Proton configurations in the hydrogen bonds ofKH2PO4as seen by resonant x-ray diffraction

Abstract

KH2PO4 (KDP) belongs to the class of hydrogen-bonded ferroelectrics, whose paraelectric to ferroelectric phase transition is driven by the ordering of the protons in the hydrogen bonds. We demonstrate that forbidden reflections of KDP, when measured at an x-ray absorption edge, are highly sensitive to the asymmetry of proton configurations. The change of average symmetry caused by the 'freezing' of the protons during the phase transition is clearly evidenced. In the paraelectric phase, we identify in the resonant spectra of the forbidden reflections a contribution related to the transient proton configurations in the hydrogen bonds, which violates the high average symmetry of the sites of the resonant atoms. The analysis of the temperature dependence reveals a change of relative probabilities of the different proton configurations. They follow the Arrhenius law and the activation energies of polar and Slater configurations are 18.6 and 7.3 meV respectively.

Helicity-dependent resonant X-ray scattering in CuB2O4

“…Interestingly, forbidden reflections that originate from interference between different scattering channels (e.g. E1.E1 with E1.E2) have been experimentally observed (Kirfel et al, 2002;Collins et al, 2003;Beutier et al, 2014Beutier et al, , 2015Richter et al, 2014;Kokubun et al, 2001;Oreshko et al, 2012). However, local chirality could not be fully investigated in these experiments since linearly polarized X-rays were used.…”

Thermal and Nuclear Quantum Effects at the Antiferroelectric to Paraelectric Phase Transition in KOH and KOD Crystals

“…In the low- T FE phase, the protons are off-center ordered in the HBs, which are nearly perpendicular to the FE c -axis. 4 KDP becomes paraelectric (PE) at higher temperatures. 5 , 6 The proton off-centering stretches the O–O distance and thus correlates to the lattice expansion and heavy-ion distortions.…”

“…The prototype material is potassium dihydrogen phosphate (KDP) discovered in 1935, where strong hydrogen bonds (HBs) link the tetrahedral PO 4 units. In the low- T FE phase, the protons are off-center ordered in the HBs, which are nearly perpendicular to the FE c -axis . KDP becomes paraelectric (PE) at higher temperatures. , The proton off-centering stretches the O–O distance and thus correlates to the lattice expansion and heavy-ion distortions.…”

Forbidden Reflections in TeO2 in the Vicinity of the Te L1 Absorption Edge

“…Contrary to the DAFS method [5,6], where the spectra of allowed Bragg reflections are studied, in the forbidden Bragg reflections any contribution from non-resonant Thomson scattering is absent making subtle resonant scattering more pronounced. In numerous studies of forbidden reflections, the great possibilities of the method were demonstrated using examples of determining the magnetic structure [7][8][9], orbital and charge ordering [10,11], local chirality [12,13] and thermal displacements of atoms [14][15][16][17] etc. Particularly exciting is the opportunity to study variations of local properties when the crystal is exposed to external influences.…”