Two PNZST samples - Pb0.99(Nb0.02Zr0.73Sn0.21Ti0.04)O3 (PNZST1) and
Pb0.99(Nb0.02Zr0.65Sn0.28Ti0.05)O3 (PNZST2) were involved in my PhD project. They were
for the properties characterization. The novel phenomenon, that structure at the near surface and bulk regions is different, was observed in PNZST1. Furthermore, the structure near the surface region is very sensitive to the surface processing and according to the in- situ NPD, the underlying reason is attributed to the mechanical force and heat-treatment induced phase transitions. In contrast, PNZST2 presents a consistent structure in surface and bulk regions. Its E-field and temperature induced crystal structure, preferred orientation and lattice strain evolutions were systematically investigated by the in-situ NPD. The results are interesting and important in understanding the role of the coupling between the various modes in these materials and how they affect bulk properties.
⚫ The XPRD results of the PNZST1 sample indicate that the virgin sample shows the AFEO phase which is quite similar to the structure of the prototype PbZrO3. After the
polishing, the sample contains both AFE and rhombohedral FE phases. With further heat-treatment, the sample presents single rhombohedral FE phase and after repolishing the surface, the state of AFE and FE coexistence returns. The NPD indicates that the sample only shows a single FE phase in regardless of the polishing and heat-treatment. Considering the penetration length of lab X-ray and neutron facilities, the diffraction patterns reflect the structural information for the near surface and bulk regions, respectively. This result not only reports the structural inconsistency between the surface and bulk regions, but also points out the near- surface structure is sensitive to the surface processing. (3.2 Figure 1)
⚫ For the sample after polishing, the PFM results show two typical regions. Some parts present uniform amplitude and phase without obvious contrasts. Other areas present clear features of the rhombohedral FE domains. This result further proves the coexistence of AFE and FE phases after mechanical polishing. The sample after heat- treatment shows labyrinthine domains with a size distributed over the range of ~100- 200 nm, similar to the domains observed in relaxor ferroelectrics. Therefore, the heat- treatment not only induces the FE phase out of AFE phase, but also reduces the domain size. (3.2 Figure 2)
⚫ The in-situ NPD characterization at different hydrostatic pressures was carried on the powder of the sample after heat-treatment. Under ambient conditions, the powder
sample presents same structure as the bulk sample – rhombohedral symmetry. When the hydrostatic pressure reaches 169 MPa, the observed NPD pattern shares the similar features with the AFE phase. Therefore, the coexistence of the AFE and FE phases in the near surface region is possibly due to the strain change induced by the mechanical polishing. The heat-treatment helps release the surface stress/strain, in turn inducing the AFE to FE phase transition. As the phase transitions only occur near the surface, the ceramic samples always present typical FE properties macroscopically. (3.2 Figure 3 and Figure 4)
⚫ The NPD pattern of PNZST2 under ambient conditions suggests that the sample shows the AFE structure which is dominated by both AFE, q1 = γ[110]p* mode and
AFD, q2 = 1/2[111]p*, mode. With in-situ applying E-field to 25 kV/cm, the AFE
phase transfers into FE phase which is dominated by FE, q = [000]p*, mode and AFD,
q2=½[111]p* mode. After withdrawal of the E-field, the FE phase is still stable. This
E-field induced structural change is in agreement with the measured P-E hysteresis loops. (3.3 Figure 1 and 2)
⚫ The associated preferred orientation and lattice strain were investigated by the ω- dependent NPD patterns. The grains/domains in the virgin AFE phase present random distribution. After being switched into the FE phase, the [111]p polar axis prefers to
align parallel to the E-field, forming a strong preferred orientation. Additionally, the lattice strain behaves elliptical distribution with elongation along the E-field. After withdrawal of the E-field, both preferred orientation and lattice strain in the metastable FE phase present little relaxation. (3.3 Figure 2)
⚫ The temperature dependent dielectric spectra and P-E hysteresis loops indicate the metastable FE phase experiences at least 2 phase transitions before entering the paraelectric phase. The in-situ neutron diffraction suggests the first phase transition around 376 K, is associated with two FE phases. During this phase transition, the AFD mode disappears, i.e., the high-temperature FE phase is simply dominated by the FE mode. With further increasing the temperature to 438 K, FE to AFE phase transition occurs. Interestingly, AFE and AFD modes reappear simultaneously during this phase transition. Furthermore, the obtained double P-E hysteresis loop also
conforms to the AFE phase. According to the ω-dependent NPD patterns, the preferred orientation formed in metastable FE phase presents minor variation during the FE-to-FE phase transition but disappears during the FE-to AFE phase transition. (3.3 Figure 3)
⚫ The in-situ NPD was carried out to investigate the E-field induced phase transitions at high temperature. With applying 20 kV/cm E-field, the induced FE phase appears, showing a pseudo-cubic symmetry without AFD mode. After withdrawal of the E- field, the AFE and AFD modes also reappear simultaneously. The temperature and E-field induced phase transitions observed in this material address that both AFE and AFD modes are required in stabilizing the AFE structure. The evolution of the preferred orientation in this E-field induced AFE-FE phase transition at high temperature are similar to that described for the PLZST sample. Once this kind of preferred orientation has been built at high temperature, it will be stored after cooling to room temperature. (3.3 Figure 4)
⚫ The dielectric and FE properties between the textured and non-textured samples show differences. The resultant textured sample has a lower critical field for AFE-FE phase transition at room temperature compared with the non-textured sample, suggesting that EAFE-FE can be tuned by the preferred orientation. (3.3 Figure 5)