November 22, 2023
PSI Villigen
Europe/Zurich timezone

Revealing the KDP soft-mode coupling mechanism with infrared spectroscopy under pressure

Nov 22, 2023, 11:00 AM
Auditorium (WHGA) (PSI Villigen)

Auditorium (WHGA)

PSI Villigen


Ana Akrap (University of Fribourg)


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).

Primary authors

Ana Akrap (University of Fribourg) Christopher Homes (National Synchrotron Light Source II, Brookhaven National Laboratory) David Santos (Department of Physics, University of Fribourg) Francesco Capitani (Synchrotron Soleil) Ricardo Lobo (ESPCI Paris) Serena Nasrallah (Department of Physics, University of Fribourg)

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