Pascal Marchet

Sodium-bismuth titanate based lead-free ferroelectric materials

Abstract In search of lead-free ferroelectric ceramics with improved properties, an investigation was carried out on barium and strontium-modified Na0.5Bi0.5TiO3 (NBT) ceramics. Many compositions among the Ba and Sr-modified ceramic materials exhibit diffuse phase transition and are characterised by a strong temperature and frequency dispersion of the permittivity which would be connected with the cation disorder in the A site of the perovskite unit cell.

Structural and dielectric study of theNa0.5Bi0.5TiO3–PbTiO3and K0.5Bi0.5TiO3–PbTiO3systems

A study of the Na 0.5 Bi 0.5 TiO 3 –PbTiO 3 and K 0.5 Bi 0.5 TiO 3 -PbTiO 3 systems has been carried out using X-ray diffraction, differential scanning calorimetry and dielectric measurements. The limits of the rhombohedral (Na 0.5 Bi 0.5 TiO 3 -rich side) and tetragonal (PbTiO 3 -rich side) solid solutions have been determined, as well as the evolution of their lattice parameters as a function of composition and temperature. Ceramic materials have been prepared by natural sintering (1090–1220 °C; 0.5 h) of powders obtained by solid-state reaction (900–1000 °C; 20 h) of the corresponding oxides and carbonates. The dielectric permittivities of these materials have been measured in a wide frequency range for temperatures between 20 and 800 °C. The results showed that they all are ferroelectric at room temperature and exhibit either a diffuse, probably second-order phase transition, or a sharp, first-order-like phase transition from the ferroelectric to the paraelectric state, depending on the composition.

Electrical properties of (110) epitaxial lead-free ferroelectric Na0.5Bi0.5TiO3 thin films grown by pulsed laser deposition: Macroscopic and nanoscale data

We report the electrical properties, measured both at the macroscopic and nanometric scales, of epitaxial (110)-Na0.5Bi0.5TiO3 (NBT) thin films grown on (110)Pt/(110)SrTiO3 by pulsed laser deposition (PLD). The influence of the A-site composition (Na and/or Bi excess) on both the structural/microstructural characteristics and the electrical properties is discussed. Whatever the composition of the NBT target, the final layers are systematically epitaxially grown, with NBT crystallites mainly (110)-oriented, and as well (100)-oriented for some minor proportion. Atomic force microscopy (AFM) images reveal the coexistence of two kinds of grains presenting different shapes: namely flat and elongated grains, corresponding to (100)- and (110)-oriented NBT crystallites, respectively. The macroscopic ferroelectric properties were measured at room temperature. A rather well-defined shape of the hysteresis loops was obtained: the incorporation of a Bi excess in the target clearly improves the saturation of the loops...

Equilibrium and non-equilibrium phase diagram within the TeO2-rich part of the TeO2-Nb2O5 system

The TeO 2 -rich part of the TeO 2 -Nb 2 O 5 system has been investigated by temperature programmed X-ray diffraction and differential scanning calorimetry. Three invariant equilibria have been observed: one eutectic reaction (8 mol% NbO 2.5 , T E =690±5 °C, L E TeO 2 +Nb 2 Te 4 O 13 ), one peritectic reaction (incongruent melting, at 766±5 °C, of the Nb 2 Te 4 O 13 compound) and one congruent melting reaction (congruent melting, at 810±5 °C, of the Nb 2 Te 3 O 11 compound). A large glass-forming domain has been evidenced (0 to 25 mol% NbO 2.5 ). The thermal behaviour of these glasses has been followed. The glass transition, crystallization temperatures and the nature of crystalline phases formed have been determined.

New investigations within the TeO2-rich part of the Tl2O–TeO2 system

The phase diagram TeO2–Tl2TeO3 has been studied under equilibrium and non-equilibrium conditions by differential scanning calorimetry and temperature programmed X-ray diffraction. Four crystalline phases, three stable (Tl2TeO3 , α-Tl2Te2O5 , Tl2Te3O7) and one metastable (β-Tl2Te2O5) have been identified and characterized. The three stable phases decompose peritectically at 377, 315 and 342 °C respectively. β-Tl2Te2O5 , which is formed by cooling of a melt, undergoes a monotropic transformation into the stable α polymorph. This study also confirmed the existence of a large glass-forming domain (15–52 mol% TlO0.5). The complex thermal behaviour of these very stable glasses has been studied. The glass transition and crystallization temperatures, and the nature of crystalline phases formed, have been determined.

The TeO2-rich part of the TeO2–Ga2O3 system: equilibrium and non-equilibrium phase diagram

The TeO2-rich part of the TeO2–Ga2O3 system has been investigated, under equilibrium and non-equilibrium conditions, by temperature programmed X-ray diffraction and differential scanning calorimetry (DSC). Two invariant equilibria have been detected: one peritectic reaction and one eutectic reaction. The peritectic reaction corresponds to the incongruent melting at 620 °C ± 5 °C of the crystalline compound Ga2Te4O11 (ΔHP = 18.1 ± 2 kJ mol−1) and to the equilibrium Ga2Te4O11 ↔ LP + Ga2O3. The eutectic reaction (23 mol% GaO1.5, TE = 600 ± 5 °C, ΔHE = 22.2 ± 2 kJ mol−1) corresponds to the equilibrium TeO2 + Ga2Te4O11 ↔ LE. The glass forming domain ranges from 5 to 30 mol% GaO1.5, and can be extended from 0 to 35 mol% by quenching the melt in a freezing mixture, consisting of ice, ethanol and NaCl (≈−14 °C). The thermal behaviour of glasses has been studied. The glass transition and crystallization temperatures, and the phases which appear during the crystallization have been identified.

The crystal structure of PbTe5O11

Abstract PbTe 5 O 11 crystallises with monoclinic symmetry (space group C2/c) and unit cell parameters: a = 18.959(4) A, b = 4.414(1) A, c = 25.798(6) A, β = 98.12 (1), Z = 8). Its crystal structure has been refined by a full matrix least-squares process to R 1 = 0.037 and wR 2 = 0.068 values, on the basis of 3121 independent single crystal X-ray reflections. It can be described as a Te 5 O 11 three-dimensional network of TeO 4 disphenoids (distorted TeO 4 E trigonal bipyramids) sharing corners and edges which contains three kinds of tunnels parallel to the Oy direction. The stereochemically active lone pairs E of tellurium atoms are directed towards the centre of two of them. Lead atoms are inserted in pairs in the third one.

Ultra-High Tunability of

We report the high-frequency electrical characterization of Ba(2/3)Sr(1/3)TiO3 (BST) thin films exhibiting a very high dielectric tunability with low losses at 2.45 GHz under very low applied electrical fields. BST layers were integrated in out-of-plane Metal-Insulator-Metal (MIM) devices with optimized Ir/MgO(100) bottom electrodes. The high frequency properties of BST films with thicknesses of 200 nm, 450 nm and 1450 nm were thoroughly investigated in the 100 MHz-10 GHz domain and exhibit extremely high capacitance tuning abilities of 82%, 81% and 70% respectively, under applied voltages as low as 10 V. MIM devices responses show the onset of acoustic resonances associated with the BST electrostrictive behavior under an electric field. By combining high tunability with low resistive losses under low applied voltages, these devices are opening promising avenues for their integration in high-performance tunable devices in the microwave domain and particularly at 2.45 GHz, corresponding to the widely used ISM (industrial, scientific and medical) frequency band.

\text{Ba}_{(2/3)}\text{Sr}_{(1/3)}\text{TiO}_{3}

The existence of domain wall motion at microwave frequencies and its contribution to the ferroelectric complex permittivity is shown by evaluating the dielectric properties of BaSrTiO 3 (BST) thin films as a function of the incident power. Even at low AC field amplitudes, the presence of the domain walls and the correlated motions (vibration and jumps) result in sensitivity of the dielectric properties to the incident field amplitude. Although the contribution of domain wall motion to the real part of the permittivity is not preponderant (less than 10 %), it represents more than 50 % of the materials global dielectric losses. This illustrates the importance to consider domain wall motion even in the microwave frequency region and the necessity to take into account the applied AC field amplitude (and thus the incident power) when characterizing ferroelectric materials. The present study has been realized on BST thin films, elaborated by pulsed laser deposition on MgO/Ir substrates.

-Based Capacitors Under Low Electric Fields

Abstract Laser ablation by varying the number of consecutive laser pulses upon two targets, one superconducting YBa 2 Cu 3 O 7 , the other insulating MCuO 2 (M=Sr, Ca), is used to build thin films of tuneable average compositions. Most of the samples are found to display high- T c superconductivity with critical temperature generally higher than the 60 K of the ‘Cava phase’, the prototypic structure with two consecutive conducting CuO 2 planes and containing same types of atoms. The properties of the samples are not linearly dependent on the concentration of species coming from the insulating target. In addition, they depend not only on the ratio of the number of pulses on each target, but also on the number of consecutive pulses on each target. Thin films are characterized by temperature-dependent DC conductivity, X-ray diffraction, Castaing microprobe and infrared reflectivity spectroscopy. Present results suggest the robustness of high- T c superconductivity, possibly related to a ‘Cava-phase-like’ skeleton.