3.1 Structure and morphology
XRD patterns are shown in Fig. 2(a) for Component A, Component B, (1-w)PMN-wPSMN-PT (w = 0.3, 0.4, 0.5, 0.6) samples, and the reference sample. As expected, the pyrochlore phase Pb2Nb2O7 (PDF card no. 40–0828) was formed with the featured diffraction peaks at 14.32° and 30.36° in the reference sample. Component A exhibited a pure perovskite tetragonal phase structure with no obvious impurities. However, a traceable pyrochlore phase in Component B with a rhombohedral structure was detected. And with increasing w, pyrochlore phase first decreases and then increases, indicating the challenge to completely eliminate the pyrochlore phase, though the contents were still far below from the reference sample. The samples prepared by the twin-crystal phase mixed co-firing method displayed higher crystal purity (99.62–99.89% perovskite structure), which is much better than the samples prepared by the conventional sintering method (98.64% perovskite structure). In this study, niobium oxide, magnesium oxide, titanium oxide, samarium oxide, and partial lead oxide were pre-sintered at 880 ℃ for 4 h, promoting the homogenization of the system and minimizing the pyrochlore phase induced by the local excessive samarium concentration[16].
By fitting the diffraction peaks of the (1-w)PMN-wPSMN-PT at 44°−46°, Fig. 2(b) shows that the overlapping degree of diffraction peaks increased gradually with w. With the increment of w, the tetragonal phase Component A decreased and the rhombohedral phase Component B increased such that the samples would gradually transform from tetragonal phase to rhombohedral phase. Consequently, double diffraction peaks of (002) and (200) transformed to a single peak of (200). When w = 0.6, (002) and (200) diffraction peaks overlapped completely. Rietveld refinement was performed for (1-w)PMN-wPSMN-PT (w = 0.3, 0.4, 0.5, 0.6) samples by GSAS software[30] and the results are shown in Figs. 2(c)−(f). Comparing the diffraction patterns of (1-w)PMN-wPSMN-PT samples and Component A, the symmetry of the diffraction peaks of (1-w)PMN-wPSMN-PT ceramics samples became worse in the tetragonal phase (111) at 2θ = 31.44°. To avoid excessive grain growth, the sintering time at 1250 ℃ was only 1.5 h such that Component A with the tetragonal phase and Component B with the rhombohedral phase cannot be completely homogenized, resulting in the distortion of diffraction peaks to some extent[16, 23]. During the mixed sintering of tetragonal Component A and rhombohedral Component B, the green body was gradually densified with the movement of grain boundary. As different grain sizes were involved in the mixture, some grains retained their original composition in the sintering process. The lattice parameters, cell volume, and density of Component A, Component B, and (1-w)PMN-wPSMN-PT samples are shown in Table 1. It can be seen that with the increment of w, the cell parameters a and b of the samples decreased first and then increased, while the corresponding c exhibited the opposite trend. The phenomenon could be explained as follows: (1) the chemical composition of crystal particles had a gradient change when the ceramic samples with the identical chemical composition and different mixing ratios were prepared by the twin-crystal mixed co-firing process, resulting in the change of cell parameters; (2) the cell parameters a and b of the samples with more pyrochlore content (w = 0.3 and w = 0.6) were relatively large, which indicates that the generation of pyrochlore phase would deviate the chemical composition of the main phase, resulting in the change of crystal lattice. In addition, the c/a ratios of (1-w)PMN-wPSMN-PT (w = 0.3, 0.4, 0.5, 0.6) ceramics were 1.0033, 1.0033, 1.0028, 1, respectively. It can be seen that the c/a of the samples was closer to 1 with higher w because the phase structure of the samples tended to the cubic phase, which agreed well with the overlapping degree of diffraction peaks in Fig. 2(b). Compared with Sm-doped Pb(ZrxTi1−x)O3 (Sm-PZT) ceramics, the c/a of Sm-PMN-PT ceramics was much closer to 1[23, 25] such that the internal stress caused by domain switching is smaller, the octahedral gap formed by oxygen atoms is relatively loose, and the micro-displacement of B-site ions is relatively easy.
The Raman peaks of lead-based ferroelectric materials with the ABO3 perovskite structure are divided into three categories: The Raman mode with the wavenumber less than 150 cm− 1 belongs to Pb-BO6 stretching mode, the one between 150 cm− 1 and 500 cm− 1 is a mixture of B-O-B bending mode and O-B-O stretching mode, and the one between 500 cm− 1 and 800 cm− 1 is related to B-O-B stretching mode[31, 32]. According to the lattice dynamics, group theory, and Raman studies of other ferroelectrics with the ABO3 structure, the number of Raman modes can be used to determine the phase structure of ferroelectrics. The rhombohedral phase (R3m) has seven Raman active modes and the tetragonal phase (P4mm) has eight Raman active modes[33]. The room-temperature Raman spectra of the (1-w)PMN-wPSMN-PT samples and the deconvolution of multiple Lorentzian/Gaussian peaks are shown in Fig. 3, in which the fitting results agree well with the measured Raman spectra. Among the vibrational bands, the mode located at ~ 800 cm− 1 can be assigned to the stretching vibration of Nb–O–Mg and B-site cations; the band at 580 cm− 1 is originated from the oxygen bending vibration; the band 500 cm− 1 is attributed to the stretching vibration of Nb–O–Nb; the band at 433 cm− 1 arises from the stretching vibration of Mg–O–Mg mode; and the most intense band at 270 cm− 1 is contributed by B site ions against O stretching vibration inside the octahedron[34]. The Raman spectra of samples with w = 0.3 and w = 0.4 can be deconvoluted into 7 Raman modes using the Lorentzian/Gaussian fitting in the wavenumber range of 180 cm− 1–840 cm− 1, while the samples with w = 0.5 and w = 0.6 have 6 Raman modes, which is related to the ratio of tetragonal Component A and rhombohedral Component B in the samples. Based on the characteristics of Raman spectra, the samples with w = 0.3 and w = 0.4 were with the tetragonal phase, while those with w = 0.5 and w = 0.6 samples were with the rhombohedral phase. Due to the detection limit of our Raman spectrometer, the Raman modes below 99 cm− 1 could not be accurately evaluated.
The ceramic samples were polished and etched at 1100 ℃ for 1 h before the morphological characterizations. Figure 4 shows that the grain size was relatively uniform in the range of 4 − 6 µm, with an average size of 5.18 µm. The SEM images of the fresh section of the sample show both transgranular fracture and intergranular fracture mechanisms. This may contribute to the comparative bonding strength between grains and grain boundaries. Besides, the grain boundaries are angular, revealing that no glass phase remained. The grains of the samples were closely packed with very few pores, which agree with the high density (95.4%−97.8%) of the samples listed in Table 1.
3.2 Dielectric properties
Figures 5(a)−(d) show the temperature-dependent dielectric constant εr and loss tangent tanδ of (1-w)PMN-wPSMN-PT samples. The characteristic temperatures Tm of (1-w)PMN-wPSMN-PT samples with w = 0.3, 0.4, 0.5 and 0.6 were 79 ℃, 79 ℃, 81 ℃ and 86 ℃, respectively, and the corresponding peak values of εr were 23789, 30190, 30192, and 27656. Above Tm, both the dielectric and loss peaks shifted to high temperature along with the increasing frequency. The results show an obvious frequency dispersion characteristic, indicating the diffusive phase transition and the relaxor ferroelectric property. The dielectric spectra of the (1-w)PMN-wPSMN-PT samples and the reference sample at 1 kHz are shown in Fig. 5(e). The characteristic temperature of the reference sample was 82.2 ℃, and the corresponding εr peak value was 19373. The sample prepared by the twin-crystal mixed co-firing process exhibited significantly improved εr. However, the Curie temperature of PMN-PT ceramics decreased when doped with Sm. Pb(ZrxTi1−x)O3 (PZT) ceramics (from 385 ℃ to 335 ℃[21, 35]) and Pb(Mg1/3Nb2/3)-PbZrO3-PbTiO3 (PMN-PZ-PT) ceramics (from 230 ℃ to 184 ℃[22, 36]) also had this characteristics after doped with Sm. The dielectric behavior of ferroelectrics above Tm can be fitted by the quadratic law for the typical relaxor ferroelectrics and normal ferroelectrics.
1/ε = 1/εmax+ (T-Tmax)γ/C’[37]
where γ is the degree of diffuseness, C’ is a constant. γ approaches 2 for an ideal relaxor ferroelectric, while γ approaches 1 for a normal ferroelectric[38]. The dielectric behavior of (1-w)PMN-wPSMN-PT samples above Tm at 1 kHz were linearly fitted according to the quadratic law. An example of the sample with w = 0.5 sample is shown in Fig. 5(f). The relaxation indexes were 1.72, 1.74, 1.83, and 1.75, respectively, showing obvious relaxation.
3.3 Piezoelectric properties
The piezoelectric and dielectric properties of (1-w)PMN-wPSMN-PT samples are shown in Fig. 6 and Table 2. It can be seen that d33, εr, electromechanical coupling factors kp (planar vibration mode) and kt (thickness vibration mode) increased along with w and reached maximum when w = 0.5. Compared to the reference sample, d33 and εr of (1-w)PMN-wPSMN-PT samples were greatly improved. Besides, the dielectric losses of all samples varied slightly while the mechanical quality factor dropped for the (1-w)PMN-wPSMN-PT samples. This could be attributed to the formation of Pb vacancy at the A-site after doped with Sm3+ such that the directional activation energy of the domain decreases. As the domain walls in the grains move easily, the coercive field reduces and makes the ceramics more easily polarized, thus improving the piezoelectric properties. At the same time, due to the easy movement of domain walls, the internal loss would inevitably increase, resulting in the decrease of mechanical quality factor Qm and the increase of dielectric loss[22].
3.4 Ferroelectric properties
The hysteresis loops and field-induced strain plots of (1-w)PMN-wPSMN-PT samples are shown in Fig. 7. It can be seen that the coercive fields Ec varied slightly among samples, giving ~ 2.3 kV/mm. The remanent polarization Pr reached the maximum value of 24.5 µC/cm2 when w = 0.5, which is consistent with the piezoelectric and dielectric performance. Similarly, the sample with w = 0.5 exhibited the largest field-induced strain of 0.25% as shown in Fig. 7(b). The butterfly curve is relatively thin, which indicates that the small field-induced strain hysteresis, good repeatability, and fast response of (1-w)PMN-wPSMN-PT ceramics[39]. This is very beneficial to the application of large displacement devices, such as atomic force microscopy, long-distance laser ranging calibration, submarine passive sonar, large displacement jacquard driver[40], and so on.