Synthesis and negative thermal expansion property of Y2−xLaxW3O12 (0≤x≤2)

Y_2−xLa_xW₃O₁₂ solid solutions were successfully synthesized by the solid state reaction method. The microstructure, hygroscopicity and thermal expansion property of the resulting samples were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM) and thermal mechanical analysis (TMA). Results indicate that the structural phase transition of the Y_2−xLa_xW₃O₁₂ changes from orthorhombic to monoclinic with increasing substituted content of lanthanum. The pure phase can form for 0≤x≤0.4 with orthorhombic structure and for 1.5≤x≤2 with monoclinic one. High lanthanum content leads to a low relative density of Y_2−xLa_xW₃O₁₂ ceramic. Thermal expansion coefficients of the Y_2−xLa_xW₃O₁₂ (0≤x≤2) ceramics also vary from −9.59×10⁻⁶ K⁻¹ to 2.06×10⁻⁶ K⁻¹ with increasing substituted content of lanthanum. The obtained Y_0.25La_1.75W₃O₁₂ ceramic shows almost zero thermal expansion and its average linear thermal expansion coefficient is −0.66×10⁻⁶ K⁻¹ from 103 °C to 700 °C.     

Synthesis and tunable thermal expansion properties of Sc2−xYxW3O12 solid solutions

A new series of rare earth solid solutions Sc_2−xY_xW₃O₁₂ was successfully synthesized by the conventional solid-state method. Effects of doping ion yttrium on the crystal structure, morphology and thermal expansion property of as-prepared Sc_2−xY_xW₃O₁₂ ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TG), field emission scanning electron microscope (FE-SEM) and thermal mechanical analyzer (TMA). Results indicate that the obtained Sc_2−xY_xW₃O₁₂ samples with Y doping of 0≤x≤0.5 are in the form of orthorhombic Sc₂W₃O₁₂-structure and show negative thermal expansion (NTE) from room temperature to 600 °C; while as-synthesized materials with Y doping of 1.5≤x≤2 take hygroscopic Y₂W₃O₁₂·nH₂O-structure at room temperature and exhibit NTE only after losing water molecules. It is suggested that the obvious difference in crystal structure leads to different thermal expansion behaviors in Sc_2−xY_xW₃O₁₂. Thus it is proposed that thermal expansion properties of Sc_2−xY_xW₃O₁₂ can be adjusted by the employment of Y dopant; the obtained Sc_1.5Y_0.5W₃O₁₂ ceramic shows almost zero thermal expansion and its average linear thermal expansion coefficient is −0.00683×10⁻⁶ °C⁻¹ in the 25–250 °C range.     

Phase transition and negative thermal expansion properties of Sc2−xCrxMo3O12

The crystal structure, phase transition and thermal expansion behaviors of solid solutions Sc_2−xCr_xMo₃O₁₂ (0≤x≤2) were investigated using X-ray diffraction (XRD) and differential scanning calorimetry (DSC). At room temperature, samples with x≤0.7 and x≥0.8 crystallize in orthorhombic and monoclinic structures, respectively. DSC result indicates that the phase transition of Sc_0.5Cr_1.5Mo₃O₁₂ from monoclinic to orthorhombic structure occurs at 203.66 °C. The linear thermal expansion coefficient of orthorhombic phases varies from −2.334×10⁻⁶ °C⁻¹ to 0.993×10⁻⁶ °C⁻¹ when x increases from 0.0 to 1.5. The near-zero linear thermal expansion coefficients of −0.512×10⁻⁶ °C⁻¹ and −0.466×10⁻⁶ °C⁻¹ are observed for compounds with x=0.5 and 0.7, respectively.     

Synthesis and transport properties in La2−xAxMo2O9−δ (A = Ca, Sr, Ba, K) series

The La_2−xA_xMo₂O_9−δ (A = Ca²⁺, Sr²⁺, Ba²⁺ and K⁺) series has been synthesised as nanocrystalline materials via a modification of the freeze-drying method. The resulting materials have been characterised by X-ray diffraction (XRD), thermal analysis (TG/DTA, DSC), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The high-temperature β-polymorph is stabilised for dopant content x > 0.01. The nanocrystalline powders were used to obtain dense ceramic materials with optimised microstructure and relative density >95%. The overall conductivity determined by impedance spectroscopy depends on both the ionic radius and dopant content. The conductivity decreases slightly as the dopant content increases in addition a maximum conductivity value was found for Sr²⁺ substitution, which show an ionic radii slightly higher than La³⁺ (e.g. 0.08 S cm⁻¹ for La₂Mo₂O₉ and 0.06 S cm⁻¹ for La_1.9Sr_0.1Mo₂O_9−δ at 973 K). The creation of extrinsic vacancies upon substitution results in a wider stability range under reducing conditions and prevents amorphisation, although the stability is not enhanced significantly when compared to samples with higher tungsten content. These materials present high thermal expansion coefficients in the range of (13-16) × 10⁻⁶ K⁻¹ between room temperature and 753 K and (18-20) × 10⁻⁶ K⁻¹ above 823 K. The ionic transport numbers determined by a modified emf method remain above 0.98 under an oxygen partial pressure gradient of O₂/air and decreases substantially under wet 5% H₂-Ar/air when approaching to the degradation temperature above 973 K due to an increase of the electronic contribution to the overall conductivity.     

Electrical conductivity and redox stability of La2Mo2−xWxO9 materials

A series of compounds La₂Mo_2−xW_xO₉ (x = 0-2) were synthesized using a freeze-dried precursor method at relatively low temperatures (673-823 K). These materials were characterised by thermogravimetric and differential thermal analysis (TG/DTA), differential scanning calorimetric (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM) and dilatometric measurements. Oxygen stoichiometry was evaluated by coulometric titration and thermogravimetric analysis at 873-1273 K. The ionic and electronic conductivities of these materials were analysed by impedance spectroscopy and a Hebb-Wagner ion-blocking method under moderately reducing conditions. The presence of W⁶⁺ leads to an increase of the stability range (about 10⁻¹⁶ Pa for La₂Mo_0.5W_1.5O₉ at 1073 K) and prevents oxygen loss and amorphisation. Within the stability range, the electronic conductivity increases gradually as the temperature increases and as the oxygen partial pressure reduces. This indicates that the electronic transport is mainly n-type as a result of the oxygen-content decreasing in the molybdate lattice. Further reduction of the oxygen partial pressure gave rise to the decomposition of La₂Mo_2−xW_xO₉, leading to the formation of new phases with molybdenum in lower oxidation states, which further enhances the electronic conductivity. The results of the coulometric titration and the thermogravimetric studies under a dry 5% H₂/Ar flow suggest that tungsten doped lanthanum molybdate materials can be used as electrolyte only at low temperature and under moderate reducing conditions.     

Synthesis of negative thermal expansion materials ZrW2−xMoxO8 (0 ≤ x ≤ 2) using hydrothermal method

Negative thermal expansion materials ZrW_2−xMo_xO₈ (0 ≤ x ≤ 2) have been successfully synthesized by the reaction of a mixture of ammonium tungstate and ammonium molybdate with zirconium oxynitrate using a hydrothermal method. Effect of substituted ion Mo on the microstructure, α-to-β and cubic to trigonal phase transition in resulting ZrW_2−xMo_xO₈ powders was examined by the XRD experiments. It was found that the structural phase transition temperature decreased slightly with increasing substituted content. The cubic to trigonal phase transition was also influenced by substituted content. The resulting products decomposed to WO₃/MoO₃ and ZrO₂ as temperature increasing when x ≤ 0.5 and while x > 0.5, the cubic phase transited to trigonal phase. The effect of substituted Mo on the morphology of resulting products was also investigated by SEM experiments.     

Ionic and electronic transport in stabilized β-La2Mo2O9 electrolytes

Increasing temperature from 973 to 1173 K leads to a substantial increase of the electronic contribution to the total conductivity of undoped lanthanum molybdate and La₂Mo₂O₉-based solid electrolytes, including La₂Mo_1.7W_0.3O₉, La₂Mo_1.95V_0.05O₉ and La_1.7Bi_0.3Mo₂O₉, where the stabilization of β-La₂Mo₂O₉ down to room temperature was confirmed by high-resolution X-ray diffraction (XRD) and differential scanning calorimetry (DSC) data. In air, the ion transference numbers determined by the modified Faradaic efficiency (FE) technique, decrease from 0.991-0.997 at 973-1023 K down to 0.977-0.984 at 1173 K. Reducing oxygen partial pressure also increases electronic conduction evaluated by the emf and oxygen permeability (OP) measurements, which indicates that the electronic transport is n-type, resulting from decreasing oxygen content in the molybdate lattice. The level of n-type electronic conductivity in air is quite similar for all La₂Mo₂O₉-based ceramics. The results show that these materials can be used as solid electrolytes only under oxidizing conditions and only at temperatures below 1073 K. Their practical applications may also be complicated due to relatively high thermal expansion coefficients (CTEs), (14.1-14.8)×10⁻⁶ K⁻¹ at 300-700 K and (16.4-22.5)×10⁻⁶ K⁻¹ at 850-1070 K, which are close to those of stabilized δ-Bi₂O₃ and γ-Bi₂VO_5.5 electrolytes.     

Polypyrrole and La1−xSrxMnO3 (0≤ x ≤0.4) composite electrodes for electroreduction of oxygen in alkaline medium

Composite G/PPy/PPy(La_1−xSr_xMnO₃)/PPy electrodes made of the perovskite La_1−xSr_xMnO₃ embedded into a polypyrrole (PPy) layer, sandwiched between two pure PPy films, electrodeposited on a graphite support were investigated for electrocatalysis of the oxygen reduction reaction (ORR). PPy and PPy(La_1−xSr_xMnO₃) (0≤ x ≤0.4) successive layers have been obtained on polished and pretreated graphite electrodes following sequential electrodeposition technique. The electrolytes used in the electrodeposition process were Ar saturated 0.1 mol dm⁻³ pyrrole (Py) plus 0.05 mol dm⁻³ K₂SO₄ with and without containing a suspension of 8.33 g L⁻¹ oxide powder. Films were characterized by XRD, SEM, linear sweep voltammetry, cyclic voltammetry (CV) and electrochemical impedance (EI) spectroscopy. Electrochemical investigations were carried out at pH 12 in a 0.5 mol dm⁻³ K₂SO₄ plus 5 mmol dm⁻³ KOH, under both oxygenated and deoxygenated conditions. Results indicate that the porosity of the PPy matrix is considerably enhanced in presence of oxide particles. Sr substitution is found to have little influence on the electrocatalytic activity of the composite electrode towards the ORR. However, the rate of oxygen reduction decreases with decreasing pH of the electrolyte from pH 12 to pH 6. It is noteworthy that in contrast to a non-composite electrode of the same oxide in film form, the composite electrode exhibits much better electrocatalytic activity for the ORR.     

Zr-doped Li[Ni0.5−xMn0.5−xZr2x]O2 (x = 0, 0.025) as cathode material for lithium ion batteries

Layered Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) have been prepared by the mixed hydroxide and molten-salt synthesis method. The individual particles of synthesized materials have a sub-microsize range of 200-500 nm, and LiNi_0.475Mn_0.475Zr_0.05O₂ has a rougher surface than that of LiNi_0.5Mn_0.5O₂. The Li/Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) electrodes were cycled between 4.5 and 2.0 V at a current density of 15 mA/g, the discharge capacity of both cells increased during the first ten cycles. The discharge capacity of the Li/LiNi_0.475Mn_0.475Zr_0.05O₂ cell increased from 150 to 220 mAh/g, which is 50 mAh/g larger than that of the Li/LiNi_0.5Mn_0.5O₂ cell. We found that the oxidation of oxygen and the Mn³⁺ ion concerned this phenomenon from the cyclic voltammetry (CV). Thermal stability of the charged Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) cathode was improved by Zr doping.     

Electrochemical and structural study of Ce0.8Sm0.2−xLaxO1.9 electrolyte materials for SOFC

Ce_0.8Sm_0.2−xLa_xO_1.9 powders, denoted as La_xSDC (for x=0, 0.01, 0.03, 0.05, 0.07 and 0.1), were synthesized via the mechanical milling reaction method. The La³⁺ doping content has a remarkable influence on structural and electrical properties. The phase identification and morphology were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Lattice parameters were calculated by the Rietveld method. It was observed that the lattice parameter values in Ce_0.8Sm_0.2−xLa_xO_1.9 systems obey Vegard's law. The pellets were then sintered at 1500 °C in air for 7 h. The relative densities of these pellets were over 93.7%.The electrical conductivity was studied using two-probe impedance spectroscopy and results showed that the conductivity of Ce_0.8Sm_0.2−xLa_xO_1.9 first increased and then decreased with La dopant content x. Results also showed that Ce_0.8Sm_0.17La_0.03O_1.9 had the highest electrical conductivity, σ₇₀₀ °C equal to 3.8×10⁻² Scm⁻¹ and an activation energy equal to 0.77 eV. It was therefore concluded that co-doping with the appropriate amount of La can further improve the electrical properties of ceria electrolytes.     

Adjustable dielectric properties of Li2CuxZn1−xTi3O8 (x=0 to 1) ceramics with low sintering temperature

Single-phase dielectric ceramics Li₂Cu_xZn_1−xTi₃O₈ (x=0–1) were synthesized by the conventional solid-state ceramic route. All the solid solutions adopted Li₂MTi₃O₈ cubic spinel structure in which Li/M and Ti show 1:3 order in octahedral sites whereas Li and M are distributed randomly in tetrahedral sites with the degree of Li/M cation mixing varying from 0.5 to 0.3. The substitution of Cu for Zn effectively lowered the sintering temperatures of the ceramics from 1050 to 850 °C and significantly affected the dielectric properties. As x increased from 0 to 0.5, τ_f gradually increased while the dielectric constant (ε_r) and quality factor value (Q×f) gradually decreased, and a near-zero τ_f of 1.6 ppm/°C with ε_r of 25.2, Q×f of 32,100 GHz could be achieved for Li₂Cu_0.1Zn_0.9Ti₃O₈ ceramic sintered at 950 °C, which make it become an attractive promising candidate for LTCC application. As x increases from 0.5 to 1, the dielectric loss significantly increases with AC conductivity increasing up to 2.3×10⁻⁴ S/cm (at 1 MHz).     

Combustion synthesis of Ca1−xCuxAl2O4 (x = 0.0, 0.4 and 0.8) copper doped calcium aluminate

We report the effect of Cu²⁺ ion on CaAl₂O₄ with different molar concentrations of 0.0, 0.4 and 0.8 M prepared by simple combustion method. The materials have been characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR) and scanning electron microscopy (SEM). DC electrical conductivity has also been measured to study the electrical behavior of the materials. The XRD patterns confirm the formation of single-phase CaAl₂O₄ along with some impurity phases like CaAl₄O₇, CaAl₁₂O₁₉ and Ca₁₂Al₁₄O₃₃. The FT-IR spectra show the stretching and bending vibrations of the synthesized compounds. DC electrical conductivity of the Ca_1−xCu_xAl₂O₄ is found to vary from 26.46 × 10⁻⁴ to 515.68 × 10⁻⁴ S cm⁻¹ for x = 0.0 to x = 0.8 at the measuring temperature of 1000 °C. SEM images show the morphological features of the compounds.     

Preparation and characterization of novel spinel Li4Ti5O12−xBrx anode materials

Br-doped Li₄Ti₅O₁₂ in the form of Li₄Ti₅O_12−xBr_x (0 ≤ x ≤ 0.3) compounds were successfully synthesized via solid state reaction. The structure and electrochemical properties of the spinel Li₄Ti₅O_12−xBr_x (0 ≤ x ≤ 0.3) materials were investigated. The Li₄Ti₅O_12−xBr_x (x = 0.2) presents the best discharge capacity among all the samples, and shows better reversibility and higher cyclic stability compared with pristine Li₄Ti₅O₁₂, especially at high current rates. When the discharge rate was 0.5 C, the Li₄Ti₅O_12−xBr_x (x = 0.2) sample presented the excellent discharge capacity of 172 mAh g⁻¹, which was very close to its theoretical capacity (175 mAh g⁻¹), while that of the pristine Li₄Ti₅O₁₂ was 123.2 mAh g⁻¹ only.     

Garnet-based red emitting phosphors Li6MLa2Nb2O12: Eu (M=Ca,Sr, Ba): Photoluminescence improvement by changing crystal lattice

A series of novel red emitting phosphors Li₆M(La_1−xEu_x)₂Nb₂O₁₂ (M=Ca, Sr, Ba; 0≤x≤0.3) were synthesized by solid state reaction, and their structures and photoluminescence properties were investigated in detail. The excitation spectrum of Li₆M(La_1−xEu_x)₂Nb₂O₁₂ revealed two mainly excitation bands at 393 nm and 464 nm, which match well with the two popular emissions from near-UV and blue LED chips. Upon the 464 nm light excitation, Li₆MLa₂Nb₂O₁₂:Eu³⁺ phosphors exhibit a red emission centered at 608 nm, originated from the ⁵D₀–⁷F₂ transition of Eu³⁺ ions. The Eu³⁺ surrounding crystal lattice environment in the garnet-based host was changed by altering the c sites element with different radii alkaline earth Ba, Sr, and Ca. The evident photoluminescence enhancement was observed in Li₆M(La_1−xEu_x)₂Nb₂O₁₂ phosphors with the decreasing of the c sites ionic radius. The emission intensity of the optimized Li₆Ca(La_0.8Eu_0.2)₂Nb₂O₁₂ (λ_exc=464 nm) phosphor is about two times higher than that of Y₂O₃:Eu³⁺ (λ_exc=467 nm) under blue light excitation. In addition, the quenching mechanism and the relationship between the structure and photoluminescence property were also discussed.     

Electrochemical properties of La1−xSrxFeO3 (x = 0.2, 0.4) as negative electrode of Ni-MH batteries

La_(1−x)Sr_xFeO₃ (x = 0.2,0.4) powders were prepared by a stearic acid combustion method, and their phase structure and electrochemical properties were investigated systematically. X-ray diffraction (XRD) analysis shows that La_(1−x)Sr_xFeO₃ perovskite-type oxides consist of single-phase orthorhombic structure (x = 0.2) and rhombohedral one (x = 0.4), respectively. The electrochemical test shows that the reaction at La_(1−x)Sr_xFeO₃ oxide electrodes are reversible. The discharge capacities of La_(1−x)Sr_xFeO₃ oxide electrodes increase as the temperature rises. With the increase of the temperature from 298 K to 333 K, their initial discharge capacity mounts up from 324.4 mA h g⁻¹ to 543.0 mA h g⁻¹ (when x = 0.2) and from 147.0 mA h g⁻¹ to 501.5 mA h g⁻¹ (when x = 0.4) at the current density of 31.25 mA g⁻¹, respectively. After 20 charge-discharge cycles, they still remain perovskite-type structure. Being similar to the relationship between the discharge capacity and the temperature, the electrochemical kinetic analysis indicates that the exchange current density and proton diffusion coefficient of La_(1−x)Sr_xFeO₃ oxide electrodes increase with the increase of the temperature. Compared with La_0.8Sr_0.2FeO₃, La_0.6Sr_0.4FeO₃ electrode is a more promising candidate for electrochemical hydrogen storage because of its higher cycle capacity at various temperatures.     

Preparation, characterization and electrical conductivity studies of NdYb1−xGdxZr2O7 (0 ≤ x ≤ 1.0) ceramics

Several compositions of NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 1.0) ceramics were prepared by pressureless-sintering method at 1973 K for 10 h in air. The relative density, microstructure and electrical conductivity of NdYb_1−xGd_xZr₂O₇ ceramics were analyzed by the Archimedes method, X-ray diffraction, scanning electron microscopy and impedance plots measurements. NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 0.3) ceramics have a single phase of defect fluorite-type structure, and NdYb_1−xGd_xZr₂O₇ (0.7 ≤ x ≤ 1.0) ceramics exhibit a single phase of pyrochlore-type structure; however, the NdYb_0.5Gd_0.5Zr₂O₇ composition shows mixed phases of both defect fluorite-type and pyrochlore-type structures. The measured values of the grain conductivity obey the Arrhenius relation. The grain conductivity of each composition in NdYb_1−xGd_xZr₂O₇ ceramics gradually increases with increasing temperature from 673 to 1173 K. NdYb_1−xGd_xZr₂O₇ ceramics are oxide-ion conductor in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The highest grain conductivity value obtained in this work is 1.79 × 10⁻² S cm⁻¹ at 1173 K for NdYb_0.3Gd_0.7Zr₂O₇ composition.     

Characterization of Mg1−xNixAl2O4 solid solutions prepared by combustion synthesis

Mg_1−xNi_xAl₂O₄ (x = 0, 0.25, 0.5, 0.75 and 1) solid solutions have been prepared by combustion synthesis. After annealing the combustion synthesized powders at 1000 °C for 3 h single-phase Mg_1−xNi_xAl₂O₄ was obtained over the entire range of compositions. The lattice parameter of Mg_1−xNi_xAl₂O₄ gradually increased from 8.049 Å (NiAl₂O₄) to 8.085 Å (MgAl₂O₄), which certified the formation of the spinel solid solutions. All samples prepared by combustion synthesis had blue color shades, denoting the inclusion of Ni²⁺ in the spinel structure in octahedral and tetrahedral configuration. The crystallite size of Mg_1−xNi_xAl₂O₄ was in the range of 35-39 nm and the specific surface area varied between 5.8 and 7.0 m²/g.     

Structure,dielectric and optical properties of Bi1.5+xZnNb1.5O7+1.5x pyrochlores

Non-stoichiometric pyrochlore ceramics with formula Bi_1.5+xZnNb_1.5O_7+1.5x were systematically investigated. Crystal structures of the compounds were studied by X-ray diffraction (XRD) technique. The structures were identified as pure cubic pyrochlores when |x| < 0.1. Dielectric and optical properties of the compositions when x = −0.1, 0 and 0.1 were studied. All samples have high resistivities and low dielectric loss. With increasing x in Bi_1.5+xZnNb_1.5O_7+1.5x, the lattice constant, permittivity, temperature coefficient of permittivity and thermal expansion coefficient increased, while dielectric loss decreased. Raman spectra indicated that the intensity of Bi–O stretching become stronger with increasing x. A vibration mode emerging at 861 cm⁻¹ when x = −0.1 means that the B–O coordination environment is significantly more disordered. Absorption spectra suggested that the bandgap energy become lower from 2.86 to 2.70 eV as lattice constants increased. Strong absorption occurs at wavelengths from 433 to 459 nm, shows that samples have the ability to respond to wavelengths in the visible light region.     

Study on the rapid synthesis of LiNi1−xCoxO2 cathode material for lithium secondary battery

LiNi_1−xCo_xO₂ (x = 0, 0.1, 0.2) cathode materials were successfully synthesized by a rheological phase reaction method with calcination time of 0.5 h at 800 °C. All obtained powders are pure phase with α-NaFeO₂ structure (R-3m space group). The samples deliver an initial discharge capacity of 182, 199 and 189 mAh g⁻¹ (25 mA g⁻¹, 4.35-3.0 V), respectively. The reaction mechanism was also discussed, which consists of a series of defect reactions. As a result of these defect reactions, the reaction of forming LiNi_1−xCo_xO₂ takes place in high speed.     

Sintering and electrical properties of La-modified (Na0.52K0.45Li0.03)1−3xLax(Nb0.88Sb0.09Ta0.03)O3 lead-free ceramics

(Na_0.52K_0.45Li_0.03)_1−3xLa_x(Nb_0.88Sb_0.09Ta_0.03)O₃ (NKLL_xNST) lead-free ceramics were prepared by normal sintering and their dielectric and piezoelectric properties were investigated. The X-ray methods indicate that the NKLL_xNST ceramics with x≤0.003 present a pure perovskite phase at room temperature. The bulk density of NKLL_xNST ceramics increases with proper amount of La₂O₃ contents, and reaches its highest value of 4.544 g/cm³ with the addition of 0.3 mol% La₂O₃. At x=0.003, remnant polarization P_r, piezoelectric constant d₃₃ and planar mode electromechanical coupling factor k_p of NKLL_xNST ceramics reach the highest values of 37.80 μC/cm², 346 pC/N and 40%, respectively, exhibiting excellent “soft” piezoelectric characteristics, demonstrating a tremendous potential of the compositions studied for device applications.