Negative piezoelectric behaviors in hybrid improper ferroelectric Ca_3‐xNa_xTi₂O₇ (x = 0, 0.01) ceramics

Abstract

Hybrid improper ferroelectric Ca3Ti2O7 (CTO) and Ca2.99Na0.01Ti2O7 (NaCTO) ceramics were synthesized using the solid‐state reaction method. X‐ray powder diffraction technique was used to determine the phase purity and crystal structure of the prepared ceramics. We investigated their ferroelectric polarization, dynamic switching current, and piezoelectric properties, and confirmed their ferroelectric reversal property with a coercive field of 60‐80 kV/cm. A negative piezoelectric behavior was observed with a maximum strain of −0.01% and −0.007% for CTO and NaCTO, respectively. This abnormal piezoelectric behavior can be used to prepare composites with conventional oxide materials that generally exhibit positive piezoelectricity. Mechanism of the negative piezoelectricity effect was ascribed to its special quasi‐two‐dimensional (quasi‐2D) crystal structure with weak van der Waals (vdW) layers alternating with covalent layers.

The novel electrical properties in quasi‐2D hybrid improper ferroelectric Sr3Sn2O7 ceramic and the structural modulation

“…The D-E loop at 130 • C has a similar trend to that at RT, as shown in Figure 7B, and the negative strain increases from −0.12% to −0.14% at 200 kV/cm, which is about a fivefold increase in the negative strain compared to the Ca 3 Ti 2 O 7 ceramic (∼−0.02% at 200 kV/cm). 20 However, when the temperature is 160 • C, the hysteresis almost disappears (Figure 7C), which is consistent with the above studies suggesting that the Sr 3 Sn 2 O 7 ceramic transforms into a paraelectric phase. Meanwhile, the negative strain rapidly decreases to −0.08%.…”

“…To effectively evaluate the piezoelectric properties, the strain value is determined by Δ L / L 0 , which is the ratio of the associated thickness change (Δ L ) to the initial thickness ( L 0 ) of the ceramic. The D – E loop at 130°C has a similar trend to that at RT, as shown in Figure 7B, and the negative strain increases from −0.12% to −0.14% at 200 kV/cm, which is about a fivefold increase in the negative strain compared to the Ca 3 Ti 2 O 7 ceramic (∼−0.02% at 200 kV/cm) 20 . However, when the temperature is 160°C, the hysteresis almost disappears (Figure 7C), which is consistent with the above studies suggesting that the Sr 3 Sn 2 O 7 ceramic transforms into a paraelectric phase.…”

“…Only a few types of ferroelectrics have been found in the literature to have the negative piezoelectric effect, including the small-molecular organic ferroelectric BTAs, 43 the 2D layered ferroelectric CuInP 2 S, 44 and the ferroelectric polymer PVDF. 45 Recently, the negative piezoelectricity was observed in the layered perovskite oxides Ca 3 Ti 2 O 7 ceramics by Wang et al 20 They analyzed the mechanism of the piezoelectric effect and explained with a qualitative view of the lattice-dipole relationship. In addition, the interesting negative piezoelectric effect was attributed the to the unique quasi-2D structure, with alternating covalent layers and weak van der Waals (vdW) layers.…”

“…In our previous work, we observed negative piezoelectric behaviors in Ca 3 Ti 2 O 7 and Na-doped Ca 3 Ti 2 O 7 ceramics. 20 The negative piezoelectrical behavior is a rare occurrence in perovskite oxides, and its origin has been debated with various proposed models.…”

Review of experimental progress of hybrid improper ferroelectricity in layered perovskite oxides

“…For example, Ca 3 (Ti,Al,Nb) 2 O 7 [31], (Ca,La) 3 (Ti,Al) 2 O 7 [34], and Ca 3 (Ti,Mn) 2 O 7 [35] ceramics have similar ferroelectric polarisation values, but the coercive field is relatively larger than that of the Ca 3 Ti 2 O 7 ceramic. Although the coercive field of (Ca,Na) 3 Ti 2 O 7 reduces to approximately 50 kV cm −1 , its polarisation value decreases simultaneously [36,77]. In contrast, (Ca,Ru) 3 Ti 2 O 7 [78] and (Ca,Sr) 3 Ti 2 O 7 [29,32] ceramics exhibit larger polarisations, but the large coercive fields (E c ∼ 120 kV cm −1 ) are still not controlled.…”