Symmetry-mode analysis for intuitive observation of structure–property relationships in the lead-free antiferroelectric (1−x)AgNbO₃–xLiTaO₃

Abstract

Functional materials are of critical importance to electronic and smart devices. A deep understanding of the structure–property relationship is essential for designing new materials. In this work, instead of utilizing conventional atomic coordinates, a symmetry-mode approach is successfully used to conduct structure refinement of the neutron powder diffraction data of (1−x)AgNbO3–xLiTaO3 (0 ≤ x ≤ 0.09) ceramics. This provides rich structural information that not only clarifies the controversial symmetry assigned to pure AgNbO3 but also explains well the detailed structural evolution of (1−x)AgNbO3–xLiTaO3 (0 ≤ x ≤ 0.09) ceramics, and builds a comprehensive and straightforward relationship between structural distortion and electrical properties. It is concluded that there are four relatively large-amplitude major modes that dominate the distorted Pmc21 structure of pure AgNbO3, namely a Λ3 antiferroelectric mode, a T4+ a − a − c 0 octahedral tilting mode, an H2 a 0 a 0 c +/a 0 a 0 c − octahedral tilting mode and a Γ4− ferroelectric mode. The H2 and Λ3 modes become progressively inactive with increasing x and their destabilization is the driving force behind the composition-driven phase transition between the Pmc21 and R3c phases. This structural variation is consistent with the trend observed in the measured temperature-dependent dielectric properties and polarization–electric field (P-E) hysteresis loops. The mode crystallography applied in this study provides a strategy for optimizing related properties by tuning the amplitudes of the corresponding modes in these novel AgNbO3-based (anti)ferroelectric materials.

Structure-Driven, Ferroelectric Wake-Up Effect for Electrical Fatigue Relief

“…34,35 However, the structural fit using a single Pmc2 1 phase does not fit other NPD patterns, such as that of pristine AN60LT, because the low angle region of the ⟨1 1 1⟩ p * peaks fails to be assigned. On the basis of our previous result, 31 Pmc2 1 + R3c, two-phase model can achieve a significantly better structural refinement, fitting well to the observed data (Figure S2 of the Supporting Information). The crystal structure of this second, FE R3c phase is schematically drawn in Figure 2b, and its ferroelectricity is directly linked with the off-center atomic displacements of the Ag + and Nb 5+ ions relative to the O 2− ion substructure (red arrows in Figure 2b).…”

“…On the basis of careful Rietveld refinement analysis (detailed refinement results are shown in Figure S2 and Tables S1–S7 of the Supporting Information), the NPD pattern of the pristine AN45LT sample is well-fitted using a structure isomorphous to that of pure AgNbO 3 with orthorhombic Pmc 2 1 space group. , However, the structural fit using a single Pmc 2 1 phase does not fit other NPD patterns, such as that of pristine AN60LT, because the low angle region of the ⟨1 1 1⟩ p * peaks fails to be assigned. On the basis of our previous result, Pmc 2 1 + R 3 c , two-phase model can achieve a significantly better structural refinement, fitting well to the observed data (Figure S2 of the Supporting Information). The crystal structure of this second, FE R 3 c phase is schematically drawn in Figure b, and its ferroelectricity is directly linked with the off-center atomic displacements of the Ag + and Nb 5+ ions relative to the O 2– ion substructure (red arrows in Figure b).…”

“…Panels d and e of Figure present the temperature-dependent dielectric spectra of the AN45LT and AN60LT samples. The dielectric behavior of pristine AN45LT is almost identical to that of pure AgNbO 3 over the temperature range from 120 to 780 K with three detectable anomalies (denoted as T 1 , T 2 , and T 3 ). , With reference to previous investigations, , the peaks at the ∼ T 1 and T 2 points are assigned to the AFE M 1 –M 2 and M 2 –M 3 phase transitions, respectively, while the peak at the T 3 point is related to the AFE M 3 to paraelectric (PE) phase transition. After AN45LT is triggered into the IM state, an additional dielectric peak appears around 400 K ( T U ), while both the dielectric constant and loss of the T 1 peak decrease.…”

“…The dielectric behavior of pristine AN45LT is almost identical to that of pure AgNbO 3 over the temperature range from 120 to 780 K with three detectable anomalies (denoted as T 1 , T 2 , and T 3 ). 29,30 With reference to previous investigations, 31,32 the peaks at the ∼T 1 and T 2 points are assigned to the AFE M 1 −M 2 and M 2 −M 3 phase transitions, respectively, while the peak at the T 3 point is related to the AFE M 3 to paraelectric (PE) phase transition.…”

“…In the (1 − x)AgNbO 3 −xLiTaO 3 material systems, increasing the LiTaO 3 doping level destabilizes the Pmc2 1 phase, disrupting the long-range ordered a 0 a 0 c + /a 0 a 0 c − octahedral tilting and weakening the antiferroelectricity. 31 The pristine AN60LT sample presents the co-existence of Pmc2 1 and R3c phases, suggesting small energy differences between these two phases in comparison to AN45LT. Therefore, to trigger the pristine sample into the woken-up state, the AN60LT sample requires a smaller E-field (30 kV/cm compared to 50 kV/cm for AN45LT) and a smaller number of cycles (3000 compared to 9000 for AN45LT).…”