Lead-free, low dielectric constant ceramics with giant electrostriction

Maxim Varenik ¹ Boyuan Xu ² Junying Li ³ Elad Gaver ¹ Ellen Wachtel ¹ David Ehre ¹ Prahlad K. Routh ³ Anatoly I. Frenkel ³ Yue Qi ⁴ Igor Lubomirsky ¹

¹Department of Molecular Chemistry and Materials Science, Weizmann Institute, Rehovot, Israel, Israel
²Department of Physics, Brown University, Providence, Rhode Island, USA
³Department of Materials Science and Chemical Engineering, Stony Brook University, Stony Brook, New York, USA
⁴Brown University, School of Engineering, Providence, Rhode Island, USA

The majority of electrostrictive ceramics in use today are based on lead manganese niobate. These ceramics display large electrostriction strain coefficients M ≈ 10^-16 m²/V² at frequencies up to a few kHz; however, they suffer from two major drawbacks: large dielectric constants (>10000), which require high driving currents, and incompatibility with thin-film Si-microfabrication techniques. We have recently reported that aliovalent doped ceria exhibits electrostriction coefficients >100-fold larger than estimated on the basis of Newnham’s scaling law for classical electrostrictors. This “non-classical” behavior has been attributed to the formation of highly polarizable, elastic dipoles reorienting under external electric field. In the present work, we find that 10mol% Zr⁴⁺-doped ceria displays M ≈ 10^-16 m²/V² throughout the 0.1-3000 Hz frequency range. However, practical application of these ceramics may be hindered by the relatively large, room-temperature electrical conductivity (> 10^-7 S/m), a result of the formation of Ce³⁺ which can promote electron hopping. Suppression Ce³⁺ of by lanthanide co-doping reduces the dielectric constant to ~ 30 but also reduces the electrostriction constant to 10^-17 m²/V². Our results imply that by systematically adjusting the composition of ceria-based solid solutions, the potential exists for development of technologically useful electrostrictive materials which are, at the same time, fully compatible with Si-microfabrication. Based on the combination of XAS data and modeling we offer a plausible mechanism of non-classical electrostriction, which in contrast to the case of non-classical electrostriction in ionic conductors, relies on dynamic, rather than permanent elastic dipoles. The elastic dipoles forms because of the contraction of the Zr-O bonds in [ZrO₈] unit, forming a “free volume” within which [ZrO₈] units have some freedom to move. This mechanism is not unique to Zr doped ceria but may be realized in other fluorite-structured hosts.