Improper Ferroelectricity in Stuffed Aluminate Sodalites for Pyroelectric Energy Harvesting

Pyroelectric materials can generate electric energy from temporal variation of temperature. Since the pyroelectric energy harvesting is complementary to thermoelectric one that converts spatial temperature gradient to the electric energy, their combination may boost efficient reuse of waste heat.
Lead-based ferroelectric oxides, typically PZT, have so far been considered for pyroelectric energy harvesters due to their large spontaneous polarization and relatively high ferroelectric phase transition temperature, which cooperatively give good pyroelectric coefficient in a useful temperature range. However, giant dielectric permittivity of the lead-based ferroelectric oxides limits the pyroelectric conversion efficiency by suppressing transduction from an induced polarization variation to an output electric field.
Here I show improper ferroelectricity in a stuffed zeolite with an excellent performance for the pyroelectric energy harvesting.1,2) Though spontaneous polarization of the stuffed zeolite is smaller by an order of magnitude than that in the conventional lead-based ferroelectrics, its low permittivity even around the phase transition temperature achieves a comparable level of figure-of-merit as that for PZT. Furthermore, if one focuses on a conversion efficiency per unit weight, the stuffed zeolite largely surpasses PZT due to its fairly light density.
Comprehensive studies employing synchrotron x-ray powder diffractions and first-principles calculations have clarified that the improper ferroelectricity of stuffed zeolite stems from multiple phonon instability in finite k-points of Brillouin zone, which synchronizes with a hopping motion of tetrahedral complex anions filled in voids of an AlO₄ zeolite framework.¹⁾
1) Y. Maeda et al., Phys. Rev. Applied 7 (2017) 034012.
2) T. Wakamatsu et al., Phys. status solidi RRL 11 1700009 (2017).