on the high resolution x-ray diffraction data. BFO films exhibited a columnar-like grain growth with rms surface roughness values of ≈5.3 nm and average grain sizes of ≈65–70 nm for samples with different thicknesses. Remanent polarization values (2Pr) of 54 μC cm−2 at room temperature were found for the BFO films with a ferroelectric behavior characteristic of an asymmetric device structure. Analysis of the leakage mechanisms for this structure in negative bias suggests Schottky injection and a dominant Poole-Frenkel trap-limited conduction at room temperature. Oxygen vacancies and Fe3+/Fe2+ trap centers are consistent with the surface chemical bonding states analysis from x-ray photoelectron spectroscopy data. The (011)-BFO/SrRuO3/Si film structure exhibits a strong magnetic interaction at the...

Synthesis, thermogravimetric and high temperature X-ray diffraction analyses of zinc-substituted nickel manganites

Abstract Stoichiometric spinel phases Mn 2.35−x Ni 0.65 Zn x O 4 were prepared by thermal decomposition of mixed oxalate precursor powders Mn 0.78−α Ni 0.22 Zn α C 2 O 4 ·nH 2 O (with 0 ≤ α ≤ 0.53) at 900°C. Cation-deficient phases Mn 2.35−x Ni 0.65 Zn x □ 3δ/4 O 4+δ were identified in the temperature range 350–500°C. The nonstoichiometric coefficient δ was found to strongly depend on the zinc content and the decomposition temperature. We showed that the introduction of zinc into the spinel phase enlarges the stability domain of the structure and inhibits oxidation at least up to 900°C. A cubic single-phase was observed for x ≤ 1.00. The lattice parameter variation of the oxides in the composition range 0 ≤ x ≤ 0.60 can be explained using Poix’s method, in terms of the distribution of Zn 2+ cations on the tetrahedral sites. However, for higher zinc content (x > 0.6) a detailed analysis of data showed that a small fraction of Zn 2+ is located on octahedral sites.

Phase Diagram at Low Temperature of the System ZrO2/Nb2O5

Various compositions of the ZrO2/Nb2O5 system were synthesized and the experimental conditions for obtaining reproducible results were established. The gel was precipitated at constant pH = 10, aged at room temperature for 18 h and, after filtration, dried at 110 °C for 24 h. The phase diagram of the ZrO2/Nb2O5 system was established between 600 and 1300 °C. Phase transitions of ZrO2 and Nb2O5 were observed with XRD; and two ternary compounds, 6 ZrO2 · Nb2O5 and 12 Nb2O5 · ZrO2, were identified. The samples with a Nb2O5 content between approximately 10 and 40 mole% showed the greatest specific area and are thus best for use as catalyst supports. Phasendiagramm des Systems ZrO2/Nb2O5 bei niedriger Temperatur Das System ZrO2/Nb2O5 wurde in verschiedener Zusammensetzung dargestellt und die experimentellen Bedingungen fur reproduzierbare Ergebnisse bestimmt. Das Gel wurde bei konstantem pH = 10 gefallt, 18 Stunden bei 20 °C gealtert und nach Filtration bei 110 °C getrocknet. Das Phasendiagramm des ZrO2/Nb2O5-Systems wurde zwischen 600° und 1300 °C bestimmt. Phasenubergange von ZrO2 und Nb2O5 wurden rontgenographisch untersucht und zwei ternare Phasen, namlich 6 ZrO2 · Nb2O5 und 12 Nb2O5 · ZrO2, identifiziert. Die Proben mit einem Nb2O5-Gehalt zwischen 10 und 40 mol-% haben die groste spezifische Oberflache und sind somit am besten fur Katalysatoren geeignet

Analysis of the Eu3+ emission in a SrWO4 laser matrix under pressure

Optical properties of the SrWO4 matrix doped with different Eu3+ concentrations (10%, 13% and 20%) have been studied. At ambient pressure, the temporal evolution of the emissions from the 5D1 level has been analyzed in the different doped crystals in order to discuss the energy transfer processes between the Eu3+ ions. This study lets conclude that the Eu3+ ions are randomly distributed in the matrix. Moreover, the 5D0→7F1, 2 emissions have been measured increasing the pressure up to 15 GPa in order to analyze the changes in the Eu3+ local structure. The results confirm the existence of a structural phase transition around 8.5 GPa.

Study on K2SeO4 at different temperatures

The crystal structure determination of K2SeO4 and the X-ray powder diffraction analysis have been carried out at different temperatures. The X-ray powder diffraction corroborate the two transitions from paraelectric to incommensurate and from this to ferroelectric phase. This study shows, also, an inhomogeneous behavior of K2SeO4 when the temperature decreases. The crystal structure determination shows the oxygen atoms of the selenate ion do not show the same effective charge. The study at different temperature shown a rotation of SeO-24 ion around the c-axis and the lengthening of Se—O(2) bond when the temperature is lower. From the structural data, the librational study and the distortion theory we deduce that K(1) is the main ion that produces the different transitions. The incommensurate phase is essentially produced by the freedom of K(1) ion to have a displacement along the a-axis together with a libration of selenate ion about the b-axis. A rotation of the selenate ion around the b-axis together wi...

Separation of electronic and ionic conductivity in mixed conductors from the ac response: Application to Pr0.56Bi0.04Li0.2TiO3

A method has been developed for separation of ionic and electronic conductivity in mixed conductors. The procedure is accomplished by using blocking electrodes and the Kramers–Kronig relations for separating the dc conductivity from total response. This method was applied to lithium inserted and deinserted sample of Pr0.56Bi0.04Li0.2TiO3. This material is an ionic conductor, and after insertion, electronic conduction appears in the sample. The deinserted sample has a similar electric behavior to the original material. This method does not need any previous model when processing the data. Hence, it avoids those modeling related mistakes and increases the possibilities of dielectric spectroscopy.

The paraelectric-incommensurate phase transition and crystal structure of the mixed crystals

A study on the mixed crystals of general formula has been carried out, using thermal analyses and x-ray diffraction on powder and single-crystal samples at different temperatures. The replacement of the potassium ion by the ammonium ion is only possible for . The transitions paraelectric-incommensurate-ferroelectric are only observed for and the temperature of the paraelectric-incommensurate transition increases when the value of x decreases. The replacement of the ion by the ammonium ion is only produced in the K(1) site. The different behaviour of the mixed crystals and the compound is explained from the structural results.

Crystal structure of the mixed conductors phases, Li0.5-3xLa0.5+x+yTi1-3yM3yO3 (M = Mn, Cr) with x = 0.133 and y = 0.20

Perovskite-like solid solutions of general formula Li 0.5-3x La 0.5+x+y Ti 1-3y M 3y O 3 (M=Mn, Cr) show three polymorphs; A, beta; and C. The crystal structure of the C polymorphs in manganese- and chromium-lanthanum systems, determined from powder neutron diffraction using Rietveld refinement, are of the ordered perovskite type. The structures of both phases are similar, containing a three dimensional framework of corner-sharing MO 6 (M=Ti or Mn/Cr) octahedra in which the structures are partially collapsed as a result of a cooperative tilting and rotation of octahedra. Orthorhombic unit cell, M=Mn: a=5.5411(11) A, b=7.8120(14) A, c=5.4924(10) A; M=Cr: a=5.5014(9) A, b=7.7735(15) A, c=5.4729(9) A; space group Pnma (no. 62). Ionic conductivity takes place by a hopping mechanism between Li + -occupied and empty A-sites, while electronic conductivity is along octahedra.

Study on the mixed crystals

A study on the mixed crystals of formula (1>x>0) has been carried out, using thermal analyses and x-ray diffraction on powder and single-crystal samples at different temperatures. It is shown that mixed crystals with have a transition from the paraelectric to the ferroelectric phase; decreases as the value of x increases. The orientation of the sulphate ion with respect to the crystallographic axes depends on the value of x. Mixed crystals with the paraelectric - ferroelectric transition turn around the b axis when the temperature decreases in the paraelectric phase, while in mixed crystals without a transition the orientation of the sulphate ion does not alter when the temperature varies. The transition is displacive, increasing the value of the parameter a without a soft mode. This is deduced from a study of the crystal structures, the translational and libration tensors and a density map of the sum of the bond valences.

Behavior of ammonium ion in β-LiRb1−x(NH4)xSO4 mixed crystals (0.5 ≤ x ≤ 1.0)

The influence of replacement of an ammonium ion by rubidium on the x-T phase diagram in the concentration range 0.50 ≤ x ≤ 1.0 has been studied by X-ray and neutron powder diffraction over a wide temperature range. It is shown that a decrease in the ammonium concentration is accompanied by an increase in the temperature of the II ⇔ I phase transition and stabilization of phase II up to low temperatures (20 K). The changes occurring in the dynamics of mixed crystals are studied by inelastic incoherent neutron scattering. The spectra of the generalized vibrational density of states obtained allowed one to establish the difference in the phonon modes corresponding to phase III of β-LiNH4SO4 and phase II of β-LiRb1−x(NH4)xSO4 mixed crystal with x = 0.91 and x = 0.77 at 20 K. It is shown that a mixed crystal with x = 0.77 at 20 K is in the orientational glass state.