Final answer:
Single crystal ferroelectrics display a 'hard' ferroelectric hysteresis curve due to their consistent crystal structure that leads to greater stability and resistance to domain switching, while polycrystalline ferroelectrics exhibit a 'soft' curve because of easier domain switching at lower fields due to grain boundaries and defects.
Step-by-step explanation:
The hysteresis behavior in ferroelectric materials such as single crystal and polycrystalline ferroelectrics is linked to their domain structures and intrinsic properties. Single crystal ferroelectrics typically exhibit a 'hard' ferroelectric hysteresis curve due to their consistent and uniform crystal structure, which requires greater applied electric fields to switch the polarization, and yields a more stable and resistant behavior against changes in domain orientation. On the other hand, polycrystalline ferroelectrics possess a 'soft' hysteresis curve, which can be attributed to the presence of grain boundaries, defects, and a range of orientation in the crystallites. These factors lead to easier switching of the domains at lower electric fields and result in a hysteresis curve that indicates greater ease of polarization reversal but less stability.
The difference in the hysteresis curves between single crystal and polycrystalline ferroelectrics extends from their microstructural differences. The uniform domain structures in single crystal ferroelectrics make them more resistant to polarization switching, resulting in a hard hysteresis loop. Conversely, the less uniform domains and the presence of various defects in polycrystalline ferroelectrics make it easier for these domains to switch, leading to a soft hysteresis loop. This distinction has implications for the applications of these materials, with hard ferroelectrics being suitable for memory storage, where stability is key, and soft ferroelectrics being used in sensors and actuators, where easy polarization switching is beneficial.