Speaker
Description
Potassium dihydrogen phosphate, KH$_2$PO$_4$ (KDP), is a classic, broadly used ferroelectric material. It is a model system of an order-disorder material, with a Curie temperature $T_C$ of 123 K. Above this temperature, it is a tetragonal paraelectric. Below, it becomes orthorhombic. In the 1940s, Slater wrote an order-disorder theory to describe rather well the physics of KDP [1]. However, his theory failed to describe why the polarization doesn’t change below the ordering temperature, and why $T_C$ increases when hydrogen is replaced by deuterium. Therefore, it was understood that phonons must also play a role, through coupling to the proton which tunnels in a double well potential [2]. How exactly this happens remained unclear for a long time [3].
In our work, which spanned more than a decade and took place across two continents, we measured the far-infrared reflectivity of KDP up to 2 GPa in its ferroelectric and paraelectric phases. We identified an infrared mode that couples the hydrogen network to the lattice modes, to create the ferroelectric polarization.
[1] J. C. Slater, Theory of the Transition in KH$_2$PO$_4$, TheJournal of Chemical Physics 9, 16 (1941).
[2] J. Pirenne, On the ferroelectricity of KH$_2$PO$_4$ and KD$_2$PO$_4$ crystals, Physica 15, 1019 (1949).
[3] P. Simon and F. Gervais, Phase-transition mechanism in RbH$_2$PO$_4$-type ferroelectrics, Phys. Rev. B 32, 468 (1985).
