Giant Piezoelectric Response in Superionic Polar Semiconductor

A lead-free piezoelectric material whose performance is comparable to that of market-dominant lead-based ones has been strongly desired for the development of environmentally friendly piezoelectric devices. In spite of intensive research activities, however, few effective alternatives have been found due to severe requirements such as the absence of conducting carriers screening the electric polarization. Here, a polar semiconductor, AgCrSe2, is demonstrated as a promising piezoelectric material even with a finite conductivity. The inverse piezoelectric effect for AgCrSe2 single crystals, of which carrier density can be tuned by subtle nonstoichiometry, is studied as a model material at room temperature. Although the inverse piezoelectric effect of the AgCrSe2 crystals tends to be suppressed as resistivity becomes lower, the maximum piezoelectric coefficients (d(33)) reach 100-1000 pC N-1, comparable to those reported for typical piezoelectric BaTiO3 and lead-based perovskites. The large piezoelectric response in AgCrSe2 is attributed to the large polarizability inherent to the intercalated Ag+ ions acting as ionic carriers. The present results demonstrate that superionic polar semiconductors have a potential to be high-performance lead-free piezoelectric materials overcoming the screening effect by the enhanced polarizability.