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A pseudocubic-tetragonal-orthorhombic (PC-T-O) multi-structure was developed in the 0.96(K0.5Na0.47Li0.03) (Nb0.92Sb0.08)O3-0.04(Ca0.5Sr0.5)ZrO3 [(KN0.47L0.03)NS-CSZ] lead-free piezoceramic. The PC structure observed in this sample was deemed to have an R3m rhombohedral structure. This sample demonstrated a large d33 (560 pC/N) owing to the existence of the PC-T-O multi-structure. The (KN0.47L0.03)NS-CSZ thick film also exhibited a large d33 (560 pC/N) and strain (0.19% at 4.0 kV/mm). The large strain was sustained even after the application of 106 electric-field cycles. Therefore, the (KN0.47L0.03)NS-CSZ thick film exhibited excellent fatigue characteristics and was utilized to fabricate a planar-type actuator, which achieved a large displacement (300 μm) and acceleration (872 G) at 125 V/mm. A cantilever-type multilayer actuator was also prepared using five layers of the (KN0.47L0.03)NS-CSZ thick film and demonstrated a large displacement (3500 μm) and acceleration (42.96 G) at 250 V/mm. Therefore, the (KN0.47L0.03)NS-CSZ thick film is a good candidate for fabricating various piezoelectric actuators.

A Fe2O3–added 0.96(K0.5Na0.5)(Nb0.94Sb0.06)–0.01SrZrO3–0.03(BiAg)ZrO3 [KNNS–0.01SZ–0.03BAZ] piezoceramic was well densified at 1090 °C. It exhibited a large piezoelectric charge constant (d33) of 650 pC/N, which is close to the largest d33 value reported in the literature. This sample has a tetragonal–orthorhombic–rhombohedral (T–O–R) structure, in which each structure has a similar proportion. Moreover, nanodomains (2 nm × 15 nm) with a low domain boundary energy were found in this sample, which exhibited relaxor properties. Therefore, the presence of a T–O–R multi-structure and nanodomains is responsible for the large d33 value of this sample. A KNNS–0.01SZ–0.03BAZ thick film was synthesized, and it also had a T–O–R multi-structure with a large d33 (630 pC/N). A planar-type actuator was produced using this thick film, and this actuator exhibited large acceleration (335 G at 120 V mm−1) and displacement (231 μm at 120 V mm−1). Hence, the KNNS–0.01SZ–0.03BAZ thick film is a good candidate for lead-free piezoelectric actuators.

Fe2O3-added 0.96(K0.5Na0.5)(Nb0.94Sb0.06)-0.01SrZrO3-0.03(BiAg)ZrO3 [KNNS-0.01SZ-0.03BAZ] piezoceramic was well densified at 1090 °C. It exhibited a large piezoelectric charge constant (d33) of 650 pC/N, which is similar to the largest d33 value reported in the literature. This sample has a tetragonal-orthorhombic-rhombohedral (T-O-R) structure, in which each structure has a similar proportion. Moreover, nanodomains (2 nm × 15 nm) with low domain boundary energy were found in this sample, which exhibited relaxor properties. Therefore, the presence of a T-O-R multi-structure and nanodomains is responsible for the large d33 value of this sample. A KNNS-0.01SZ-0.03BAZ thick film was synthesized, and it also had a T-O-R multi-structure with a large d33 (630 pC/N). A planar-type actuator was produced using this thick film, and this actuator exhibited a large acceleration (335 G at 120 V/mm) and displacement (231 μm at 120 V/mm). Hence, the KNNS-0.01SZ-0.03BAZ thick film is a good candidate for lead-free piezoelectric actuators.

Fe2O3-containing 0.96(K0.5Na0.5)(Nb0.93Sb0.07)-(0.04-x)CaZrO3-x(Bi, Ag)ZrO3 piezoceramics with 0.0 ≤ x ≤ 0.04 fabricated at 1090 °C show a dense microstructure without any secondary phases. The piezoceramic with x = 0.03 has an ideal rhombohedral–orthorhombic–tetragonal (R-O-T) multi-structure because its constituent structure proportions are similar. Its R-O-T phase transition temperature (TR-O-T) increases with increasing measuring frequency, indicating that this piezoceramic exhibits relaxor properties which develop near its ferroelectric-to-ferroelectric phase transition temperature, that is, its TR-O-T. This piezoceramic has nanodomains that are approximately 3 nm × 20 nm in size. The associated low boundary energies of these domains are expected to facilitate domain rotation. Furthermore, the piezoceramic has an extremely high piezoelectric constant (d33) of 680 ± 10 pC/N, which exceeds the maximum d33 reported in the literature for similar materials. This high d33 is attributed to the existence of an ideal R-O-T multi-structure and nanodomains.

A 0.96(K0.5Na0.5)(Nb1−ySby)O3-0.04SrZrO3 [KN(N1−ySy)-SZ] thick films with 0.045 ≤ y ≤ 0.08 were textured along the [001] direction using NaNbO3 (NN) templates. A 3.0 mol% NN seed-added KN(N1−ySy)-SZ thick film with y = 0.055 exhibited a large Lotgering factor of 97% and d33 value of 502 pC/N. This thick film had an orthorhombic-pseudocubic polymorphic phase transition structure. Rietveld analysis revealed that the pseudocubic structure was similar to the R3m rhombohedral structure. The proportion of the pseudocubic phase in this thick film was significantly lower than that of the orthorhombic phase. The proportion of the pseudocubic phase increased with increasing Sb2O5. A 3.0 mol% NN seed-added KN(N0.935S0.065)-SZ thick film (y = 0.065) exhibited a large d33 value of 620 pC/N because it had similar proportions of orthorhombic and pseudocubic structure. The large d33 value (≥580 pC/N) was maintained up to 110 °C, indicating that this thick film exhibits excellent thermal stability. Furthermore, the thick films with 0.045 ≤ y ≤ 0.055 are expected to be good candidates for piezoelectric energy harvesters (PEHs) because they exhibited large d33 × g33 values (≥19.3 pm2/N), which represent the figure of merit of a PEH.