Orthorhombic-tetragonal Phase Transition Temperature Research Articles

Enhanced Thermal Stability in Dielectric Properties of (K0.8Na0.2)NbO3–Based Ceramics for X9R-Mlcc Applications

(1−x)(K0.8Na0.2)NbO3–xBaTiO3 [(1−x)(K0.8N0.2)N–xBT] ceramics are well densified at 1080 °C for 2 h. The Curie temperature (TC ) and orthorhombic–tetragonal phase transition temperature (TO-T ) of the ceramics considerably decrease and broaden with increasing x. Moreover, the ceramics (x≥0.08) have large relaxor properties. The (1−x)(K0.8N0.2)N–xBT ceramics (0.09≤x≤0.11) exhibit excellent thermal stability of dielectric properties, satisfying the X9R multilayer ceramic capacitor (MLCC) specification requirements. However, the (1−x)(K1-yNy)N–xBT ceramics (0.0≤x≤0.3 and y = 0.5 and 0.8), which contain a large amount of Na+ ions, cannot satisfy the X9R MLCC specification requirements owing to the weakened relaxor properties and reduced decreasing rate of TC and TO-T . An MLCC is fabricated using the 0.9(K0.8N0.2)N–0.1BT ceramic. The MLCC complies with the X9R MLCC regulation requirements and has excellent stability of the applied electric field. Therefore, the 0.9(K0.8N0.2)N–0.1BT ceramic serves as a promising candidate for X9R MLCC devices.

Good Thermal Stability in Dielectric Properties of (1-x)(K0.8Na0.2)NbO3-xSrTiO3 Ceramics for the Application to X9R MLCC

(1 − x)(K0.8Na0.2)NbO3–xSrTiO3 [(1 − x)(K0.8N0.2)N–xST] ceramics are well sintered at 1120 – 1200 ˚C for 2 h. The Curie temperature (T C) and the orthorhombic–tetragonal phase transition temperature (T O–T) of the ceramics considerably shifts toward low temperature region and the T C peak is flattened with increasing amount of x. Moreover, the ceramics (x ≥ 0.09) show relatively large relaxor ferroelectric properties. The 0.9(K0.8N0.2)N–0.1ST ceramics exhibit a remarkable thermal stability in dielectric properties, and it complies with the requirement of the EIA standard X9R multilayer ceramic capacitor (MLCC) specification. However, the (1 − x)(KyN1 − y)N−xST ceramics (x = 0.10, 0.0 ≤ y ≤ 0.6), which contain relatively large amount of Na+ ions, cannot satisfy the requirement of the X9R MLCC specification, because they have relatively small relaxor ferroelectric properties and the reduced decreasing rate of the T C. An 11 layered MLCC is fabricated by using the 0.9(K0.8N0.2)N–0.1ST ceramic. The fabricated MLCC also complies with the requirement of the X9R MLCC specification with an excellent stability to the applied electric field. Therefore, the 0.9(K0.8N0.2)N–0.1ST ceramic can be one of the excellent dielectric ceramic materials for the application to X9R MLCC devices.

Multisource energy harvesting using (Ba,Ca)(Zr,Ti)O3 oscillating under temperature gradient

AbstractThe concept of multisource energy harvesting has attracted attention in order to harvest multiple types of energy in a single material. In this work, Pb‐free (Ba,Ca)(Zr,Ti)O3 (BCZT) ceramics were used to investigate a combination of piezoelectric and pyroelectric energy harvesting behavior. The temperature dependence of the permittivity and the pyroelectric coefficient was measured, and the pyroelectric properties of BCZT were expected to improve near its orthorhombic–tetragonal phase transition temperature TO–T (∼40°C). The output voltage and current were measured using a BCZT plate, where the BCZT was oscillated under a temperature gradient to apply mechanical stress and temperature fluctuation simultaneously. The measured output voltage from the combined piezo‐ and pyroelectric contributions at a frequency of 2 Hz and with a load resistance of 10 MΩ was 1.4 times higher than the sole piezoelectric output voltage. The resulting combined output voltage was confirmed by finite element simulation.

High inverse piezoelectric coefficient achieved in Sr2+‐doped KNN‐based lead‐free piezoceramics

AbstractHigh inverse piezoelectric coefficients are crucial for the effective use of piezoelectric ceramics as actuators. In this study, we focused on the design and preparation of (K0.5−xNa0.5−xSrx) (Nb0.8Ta0.2)O3 (x = 0, 0.01, 0.02, 0.03) by defect engineering and investigated the effect of Sr2+ doping on its electrical properties. The results showed that a single perovskite phase can be achieved when Sr2+ doping is less than 3 mol%. Furthermore, Sr2+ doping was found to cause a linear decrease in both the Curie temperature (TC) and the orthorhombic–tetragonal phase transition temperature (TO‐T). The ceramic with x = 1% showed optimal piezoelectric properties, exhibiting an electric field‐induced strain of 0.124% under a low driving electric field of 20 kV/cm. This corresponds to an inverse piezoelectric coefficient () of 620 pm/V, which is comparable to that of commercial PZT piezoelectric ceramics. This study not only provides potential piezoelectric ceramics for actuators but also serves as a reference for developing high‐performance piezoelectric ceramics.

Recent Developments in (K, Na)NbO3-Based Lead-Free Piezoceramics.

(K0.5Na0.5)NbO3 (KNN)-based ceramics have been extensively investigated as replacements for Pb(Zr, Ti)O3-based ceramics. KNN-based ceramics exhibit an orthorhombic structure at room temperature and a rhombohedral-orthorhombic (R-O) phase transition temperature (TR-O), orthorhombic-tetragonal (O-T) phase transition temperature (TO-T), and Curie temperature of -110, 190, and 420 °C, respectively. Forming KNN-based ceramics with a multistructure that can assist in domain rotation is one technique for enhancing their piezoelectric properties. This review investigates and introduces KNN-based ceramics with various multistructures. A reactive-templated grain growth method that aligns the grains of piezoceramics in a specific orientation is another approach for improving the piezoelectric properties of KNN-modified ceramics. The piezoelectric properties of the [001]-textured KNN-based ceramics are improved because their microstructures are similar to those of the [001]-oriented single crystals. The improvement in the piezoelectric properties after [001] texturing is largely influenced by the crystal structure of the textured ceramics. In this review, [001]-textured KNN-based ceramics with different crystal structures are investigated and systematically summarized.

Excellent thermal stability of dielectric properties of (1− x)(K0.8Na0.2)NbO3–xBaTiO3 ceramics for application in X9R multilayer ceramic capacitors

(1 −x)(K0.8Na0.2)NbO3–xBaTiO3 [(1 −x)(K0.8N0.2)N–xBT] ceramics are well densified at 1080 °C for 2 h. The Curie temperature (TC) and orthorhombic–tetragonal phase transition temperature (TO-T) of the ceramics considerably decrease and broaden with increasing x. Moreover, the ceramics (x ≥ 0.08) have large relaxor properties. The (1 −x)(K0.8N0.2)N–xBT ceramics (0.09 ≤x ≤ 0.11) exhibit excellent thermal stability of dielectric properties, satisfying the X9R multilayer ceramic capacitor (MLCC) specification requirements. However, the (1 −x)(K1−yNy)N–xBT ceramics (0.0 ≤x ≤ 0.3 and y = 0.5 and 0.8), which contain a large amount of Na+ ions, cannot satisfy the X9R MLCC specification requirements owing to the weakened relaxor properties and reduced decreasing rate of TC and TO-T. An MLCC is fabricated using the 0.9(K0.8N0.2)N–0.1BT ceramic. The MLCC complies with the X9R MLCC regulation requirements and has excellent stability of the applied electric field. Therefore, the 0.9(K0.8N0.2)N–0.1BT ceramic serves as a promising candidate for X9R MLCC devices.

Deciphering the role of A-site ions of AZrO3-type dopants in (K, Na)NbO3 ceramics

As one of the most important lead-free piezoelectric candidates, potassium sodium niobate [(K, Na)NbO3, KNN] has gained popularity due to its attractive functional properties, which are much improved by chemical modifications. However, there still remains an insufficient understanding of how these dopants influence the structure and performance of KNN ceramics. Herein, we comparatively studied a series of changes triggered by introducing (Bi0.5Na0.5)ZrO3, BaZrO3, and PbZrO3 into the KNN matrix. The analysis highlights their different roles in modulating the orthorhombic-tetragonal phase transition temperature (TO-T) by considering discrepancies in the electronegativity and radius among Bi3+, Ba2+, and Pb2+ ions. The synergistic effects of the large electronegativity and the small radius of the Bi3+ ion are the key to regulating TO-T, which is relatively unaffected by Pb2+ and Ba2+ ions. Based on experimental results and first-principles calculations, a multi-scale model is proposed to summary the general law of how the electronegativity and radius of the A-site ion in AZrO3-type dopants synergistically affect the phase structure, ferroelectric domains, and electrical properties of KNN ceramics. Therefore, this work helps understand the role of chemical dopants in the structure and performance of KNN ceramics and promote the future composition design for high performance.

Phase structure regulation and enhanced Curie temperature of (K,Na)NbO3-based piezoelectric ceramics

ABSTRACT (K,Na)NbO3 (KNN) ceramics can achieve high piezoelectric properties under the action of polymorphic phase boundary (PPB), but their application is always limited by low Curie temperature (T c). In this work, a new system of lead‐free (1-x)K0.4Na0.6Nb0.96Sb0.04O3-xLi0.5Bi0.45La0.05ZrO3 (KNNS-xLBLZ) ceramics is first designed and prepared to search for good piezoelectric properties and high Curie temperature. The crystal structure, dielectric, piezoelectric, microstructure and conductivity of the system were systematically investigated. The analysis of the dielectric temperature dependence reveals that doping Li0.5Bi0.45La0.05ZrO3 increases rhombohedral-orthorhombic phase temperature (T R-O) and decreases orthorhombic-tetragonal phase transition temperature (T O-T), and then achieves O-T phase coexistence at room temperature. In particular, a piezoelectric coefficient (d 33) of 320 pC/N and T c = 290°C are achieved at 0.02. Ceramics prepared by this ion doping and phase regulation method possess high d 33 and T c. These results show that KNNS-xLBLZ ceramics have great potential for application in high-temperature environments.

The Role of Adding Bi0.5A0.5ZrO3 in Affecting Orthorhombic-Tetragonal Phase Transition Temperature and Electrical Properties in Potassium Sodium Niobate Ceramics

By studying the alone and synergetic effects of Zr4+ and Bi3+ on potassium sodium niobate ((K, Na)NbO3, KNN) ceramics, we revealed how Bi0.5A0.5ZrO3 (A = K, Na, Ag, and (Na0.82K0.18)) reduces the orthorhombic-tetragonal phase transition temperature (TO–T) of KNN ceramics. Investigations into the alone effects reveal that aliovalent substitutions on K+/Na+ (Nb5+) with Bi3+ (Zr4+) inevitably destroy long-range ordering (LRO) and thus worsen piezo/ferroelectric properties. Despite this, Zr4+ can replace Nb5+ within a high content, remain an orthorhombic (O) phase, and slightly increase TO–T. Although substituting on K+/Na+ with Bi3+ decreases TO–T, it already significantly destroyed LRO before shifting TO–T to room temperature. Then, investigations into the synergetic effects show that Zr4+ acts as a buffer, Bi3+ is an accelerator, and A+ is a stabilizer. The buffer can exist in KNN ceramics within a high content and neutralizes the charges caused by the accelerator that concentrates on decreasing TO–T, and the stabilizer compensates for the stability of the perovskite phase. Their synergetic effects explain why Bi0.5A0.5ZrO3 can gradually reduce the TO–T of KNN ceramics without significantly destroying LRO. Therefore, this work helps understand how Bi0.5A0.5ZrO3 decreases TO–T and further design the phase boundary for KNN ceramics.

Looking beyond the Surface: The Band Gap of Bulk Methylammonium Lead Iodide.

Despite the intense research on photovoltaic lead halide perovskites, reported optical properties as basic as the absorption onset and the optical band gap vary significantly. To unambiguously answer the question whether the discrepancies are a result of differences between bulk and "near-surface" material, we perform two nonlinear spectroscopies with drastically different information depths on single crystals of the prototypical (CH3NH3)PbI3 methylammonium lead iodide. Two-photon absorption, detected via the resulting generation of carriers and photocurrents (2PI-PC), probes the interband transitions with an information depth in the millimeter range relevant for bulk (single-crystal) material. In contrast, the transient magneto-optical Kerr effect (trMOKE) measured in a reflection geometry determines the excitonic transition energies in the region near (hundreds of nm) the surface which also determine the optical properties in typical thin films. To identify differences between structural phases, we sweep the sample temperature across the orthorhombic-tetragonal phase transition temperature. In the application-relevant room-temperature tetragonal phase (at 170 K), we find a bulk band gap of 1.55 ± 0.01 eV, whereas in the near-surface region excitonic transitions occur at 1.59 ± 0.01 eV. The latter value is consistent with previous reflectance measurements by other groups and considerably higher than the bulk band gap. The small band gap of the bulk material explains the extended infrared absorption of crystalline perovskite solar cells, the low-energy bands which carry optically driven spin-polarized currents, and the narrow bandwidth of crystalline perovskite photodetectors making use of the spectral filtering at the surface.

Reduced degree of phase coexistence in KNN-Based ceramics by competing additives

This work reveals the role of Bi3+ in the controlling ability of (Bi0.5Ag0.5)ZrO3 to the phase structure of potassium sodium niobate based ceramics by introducing Ba2+. The substitution on (Bi0.5Ag0.5)2+ with Ba2+ results in the reduced degree of phase coexistence, accompanying the phase transition from coexisting orthorhombic-tetragonal (O–T) phases to an orthorhombic (O) phase, which is demonstrated by the systematic phase structure analysis and variations of electrical properties. The analysis reveals that the high electronegativity of Bi3+ results in the increased covalent bond character and then increases the stabilization of the T phase, leading to a decrease in orthorhombic-tetragonal phase transition temperature (TO-T). Therefore, this work not only compares the controlling ability of Ba2+ and (Bi0.5Ag0.5)2+ to TO-T but also reveals which and how elements cause the decreased TO-T, promoting the understanding of the effect of additives on the phase structure.

Remarkably strong piezoelectricity, rhombohedral-orthorhombic-tetragonal phase coexistence and domain structure of (K,Na)(Nb,Sb)O3–(Bi,Na)ZrO3–BaZrO3 ceramics

Demands for lead-free piezoelectric ceramics increase rapidly owing to environmental concerns. Strong piezoelectricity in (K,Na)Nb-based ceramics was usually realized by taking the advantage of rhombohedral-tetragonal (R-T) phase coexistence associating with the R-T phase transition. In this paper, we report the study results on a series of dense (0.96-x)(K0.48Na0.52)(Nb0.96Sb0.04)O3−0.04(Bi0.50Na0.50)ZrO3−xBaZrO3 ceramics (abbreviated as KNNS-BNZ-xBZ ceramics), in which the one with x = 0.01 shows a giant piezoelectric coefficient d33 of 610 pC/N with quite large electromechanical coupling coefficients of kp = 0.58 and k33 = 0.69. Dielectric measurement indicated that the rhombohedral-orthorhombic (R–O) phase transition temperature TR-O, orthorhombic-tetragonal (O-T) phase transition temperature TO-T and Curie temperature TC of this ceramic are −29 °C, 43 °C and 241 °C, respectively. Meanwhile, XRD analysis unveiled that this ceramic is in rhombohedral-orthorhombic-tetragonal (R-O-T) phase coexistence at room temperature. Acid etching revealed that domain structure inside the polycrystalline grains consists of lamellar domain stacks with a large amount of hierarchical nanodomain structure existing in part of the lamellar domains. The observed strong piezoelectricity is ascribed to the R-O-T phase coexistence and the corresponding characteristic domain structure.

Giant piezoelectricity, rhombohedral-orthorhombic-tetragonal phase coexistence and domain configurations of (K,Na)(Nb,Sb)O3–BiFeO3–(Bi, Na)ZrO3 ceramics

Research on lead-free piezoelectric ceramics has been an important subject in recent years due to increasingly strong environmental concerns. In this paper, we report the piezoelectric performance, phase transitions and domain structure for a series of dense (0.994-x)(K0.40Na0.60)(Nb0.955Sb0.045)O3–0.006BiFeO3–x(Bi0.50Na0.50)ZrO3 ceramics prepared by two step-sintering through solid-state reaction. Among these ceramics, the one with x = 0.03 shows a giant piezoelectricity with remarkably high piezoelectric coefficient d33 of 550 pC/N at room temperature. Concurrent measurements of ε′ and ε′′ vs. temperature dependences and X-ray diffraction analysis showed that ceramics with x ≤ 0.04 are in rhombohedral-orthorhombic-tetragonal (R-O-T) phase coexistence at room temperature. More intriguingly, the rhombohedral-orthorhombic phase transition temperature, TR-O, is almost independent of the (Bi0.50Na0.50)ZrO3 content, while orthorhombic-tetragonal phase transition temperature, TO-T, decreases largely with increasing the (Bi0.50Na0.50)ZrO3 content. Domain configurations of the ceramic with x = 0.03 were investigated by acid-etching. Complicated domain patterns consisting of watermark-shaped domains of long parallel stripes separated by irregularly shaped boundaries are seen before poling. In contrast, relatively simple domain patterns of long parallel stripes with some nanodomains appearing in a part of broad stripes are observed after poling. The obtained excellent piezoelectric properties are ascribed to the R-O-T phase coexistence and the corresponding characteristic domain structure.

Co-Doping Effect of BiGaO3 and (Bi,Na,K,Li)ZrO3 on Multi-Phase Structure and Piezoelectric Properties of (K,Na)NbO3 Lead-Free Ceramics

The phase boundary structure of (K,Na)NbO3 piezoelectric ceramic was modified by doping with Bi(Na,K,Li)ZrO3 and BiGaO3 through normal solid-state sintering. Rietveld refinements by X-ray diffraction revealed that the Bi(Na,K,Li)ZrO3/BiGaO3 co-doping in (K,Na)NbO3 led to a multi-phase structure at room-temperature, effectively moving the rhombohedral-orthorhombic (R-O) and orthorhombic-tetragonal (O-T) polymorphic phase transition temperatures close to the room temperature region. Increased levels of doping also generated a structural transition, i.e., triphasic R-O-T to diphasic R-T (T-rich) and finally to R-T (R-rich), contributing to shrinkage of the O phase as well as the increase of R phase fraction. A sensitive influence of the BiGaO3 doping (0.001 mole fraction level) on the structural properties such as the phase and microstructure was shown, resulting from the effect of the super-tetragonal structure of BiGaO3. The d33 property was strongly dependent on the phase and its volume fraction, in addition to the grain sizes. Eventually, enhanced and balanced properties of the piezoelectric coefficient and Curie temperature (d33 = 309 pC/N, TC = 343 °C) were obtained when the doped ceramic had a T-rich (86%) R-T structure.

Structural evolution and electrical properties of lead-free (1-x) (K0.4Na0.6)Nb0.96Sb0.04O3-xBa0.1(Bi0.5Na0.5)0.9ZrO3 ceramics

New lead-free piezoelectric ceramic system based on (1-x) (K0.4Na0.6) Nb0.96Sb0.04O3-xBa0.1(Bi0.5Na0.5)0.9ZrO3 was designed and fabricated by using conventional ceramic processing techniques. An investigation was carried out with a focus on the effects of Ba0.1(Bi0.5Na0.5)0.9ZrO3 content on the phase structure, dielectric and piezoelectric properties of the ceramics. It was found that adding Ba0.1(Bi0.5Na0.5)0.9ZrO3 could move the rhombohedral-orthorhombic phase transition temperature upwards and simultaneously shift the orthorhombic-tetragonal phase transition temperature downwards, and thereby resulted in the formation of a rhombohedral-tetragonal phase boundary with 0.045 ≤ x ≤ 0.05. The piezoelectric properties were obviously enhanced for the ceramics with compositions near the rhombohedral-tetragonal phase boundary. Furthermore, an analysis was as well conducted on the main characteristics of the ferroelectric-paraelectric phase transition of the ceramics. Our experimental results suggest that the Ba0.1(Bi0.5Na0.5)0.9ZrO3 is an effective additive for the construction of phase boundaries in (K,Na)NbO3-based ceramics, therefore benefiting the improvement of piezoelectric activity.

Microstructures, phase evolution and electrical properties of (1-x)K0.40Na0.60Nb0.96Sb0.04O3-xBi0.5K0.5HfO3 lead-free ceramics

An investigation was carried out on the surface morphologies, crystal structures and electrical properties of lead-free piezoelectric (1-x)K0.40Na0.60Nb0.96Sb0.04O3-xBi0.5K0.5HfO3 ceramics, prepared by conventional solid-state reaction route. Based on X-ray diffraction results and temperature-dependent dielectric properties, we found that the ceramics experienced a structural evolution from orthorhombic phase to multi-phase coexistence and then to rhombohedral phase, with the increase of Bi0.5K0.5HfO3 content. Furthermore, a rhombohedral–tetragonal phase boundary was identified at x = 0.035, which was considered to be caused by the merging of rhombohedral-orthorhombic and orthorhombic-tetragonal phase transition temperatures. The ceramics exhibited an enhanced piezoelectric activity near the rhombohedral–tetragonal phase boundary, with a piezoelectric constant d33 of 230 pC/N and a planar electromechanical coupling coefficient kp of 0.340. In addition, a clear composition dependence was observed for the microstructure of the ceramics, which could be explained by the combined effect of vacancies and phase segregations.

Structure and property of lead-free (K,Na)NbO3–(Bi1/2Na1/2)ZrO3–CaTiO3 piezoelectric ceramics

Simultaneous improvement of piezoelectricity and temperature stability is vital for developing high-performance KNN-based lead-free materials. Here, effects of CaTiO3 on phase structure, piezoelectricity and temperature stability of (1 – x)[0.95(K0.5Na0.5)NbO3–0.05(Bi0.5Na0.5)ZrO3]–xCaTiO3–0.2%MnO2 ceramics were studied. It was found that the orthorhombic-tetragonal phase transition temperature can be gradually decreased by increasing CaTiO3 content. Particularly, the improved strain temperature stability (strain varied < 15% when measurement temperature reaches 110 °C) and enhanced electrical properties (d33 ~ 296 pC/N and strain ~ 0.14%) can be observed in the ceramics with x = 0.02. Therefore, shifting the TO−T below room temperature is an effective way to promote temperature stability and electrical properties of KNN-based piezoelectric materials.

Room temperature multiferroic behavior and magnetoelectric coupling in (K,Na)NbO3-based ceramics

Polycrystalline (K0.48Na0.52)0.95Li0.05(Nb0.95Sb0.05)1-xFexO3 (0 ≤ x ≤ 0.030) ceramics are synthesized by the conventional solid state reaction method. The samples with 0.010 ≤ x ≤ 0.030 are found to possess good ferroelectricity, weak ferromagnetism and large magnetodielectric effect simultaneously at room temperature. Temperature dependence of magnetization and dielectric properties show that there are both magnetic and magnetodielectric anomalies in the vicinity of the ferroelectric rhombohedral-orthorhombic (TR-O) and orthorhombic-tetragonal (TO-T) phase transition temperatures. Furthermore, magnetodielectric effect as large as 16% is observed at 0.9 T near TR-O when x = 0.030. According to these results, a strong spin-lattice coupling is considered to exist in these samples. In the structure transition process, the spin interactions between Fe ions will alter for the severe distortion of the FeO6 octahedron. Moreover, since there are more possible polarization vectors due to polymorphic phase transition (PPT), the local polarization of FeO6 will also be greatly affected when the external magnetic field changes the spin interactions.

Piezoelectric properties and domain structure of the orthorhombic (K,Na,Li)(Nb,Ta,Sb)O3 single crystal

ABSTRACTIn this work, (K,Na,Li)(Nb,Ta,Sb)O3 (KNLNTS) crystal is in the orthorhombic phase at room temperature. The orthorhombic-tetragonal phase transition temperature is 50.0°C, and the Curie temperature TC of the tetragonal-cubic phase transition temperature is 253.8°C. The crystal is poled, the defects were “pinned” in specific position, and has positive effect in domain wall motions and better ferroelectric property (Pr is 6.49 µC/cm2, Ec is 6.66 kV/cm). The domain configrations of the crystal were studied by means of a polarizing light microscopy (PLM), with poling along [100]C direction.

Ferroelastic behavior across the orthorhombic-to-tetragonal phase transition region of NKN-based lead-free ferroelectrics

In this study, the macroscopic mechanical behavior was characterized as a function of temperature (−150 °C to 400 °C) for polycrystalline (Na0.5K0.5)NbO3 with three dopant concentrations. Dopants can improve certain electromechanical properties and, in the case of NKN and Li+, shift the orthorhombic-to-tetragonal phase transition temperature to lower temperatures. In this study, the mechanical behavior of undoped NKN, LNKN6 with 6 mol. % Li+, and LNKN6 with additional dopants was characterized and compared with the temperature dependent dielectric response and crystal structure. During mechanical loading, the samples showed a nonlinear hysteretic response. At low temperatures, this is understood to be due to ferroelasticity. At temperatures in the vicinity of the orthorhombic-tetragonal phase transition temperature, a closed hysteresis behavior was observed, corresponding to a local maximum of the critical ferroelastic stress and a minimum in the remanent strain. The observed closed hysteresis behavior is suggested to be due to a stress-induced structural phase transformation.