J. M. Siqueiros

Frequency-temperature response of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors. Part I. Structural and thermo-electrical characterization

With the purpose of fabricating ceramics where ferroelectric and magnetic order coexist, ceramics of Pb(Fe1∕2Nb1∕2)O3 have been prepared using the traditional ceramic method following three different routes. The first is a direct via starting from oxide reagents and the other two use different kinds of FeNbO4 precursors with either monoclinic or orthorhombic structures. Crystallographic and surface morphological studies were carried out by the powder x-ray diffraction and scanning microscopy techniques. The presence of Fe2+, detrimental to the ferroelectric and magnetic performance, was evaluated by x-ray photoelectron spectroscopy. The samples showed no structural differences, uniformly distributed grains, a ferro-paraelectric transition temperature at 110°C and a normal diffuse phase transition (nonrelaxor behavior). Differences in the degree of diffuseness, densities and grain size were observed depending on the kind of precursor. Measurements of dc and ac electrical resistivity, dielectric constant an...

Frequency-temperature response of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors. Part II. Impedance spectroscopy characterization

The dielectric behavior of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained using the traditional ceramic method employing three different precursors was investigated by impedance spectroscopy in the temperature range of 200–300°C. This study was carried out by means of the simultaneous analysis of the complex impedance Z, electric modulus M, and admittance Y functions from the measurements in the frequency range of 20Hz–1MHz. In correspondence to a previous structural, morphological, and temperature response study, appropriate microstructural and equivalent circuit models were established. Based on the brick layer model, three series of interconnected electrically distinct regions are considered: a conductive grain boundary layer, a capacitive grain boundary surface layer, and a resistive-ferroelectric bulk layer. Two conduction mechanisms were identified: a dielectric relaxation process due to localized conduction associated with the presence of oxygen vacancies and the nonlocalized conduction...

Ferroelectricity at room temperature in Pr-doped SrTiO3

The influence of Pr ion on the structural and dielectric properties was studied in Sr1−xPrxTiO3 for 0<x<0.075 composition from room temperature to 375°C. X-ray diffraction and x-ray photoelectron spectroscopy studies revealed that the praseodymium ion enters substitutionally in the Sr site with mixed valence in a Pr+3∕Pr+4=2:1 ratio. This substitution yields compressive chemical pressure in the lattice. The polarization measurements at room temperature display hysteresis loops, indicating that the Pr ion induces ferroelectric behavior at room temperature. The permittivity shows a well-defined peak at about 240°C, indicative of a ferroelectric-paraelectric transition with Curie temperature of ∼118°C.

Frequency-temperature response of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors. III. Dielectric relaxation near the transition temperature

Dielectric relaxation processes occurring near the ferroelectric-paraelectric phase transition of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors are discussed using microstructural and equivalent circuit modeling and the impedance spectroscopy technique. The frequency-temperature response was obtained from room temperature to 300°C and from 20Hzto1Mz. In correspondence with a previous structural, morphological, and temperature response study, appropriate microstructural, and equivalent circuit models were established. The frequency response study was carried out by means of the simultaneous analysis of the complex dielectric constant e and admittance Y functions and the dielectric loss, tanδ. A strong absorption near the transition temperature region at a frequency around 1MHz is discussed and is attributed with relaxation processes associated with domain reorientation, domain wall (DW) motion, and the dipolar behavior of ferroelectric materials. Such processes were foun...

Effects of two kinds of FeNbO4 precursors in the obtainment and dielectric properties of PFN ceramics

We report here a study of the obtainment, structural and dielectric characterization of the monoclinic and orthorhombic phases of FeNbO4 precursors and their effects in the processing and the dielectric properties of Pb(Fe1/2Nb1/2)O3 ceramics. Crystallographic and surface morphological studies were carried out by powder X-ray diffraction and scanning electron microscopy techniques. For structural determination, runs of Rietveld refinement were realized employing FULLPROF software. Measurements of electrical resistivity, dielectric constant and dielectric loss as functions of temperature were realized in PFN ceramics. An analysis and a comparative discussion of results are presented.

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...

ac conductivity in Gd doped Pb(Zr0.53Ti0.47)O3 ceramics

This study is focused in the conduction processes taking place in 0.6 wt % Gd doped lead zirconate titanate samples PbZr0.53Ti0.47O3:Gd (PZT53/47:Gd) in the vicinity of the morphotropic phase boundary. Doped samples show very large dielectric permittivity with respect to that of undoped ones near the transition temperature. The frequency dependent ac conductivity of PZT53/47:Gd ceramics was studied in the 30–450 °C temperature range. X-ray diffraction analyses indicate the incorporation of Gd atoms to the structure. The changes in the dielectric properties as functions of temperature of the doped samples are taken as additional evidence of the incorporation of Gd into the crystal structure. Gd acts as donor center promoting extrinsic n-type conduction. The ac conductivity behavior obeys Jonscher universal relation in the 100 Hz–1 MHz frequency range for temperatures between 30 and 300 °C. The measured conductivity values for Gd doped PZT53/47 are higher than those of pure PZT53/47. According to the correl...

Specific heat anomalies at 37, 105 and 455 K in SrTiO3:Pr

Strontium titanate modified with praseodymium was investigated by means of a combination of synchrotron x-ray diffraction and thermal analysis at high and low temperatures. The x-ray diffraction data obtained at room temperature revealed a breakdown of the cubic symmetry via the (002)/(200) split peaks. Thus, both the diffuse anomaly in the DTA measurement and the permittivity peak indicate that cubic symmetry is reached at about 455 K. At low temperatures, the specific heat reveals two anomalies at ~105 and ~37 K. Both anomalies were sensed for the phonon part of the total specific heat. The soft acoustic phonon modes are responsible for the plateau region in the specific heat at about 105 K, which is slightly larger for undoped samples. Furthermore, a clear anomaly at ~37 K in the specific heat is sensed by the Pr impurity in the host lattice of SrTiO3 and a minimum in the temperature dependence of the Debye temperature, ΘD around 30 K is observed. A maximum in the CT−3 versus T2 curve seems to indicate that the low-lying vibrational modes dominate at 37 K.

Ba0.5Sr0.5TiO3 thin films deposited by PLD on SiO2/Si RuO2/Si and Pt/Si electrodes

Abstract Stoichiometric barium strontium titanate (Ba 0.5 Sr 0.5 TiO 3 or BST) 220-nm thick thin films were deposited by pulsed laser ablation on SiO 2 /Si, RuO 2 /Ta/SiO 2 /Si and Pt/Ti/SiO 2 /Si substrates at 400°C. The films were weakly crystalline as-deposited. Crystallization was induced by annealing the films in the range of 550–650°C. The BST films deposited on Pt/Ti/SiO 2 /c-Si substrates presented wide cracks that were promoted during the annealing process due to the thermal expansion mismatch between the BST films (α BST =4×10 −6 °C −1 ) and the Pt (α Pt =9×10 −6 °C −1 ). Smooth films showing slightly cracked areas were obtained on SiO 2 /c-Si and RuO 2 /Ta/SiO 2 /Si substrates. The ruthenium oxide thermal expansion coefficient is α RuO 2 =5.2×10 −6 °C −1 . A cross-sectional analysis at the ferroelectric/substrate interface showed that for the lower annealing temperature (550°C) a mixed amorphous/nanocrystalline microstructure is formed. For temperatures above 600°C a randomly oriented polycrystalline material is obtained. However, an amorphous layer of 4–6 nm still remains on the substrate even after heat-treatments up to 650°C. The dielectric constant of the BST films varied in the range of 30–325.

Dielectric properties of the La3+ doped Sr0.3-3y/2LayBa0.7Nb2O6 ceramic system

Abstract A process for obtaining Sr 0.3− 3y 2 La y Ba 0.7 Nb 2 O 6 (LSBN) ceramics at different concentrations of the La3+ cation and different sintering times is presented. The dielectric properties of the LSBN system where Sr2+ ions are substituted by La3+ cations follow the pattern of a modification by cationic substitution for a constant Sr Ba ratio. This behavior of the dielectric properties indicate that the La3+ cations occupy the A1 as well as the A2 sites which favors the formation of the LSBN system up to y