Investigation of Structural Phase Transitions in K0.5Na0.5NbO3 Based Ferroelectric Systems due to Chemical Modifications

Abstract

Perovskite-based ferroelectric materials have strong potential to be utilized in numerous electrical and electronic devices due to their high piezoelectric coefficients, excellent dielectric and ferroelectric properties, and strong electromechanical coupling. These outstanding functional properties make ferroelectric oxides highly suitable for device applications including piezoelectric sensors, electrostrictive actuators, electromechanical transducers, capacitors, underwater acoustic devices, ultrasonic medical imaging, non-volatile memory, energy harvesting, and energy storage devices. At the same time, the functional properties of perovskite-based ferroelectric oxides can be easily tuned as per the requirement of device applications. The exploration of lead-free ferroelectric has significantly expanded in recent years due to the toxicity of lead (Pb), with a focus on three main groups of materials currently under consideration: BaTiO3 (BT)-based, (Na0.5Bi0.5)TiO3 NBT-based, and K0.5Na0.5NbO3 (KNN)-based piezoelectrics. Among these, KNN-based lead-free ferroelectric systems received significant attention since 2003 after the famous work on textured (Li, Ta, Sb) modified KNN ceramic by Saito et al. with excellent piezoelectric properties. Although pure KNN exhibits high Curie temperature (TC ~ 420°C) and high remanent polarization (Pr = 33 μC/cm2), synthesizing high-quality KNN with precise stoichiometry, material stability, and reproducibility using ordinary synthesis conditions is a challenge. Additionally, KNN has limitations such as moderate piezoelectric property, higher coercive field, and poor electromechanical coefficient. To enhance its density, microstructure and piezoelectric properties, various strategies have been adopted by different research groups. The fabrication of solid solutions with different perovskites and chemical modifications at different sites are the most effective ways to improve the density and functional properties of KNN ceramics. The 1st part of this work is mainly focused on the fabrication of solid solutions of KNN with Ba0.5Sr0.5TiO3 (BST) and CaTiO3 (CT). In the second part, we focus on the chemical substitution, such as Sm substituted KNN and Li/Ta substituted KNN. The lead- free ferroelectric solid solution of (1–x)(K0.5Na0.5)NbO3-x(Ba0.5Sr0.5)TiO3 (KNN-xBST, where x = 0.00, 0.025, 0.05, 0.10, 0.15, 0.20, 0.30) were synthesized by solid-state reaction technique. The Rietveld refinement of the XRD data and Raman spectroscopic studies suggest a compositional-driven structural phase transition from an orthorhombic (Amm2) phase for x = 0.00 to the orthorhombic+tetragonal (Amm2+P4mm) dual-phase for 0.025 ≤ x ≤ 0.15, then to the tetragonal+cubic (P4mm+ 𝑃𝑚3̅𝑚) dual-phase (x = 0.20) and finally to cubic (𝑃𝑚3̅ 𝑚) phase with increase in BST concentration at room RT. FESEM micrograph for pure KNN consists of well-defined grains of different sizes due to abnormal grain growth process. However, with the increase in BST concentration, the average grain size decreases, resulting in a compact microstructure and uniform distribution of grains. The temperature-dependent dielectric properties show two dielectric anomalies correspond to orthorhombic to tetragonal (TO-T) and tetragonal to cubic phase transition (TC) for pure KNN. However, with increasing BST concentration, both the transition temperatures decrease, the transition peaks become broaden, and for x ≥ 0.10, TO-T is expected to be below the RT. The dielectric constant at RT increases with BST concentration up to x = 0.10 and after that, it decreases. The P-E hysteresis loops show ferroelectric behavior up to x = 0.15 and paraelectric behavior for x = 0.20 and 0.30. With increasing BST concentration, the EC value decreases, is a minimum for x = 0.10 and subsequently increases. Similarly, the remanent polarization initially decreases with increasing composition, then increases, becomes maximum for x = 0.10, and after that, it decreases. The d33 increases with increasing BST concentration and the maximum value is found for x = 0.025. A phase diagram has been presented based on the temperature-dependent dielectric, RT XRD, and Raman data. Lead-free piezoelectric ceramics of (1-x)K0.5Na0.5NbO3-xCaTiO3 (KNN-xCT, where x = 0.00, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08, 0.10, and 0.15) were fabricated using solid-state synthesis technique. The X-ray diffraction and Raman spectroscopic analysis revealed a composition-dependent structural phase transitions: three phase transitions, namely from a pure orthorhombic (Amm2) phase for x ≤ 0.02 to a mixed phase of orthorhombic and tetragonal (Amm2+P4mm) phases (0.03 ≤ x ≤ 0.08), and finally another mixed phase of tetragonal+cubic (P4mm+𝑃𝑚3̅𝑚) for x = 0.10 and 0.15 at RT. The morphological study reveals a decrease in grain size along with a more uniform distribution of grains as the concentration of CaTiO3 (CT) increases. The temperature- dependent dielectric properties show that, with an increase in CT substitution, both the phase transition temperatures (TO-T and TC) decrease, the transition peaks broaden, and for x > 0.06, the TO-T shifted below RT. Among the prepared samples, the 5 mol.% CT modified KNN shows the optimum electrical properties (d33 = 114 pC/N, Ɛr = 412, 2Pr = 15.25 μC/cm2) at RT. A phase diagram has been constructed based on the information gathered from the temperature-dependent dielectric measurements, RT X-ray diffraction, and Raman spectroscopy data. Substitution of suitable metal ions in ferroelectric oxides modifies the physical properties and can induce additional functionalities. Sm (samarium) substitution in ((K0.5Na0.5)1-3xSmx)NbO3 (KNSN) ceramics is expected to alter the crystal structure and induce local structural heterogeneity, influencing dielectric, ferroelectric, and piezoelectric properties. High-density KNSN (0.00 ≤ x ≤ 0.02) ceramics were fabricated by the conventional solid-state reaction route. X-ray diffraction and Raman spectroscopic analysis indicate that KNSN ceramics exhibit the single-phase orthorhombic (Amm2) structure for x = 0.00 and the coexistence of orthorhombic and tetragonal (Amm2+P4mm) structure for the composition range 0.005 ≤ x ≤ 0.02. Increasing Sm concentration leads to a slight increase in the values of TO-T, while the TC value remains constant. However, the broadening of the TO-T peak at the phase transition is observed as Sm concentration increases. We observed dielectric relaxation behavior in KNSN at orthorhombic to tetragonal (TO-T) phase transition temperature, and its origin can be attributed to the structural heterogeneity at the inter-ferroelectric phase boundary. With an increase in Sm concentration, the dielectric constant at RT increases, reaches a maximum at x = 0.005 (Ɛr = 583), and then decreases. The ceramic with x = 0.005 exhibits the maximum ferroelectric properties (2Pr = 57.02 μC/cm2) and the highest piezoelectric coefficient (d33 = 94 pC/N) at RT among the prepared samples of KNSN. The enhanced piezoelectric property for the critical composition x = 0.005 is due to the increase in the degree of local structural heterogeneity caused by Sm substitution. Lead-free ferroelectric ceramic of (K0.48Na0.48 Li0.04)(Nb1-xTax)O3 (KNLNT-x, where x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.40) were prepared using solid-state synthesis technique. The XRD analysis using Rietveld refinement and Raman spectroscopic studies suggests a composition-driven structural phase transition from the orthorhombic (Amm2) phase for x < 0.10 to the orthorhombic+tetragonal (Amm2+P4mm) dual-phase in the composition range of 0.10 ≤ x ≤ 0.20 and finally to a single-phase tetragonal (P4mm) structure for x > 0.20. SEM micrograph of KNLN (x = 0.00) shows a highly dense ceramic with inhomogeneous distribution of grains of different sizes. However, with an increase in Ta concentration, the grain size decreases, resulting in a more compact microstructure with a uniform distribution of grains. With increasing Ta concentration, both TO-T and TC systematically decrease, the transition peaks broaden, and TO-T shifts below RT for x > 0.15. For all the compositions, the P-E loops show well defined, nonlinear and saturated loops suggesting good ferroelectric nature. The 2Pr value intially decreases, then increases with increasing Ta concentration, becomes maximum for x = 0.20 (40.62 μC/cm2) and decreases after that. On the other hand, the coercive field EC ecreases with increasing x, is a minimum for x = 0.20 ( 9.3 kV/cm) and subsequently increases. However, the dielectric constant and piezoelectric coefficient at RT increase with increasing x, reaches maxima at x = 0.20 and subsequently decreases. Among the Ta- modified samples, the ceramic with x = 0.20 shows the highest dielectric constant (Ɛr = 556) and piezoelectric coefficient (d33 = 159 pC/N). The enhanced piezoelectric property is attributed to the morphotropic phase boundary (MPB) composition. Based on the RT XRD data, Raman spectra, and temperature-dependent dielectric properties, a phase diagram has been constructed.