Octahedral distortion,phase structural stability,and microwave dielectric properties in equivalently substituted LaTiNbO6 ceramics

The room‐temperature phase composition, microstructural characteristic, and microwave dielectric properties of equivalently substituted LaTiNbO₆ ceramics were investigated by means of the Rietveld structural refinement, scanning electron microscopy, and network analyzer. A special interest was focused on the influence of the variation in bond length and octahedral distortion on the phase structural stability. A monoclinic (M) aeschynite phase was obtained in B‐site ions (Ta⁵⁺ and [W_0.5Ti_0.5]⁵⁺) substituted LaTiNbO₆ ceramics while an orthorhombic (O) aeschynite structure appeared in B‐site ions (Zr⁴⁺) or A‐site ions (Ce³⁺ and Sm³⁺) substituted LaTiNbO₆ ceramics, which owned typical polygonal and short rod‐like grain morphologies, respectively. Compared with the octahedral bond length, a straightforward relation between the phase structural stability and the octahedral distortion was established, in which a reduced octahedral distortion was directly correlated with the destabilization of O or M phases or vice versa, irrespective of the substitution ionic radius. The experimentally observed giant difference in dielectric performance of LaTiNbO₆ based ceramics was believed to mainly originate from their distinct phase structure and grain morphology.     

Variation of crystal structure,magnetization, and dielectric properties of Nd and Ba co-doped BiFeO3 multiferroics

Multiferroics having composition Bi_0.80Nd_0.20-xBa_xFeO₃ were prepared to investigate the effect of doping on crystal structure, magnetic, and dielectric properties. The Rietveld refinement deduces the formation of mixed structural symmetry. With larger content of Nd, crystal structure consisting of major rhombohedral R3c and minor orthorhombic Pnma has been accomplished. The fraction of rhombohedral phase has been found to increase with doping of Ba up to x = 0.10. At composition x = 0.15, the orthorhombic phase Pnma disappears, and there is evolution of triclinic phase P1 in place of it. The mixed structure now accomplished contains ≈61% rhombohedral R3c and rest 39% triclinic P1. In solely Ba-doped sample (ie, at x = 0.20), the fraction of rhombohedral R3c phase again rises and attains ≈92% fraction of the structure along with rest triclinic P1 phase. The M-H loops depict enormous enhancement in magnetic properties with increasing doping of Ba. Dielectric constant (ε′) and dielectric loss (tan δ) both were found to increase with doping of Ba. The anomalies present in the dielectric constant and dielectric loss with temperature may be regarded to the hopping conduction of e⁻ between Fe³⁺ and Fe²⁺ and their interaction with oxygen vacancies.     

Crystal structure and electrical properties of (Li,Ce, Nd)-multidoped CaBi2Nb2O9 high temperature ceramics

(Li, Ce, and Nd)-multidoped CaBi₂Nb₂O₉ (CBN) Aurivillius phase ceramics were prepared via a conventional solid-state sintering route. The crystal structure including bond lengths and bond angles, microstructure, dielectric constant, DC resistivity, and piezoelectric properties were systematically investigated. Rietveld-refinements of X-ray results indicated that small quantity of (Li, Ce, Nd) doping (< 2.5 mol%) increases orthorhombic distortion, because of the smaller ionic radii of doping ions. However, orthorhombic distortion obviously decreased with increasing (Li, Ce, Nd) doping concentration from 5 to 25 mol%. The replacement of asymmetric A-site Bi³⁺ with 6s2 lone pair electrons by symmetric Li⁺, Ce³⁺ and Nd³⁺ ions decreased the orthorhombic distortion. The morphologies and electrical properties of sintered ceramics were tailored by the introducing (Li, Ce, Nd) multi-dopants. The improvement of piezoelectric properties of modified-CBN ceramics were attributed to decreasing grain sizes and morphotropic phase boundary (MPB). Ca_0.85(Li_0.5Ce_0.25Nd_0.25)_0.15Bi₂Nb₂O₉ (CBNLCN-15) ceramics had optimum properties, and d₃₃ and T_c values were found to be ~ 13.1 pC/N and ~ 900 °C, respectively.     

Microwave dielectric properties of (Bi1 − xRx)NbO4 ceramics (R = Ce,Nd,Dy,Er)

(Bi_1 − xR_x)NbO₄ (R = Ce,Nd,Dy,Er; x = 0.03 mol) ceramics with 0.1 wt.% CuO + V₂O₅ as a sintering aid were prepared by conventional solid state reaction process. The microwave dielectric properties of (Bi_1 − xR_x)NbO₄ ceramics were investigated as a functional of R ions. The microwave dielectric properties such as dielectric constant, Q value and temperature coefficient of resonant frequency (TCF) are found to correlate with the R ions. When R = Ce, the dielectric constant, Q value and TCF are 44.7, 5000 and −4.9 ppm/°C, respectively at about 5 GHz. The relation of microwave dielectric properties and the ionic radii, polarizability and bond valence of R (R = Ce, Nd, Dy, Er) was discussed. The Q value decreased with decreasing the bond valence and the size of R ions. The dielectric constant decreased with decreasing polarizability.     

Size and Lone Pair Effects on the Multiferroic Properties of Bi0.75A0.25FeO3−δ (A = Sr,Pb, and Ba) Ceramics

The work presents a comparative study of the effects of divalent Ba, Sr, and Pb substituents on the multiferroic properties of BiFeO₃. The multiferroic properties of Bi_0.75A_0.25FeO₃ (A = Sr, Pb, Ba) solid solution have been explained taking into account the effects of size differences and electronic configuration differences between the host element (Bi) and the substituent. X‐ray diffraction studies revealed that Sr and Pb substitution at Bi‐site transforms the rhombohedral phase (R3c) to cubic phase (Pm3m), whereas the Ba‐substituted sample exhibited the presence of both rhombohedral and cubic phases (R3c + Pm3m). Electronic structure studies through XPS revealed that charge imbalance induced by divalent substitution was being compensated by the formation of oxygen vacancies, while the Fe ions exist in Fe²⁺ and Fe³⁺ states. Replacement of volatile Bi by Sr, Pb, and Ba reduces the concentration of oxygen vacancies (V_O²⁺) and helps to improve the dielectric properties. Strong magnetization enhancement was observed in the substituted compositions and was seen to be consistent with the suppression of cycloid spin structure due to structural transformation as well as possible changes in Fe–O local environment leading to local lattice distortion effects. Furthermore, the observed decrease in the values of magnetic coercivity at low temperature in all the substituted samples is explained in terms of reduced effective single ion anisotropy, originating in the magnetoelectric coupling and being a particularly stronger effect in the case of the lone pair dopant Pb, consistent with theoretical predictions. The lone pair substituent Pb leads to the largest dielectric constant, enhanced magnetization, and large effects on the low‐temperature hysteresis.     

Structural,magnetic, vibrational and impedance properties of Pr and Ti codoped BiFeO3 multiferroic ceramics

Single-phase (Bi_1−xPr_x)(Fe_1−xTi_x)O₃ ceramics (x=0.03, 0.06, and 0.10 as BPFT-3, BPFT-6 and BPFT-10, respectively) were synthesized by conventional solid state reaction method. The effect of varying Pr and Ti codoping concentration on the structural, magnetic, dielectric and optical properties of the BPFT ceramics have been investigated. X-ray diffraction indicated pure rhombohedral phase formation for BPFT-3 and BPFT-6 ceramics, however, a structural phase transition from a rhombohedral to an orthorhombic phase has been observed for BPFT-10 ceramic. The maximum remnant magnetization of 0.1824 emu/g has been observed in BPFT-6. With increasing codoping concentration the room temperature dielectric measurements showed enhancement in dielectric properties with reduced dielectric loss. UV–vis diffuse reflectance spectra demonstrated the strong absorption of light in the visible region for a band gap variation 2.31–2.34 eV. Infrared spectroscopy indicated the shifting of Bi/Pr–O and Fe/Ti–O bonds vibrations and change in Fe/Ti–O bond lengths. Decrease in the conductivity on increasing Pr and Ti concentration in BFO is attributed to an enhancement in the barrier properties leading to suppression of lattice conduction path arising due to lattice distortion as confirmed from impedance analysis.     

Correlation between structure,dielectric and multiferroic properties of lead free Ni modified BaTiO3 solid solution

Present study highlights the influence of Ni doping on the structural, optical, dielectric, ferroelectric and magnetic properties of nanocrystalline BaTi_1-xNi_xO₃ (0 ≤ x ≤ 0.06) ceramics synthesized by sol-gel auto combustion process. Phase identification and crystal structure are examined through Rietveld refinement analysis that ensures mono-phase nature with tetragonal crystal structure in P4mm space group. The observed variation in the structural parameters viz. lattice constants, unit cell volume, bond lengths (Ti-O) and bond angles (Ti-O-Ti) are the direct evidence of distortion produce in the unit cell under the effect of Ni doping. The FTIR studies confirm perovskite structure and presence of stretching/bending vibrations of the various bands present in the samples. The optical properties divulge a minor alteration in optical bandgap under the influence of Ni content. Dielectric studies reveal higher value of the dielectric constant for pristine sample in the low frequency region, but its value decreases on Ni doping. The dielectric response, analyzed through UDR model, exhibits deviation from linear behavior at higher frequencies. Ferroelectric measurements demonstrate that the pristine sample has higher values of remnant polarization (P_r) and maximum polarization (P_m) which decrease linearly with the increase in Ni doping. Magnetic hysteresis loops at room temperature establish a weak ferromagnetic nature of the samples that arises due to the carrier-mediated exchange interactions with smaller values of magnetic parameters. These investigations ensure that the ferromagnetism can be induced in BaTiO₃ by appropriate doping of Ni ions, which may find their potential use in the field of multiferroics.     

Synthesis,structural and electrical properties of Bi1−xDyxFeO3 multiferroic ceramics

Polycrystalline ceramic samples of dysprosium (Dy³⁺) doped bismuth ferrite of general formula Bi_1?xDy_xFeO₃ (x=0.00, 0.01, 0.05 and 0.1) have been prepared by standard solid state reaction method. Powder X-ray diffraction (XRD) analysis reveals that all the samples crystallize in the rhombohedral structure with noncentrosymmetric R3c space group. The refined lattice parameters decrease with the increase of Dy concentration within the same structure symmetry. The bond lengths among atoms for all the compounds were calculated by the Rietveld analysis. The frequency and temperature dependent dielectric constants (real and imaginary parts) have been measured. The real part of dielectric constant reveals that the Neel temperature decreases with the increase of Dy-substitution down to ～200 °C for 10% substitution to the Bi site.     

Improving the photostriction of BiFeO3-based ceramics by bandgap and domain size engineering

The photostrictive properties of (1?x)BiFe_0.96Mn_0.04O₃-xBaTiO₃ (0.23 ≤ x ≤ 0.38) ceramics were investigated using the solid-state synthesis method. Appropriate addition of manganese significantly reduces the bandgap, while the introduction of BaTiO₃ changes the phase structure from rhombohedral to pseudo-cubic and significantly optimizes the ferroelectric domain size. The photostriction was observed in the visible light wavelength range with a response time of around 45 s. Specifically, both enhanced photo-induced deformation around 1.27×10^?3 and high photostrictive efficiency of 8.40×10^?12 m³ W^?1 were obtained for the 0.67BiFe_0.96Mn_0.04O₃-0.33BaTiO₃ ceramics. The significantly narrow bandgap (～1.89 eV) and the increased domain wall density due to reduction in ferroelectric domain size enhance the separation and motion of photo-generated carriers, and consequently improve the photostrictive performance. Besides, the prominent Raman peak redshift with the increasing of Raman power reveals the enhanced FeO₆ octahedral distortion and stretching vibration of Fe–O bond, which indicates the lattice expansion caused by the photoexcited charge carriers.     

Investigation of lanthanum substitution in lead-free BNBT ceramics for transducer applications

The influence of lanthanum on structural, dielectric, hysteresis and electric-field-induced strain behavior in lead-free, Bismuth Sodium Barium Titanate (BNBT7) system Bi_0.465−xLa_xNa_0.465Ba_0.07TiO₃ (x=0.00 to 0.10 in steps of 0.02) near the morphotrophic phase boundary has been investigated. X-ray diffraction confirms the single-phase perovskite structure with co-existence of rhombohedral and tetragonal phases has been observed in all densely packed materials. The diffuse phase transition is evident from the study of the temperature dependence of dielectric constant at different frequencies. All the samples have shown relaxor type behavior. The compositions BNBT7 and lanthanum substituted BNBT7 (x=0.02) have been considered to be suitable for transducer applications on the basis of measured parameters like dynamic piezoelectric constant d^*₃₃, 152 and 147 (pm/V), piezoelectric voltage constant g_33, 14.36 and 13.37 (10⁻³ mV/N) and coercive field (E_c) is, 14.78 and 9.24 (kV/cm) respectively.     

Investigation of structural,physical, spectral,photoluminescence, Raman,and dielectric properties of Ba2Co2GdxFe28-xO46 hexaferrites

Nano-crystalline Gd-substituted X-type hexaferrites with the composition Ba₂Co₂Gd_xFe_28-xO₄₆ (x = 0.00, 0.08, 0.16, 0.24, and 0.32) were synthesized using sol-gel auto-combustion procedure. The establishment of single and pure phases of X-type hexagonal ferrites is verified by X-ray diffraction. The crystallite size ranging from 25 to 31 nm showed the increasing trend with the enhancement of Gd-substitution. The unit cell volume was found in 2502.83–2516.07 Å³. The alteration in lattice parameters and cell volume is due to the difference in ionic radii of the host (Fe) and substituent (Gd) element. FTIR spectra ranging 590-3000 cm⁻¹ revealed the emergence of specific absorption bands verifying the formation of hexagonal ferrites. The behavior of dielectric properties is frequency dependent. The dielectric constant at low frequency for all substitution was high, but over the large frequency values, the dielectric constant was decreased. The dielectric loss factor increases over low frequencies and decreases as frequency rises with a higher concentration level. Higher dielectric parameter values suggest that synthetic material might be good for high-frequency applications and super-capacitors fabrication. The changes in Raman spectra as a function of substitution content are ascribed to the development of strain in the unit cell (x). The XRD investigation well corroborates this conclusion. FESEM images depicted the hexagon shaped particles of the X type hexagonal ferrites. In the PL spectra of synthesized compounds, all emissions invisible (red) zone with PL intensity at 661 nm were observed.     

Comparative study on structure,dielectric, and piezoelectric properties of (Na0.47Bi0.47Ba0.06)0.95A0.05TiO3 (A = Ca2+/Sr2+) ceramics: Effect of radii of A-site cations

Lead-free ceramics (Na_0.47Bi_0.47Ba_0.06)TiO₃, (Na_0.47Bi_0.47Ba_0.06)_0.95Ca_0.05TiO₃, and (Na_0.47Bi_0.47Ba_0.06)_0.95Sr_0.05TiO₃ (BNBT, BNBT-Ca, and BNBT-Sr, respectively) were obtained via solid-state sintering. Due to the different radii between Ca²⁺ and Sr²⁺, the mean radii of the A-site cations in BNBT-Ca, BNBT, and BNBT-Sr are different. Effects of radii of the A-site cations on crystallite structure, microstructure, piezoelectric, and dielectric properties of the ceramics were investigated comparatively. X-ray diffraction and Raman spectroscopy results reveal the lattice distortion due to the doping. With the increase in the mean radii of the A-site cations, lattice parameters, tolerance factor, band-gap energy, average size of grains, dielectric constant, and piezoelectric constant of the ceramics increase. The results imply that structure and electrical properties of BNBT-based ceramics are associated with radii of the A-site cations, which provides a possible route to tune structure and electrical properties of BNBT-based materials by changing radii of the A-site cations.     

Niobium doping of Li1.2Mn0.54Ni0.13Co0.13O2 cathode materials with enhanced structural stability and electrochemical performance

Lithium-rich layered oxides with high energy density have been intensively investigated as advanced lithium-ion batteries cathode materials. However, capacity degradation and voltage decay caused by irreversible lattice oxygen loss and structural transformation during cycling restrict their application. Herein, we proposed a high valance cations Nb⁵⁺ doping strategy and synthesized a series of Li_1.2Mn_0.54-x/3Ni_0.13-x/3Co_0.13-x/3Nb_xO₂ (x = 0, 0.01, 0.02 and 0.03) cathode materials. The effects of Nb⁵⁺ doping on crystallographic structure and electrochemical property were systematically studied. In virtue of the large ionic radii and strengthened Nb–O bonds, the doped samples present commendable structural stability and expanded interlayer spacing for Li-ions migration, which ensures the upgraded cyclic stability and rate performance. In particular, the electrode with x = 0.02 delivers a discharge specific capacity of 265.8 mAh g^-1 at 0.2 C with decelerated voltage decay, while 86.9% capacity are remained after long-term cycles. Moreover, excellent discharge specific capacity of 153.4 mAh g⁻¹ is still attained at 5 C accompanied with enhanced Li-ion diffusion kinetics.     

Microwave dielectric properties of ATiO3 (A = Ni,Mg, Co,Mn) ceramics

Relationships between structural characteristics and microwave dielectric properties of ATiO₃ (A = Ni, Mg, Co, Mn) with ilmenite structure were investigated. The oxygen octahedral distortion was dependent on the type of A-site ions which affected to the temperature coefficient of the resonant frequency (TCF) of ATiO₃ ceramics. The quality factor (Qf) of ATiO₃ (A = Mn, Ni, Co) specimens was appreciably lowered than that of MgTiO₃ specimens due to the degree of covalency of cation–oxygen ion bond and the ordering of A-site ions. Also, the dielectric constant (K) was dependent on the electronic oxide polarizabilities of ATiO₃ ceramics.     

Structural,electrical, optical and dielectric properties of yttrium substituted cadmium ferrites prepared by Co-Precipitation method

The yttrium substituted cadmium ferrites having composition Cd_1-xY_xFe₂O₄ (X = 0.00, 0.125, 0.250, 0.375, 0.500) were synthesized by the co-precipitation method and sintered at 1100 °C for 6 h. Structural, morphological, electrical, optical and dielectric characteristics were explored by XRD, SEM, EDS, FTIR, I–V two probes, UV–Vis and LCR techniques.XRD results confirmed the cubic structure of spinel ferrites. A decrease in lattice constants of the prepared samples was observed with the substitution of Y ions and was attributed to the difference in ionic radii of Y³⁺ (0.95 Å) and Cd²⁺ (0.97 Å) ions. Cationic distributions, ionic radii of both tetrahedral and octahedral sites, tolerance factor, oxygen positional parameters, bond lengths, interatomic distances, positional parameters and bond length angles were calculated from XRD data. The morphology of the prepared ferrites was studied using SEM and results ratified the XRD results. EDS confirmed the presence of all inserted elements in Cd_1-xY_xFe₂O₄ composition. DC resistivity and drift mobility of soft-ferrites were found to be increased from 1.047 × 10⁸–4.822 × 10¹⁰ Ω-cm and 5.87 × 10⁻¹² – 1.045 × 10⁻¹⁴ cm²V⁻¹s⁻¹, respectively, at 523 K with yttrium content confirming the behavior of semiconductor materials. The optical band gap energy calculated from the UV–Vis pattern of the Cd_1-xY_xFe₂O₄ system was decreased from 3.6011 to 2.8153 eV. DC resistivity and optical band gaps exposed inverse relation. FTIR results revealed lower and upper-frequency absorption bands in the ranges of 419.31–417.01 cm⁻¹ and 540.95–565.70 cm⁻¹, respectively. Dielectric constant and dielectric losses were in decreasing order, while ac conductivity revealed rising behavior with increasing frequency. Results showed the potential of yttrium doped Cd nanoferrites for applications in high-frequency microwave absorbing devices.     

Co-substitution tailored dielectric relaxation and electrical conduction in lanthanum orthoferrite

Effect of co-substitution on structural, dielectric and electrical conduction property of LaFeO₃ is reported. Partial co-substitution of La by Na and Fe by Mn has been observed to cause substantial modification in structural property including lattice distortion in LaFeO_3. Evidence of Jahn teller distortion has been noticed in Raman analysis due to presence of Mn³⁺ Jahn teller active ion at Fe lattice site. Raman analysis also indicated Fe–O–Fe bond weakening in LaFeO₃ on co-substitution. Dielectric response provides evidence of temperature dependent polydispersive relaxations contributed by combined effect of dipolar and space charge relaxations. Temperature response of dielectric parameter (ε_r and tan δ) provided indication of phase transition that is similar to found in ferroelectric materials. The dielectric relaxations are studied in the framework of complex impedance and electrical modulus spectrum. The electrical response comprises both grain and grain boundary contribution in the reported polycrystalline sample and it seems consistent with brick layer model of electrical equivalent circuit. It is evident from the activation energy estimation from modulus and conductivity plot that the small polarons hoping are the key factor in both dielectric ordering as well as electrical conduction mechanism in co-substituted LaFeO₃.     

Crystal structure,spectra analysis and dielectric characteristics of Ba4M28/3Ti18O54 (M= La,Pr, Nd,and Sm) microwave ceramics

High dielectric constant Ba₄M_28/3Ti₁₈O₅₄ (M = La, Pr, Nd, and Sm) ceramics were synthesized via standard solid-state reaction route. All the samples possessed single-phase orthorhombic tungsten-bronze structure, and the unit cell volumes decreased monotonously when rare-earth ion changed from La to Sm. With the decreasing radius of rare-earth ions, the dielectric constant (ε_r) gradually decreased, which was correlated with the decrease of total ionic polarizability and blue shift of Raman vibration modes. The increase of the quality factor (Q × f) was associated with the increase of packing fraction and the decrease of FWHM of Raman vibration modes (A_g and B_1g). The temperature coefficient of resonant frequency (TCF) shifted in the negative direction, which was highly related to the decrease of the TiO₆ octahedral tilt angle. Infrared reflection (IR) spectroscopy analysis indicated that the infrared vibration modes between TiO₆ octahedron and A-site cation (M³⁺ and Ba²⁺) at low frequency dominated microwave dielectric polarization in the Ba₄M_28/3Ti₁₈O₅₄ ceramics.     

Effects of Nd3+ and high-valence Nb5+ co-doping on the structural,dielectric and magnetic properties of BiFeO3 multiferroics

Bi_0.90Nd_0.10Fe_1?xNb_xO₃ (0 ≤ x ≤ 0.05) multiferroics have been studied to reveal the effect of Nb doping on the physical properties of the neodymium modified BiFeO₃. These samples have been synthesized via conventional solid state reaction method. The structural characterization was performed by XRD technique and Rietveld refinement. Rietveld refinement results confirmed that all samples crystallized in rhombohedral symmetry. In the vicinity of anti-ferromagnetic Neel-temperature (T_N), an anomaly was observed in dielectric constant (?′) and loss tangent (tan δ) which indicates the existence of magnetoelectric coupling. It is observed that with Nb doping dielectric constant was reduced and Neel temperature shifted towards higher temperature. The impedance (Nyquist plots) and modulus spectroscopy revealed that materials possess non-Debye type of relaxation. The doping of donor ion is able to suppress the existence of oxygen vacancies which results in increase in resistivity. The B-site doping by higher valence ion suppresses the existing modulated spin structure by structural distortion, results in released net magnetization. The room temperature remanent magnetization increased with Nb doping and all powder samples possess weak ferromagnetism. The possible reasons for the notable magnetic and dielectric performance of prepared samples were discussed.     

Structural characteristics and dielectric properties of Ti4+-substituted Li2Mg3SnO6 ceramics

To analyze the impacts of intrinsic factors on the dielectric properties, LiF was introduced to eliminate impurity phase Mg₂SnO₄, and a set of Li₂Mg₃Sn_1-xTi_xO₆-4 wt.% LiF (x = 0.0–1.0) ceramics were prepared by the solid-state reaction method. The XRD and SEM results indicated that all the compositions displayed a pure cubic phase (space group: Fm-3m, 225) and compact microstructure with increasing Ti⁴⁺ concentration, accompanied by a decrease in the average crystallite size from 2.54 μm to 1.92 μm. Based on the chemical bond theory and refinement data, several structural factors were determined to evaluate the relationship between the structural characteristics and dielectric properties. The increase in the dielectric constant (ε_r) was ascribed to the improved average ionic polarizability α_th/V_m. The trend of the change in the quality factor (Q × f) was closely related to the packing fraction and lattice energy. Additionally, the temperature coefficient of the resonant frequency (τ_f) changed within a narrow range, which could be attributed to the minor deviation in the octahedron bond energy. These results demonstrated that structural characteristics are key factors influencing the dielectric properties.