Enhanced piezoelectric properties of Sr(1.7)(Na0.5Bi0.5)0.3Bi4Ti5O18 ceramics in the system Sr(2−x)(Na0.5Bi0.5)xBi4Ti5O18 (where 0 ≤ x ≤ 0.5)

Fine powders synthesized via sol-gel route were employed to fabricate Sr_(2−x)(Na_0.5Bi_0.5)_xBi₄Ti₅O₁₈ (SNBT, where x = 0, 0.1, 0.25, 0.3, 0.4, and 0.5) ceramics. The composition (x)-dependent structural changes associated with SNBT ceramics were analyzed using X-ray powder diffraction, transmission electron microscopy, and Raman spectroscopic techniques. Average grain size analyses carried out on the SNBT ceramics by scanning electron microscopy revealed an important role played by the dopants in inhibiting the grain growth. Dielectric constants and the Curie temperature of the ceramics were found to decrease and increase, respectively, with increase in x. The increase in Curie temperature with increase in x was attributed to the decrease in the tolerance factor. The specific composition (x = 0.3) of the SNBT ceramics exhibited improved piezo- and ferroelectric properties associated with a higher Curie temperature (569 K). The piezoelectric coefficient (d₃₃) and the planar electromechanical coupling coefficient (k_p) of SNBT(x = 0.3) were enhanced by 25% and 42%, respectively, as compared to the undoped ceramics.     

Structure and multiferroic properties of ternary (1−x)(0.8BiFeO3-0.2BaTiO3)-xK0.5Na0.5NbO3 (0 ≤ x ≤ 0.5) solid solutions

The ternary (1?x)(0.8BiFeO_3-0.2BaTiO₃)-xK_0.5Na_0.5NbO₃ (0?≤?x?≤?0.5) solid solutions have been successfully synthesized by a solid-state reaction route. X-ray diffraction and Rietveld refinement studies reveal the phase transition from the rhombohedral and tetragonal phases to the single tetragonal phase with x increasing. The average grain size decreases initially and then increases as x increases, whereas the remnant magnetization shows an opposite trend and reaches the maximum value of ~2.09?emu/g at x?=?0.3. An enhanced remnant polarization of ~8.6?μC/cm² appears at x?=?0.3 due to the structure distortion and the decrement of defects. Moreover, the remanent polarization and the relative permittivity reach the maximum value of ~20.14 μC/cm² (10?Hz) and ~644 (1?kHz) at x?=?0.5, respectively, and the corresponding dielectric loss decreases to the lowest value of ~0.022 (1?kHz). These results indicate that the properties of ternary BFO-BTO-KNN solid solutions can be modulated by adjusting the K_0.5Na_0.5NbO₃ content to adapt to different application needs.     

State of manganese atoms in the Nd1+x Sr2−x Mn2O7−δ solid solution

The oxidation state of manganese in the Nd_2?x Sr_1+x Mn₂O_7?δ solid solution was determined by X-ray photoelectron spectroscopy and by calculating the oxygen nonstoichiometry based on the gravimetric data. As a result of the heterovalent replacement of Nd³⁺ with Sr²⁺, the change in the oxidation state of manganese occurs in different ways, i.e., it increases at x > 0 and decreases at x < 0. In the latter case, some oxygen ions acquire the oxidation state of ?1. The samples slowly cooled under oxidative conditions possess a significant positive oxygen non-stoichiometry, which tends to decrease after Nd³⁺ is replaced with Sr²⁺. An excess of oxygen stabilizes the crystal structure of Nd_1+x Sr_2?x Mn₂O_7?δ.     

Temperature dependent phase transition of (Bi0.5Na0.5)1−xBaxTiO3 lead-free piezoelectric

Phase transformation and electric properties of lead-free piezoceramics (Bi_0.5Na_0.5)_1?xBa_xTiO₃ with x=0.05, 0.06, and 0.07(BNB5T, BNB6T and BNB7T) were investigated using dielectric, piezoelectric and ferroelectric measurements. Electric field induced strain measurement shows “W” shape bipolar strain characteristics for BNB5T with typical ferroelectric P–E curve, while BNB6T and BNB7T, possessing pinch-off P–E, exhibit “V” shape field-induced strain. All the BNBxT specimens exhibit relaxor characteristic, identified by the Debye Law. Dielectric properties measured at elevated temperatures with the frequency variation (10–500 kHz) reveal frequency dispersion below the T_d point, but no dispersion between T_d and T_m, which may be ascribed to an intermediate phase transition. By adding more Ba²⁺ ions, the region of intermediate phase, distinguished by frequency dependence dielectric constant, expands to lower temperature. Moreover, the ferroelectric properties measured at elevated temperature were carried out below and at the depolarization temperature to well investigate the existence of this phase. Much less ε–T profile dispersion were observed during the investigation of BNB6T and BNB7T, leading to possible existence of an intermediate phase in the investigated compositions. The results suggest that the linear field-induced-strain of (Bi_0.5Na_0.5)_1?xBa_xTiO₃ are expected to be attributed to the intermediate phase.     

Surface Exchange of Oxygen in La1−x Sr x FeO3−δ (x = 0, 0.1)

The electronic charge carrier concentration in La_1?x Sr _x FeO_3?δ was shown to depend on the partial pressure of O₂ (pO ₂). Chemical diffusion coefficient and surface exchange coefficient, k _chem, were determined by conductivity relaxation in O₂/N₂ and CO/CO₂ mixtures. k _chem was proportional to pO ₂ ^1.06 in O₂/N₂, while in CO/CO₂ k _chem was controlled by a reaction mechanism involving both CO and CO₂.     

Finite size effect on Sm3+ doped Mn0.5Zn0.5SmxFe2−xO4 (0≤x≤0.5) ferrite nanoparticles

Samarium doped Mn–Zn ferrite nanoparticles of composition Mn_0.5Zn_0.5Sm_xFe_2−xO₄ (0≤x≤0.5) have been synthesized by a chemical co-precipitation method for developing low Curie temperature stable ferrofluid. These samples were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Electron Paramagnetic Resonance (EPR) spectroscopy and search coil method analytical techniques for their structural, morphological and magnetic properties. X-ray diffraction patterns confirmed the formation of crystalline single spinel phase of as grown nanoparticles. Lattice parameter and lattice strain increases with the increase in Sm³⁺ content. SEM images revealed the presence of ultrafine particles and their agglomerated structures in higher Sm³⁺ ions concentration analogues. The stoichiometry of the final product agreed well with the initial substitution composition as evidenced by EDS data. Electron paramagnetic resonance (EPR) spectra proved the ferromagnetic nature of nanoparticles. The magnetic measurements by search coil method showed superparamagnetism for x=0, 0.1 the samples with saturation magnetization of 23.95 emu/g for Mn_0.5Zn_0.5Fe₂O₄ sample which increases with rise in Sm³⁺ ions content. The results are explained and correlated with the structural, morphological and magnetic properties for developing stable kerosene based ferrofluid by using these nanoparticles.     

Green and blue materials for the ceramic industry from pink MgCoxNi1-xSiO4 (0 ≤ x ≤ 1) solid solutions

In this study, MgCo_xNi_1-xSiO₄ (0.0 ≤ x ≤ 1.0) solid solutions with an olivine structure were synthetized via the chemical coprecipitation method and materials with a smaller M(II) (M = Co, Ni) amount than Co₂SiO₄ and Ni₂SiO₄ compounds were obtained. At 1200 °C, the Co(II) and Ni(II) were randomly distributed in the MgCo_xNi_1-xSiO₄ (0.0 ≤ x ≤ 1.0) solid solutions with the olivine structure, but the occupation of Co(II) and Ni(II) ions in M1 (4a) octahedral sites was obtained at a higher level than in M2 (4c) octahedral sites. The Mg(II) ions prefer the M2 sites. This preference explains the main contribution of the M1 sites in spectra of octahedral Co(II) ions and the M1-O and M2-O distances jointly explain the pink colour of the MgCo_xNi_1-xSiO₄ (0.0 ≤ x ≤ 1.0) solid solutions, while the colour of Co₂SiO₄ is blue. Spectra can be interpreted as the sum of Ni(II) and Co(II) ions in octahedral sites. When these solid solutions are enamelled, the pink colouring changes to green or blue because of the presence of tetrahedral Co(II).     

Ceramic pigments from CoxNi3-xP2O8 (0 ≤ x ≤ 3) solid solutions

Co_xNi_3-xP₂O₈ (0 ≤ x ≤ 3) solid solutions were synthesized via the chemical co-precipitation method. Variation of unit cell parameters and interatomic distances indicated that these solid solutions with the Ni₃P₂O₈ structure are stable between 800 and 1200 °C in compositions with 0 ≤ x ≤ 1.5 and between 800 and 1000 °C when (0 ≤ x ≤ 3). When (2.5 ≤ x ≤ 3.0), the solid solutions lead to the Co₃P₂O₈ structure, being stable between 800 and 1000 °C.The yellow colour of the Ni₃P₂O₈ compound changes to pink or red when Co(II) ions are incorporated in the structure as Ni₃P₂O₈ solid solutions are formed. Bands corresponding to second and third electronic transitions of the Co(II) ions in octahedral coordination appear in the 450–600 nm in the UV-V spectra, and they are responsible of the observed changes in the colour. Absorbance in the visible spectra was also obtained from enamelled samples but a new band at 650 nm with considerable absorbance when x > 1.0 increased the blue amount, and colour of the enamelled samples was yellowish brown, brown, green and blue.     

Role of iron in the ferromagnetic-antiferromagnetic boundary of La0.5Ca0.5Mn1-xFexO3+δ (0≤x≤0.5) manganites

Samples of La_0.5Ca_0.5Mn_1−xFe_xO_3+δ (0≤x≤0.5) were synthesized using a solid-state reaction method involving a milling process and thermal treatment up to 1200 °C in an air atmosphere. Samples were characterized structurally with X-ray diffraction analysis and Rietveld refinement with morphology characterization using scanning electron microscopy. Magnetic properties were investigated using a physical property measurement system to obtain zero field cooling and the associated curves to plot hysteresis loops. Our results revealed the interplay between the structural and magnetic properties as Fe ions attached to the crystalline structure. A mechanism based on the substitution of Mn³⁺ and Mn⁴⁺ by low-spin Fe³⁺ and Fe⁴⁺ ions, respectively, was hypothesized to interpret the experimental data. More specifically, the temperature at which the transition from ferromagnetic to paramagnetic occurred increased with increasing Fe content as a result of a greater density of oxygen-mediated ferromagnetic bonds. Conversely, the magnetization weakened because the t_2g electrons were distributed in the respective d orbitals by adopting a low-spin configuration. Such a configuration is preferred as a result of the unit cell distortion in the milling process where the greater ionic radius of the Fe⁴⁺ ions leads to an elongated c-axis tetragonal symmetry and a greater unit cell volume. Finally, low-temperature magnetic behavior revealed the occurrence of a reentrant spin-glass type state within the ferromagnetic matrix favored by a milling-driven structural disorder and the existence of competitive superexchange interactions.     

Structure,dielectric, and ferroelectric properties of (1-x)Bi0.5Na0.5TiO3 – X K0.5Na0.5NbO3 (0 ≤ x ≤ 0.1) solid solution

The correlation of the phase structure, dielectric, and ferroelectric properties of lead-free (1-x)(Na_0.5Bi_0.5)TiO₃–xK_0.5Na_0.5NbO₃ (NBTKNx) (0 = x ≤ 0.1) polycrystalline ceramics, fabricated via a solid state reaction technique, were investigated. The Rietveld refinement allowed identifying the crystallographic transformation from a rhombohedral to a coexisting rhombohedral-tetragonal or tetragonal long range-ordered ferroelectric (FE) phase. The dielectric investigations showed an increase of the dielectric diffuseness (1.53 = γ ≤ 1.73) and a clear shift of the depolarization temperature (T_d) to a lower temperature while increasing substitution. More importantly, the lattice disorder also generated a plateau-like dielectric anomaly, leading to a thermally stable ϵ_r ∼2859 ± 20% (120–500 °C) and ∼3112 ± 10% (120–420 °C) for x = 0.075 and 0.1 samples, respectively. At room temperature (RT), Raman spectroscopy investigations revealed a downshift of the frequencies as a function of the composition with an inhomogeneous broadening of the Raman lines. On heating, Raman spectra showed changes in the region where the dielectric transitions are observed. Moreover, the composition dependence of the current peaks in the I-E loops confirmed the occurrence of a phase transition from a non-ergodic polar phase to an ergodic weakly polar after the applying of an electric field of 60 kV/cm⁻¹.     

Temperature-independent permittivity of xBaTiO3–(1−x)(0.5Bi(Mg1/2Ti1/2)O3–0.5BiScO3) ceramics

xBaTiO₃–(1−x)(0.5Bi(Mg_1/2Ti_1/2)O₃-0.5BiScO₃) or xBT–(1−x)(0.5BMT–0.5BS) (x=0.45–0.60) ceramics were prepared by using the conventional mixed oxide method. Perovskite structure with pseudo-cubic symmetry was observed in all the compositions. Dielectric measurement results indicated that all the samples showed dielectric relaxation behavior. As the content BaTiO₃ was decreased from 0.60 to 0.45, temperature coefficient of permittivity (TCε) in the range of 200–400 °C was improved from −706 to −152 ppm/°C, while the permittivity at 400 °C was increased from 1208 to 1613. The temperature stability of permittivity was further improved by using 2 mol% Ba-deficiency. It was found that lattice parameter and grain size of the 2 mol% Ba-deficient ceramics were smaller than those of their corresponding stoichiometric (S) counterparts, with TCε in the range of 200–400 °C to be improved noticeably. For example, TCε of the Ba-deficiency sample with x=0.45 was −75 ppm/°C in the temperature range of 200–400 °C and the permittivity was 1567 at 400 °C. The results obtained in this work indicated that xBT–(1−x)(0.5BMT–0.5BS) ceramics are very promising candidates for high temperature capacitor applications.     

The structure evolution and microwave dielectric properties of MgAl2-x(Mg0·5Ti0.5)xO4 solid solutions

In the present work, MgAl_2-x(Mg_0·5Ti_0.5)_xO₄ (x = 0.02, 0.04, 0.06, 0.08, 0.10) solid solutions were synthesized via the traditional solid-state reaction route. The valence state of Ti ions, crystal structural characteristics, and microwave dielectric properties were discussed. A solid solution with spinel structure was revealed by the Rietveld refinement results. The partial substitution of (Mg_0·5Ti_0.5)³⁺ for Al³⁺ lowered the sintering temperature and improved the Q × f value of MgAl₂O₄ ceramic. The MgAl_2-x(Mg_0·5Ti_0.5)_xO₄ solid solutions with x = 0.06 can be well sintered at 1425 °C in an oxygen atmosphere for 8 h and exhibits excellent microwave dielectric properties with ε_r = 9.1, Q × f = 98,000 GHz, τ_f = −61.36 ppm/°C. The sintering temperature of MgAl_1·94(Mg_0·5Ti_0.5)_0.06O₄ microwave dielectric ceramics was approximately 200 °C lower than that of conventional MgAl₂O₄ ceramics.     

Composition-dependent microstructure and electrical property of (1−x)SBN-xBNBT solid solutions

In this study, (1−x)Sr_0.75Ba_0.25Nb₂O₆-x[0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃] solid solution ceramics were prepared and investigated. The length-diameter ratio of pillar-like grain increased with increase in x, reaching maximum value of 10.5 at average length of 1.36 μm and diameter of 0.13 μm for x = 0.20. The maximum dielectric constant, temperature corresponding to the maximum dielectric constant, and saturated polarization decreased, respectively, from 507, 36°C, and 3.2 μC/cm² for x = 0 to 365, −61°C, and 2.0 μC/cm² fort x = 0.10, and then increased to 1167, 43°C, and 5.5 μC/cm² for x = 0.20. These results indicate the competitive effects of B-site Ti⁴⁺ and A-site Ba²⁺ on ferroelectricity.     

Structure and microwave dielectric characteristics of Sr2[Ti1−x(Al0.5Nb0.5)x]O4 (x ≤ 0.50) ceramics

Sr₂[Ti_1−x(Al_0.5Nb_0.5)_x]O₄ (x = 0, 0.10, 0.25, 0.30, 0.5) ceramics were synthesized by a standard solid-state reaction process. Sr₂[Ti_1−x(Al_0.5Nb_0.5)_x]O₄ solid solutions with tetragonal Ruddlesdon-Popper (R-P) structure in space group I4/mmm were obtained within x ≤ 0.50, and only minor amount (1-2 wt%) of Sr₃Ti₂O₇ secondary phase was detected for the compositions x ≥ 0.25. The temperature coefficient of resonant frequency τ_f of Sr₂[Ti_1−x(Al_0.5Nb_0.5)_x]O₄ ceramics was significantly improved from 132 to 14 ppm/°C correlated with the increase in degree of covalency (%) with increasing x. The dielectric constant ɛ_r decreased linearly with increasing x, while high Qf value was maintained though it decreased firstly. The variation tendency of Qf value was dependent on the trend of packing fraction combined with the microstructure. Good combination of microwave dielectric properties was achieved for x = 0.50: ɛ_r = 25.1, Qf = 77 580 GHz, τ_f = 14 ppm/°C. The present ceramics could be expected as new candidates of ultra-high Q microwave dielectric materials without noble element such as Ta.     

The PZT system (PbTixZr1–xO3, 0≤x≤1.0): The real phase diagram of solid solutions (room temperature) (Part 2)

The sequence of phase transformations in the ceramic system PbTi_xZr_1?xO₃ (0≤x≤1.0) is determined and the real phase diagram of solid solutions is built. The observed periodicity of phase formation processes in the rhombohedral and tetragonal regions is explained by the real (defective) structure of PZT system ceramics, which is in many respects related to the variable valence of Ti ions and, as a result, to formation, accumulation, and ordering of point defects (oxygen vacancies) and their elimination by crystallographic shifts. The obtained results are useful in interpretation of the macroscopic properties of ceramics based on the PZT system.     

Structural evolution and ferroelectric properties in lead-free (1−x)(Bi0.5Na0.4K0.1)TiO3-xSrTiO3 solid solutions

Bi_0.5Na_0.5TiO₃ (BNT)-based dielectric ceramics have received a lot of attention due to the increased demand for pulse ceramic capacitors. However, comprehensive study on the relationship between their internal phase structure, dielectric characteristics, and ferroelectric properties is still lacking. The phase evolution and its impact on dielectric and ferroelectric properties of an important BNT-based solid solution, Bi_0.5Na_0.4K_0.1TiO₃-xSrTiO₃ (x = 0, 0.1, 0.2, 0.3 and 0.4), were investigated systematically in this work using structural, dielectric, and ferroelectric characterization techniques. X-ray diffraction indicated the coexistence of rhombohedral and tetragonal phases. The frequency- and temperature-dielectric characterization was then used to derive the characteristic temperatures T_B, T_m, T_s, and T_d, and a phase diagram was developed. Furthermore, the temperature-dependent current against electric field curves and polarization versus electric field loops were used to derive the characteristic temperatures connected to high electric field features. This study not only explains the phase evolution of the Bi_0.5Na_0.4K_0.1TiO₃-xSrTiO₃ solid solution, but it also correlates microscopic domains and polar nanoregions to macroscopic dielectric and ferroelectric properties.     

Lithium ion conductivity in the solid electrolytes (Li0.25La0.25)1−xM0.5xNbO3 (M=Sr,Ba, Ca,x=0.125) with perovskite-type structure

Perovskite-type structured solid electrolytes with the general formula (Li_0.25La_0.25)_1−xM_0.5xNbO₃ (M=Sr, Ba, Ca, x=0.125) have been prepared by solid-state reaction. Their crystal structure and ionic conductivity were examined by means of X-ray diffraction analysis (XRD), scanning electron microscope (SEM), and alternating current (AC) impedance technique. All sintered compounds are isostructural with the parent compound Li_0.5La_0.5Nb₂O₆. Some impurity phase is detected at the grain boundary in the Ba- and Ca-substituted compounds. The substitution of partial Li⁺ by alkaline earth metal ions has responsibility for the cell volume expansion as determined by the XRD data. The densification is accelerated, with the overall porosity and grain boundary minimized as Sr²⁺ ions are doped. Among the investigated compounds, the perovskite (Li_0.25La_0.25)_0.875Sr_0.0625NbO₃ shows a remarkable ionic conductivity of 1.02×10⁻⁵ S/cm at room temperature (20 °C) and the lowest activation energy of 0.34 eV in comparison with 0.38 eV and 0.44 eV for the corresponding Ba- and Ca-doped samples, respectively. It is identified that the enhancement of ionic conductivity is attributed to a reduction in activation energy for ionic conduction which is related to an increase in the cell volume.     

Influence of La addition on the structural,magnetic and magnetocaloric properties in Sr2−xLaxFeMoO6 (0≤x≤0.3) double perovskite

We report the effect of carrier doping via partial substitution of La³⁺ for Sr²⁺ on the structural, magnetic and magnetocaloric properties of Sr₂FeMoO₆ double perovskite. Polycrystalline Sr_2−xLa_xFeMoO₆ (x=0.0, 0.1, 0.2, 0.3) samples were prepared using the conventional solid state reaction method. Using the X-ray diffraction (XRD) analysis it was established that all the samples crystallized in a tetragonal structure with I4/mmm space group. An increase in the La doping lead to an increase in the lattice parameter ‘a’ and the volume of the unit cell. The lattice parameter ‘c’ however remained unchanged. The temperature variation in magnetization and Arrott analysis suggested a second order of ferromagnetic phase transition in all samples with Curie temperature, T_C increasing from 358 K for x=0.0–365 K for x=0.3. A gradual increase in magnetization was also observed with the increasing La content up to x=0.2. The magnetic entropy change was calculated from the measurement of isothermal magnetization versus magnetic field at different temperatures. The tunability of magnetization and T_C simply by adjusting the concentration of La and synthesis conditions makes Sr₂FeMoO₆ an attractive material for magnetic refrigeration at desired temperature.     

Ionic conductivity of apatite-type solid electrolyte ceramics Ca2−xBaxLa4Bi4(SiO4)6O2 (0≤x≤2)

A complex impedance of oxyapatites Ca_2?xBa_xLa₄Bi₄(SiO₄)₆O₂ (0≤x≤2) prepared by solid state reaction has been investigated. The formation of apatites has been checked by X-ray diffraction, FTIR, Raman and ²⁹Si MAS-NMR techniques. The electric impedance data indicate that relaxation phenomena are strongly dependent on temperature in the 923–1048 K range. The bulk resistance decreases with increasing temperature, showing a typical negative temperature coefficient of resistance (NTCR). ac-Conductivity measurements have been performed on a wide range of frequencies and temperatures. The complex modulus plots have confirmed the presence of bulk contributions. The complex impedance analysis suggests the presence of non-Debye relaxations that would be associated with correlation on ions motion.