Structural,magnetic and dielectric properties of Cr substituted yttrium iron garnets

Chromium substituted polycrystalline Yttrium Iron Garnets (Y₃Fe_5–xCr_xO₁₂ with x = 0 to 0.5) were prepared in single‐phase form with lattice constant in the range of 12.3775 Å to 12.3560 Å. All samples exhibit ferrimagnetic transition with transition temperature (T_c) in the range of 547 K for x = 0 to 494 K for x = 0.5. The saturation magnetization value at room temperature is found to increase with Cr concentration that is, from 24.8 to 26.6 emu/g and this is attributed to the preferential occupation of Cr³⁺ ions at octahedral site of Fe³⁺ ions. The frequency dependence of impedance data shows the relaxation and thermal activation of charge carriers across grains and grain boundaries. The complex impedance spectra were modeled by considering equivalent circuits having contributions from the resistance and constant phase element due to grains and grain boundaries and capacitance across grain boundaries. The dielectric constant is found to increase from 20 to 52 as the Cr concentration is increased and it is attributed to hopping of charge carriers across Fe²⁺–Fe³⁺ centers. The Arrhenius plots of relaxation time of charge carriers and conductivity show an anomaly in the vicinity of ferrimagnetic transition temperature and it highlights the presence of magneto‐electric coupling.     

Enhanced magnetic and dielectric properties of Ti‐doped YFeO3 ceramics

Polycrystalline YFeO₃ (YFO) and YFe_1?(4/3)xTi_xO₃(YFTO) ceramics were prepared using the powder synthesized from the sol‐gel route. X‐ray diffraction analyses of the polycrystalline ceramics revealed the crystallization of the phase in orthorhombic crystal structure associated with the space group Pnma. The magnetization versus magnetic field hysteresis loops were obtained at room temperature for YFO and YFTO ceramics. The magnetic property changes from weak ferromagnetic in YFO to ferromagnetic in YFTO ceramics. The dielectric constant recorded at room temperature for YFTO ceramics was six times higher than that of YFO, whereas the dielectric loss gets reduced to 0.06 from 0.3 for YFO at 1 kHz. Impedance spectroscopy study carried out on YFO and YFTO ceramics confirmed the existence of non‐Debye‐type relaxation. Observed single semicircle in Z′ vs ?Z′′ plot established the incidence of intrinsic (bulk) effect and ruled out any grain boundary or electrode effects. The mechanism for the dielectric relaxation and electrical conduction process observed in YFO and YFTO ceramics was discussed by invoking electric modulus formalisms. Activation energy obtained by ac conductivity study suggested that the conduction process in YFO was linked up with the existence of the polaron and oxygen vacancies, whereas only oxygen vacancies contribute to the conduction process in YFTO ceramics.     

Phase composition,crystal structure,and microwave dielectric properties of Nb-doped and Y-deficient yttrium aluminum garnet ceramics

Nb-doped and Y-deficient yttrium aluminum garnet ceramics were designed and synthesized using the solid-state reaction method according to the chemical equation Y_3?xAl₅Nb_xO_12+x (0 ≤ x ≤ 0.16). The phase composition, sintering behavior, microstructure, and microwave dielectric properties were investigated as functions of the composition and sintering temperature. A single-phase solid solution of yttrium aluminum garnet structure formation was observed in the range of 0 ≤ x ≤ 0.1. Further increments in x prompted the precipitation of the YNbO₄ secondary phase at the grain boundary of Y₃Al₅O₁₂. The complexity of the phase composition degrades the micromorphology and dielectric properties of the ceramics to varying degrees. Transmission electron microscopy results show that the lattice exhibits additional symmetry, which is closely related to the ultrahigh Q×f values of the ceramics. Effectively improving the sintering behaviour and suppressing the secondary phase by simultaneously doping with Nb⁵⁺ and reducing the yttrium stoichiometry. Finally, excellent microwave dielectric properties of ε_r ~ 10.99, Q×f ~ 280,387 GHz (13.5 GHz), and τ_f ~ ? 34.7 ppm/°C can be obtained in x = 0.1 (Y_2.9Al₅Nb_0.1O_12.1) sintered at 1700 °C for 6 h.     

Luminescent and magnetic properties of cerium-doped yttrium aluminum garnet and yttrium iron garnet composites

Functional materials exhibiting magnetic and luminescent properties have been recognized as an emerging class of materials with great potential in advanced applications. Herein, properties of multifunctional ceramic composites consisting of two garnets, luminescent cerium-doped Y₃Al₅O₁₂ (Ce:YAG) and magnetic Y₃Fe₅O₁₂ (YIG), are reported. On increasing the sintering temperature, both the photoluminescence and saturation magnetization of the Ce:YAG-YIG composites decreased gradually because of the interdiffusion of trivalent ions such as Al³⁺ and Fe³⁺. At a constant sintering temperature of 1100?°C, the YIG contents in the composites increased, thereby causing their luminescent properties to degrade and the saturation magnetizations to increase. For application to electronics, Ce:YAG-YIG composite thin films were integrated on quartz substrates by sputtering the ceramic target. The composite thin films exhibited both magnetic and luminescent properties after annealing. These techniques facilitate the incorporation of multifunctional nanocomposites into various devices.     

Lattice evolution and microwave dielectric properties of La-doped yttrium aluminum garnet ceramics

In this study, Y_3−xLa_xAl₅O₁₂ (0 ≤ x ≤ 0.09) ceramics were synthesized, and the phase composition, lattice evolution, and microwave dielectric properties were investigated in detail. Scanning electron microscopy confirms that the addition of moderate amounts of La₂O₃ improves the grain development of YAG ceramics, but excessive doping destabilizes the crystal structure. Transmission electron microscopy characterization shows that the variation of the dielectric properties of the samples with x-value is related to the occurrence of benign dislocation structures caused by modifications in the type and content of the A-site rare-earth ions. The variations in relative density, dielectric constant, and quality factor remain basically coordinated. The optimum microwave dielectric properties of La³⁺ doped YAG samples are exhibited as ε_r = 10.61, Q × f = 187, 542 GHz, τ_f = −31.2 ppm/°C when La₂O₃ is doped at x = 0.015.     

Phase stability,microstructure, and dielectric properties of quaternary oxides In12Ti10A2BO42 (A: Ga or Al; B: Mg or Zn)

Quaternary oxides In₁₂Ti₁₀Al₂MgO₄₂ (ITAM), In₁₂Ti₁₀Al₂ZnO₄₂ (ITAZ), In₁₂Ti₁₀Ga₂MgO₄₂ (ITGM), and In₁₂Ti₁₀Ga₂ZnO₄₂ (ITGZ), were synthesized by the solid-state reaction method and the dielectric behavior is reported. The samples were submitted to different sintering temperatures (1473-1773 K) for 24 hours and the phase stability and microstructure were analyzed by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). It is found that the phase decomposition occurs above of 1573 K. Microstructure images showed an increase in the grains size as sintering temperature was raised. The dielectric permittivity as a function of temperature and frequency showed acceptable dielectric constant (15-30) and low dielectric loss (tan δ << 1) values in a wide range of temperature. The band gap obtained by the optical spectrum analysis is about 3.5 eV indicating good dielectric insulating compounds. Furthermore, the electrical conductivity, the activation energy, and the conduction mechanisms are analyzed and discussed in a whole range of temperature. The good dielectric values, ε′ (15-20) and tan δ (~0.004), and their behavior (almost independent of the frequency and temperature) almost constant within a wide range of temperature make these quaternary oxides interesting in electroceramic applications.     

Ionic occupation,structures, and microwave dielectric properties of Y3MgAl3SiO12 garnet‐type ceramics

The Y₃MgAl₃SiO₁₂ ceramics with pure phase were successfully synthesized by solid‐state sintering reaction method for the first time. Their microwave dielectric properties were investigated as a function of sintering temperature. Their microstructure characteristics and ionic occupation sites of tetrahedral and octahedral units were characterized and analyzed by SEM& energy dispersive spectrometer (EDS) and Rietveld refinement of X‐ray powder diffraction data. Crystal structure of Y₃MgAl₃SiO₁₂ is isostructural to Y₃Al₅O₁₂ with a cubic garnet structure and space group of Ia‐3d, which contains YO₈ dodecahedra, (Mg/Al_oct)O₆ octahedral, and (Si/Al_tet)O₄ tetrahedral units. The Qf and ε_r values of different samples are strongly dependent on the distribution of grain sizes, grain sizes, and porosity. The samples sintered at 1550°C exhibit optimized microwave dielectric properties with relative permittivity (?_r) of 10.1, Qf values of 57 340GHz (at 9.5 GHz), and τ_f values of ?32 ppm/°C. Such properties indicate potential application of Y₃MgAl₃SiO₁₂ as microwave substrates.     

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.     

Double perovskite Sr2FeReO6 oxides: Structural,dielectric, magnetic,electrical, and optical properties

Double perovskite Sr₂FeReO₆ (SFRO) powders were synthesized by so-gel process and annealed in argon atmosphere. Their structural, dielectric, magnetic, electrical, and optical properties were comprehensively investigated. It was found that the SFRO powders possessed a tetragonal crystal structure with I4/m space group and exhibited spherical shapes with some agglomeration due to the magnetic interactions between particles of the powders. Quantitative energy dispersive X-ray spectrometer data revealed the atomic ratio of Sr, Fe, Re, and O elements close to the nominal values of 2:1:1:6. X-ray photoemission spectroscopy spectra reveal two species of Re⁵⁺ and Re^6-7+ coexist in the SFRO powders. Sr, Fe, and O elements are present as Sr²⁺, Fe³⁺, and lattice oxygen, respectively. Dielectric property measurements revealed a Maxwell–Wagner type dielectric dispersion in the SFRO ceramics. Ferromagnetic behavior was verified by the observed magnetic hysteresis loops in the SFRO powders at 2 K and 300 K. The remanent magnetization and coercive field at 2 K were 8.23 emu/g and 3152 Oe, respectively, and the saturated magnetization was estimated to be 21.8 emu/g (or 2.0 μ_B/f.u.), smaller than the theoretical value of 3.0 μ_B/f.u. owing to the presence of the anti-site defects. Magnetic Curie temperature (T_C) was estimated to be 432.3 K. Intergranular tunneling magnetoresistance and hysteresis phenomena were observed in the SFRO powders at low temperatures, and the MR (2 K, 6 T) was measured to be −15% and −10% for MR (100 K, 6 T). Electrical transport and optical absorption measurements demonstrate the semiconducting nature of the SFRO with optical band gap of 1.39 eV. The electrical transport process follows the small polaron variable range hopping theory. The unique combination of high T_C ferromagnetism with the semiconductivity enables the SFRO to be a promising candidate for spintronic devices.     

Structural,vibrational, thermal,and magnetic properties of mullite-type NdMnTiO5 ceramic

Mullite-type RMn₂O₅ (R = Y, rare-earth element) ceramics are of ongoing research attention because of their interesting crystal-chemical and magnetic properties. We report nuclear and magnetic structures of NdMnTiO₅ together with its spectroscopic, thermogravimetric, and magnetic properties. The polycrystalline sample is prepared by solid-state synthesis and characterized from neutron and X-ray powder diffraction data Rietveld refinements. NdMnTiO₅ crystallizes in the orthorhombic space group Pbam with metric parameter a  =  755.20(1) pm, b  =  869.91(1) pm, c  =  582.42(1) pm, and V  =  382.62(1) 10⁶ pm³. The Mn³⁺ and Ti⁴⁺ cations are observed to be located in the octahedral and pyramidal sites, respectively. The vibrational features in these polyhedral sites are characterized by Raman and Fourier transform infrared spectroscopes. The higher decomposition temperature of NdMnTiO₅, compared to other RMn₂O₅ phases, is explained in terms of the higher bond strength of Ti-O bonds than those of Mn-O bonds. Temperature-dependent DC magnetic susceptibility suggests a paramagnetic to antiferromagnetic phase transition at 43(1) K. Inverse susceptibility in the paramagnetic region above 120 K follows the Curie-Weiss law, resulting in a magnetic moment of 6.33(1) μ_B per formula unit. Neutron diffraction data collected at 7.5 K reveal that the magnetic moments of Nd³⁺ and Mn³⁺ in NdMnTiO₅ are incommensurately ordered with a propagation vector k  = (0, 0.238, 0.117).     

Microwave dielectric properties of low temperature fired Ba5Nb4O15 ‐ BaWO4 ceramics supplemented with their own nanoparticles for LTCC applications

Polycrystalline Ba₅Nb₄O₁₅ (BNO) ‐ BaWO₄ (BWO) composite ceramics are prepared by adding their own nanoparticles as a sintering aid. The prepared samples exhibited the maximum relative density of 98.2% at 900°C. The complex dielectric response of these composite ceramics are analysed by Havriliak ‐ Negami (HN) equation. The BNO ‐ BWO composite added with x=3 wt% of their nanoparticles, fired at 900oC displayed the best microwave dielectric properties; εr=39 and Q×f0=59.8 THz at 9.44 GHz. The obtained results of BNO ‐ BWO composite makes this material as a potential candidate for LTCC applications.     

Structural,magnetic, and optical properties of nano-sized Ni0.85Se

Nanocrystalline Ni_0.85Se was synthesized by a hydrothermal method. X-ray diffraction analysis by Rietveld method indicated that Ni_0.85Se has a NiAs-type hexagonal structure. Narrow crystallite size distribution with an average area-weighted size of 〈ɛ_F〉 = 8.5 nm is obtained by Warren-Averbach method. Structural analysis revealed deformed Se atoms octahedron with the shortest distance between Se atoms in adjacent planes 〈Se–Se〉_adj smaller than between nearest neighbors in layer plane 〈Se–Se〉_nea, and a rather short interatomic distance between the transition metal atoms 〈Ni–Ni〉. The optical property of Ni_0.85Se was studied by UV-spectroscopy. Magnetic measurement shows a ferromagnetic phase transition for Ni_0.85Se below 14 K.     

Improved dielectric properties in A′‐site nickel‐doped CaCu3Ti4O12 ceramics

The improved dielectric properties and voltage‐current nonlinearity of nickel‐doped CaCu₃Ti₄O₁₂ (CCNTO) ceramics prepared by solid‐state reaction were investigated. The approach of A′‐site Ni doping resulted in improved dielectric properties in the CaCu₃Ti₄O₁₂ (CCTO) system, with a dielectric constant ε′≈1.51×10⁵ and dielectric loss tanδ≈0.051 found for the sample with a Ni doping of 20% (CCNTO20) at room temperature and 1 kHz. The X‐ray photoelectron spectroscopy (XPS) analysis of the CCTO and the specimen with a Ni doping of 25% (CCNTO25) verified the co‐existence of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺. A steady increase in ε′(f) and a slight increase in α observed upon initial Ni doping were ascribed to a more Cu‐rich phase in the intergranular phase caused by the Ni substitution in the grains. The low‐frequency relaxation leading to a distinct enhancement in ε′(f) beginning with CCNTO25 was confirmed to be a Maxwell‐Wagner‐type relaxation strongly affected by the Ni‐related phase with the formation of a core‐shell structure. The decrease of the dielectric loss was associated with the promoted densification of CCNTO and the increase of Cu vacancies, due to Ni doping on the Cu sites. In addition, the Ni dopant had a certain effect on tuning the current‐voltage characteristics of the CCTO ceramics. The present A′‐site Ni doping experiments demonstrate the extrinsic effect underlying the giant dielectric constant and provides a promising approach for developing practical applications.     

Nonlinear dielectric response and positive dielectric tunability in antipolar CaTiSiO5

Dielectric tunability has been extensively investigated in ferroelectric materials, which exhibit a negative tunability of dielectric permittivity in an external electric field. In contrast, positive tunability is rare and has been reported only in a few antiferroelectric materials. We present positive (and negative) tunability in the titanite, CaTiSiO₅. The dielectric property of CaTiSiO₅ was measured up to an extraordinarily high electric field of 40 MV/m. A nonlinear polarization field loop with no hysteresis was obtained. The dielectric permittivity of ε_r ~ 25 increases up to ε_r ~ 40 at 20 MV/m and room temperature. Although titanite has an antipolar structure and is expected to be “antiferroelectric,” its dielectric response in high electric fields up to ~40 MV/m differs from that of conventional antiferroelectrics. We demonstrate that the phase-transition temperature and dielectric tunability could be modulated through the chemical substitution of Ca_1−xLa_xTiSi_1−xAl_xO₅, in which the destabilization of the long-range antipolar order is revealed by transmission electron microscopy analysis. These results indicate that the observed dielectric response to an electric field may originate from the unique features of the antipolar and domain structures in CaTiSiO₅.     

Structure development and dielectric properties of Zn-doped Pb([Mg,Fe],W)O3 perovskite ceramics

Ceramic samples of a pseudo-binary system Pb(Mg_1/2W_1/2)O₃-Pb(Fe_2/3W_1/3)O₃ (PMW-PFW) were prepared by solid-state reaction. In addition, their compositions were modified by 20 mol% Pb(Zn_1/2W_1/2)O₃ (PZW) doping in order to investigate the role of zinc in the perovskite formation and dielectric properties. The perovskite contents were ≥95.1% at the overall composition range, except for a significantly low value of 79.0% at 0.8PFW-0.2PZW. The extension of sintering time for PMW accelerated superstructure formation, resulting in the ordering factor increased up to 0.40 after 12 hours heat treatment. The maximum dielectric constant values increased with increasing PFW fractions. In comparison, the phase transition temperatures decreased sharply (by up to 161°C) at low concentrations of PFW, followed by mild changes (by up to 47°C) afterward. The dielectric constant spectra were analyzed in terms of diffuseness characteristics, which reflected the phase transition modes quite well.     

Electric and magnetic properties of lanthanum barium cobaltite

The cubic Ba_0.5La_0.5CoO_3-δ was synthesized using solid state reaction. The structural properties were determined by the simultaneous refinement of Synchrotron Powder X-ray Diffraction and Neutron Powder Diffraction data. Iodometric titration was used to examine the oxygen stoichiometry and average cobalt oxidation state. Low-temperature magnetic studies show soft ferromagnetic character of fully oxidized material, with θ_P = 198(3) K and µ_eff = 2.11(2) µ_B. Electric measurements show the thermally activated nature of conductivity at low temperatures, whereas, due to the variable oxidation and spin state of cobalt, a single charge transport mechanism cannot be distinguished. Around room temperature, a wide transition from thermally activated conductivity to semi-metallic behavior is observed. Under the inert atmosphere, the oxygen content lowers and the cation ordering takes place, leading to coexistence of two, ordered and disordered, phases. As a result of this change, thermally activated conductivity is observed also at high temperatures in inert atmosphere.     

Enhanced dielectric and microwave absorption properties of Cr/Al2O3 coatings deposited by low‐power plasma spraying

Low‐power plasma‐sprayed Cr/Al₂O₃ coatings have been developed for their potential application as broad bandwidth, thin thickness, lightweight, and strong microwave‐absorbing materials. The dielectric and microwave absorption properties of the as‐sprayed coatings were studied in the X‐band (from 8.2 to 12.4 GHz). High complex permittivity of the coatings was obtained because of a large number of internal boundaries and the conductive networks. Meanwhile, a significant enhancement of microwave absorption properties of the coating was achieved due to the enhanced interfacial polarization and conductance loss. The reflection loss (RL) <?10 dB of the Al₂O₃–15Cr coating was obtained from 9.8 to 11.4 GHz by choosing an appropriate coating thickness, and an optimal minimum reflection loss (RL_min) of ?45.35 dB was achieved at 10.3 GHz with a thin thickness of 1.32 mm.     

Study of the microwave dielectric properties of (La1−xSmx)NbO4 (x=0‐0.10) ceramics via bond valence and packing fraction

The (La_1?xSm_x)NbO₄ (x=0‐0.10) ceramics were prepared by the conventional solid‐state reaction method. The microstructure and the microwave dielectric properties were discussed in detail. The X‐ray diffraction patterns of (La_1?xSm_x)NbO₄ (x=0‐0.10) showed that only a single monoclinic fergusonite structure of LaNbO₄ could be found. The dielectric constant (ε_r) was affected by the dielectric polarizabilities and the B‐site bond valence. The variation trend of Q×f₀ was in accordance with packing fraction. The temperature coefficient of resonant frequency (τ_f) had a close relationship with the B‐site bond valence, which was determined by the bond strength and bond length. When sintered at 1325°C for 4 hours, the (La_1?xSm_x)NbO₄ ceramics with x=0.08 exhibited enhanced microwave dielectric properties: ε_r=19.37, Q×f₀=62203 GHz and τ_f=2.57 ppm/°C. In addition, we made an overview about the ceramics that possess the same packing fraction and bond valence relationships, the results show that this structure‐property relationship has a wide applicability.