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31.12.07 - Phase transitions, anisotropy and domain engineering : the piezoelectric properties of relaxor-ferroelectric single crystals. Thèse EPFL, n° 3513 (2006). Dirs.: Profs N. Setter and D. Damjanovic.

"For his significant contribution to the understanding of relationships among domain engineering, crystal symmetry, phase transitions and piezoelectric properties of relaxor-ferroelectrics crystals and ferroelectric materials in general"

Phase transitions, anisotropy and domain engineering : the piezoelectric properties of relaxor-ferroelectric single crystals.

Relaxor-ferroelectric single crystals PZN-xPT [(1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3] and PMN-xPT [(1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3] look extremely promising for next-generation actuator and sensor applications due to their "giant" piezoelectric properties ( > 2000 pm/V) when poled along special, non-polar directions. Such properties are observed in a special region of the phase diagram where various crystallographic phases are nearly degenerate leading to extremely complex microstructures and electromechanical behavior, and fragile phase stabilities.This complexity is investigated by measurement of bulk electromechanical properties and optical microscopy. Electric-field induced phase transitions are evidenced by microstrain measurement and in situ microscopy showing that the phase present is strongly dependent on the strength and direction of the applied field, as well as any prior exposure to field (poling); the latter is due to the presence of residual electric fields and/or residual stresses. The (direct) piezoelectric response is investigated under compressive stresses both along and perpendicular to the poling direction ( and modes, respectively); the former is hysteretic, obeying the Rayleigh Law, due to domain wall motion driven by a local stress induced phase transition. It is concluded that strong piezoelectric anisotropy close to phase instability, and an electromechanical softness due to the complex "relaxor" chemistry, can explain the giant response of PMN-xPT and PZN-xPT.

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