Mentions: Sample C was glued to a flexible substrate, which was bent upward or downward to create tensile or compressive strain in the crystal, respectively. The evolution of the domain structure in the MAPbI3 crystal under stress, an inherent feature of ferroelastic materials, is reported in Fig. 2. We have chosen the domain group area fraction (F) with respect to the whole field of view to be a relevant descriptor of the sample behavior: . Thus, F = 1 (−1) implies that the entire field of view is filled with a bright (dark) single domain, and F = 0 implies equal areas for dark and bright domains. Figure 2 shows the domain area fraction–versus–stress plot together with optical images at selected points of the stress cycle (points A to H). Although the F-versus-stress plot has a complex shape and differs from the classic strain-stress hysteresis curve [note that its relationship with strain is nontrivial (31)] (21), it represents and consolidates the rich data displayed by multiple images well (Fig. 2). Starting with an approximately equal domain population (Fig. 2A; F ≈ 0), the system evolved into a state with predominantly bright domains under tensile stress (F increases). The most important observation is that a new family of bright domains with walls tilted 70° and 109° with respect to the old domains appears in response to the application of a large tensile stress (Fig. 2B). The formation of these domains is associated with a minimum in the domain area fraction (at stress ≈ 0.5 a.u.). Consequently, the preceding F maximum (at stress = 0.3 a.u.) must arise because of minimization of the system’s energy in the wake of transition that yields the 70° domains. Releasing the stress gradually reduces the density of the new domains (reflecting a decrease in the elastic energy stored in the material) until their disappearance (Fig. 2, B to E) and yields a negative F (Fig. 2, C to F). Upon further increase in compressive stress, the system undergoes a sharp transition characterized by the complete disappearance of bright domains (Fig. 2, F and G). Beyond this point, the crystal yielded, cracking and relieving some of the strain. The nearly zero stress domain pattern reappeared (Fig. 2H), and the system became only weakly responsive to the applied stress afterward. The plot is clearly hysteretic and nonlinear, which is typical for ferroelastic materials, and indicates that the domains in sample C (i) are affected by external stress, (ii) exist at nominally zero stress, and (iii) do not return to the same state upon stress cycling, that is, revealing the hysteretic nature of material’s response to the applied stress.
Evidence and control of ferroelastic (but not ferroelectric) domains in CH3NH3PbI3 perovskite are provided.
No MeSH data available.